feat: complete v0.3.1 agent workflow

.gitignore

@@ -3,3 +3,4 @@ agentu*.yaml
 /coverage.out
 *.swp
 chat-history.txt
+dist/

README.md

@@ -1,19 +1,24 @@
 # agentu
 
-agentu is a small local agent written in Go. The current MVP is a terminal chat
+agentu is a small local coding agent written in Go. It provides a terminal chat
 UI with OpenAI-compatible providers, streaming responses, session-level
-model/provider switching, and local tools.
+model/provider switching, skills, approvals, and local tools.
 
-## Current MVP
+## v0.3.1
 
 - TUI-first chat experience with a light theme by default.
 - OpenAI-compatible `/v1/chat/completions` provider support.
 - OpenAI Responses API (`/v1/responses`) provider support.
 - Multiple providers from `~/.agentu/config.yaml`.
 - Session-only provider, model, and thinking-level switching.
-- Read-only project tools enabled by default.
+- Interactive approval for model-requested file writes and shell commands.
+- Atomic file writes, per-file rollback snapshots, and multi-file patches.
+- Project outlines with Go, TypeScript/JavaScript, and Rust symbols.
+- Global Codex-style skills loaded from `~/.agentu/skills`.
+- Persistent session index and in-memory task tracking.
 - Built-in web search and URL fetching.
-- `--yolo` mode for file writes and shell execution.
+- `--yolo` mode for automatic file writes and shell execution.
+- Temporary `/btw` side questions that can run while the main task continues.
 
 ## Quick Start
 
@@ -30,7 +35,9 @@ Use dark mode when your terminal is dark:
 go run . --theme dark
 ```
 
-Use `--yolo` only when you want agentu to write files or run shell commands:
+By default, the TUI asks for approval before every model-requested file change
+or shell command. Use `--yolo` only when those operations should run without
+confirmation:
 
 ```sh
 go run . --yolo
@@ -114,7 +121,7 @@ go run . [flags]
 - `--config <path>` loads a YAML config file. Default: `~/.agentu/config.yaml`.
 - `--theme light|dark` selects the TUI theme. Default: `light`.
 - `--plain` uses the simple line-based REPL instead of the TUI.
-- `--yolo` enables automatic file writes and shell execution.
+- `--yolo` automatically approves model-requested file writes and shell execution.
 - `--resume <id>` resumes a saved session from `~/.agentu/sessions`.
 
 ## TUI Controls
@@ -125,16 +132,23 @@ go run . [flags]
 - The input starts at one line and grows up to six lines.
 - `page up` / `page down` scrolls the transcript.
 - `esc` cancels a running response.
+- While a task is running, `/btw <question>` asks a concurrent side question.
+- Entering only `.` means “continue with the current task.”
+- At a tool approval prompt, `y` approves and `n` or `esc` denies the operation.
 - Type `/` to show available commands.
 - In `--yolo` mode, type `! <command>` to run a shell command from the configured working directory.
 
 Slash commands:
 
+- `/btw <question>` answers from a snapshot of the current context without adding the question or answer to conversation history. It never exposes tools and can run alongside the main task.
 - `/clear` resets conversation history.
 - `/compact` summarizes older conversation history and keeps the newest ~20% of context, with a small recent-message floor.
 - `/status` shows changed files with `git status --short`.
 - `/diff [--stat] [path...]` shows the unstaged git diff.
 - `/test [command...]` runs a project test command; defaults to `go test ./...` for Go modules or `npm test` for Node projects.
+- `/skills` lists global skills.
+- `/skills install <path-or-url>` installs a skill and activates it in the current conversation.
+- `/rollback [id]` restores a file snapshot; without an ID it restores the newest snapshot.
 - `/model` shows current provider, model, thinking, and switch usage.
 - `/model provider <name>` switches provider for the current session.
 - `/model model <name>` or `/model <name>` switches model for the current session.
@@ -148,26 +162,36 @@ Slash commands:
 
 ## Local Tools
 
-Read-only tools are available by default:
+These tools are always available:
 
 - `file_read`
 - `file_list`
 - `file_search` with optional `context_lines` and `max_results`
 - `code_symbols` for Go packages, imports, types, functions, and methods
+- `project_outline` for a gitignore-aware tree with Go, TypeScript/JavaScript, and Rust symbols
+- `todo` for conversation-local task tracking
 - `git_status` shows changed files
 - `git_diff` shows unstaged or staged diff output
 - `git_log` shows recent commit history
 
-Web tools are also available in read-only mode:
+Web tools are also available:
 
 - `web_search`
 - `web_fetch`
 
-`--yolo` additionally enables:
+In the TUI, these tools require approval by default and run automatically with
+`--yolo`:
 
 - `file_write` for creating or replacing whole files
 - `file_edit` for surgical line-range or pattern edits to existing files
-- `shell_run` for non-interactive shell commands
+- `apply_patch` for atomic-style multi-file V4 patches, including dry runs
+- `shell_run` for non-interactive shell commands; output streams live into the
+  TUI while the command runs instead of appearing only after completion
+
+Before a file mutation, agentu stores an in-memory snapshot and includes its
+`rb_...` ID in the tool result. Use `/rollback` or `/rollback <id>` to restore
+it. Snapshots last for the lifetime of the process. The plain REPL remains
+read-only unless `--yolo` is supplied because it has no interactive approval UI.
 
 Use `--yolo` for prompts that ask agentu to run or inspect local commands, for
 example:
@@ -176,7 +200,20 @@ example:
 请帮我运行 `go test ./...` 并总结结果
 ```
 
+## Global Skills
 
+At startup, agentu loads both Codex-style directories containing `SKILL.md`
+and legacy flat Markdown files from `~/.agentu/skills`. Skill instructions are
+added to the system prompt. Install a local directory, Markdown file, or direct
+HTTP(S) Markdown URL from the TUI:
+
+```text
+/skills install ./my-skill
+/skills install https://example.com/SKILL.md
+```
+
+Directory installs preserve supporting files such as `references/`; symlinks
+inside a source directory are rejected.
 
 ## Development
 
@@ -199,3 +236,5 @@ Local config and build outputs are intentionally ignored by git.
   API type.
 - The config schema is intentionally strict during MVP development. Deprecated
   fields such as `provider` or `active_provider` are rejected.
+- Session metadata is indexed in `~/.agentu/sessions/index.json`; complete
+  conversations remain stored as one JSON file per session.

internal/agent/agent.go

@@ -15,15 +15,17 @@ import (
 )
 
 const (
-	defaultMaxToolRounds      = 50
+	defaultMaxToolRounds          = 50
-	doomLoopThreshold         = 3
+	doomLoopThreshold             = 3
 	compactMinRecentMessages      = 6
 	compactRecentRetentionPercent = 20
-	summaryMessageLimit       = 8000
+	summaryMessageLimit           = 8000
-	contextSummaryPrefix      = "[context summary]"
+	contextSummaryPrefix          = "[context summary]"
-	toolLogArgumentLimit      = 400
+	toolLogArgumentLimit          = 400
-	toolLogOutputPreviewBytes = 4096
+	toolLogOutputPreviewBytes     = 4096
-	toolLogOutputMarker       = "[output]"
+	toolLogOutputMarker           = "[output]"
+	toolStreamLogLimit            = tools.MaxToolOutputBytes
+	continuationPrompt            = "Continue with the current task."
 
 	summarizerSystemPrompt = "You are a conversation summarizer. Summarize the following conversation history into a concise but detailed summary. Preserve:\n" +
 		"- Key facts, decisions, and conclusions\n" +
@@ -40,6 +42,7 @@ type toolExecutionResult struct {
 }
 
 type Agent struct {
+	mu               sync.RWMutex
 	provider         llm.Provider
 	providerName     string
 	model            string
@@ -48,6 +51,8 @@ type Agent struct {
 	thinkingEnabled  bool
 	systemPrompt     string
 	toolRegistry     *tools.Registry
+	authorizer       ToolAuthorizer
+	rollbackStore    *tools.RollbackStore
 	toolTimeout      time.Duration
 	maxToolRounds    int
 	maxContextTokens int
@@ -64,6 +69,8 @@ type Options struct {
 	ThinkingEnabled   bool
 	SystemPrompt      string
 	ToolRegistry      *tools.Registry
+	Authorizer        ToolAuthorizer
+	RollbackStore     *tools.RollbackStore
 	ToolTimeout       time.Duration
 	MaxToolIterations int
 	MaxContextTokens  int
@@ -87,6 +94,8 @@ func New(opts Options) *Agent {
 		thinkingEnabled:  opts.ThinkingEnabled,
 		systemPrompt:     opts.SystemPrompt,
 		toolRegistry:     opts.ToolRegistry,
+		authorizer:       opts.Authorizer,
+		rollbackStore:    opts.RollbackStore,
 		toolTimeout:      toolTimeout,
 		maxToolRounds:    maxToolRounds,
 		maxContextTokens: opts.MaxContextTokens,
@@ -115,6 +124,8 @@ type RuntimeInfo struct {
 }
 
 func (a *Agent) SetRuntime(opts RuntimeOptions) {
+	a.mu.Lock()
+	defer a.mu.Unlock()
 	if opts.Provider != nil {
 		a.provider = opts.Provider
 	}
@@ -131,6 +142,8 @@ func (a *Agent) SetRuntime(opts RuntimeOptions) {
 }
 
 func (a *Agent) RuntimeInfo() RuntimeInfo {
+	a.mu.RLock()
+	defer a.mu.RUnlock()
 	return RuntimeInfo{
 		ProviderName:     a.providerName,
 		Model:            a.model,
@@ -142,6 +155,12 @@ func (a *Agent) RuntimeInfo() RuntimeInfo {
 }
 
 func (a *Agent) Clear() {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	a.clear()
+}
+
+func (a *Agent) clear() {
 	a.lastUsage = nil
 	a.messages = nil
 	if strings.TrimSpace(a.systemPrompt) != "" {
@@ -152,8 +171,27 @@ func (a *Agent) Clear() {
 	}
 }
 
+// AddSystemInstructions makes newly installed instructions effective for the
+// current conversation and for future Clear calls.
+func (a *Agent) AddSystemInstructions(instructions string) {
+	instructions = strings.TrimSpace(instructions)
+	if instructions == "" {
+		return
+	}
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	a.systemPrompt = strings.TrimSpace(a.systemPrompt + "\n\n" + instructions)
+	if len(a.messages) > 0 && a.messages[0].Role == llm.RoleSystem {
+		a.messages[0].Content = a.systemPrompt
+		return
+	}
+	a.messages = append([]llm.Message{{Role: llm.RoleSystem, Content: a.systemPrompt}}, a.messages...)
+}
+
 // Messages returns a copy of the current conversation messages.
 func (a *Agent) Messages() []llm.Message {
+	a.mu.RLock()
+	defer a.mu.RUnlock()
 	out := make([]llm.Message, len(a.messages))
 	copy(out, a.messages)
 	return out
@@ -161,11 +199,23 @@ func (a *Agent) Messages() []llm.Message {
 
 // SetMessages replaces the conversation messages (used for session restore).
 func (a *Agent) SetMessages(messages []llm.Message) {
+	a.mu.Lock()
+	defer a.mu.Unlock()
 	a.messages = append([]llm.Message(nil), messages...)
+	if strings.TrimSpace(a.systemPrompt) == "" {
+		return
+	}
+	if len(a.messages) > 0 && a.messages[0].Role == llm.RoleSystem {
+		a.messages[0].Content = a.systemPrompt
+		return
+	}
+	a.messages = append([]llm.Message{{Role: llm.RoleSystem, Content: a.systemPrompt}}, a.messages...)
 }
 
 // LastUsage returns a copy of the most recent provider token usage data.
 func (a *Agent) LastUsage() *llm.Usage {
+	a.mu.RLock()
+	defer a.mu.RUnlock()
 	if a.lastUsage == nil {
 		return nil
 	}
@@ -175,6 +225,12 @@ func (a *Agent) LastUsage() *llm.Usage {
 
 // SetLastUsage restores the most recent provider token usage data.
 func (a *Agent) SetLastUsage(usage *llm.Usage) {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	a.setLastUsage(usage)
+}
+
+func (a *Agent) setLastUsage(usage *llm.Usage) {
 	if usage == nil {
 		a.lastUsage = nil
 		return
@@ -182,6 +238,7 @@ func (a *Agent) SetLastUsage(usage *llm.Usage) {
 	copy := *usage
 	a.lastUsage = &copy
 }
+
 // compactRetainedTokenBudget returns the number of estimated tokens to retain
 // based on compactRecentRetentionPercent, using integer ceiling division.
 func compactRetainedTokenBudget(totalTokens int) int {
@@ -237,31 +294,35 @@ func compactRecentStart(messages []llm.Message, prefixEnd int) int {
 	return compactTurnBoundary(messages, prefixEnd, start)
 }
 
-
 // Compact summarizes older conversation messages while preserving the system
 // prompt and recent messages intact.
 func (a *Agent) Compact(ctx context.Context) error {
-	if len(a.messages) < 4 {
+	a.mu.RLock()
+	messages := append([]llm.Message(nil), a.messages...)
+	provider := a.provider
+	model := a.model
+	a.mu.RUnlock()
+	if len(messages) < 4 {
 		return nil
 	}
-	if a.provider == nil {
+	if provider == nil {
 		return fmt.Errorf("provider is not configured")
 	}
 
 	prefixEnd := 0
-	if a.messages[0].Role == llm.RoleSystem {
+	if messages[0].Role == llm.RoleSystem {
 		prefixEnd = 1
 	}
-	recentStart := compactRecentStart(a.messages, prefixEnd)
+	recentStart := compactRecentStart(messages, prefixEnd)
-	if recentStart <= prefixEnd || recentStart >= len(a.messages) {
+	if recentStart <= prefixEnd || recentStart >= len(messages) {
 		return nil
 	}
-	middle := append([]llm.Message(nil), a.messages[prefixEnd:recentStart]...)
+	middle := append([]llm.Message(nil), messages[prefixEnd:recentStart]...)
 	if len(middle) == 0 {
 		return nil
 	}
 
-	summary, usage, err := a.summarizeMessages(ctx, middle)
+	summary, usage, err := summarizeMessages(ctx, provider, model, middle)
 	if err != nil {
 		return fmt.Errorf("compact context: %w", err)
 	}
@@ -270,13 +331,15 @@ func (a *Agent) Compact(ctx context.Context) error {
 		return fmt.Errorf("compact context: summary was empty")
 	}
 
-	compressed := make([]llm.Message, 0, prefixEnd+1+len(a.messages[recentStart:]))
+	compressed := make([]llm.Message, 0, prefixEnd+1+len(messages[recentStart:]))
-	compressed = append(compressed, a.messages[:prefixEnd]...)
+	compressed = append(compressed, messages[:prefixEnd]...)
 	compressed = append(compressed, llm.Message{
 		Role:    llm.RoleAssistant,
 		Content: contextSummaryPrefix + "\n" + summary,
 	})
-	compressed = append(compressed, a.messages[recentStart:]...)
+	compressed = append(compressed, messages[recentStart:]...)
+	a.mu.Lock()
+	defer a.mu.Unlock()
 	a.messages = compressed
 	if usage != nil {
 		a.lastUsage = usage
@@ -285,6 +348,8 @@ func (a *Agent) Compact(ctx context.Context) error {
 }
 
 func (a *Agent) shouldCompact() bool {
+	a.mu.RLock()
+	defer a.mu.RUnlock()
 	if a.maxContextTokens <= 0 {
 		return false
 	}
@@ -294,9 +359,9 @@ func (a *Agent) shouldCompact() bool {
 	return llm.EstimateTokens(a.messages, a.toolDefinitions()) >= a.maxContextTokens
 }
 
-func (a *Agent) summarizeMessages(ctx context.Context, messages []llm.Message) (string, *llm.Usage, error) {
+func summarizeMessages(ctx context.Context, provider llm.Provider, model string, messages []llm.Message) (string, *llm.Usage, error) {
 	req := llm.ChatRequest{
-		Model: a.model,
+		Model: model,
 		Messages: []llm.Message{
 			{Role: llm.RoleSystem, Content: summarizerSystemPrompt},
 			{Role: llm.RoleUser, Content: formatMessagesForSummary(messages)},
@@ -305,7 +370,7 @@ func (a *Agent) summarizeMessages(ctx context.Context, messages []llm.Message) (
 
 	var summary strings.Builder
 	var usage *llm.Usage
-	if err := a.provider.ChatStream(ctx, req, func(event llm.StreamEvent) error {
+	if err := provider.ChatStream(ctx, req, func(event llm.StreamEvent) error {
 		if event.Content != "" {
 			summary.WriteString(event.Content)
 		}
@@ -353,12 +418,93 @@ func roundSignature(calls []llm.ToolCall) string {
 	return b.String()
 }
 
+// BTW answers a temporary side question using a snapshot of the current
+// conversation. It intentionally exposes no tools and does not mutate the
+// conversation, usage, or compaction state.
+func (a *Agent) BTW(ctx context.Context, input string, out io.Writer) error {
+	input = strings.TrimSpace(input)
+	if input == "" {
+		return fmt.Errorf("btw question is required")
+	}
+
+	a.mu.RLock()
+	provider := a.provider
+	model := a.model
+	messages := append([]llm.Message(nil), a.messages...)
+	thinking := a.thinking
+	thinkingParam := a.thinkingParam
+	thinkingEnabled := a.thinkingEnabled
+	a.mu.RUnlock()
+	if provider == nil {
+		return fmt.Errorf("provider is not configured")
+	}
+
+	// A snapshot taken while tools are running may end with unanswered tool
+	// calls, which providers reject. Keep only the latest complete boundary.
+	messages = completeConversationPrefix(messages)
+	messages = append(messages, llm.Message{Role: llm.RoleUser, Content: input})
+	messages = expandContinuationMessages(messages)
+	req := llm.ChatRequest{Model: model, Messages: messages}
+	if thinkingEnabled {
+		if thinkingParam == "" {
+			thinkingParam = "thinking"
+		}
+		req.Extra = map[string]any{thinkingParam: thinking}
+	}
+
+	return provider.ChatStream(ctx, req, func(event llm.StreamEvent) error {
+		if event.Content == "" || out == nil {
+			return nil
+		}
+		_, err := io.WriteString(out, event.Content)
+		return err
+	})
+}
+
+func completeConversationPrefix(messages []llm.Message) []llm.Message {
+	safeEnd := 0
+	pending := make(map[string]struct{})
+	for i, msg := range messages {
+		if len(pending) > 0 && msg.Role != llm.RoleTool {
+			break
+		}
+		switch {
+		case msg.Role == llm.RoleAssistant && len(msg.ToolCalls) > 0:
+			for _, call := range msg.ToolCalls {
+				pending[call.ID] = struct{}{}
+			}
+		case msg.Role == llm.RoleTool:
+			delete(pending, msg.ToolCallID)
+		}
+		if len(pending) == 0 {
+			safeEnd = i + 1
+		}
+	}
+	return append([]llm.Message(nil), messages[:safeEnd]...)
+}
+
+func expandContinuationMessages(messages []llm.Message) []llm.Message {
+	expanded := append([]llm.Message(nil), messages...)
+	for i := range expanded {
+		if expanded[i].Role == llm.RoleUser && strings.TrimSpace(expanded[i].Content) == "." {
+			expanded[i].Content = continuationPrompt
+		}
+	}
+	return expanded
+}
+
+func (a *Agent) appendMessage(message llm.Message) {
+	a.mu.Lock()
+	a.messages = append(a.messages, message)
+	a.mu.Unlock()
+}
+
 func (a *Agent) RunTurn(ctx context.Context, input string, out io.Writer, logs io.Writer) error {
 	input = strings.TrimSpace(input)
 	if input == "" {
 		return nil
 	}
-	a.messages = append(a.messages, llm.Message{Role: llm.RoleUser, Content: input})
+	a.appendMessage(llm.Message{Role: llm.RoleUser, Content: input})
 	if a.shouldCompact() {
 		if err := a.Compact(ctx); err != nil {
 			if logs != nil {
@@ -375,14 +521,14 @@ func (a *Agent) RunTurn(ctx context.Context, input string, out io.Writer, logs i
 			return err
 		}
 		if len(calls) == 0 {
-			a.messages = append(a.messages, llm.Message{
+			a.appendMessage(llm.Message{
 				Role:    llm.RoleAssistant,
 				Content: content,
 			})
 			return nil
 		}
 
-		a.messages = append(a.messages, llm.Message{
+		a.appendMessage(llm.Message{
 			Role:      llm.RoleAssistant,
 			Content:   content,
 			ToolCalls: calls,
@@ -417,7 +563,7 @@ func (a *Agent) RunTurn(ctx context.Context, input string, out io.Writer, logs i
 					strings.Join(toolNames, ", "),
 					doomLoopThreshold,
 				)
-				a.messages = append(a.messages, llm.Message{
+				a.appendMessage(llm.Message{
 					Role:    llm.RoleAssistant,
 					Content: msg,
 				})
@@ -427,7 +573,7 @@ func (a *Agent) RunTurn(ctx context.Context, input string, out io.Writer, logs i
 
 		results := a.executeTools(ctx, calls, logs)
 		for _, result := range results {
-			a.messages = append(a.messages, llm.Message{
+			a.appendMessage(llm.Message{
 				Role:       llm.RoleTool,
 				ToolCallID: result.call.ID,
 				Content:    result.output,
@@ -443,23 +589,33 @@ func (a *Agent) streamAssistant(ctx context.Context, out io.Writer) (string, []l
 	var content strings.Builder
 	calls := make(map[int]*toolCallBuilder)
 
+	a.mu.RLock()
+	provider := a.provider
+	model := a.model
+	messages := append([]llm.Message(nil), a.messages...)
+	thinking := a.thinking
+	thinkingParam := a.thinkingParam
+	thinkingEnabled := a.thinkingEnabled
+	a.mu.RUnlock()
+	if provider == nil {
+		return "", nil, fmt.Errorf("provider is not configured")
+	}
 	req := llm.ChatRequest{
-		Model:    a.model,
+		Model:    model,
-		Messages: append([]llm.Message(nil), a.messages...),
+		Messages: expandContinuationMessages(messages),
 		Tools:    a.toolDefinitions(),
 	}
-	if a.thinkingEnabled {
+	if thinkingEnabled {
-		thinkingParam := a.thinkingParam
 		if thinkingParam == "" {
 			thinkingParam = "thinking"
 		}
-		req.Extra = map[string]any{thinkingParam: a.thinking}
+		req.Extra = map[string]any{thinkingParam: thinking}
 	}
 	if len(req.Tools) > 0 {
 		req.ToolChoice = "auto"
 	}
 
-	err := a.provider.ChatStream(ctx, req, func(event llm.StreamEvent) error {
+	err := provider.ChatStream(ctx, req, func(event llm.StreamEvent) error {
 		if event.Usage != nil {
 			a.SetLastUsage(event.Usage)
 		}
@@ -495,32 +651,144 @@ func (a *Agent) toolDefinitions() []llm.Tool {
 	return a.toolRegistry.Definitions()
 }
 
-func (a *Agent) executeTool(ctx context.Context, call llm.ToolCall) string {
+func (a *Agent) executeTool(ctx context.Context, call llm.ToolCall, logs io.Writer) (string, bool) {
 	name := call.Function.Name
 
 	if a.toolRegistry == nil {
-		return tools.FormatError(fmt.Errorf("tool registry is disabled"))
+		return tools.FormatError(fmt.Errorf("tool registry is disabled")), false
 	}
 	tool, ok := a.toolRegistry.Get(name)
 	if !ok {
-		return tools.FormatError(fmt.Errorf("unknown tool: %s", name))
+		return tools.FormatError(fmt.Errorf("unknown tool: %s", name)), false
 	}
 
 	args := strings.TrimSpace(call.Function.Arguments)
 	if args == "" {
 		args = "{}"
 	}
+	raw := json.RawMessage(args)
+
+	rollbackNotes, err := a.prepareMutation(ctx, tool, call, raw, args)
+	if err != nil {
+		return tools.FormatError(err), false
+	}
 
 	toolCtx, cancel := context.WithTimeout(ctx, a.toolTimeout)
 	defer cancel()
-	output, err := tool.Execute(toolCtx, json.RawMessage(args))
+
+	if streaming, ok := tool.(tools.StreamingTool); ok {
+		output, streamed := a.executeStreamingTool(streaming, toolCtx, raw, name, args, logs)
+		return appendRollbackNotes(output, rollbackNotes), streamed
+	}
+
+	output, err := tool.Execute(toolCtx, raw)
 	if err != nil {
 		if output != "" {
-			return output + "\n" + tools.FormatError(err)
+			return appendRollbackNotes(output+"\n"+tools.FormatError(err), rollbackNotes), false
 		}
-		return tools.FormatError(err)
+		return appendRollbackNotes(tools.FormatError(err), rollbackNotes), false
 	}
-	return output
+	return appendRollbackNotes(output, rollbackNotes), false
+}
+
+func (a *Agent) prepareMutation(ctx context.Context, tool tools.Tool, call llm.ToolCall, raw json.RawMessage, args string) ([]string, error) {
+	if !tools.IsMutatingCall(tool, raw) {
+		return nil, nil
+	}
+
+	var targets []string
+	if targetTool, ok := tool.(tools.TargetPathTool); ok {
+		var err error
+		targets, err = targetTool.TargetPaths(raw)
+		if err != nil {
+			return nil, fmt.Errorf("resolve mutation targets: %w", err)
+		}
+	}
+
+	preview := compact(args, toolLogArgumentLimit)
+	if previewTool, ok := tool.(tools.PreviewableTool); ok {
+		value, err := previewTool.Preview(ctx, raw)
+		if err != nil {
+			return nil, fmt.Errorf("preview mutation: %w", err)
+		}
+		preview = value
+	}
+
+	if a.authorizer != nil {
+		decision, err := a.authorizer.AuthorizeToolCall(ctx, ToolAuthorizationRequest{
+			ID:                call.ID,
+			ToolName:          call.Function.Name,
+			Arguments:         args,
+			Preview:           preview,
+			RollbackAvailable: a.rollbackStore != nil && len(targets) > 0,
+		})
+		if err != nil {
+			return nil, fmt.Errorf("authorize %s: %w", call.Function.Name, err)
+		}
+		if decision != ToolAuthorizationAllow {
+			return nil, fmt.Errorf("tool call denied by user: %s", call.Function.Name)
+		}
+	}
+
+	if a.rollbackStore == nil {
+		return nil, nil
+	}
+	notes := make([]string, 0, len(targets))
+	for _, path := range targets {
+		id, err := a.rollbackStore.Capture(call.Function.Name, path, path)
+		if err != nil {
+			return nil, fmt.Errorf("capture rollback snapshot for %s: %w", path, err)
+		}
+		notes = append(notes, fmt.Sprintf("[rollback] %s %s", id, path))
+	}
+	return notes, nil
+}
+
+func appendRollbackNotes(output string, notes []string) string {
+	if len(notes) == 0 {
+		return output
+	}
+	if strings.TrimSpace(output) == "" {
+		return strings.Join(notes, "\n")
+	}
+	return output + "\n" + strings.Join(notes, "\n")
+}
+
+func (a *Agent) executeStreamingTool(streaming tools.StreamingTool, ctx context.Context, raw json.RawMessage, name, args string, logs io.Writer) (string, bool) {
+	var streamed int
+	emit := func(chunk string) error {
+		if chunk == "" || logs == nil {
+			return nil
+		}
+		if streamed >= toolStreamLogLimit {
+			return nil
+		}
+		if len(chunk) > toolStreamLogLimit-streamed {
+			chunk = chunk[:toolStreamLogLimit-streamed]
+			streamed = toolStreamLogLimit
+			_, err := fmt.Fprintf(logs, "\n[tool] %s %s\n%s\n%s\n[streaming output truncated after %d bytes]", name, compact(args, toolLogArgumentLimit), toolLogOutputMarker, chunk, toolStreamLogLimit)
+			return err
+		}
+		streamed += len(chunk)
+		_, err := fmt.Fprintf(logs, "\n[tool] %s %s\n%s\n%s", name, compact(args, toolLogArgumentLimit), toolLogOutputMarker, chunk)
+		return err
+	}
+
+	output, err := streaming.ExecuteStream(ctx, raw, emit)
+	if logs != nil {
+		if err != nil {
+			_ = emit("\n" + tools.FormatError(err))
+		} else if streamed == 0 {
+			_ = emit("(no output)")
+		}
+	}
+	if err != nil {
+		if output != "" {
+			return output + "\n" + tools.FormatError(err), true
+		}
+		return tools.FormatError(err), true
+	}
+	return output, true
 }
 
 func (a *Agent) hasMutatingCall(calls []llm.ToolCall) bool {
@@ -529,7 +797,11 @@ func (a *Agent) hasMutatingCall(calls []llm.ToolCall) bool {
 	}
 	for _, call := range calls {
 		if tool, ok := a.toolRegistry.Get(call.Function.Name); ok {
-			if tools.IsMutating(tool) {
+			args := strings.TrimSpace(call.Function.Arguments)
+			if args == "" {
+				args = "{}"
+			}
+			if tools.IsMutatingCall(tool, json.RawMessage(args)) {
 				return true
 			}
 		}
@@ -550,9 +822,9 @@ func (a *Agent) executeTools(ctx context.Context, calls []llm.ToolCall, logs io.
 	if a.hasMutatingCall(calls) {
 		// Sequential: avoid racing mutating tools.
 		for i, call := range calls {
-			output := a.executeTool(ctx, call)
+			output, streamed := a.executeTool(ctx, call, logs)
 			results[i] = toolExecutionResult{call: call, output: output}
-			if logs != nil {
+			if logs != nil && !streamed {
 				logToolOutput(logs, call, output)
 			}
 		}
@@ -566,9 +838,9 @@ func (a *Agent) executeTools(ctx context.Context, calls []llm.ToolCall, logs io.
 		wg.Add(1)
 		go func(idx int, c llm.ToolCall) {
 			defer wg.Done()
-			output := a.executeTool(ctx, c)
+			output, streamed := a.executeTool(ctx, c, logs)
 			results[idx] = toolExecutionResult{call: c, output: output}
-			if logs != nil {
+			if logs != nil && !streamed {
 				mu.Lock()
 				logToolOutput(logs, c, output)
 				mu.Unlock()

internal/agent/agent_test.go

@@ -3,8 +3,13 @@ package agent
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
+	"os"
+	"path/filepath"
+	"reflect"
 	"strings"
+	"sync"
 	"testing"
 
 	"agentu/internal/tools"
@@ -87,6 +92,86 @@ func TestAgentRunsToolLoop(t *testing.T) {
 	}
 }
 
+type streamToolProvider struct {
+	calls int
+	t     *testing.T
+}
+
+func (p *streamToolProvider) ChatStream(ctx context.Context, req llm.ChatRequest, emit func(llm.StreamEvent) error) error {
+	p.calls++
+	switch p.calls {
+	case 1:
+		return emit(llm.StreamEvent{ToolCalls: []llm.ToolCallDelta{
+			{Index: 0, ID: "call_1", Type: "function", Name: "test_stream", Arguments: `{}`},
+		}})
+	case 2:
+		last := req.Messages[len(req.Messages)-1]
+		if last.Role != llm.RoleTool || last.Content != "full result" {
+			p.t.Fatalf("last message = %#v", last)
+		}
+		return emit(llm.StreamEvent{Content: "done"})
+	default:
+		p.t.Fatalf("unexpected call count %d", p.calls)
+		return nil
+	}
+}
+
+type streamEchoTool struct{}
+
+func (streamEchoTool) Definition() llm.Tool {
+	return llm.Tool{
+		Type: "function",
+		Function: llm.ToolFunction{
+			Name:        "test_stream",
+			Description: "streaming test",
+			Parameters:  json.RawMessage(`{"type":"object"}`),
+		},
+	}
+}
+
+func (streamEchoTool) Execute(ctx context.Context, raw json.RawMessage) (string, error) {
+	return "full result", nil
+}
+
+func (streamEchoTool) ExecuteStream(ctx context.Context, raw json.RawMessage, emit func(string) error) (string, error) {
+	if err := emit("one\n"); err != nil {
+		return "", err
+	}
+	if err := emit("two\n"); err != nil {
+		return "", err
+	}
+	return "full result", nil
+}
+
+func TestAgentStreamsToolOutputToLogs(t *testing.T) {
+	provider := &streamToolProvider{t: t}
+	a := New(Options{
+		Provider:     provider,
+		Model:        "test",
+		SystemPrompt: "system",
+		ToolRegistry: tools.NewRegistry(streamEchoTool{}),
+	})
+
+	var out strings.Builder
+	var logs strings.Builder
+	if err := a.RunTurn(context.Background(), "stream", &out, &logs); err != nil {
+		t.Fatal(err)
+	}
+	if out.String() != "done" {
+		t.Fatalf("out = %q", out.String())
+	}
+	logText := logs.String()
+	if strings.Count(logText, "[output]") != 2 {
+		t.Fatalf("logs = %q", logText)
+	}
+	if !strings.Contains(logText, "one\n") || !strings.Contains(logText, "two\n") {
+		t.Fatalf("logs missing streamed chunks: %q", logText)
+	}
+	if strings.Contains(logText, "[output]\nfull result") {
+		t.Fatalf("logs contain duplicate full output: %q", logText)
+	}
+}
+
 type contentProvider struct {
 	called bool
 	text   string
@@ -97,6 +182,173 @@ func (p *contentProvider) ChatStream(ctx context.Context, req llm.ChatRequest, e
 	return emit(llm.StreamEvent{Content: p.text})
 }
 
+type btwProvider struct {
+	req llm.ChatRequest
+	err error
+}
+
+func (p *btwProvider) ChatStream(ctx context.Context, req llm.ChatRequest, emit func(llm.StreamEvent) error) error {
+	p.req = req
+	if p.err != nil {
+		return p.err
+	}
+	return emit(llm.StreamEvent{Content: "side answer", Usage: &llm.Usage{TotalTokens: 999}})
+}
+
+func TestBTWUsesContextWithoutMutatingConversation(t *testing.T) {
+	provider := &btwProvider{}
+	a := New(Options{Provider: provider, Model: "test", Thinking: "high", ThinkingEnabled: true})
+	a.SetMessages([]llm.Message{
+		{Role: llm.RoleUser, Content: "main question"},
+		{Role: llm.RoleAssistant, Content: "main answer"},
+	})
+	a.SetLastUsage(&llm.Usage{TotalTokens: 7})
+	before := a.Messages()
+
+	var out strings.Builder
+	if err := a.BTW(context.Background(), "what does that mean?", &out); err != nil {
+		t.Fatal(err)
+	}
+	if out.String() != "side answer" {
+		t.Fatalf("output = %q", out.String())
+	}
+	if !reflect.DeepEqual(a.Messages(), before) {
+		t.Fatalf("conversation changed:\nbefore: %+v\nafter:  %+v", before, a.Messages())
+	}
+	if usage := a.LastUsage(); usage == nil || usage.TotalTokens != 7 {
+		t.Fatalf("usage changed: %+v", usage)
+	}
+	if len(provider.req.Tools) != 0 || provider.req.ToolChoice != "" {
+		t.Fatalf("btw exposed tools: %+v / %q", provider.req.Tools, provider.req.ToolChoice)
+	}
+	last := provider.req.Messages[len(provider.req.Messages)-1]
+	if last.Role != llm.RoleUser || last.Content != "what does that mean?" {
+		t.Fatalf("last request message = %+v", last)
+	}
+	if provider.req.Extra["thinking"] != "high" {
+		t.Fatalf("thinking extra = %+v", provider.req.Extra)
+	}
+}
+
+func TestBTWRejectsEmptyQuestionAndReturnsProviderError(t *testing.T) {
+	provider := &btwProvider{err: errors.New("boom")}
+	a := New(Options{Provider: provider, Model: "test"})
+	if err := a.BTW(context.Background(), "  ", nil); err == nil {
+		t.Fatal("empty question did not fail")
+	}
+	if err := a.BTW(context.Background(), "question", nil); err == nil || !strings.Contains(err.Error(), "boom") {
+		t.Fatalf("provider error = %v", err)
+	}
+}
+
+func TestDotContinuesCurrentTaskWithoutRewritingHistory(t *testing.T) {
+	provider := &btwProvider{}
+	a := New(Options{Provider: provider, Model: "test"})
+	if err := a.RunTurn(context.Background(), "  .  ", nil, nil); err != nil {
+		t.Fatal(err)
+	}
+
+	requestMessage := provider.req.Messages[len(provider.req.Messages)-1]
+	if requestMessage.Content != continuationPrompt {
+		t.Fatalf("request content = %q, want %q", requestMessage.Content, continuationPrompt)
+	}
+	history := a.Messages()
+	if history[0].Content != "." {
+		t.Fatalf("stored input = %q, want literal dot", history[0].Content)
+	}
+}
+
+func TestExpandContinuationMessagesOnlyExpandsSingleDot(t *testing.T) {
+	messages := []llm.Message{
+		{Role: llm.RoleUser, Content: "."},
+		{Role: llm.RoleUser, Content: " . "},
+		{Role: llm.RoleUser, Content: ".."},
+		{Role: llm.RoleUser, Content: ". continue"},
+		{Role: llm.RoleAssistant, Content: "."},
+	}
+	expanded := expandContinuationMessages(messages)
+	if expanded[0].Content != continuationPrompt || expanded[1].Content != continuationPrompt {
+		t.Fatalf("single dots were not expanded: %+v", expanded)
+	}
+	for _, index := range []int{2, 3, 4} {
+		if expanded[index].Content != messages[index].Content {
+			t.Fatalf("message %d changed from %q to %q", index, messages[index].Content, expanded[index].Content)
+		}
+	}
+	if messages[0].Content != "." || messages[1].Content != " . " {
+		t.Fatalf("source messages were mutated: %+v", messages)
+	}
+}
+
+func TestCompleteConversationPrefixDropsPendingToolRound(t *testing.T) {
+	messages := []llm.Message{
+		{Role: llm.RoleUser, Content: "first"},
+		{Role: llm.RoleAssistant, Content: "answer"},
+		{Role: llm.RoleUser, Content: "main"},
+		{Role: llm.RoleAssistant, ToolCalls: []llm.ToolCall{{ID: "call_1"}, {ID: "call_2"}}},
+		{Role: llm.RoleTool, ToolCallID: "call_1", Content: "one"},
+	}
+	got := completeConversationPrefix(messages)
+	if len(got) != 3 || got[len(got)-1].Content != "main" {
+		t.Fatalf("complete prefix = %+v", got)
+	}
+}
+
+type concurrentBTWProvider struct {
+	mainStarted chan struct{}
+	releaseMain chan struct{}
+	once        sync.Once
+	mu          sync.Mutex
+	btwRequest  llm.ChatRequest
+}
+
+func (p *concurrentBTWProvider) ChatStream(ctx context.Context, req llm.ChatRequest, emit func(llm.StreamEvent) error) error {
+	last := req.Messages[len(req.Messages)-1].Content
+	if last == "main task" {
+		p.once.Do(func() { close(p.mainStarted) })
+		select {
+		case <-p.releaseMain:
+			return emit(llm.StreamEvent{Content: "main answer"})
+		case <-ctx.Done():
+			return ctx.Err()
+		}
+	}
+	p.mu.Lock()
+	p.btwRequest = req
+	p.mu.Unlock()
+	return emit(llm.StreamEvent{Content: "side answer"})
+}
+
+func TestBTWCanRunDuringMainTurn(t *testing.T) {
+	provider := &concurrentBTWProvider{mainStarted: make(chan struct{}), releaseMain: make(chan struct{})}
+	a := New(Options{Provider: provider, Model: "test"})
+	mainDone := make(chan error, 1)
+	go func() { mainDone <- a.RunTurn(context.Background(), "main task", nil, nil) }()
+	<-provider.mainStarted
+
+	var side strings.Builder
+	if err := a.BTW(context.Background(), "side question", &side); err != nil {
+		t.Fatal(err)
+	}
+	if side.String() != "side answer" {
+		t.Fatalf("side output = %q", side.String())
+	}
+	provider.mu.Lock()
+	req := provider.btwRequest
+	provider.mu.Unlock()
+	if len(req.Messages) < 2 || req.Messages[len(req.Messages)-2].Content != "main task" {
+		t.Fatalf("btw request did not include active main turn: %+v", req.Messages)
+	}
+
+	close(provider.releaseMain)
+	if err := <-mainDone; err != nil {
+		t.Fatal(err)
+	}
+	if len(a.Messages()) != 2 {
+		t.Fatalf("main conversation contains btw messages: %+v", a.Messages())
+	}
+}
+
 type compactProvider struct {
 	summaryCalls int
 	answerCalls  int
@@ -549,6 +801,120 @@ type parallelProvider struct {
 	calls        int
 }
 
+type authorizationProvider struct {
+	call       llm.ToolCallDelta
+	calls      int
+	toolOutput string
+}
+
+func (p *authorizationProvider) ChatStream(ctx context.Context, req llm.ChatRequest, emit func(llm.StreamEvent) error) error {
+	p.calls++
+	if p.calls == 1 {
+		return emit(llm.StreamEvent{ToolCalls: []llm.ToolCallDelta{p.call}})
+	}
+	p.toolOutput = req.Messages[len(req.Messages)-1].Content
+	return emit(llm.StreamEvent{Content: "done"})
+}
+
+type recordingAuthorizer struct {
+	decision ToolAuthorizationDecision
+	requests []ToolAuthorizationRequest
+}
+
+func (a *recordingAuthorizer) AuthorizeToolCall(ctx context.Context, req ToolAuthorizationRequest) (ToolAuthorizationDecision, error) {
+	a.requests = append(a.requests, req)
+	return a.decision, ctx.Err()
+}
+
+func TestAgentAuthorizesMutationAndCreatesRollbackSnapshot(t *testing.T) {
+	dir := t.TempDir()
+	provider := &authorizationProvider{call: llm.ToolCallDelta{
+		Index: 0, ID: "write_1", Type: "function", Name: "file_write",
+		Arguments: `{"path":"new.txt","content":"hello"}`,
+	}}
+	authorizer := &recordingAuthorizer{decision: ToolAuthorizationAllow}
+	rollback := tools.NewRollbackStore()
+	a := New(Options{
+		Provider:      provider,
+		Model:         "test",
+		ToolRegistry:  tools.Builtins(dir),
+		Authorizer:    authorizer,
+		RollbackStore: rollback,
+	})
+
+	if err := a.RunTurn(context.Background(), "write it", nil, nil); err != nil {
+		t.Fatal(err)
+	}
+	if len(authorizer.requests) != 1 || !authorizer.requests[0].RollbackAvailable {
+		t.Fatalf("authorization requests = %+v", authorizer.requests)
+	}
+	if !strings.Contains(authorizer.requests[0].Preview, "write 5 bytes to new.txt") {
+		t.Fatalf("preview = %q", authorizer.requests[0].Preview)
+	}
+	rollbackID := rollback.LastID()
+	if rollbackID == "" || !strings.Contains(provider.toolOutput, rollbackID) {
+		t.Fatalf("tool output %q missing rollback ID %q", provider.toolOutput, rollbackID)
+	}
+	if _, err := rollback.Restore(rollbackID); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := os.Stat(filepath.Join(dir, "new.txt")); !os.IsNotExist(err) {
+		t.Fatalf("new file still exists after rollback: %v", err)
+	}
+}
+
+func TestAgentDeniedMutationDoesNotExecute(t *testing.T) {
+	dir := t.TempDir()
+	provider := &authorizationProvider{call: llm.ToolCallDelta{
+		Index: 0, ID: "write_1", Type: "function", Name: "file_write",
+		Arguments: `{"path":"denied.txt","content":"no"}`,
+	}}
+	authorizer := &recordingAuthorizer{decision: ToolAuthorizationDeny}
+	a := New(Options{Provider: provider, Model: "test", ToolRegistry: tools.Builtins(dir), Authorizer: authorizer})
+
+	if err := a.RunTurn(context.Background(), "write it", nil, nil); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := os.Stat(filepath.Join(dir, "denied.txt")); !os.IsNotExist(err) {
+		t.Fatalf("denied file exists: %v", err)
+	}
+	if !strings.Contains(provider.toolOutput, "denied by user") {
+		t.Fatalf("tool output = %q", provider.toolOutput)
+	}
+}
+
+func TestAgentDoesNotAuthorizeApplyPatchDryRun(t *testing.T) {
+	dir := t.TempDir()
+	provider := &authorizationProvider{call: llm.ToolCallDelta{
+		Index: 0, ID: "patch_1", Type: "function", Name: "apply_patch",
+		Arguments: `{"patch":"*** Begin Patch\n*** Add File: dry.txt\n+dry\n*** End Patch","dry_run":true}`,
+	}}
+	authorizer := &recordingAuthorizer{decision: ToolAuthorizationDeny}
+	a := New(Options{Provider: provider, Model: "test", ToolRegistry: tools.Builtins(dir), Authorizer: authorizer})
+
+	if err := a.RunTurn(context.Background(), "preview", nil, nil); err != nil {
+		t.Fatal(err)
+	}
+	if len(authorizer.requests) != 0 {
+		t.Fatalf("dry run requested authorization: %+v", authorizer.requests)
+	}
+	if _, err := os.Stat(filepath.Join(dir, "dry.txt")); !os.IsNotExist(err) {
+		t.Fatalf("dry run created a file: %v", err)
+	}
+}
+
+func TestSetMessagesRefreshesCurrentSystemInstructions(t *testing.T) {
+	a := New(Options{SystemPrompt: "current instructions"})
+	a.SetMessages([]llm.Message{
+		{Role: llm.RoleSystem, Content: "old instructions"},
+		{Role: llm.RoleUser, Content: "hello"},
+	})
+	messages := a.Messages()
+	if messages[0].Content != "current instructions" || messages[1].Content != "hello" {
+		t.Fatalf("messages = %+v", messages)
+	}
+}
+
 func (p *parallelProvider) ChatStream(ctx context.Context, req llm.ChatRequest, emit func(llm.StreamEvent) error) error {
 	p.calls++
 	if p.calls <= len(p.responses) {

internal/agent/authorization.go

@@ -0,0 +1,26 @@
+package agent
+
+import "context"
+
+// ToolAuthorizationDecision is the result of an authorization check.
+type ToolAuthorizationDecision string
+
+const (
+	ToolAuthorizationAllow ToolAuthorizationDecision = "allow"
+	ToolAuthorizationDeny  ToolAuthorizationDecision = "deny"
+)
+
+// ToolAuthorizationRequest is sent to an authorizer before executing a
+// mutating tool call.
+type ToolAuthorizationRequest struct {
+	ID                string
+	ToolName          string
+	Arguments         string
+	Preview           string
+	RollbackAvailable bool
+}
+
+// ToolAuthorizer decides whether a mutating tool call should proceed.
+type ToolAuthorizer interface {
+	AuthorizeToolCall(ctx context.Context, req ToolAuthorizationRequest) (ToolAuthorizationDecision, error)
+}

internal/app/app.go

@@ -13,6 +13,7 @@ import (
 
 	"agentu/internal/agent"
 	"agentu/internal/session"
+	"agentu/internal/skills"
 	"agentu/internal/tools"
 	"agentu/internal/tui"
 	"agentu/pkg/config"
@@ -40,8 +41,20 @@ func Run() error {
 		return err
 	}
 
+	globalSkills, err := skills.LoadGlobal()
+	if err != nil {
+		return fmt.Errorf("load global skills: %w", err)
+	}
+	rollbackStore := tools.NewRollbackStore()
+	var approvalBroker *tui.ApprovalBroker
+	var authorizer agent.ToolAuthorizer
+	if !opts.yolo && !opts.plain {
+		approvalBroker = tui.NewApprovalBroker()
+		authorizer = approvalBroker
+	}
+
 	providerName, providerConfig, provider := defaultProvider(cfg, http.DefaultClient)
-	assistant := newAssistant(cfg, providerName, providerConfig, provider, opts.yolo, http.DefaultClient)
+	assistant := newAssistant(cfg, providerName, providerConfig, provider, opts, globalSkills, authorizer, rollbackStore, http.DefaultClient)
 	modelManager, err := newSessionManager(cfg, assistant, http.DefaultClient)
 	if err != nil {
 		return err
@@ -54,7 +67,7 @@ func Run() error {
 	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt)
 	defer stop()
 
-	return runInterface(ctx, assistant, modelManager, providerConfig, cfg.Agent.WorkingDir, opts, themeMode)
+	return runInterface(ctx, assistant, modelManager, providerConfig, cfg.Agent.WorkingDir, opts, themeMode, approvalBroker, rollbackStore)
 }
 
 func parseCLIOptions() cliOptions {
@@ -102,7 +115,8 @@ func newProviderClient(providerConfig config.ProviderConfig, httpClient *http.Cl
 	return llm.NewOpenAICompatibleClient(providerConfig.BaseURL, providerConfig.APIKey, httpClient)
 }
 
-func newAssistant(cfg *config.Config, providerName string, providerConfig config.ProviderConfig, provider llm.Provider, yolo bool, httpClient *http.Client) *agent.Agent {
+func newAssistant(cfg *config.Config, providerName string, providerConfig config.ProviderConfig, provider llm.Provider, opts cliOptions, globalSkills []skills.Skill, authorizer agent.ToolAuthorizer, rollbackStore *tools.RollbackStore, httpClient *http.Client) *agent.Agent {
+	writesEnabled := opts.yolo || !opts.plain
 	return agent.New(agent.Options{
 		Provider:         provider,
 		ProviderName:     providerName,
@@ -110,16 +124,21 @@ func newAssistant(cfg *config.Config, providerName string, providerConfig config
 		Thinking:         providerConfig.Thinking,
 		ThinkingParam:    providerConfig.ThinkingParam,
 		ThinkingEnabled:  providerConfig.ThinkingConfigured,
-		SystemPrompt:     systemPrompt(cfg, yolo),
+		SystemPrompt:     systemPrompt(cfg, writesEnabled, opts.yolo, globalSkills),
-		ToolRegistry:     newToolRegistry(cfg.Agent.WorkingDir, yolo, httpClient),
+		ToolRegistry:     newToolRegistry(cfg.Agent.WorkingDir, writesEnabled, httpClient),
+		Authorizer:       authorizer,
+		RollbackStore:    rollbackStore,
 		ToolTimeout:      cfg.Agent.ToolTimeout,
 		MaxContextTokens: maxContextTokens(cfg, providerConfig),
 	})
 }
 
-func systemPrompt(cfg *config.Config, yolo bool) string {
+func systemPrompt(cfg *config.Config, writesEnabled bool, yolo bool, globalSkills []skills.Skill) string {
-	instructions := tools.AgentInstructions(cfg.Agent.WorkingDir, yolo)
+	parts := []string{cfg.Agent.SystemPrompt, tools.AgentInstructionsForMode(cfg.Agent.WorkingDir, writesEnabled, yolo)}
-	return strings.TrimSpace(cfg.Agent.SystemPrompt + "\n\n" + instructions)
+	if formatted := skills.FormatForSystemPrompt(globalSkills); formatted != "" {
+		parts = append(parts, formatted)
+	}
+	return strings.TrimSpace(strings.Join(parts, "\n\n"))
 }
 
 func maxContextTokens(cfg *config.Config, providerConfig config.ProviderConfig) int {
@@ -129,9 +148,9 @@ func maxContextTokens(cfg *config.Config, providerConfig config.ProviderConfig)
 	return providerConfig.ContextTokens
 }
 
-func newToolRegistry(workingDir string, yolo bool, httpClient *http.Client) *tools.Registry {
+func newToolRegistry(workingDir string, writesEnabled bool, httpClient *http.Client) *tools.Registry {
 	var registry *tools.Registry
-	if yolo {
+	if writesEnabled {
 		registry = tools.Builtins(workingDir)
 	} else {
 		registry = tools.ReadOnlyBuiltins(workingDir)
@@ -154,7 +173,7 @@ func newSessionManager(cfg *config.Config, assistant *agent.Agent, httpClient *h
 	return session.NewManager(cfg, assistant, httpClient, store)
 }
 
-func runInterface(ctx context.Context, assistant *agent.Agent, modelManager *session.Manager, providerConfig config.ProviderConfig, workingDir string, opts cliOptions, themeMode tui.ThemeMode) error {
+func runInterface(ctx context.Context, assistant *agent.Agent, modelManager *session.Manager, providerConfig config.ProviderConfig, workingDir string, opts cliOptions, themeMode tui.ThemeMode, approvalBroker *tui.ApprovalBroker, rollbackStore *tools.RollbackStore) error {
 	if opts.plain {
 		return repl(ctx, assistant, modelManager)
 	}
@@ -165,6 +184,8 @@ func runInterface(ctx context.Context, assistant *agent.Agent, modelManager *ses
 		ThemeMode:    themeMode,
 		ModelManager: modelManager,
 		WorkingDir:   workingDir,
+		Approval:     approvalBroker,
+		Rollback:     rollbackStore,
 	})
 	printExitSession(modelManager)
 	return err

internal/app/main_test.go

@@ -3,10 +3,12 @@ package app
 import (
 	"context"
 	"path/filepath"
+	"strings"
 	"testing"
 
 	"agentu/internal/agent"
 	"agentu/internal/session"
+	"agentu/internal/skills"
 	"agentu/pkg/config"
 	"agentu/pkg/llm"
 )
@@ -99,3 +101,13 @@ func TestInitializeSessionResumesExplicitID(t *testing.T) {
 		t.Fatal("explicit resume did not restore saved context")
 	}
 }
+
+func TestSystemPromptIncludesGlobalSkillsAndApprovalMode(t *testing.T) {
+	cfg := &config.Config{Agent: config.AgentConfig{SystemPrompt: "base", WorkingDir: "/tmp/project"}}
+	prompt := systemPrompt(cfg, true, false, []skills.Skill{{Name: "review", Path: "/tmp/review/SKILL.md", Content: "Review carefully."}})
+	for _, want := range []string{"base", "interactive user approval", "### review", "Review carefully."} {
+		if !strings.Contains(prompt, want) {
+			t.Fatalf("prompt missing %q:\n%s", want, prompt)
+		}
+	}
+}

internal/app/repl.go

@@ -9,6 +9,7 @@ import (
 
 	"agentu/internal/agent"
 	"agentu/internal/session"
+	"agentu/internal/skills"
 )
 
 func repl(ctx context.Context, assistant *agent.Agent, modelManager *session.Manager) error {
@@ -16,7 +17,7 @@ func repl(ctx context.Context, assistant *agent.Agent, modelManager *session.Man
 	scanner.Buffer(make([]byte, 0, 64*1024), 1024*1024)
 
 	fmt.Println("agentu")
-	fmt.Println("Type /exit to quit, /clear to reset context, /compact to compress context.")
+	fmt.Println("Type /exit to quit, /clear to reset context, /compact to compress context, /btw <question> for a side question.")
 	for {
 		fmt.Print("> ")
 		if !scanner.Scan() {
@@ -29,6 +30,7 @@ func repl(ctx context.Context, assistant *agent.Agent, modelManager *session.Man
 		}
 
 		input := strings.TrimSpace(scanner.Text())
+		fields := strings.Fields(input)
 		switch input {
 		case "":
 			continue
@@ -50,6 +52,39 @@ func repl(ctx context.Context, assistant *agent.Agent, modelManager *session.Man
 			}
 			continue
 		}
+		if len(fields) > 0 && fields[0] == "/btw" {
+			question := strings.TrimSpace(strings.TrimPrefix(input, "/btw"))
+			if question == "" {
+				fmt.Fprintln(os.Stderr, "usage: /btw <question>")
+				continue
+			}
+			fmt.Print("[btw] ")
+			if err := assistant.BTW(ctx, question, os.Stdout); err != nil {
+				fmt.Fprintln(os.Stderr, "\nbtw error:", err)
+			} else {
+				fmt.Println()
+			}
+			continue
+		}
+		if input == "/skills" || input == "/skills list" {
+			items, err := skills.LoadGlobal()
+			if err != nil {
+				fmt.Fprintln(os.Stderr, "skills error:", err)
+			} else {
+				fmt.Println(skills.FormatList(items))
+			}
+			continue
+		}
+		if len(fields) >= 3 && fields[0] == "/skills" && fields[1] == "install" {
+			result, err := skills.InstallGlobal(ctx, strings.Join(fields[2:], " "))
+			if err != nil {
+				fmt.Fprintln(os.Stderr, "skills error:", err)
+			} else {
+				assistant.AddSystemInstructions(skills.FormatForSystemPrompt([]skills.Skill{result.Skill}))
+				fmt.Printf("installed skill %q; active in this conversation\n", result.Skill.Name)
+			}
+			continue
+		}
 
 		if err := assistant.RunTurn(ctx, input, os.Stdout, os.Stderr); err != nil {
 			fmt.Fprintln(os.Stderr, "\nerror:", err)

internal/safeio/atomic.go

@@ -0,0 +1,56 @@
+package safeio
+
+import (
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+)
+
+// AtomicWriteFile writes data to path atomically by writing to a temp file in
+// the same directory and renaming it into place. On any error before rename,
+// the temp file is removed. The parent directory is created if needed.
+func AtomicWriteFile(path string, data []byte, perm fs.FileMode) error {
+	dir := filepath.Dir(path)
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		return fmt.Errorf("create parent directory: %w", err)
+	}
+
+	tmp, err := os.CreateTemp(dir, ".atomic-*")
+	if err != nil {
+		return fmt.Errorf("create temp file: %w", err)
+	}
+	tmpPath := tmp.Name()
+	cleanup := true
+	defer func() {
+		if cleanup {
+			os.Remove(tmpPath)
+		}
+	}()
+
+	if _, err := tmp.Write(data); err != nil {
+		tmp.Close()
+		return fmt.Errorf("write temp file: %w", err)
+	}
+	if err := tmp.Sync(); err != nil {
+		tmp.Close()
+		return fmt.Errorf("sync temp file: %w", err)
+	}
+	if err := tmp.Close(); err != nil {
+		return fmt.Errorf("close temp file: %w", err)
+	}
+	if err := os.Chmod(tmpPath, perm); err != nil {
+		return fmt.Errorf("chmod temp file: %w", err)
+	}
+	if err := os.Rename(tmpPath, path); err != nil {
+		return fmt.Errorf("rename temp file: %w", err)
+	}
+	cleanup = false
+
+	// Best-effort sync of the parent directory.
+	if f, err := os.Open(dir); err == nil {
+		_ = f.Sync()
+		f.Close()
+	}
+	return nil
+}

internal/safeio/atomic_test.go

@@ -0,0 +1,58 @@
+package safeio
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestAtomicWriteFileWritesContentAndReplacesExisting(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "data.txt")
+
+	// Write initial content.
+	if err := os.WriteFile(path, []byte("old"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	// Atomic replace.
+	if err := AtomicWriteFile(path, []byte("new"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != "new" {
+		t.Fatalf("content = %q, want %q", string(got), "new")
+	}
+
+	// No temp files left behind.
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, e := range entries {
+		if e.Name() != "data.txt" {
+			t.Fatalf("unexpected file in directory: %s", e.Name())
+		}
+	}
+}
+
+func TestAtomicWriteFileCreatesParentDirs(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "sub", "dir", "file.txt")
+
+	if err := AtomicWriteFile(path, []byte("hello"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != "hello" {
+		t.Fatalf("content = %q, want %q", string(got), "hello")
+	}
+}

internal/session/index.go

@@ -0,0 +1,58 @@
+package session
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sort"
+
+	"agentu/internal/safeio"
+)
+
+const sessionIndexFile = "index.json"
+
+type sessionIndex struct {
+	Sessions []SessionMeta `json:"sessions"`
+}
+
+func metaFromSession(sess *Session) SessionMeta {
+	return SessionMeta{
+		ID:           sess.ID,
+		Name:         sess.Name,
+		CreatedAt:    sess.CreatedAt,
+		UpdatedAt:    sess.UpdatedAt,
+		Provider:     sess.Provider,
+		Model:        sess.Model,
+		MessageCount: len(sess.Messages),
+	}
+}
+
+func (s *Store) saveIndexFromSessions(metas []SessionMeta) error {
+	sort.Slice(metas, func(i, j int) bool {
+		return metas[i].UpdatedAt.After(metas[j].UpdatedAt)
+	})
+	idx := sessionIndex{Sessions: metas}
+	data, err := json.MarshalIndent(idx, "", "  ")
+	if err != nil {
+		return fmt.Errorf("marshal session index: %w", err)
+	}
+	path := filepath.Join(s.dir, sessionIndexFile)
+	return safeio.AtomicWriteFile(path, data, 0o644)
+}
+
+func (s *Store) loadIndex() ([]SessionMeta, error) {
+	path := filepath.Join(s.dir, sessionIndexFile)
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+	var idx sessionIndex
+	if err := json.Unmarshal(data, &idx); err != nil {
+		return nil, err
+	}
+	sort.Slice(idx.Sessions, func(i, j int) bool {
+		return idx.Sessions[i].UpdatedAt.After(idx.Sessions[j].UpdatedAt)
+	})
+	return idx.Sessions, nil
+}

internal/session/store.go

@@ -11,6 +11,7 @@ import (
 	"strings"
 	"time"
 
+	"agentu/internal/safeio"
 	"agentu/pkg/llm"
 )
 
@@ -54,8 +55,11 @@ func NewStore(dir string) (*Store, error) {
 
 // Save writes a session to disk.
 func (s *Store) Save(sess *Session) error {
-	if sess.ID == "" {
+	if sess == nil {
-		return fmt.Errorf("session ID is required")
+		return fmt.Errorf("session is required")
+	}
+	if err := validateSessionID(sess.ID); err != nil {
+		return err
 	}
 	sess.UpdatedAt = time.Now().UTC()
 	if sess.CreatedAt.IsZero() {
@@ -66,14 +70,17 @@ func (s *Store) Save(sess *Session) error {
 		return fmt.Errorf("marshal session: %w", err)
 	}
 	path := s.filePath(sess.ID)
-	if err := os.WriteFile(path, data, 0o644); err != nil {
+	if err := safeio.AtomicWriteFile(path, data, 0o644); err != nil {
 		return fmt.Errorf("write session: %w", err)
 	}
-	return nil
+	return s.updateIndex(metaFromSession(sess))
 }
 
 // Load reads a session by ID.
 func (s *Store) Load(id string) (*Session, error) {
+	if err := validateSessionID(id); err != nil {
+		return nil, err
+	}
 	path := s.filePath(id)
 	data, err := os.ReadFile(path)
 	if err != nil {
@@ -91,13 +98,27 @@ func (s *Store) Load(id string) (*Session, error) {
 
 // List returns metadata for all sessions, sorted by updated_at descending.
 func (s *Store) List() ([]SessionMeta, error) {
+	if metas, err := s.loadIndex(); err == nil {
+		return metas, nil
+	}
+	metas, err := s.scanSessions()
+	if err != nil {
+		return nil, err
+	}
+	if err := s.saveIndexFromSessions(metas); err != nil {
+		return nil, err
+	}
+	return metas, nil
+}
+
+func (s *Store) scanSessions() ([]SessionMeta, error) {
 	entries, err := os.ReadDir(s.dir)
 	if err != nil {
 		return nil, fmt.Errorf("read session dir: %w", err)
 	}
 	var metas []SessionMeta
 	for _, entry := range entries {
-		if entry.IsDir() || !strings.HasSuffix(entry.Name(), ".json") {
+		if entry.IsDir() || entry.Name() == sessionIndexFile || !strings.HasSuffix(entry.Name(), ".json") {
 			continue
 		}
 		path := filepath.Join(s.dir, entry.Name())
@@ -127,6 +148,9 @@ func (s *Store) List() ([]SessionMeta, error) {
 
 // Delete removes a session file.
 func (s *Store) Delete(id string) error {
+	if err := validateSessionID(id); err != nil {
+		return err
+	}
 	path := s.filePath(id)
 	if err := os.Remove(path); err != nil {
 		if os.IsNotExist(err) {
@@ -134,13 +158,64 @@ func (s *Store) Delete(id string) error {
 		}
 		return fmt.Errorf("delete session: %w", err)
 	}
-	return nil
+	metas, err := s.loadIndex()
+	if err != nil {
+		metas, err = s.scanSessions()
+		if err != nil {
+			return err
+		}
+	}
+	filtered := metas[:0]
+	for _, meta := range metas {
+		if meta.ID != id {
+			filtered = append(filtered, meta)
+		}
+	}
+	return s.saveIndexFromSessions(filtered)
 }
 
 func (s *Store) filePath(id string) string {
 	return filepath.Join(s.dir, id+".json")
 }
 
+func validateSessionID(id string) error {
+	if id == "" {
+		return fmt.Errorf("session ID is required")
+	}
+	if id == "." || id == ".." {
+		return fmt.Errorf("invalid session ID %q", id)
+	}
+	for _, r := range id {
+		if (r >= 'a' && r <= 'z') || (r >= 'A' && r <= 'Z') || (r >= '0' && r <= '9') || r == '-' || r == '_' || r == '.' {
+			continue
+		}
+		return fmt.Errorf("invalid session ID %q", id)
+	}
+	return nil
+}
+
+func (s *Store) updateIndex(meta SessionMeta) error {
+	metas, err := s.loadIndex()
+	if err != nil {
+		metas, err = s.scanSessions()
+		if err != nil {
+			return err
+		}
+	}
+	found := false
+	for i := range metas {
+		if metas[i].ID == meta.ID {
+			metas[i] = meta
+			found = true
+			break
+		}
+	}
+	if !found {
+		metas = append(metas, meta)
+	}
+	return s.saveIndexFromSessions(metas)
+}
+
 // GenerateID creates a random 8-character hex session ID.
 func GenerateID() string {
 	b := make([]byte, 4)

internal/session/store_test.go

@@ -119,6 +119,9 @@ func TestDeleteNotFound(t *testing.T) {
 
 func TestSaveRequiresID(t *testing.T) {
 	store := tempStore(t)
+	if err := store.Save(nil); err == nil {
+		t.Fatal("expected error for nil session")
+	}
 	err := store.Save(&Session{Name: "no id"})
 	if err == nil {
 		t.Fatal("expected error for empty ID")
@@ -172,3 +175,40 @@ func TestNewStoreCreatesDir(t *testing.T) {
 		t.Error("expected directory")
 	}
 }
+
+func TestStoreMaintainsSessionIndex(t *testing.T) {
+	store := tempStore(t)
+	if err := store.Save(&Session{ID: "indexed", Name: "Indexed"}); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := os.Stat(filepath.Join(store.dir, sessionIndexFile)); err != nil {
+		t.Fatalf("index not created: %v", err)
+	}
+	metas, err := store.loadIndex()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(metas) != 1 || metas[0].ID != "indexed" {
+		t.Fatalf("index metadata = %+v", metas)
+	}
+	if err := store.Delete("indexed"); err != nil {
+		t.Fatal(err)
+	}
+	metas, err = store.loadIndex()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(metas) != 0 {
+		t.Fatalf("index after delete = %+v", metas)
+	}
+}
+
+func TestStoreRejectsUnsafeSessionID(t *testing.T) {
+	store := tempStore(t)
+	if _, err := store.Load("../outside"); err == nil {
+		t.Fatal("expected unsafe ID rejection")
+	}
+	if err := store.Save(&Session{ID: "a/b"}); err == nil {
+		t.Fatal("expected unsafe ID rejection")
+	}
+}

internal/skills/skills.go

@@ -0,0 +1,583 @@
+package skills
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"path/filepath"
+	"regexp"
+	"sort"
+	"strings"
+	"time"
+
+	"agentu/internal/safeio"
+	"agentu/pkg/config"
+
+	"gopkg.in/yaml.v3"
+)
+
+// DefaultDir is the default directory for global skills.
+const DefaultDir = "~/.agentu/skills"
+
+// SkillFileName is the required instruction file inside a Codex-style skill directory.
+const SkillFileName = "SKILL.md"
+
+// maxInstallDownloadBytes caps HTTP downloads at 1 MiB.
+const maxInstallDownloadBytes = 1 << 20
+
+// Skill is a single global Markdown skill.
+type Skill struct {
+	Name        string
+	Description string
+	Path        string
+	Content     string
+}
+
+// InstallResult describes a successful skill installation.
+type InstallResult struct {
+	Skill     Skill
+	TargetDir string
+}
+
+// validNameRe allows ASCII letters, digits, dots, underscores, and hyphens.
+var validNameRe = regexp.MustCompile(`^[A-Za-z0-9._-]+$`)
+
+// validateSkillName rejects empty, path-like, and otherwise unsafe names.
+func validateSkillName(name string) error {
+	if name == "" || name == "." || name == ".." {
+		return fmt.Errorf("invalid skill name %q: use letters, numbers, dots, underscores, or hyphens", name)
+	}
+	if strings.ContainsAny(name, `/\`) {
+		return fmt.Errorf("invalid skill name %q: use letters, numbers, dots, underscores, or hyphens", name)
+	}
+	if strings.ContainsAny(name, " \t\n\r") {
+		return fmt.Errorf("invalid skill name %q: use letters, numbers, dots, underscores, or hyphens", name)
+	}
+	if !validNameRe.MatchString(name) {
+		return fmt.Errorf("invalid skill name %q: use letters, numbers, dots, underscores, or hyphens", name)
+	}
+	return nil
+}
+
+// skillFrontmatter is the decoded YAML frontmatter.
+type skillFrontmatter struct {
+	Name        string `yaml:"name"`
+	Description string `yaml:"description"`
+}
+
+// parseSkillFile reads raw bytes and returns the skill body, frontmatter name,
+// and frontmatter description. hasFrontmatter indicates whether the source
+// bytes started with a --- delimiter pair.
+func parseSkillFile(path string, data []byte) (body string, fmName string, fmDesc string, hasFrontmatter bool, err error) {
+	normalized := strings.ReplaceAll(string(data), "\r\n", "\n")
+	lines := strings.Split(normalized, "\n")
+	if len(lines) >= 2 && lines[0] == "---" {
+		for i := 1; i < len(lines); i++ {
+			if lines[i] == "---" {
+				yamlBlock := strings.Join(lines[1:i], "\n")
+				var fm skillFrontmatter
+				if err := yaml.Unmarshal([]byte(yamlBlock), &fm); err != nil {
+					return "", "", "", false, fmt.Errorf("parse skill %s: %w", path, err)
+				}
+				bodyText := strings.TrimSpace(strings.Join(lines[i+1:], "\n"))
+				return bodyText, fm.Name, fm.Description, true, nil
+			}
+		}
+		return "", "", "", false, fmt.Errorf("parse skill %s: missing closing ---", path)
+	}
+	return strings.TrimSpace(normalized), "", "", false, nil
+}
+
+// LoadGlobal loads global skills from DefaultDir.
+func LoadGlobal() ([]Skill, error) {
+	return LoadDir(DefaultDir)
+}
+
+// LoadDir loads skills from a directory. Missing directories return nil, nil.
+func LoadDir(dir string) ([]Skill, error) {
+	resolved, err := config.ResolvePath(dir)
+	if err != nil {
+		return nil, err
+	}
+	resolved, err = filepath.Abs(resolved)
+	if err != nil {
+		return nil, err
+	}
+
+	info, err := os.Stat(resolved)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil
+		}
+		return nil, fmt.Errorf("read skills directory: %w", err)
+	}
+	if !info.IsDir() {
+		return nil, fmt.Errorf("skills path is not a directory: %s", resolved)
+	}
+
+	entries, err := os.ReadDir(resolved)
+	if err != nil {
+		return nil, fmt.Errorf("read skills directory: %w", err)
+	}
+
+	var skills []Skill
+	for _, entry := range entries {
+		name := entry.Name()
+		if strings.HasPrefix(name, ".") {
+			continue
+		}
+
+		entryPath := filepath.Join(resolved, name)
+
+		// Shape 1: Codex-style directory with SKILL.md.
+		isDir, err := isDirectory(entryPath)
+		if err != nil {
+			return nil, err
+		}
+		if isDir {
+			skillFile := filepath.Join(entryPath, SkillFileName)
+			fi, err := os.Stat(skillFile)
+			if err != nil {
+				if os.IsNotExist(err) {
+					continue
+				}
+				return nil, fmt.Errorf("read skill %s: %w", skillFile, err)
+			}
+			if !fi.Mode().IsRegular() {
+				continue
+			}
+			skill, err := loadSkillFile(name, skillFile, true)
+			if err != nil {
+				return nil, err
+			}
+			if skill != nil {
+				skills = append(skills, *skill)
+			}
+			continue
+		}
+
+		// Shape 2: Legacy flat .md file.
+		if !entry.Type().IsRegular() {
+			continue
+		}
+		if !strings.EqualFold(filepath.Ext(name), ".md") {
+			continue
+		}
+		skill, err := loadSkillFile(name[:len(name)-len(filepath.Ext(name))], entryPath, false)
+		if err != nil {
+			return nil, err
+		}
+		if skill != nil {
+			skills = append(skills, *skill)
+		}
+	}
+
+	sort.Slice(skills, func(i, j int) bool {
+		if skills[i].Name != skills[j].Name {
+			return skills[i].Name < skills[j].Name
+		}
+		return skills[i].Path < skills[j].Path
+	})
+
+	return skills, nil
+}
+
+// isDirectory reports whether path is a directory. For symlinks it resolves
+// the target and checks the resolved type.
+func isDirectory(path string) (bool, error) {
+	fi, err := os.Lstat(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return false, nil
+		}
+		return false, fmt.Errorf("read skill %s: %w", path, err)
+	}
+	if fi.IsDir() {
+		return true, nil
+	}
+	if fi.Mode()&os.ModeSymlink != 0 {
+		resolved, err := filepath.EvalSymlinks(path)
+		if err != nil {
+			return false, nil
+		}
+		target, err := os.Stat(resolved)
+		if err != nil {
+			return false, nil
+		}
+		return target.IsDir(), nil
+	}
+	return false, nil
+}
+
+// loadSkillFile reads one skill markdown file and returns a Skill or nil when
+// the instruction body is empty. legacyBaseName is used as the default name
+// when frontmatter does not supply one and isDir is false.
+func loadSkillFile(legacyBaseName string, path string, isDirSkill bool) (*Skill, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("read skill %s: %w", path, err)
+	}
+	body, fmName, fmDesc, _, err := parseSkillFile(path, data)
+	if err != nil {
+		return nil, err
+	}
+	if body == "" {
+		return nil, nil
+	}
+
+	name := fmName
+	if name == "" {
+		if isDirSkill {
+			name = filepath.Base(filepath.Dir(path))
+		} else {
+			name = legacyBaseName
+		}
+	}
+	if err := validateSkillName(name); err != nil {
+		return nil, err
+	}
+
+	return &Skill{
+		Name:        name,
+		Description: strings.TrimSpace(fmDesc),
+		Path:        path,
+		Content:     body,
+	}, nil
+}
+
+// FormatForSystemPrompt formats skills for injection into the system prompt.
+// Returns an empty string if items is empty.
+func FormatForSystemPrompt(items []Skill) string {
+	if len(items) == 0 {
+		return ""
+	}
+	var b strings.Builder
+	b.WriteString("Global skills:\n")
+	b.WriteString("The following global skills are available for every conversation. Apply them when relevant.")
+	for _, skill := range items {
+		b.WriteString("\n\n### ")
+		b.WriteString(skill.Name)
+		b.WriteString("\nSource: ")
+		b.WriteString(skill.Path)
+		if skill.Description != "" {
+			b.WriteString("\nDescription: ")
+			b.WriteString(skill.Description)
+		}
+		b.WriteString("\n\n")
+		b.WriteString(skill.Content)
+	}
+	return b.String()
+}
+
+// FormatList formats skills for slash-command output.
+func FormatList(items []Skill) string {
+	if len(items) == 0 {
+		return "No global skills installed."
+	}
+	var b strings.Builder
+	b.WriteString("Global skills:\n")
+	for _, skill := range items {
+		b.WriteString("- ")
+		b.WriteString(skill.Name)
+		if skill.Description != "" {
+			b.WriteString(" — ")
+			b.WriteString(skill.Description)
+		}
+		b.WriteString("\n  Source: ")
+		b.WriteString(skill.Path)
+		b.WriteString("\n")
+	}
+	return b.String()
+}
+
+// InstallGlobal installs a skill into DefaultDir.
+func InstallGlobal(ctx context.Context, source string) (InstallResult, error) {
+	return InstallDir(ctx, DefaultDir, source)
+}
+
+// InstallDir installs a Codex-style skill directory under the target root.
+func InstallDir(ctx context.Context, dir string, source string) (InstallResult, error) {
+	source = strings.TrimSpace(source)
+	if source == "" {
+		return InstallResult{}, fmt.Errorf("skill source is required")
+	}
+
+	resolvedRoot, err := config.ResolvePath(dir)
+	if err != nil {
+		return InstallResult{}, err
+	}
+	resolvedRoot, err = filepath.Abs(resolvedRoot)
+	if err != nil {
+		return InstallResult{}, err
+	}
+
+	// HTTP(S) source.
+	if strings.HasPrefix(source, "http://") || strings.HasPrefix(source, "https://") {
+		return installFromHTTP(ctx, resolvedRoot, source)
+	}
+
+	// Local source.
+	resolvedSource, err := config.ResolvePath(source)
+	if err != nil {
+		return InstallResult{}, err
+	}
+	resolvedSource, err = filepath.Abs(resolvedSource)
+	if err != nil {
+		return InstallResult{}, err
+	}
+
+	fi, err := os.Stat(resolvedSource)
+	if err != nil {
+		return InstallResult{}, fmt.Errorf("read skill source: %w", err)
+	}
+	if fi.IsDir() {
+		return installFromDirectory(ctx, resolvedRoot, resolvedSource)
+	}
+	if fi.Mode().IsRegular() && strings.EqualFold(filepath.Ext(resolvedSource), ".md") {
+		return installFromFile(ctx, resolvedRoot, resolvedSource)
+	}
+	return InstallResult{}, fmt.Errorf("skill source must be a directory, .md file, or http(s) URL")
+}
+
+func installFromHTTP(ctx context.Context, root string, url string) (InstallResult, error) {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return InstallResult{}, fmt.Errorf("download skill: %w", err)
+	}
+	client := &http.Client{Timeout: 30 * time.Second}
+	resp, err := client.Do(req)
+	if err != nil {
+		return InstallResult{}, fmt.Errorf("download skill: %w", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		return InstallResult{}, fmt.Errorf("download skill: HTTP %d", resp.StatusCode)
+	}
+	limited := io.LimitReader(resp.Body, int64(maxInstallDownloadBytes)+1)
+	data, err := io.ReadAll(limited)
+	if err != nil {
+		return InstallResult{}, fmt.Errorf("download skill: %w", err)
+	}
+	if len(data) > maxInstallDownloadBytes {
+		return InstallResult{}, fmt.Errorf("skill file is too large: max %d bytes", maxInstallDownloadBytes)
+	}
+	return installMarkdownBytes(root, data, deriveNameFromURL(url))
+}
+
+func installFromDirectory(ctx context.Context, root string, sourceDir string) (InstallResult, error) {
+	skillFile := filepath.Join(sourceDir, SkillFileName)
+	fi, err := os.Stat(skillFile)
+	if err != nil {
+		return InstallResult{}, fmt.Errorf("read skill source: %w", err)
+	}
+	if !fi.Mode().IsRegular() {
+		return InstallResult{}, fmt.Errorf("skill source must contain a regular %s file", SkillFileName)
+	}
+	data, err := os.ReadFile(skillFile)
+	if err != nil {
+		return InstallResult{}, fmt.Errorf("read skill source: %w", err)
+	}
+
+	body, fmName, _, _, err := parseSkillFile(skillFile, data)
+	if err != nil {
+		return InstallResult{}, err
+	}
+	_ = body
+	name := fmName
+	if name == "" {
+		name = filepath.Base(sourceDir)
+	}
+	if err := validateSkillName(name); err != nil {
+		return InstallResult{}, err
+	}
+
+	targetDir := filepath.Join(root, name)
+	if err := createSkillTarget(root, targetDir); err != nil {
+		return InstallResult{}, err
+	}
+
+	// Copy tree.
+	if err := copyTree(ctx, sourceDir, targetDir); err != nil {
+		os.RemoveAll(targetDir)
+		return InstallResult{}, err
+	}
+
+	installedPath := filepath.Join(targetDir, SkillFileName)
+	installedData, _ := os.ReadFile(installedPath)
+	instBody, instName, instDesc, _, _ := parseSkillFile(installedPath, installedData)
+	if instName == "" {
+		instName = name
+	}
+
+	return InstallResult{
+		Skill: Skill{
+			Name:        instName,
+			Description: strings.TrimSpace(instDesc),
+			Path:        installedPath,
+			Content:     instBody,
+		},
+		TargetDir: targetDir,
+	}, nil
+}
+
+func installFromFile(ctx context.Context, root string, sourcePath string) (InstallResult, error) {
+	data, err := os.ReadFile(sourcePath)
+	if err != nil {
+		return InstallResult{}, fmt.Errorf("read skill source: %w", err)
+	}
+	baseName := filepath.Base(sourcePath)
+	baseNameNoExt := baseName[:len(baseName)-len(filepath.Ext(baseName))]
+	// When the source is literally SKILL.md, use the parent directory name.
+	if strings.EqualFold(baseName, SkillFileName) {
+		baseNameNoExt = filepath.Base(filepath.Dir(sourcePath))
+	}
+	return installMarkdownBytes(root, data, baseNameNoExt)
+}
+
+// installMarkdownBytes parses markdown bytes and writes a Codex-style
+// SKILL.md under <root>/<name>/. If the source had frontmatter, the original
+// bytes are preserved. Otherwise a normalized header is prepended.
+func installMarkdownBytes(root string, data []byte, fallbackName string) (InstallResult, error) {
+	normalized := strings.ReplaceAll(string(data), "\r\n", "\n")
+	lines := strings.Split(normalized, "\n")
+
+	hasFrontmatter := false
+	if len(lines) >= 2 && lines[0] == "---" {
+		for i := 1; i < len(lines); i++ {
+			if lines[i] == "---" {
+				hasFrontmatter = true
+				break
+			}
+		}
+	}
+
+	var body string
+	var fmName string
+	var fmDesc string
+	var err error
+
+	if hasFrontmatter {
+		body, fmName, fmDesc, _, err = parseSkillFile("<source>", data)
+		if err != nil {
+			return InstallResult{}, err
+		}
+	} else {
+		body = strings.TrimSpace(normalized)
+	}
+	name := fmName
+	if name == "" {
+		name = fallbackName
+	}
+	if err := validateSkillName(name); err != nil {
+		return InstallResult{}, err
+	}
+	if body == "" {
+		return InstallResult{}, fmt.Errorf("skill body is empty")
+	}
+
+	targetDir := filepath.Join(root, name)
+	if err := createSkillTarget(root, targetDir); err != nil {
+		return InstallResult{}, err
+	}
+
+	var writeData []byte
+	if hasFrontmatter {
+		writeData = data
+	} else {
+		var b strings.Builder
+		b.WriteString("---\n")
+		fmt.Fprintf(&b, "name: %s\n", name)
+		b.WriteString("description: \"\"\n")
+		b.WriteString("---\n\n")
+		b.WriteString(body)
+		b.WriteString("\n")
+		writeData = []byte(b.String())
+	}
+
+	targetFile := filepath.Join(targetDir, SkillFileName)
+	if err := safeio.AtomicWriteFile(targetFile, writeData, 0o644); err != nil {
+		os.RemoveAll(targetDir)
+		return InstallResult{}, err
+	}
+
+	return InstallResult{
+		Skill: Skill{
+			Name:        name,
+			Description: strings.TrimSpace(fmDesc),
+			Path:        targetFile,
+			Content:     body,
+		},
+		TargetDir: targetDir,
+	}, nil
+}
+
+func createSkillTarget(root, targetDir string) error {
+	if err := os.MkdirAll(root, 0o755); err != nil {
+		return fmt.Errorf("create skills directory: %w", err)
+	}
+	if err := os.Mkdir(targetDir, 0o755); err != nil {
+		if os.IsExist(err) {
+			return fmt.Errorf("skill already exists: %s", targetDir)
+		}
+		return fmt.Errorf("create skill directory: %w", err)
+	}
+	return nil
+}
+
+// deriveNameFromURL extracts a filename stem from a URL for use as a fallback name.
+func deriveNameFromURL(rawURL string) string {
+	// Strip query and fragment.
+	u := rawURL
+	if idx := strings.IndexAny(u, "?#"); idx >= 0 {
+		u = u[:idx]
+	}
+	base := filepath.Base(u)
+	ext := filepath.Ext(base)
+	if ext != "" {
+		base = base[:len(base)-len(ext)]
+	}
+	if base == "" || strings.EqualFold(base, "SKILL") {
+		return "downloaded-skill"
+	}
+	return base
+}
+
+// copyTree copies srcDir to dstDir, rejecting symlinks inside the source tree.
+func copyTree(ctx context.Context, srcDir string, dstDir string) error {
+	return filepath.Walk(srcDir, func(path string, info os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+		if err := ctx.Err(); err != nil {
+			return err
+		}
+		rel, err := filepath.Rel(srcDir, path)
+		if err != nil {
+			return err
+		}
+		target := filepath.Join(dstDir, rel)
+
+		// Reject any symlink inside the source tree.
+		lfi, lerr := os.Lstat(path)
+		if lerr != nil {
+			return lerr
+		}
+		if lfi.Mode()&os.ModeSymlink != 0 {
+			return fmt.Errorf("skill source contains symlink: %s", path)
+		}
+
+		if lfi.IsDir() {
+			return os.MkdirAll(target, 0o755)
+		}
+		if !lfi.Mode().IsRegular() {
+			return nil
+		}
+		data, err := os.ReadFile(path)
+		if err != nil {
+			return err
+		}
+		return safeio.AtomicWriteFile(target, data, 0o644)
+	})
+}

internal/skills/skills_test.go

@@ -0,0 +1,404 @@
+package skills
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestLoadDirMissingReturnsEmpty(t *testing.T) {
+	dir := filepath.Join(t.TempDir(), "missing")
+	skills, err := LoadDir(dir)
+	if err != nil {
+		t.Fatalf("LoadDir(missing) returned error: %v", err)
+	}
+	if len(skills) != 0 {
+		t.Fatalf("expected 0 skills, got %d", len(skills))
+	}
+}
+
+func TestLoadDirRejectsFilePath(t *testing.T) {
+	file := filepath.Join(t.TempDir(), "not-a-dir")
+	if err := os.WriteFile(file, []byte("x"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	_, err := LoadDir(file)
+	if err == nil {
+		t.Fatal("expected error for file path")
+	}
+	if !strings.Contains(err.Error(), "skills path is not a directory") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestLoadDirLoadsDirectorySkillsSorted(t *testing.T) {
+	dir := t.TempDir()
+
+	// b-skill loads second by name.
+	bDir := filepath.Join(dir, "b-skill")
+	if err := os.MkdirAll(bDir, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(bDir, SkillFileName), []byte("---\nname: b-skill\ndescription: Beta skill\n---\n\nBeta content.\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	// a-skill loads first by name.
+	aDir := filepath.Join(dir, "a-skill")
+	if err := os.MkdirAll(aDir, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(aDir, SkillFileName), []byte("---\nname: a-skill\ndescription: Alpha skill\n---\n\nAlpha content.\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	// Visible directory without SKILL.md — ignored.
+	noSkillDir := filepath.Join(dir, "no-skill-here")
+	if err := os.MkdirAll(noSkillDir, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(noSkillDir, "README.md"), []byte("not a skill"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	skills, err := LoadDir(dir)
+	if err != nil {
+		t.Fatalf("LoadDir: %v", err)
+	}
+	if len(skills) != 2 {
+		t.Fatalf("expected 2 skills, got %d: %+v", len(skills), skills)
+	}
+
+	if skills[0].Name != "a-skill" {
+		t.Fatalf("skills[0].Name = %q", skills[0].Name)
+	}
+	if skills[0].Description != "Alpha skill" {
+		t.Fatalf("skills[0].Description = %q", skills[0].Description)
+	}
+	if skills[0].Content != "Alpha content." {
+		t.Fatalf("skills[0].Content = %q", skills[0].Content)
+	}
+	if !strings.HasSuffix(skills[0].Path, SkillFileName) {
+		t.Fatalf("skills[0].Path = %q, want suffix %s", skills[0].Path, SkillFileName)
+	}
+
+	if skills[1].Name != "b-skill" {
+		t.Fatalf("skills[1].Name = %q", skills[1].Name)
+	}
+	if skills[1].Description != "Beta skill" {
+		t.Fatalf("skills[1].Description = %q", skills[1].Description)
+	}
+}
+
+func TestLoadDirKeepsLegacyMarkdownSkills(t *testing.T) {
+	dir := t.TempDir()
+
+	write := func(name, content string) {
+		if err := os.WriteFile(filepath.Join(dir, name), []byte(content), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	write("legacy.md", "  legacy content  ")
+	write(".hidden.md", "hidden")
+	write("notes.txt", "not a skill")
+	write("empty.md", "  \n  ")
+
+	skills, err := LoadDir(dir)
+	if err != nil {
+		t.Fatalf("LoadDir: %v", err)
+	}
+	if len(skills) != 1 {
+		t.Fatalf("expected 1 skill, got %d: %+v", len(skills), skills)
+	}
+	if skills[0].Name != "legacy" {
+		t.Fatalf("skill.Name = %q", skills[0].Name)
+	}
+	if skills[0].Content != "legacy content" {
+		t.Fatalf("skill.Content = %q", skills[0].Content)
+	}
+}
+
+func TestInstallDirInstallsMarkdownFileAsSkillDirectory(t *testing.T) {
+	sourceDir := t.TempDir()
+	targetRoot := filepath.Join(t.TempDir(), "skills")
+
+	sourcePath := filepath.Join(sourceDir, "source.md")
+	sourceContent := "---\nname: uiux-pro-max\ndescription: UI workflow\n---\n\nReview the UI carefully.\n"
+	if err := os.WriteFile(sourcePath, []byte(sourceContent), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	result, err := InstallDir(context.Background(), targetRoot, sourcePath)
+	if err != nil {
+		t.Fatalf("InstallDir: %v", err)
+	}
+
+	absRoot, _ := filepath.Abs(targetRoot)
+	wantDir := filepath.Join(absRoot, "uiux-pro-max")
+	if result.TargetDir != wantDir {
+		t.Fatalf("TargetDir = %q, want %q", result.TargetDir, wantDir)
+	}
+
+	skillFile := filepath.Join(wantDir, SkillFileName)
+	if _, err := os.Stat(skillFile); err != nil {
+		t.Fatalf("SKILL.md not created: %v", err)
+	}
+
+	loaded, err := LoadDir(targetRoot)
+	if err != nil {
+		t.Fatalf("LoadDir: %v", err)
+	}
+	if len(loaded) != 1 {
+		t.Fatalf("expected 1 loaded skill, got %d", len(loaded))
+	}
+	if loaded[0].Name != "uiux-pro-max" {
+		t.Fatalf("loaded name = %q", loaded[0].Name)
+	}
+	if loaded[0].Content != "Review the UI carefully." {
+		t.Fatalf("loaded content = %q", loaded[0].Content)
+	}
+}
+
+func TestInstallDirCopiesDirectorySkill(t *testing.T) {
+	sourceDir := t.TempDir()
+	targetRoot := filepath.Join(t.TempDir(), "skills")
+
+	skillSrc := filepath.Join(sourceDir, "grill-me")
+	if err := os.MkdirAll(filepath.Join(skillSrc, "references"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(skillSrc, SkillFileName), []byte("---\nname: grill-me\ndescription: Grill skill\n---\n\nGrill content.\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(skillSrc, "references", "guide.md"), []byte("# Guide\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	result, err := InstallDir(context.Background(), targetRoot, skillSrc)
+	if err != nil {
+		t.Fatalf("InstallDir: %v", err)
+	}
+
+	skillFile := filepath.Join(result.TargetDir, SkillFileName)
+	if _, err := os.Stat(skillFile); err != nil {
+		t.Fatalf("SKILL.md not copied: %v", err)
+	}
+	guideFile := filepath.Join(result.TargetDir, "references", "guide.md")
+	if _, err := os.Stat(guideFile); err != nil {
+		t.Fatalf("references/guide.md not copied: %v", err)
+	}
+
+	loaded, err := LoadDir(targetRoot)
+	if err != nil {
+		t.Fatalf("LoadDir: %v", err)
+	}
+	if len(loaded) != 1 || loaded[0].Name != "grill-me" {
+		t.Fatalf("loaded = %+v", loaded)
+	}
+}
+
+func TestInstallDirRejectsExistingSkill(t *testing.T) {
+	targetRoot := filepath.Join(t.TempDir(), "skills")
+	existingDir := filepath.Join(targetRoot, "grill-me")
+	if err := os.MkdirAll(existingDir, 0o755); err != nil {
+		t.Fatal(err)
+	}
+
+	sourceDir := t.TempDir()
+	sourcePath := filepath.Join(sourceDir, "source.md")
+	if err := os.WriteFile(sourcePath, []byte("---\nname: grill-me\ndescription: d\n---\n\nbody\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	_, err := InstallDir(context.Background(), targetRoot, sourcePath)
+	if err == nil {
+		t.Fatal("expected error for existing skill")
+	}
+	if !strings.Contains(err.Error(), "skill already exists") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestInstallDirRejectsInvalidName(t *testing.T) {
+	targetRoot := filepath.Join(t.TempDir(), "skills")
+	sourceDir := t.TempDir()
+	sourcePath := filepath.Join(sourceDir, "source.md")
+	if err := os.WriteFile(sourcePath, []byte("---\nname: ../bad\ndescription: d\n---\n\nbody\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	_, err := InstallDir(context.Background(), targetRoot, sourcePath)
+	if err == nil {
+		t.Fatal("expected error for invalid name")
+	}
+	if !strings.Contains(err.Error(), "invalid skill name") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestInstallDirRequiresSource(t *testing.T) {
+	targetRoot := filepath.Join(t.TempDir(), "skills")
+	_, err := InstallDir(context.Background(), targetRoot, "")
+	if err == nil {
+		t.Fatal("expected error for empty source")
+	}
+	if !strings.Contains(err.Error(), "skill source is required") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestInstallDirRejectsBadSourceType(t *testing.T) {
+	targetRoot := filepath.Join(t.TempDir(), "skills")
+	sourceDir := t.TempDir()
+	sourcePath := filepath.Join(sourceDir, "something.yaml")
+	if err := os.WriteFile(sourcePath, []byte("key: value\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	_, err := InstallDir(context.Background(), targetRoot, sourcePath)
+	if err == nil {
+		t.Fatal("expected error for non-.md non-dir source")
+	}
+	if !strings.Contains(err.Error(), "skill source must be a directory") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestInstallDirWithoutFrontmatterCreatesNormalizedHeader(t *testing.T) {
+	targetRoot := filepath.Join(t.TempDir(), "skills")
+	sourceDir := t.TempDir()
+	sourcePath := filepath.Join(sourceDir, "my-skill.md")
+	if err := os.WriteFile(sourcePath, []byte("Just body content.\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	result, err := InstallDir(context.Background(), targetRoot, sourcePath)
+	if err != nil {
+		t.Fatalf("InstallDir: %v", err)
+	}
+	if result.Skill.Name != "my-skill" {
+		t.Fatalf("name = %q", result.Skill.Name)
+	}
+
+	installed, err := os.ReadFile(filepath.Join(result.TargetDir, SkillFileName))
+	if err != nil {
+		t.Fatal(err)
+	}
+	content := string(installed)
+	if !strings.HasPrefix(content, "---\nname: my-skill\n") {
+		t.Fatalf("installed SKILL.md missing normalized header: %q", content)
+	}
+	if !strings.Contains(content, "Just body content.") {
+		t.Fatalf("installed SKILL.md missing body: %q", content)
+	}
+}
+
+func TestFormatForSystemPromptIncludesDescription(t *testing.T) {
+	items := []Skill{
+		{Name: "first", Path: "/tmp/first.md", Content: "skill one"},
+		{Name: "second", Description: "Described", Path: "/tmp/second.md", Content: "skill two"},
+	}
+	out := FormatForSystemPrompt(items)
+	if out == "" {
+		t.Fatal("FormatForSystemPrompt returned empty string")
+	}
+	if !strings.Contains(out, "Global skills:") {
+		t.Fatal("missing 'Global skills:' header")
+	}
+	if !strings.Contains(out, "### first") {
+		t.Fatal("missing '### first'")
+	}
+	if !strings.Contains(out, "### second") {
+		t.Fatal("missing '### second'")
+	}
+	if !strings.Contains(out, "Source: /tmp/first.md") {
+		t.Fatal("missing source for first")
+	}
+	if !strings.Contains(out, "Source: /tmp/second.md") {
+		t.Fatal("missing source for second")
+	}
+	if !strings.Contains(out, "skill one") {
+		t.Fatal("missing content for first")
+	}
+	if !strings.Contains(out, "skill two") {
+		t.Fatal("missing content for second")
+	}
+	// First skill has no description — the Description: line must be absent.
+	firstBlock := out[:strings.Index(out, "### second")]
+	if strings.Contains(firstBlock, "Description:") {
+		t.Fatal("first skill should not have Description line")
+	}
+	// Second skill has description — the line must appear.
+	if !strings.Contains(out, "Description: Described") {
+		t.Fatal("missing Description line for second skill")
+	}
+}
+
+func TestFormatForSystemPromptEmptyReturnsEmpty(t *testing.T) {
+	if got := FormatForSystemPrompt(nil); got != "" {
+		t.Fatalf("expected empty string, got %q", got)
+	}
+}
+
+func TestFormatListNonEmpty(t *testing.T) {
+	items := []Skill{
+		{Name: "alpha", Description: "Alpha desc", Path: "/tmp/alpha/SKILL.md", Content: "..."},
+		{Name: "beta", Path: "/tmp/beta/SKILL.md", Content: "..."},
+	}
+	out := FormatList(items)
+	if !strings.Contains(out, "Global skills:") {
+		t.Fatal("missing header")
+	}
+	if !strings.Contains(out, "- alpha — Alpha desc") {
+		t.Fatal("missing alpha with description")
+	}
+	if !strings.Contains(out, "  Source: /tmp/alpha/SKILL.md") {
+		t.Fatal("missing alpha source")
+	}
+	if !strings.Contains(out, "- beta\n") {
+		t.Fatal("missing beta without description")
+	}
+	// Ensure beta has no em-dash line.
+	if strings.Contains(out, "- beta —") {
+		t.Fatal("beta should not have description separator")
+	}
+}
+
+func TestFormatListEmpty(t *testing.T) {
+	out := FormatList(nil)
+	if out != "No global skills installed." {
+		t.Fatalf("unexpected: %q", out)
+	}
+}
+
+func TestLoadDirRejectsInvalidFrontmatterName(t *testing.T) {
+	dir := t.TempDir()
+	skillDir := filepath.Join(dir, "bad-skill")
+	if err := os.MkdirAll(skillDir, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(skillDir, SkillFileName), []byte("---\nname: ../bad\n---\n\nbody\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	_, err := LoadDir(dir)
+	if err == nil {
+		t.Fatal("expected error for invalid frontmatter name")
+	}
+	if !strings.Contains(err.Error(), "invalid skill name") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestParseSkillFileMissingClosingDelimiter(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "bad.md")
+	_, _, _, _, err := parseSkillFile(path, []byte("---\nname: foo\nbody\n"))
+	if err == nil {
+		t.Fatal("expected error for missing closing ---")
+	}
+	if !strings.Contains(err.Error(), "missing closing ---") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}

internal/tools/apply_patch.go

@@ -0,0 +1,153 @@
+package tools
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"strings"
+
+	"agentu/pkg/llm"
+)
+
+// ────────────────────────────────────────────────────────────────────────────
+// ApplyPatchTool — multi-file patch using V4 format
+// ────────────────────────────────────────────────────────────────────────────
+
+type ApplyPatchTool struct {
+	workingDir string
+}
+
+func NewApplyPatchTool(workingDir string) *ApplyPatchTool {
+	return &ApplyPatchTool{workingDir: workingDir}
+}
+
+// MutatingTool interface.
+
+func (*ApplyPatchTool) Mutates() bool { return true }
+
+// CallMutatingTool — dry_run is non-mutating.
+func (t *ApplyPatchTool) MutatesCall(args json.RawMessage) bool {
+	var a applyPatchArgs
+	if err := json.Unmarshal(args, &a); err != nil {
+		return true // conservative
+	}
+	return !a.DryRun
+}
+
+// PreviewableTool interface.
+
+func (t *ApplyPatchTool) Preview(ctx context.Context, args json.RawMessage) (string, error) {
+	var a applyPatchArgs
+	if err := json.Unmarshal(args, &a); err != nil {
+		return "", fmt.Errorf("parse arguments: %w", err)
+	}
+	return summarizePatch(a.Patch)
+}
+
+// TargetPathTool interface.
+
+func (t *ApplyPatchTool) TargetPaths(args json.RawMessage) ([]string, error) {
+	var a applyPatchArgs
+	if err := json.Unmarshal(args, &a); err != nil {
+		return nil, fmt.Errorf("parse arguments: %w", err)
+	}
+	return extractTargetPaths(t.workingDir, a.Patch)
+}
+
+// Definition.
+
+func (t *ApplyPatchTool) Definition() llm.Tool {
+	return llm.Tool{
+		Type: "function",
+		Function: llm.ToolFunction{
+			Name:        "apply_patch",
+			Description: "Apply a multi-file patch to create, update, or delete files. Uses a V4 diff format with *** Begin Patch / *** Add File / *** Update File / *** Delete File markers. Prefer this over multiple file_edit calls when changing 2+ files.",
+			Parameters: JSONSchema(`{
+  "type": "object",
+  "properties": {
+    "patch": {"type": "string", "description": "V4 format patch text. Must start with *** Begin Patch and end with *** End Patch."},
+    "dry_run": {"type": "boolean", "description": "Preview the patch without writing any files."}
+  },
+  "required": ["patch"],
+  "additionalProperties": false
+}`),
+		},
+	}
+}
+
+type applyPatchArgs struct {
+	Patch  string `json:"patch"`
+	DryRun bool   `json:"dry_run"`
+}
+
+// Execute.
+
+func (t *ApplyPatchTool) Execute(ctx context.Context, raw json.RawMessage) (string, error) {
+	var args applyPatchArgs
+	if err := json.Unmarshal(raw, &args); err != nil {
+		return "", fmt.Errorf("parse arguments: %w", err)
+	}
+
+	if strings.TrimSpace(args.Patch) == "" {
+		return "", fmt.Errorf("patch is empty")
+	}
+
+	pp, err := parsePatch(args.Patch)
+	if err != nil {
+		return "", err
+	}
+
+	if args.DryRun {
+		summary, err := summarizePatch(args.Patch)
+		if err != nil {
+			return "", err
+		}
+		return "dry run — " + summary, nil
+	}
+
+	rollback := NewRollbackStore()
+	var snapshotIDs []string
+	for _, pf := range pp.files {
+		path, err := resolvePathForWrite(t.workingDir, pf.path)
+		if err != nil {
+			return "", err
+		}
+		id, err := rollback.Capture("apply_patch", pf.path, path)
+		if err != nil {
+			return "", err
+		}
+		snapshotIDs = append(snapshotIDs, id)
+	}
+
+	var results []string
+	for _, pf := range pp.files {
+		if err := ctx.Err(); err != nil {
+			if rollbackErr := restoreSnapshots(rollback, snapshotIDs); rollbackErr != nil {
+				return "", fmt.Errorf("%w; rollback failed: %v", err, rollbackErr)
+			}
+			return "", err
+		}
+		result, err := applyPatchFile(t.workingDir, pf)
+		if err != nil {
+			if rollbackErr := restoreSnapshots(rollback, snapshotIDs); rollbackErr != nil {
+				return "", fmt.Errorf("apply patch: %w; rollback failed: %v", err, rollbackErr)
+			}
+			return "", fmt.Errorf("apply patch: %w", err)
+		}
+		results = append(results, result)
+	}
+
+	summary := fmt.Sprintf("applied patch: %d file(s) changed\n%s", len(results), strings.Join(results, "\n"))
+	return summary, nil
+}
+
+func restoreSnapshots(store *RollbackStore, ids []string) error {
+	var errs []error
+	for i := len(ids) - 1; i >= 0; i-- {
+		if _, err := store.Restore(ids[i]); err != nil {
+			errs = append(errs, err)
+		}
+	}
+	return errors.Join(errs...)
+}

internal/tools/file.go

@@ -13,6 +13,7 @@ import (
 	"sort"
 	"strings"
 
+	"agentu/internal/safeio"
 	"agentu/pkg/llm"
 )
 
@@ -83,6 +84,33 @@ func NewFileWriteTool(workingDir string) *FileWriteTool {
 }
 func (*FileWriteTool) Mutates() bool { return true }
 
+func (t *FileWriteTool) Preview(ctx context.Context, raw json.RawMessage) (string, error) {
+	var args fileWriteArgs
+	if err := json.Unmarshal(raw, &args); err != nil {
+		return "", fmt.Errorf("parse arguments: %w", err)
+	}
+	action := "write"
+	if args.Append {
+		action = "append"
+	}
+	return fmt.Sprintf("%s %d bytes to %s", action, len(args.Content), args.Path), ctx.Err()
+}
+
+func (t *FileWriteTool) TargetPaths(raw json.RawMessage) ([]string, error) {
+	var args fileWriteArgs
+	if err := json.Unmarshal(raw, &args); err != nil {
+		return nil, fmt.Errorf("parse arguments: %w", err)
+	}
+	if strings.TrimSpace(args.Path) == "" {
+		return nil, errors.New("path is required")
+	}
+	path, err := resolvePathForWrite(t.workingDir, args.Path)
+	if err != nil {
+		return nil, err
+	}
+	return []string{path}, nil
+}
+
 func (t *FileWriteTool) Definition() llm.Tool {
 	return llm.Tool{
 		Type: "function",
@@ -117,28 +145,28 @@ func (t *FileWriteTool) Execute(ctx context.Context, raw json.RawMessage) (strin
 	if strings.TrimSpace(args.Path) == "" {
 		return "", errors.New("path is required")
 	}
-	path, err := resolvePath(t.workingDir, args.Path)
+	path, err := resolvePathForWrite(t.workingDir, args.Path)
 	if err != nil {
 		return "", err
 	}
-	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
-		return "", err
-	}
-	flag := os.O_CREATE | os.O_WRONLY | os.O_TRUNC
-	if args.Append {
-		flag = os.O_CREATE | os.O_WRONLY | os.O_APPEND
-	}
-	file, err := os.OpenFile(path, flag, 0o644)
-	if err != nil {
-		return "", err
-	}
-	defer file.Close()
-	if _, err := file.WriteString(args.Content); err != nil {
-		return "", err
-	}
 	if err := ctx.Err(); err != nil {
 		return "", err
 	}
+	content := []byte(args.Content)
+	perm := os.FileMode(0o644)
+	if info, statErr := os.Stat(path); statErr == nil {
+		perm = info.Mode().Perm()
+	}
+	if args.Append {
+		existing, readErr := os.ReadFile(path)
+		if readErr != nil && !os.IsNotExist(readErr) {
+			return "", readErr
+		}
+		content = append(existing, content...)
+	}
+	if err := safeio.AtomicWriteFile(path, content, perm); err != nil {
+		return "", err
+	}
 	action := "wrote"
 	if args.Append {
 		action = "appended"

internal/tools/file_edit.go

@@ -8,6 +8,7 @@ import (
 	"os"
 	"strings"
 
+	"agentu/internal/safeio"
 	"agentu/pkg/llm"
 )
 
@@ -21,6 +22,36 @@ func NewFileEditTool(workingDir string) *FileEditTool {
 
 func (*FileEditTool) Mutates() bool { return true }
 
+func (t *FileEditTool) Preview(ctx context.Context, raw json.RawMessage) (string, error) {
+	var args fileEditArgs
+	if err := json.Unmarshal(raw, &args); err != nil {
+		return "", fmt.Errorf("parse arguments: %w", err)
+	}
+	if args.StartLine > 0 {
+		end := args.EndLine
+		if end == 0 {
+			end = args.StartLine
+		}
+		return fmt.Sprintf("edit %s lines %d-%d", args.Path, args.StartLine, end), ctx.Err()
+	}
+	return fmt.Sprintf("replace text in %s", args.Path), ctx.Err()
+}
+
+func (t *FileEditTool) TargetPaths(raw json.RawMessage) ([]string, error) {
+	var args fileEditArgs
+	if err := json.Unmarshal(raw, &args); err != nil {
+		return nil, fmt.Errorf("parse arguments: %w", err)
+	}
+	if strings.TrimSpace(args.Path) == "" {
+		return nil, errors.New("path is required")
+	}
+	path, err := resolvePath(t.workingDir, args.Path)
+	if err != nil {
+		return nil, err
+	}
+	return []string{path}, nil
+}
+
 func (t *FileEditTool) Definition() llm.Tool {
 	return llm.Tool{
 		Type: "function",
@@ -111,7 +142,7 @@ func (t *FileEditTool) Execute(ctx context.Context, raw json.RawMessage) (string
 	if err := ctx.Err(); err != nil {
 		return "", err
 	}
-	if err := os.WriteFile(path, []byte(result), perm); err != nil {
+	if err := safeio.AtomicWriteFile(path, []byte(result), perm); err != nil {
 		return "", err
 	}
 

internal/tools/outline.go

@@ -0,0 +1,244 @@
+package tools
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"agentu/pkg/llm"
+)
+
+const defaultOutlineMaxFiles = 200
+
+// ────────────────────────────────────────────────────────────────────────────
+// ProjectOutlineTool — gitignore-aware file tree + symbol outline
+// ────────────────────────────────────────────────────────────────────────────
+
+type ProjectOutlineTool struct {
+	workingDir string
+}
+
+func NewProjectOutlineTool(workingDir string) *ProjectOutlineTool {
+	return &ProjectOutlineTool{workingDir: workingDir}
+}
+
+func (t *ProjectOutlineTool) Definition() llm.Tool {
+	return llm.Tool{
+		Type: "function",
+		Function: llm.ToolFunction{
+			Name:        "project_outline",
+			Description: "Get the project file tree and code symbol outline in one call. Respects .gitignore. Use this first to understand project structure before reading individual files.",
+			Parameters: JSONSchema(`{
+  "type": "object",
+  "properties": {
+    "path": {"type": "string", "description": "Subdirectory to outline. Defaults to the project root."},
+    "max_files": {"type": "integer", "description": "Maximum files to include in the outline. Defaults to 200."}
+  },
+  "additionalProperties": false
+}`),
+		},
+	}
+}
+
+type outlineArgs struct {
+	Path     string `json:"path"`
+	MaxFiles int    `json:"max_files"`
+}
+
+func (t *ProjectOutlineTool) Execute(ctx context.Context, raw json.RawMessage) (string, error) {
+	var args outlineArgs
+	if err := json.Unmarshal(raw, &args); err != nil {
+		return "", fmt.Errorf("parse arguments: %w", err)
+	}
+
+	root, err := resolvePath(t.workingDir, args.Path)
+	if err != nil {
+		return "", err
+	}
+
+	limit := args.MaxFiles
+	if limit <= 0 {
+		limit = defaultOutlineMaxFiles
+	}
+
+	// Parse .gitignore files from the root upward.
+	ignorer := newGitignoreMatcher(root)
+
+	var out strings.Builder
+	out.WriteString(root)
+	out.WriteString("\n")
+
+	count := 0
+	truncated := false
+
+	err = filepath.WalkDir(root, func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return nil
+		}
+		if ctx.Err() != nil {
+			return ctx.Err()
+		}
+
+		// Skip always-ignored directories.
+		if d.IsDir() {
+			name := d.Name()
+			// Always skip VCS and common build-artifact directories.
+			switch name {
+			case ".git", ".hg", ".svn", "vendor", "node_modules",
+				"dist", "build", "__pycache__", "target":
+				return filepath.SkipDir
+			}
+			// Skip hidden directories other than .github and similar.
+			if strings.HasPrefix(name, ".") && name != ".github" && name != ".vscode" && name != ".devcontainer" {
+				return filepath.SkipDir
+			}
+		}
+
+		rel, _ := filepath.Rel(root, path)
+		if rel == "." {
+			return nil
+		}
+
+		// Check gitignore.
+		if ignorer.shouldIgnore(rel, d.IsDir()) {
+			if d.IsDir() {
+				return filepath.SkipDir
+			}
+			return nil
+		}
+
+		if count >= limit {
+			truncated = true
+			return filepath.SkipAll
+		}
+
+		// Build indented tree entry.
+		depth := strings.Count(rel, string(filepath.Separator))
+		indent := strings.Repeat("  ", depth)
+		name := d.Name()
+
+		if d.IsDir() {
+			out.WriteString(fmt.Sprintf("%s%s/\n", indent, name))
+			return nil
+		}
+
+		count++
+		out.WriteString(indent + name)
+
+		// Extract symbols for source files.
+		ext := filepath.Ext(name)
+		if sourceFileExtensions[ext] {
+			data, readErr := os.ReadFile(path)
+			if readErr == nil && len(data) <= 256*1024 {
+				symbols, symErr := extractSymbols(path, data)
+				if symErr == nil && len(symbols) > 0 {
+					out.WriteString(" — ")
+					out.WriteString(compactSymbolSummary(symbols))
+				}
+			}
+		}
+
+		out.WriteString("\n")
+		return nil
+	})
+
+	if err != nil && !errors.Is(err, filepath.SkipAll) {
+		return "", err
+	}
+
+	if truncated {
+		out.WriteString(fmt.Sprintf("\n... (%d files shown, more available; use file_list or code_symbols for details)\n", count))
+	}
+
+	return FormatResult(out.String()), nil
+}
+
+// ────────────────────────────────────────────────────────────────────────────
+// Compact symbol summary for inline display in the file tree
+// ────────────────────────────────────────────────────────────────────────────
+
+func compactSymbolSummary(symbols []symbolInfo) string {
+	var parts []string
+
+	// Group by kind.
+	funcs := []string{}
+	methods := []string{}
+	types := []string{}
+
+	for _, s := range symbols {
+		switch s.Kind {
+		case "function":
+			funcs = append(funcs, s.Name)
+		case "method":
+			methods = append(methods, s.Name)
+		case "class", "interface", "struct", "enum", "trait", "alias", "func", "type", "impl":
+			types = append(types, s.Name)
+		}
+		// Skip imports and package for compact view.
+	}
+
+	if len(funcs) > 0 {
+		parts = append(parts, fmt.Sprintf("fn: %s", strings.Join(funcs, ", ")))
+	}
+	if len(methods) > 0 {
+		parts = append(parts, fmt.Sprintf("method: %s", strings.Join(methods, ", ")))
+	}
+	if len(types) > 0 {
+		parts = append(parts, fmt.Sprintf("type: %s", strings.Join(types, ", ")))
+	}
+
+	return strings.Join(parts, "; ")
+}
+
+// ────────────────────────────────────────────────────────────────────────────
+// Simple .gitignore matcher
+// ────────────────────────────────────────────────────────────────────────────
+
+type gitignoreMatcher struct {
+	patterns []string
+}
+
+func newGitignoreMatcher(root string) *gitignoreMatcher {
+	m := &gitignoreMatcher{}
+	// Walk up from root, collecting .gitignore patterns.
+	for dir := root; dir != "" && dir != string(filepath.Separator); dir = filepath.Dir(dir) {
+		giPath := filepath.Join(dir, ".gitignore")
+		data, err := os.ReadFile(giPath)
+		if err != nil {
+			continue
+		}
+		for _, line := range strings.Split(string(data), "\n") {
+			line = strings.TrimSpace(line)
+			if line == "" || strings.HasPrefix(line, "#") {
+				continue
+			}
+			// Remove leading / for anchored patterns; we handle all as relative.
+			line = strings.TrimPrefix(line, "/")
+			// Remove trailing / for directory-only patterns.
+			line = strings.TrimSuffix(line, "/")
+			m.patterns = append(m.patterns, line)
+		}
+	}
+	return m
+}
+
+func (m *gitignoreMatcher) shouldIgnore(path string, isDir bool) bool {
+	name := filepath.Base(path)
+	for _, pat := range m.patterns {
+		// Simple match against full relative path or just the basename.
+		matched, _ := filepath.Match(pat, path)
+		if matched {
+			return true
+		}
+		matched, _ = filepath.Match(pat, name)
+		if matched {
+			return true
+		}
+	}
+	return false
+}

internal/tools/patch.go

@@ -0,0 +1,430 @@
+package tools
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"agentu/internal/safeio"
+)
+
+// ────────────────────────────────────────────────────────────────────────────
+// V4 patch format data structures
+// ────────────────────────────────────────────────────────────────────────────
+
+type patchAction string
+
+const (
+	actionAdd    patchAction = "add"
+	actionUpdate patchAction = "update"
+	actionDelete patchAction = "delete"
+)
+
+type patchFile struct {
+	action     patchAction
+	path       string
+	newContent string   // for "add" — full file content
+	lines      []string // for "update" — lines to match/replace; for "delete" — unused
+}
+
+type parsedPatch struct {
+	files []patchFile
+}
+
+// ────────────────────────────────────────────────────────────────────────────
+// Parser
+// ────────────────────────────────────────────────────────────────────────────
+
+// parsePatch parses a V4-format patch string.
+//
+// Format:
+//
+//	*** Begin Patch
+//	*** Add File: <path>
+//	<file content>
+//	*** Update File: <path>
+//	@@
+//	<context line>
+//	@@
+//	-<old line>
+//	+<new line>
+//	*** Delete File: <path>
+//	*** End Patch
+func parsePatch(patch string) (*parsedPatch, error) {
+	patch = strings.TrimSpace(patch)
+	if patch == "" {
+		return nil, fmt.Errorf("patch is empty")
+	}
+
+	lines := strings.Split(patch, "\n")
+	if len(lines) == 0 {
+		return nil, fmt.Errorf("patch is empty")
+	}
+
+	if strings.TrimSpace(lines[0]) != "*** Begin Patch" {
+		return nil, fmt.Errorf("patch must start with *** Begin Patch")
+	}
+
+	var files []patchFile
+	seenPaths := make(map[string]struct{})
+	seenEnd := false
+	i := 1
+
+	for i < len(lines) {
+		line := strings.TrimSpace(lines[i])
+
+		switch {
+		case line == "*** End Patch":
+			seenEnd = true
+			i = len(lines)
+		case strings.HasPrefix(line, "*** Add File:"):
+			path := strings.TrimSpace(strings.TrimPrefix(line, "*** Add File:"))
+			if err := validatePatchPath(path, seenPaths); err != nil {
+				return nil, err
+			}
+			i++
+			var contentLines []string
+			for i < len(lines) {
+				next := strings.TrimSpace(lines[i])
+				if strings.HasPrefix(next, "***") {
+					break
+				}
+				content := lines[i]
+				if strings.HasPrefix(content, "+") {
+					content = strings.TrimPrefix(content, "+")
+				}
+				contentLines = append(contentLines, content)
+				i++
+			}
+			files = append(files, patchFile{
+				action:     actionAdd,
+				path:       path,
+				newContent: strings.Join(contentLines, "\n") + "\n",
+			})
+
+		case strings.HasPrefix(line, "*** Delete File:"):
+			path := strings.TrimSpace(strings.TrimPrefix(line, "*** Delete File:"))
+			if err := validatePatchPath(path, seenPaths); err != nil {
+				return nil, err
+			}
+			files = append(files, patchFile{
+				action: actionDelete,
+				path:   path,
+			})
+			i++
+
+		case strings.HasPrefix(line, "*** Update File:"):
+			path := strings.TrimSpace(strings.TrimPrefix(line, "*** Update File:"))
+			if err := validatePatchPath(path, seenPaths); err != nil {
+				return nil, err
+			}
+			i++
+			var hunkLines []string
+			for i < len(lines) {
+				next := strings.TrimSpace(lines[i])
+				if strings.HasPrefix(next, "***") {
+					break
+				}
+				hunkLines = append(hunkLines, lines[i])
+				i++
+			}
+			files = append(files, patchFile{
+				action: actionUpdate,
+				path:   path,
+				lines:  hunkLines,
+			})
+
+		default:
+			i++
+		}
+	}
+
+	if len(files) == 0 {
+		return nil, fmt.Errorf("patch contains no file operations")
+	}
+	if !seenEnd {
+		return nil, fmt.Errorf("patch must end with *** End Patch")
+	}
+
+	return &parsedPatch{files: files}, nil
+}
+
+func validatePatchPath(path string, seen map[string]struct{}) error {
+	if path == "" {
+		return fmt.Errorf("patch file path is required")
+	}
+	if _, ok := seen[path]; ok {
+		return fmt.Errorf("patch contains duplicate file path: %s", path)
+	}
+	seen[path] = struct{}{}
+	return nil
+}
+
+// ────────────────────────────────────────────────────────────────────────────
+// Applier
+// ────────────────────────────────────────────────────────────────────────────
+
+// applyPatchFile applies a single patch file operation.
+func applyPatchFile(baseDir string, pf patchFile) (string, error) {
+	switch pf.action {
+	case actionAdd:
+		return applyAdd(baseDir, pf)
+	case actionUpdate:
+		return applyUpdate(baseDir, pf)
+	case actionDelete:
+		return applyDelete(baseDir, pf)
+	default:
+		return "", fmt.Errorf("unknown patch action: %s", pf.action)
+	}
+}
+
+func applyAdd(baseDir string, pf patchFile) (string, error) {
+	path, err := resolvePathForWrite(baseDir, pf.path)
+	if err != nil {
+		return "", err
+	}
+	if _, err := os.Stat(path); err == nil {
+		return "", fmt.Errorf("cannot add existing file: %s", pf.path)
+	} else if !os.IsNotExist(err) {
+		return "", fmt.Errorf("stat new file %s: %w", pf.path, err)
+	}
+	if err := safeio.AtomicWriteFile(path, []byte(pf.newContent), 0o644); err != nil {
+		return "", fmt.Errorf("write new file %s: %w", pf.path, err)
+	}
+	return fmt.Sprintf("+ %s", pf.path), nil
+}
+
+func applyDelete(baseDir string, pf patchFile) (string, error) {
+	path, err := resolvePath(baseDir, pf.path)
+	if err != nil {
+		return "", err
+	}
+	if info, err := os.Stat(path); err != nil {
+		if os.IsNotExist(err) {
+			return "", fmt.Errorf("cannot delete non-existent file: %s", pf.path)
+		}
+		return "", err
+	} else if info.IsDir() {
+		return "", fmt.Errorf("cannot delete directory: %s", pf.path)
+	}
+	if err := os.Remove(path); err != nil {
+		return "", fmt.Errorf("delete file %s: %w", pf.path, err)
+	}
+	// Try to remove empty parent directories.
+	dir := filepath.Dir(path)
+	for dir != baseDir && dir != "." {
+		if entries, err := os.ReadDir(dir); err == nil && len(entries) == 0 {
+			os.Remove(dir)
+			dir = filepath.Dir(dir)
+		} else {
+			break
+		}
+	}
+	return fmt.Sprintf("- %s", pf.path), nil
+}
+
+func applyUpdate(baseDir string, pf patchFile) (string, error) {
+	path, err := resolvePath(baseDir, pf.path)
+	if err != nil {
+		return "", fmt.Errorf("cannot update non-existent file: %s: %w", pf.path, err)
+	}
+	existing, err := os.ReadFile(path)
+	if err != nil {
+		return "", fmt.Errorf("cannot update non-existent file: %s", pf.path)
+	}
+	info, err := os.Stat(path)
+	if err != nil {
+		return "", fmt.Errorf("stat file %s: %w", pf.path, err)
+	}
+
+	fileLines := strings.Split(string(existing), "\n")
+
+	// Parse the hunk lines into: context blocks + old lines + new lines.
+	result, err := applyHunkLines(fileLines, pf.lines, pf.path)
+	if err != nil {
+		return "", err
+	}
+
+	newContent := strings.Join(result, "\n")
+	if err := safeio.AtomicWriteFile(path, []byte(newContent), info.Mode().Perm()); err != nil {
+		return "", fmt.Errorf("write updated file %s: %w", pf.path, err)
+	}
+	return fmt.Sprintf("~ %s", pf.path), nil
+}
+
+// applyHunkLines applies a sequence of hunk lines to the file content.
+// The hunk lines are a mix of:
+//
+//	@@           — hunk separator (a context line block starts)
+//	<plain line> — context line (must appear in both old and new)
+//	-<line>      — line to remove
+//	+<line>      — line to add
+//
+// Hunks are matched greedily: for each group of contiguous context + - + + lines,
+// find the first position in the file where context + - lines match,
+// replace with context + + lines.
+func applyHunkLines(fileLines []string, hunkLines []string, path string) ([]string, error) {
+	if len(hunkLines) == 0 {
+		return fileLines, nil
+	}
+
+	// Split the hunk lines into separate hunks at @@ markers.
+	type hunk struct {
+		lines []string
+	}
+	var hunks []hunk
+	var current hunk
+
+	for _, hl := range hunkLines {
+		trimmed := strings.TrimSpace(hl)
+		if strings.HasPrefix(trimmed, "@@") {
+			if len(current.lines) > 0 {
+				hunks = append(hunks, current)
+				current = hunk{}
+			}
+			continue
+		}
+		current.lines = append(current.lines, hl)
+	}
+	if len(current.lines) > 0 {
+		hunks = append(hunks, current)
+	}
+
+	if len(hunks) == 0 {
+		return fileLines, nil
+	}
+
+	result := make([]string, len(fileLines))
+	copy(result, fileLines)
+	offset := 0 // tracks net line count change from previous hunks
+
+	for hi, h := range hunks {
+		// Separate the hunk lines into a search pattern and replacement.
+		searchLines, replaceLines := buildHunkSearchReplace(h.lines)
+		if len(searchLines) == 0 && len(replaceLines) == 0 {
+			continue
+		}
+
+		matchStart := findHunkMatch(result, searchLines, offset)
+		if matchStart < 0 {
+			return nil, fmt.Errorf("hunk context not found in %s", path)
+		}
+
+		// Replace: remove searchLines, insert replaceLines.
+		matchEnd := matchStart + len(searchLines)
+		newResult := make([]string, 0, len(result)-len(searchLines)+len(replaceLines))
+		newResult = append(newResult, result[:matchStart]...)
+		newResult = append(newResult, replaceLines...)
+		newResult = append(newResult, result[matchEnd:]...)
+		result = newResult
+
+		offset = matchStart + len(replaceLines)
+
+		// Multiple matches check for this hunk only.
+		secondMatch := findHunkMatch(result, searchLines, offset)
+		if secondMatch >= 0 {
+			return nil, fmt.Errorf("hunk context matched multiple times in %s", path)
+		}
+
+		_ = hi
+	}
+
+	return result, nil
+}
+
+// buildHunkSearchReplace splits a hunk's lines into the search pattern
+// (context + old lines) and replacement (context + new lines).
+func buildHunkSearchReplace(lines []string) (search, replace []string) {
+	for _, l := range lines {
+		if strings.HasPrefix(l, "-") {
+			search = append(search, l[1:])
+		} else if strings.HasPrefix(l, "+") {
+			replace = append(replace, l[1:])
+		} else {
+			contextLine := strings.TrimPrefix(l, " ")
+			search = append(search, contextLine)
+			replace = append(replace, contextLine)
+		}
+	}
+	return
+}
+
+// findHunkMatch finds the first position >= offset in fileLines where
+// searchLines matches exactly (line-by-line comparison).
+func findHunkMatch(fileLines []string, searchLines []string, offset int) int {
+	for i := offset; i <= len(fileLines)-len(searchLines); i++ {
+		match := true
+		for j, sl := range searchLines {
+			if fileLines[i+j] != sl {
+				match = false
+				break
+			}
+		}
+		if match {
+			return i
+		}
+	}
+	return -1
+}
+
+// ────────────────────────────────────────────────────────────────────────────
+// Preview
+// ────────────────────────────────────────────────────────────────────────────
+
+// summarizePatch returns a human-readable summary of the patch.
+func summarizePatch(patch string) (string, error) {
+	pp, err := parsePatch(patch)
+	if err != nil {
+		return "", err
+	}
+
+	var b strings.Builder
+	added, updated, deleted := 0, 0, 0
+	for _, pf := range pp.files {
+		switch pf.action {
+		case actionAdd:
+			added++
+			fmt.Fprintf(&b, "+ add %s\n", pf.path)
+		case actionUpdate:
+			updated++
+			fmt.Fprintf(&b, "~ update %s\n", pf.path)
+		case actionDelete:
+			deleted++
+			fmt.Fprintf(&b, "- delete %s\n", pf.path)
+		}
+	}
+
+	summary := fmt.Sprintf("patch: %d file(s) changed", len(pp.files))
+	if added > 0 {
+		summary += fmt.Sprintf(" (%d added)", added)
+	}
+	if updated > 0 {
+		summary += fmt.Sprintf(" (%d updated)", updated)
+	}
+	if deleted > 0 {
+		summary += fmt.Sprintf(" (%d deleted)", deleted)
+	}
+
+	return summary + "\n" + b.String(), nil
+}
+
+// extractTargetPaths returns all file paths referenced in the patch.
+func extractTargetPaths(baseDir, patch string) ([]string, error) {
+	pp, err := parsePatch(patch)
+	if err != nil {
+		return nil, err
+	}
+
+	var paths []string
+	for _, pf := range pp.files {
+		// Use resolvePathForWrite for all files — TargetPaths needs paths
+		// even for files that don't exist yet (rollback snapshots).
+		resolved, err := resolvePathForWrite(baseDir, pf.path)
+		if err != nil {
+			return nil, fmt.Errorf("file %s: %w", pf.path, err)
+		}
+		paths = append(paths, resolved)
+	}
+	return paths, nil
+}

internal/tools/paths.go

@@ -2,22 +2,85 @@ package tools
 
 import (
 	"fmt"
+	"os"
 	"path/filepath"
 	"strings"
 )
 
 func resolvePath(baseDir, path string) (string, error) {
+	resolved, canonicalBase, err := resolveLexicalPath(baseDir, path)
+	if err != nil {
+		return "", err
+	}
+	canonical, err := filepath.EvalSymlinks(resolved)
+	if err != nil {
+		return "", err
+	}
+	if !isWithinDir(canonical, canonicalBase) {
+		return "", fmt.Errorf("path escapes the project directory through a symbolic link: %s", path)
+	}
+	return canonical, nil
+}
+
+// resolvePathForWrite resolves a path that may not exist yet while preventing
+// existing parent-directory symlinks from escaping the project directory.
+func resolvePathForWrite(baseDir, path string) (string, error) {
+	resolved, canonicalBase, err := resolveLexicalPath(baseDir, path)
+	if err != nil {
+		return "", err
+	}
+
+	existing := resolved
+	var suffix []string
+	for {
+		_, err := os.Lstat(existing)
+		if err == nil {
+			break
+		}
+		if !os.IsNotExist(err) {
+			return "", err
+		}
+		parent := filepath.Dir(existing)
+		if parent == existing {
+			return "", err
+		}
+		suffix = append(suffix, filepath.Base(existing))
+		existing = parent
+	}
+
+	canonical, err := filepath.EvalSymlinks(existing)
+	if err != nil {
+		return "", err
+	}
+	for i := len(suffix) - 1; i >= 0; i-- {
+		canonical = filepath.Join(canonical, suffix[i])
+	}
+	if !isWithinDir(canonical, canonicalBase) {
+		return "", fmt.Errorf("path escapes the project directory through a symbolic link: %s", path)
+	}
+	return canonical, nil
+}
+
+func resolveLexicalPath(baseDir, path string) (string, string, error) {
 	if path == "" {
 		path = "."
 	}
 	if filepath.IsAbs(path) {
-		return "", fmt.Errorf("absolute paths are not allowed; use a path relative to the project directory")
+		return "", "", fmt.Errorf("absolute paths are not allowed; use a path relative to the project directory")
 	}
-	resolved := filepath.Clean(filepath.Join(baseDir, path))
+	absBase, err := filepath.Abs(baseDir)
-	if !isWithinDir(resolved, baseDir) {
+	if err != nil {
-		return "", fmt.Errorf("path escapes the project directory: %s", path)
+		return "", "", err
 	}
-	return resolved, nil
+	canonicalBase, err := filepath.EvalSymlinks(absBase)
+	if err != nil {
+		return "", "", err
+	}
+	resolved := filepath.Clean(filepath.Join(absBase, path))
+	if !isWithinDir(resolved, absBase) {
+		return "", "", fmt.Errorf("path escapes the project directory: %s", path)
+	}
+	return resolved, canonicalBase, nil
 }
 
 func isWithinDir(path, dir string) bool {

internal/tools/rollback.go

@@ -0,0 +1,117 @@
+package tools
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"fmt"
+	"io/fs"
+	"os"
+	"sync"
+	"time"
+
+	"agentu/internal/safeio"
+)
+
+// RollbackSnapshot captures the state of a file before a mutation.
+type RollbackSnapshot struct {
+	ID           string
+	Tool         string
+	Path         string
+	ResolvedPath string
+	Existed      bool
+	Data         []byte
+	Mode         fs.FileMode
+	CreatedAt    time.Time
+}
+
+// RollbackStore manages in-memory rollback snapshots for the current process.
+type RollbackStore struct {
+	mu        sync.Mutex
+	snapshots []RollbackSnapshot
+}
+
+// NewRollbackStore creates an empty rollback store.
+func NewRollbackStore() *RollbackStore {
+	return &RollbackStore{}
+}
+
+// generateSnapshotID creates a rollback ID: "rb_" + 12 lowercase hex chars.
+func generateSnapshotID() (string, error) {
+	b := make([]byte, 6)
+	if _, err := rand.Read(b); err != nil {
+		return "", fmt.Errorf("generate snapshot id: %w", err)
+	}
+	return "rb_" + hex.EncodeToString(b), nil
+}
+
+// Capture stores the current state of the file at resolvedPath before a tool
+// modifies it. Returns the snapshot ID.
+func (s *RollbackStore) Capture(tool string, displayPath string, resolvedPath string) (string, error) {
+	id, err := generateSnapshotID()
+	if err != nil {
+		return "", err
+	}
+
+	snap := RollbackSnapshot{
+		ID:           id,
+		Tool:         tool,
+		Path:         displayPath,
+		ResolvedPath: resolvedPath,
+		CreatedAt:    time.Now().UTC(),
+	}
+
+	info, err := os.Stat(resolvedPath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			snap.Existed = false
+		} else {
+			return "", fmt.Errorf("stat file: %w", err)
+		}
+	} else {
+		snap.Existed = true
+		snap.Mode = info.Mode().Perm()
+		data, err := os.ReadFile(resolvedPath)
+		if err != nil {
+			return "", fmt.Errorf("read file for snapshot: %w", err)
+		}
+		snap.Data = data
+	}
+
+	s.mu.Lock()
+	s.snapshots = append(s.snapshots, snap)
+	s.mu.Unlock()
+	return id, nil
+}
+
+// Restore rolls back a file to the state captured in the given snapshot.
+func (s *RollbackStore) Restore(id string) (RollbackSnapshot, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	for i := len(s.snapshots) - 1; i >= 0; i-- {
+		if s.snapshots[i].ID == id {
+			snap := s.snapshots[i]
+			if snap.Existed {
+				if err := safeio.AtomicWriteFile(snap.ResolvedPath, snap.Data, snap.Mode); err != nil {
+					return RollbackSnapshot{}, fmt.Errorf("restore file: %w", err)
+				}
+			} else {
+				if err := os.Remove(snap.ResolvedPath); err != nil && !os.IsNotExist(err) {
+					return RollbackSnapshot{}, fmt.Errorf("remove created file: %w", err)
+				}
+			}
+			s.snapshots = append(s.snapshots[:i], s.snapshots[i+1:]...)
+			return snap, nil
+		}
+	}
+	return RollbackSnapshot{}, fmt.Errorf("rollback snapshot %s not found", id)
+}
+
+// LastID returns the most recent snapshot ID, or "" if empty.
+func (s *RollbackStore) LastID() string {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if len(s.snapshots) == 0 {
+		return ""
+	}
+	return s.snapshots[len(s.snapshots)-1].ID
+}

internal/tools/shell.go

@@ -1,6 +1,7 @@
 package tools
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"errors"
@@ -8,6 +9,7 @@ import (
 	"os"
 	"os/exec"
 	"strings"
+	"sync"
 
 	"agentu/pkg/llm"
 )
@@ -21,6 +23,17 @@ func NewShellRunTool(workingDir string) *ShellRunTool {
 }
 func (*ShellRunTool) Mutates() bool { return true }
 
+func (t *ShellRunTool) Preview(ctx context.Context, raw json.RawMessage) (string, error) {
+	var args shellRunArgs
+	if err := json.Unmarshal(raw, &args); err != nil {
+		return "", fmt.Errorf("parse arguments: %w", err)
+	}
+	if strings.TrimSpace(args.Command) == "" {
+		return "", errors.New("command is required")
+	}
+	return "$ " + strings.TrimSpace(args.Command), ctx.Err()
+}
+
 func (t *ShellRunTool) Definition() llm.Tool {
 	return llm.Tool{
 		Type: "function",
@@ -46,6 +59,10 @@ type shellRunArgs struct {
 }
 
 func (t *ShellRunTool) Execute(ctx context.Context, raw json.RawMessage) (string, error) {
+	return t.ExecuteStream(ctx, raw, nil)
+}
+
+func (t *ShellRunTool) ExecuteStream(ctx context.Context, raw json.RawMessage, emit func(string) error) (string, error) {
 	var args shellRunArgs
 	if err := json.Unmarshal(raw, &args); err != nil {
 		return "", fmt.Errorf("parse arguments: %w", err)
@@ -66,16 +83,45 @@ func (t *ShellRunTool) Execute(ctx context.Context, raw json.RawMessage) (string
 	cmd := exec.CommandContext(ctx, shell, "-lc", args.Command)
 	cmd.Dir = workdir
 
-	output, err := cmd.CombinedOutput()
+	var mu sync.Mutex
-	text := FormatResult(string(output))
+	var combined bytes.Buffer
+	writer := &shellStreamWriter{mu: &mu, buf: &combined, emit: emit}
+	cmd.Stdout = writer
+	cmd.Stderr = writer
+
+	runErr := cmd.Run()
+	text := FormatResult(combined.String())
 	if errors.Is(ctx.Err(), context.DeadlineExceeded) {
 		return text, fmt.Errorf("command timed out: %w", ctx.Err())
 	}
-	if err != nil {
+	if runErr != nil {
 		if text != "" {
-			return text, fmt.Errorf("command failed: %w", err)
+			return text, fmt.Errorf("command failed: %w", runErr)
 		}
-		return "", fmt.Errorf("command failed: %w", err)
+		return "", fmt.Errorf("command failed: %w", runErr)
 	}
 	return text, nil
 }
+
+// shellStreamWriter captures command output for the final result while
+// forwarding every write to the streaming callback as it happens.
+type shellStreamWriter struct {
+	mu   *sync.Mutex
+	buf  *bytes.Buffer
+	emit func(string) error
+}
+
+func (w *shellStreamWriter) Write(p []byte) (int, error) {
+	w.mu.Lock()
+	n, writeErr := w.buf.Write(p)
+	w.mu.Unlock()
+	if writeErr != nil {
+		return n, writeErr
+	}
+	if w.emit != nil {
+		if err := w.emit(string(p)); err != nil {
+			return 0, err
+		}
+	}
+	return len(p), nil
+}

internal/tools/shell_test.go

@@ -0,0 +1,71 @@
+package tools
+
+import (
+	"context"
+	"encoding/json"
+	"strings"
+	"sync"
+	"testing"
+)
+
+func TestShellRunStreamsOutputWhileRunning(t *testing.T) {
+	tool := NewShellRunTool(t.TempDir())
+	var mu sync.Mutex
+	var chunks []string
+	output, err := tool.ExecuteStream(context.Background(), json.RawMessage(`{"command":"printf 'a\\n'; sleep 0.05; printf 'b\\n'"}`), func(chunk string) error {
+		mu.Lock()
+		chunks = append(chunks, chunk)
+		mu.Unlock()
+		return nil
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(output, "a") || !strings.Contains(output, "b") {
+		t.Fatalf("output = %q", output)
+	}
+	mu.Lock()
+	joined := strings.Join(chunks, "")
+	mu.Unlock()
+	if len(chunks) == 0 {
+		t.Fatal("expected at least one streamed chunk")
+	}
+	if !strings.Contains(joined, "a") || !strings.Contains(joined, "b") {
+		t.Fatalf("streamed chunks = %q", joined)
+	}
+}
+
+func TestShellRunExecuteCollectsFullOutput(t *testing.T) {
+	tool := NewShellRunTool(t.TempDir())
+	output, err := tool.Execute(context.Background(), json.RawMessage(`{"command":"printf 'hello world'"}`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.TrimSpace(output) != "hello world" {
+		t.Fatalf("output = %q", output)
+	}
+}
+
+func TestShellRunStreamsStderrAndStdout(t *testing.T) {
+	tool := NewShellRunTool(t.TempDir())
+	var mu sync.Mutex
+	var chunks []string
+	output, err := tool.ExecuteStream(context.Background(), json.RawMessage(`{"command":"echo out; echo err >&2"}`), func(chunk string) error {
+		mu.Lock()
+		chunks = append(chunks, chunk)
+		mu.Unlock()
+		return nil
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(output, "out") || !strings.Contains(output, "err") {
+		t.Fatalf("output = %q", output)
+	}
+	mu.Lock()
+	joined := strings.Join(chunks, "")
+	mu.Unlock()
+	if !strings.Contains(joined, "out") || !strings.Contains(joined, "err") {
+		t.Fatalf("streamed chunks = %q", joined)
+	}
+}

internal/tools/symbols.go

@@ -0,0 +1,530 @@
+package tools
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/printer"
+	"go/token"
+	"regexp"
+	"sort"
+	"strings"
+)
+
+// symbolInfo holds extracted symbol metadata.
+type symbolInfo struct {
+	Name      string
+	Kind      string // "import", "function", "method", "class", "type", "interface", "trait", "enum", "struct"
+	StartLine int
+}
+
+// sourceFileExtensions lists extensions that we can extract symbols from.
+var sourceFileExtensions = map[string]bool{
+	".go":  true,
+	".ts":  true,
+	".tsx": true,
+	".js":  true,
+	".jsx": true,
+	".mjs": true,
+	".py":  true,
+	".rs":  true,
+}
+
+// extractSymbols extracts symbols from a source file using language-specific
+// parsing (go/ast for Go, regex for other languages).
+func extractSymbols(path string, data []byte) ([]symbolInfo, error) {
+	ext := ""
+	if idx := strings.LastIndex(path, "."); idx >= 0 {
+		ext = path[idx:]
+	}
+
+	switch ext {
+	case ".go":
+		return extractGoSymbols(path, data)
+	case ".py":
+		return extractPythonsymbols(data)
+	case ".ts", ".tsx", ".js", ".jsx", ".mjs":
+		return extractTSSymbols(data)
+	case ".rs":
+		return extractRustSymbols(data)
+	default:
+		return nil, nil
+	}
+}
+
+// ────────────────────────────────────────────────────────────────────────────
+// Go symbol extraction (using go/ast)
+// ────────────────────────────────────────────────────────────────────────────
+
+func extractGoSymbols(path string, data []byte) ([]symbolInfo, error) {
+	fset := token.NewFileSet()
+	// Parse imports only first for the import list.
+	importFile, err := parser.ParseFile(fset, path, data, parser.ImportsOnly|parser.ParseComments)
+	if err != nil {
+		return nil, err
+	}
+	full, err := parser.ParseFile(fset, path, data, 0)
+	if err != nil {
+		return nil, err
+	}
+
+	var symbols []symbolInfo
+
+	// Package
+	symbols = append(symbols, symbolInfo{
+		Name:      full.Name.Name,
+		Kind:      "package",
+		StartLine: fset.Position(full.Name.Pos()).Line,
+	})
+
+	// Imports
+	for _, imp := range importFile.Imports {
+		symbols = append(symbols, symbolInfo{
+			Name:      strings.Trim(imp.Path.Value, "\""),
+			Kind:      "import",
+			StartLine: fset.Position(imp.Pos()).Line,
+		})
+	}
+
+	// Declarations
+	for _, decl := range full.Decls {
+		switch d := decl.(type) {
+		case *ast.GenDecl:
+			if d.Tok != token.TYPE {
+				continue
+			}
+			for _, spec := range d.Specs {
+				if ts, ok := spec.(*ast.TypeSpec); ok {
+					kind := goTypeKind(ts.Type)
+					symbols = append(symbols, symbolInfo{
+						Name:      ts.Name.Name,
+						Kind:      kind,
+						StartLine: fset.Position(ts.Pos()).Line,
+					})
+				}
+			}
+		case *ast.FuncDecl:
+			line := fset.Position(d.Pos()).Line
+			if d.Recv != nil {
+				symbols = append(symbols, symbolInfo{
+					Name:      fmt.Sprintf("%s.%s", goReceiverName(d.Recv), d.Name.Name),
+					Kind:      "method",
+					StartLine: line,
+				})
+			} else {
+				symbols = append(symbols, symbolInfo{
+					Name:      d.Name.Name,
+					Kind:      "function",
+					StartLine: line,
+				})
+			}
+		}
+	}
+
+	sortSymbols(symbols)
+	return symbols, nil
+}
+
+func goTypeKind(expr ast.Expr) string {
+	switch expr.(type) {
+	case *ast.StructType:
+		return "struct"
+	case *ast.InterfaceType:
+		return "interface"
+	case *ast.FuncType:
+		return "func"
+	default:
+		return "alias"
+	}
+}
+
+func goReceiverName(recv *ast.FieldList) string {
+	if recv == nil || len(recv.List) == 0 {
+		return "?"
+	}
+	var b bytes.Buffer
+	if err := printer.Fprint(&b, token.NewFileSet(), recv.List[0].Type); err != nil {
+		return "?"
+	}
+	return b.String()
+}
+
+// ────────────────────────────────────────────────────────────────────────────
+// Python symbol extraction (regex-based)
+// ────────────────────────────────────────────────────────────────────────────
+
+var (
+	pyFuncRe      = regexp.MustCompile(`^\s*def\s+(\w+)\s*\(`)
+	pyClassRe     = regexp.MustCompile(`^\s*class\s+(\w+)\s*[(:]`)
+	pyImportRe    = regexp.MustCompile(`^\s*(?:from\s+\S+\s+)?import\s+`)
+	pyDecoratorRe = regexp.MustCompile(`^\s*@`)
+)
+
+func extractPythonsymbols(data []byte) ([]symbolInfo, error) {
+	var symbols []symbolInfo
+	scanner := bufio.NewScanner(bytes.NewReader(data))
+	lineNum := 0
+	inDecorator := false
+
+	for scanner.Scan() {
+		lineNum++
+		line := scanner.Text()
+
+		// Track decorator stacks
+		if pyDecoratorRe.MatchString(line) {
+			inDecorator = true
+			continue
+		}
+
+		// Import lines
+		if pyImportRe.MatchString(line) {
+			// Extract the module name for a simple "import foo" or "from foo import bar"
+			importPath := strings.TrimSpace(line)
+			symbols = append(symbols, symbolInfo{
+				Name:      importPath,
+				Kind:      "import",
+				StartLine: lineNum,
+			})
+			inDecorator = false
+			continue
+		}
+
+		matches := pyClassRe.FindStringSubmatch(line)
+		if matches != nil {
+			symbols = append(symbols, symbolInfo{
+				Name:      matches[1],
+				Kind:      "class",
+				StartLine: lineNum,
+			})
+			inDecorator = false
+			continue
+		}
+
+		matches = pyFuncRe.FindStringSubmatch(line)
+		if matches != nil {
+			kind := "function"
+			if inDecorator {
+				// Could be a decorated function or method — still mark as function
+				// (class-method detection requires indentation tracking)
+			}
+			symbols = append(symbols, symbolInfo{
+				Name:      matches[1],
+				Kind:      kind,
+				StartLine: lineNum,
+			})
+			inDecorator = false
+			continue
+		}
+
+		inDecorator = false
+	}
+
+	sortSymbols(symbols)
+	return symbols, scanner.Err()
+}
+
+// ────────────────────────────────────────────────────────────────────────────
+// TypeScript / JavaScript symbol extraction (regex-based)
+// ────────────────────────────────────────────────────────────────────────────
+
+var (
+	tsFuncRe      = regexp.MustCompile(`(?:export\s+(?:default\s+)?)?(?:async\s+)?function\s+(\w+)\s*[<(]`)
+	tsArrowRe     = regexp.MustCompile(`(?:export\s+(?:default\s+)?)?(?:const|let|var)\s+(\w+)\s*=\s*(?:async\s*)?\s*\(`)
+	tsClassRe     = regexp.MustCompile(`(?:export\s+(?:default\s+)?)?class\s+(\w+)`)
+	tsInterfaceRe = regexp.MustCompile(`(?:export\s+(?:default\s+)?)?interface\s+(\w+)`)
+	tsTypeRe      = regexp.MustCompile(`(?:export\s+(?:default\s+)?)?type\s+(\w+)\s*=`)
+	tsEnumRe      = regexp.MustCompile(`(?:export\s+(?:default\s+)?)?enum\s+(\w+)`)
+	tsImportRe    = regexp.MustCompile(`^\s*import\s+`)
+	tsMethodRe    = regexp.MustCompile(`^\s*(?:public\s+|private\s+|protected\s+|static\s+|async\s+)*(\w+)\s*[<(][^)]*\)\s*(?::\s*\S+\s*)?\{`)
+)
+
+func extractTSSymbols(data []byte) ([]symbolInfo, error) {
+	var symbols []symbolInfo
+	scanner := bufio.NewScanner(bytes.NewReader(data))
+	lineNum := 0
+
+	// Track brace depth to detect class body methods.
+	braceDepth := 0
+	inClass := false
+
+	for scanner.Scan() {
+		lineNum++
+		line := scanner.Text()
+		trimmed := strings.TrimSpace(line)
+
+		// Count braces on this line (applied at end of iteration).
+		opens := strings.Count(line, "{")
+		closes := strings.Count(line, "}")
+
+		// Import lines — skip everything else, just track braces.
+		if tsImportRe.MatchString(line) {
+			importPath := trimmed
+			symbols = append(symbols, symbolInfo{
+				Name:      importPath,
+				Kind:      "import",
+				StartLine: lineNum,
+			})
+			goto applyBraces
+		}
+
+		// Skip empty and comment-only lines.
+		if trimmed == "" || strings.HasPrefix(trimmed, "//") || strings.HasPrefix(trimmed, "/*") {
+			goto applyBraces
+		}
+
+		// Interface
+		if matches := tsInterfaceRe.FindStringSubmatch(line); matches != nil {
+			symbols = append(symbols, symbolInfo{
+				Name:      matches[1],
+				Kind:      "interface",
+				StartLine: lineNum,
+			})
+			goto applyBraces
+		}
+
+		// Type alias
+		if matches := tsTypeRe.FindStringSubmatch(line); matches != nil {
+			symbols = append(symbols, symbolInfo{
+				Name:      matches[1],
+				Kind:      "type",
+				StartLine: lineNum,
+			})
+			goto applyBraces
+		}
+
+		// Enum
+		if matches := tsEnumRe.FindStringSubmatch(line); matches != nil {
+			symbols = append(symbols, symbolInfo{
+				Name:      matches[1],
+				Kind:      "enum",
+				StartLine: lineNum,
+			})
+			goto applyBraces
+		}
+
+		// Class
+		if matches := tsClassRe.FindStringSubmatch(line); matches != nil {
+			symbols = append(symbols, symbolInfo{
+				Name:      matches[1],
+				Kind:      "class",
+				StartLine: lineNum,
+			})
+			// Set inClass BEFORE applying braces for this line, so that
+			// methods on the same line (unlikely) or next line are detected.
+			if opens > 0 {
+				inClass = true
+			}
+			goto applyBraces
+		}
+
+		// Method inside class body (at depth 1 brace level).
+		if inClass && braceDepth == 1 {
+			if matches := tsMethodRe.FindStringSubmatch(line); matches != nil {
+				name := matches[1]
+				if !isKeyword(name) {
+					symbols = append(symbols, symbolInfo{
+						Name:      name,
+						Kind:      "method",
+						StartLine: lineNum,
+					})
+				}
+			}
+		}
+
+		// Function declaration (top-level only, or nested function).
+		if !inClass || braceDepth > 1 {
+			if matches := tsFuncRe.FindStringSubmatch(line); matches != nil {
+				symbols = append(symbols, symbolInfo{
+					Name:      matches[1],
+					Kind:      "function",
+					StartLine: lineNum,
+				})
+			}
+		}
+
+		// Arrow function assigned to const/let/var.
+		if !inClass || braceDepth > 1 {
+			if matches := tsArrowRe.FindStringSubmatch(line); matches != nil {
+				symbols = append(symbols, symbolInfo{
+					Name:      matches[1],
+					Kind:      "function",
+					StartLine: lineNum,
+				})
+			}
+		}
+
+	applyBraces:
+		braceDepth += opens - closes
+		if braceDepth <= 0 {
+			braceDepth = 0
+			inClass = false
+		}
+	}
+
+	sortSymbols(symbols)
+	return symbols, scanner.Err()
+}
+
+func isKeyword(s string) bool {
+	switch s {
+	case "if", "else", "for", "while", "switch", "case", "return", "throw",
+		"try", "catch", "finally", "new", "delete", "typeof", "instanceof",
+		"constructor", "get", "set":
+		return true
+	}
+	return false
+}
+
+// ────────────────────────────────────────────────────────────────────────────
+// Rust symbol extraction (regex-based)
+// ────────────────────────────────────────────────────────────────────────────
+
+var (
+	rsFnRe     = regexp.MustCompile(`^\s*(?:pub(?:\s*\(\s*crate\s*\))?\s+)?(?:async\s+)?(?:unsafe\s+)?fn\s+(\w+)\s*[<(]`)
+	rsStructRe = regexp.MustCompile(`^\s*(?:pub(?:\s*\(\s*crate\s*\))?\s+)?struct\s+(\w+)`)
+	rsEnumRe   = regexp.MustCompile(`^\s*(?:pub(?:\s*\(\s*crate\s*\))?\s+)?enum\s+(\w+)`)
+	rsTraitRe  = regexp.MustCompile(`^\s*(?:pub(?:\s*\(\s*crate\s*\))?\s+)?(?:unsafe\s+)?trait\s+(\w+)`)
+	rsImplRe   = regexp.MustCompile(`^\s*impl\s+`)
+	rsUseRe    = regexp.MustCompile(`^\s*(?:pub\s+)?use\s+`)
+)
+
+func extractRustSymbols(data []byte) ([]symbolInfo, error) {
+	var symbols []symbolInfo
+	scanner := bufio.NewScanner(bytes.NewReader(data))
+	lineNum := 0
+
+	for scanner.Scan() {
+		lineNum++
+		line := scanner.Text()
+		trimmed := strings.TrimSpace(line)
+
+		// Skip empty and comments
+		if trimmed == "" || strings.HasPrefix(trimmed, "//") || strings.HasPrefix(trimmed, "/*") {
+			continue
+		}
+
+		// Use
+		if rsUseRe.MatchString(line) {
+			symbols = append(symbols, symbolInfo{
+				Name:      trimmed,
+				Kind:      "import",
+				StartLine: lineNum,
+			})
+			continue
+		}
+
+		// Struct
+		if matches := rsStructRe.FindStringSubmatch(line); matches != nil {
+			symbols = append(symbols, symbolInfo{
+				Name:      matches[1],
+				Kind:      "struct",
+				StartLine: lineNum,
+			})
+			continue
+		}
+
+		// Enum
+		if matches := rsEnumRe.FindStringSubmatch(line); matches != nil {
+			symbols = append(symbols, symbolInfo{
+				Name:      matches[1],
+				Kind:      "enum",
+				StartLine: lineNum,
+			})
+			continue
+		}
+
+		// Trait
+		if matches := rsTraitRe.FindStringSubmatch(line); matches != nil {
+			symbols = append(symbols, symbolInfo{
+				Name:      matches[1],
+				Kind:      "trait",
+				StartLine: lineNum,
+			})
+			continue
+		}
+
+		// Impl
+		if rsImplRe.MatchString(line) {
+			// Extract type name from "impl Foo" or "impl<T> Foo" or "impl Trait for Foo"
+			rest := strings.TrimPrefix(trimmed, "impl")
+			rest = strings.TrimSpace(rest)
+			// Remove generics
+			if idx := strings.Index(rest, "<"); idx >= 0 {
+				depth := 1
+				for i := idx + 1; i < len(rest); i++ {
+					if rest[i] == '<' {
+						depth++
+					} else if rest[i] == '>' {
+						depth--
+						if depth == 0 {
+							rest = strings.TrimSpace(rest[:idx] + rest[i+1:])
+							break
+						}
+					}
+				}
+			}
+			// Handle "for Type"
+			if idx := strings.Index(rest, " for "); idx >= 0 {
+				rest = strings.TrimSpace(rest[idx+5:])
+			}
+			name := strings.Fields(rest)[0]
+			symbols = append(symbols, symbolInfo{
+				Name:      name,
+				Kind:      "impl",
+				StartLine: lineNum,
+			})
+			continue
+		}
+
+		// Function
+		if matches := rsFnRe.FindStringSubmatch(line); matches != nil {
+			symbols = append(symbols, symbolInfo{
+				Name:      matches[1],
+				Kind:      "function",
+				StartLine: lineNum,
+			})
+		}
+	}
+
+	sortSymbols(symbols)
+	return symbols, scanner.Err()
+}
+
+// ────────────────────────────────────────────────────────────────────────────
+// Helpers
+// ────────────────────────────────────────────────────────────────────────────
+
+func sortSymbols(symbols []symbolInfo) {
+	sort.Slice(symbols, func(i, j int) bool {
+		return symbols[i].StartLine < symbols[j].StartLine
+	})
+}
+
+// groupSymbols groups symbols by kind and returns formatted strings.
+func groupSymbols(symbols []symbolInfo) map[string][]string {
+	groups := make(map[string][]string)
+	for _, s := range symbols {
+		kind := s.Kind
+		switch kind {
+		case "package":
+			groups["package"] = append(groups["package"], s.Name)
+		case "import":
+			groups["imports"] = append(groups["imports"], s.Name)
+		case "function":
+			groups["functions"] = append(groups["functions"], s.Name)
+		case "method":
+			groups["methods"] = append(groups["methods"], s.Name)
+		case "class", "interface", "struct", "enum", "trait", "alias", "func":
+			// All structural types go under "types" for consistency.
+			groups["types"] = append(groups["types"], fmt.Sprintf("%s %s", s.Name, kind))
+		case "type":
+			// TS type alias
+			groups["types"] = append(groups["types"], fmt.Sprintf("%s type", s.Name))
+		case "impl":
+			groups["types"] = append(groups["types"], fmt.Sprintf("impl %s", s.Name))
+		}
+	}
+	return groups
+}

internal/tools/todo.go

@@ -0,0 +1,173 @@
+package tools
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"strings"
+	"sync"
+
+	"agentu/pkg/llm"
+)
+
+// ────────────────────────────────────────────────────────────────────────────
+// TodoTool — in-memory task tracking
+// ────────────────────────────────────────────────────────────────────────────
+
+type todoItem struct {
+	Content string `json:"content"`
+	Status  string `json:"status"` // "pending", "in_progress", "completed"
+}
+
+type TodoTool struct {
+	mu    sync.Mutex
+	items []todoItem
+}
+
+func NewTodoTool() *TodoTool {
+	return &TodoTool{}
+}
+
+func (t *TodoTool) Definition() llm.Tool {
+	return llm.Tool{
+		Type: "function",
+		Function: llm.ToolFunction{
+			Name:        "todo",
+			Description: "Manage a task list for the current conversation. Create, update, and track multi-step work. Use this to plan complex tasks and show progress.",
+			Parameters: JSONSchema(`{
+  "type": "object",
+  "properties": {
+    "action": {"type": "string", "enum": ["list", "create", "update", "start", "complete", "clear"], "description": "Operation to perform."},
+    "items": {"type": "array", "items": {"type": "string"}, "description": "For 'create': list of task descriptions to add."},
+    "task": {"type": "string", "description": "For 'update'/'start'/'complete': the exact task content string to target."},
+    "status": {"type": "string", "enum": ["pending", "in_progress", "completed"], "description": "For 'update': new status for the task."}
+  },
+  "required": ["action"],
+  "additionalProperties": false
+}`),
+		},
+	}
+}
+
+type todoArgs struct {
+	Action string   `json:"action"`
+	Items  []string `json:"items"`
+	Task   string   `json:"task"`
+	Status string   `json:"status"`
+}
+
+func (t *TodoTool) Execute(ctx context.Context, raw json.RawMessage) (string, error) {
+	var args todoArgs
+	if err := json.Unmarshal(raw, &args); err != nil {
+		return "", fmt.Errorf("parse arguments: %w", err)
+	}
+
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	if err := ctx.Err(); err != nil {
+		return "", err
+	}
+
+	switch args.Action {
+	case "list":
+		return t.listTasks(), nil
+	case "create":
+		return t.createTasks(args.Items)
+	case "start":
+		return t.startTask(args.Task)
+	case "complete":
+		return t.completeTask(args.Task)
+	case "update":
+		return t.updateTask(args.Task, args.Status)
+	case "clear":
+		t.items = nil
+		return "Tasks cleared.", nil
+	default:
+		return "", fmt.Errorf("unknown todo action: %s", args.Action)
+	}
+}
+
+func (t *TodoTool) listTasks() string {
+	if len(t.items) == 0 {
+		return "No tasks."
+	}
+
+	var b strings.Builder
+	b.WriteString("Tasks:\n")
+	for _, item := range t.items {
+		fmt.Fprintf(&b, "[%s] %s\n", item.Status, item.Content)
+	}
+	return b.String()
+}
+
+func (t *TodoTool) createTasks(items []string) (string, error) {
+	if len(items) == 0 {
+		return "", fmt.Errorf("items is required for create")
+	}
+	for _, content := range items {
+		content = strings.TrimSpace(content)
+		if content == "" {
+			return "", fmt.Errorf("task content must not be empty")
+		}
+		t.items = append(t.items, todoItem{
+			Content: content,
+			Status:  "pending",
+		})
+	}
+	return t.listTasks(), nil
+}
+
+func (t *TodoTool) startTask(task string) (string, error) {
+	idx := -1
+	for i, item := range t.items {
+		if item.Content == task {
+			idx = i
+			break
+		}
+	}
+	if idx < 0 {
+		return "", fmt.Errorf("task not found: %s", task)
+	}
+
+	// Set all in_progress items to pending, then mark the target as in_progress.
+	for i := range t.items {
+		if t.items[i].Status == "in_progress" {
+			t.items[i].Status = "pending"
+		}
+	}
+	t.items[idx].Status = "in_progress"
+	return t.listTasks(), nil
+}
+
+func (t *TodoTool) completeTask(task string) (string, error) {
+	idx := -1
+	for i, item := range t.items {
+		if item.Content == task {
+			idx = i
+			break
+		}
+	}
+	if idx < 0 {
+		return "", fmt.Errorf("task not found: %s", task)
+	}
+	t.items[idx].Status = "completed"
+	return t.listTasks(), nil
+}
+
+func (t *TodoTool) updateTask(task, status string) (string, error) {
+	if status != "pending" && status != "in_progress" && status != "completed" {
+		return "", fmt.Errorf("status must be one of: pending, in_progress, completed")
+	}
+	idx := -1
+	for i, item := range t.items {
+		if item.Content == task {
+			idx = i
+			break
+		}
+	}
+	if idx < 0 {
+		return "", fmt.Errorf("task not found: %s", task)
+	}
+	t.items[idx].Status = status
+	return t.listTasks(), nil
+}

internal/tools/tools.go

@@ -19,18 +19,54 @@ type Tool interface {
 	Execute(ctx context.Context, args json.RawMessage) (string, error)
 }
 
+// StreamingTool is optionally implemented by tools that can report partial
+// output while they execute. emit receives chunks of the tool's output as
+// they are produced; the returned string is still the complete result that
+// gets sent back to the model.
+type StreamingTool interface {
+	Tool
+	ExecuteStream(ctx context.Context, args json.RawMessage, emit func(string) error) (string, error)
+}
+
 // MutatingTool is optionally implemented by tools that modify external state.
 // It is used by the agent to determine whether tool calls can run in parallel.
 type MutatingTool interface {
 	Mutates() bool
 }
 
+// CallMutatingTool can refine a tool's mutation classification per call.
+// This is useful for tools such as apply_patch whose dry-run mode is read-only.
+type CallMutatingTool interface {
+	MutatesCall(args json.RawMessage) bool
+}
+
+// PreviewableTool describes a pending mutation for interactive approval.
+type PreviewableTool interface {
+	Preview(ctx context.Context, args json.RawMessage) (string, error)
+}
+
+// TargetPathTool reports files that should be snapshotted before a mutation.
+type TargetPathTool interface {
+	TargetPaths(args json.RawMessage) ([]string, error)
+}
+
 // IsMutating reports whether tool implements MutatingTool and returns true.
 func IsMutating(tool Tool) bool {
 	mutating, ok := tool.(MutatingTool)
 	return ok && mutating.Mutates()
 }
 
+// IsMutatingCall reports whether this invocation modifies external state.
+func IsMutatingCall(tool Tool, args json.RawMessage) bool {
+	if !IsMutating(tool) {
+		return false
+	}
+	if call, ok := tool.(CallMutatingTool); ok {
+		return call.MutatesCall(args)
+	}
+	return true
+}
+
 type Registry struct {
 	tools   map[string]Tool
 	primary []string
@@ -81,8 +117,10 @@ func ReadOnlyBuiltins(workingDir string) *Registry {
 	gitStatus := NewGitStatusTool(workingDir)
 	gitDiff := NewGitDiffTool(workingDir)
 	gitLog := NewGitLogTool(workingDir)
+	projectOutline := NewProjectOutlineTool(workingDir)
+	todo := NewTodoTool()
 
-	registry := NewRegistry(fileRead, fileList, fileSearch, codeSymbols, gitStatus, gitDiff, gitLog)
+	registry := NewRegistry(fileRead, fileList, fileSearch, codeSymbols, projectOutline, todo, gitStatus, gitDiff, gitLog)
 	registry.RegisterAlias("file.read", fileRead)
 	registry.RegisterAlias("file.list", fileList)
 	registry.RegisterAlias("file.search", fileSearch)
@@ -97,36 +135,48 @@ func Builtins(workingDir string) *Registry {
 	registry := ReadOnlyBuiltins(workingDir)
 	fileWrite := NewFileWriteTool(workingDir)
 	fileEdit := NewFileEditTool(workingDir)
+	applyPatch := NewApplyPatchTool(workingDir)
 	shellRun := NewShellRunTool(workingDir)
 	registry.Register(fileWrite)
 	registry.Register(fileEdit)
+	registry.Register(applyPatch)
 	registry.Register(shellRun)
 	registry.RegisterAlias("file.write", fileWrite)
 	registry.RegisterAlias("file.edit", fileEdit)
+	registry.RegisterAlias("apply.patch", applyPatch)
 	registry.RegisterAlias("shell.run", shellRun)
 	return registry
 }
 
 func AgentInstructions(workingDir string, yolo bool) string {
+	return AgentInstructionsForMode(workingDir, yolo, yolo)
+}
+
+func AgentInstructionsForMode(workingDir string, writesEnabled bool, yolo bool) string {
 	searchInstructions := webSearchInstructions()
-	readOnlyTools := "file_list, file_read, file_search, code_symbols, git_status, git_diff, git_log, web_search, web_fetch"
+	readOnlyTools := "file_list, file_read, file_search, code_symbols, project_outline, todo, git_status, git_diff, git_log, web_search, web_fetch"
-	yoloTools := "file_list, file_read, file_search, code_symbols, file_write, file_edit, shell_run, git_status, git_diff, git_log, web_search, web_fetch"
+	writeTools := "file_list, file_read, file_search, code_symbols, project_outline, todo, file_write, file_edit, apply_patch, shell_run, git_status, git_diff, git_log, web_search, web_fetch"
-	if yolo {
+	if writesEnabled {
+		approval := "Mutating tools require interactive user approval before execution. Explain the intended change through the tool call and wait for the decision."
+		if yolo {
+			approval = "Mutating tools execute automatically because --yolo is enabled."
+		}
 		return fmt.Sprintf(`Local project context:
 - The project working directory is %q.
 - Tools are available, but use them only when needed to satisfy the user's request.
+- %s
 - Do not call tools for greetings, small talk, simple Q&A, or general questions that do not require local project or command context.
 - Do not explore the project preemptively. Do not call file_list just to discover context unless the user asks about project files or the answer truly depends on local files.
 - When the user asks to inspect, list, search, or read project files, use file_list, file_search, and file_read instead of saying you cannot access local files.
 - When the user asks about code structure, symbols, functions, methods, types, or definitions in Go files, use code_symbols before reading whole files.
 - When the user asks to run, execute, test, check, debug, or inspect a shell command, use shell_run. This includes local commands written in backticks, such as go, make, git, pwd, ls, ps, and similar terminal commands.
-- When editing existing files, prefer file_edit for small or localized changes; use file_write only for creating files, replacing whole files intentionally, or appending.
+- Use project_outline first when the user asks for a broad understanding of a project; prefer file_edit for localized changes, apply_patch for related multi-file changes, and file_write for whole-file creation or replacement.
 - Do not use file tools as a substitute for a command execution request.
 - When the user asks about changed files, diffs, or git history, use git_status, git_diff, or git_log instead of shell_run.
 - Do not start interactive or long-running sessions. Prefer bounded, non-interactive local commands with clear completion conditions.
 - %s
 - Tool paths should normally be relative to the project working directory.
-- Available tools: %s.`, workingDir, searchInstructions, yoloTools)
+- Available tools: %s.`, workingDir, approval, searchInstructions, writeTools)
 	}
 	return fmt.Sprintf(`Local project context:
 - The project working directory is %q.

internal/tools/tools_test.go

@@ -104,7 +104,7 @@ func TestReadOnlyBuiltinsExposeOpenAICompatibleNamesAndAliases(t *testing.T) {
 		}
 	}
 
-	for _, want := range []string{"file_list", "file_read", "file_search", "code_symbols", "git_status", "git_diff", "git_log"} {
+	for _, want := range []string{"file_list", "file_read", "file_search", "code_symbols", "project_outline", "todo", "git_status", "git_diff", "git_log"} {
 		if !slices.Contains(names, want) {
 			t.Fatalf("definitions missing %s: %#v", want, names)
 		}
@@ -173,7 +173,7 @@ func TestRegistryCanExposeWebSearch(t *testing.T) {
 
 func TestBuiltinsExposeYoloTools(t *testing.T) {
 	registry := Builtins(t.TempDir())
-	for _, name := range []string{"file_write", "file_edit", "shell_run", "file.write", "file.edit", "shell.run"} {
+	for _, name := range []string{"file_write", "file_edit", "apply_patch", "shell_run", "file.write", "file.edit", "apply.patch", "shell.run"} {
 		if _, ok := registry.Get(name); !ok {
 			t.Fatalf("tool missing: %s", name)
 		}
@@ -198,7 +198,7 @@ func TestAgentInstructionsExplainCommandModes(t *testing.T) {
 
 func TestAgentInstructionsExplainWebSearch(t *testing.T) {
 	withSearch := AgentInstructions("/tmp/project", false)
-	for _, want := range []string{"use web_search", "Use web_fetch", "latest information", "source URLs", "Available tools: file_list, file_read, file_search, code_symbols, git_status, git_diff, git_log, web_search, web_fetch"} {
+	for _, want := range []string{"use web_search", "Use web_fetch", "latest information", "source URLs", "Available tools: file_list, file_read, file_search, code_symbols, project_outline, todo, git_status, git_diff, git_log, web_search, web_fetch"} {
 		if !strings.Contains(withSearch, want) {
 			t.Fatalf("web search instructions missing %q:\n%s", want, withSearch)
 		}

internal/tools/v03_test.go

@@ -0,0 +1,200 @@
+package tools
+
+import (
+	"context"
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestApplyPatchChangesMultipleFiles(t *testing.T) {
+	dir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(dir, "old.txt"), []byte("before\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	patch := `*** Begin Patch
+*** Update File: old.txt
+@@
+-before
++after
+*** Add File: new.txt
++created
+*** End Patch`
+	out, err := NewApplyPatchTool(dir).Execute(context.Background(), mustJSON(t, map[string]any{"patch": patch}))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(out, "2 file(s) changed") {
+		t.Fatalf("output = %q", out)
+	}
+	assertFileContent(t, filepath.Join(dir, "old.txt"), "after\n")
+	assertFileContent(t, filepath.Join(dir, "new.txt"), "created\n")
+	if info, err := os.Stat(filepath.Join(dir, "old.txt")); err != nil || info.Mode().Perm() != 0o600 {
+		t.Fatalf("updated mode = %v, err = %v", info.Mode().Perm(), err)
+	}
+}
+
+func TestApplyPatchRollsBackEarlierFilesOnFailure(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "first.txt")
+	if err := os.WriteFile(path, []byte("original\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	patch := `*** Begin Patch
+*** Update File: first.txt
+@@
+-original
++changed
+*** Update File: missing.txt
+@@
+-missing
++changed
+*** End Patch`
+	_, err := NewApplyPatchTool(dir).Execute(context.Background(), mustJSON(t, map[string]any{"patch": patch}))
+	if err == nil {
+		t.Fatal("expected patch failure")
+	}
+	assertFileContent(t, path, "original\n")
+}
+
+func TestApplyPatchDryRunDoesNotWrite(t *testing.T) {
+	dir := t.TempDir()
+	patch := "*** Begin Patch\n*** Add File: dry.txt\n+dry\n*** End Patch"
+	out, err := NewApplyPatchTool(dir).Execute(context.Background(), mustJSON(t, map[string]any{"patch": patch, "dry_run": true}))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(out, "dry run") {
+		t.Fatalf("output = %q", out)
+	}
+	if _, err := os.Stat(filepath.Join(dir, "dry.txt")); !os.IsNotExist(err) {
+		t.Fatalf("dry run created file: %v", err)
+	}
+}
+
+func TestApplyPatchRejectsMalformedEnvelopeAndDuplicatePaths(t *testing.T) {
+	for _, patch := range []string{
+		"*** Begin Patch\n*** Add File: x.txt\n+x",
+		"prefix\n*** Begin Patch\n*** Add File: x.txt\n+x\n*** End Patch",
+		"*** Begin Patch\n*** Add File: x.txt\n+x\n*** Delete File: x.txt\n*** End Patch",
+	} {
+		if _, err := parsePatch(patch); err == nil {
+			t.Fatalf("expected parse failure for %q", patch)
+		}
+	}
+}
+
+func TestResolvePathRejectsSymlinkEscape(t *testing.T) {
+	parent := t.TempDir()
+	base := filepath.Join(parent, "project")
+	outside := filepath.Join(parent, "outside")
+	if err := os.MkdirAll(base, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.MkdirAll(outside, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(outside, "secret.txt"), []byte("secret"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Symlink(outside, filepath.Join(base, "link")); err != nil {
+		t.Skipf("symlinks unavailable: %v", err)
+	}
+	if _, err := resolvePath(base, "link/secret.txt"); err == nil {
+		t.Fatal("expected read symlink escape rejection")
+	}
+	if _, err := resolvePathForWrite(base, "link/new.txt"); err == nil {
+		t.Fatal("expected write symlink escape rejection")
+	}
+}
+
+func TestTodoRejectsInvalidData(t *testing.T) {
+	tool := NewTodoTool()
+	if _, err := tool.Execute(context.Background(), json.RawMessage(`{"action":"create","items":[""]}`)); err == nil {
+		t.Fatal("expected empty task rejection")
+	}
+	if _, err := tool.Execute(context.Background(), json.RawMessage(`{"action":"create","items":["one"]}`)); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := tool.Execute(context.Background(), json.RawMessage(`{"action":"update","task":"one","status":"invalid"}`)); err == nil {
+		t.Fatal("expected invalid status rejection")
+	}
+}
+
+func TestProjectOutlineIncludesGoTypeScriptAndRustSymbols(t *testing.T) {
+	dir := t.TempDir()
+	files := map[string]string{
+		"main.go": "package demo\nfunc Run() {}\n",
+		"ui.ts":   "export class Panel {}\nexport function render() {}\n",
+		"lib.rs":  "pub struct Worker;\npub fn execute() {}\n",
+	}
+	for name, content := range files {
+		if err := os.WriteFile(filepath.Join(dir, name), []byte(content), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+	out, err := NewProjectOutlineTool(dir).Execute(context.Background(), json.RawMessage(`{}`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, want := range []string{"main.go", "Run", "ui.ts", "Panel", "render", "lib.rs", "Worker", "execute"} {
+		if !strings.Contains(out, want) {
+			t.Fatalf("outline missing %q:\n%s", want, out)
+		}
+	}
+}
+
+func TestRollbackStoreRestoresInReverseOrder(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "state.txt")
+	if err := os.WriteFile(path, []byte("one"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	store := NewRollbackStore()
+	first, err := store.Capture("edit", "state.txt", path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(path, []byte("two"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	second, err := store.Capture("edit", "state.txt", path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(path, []byte("three"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if store.LastID() != second {
+		t.Fatalf("last ID = %q", store.LastID())
+	}
+	if _, err := store.Restore(second); err != nil {
+		t.Fatal(err)
+	}
+	assertFileContent(t, path, "two")
+	if store.LastID() != first {
+		t.Fatalf("last ID after restore = %q, want %q", store.LastID(), first)
+	}
+}
+
+func mustJSON(t *testing.T, value any) json.RawMessage {
+	t.Helper()
+	data, err := json.Marshal(value)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return data
+}
+
+func assertFileContent(t *testing.T, path, want string) {
+	t.Helper()
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(data) != want {
+		t.Fatalf("%s content = %q, want %q", path, string(data), want)
+	}
+}

internal/tui/app.go

@@ -2,6 +2,7 @@ package tui
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -9,8 +10,11 @@ import (
 	"sort"
 	"strings"
 	"unicode"
+	"unicode/utf8"
 
 	"agentu/internal/agent"
+	"agentu/internal/skills"
+	"agentu/internal/tools"
 	"agentu/pkg/llm"
 
 	"github.com/charmbracelet/bubbles/spinner"
@@ -33,6 +37,8 @@ type Options struct {
 	ThemeMode    ThemeMode
 	ModelManager ModelManager
 	WorkingDir   string
+	Approval     *ApprovalBroker
+	Rollback     *tools.RollbackStore
 }
 
 type ModelManager interface {
@@ -67,6 +73,8 @@ const (
 	roleTool      role = "tool"
 	roleError     role = "error"
 	roleSystem    role = "system"
+	roleBTWUser   role = "btw-user"
+	roleBTW       role = "btw"
 )
 
 type message struct {
@@ -75,14 +83,16 @@ type message struct {
 }
 
 type model struct {
-	ctx        context.Context
+	ctx            context.Context
-	agent      *agent.Agent
+	agent          *agent.Agent
-	modelName  string
+	modelName      string
-	yolo       bool
+	yolo           bool
-	models     ModelManager
+	models         ModelManager
-	workingDir string
+	workingDir     string
-	theme      theme
+	approvalBroker *ApprovalBroker
-	styles     styles
+	rollback       *tools.RollbackStore
+	theme          theme
+	styles         styles
 
 	viewport viewport.Model
 	input    textarea.Model
@@ -99,9 +109,12 @@ type model struct {
 	draftInput    string
 	commandCursor int
 
-	running bool
+	running    bool
-	cancel  context.CancelFunc
+	cancel     context.CancelFunc
-	events  <-chan tea.Msg
+	btwRunning bool
+	btwCancel  context.CancelFunc
+	events     chan tea.Msg
+	approval   *toolApprovalMsg
 
 	// Session picker state
 	picking      bool
@@ -117,11 +130,15 @@ type pickerItem struct {
 }
 
 type assistantChunkMsg string
+type btwChunkMsg string
 type toolLogMsg string
 type turnStartedMsg struct{}
 type turnDoneMsg struct {
 	err error
 }
+type btwDoneMsg struct {
+	err error
+}
 
 var errOpenPicker = fmt.Errorf("open picker")
 
@@ -142,7 +159,10 @@ type commandSuggestion struct {
 	executable  bool
 }
 
+var btwSlashCommand = slashCommand{Name: "/btw", Description: "ask a temporary side question"}
+
 var slashCommands = []slashCommand{
+	btwSlashCommand,
 	{Name: "/clear", Description: "reset context"},
 	{Name: "/compact", Description: "compress context"},
 	{Name: "/exit", Description: "quit"},
@@ -154,6 +174,9 @@ var slashCommands = []slashCommand{
 	{Name: "/new", Description: "new session"},
 	{Name: "/status", Description: "show changed files"},
 	{Name: "/diff", Description: "show unstaged diff"},
+	{Name: "/test", Description: "run project tests"},
+	{Name: "/skills", Description: "list or install global skills"},
+	{Name: "/rollback", Description: "restore the latest file snapshot"},
 	{Name: "/theme", Description: "switch theme light|dark"},
 }
 
@@ -184,19 +207,21 @@ func newModel(ctx context.Context, assistant *agent.Agent, opts Options) model {
 	vp.SetContent("")
 
 	m := model{
-		ctx:          ctx,
+		ctx:            ctx,
-		agent:        assistant,
+		agent:          assistant,
-		modelName:    opts.ModelName,
+		modelName:      opts.ModelName,
-		yolo:         opts.Yolo,
+		yolo:           opts.Yolo,
-		models:       opts.ModelManager,
+		models:         opts.ModelManager,
-		workingDir:   opts.WorkingDir,
+		workingDir:     opts.WorkingDir,
-		theme:        th,
+		approvalBroker: opts.Approval,
-		styles:       st,
+		rollback:       opts.Rollback,
-		viewport:     vp,
+		theme:          th,
-		input:        input,
+		styles:         st,
-		spinner:      spin,
+		viewport:       vp,
-		status:       "ready",
+		input:          input,
-		historyIndex: -1,
+		spinner:        spin,
+		status:         "ready",
+		historyIndex:   -1,
 	}
 	m.refreshContextUsage()
 	return m
@@ -242,7 +267,7 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 		return m, nil
 
 	case spinner.TickMsg:
-		if !m.running {
+		if !m.active() {
 			return m, nil
 		}
 		var cmd tea.Cmd
@@ -250,6 +275,9 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 		return m, cmd
 
 	case tea.KeyMsg:
+		if m.approval != nil {
+			return m.updateApproval(msg)
+		}
 		if m.picking {
 			return m.updatePicker(msg)
 		}
@@ -261,31 +289,30 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 			if m.cancel != nil {
 				m.cancel()
 			}
+			if m.btwCancel != nil {
+				m.btwCancel()
+			}
 			return m, tea.Quit
 		case "esc":
-			if m.running && m.cancel != nil {
+			if m.cancel != nil {
 				m.cancel()
+			}
+			if m.btwCancel != nil {
+				m.btwCancel()
+			}
+			if m.active() {
 				m.status = "canceling..."
 				return m, nil
 			}
 		case "enter":
-			if m.running {
-				return m, nil
-			}
 			return m.submit()
 		case "up":
-			if m.running {
-				return m, nil
-			}
 			if m.input.Line() == 0 {
 				m.previousInputHistory()
 				m.afterInputChanged()
 				return m, nil
 			}
 		case "down":
-			if m.running {
-				return m, nil
-			}
 			value := m.input.Value()
 			lineCount := 1
 			if value != "" {
@@ -297,16 +324,10 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 				return m, nil
 			}
 		case "ctrl+p":
-			if m.running {
-				return m, nil
-			}
 			m.previousInputHistory()
 			m.afterInputChanged()
 			return m, nil
 		case "ctrl+n":
-			if m.running {
-				return m, nil
-			}
 			m.nextInputHistory()
 			m.afterInputChanged()
 			return m, nil
@@ -320,6 +341,14 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 		m.refreshViewport(true)
 		return m, m.waitForEvent()
 
+	case btwChunkMsg:
+		m.appendBTWChunk(string(msg))
+		if !m.running {
+			m.status = "answering btw..."
+		}
+		m.refreshViewport(true)
+		return m, m.waitForEvent()
+
 	case toolLogMsg:
 		text := cleanToolLog(string(msg))
 		if text != "" {
@@ -331,13 +360,25 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 		}
 		return m, m.waitForEvent()
 
+	case toolApprovalMsg:
+		m.approval = &msg
+		m.status = "approval required"
+		m.resize()
+		m.refreshViewport(true)
+		return m, m.waitForEvent()
+
 	case turnDoneMsg:
 		m.running = false
-		m.events = nil
 		m.cancel = nil
+		m.approval = nil
+		if m.approvalBroker != nil {
+			m.approvalBroker.ClearEvents()
+		}
 		if msg.err != nil {
 			m.messages = append(m.messages, message{role: roleError, content: msg.err.Error()})
 			m.status = "error"
+		} else if m.btwRunning {
+			m.status = "answering btw..."
 		} else {
 			m.status = "ready"
 		}
@@ -349,6 +390,29 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 		m.resize()
 		m.refreshViewport(true)
 		cmds = append(cmds, textarea.Blink)
+		if m.active() {
+			cmds = append(cmds, m.waitForEvent())
+		} else {
+			m.events = nil
+		}
+
+	case btwDoneMsg:
+		m.btwRunning = false
+		m.btwCancel = nil
+		if msg.err != nil && !errors.Is(msg.err, context.Canceled) {
+			m.messages = append(m.messages, message{role: roleBTW, content: "Error: " + msg.err.Error()})
+		}
+		if !m.running {
+			m.status = "ready"
+		}
+		m.resize()
+		m.refreshViewport(true)
+		cmds = append(cmds, textarea.Blink)
+		if m.active() {
+			cmds = append(cmds, m.waitForEvent())
+		} else {
+			m.events = nil
+		}
 	}
 
 	if keyMsg, ok := msg.(tea.KeyMsg); ok {
@@ -357,18 +421,15 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 			m.viewport = nextViewport
 			return m, cmd
 		}
-		if !m.running {
+		nextInput, cmd := m.input.Update(msg)
-			nextInput, cmd := m.input.Update(msg)
+		m.input = nextInput
-			m.input = nextInput
+		m.historyIndex = -1
-			m.historyIndex = -1
+		m.draftInput = ""
-			m.draftInput = ""
+		m.afterInputChanged()
-			m.afterInputChanged()
+		return m, cmd
-			return m, cmd
-		}
-		return m, nil
 	}
 
-	if !m.running {
+	if !m.active() {
 		nextInput, cmd := m.input.Update(msg)
 		m.input = nextInput
 		cmds = append(cmds, cmd)
@@ -392,6 +453,15 @@ func (m model) View() string {
 		body := lipgloss.JoinVertical(lipgloss.Left, parts...)
 		return m.styles.App.Width(m.width).Height(m.height).Render(body)
 	}
+	if m.approval != nil {
+		parts := []string{m.viewport.View()}
+		if activity := m.activityView(); activity != "" {
+			parts = append(parts, activity)
+		}
+		parts = append(parts, m.approvalView(), m.footerView())
+		body := lipgloss.JoinVertical(lipgloss.Left, parts...)
+		return m.styles.App.Width(m.width).Height(m.height).Render(body)
+	}
 
 	parts := []string{m.viewport.View()}
 	if activity := m.activityView(); activity != "" {
@@ -407,6 +477,14 @@ func (m *model) submit() (tea.Model, tea.Cmd) {
 	if input == "" {
 		return *m, nil
 	}
+	fields := strings.Fields(input)
+	if len(fields) > 0 && fields[0] == "/btw" {
+		m.rememberInput(input)
+		return m.submitBTW(strings.TrimSpace(strings.TrimPrefix(input, "/btw")))
+	}
+	if m.active() {
+		return *m, nil
+	}
 	m.rememberInput(input)
 
 	switch input {
@@ -446,20 +524,19 @@ func (m *model) submit() (tea.Model, tea.Cmd) {
 
 	events := make(chan tea.Msg, 256)
 	m.events = events
+	if m.approvalBroker != nil {
+		m.approvalBroker.SetEvents(events)
+	}
 
 	start := func() tea.Msg {
 		go func() {
-			defer close(events)
 			out := eventWriter{ctx: turnCtx, events: events, assistant: true}
 			logs := eventWriter{ctx: turnCtx, events: events}
 			err := m.agent.RunTurn(turnCtx, input, out, logs)
 			if errors.Is(err, context.Canceled) {
 				err = nil
 			}
-			select {
+			events <- turnDoneMsg{err: err}
-			case events <- turnDoneMsg{err: err}:
-			case <-turnCtx.Done():
-			}
 		}()
 		return turnStartedMsg{}
 	}
@@ -467,6 +544,54 @@ func (m *model) submit() (tea.Model, tea.Cmd) {
 	return *m, tea.Batch(start, m.waitForEvent(), m.spinner.Tick)
 }
 
+func (m *model) submitBTW(question string) (tea.Model, tea.Cmd) {
+	if question == "" {
+		m.messages = append(m.messages, message{role: roleBTW, content: "Usage: /btw <question>"})
+		m.input.SetValue("")
+		m.afterInputChanged()
+		m.refreshViewport(true)
+		return *m, nil
+	}
+	if m.agent == nil {
+		m.messages = append(m.messages, message{role: roleBTW, content: "Error: agent is not configured"})
+		return *m, nil
+	}
+	if m.btwRunning {
+		m.messages = append(m.messages, message{role: roleBTW, content: "A /btw question is already running."})
+		return *m, nil
+	}
+
+	hadActive := m.active()
+	m.messages = append(m.messages, message{role: roleBTWUser, content: question})
+	m.input.SetValue("")
+	m.afterInputChanged()
+	m.btwRunning = true
+	if !m.running {
+		m.status = "answering btw..."
+	}
+	m.resize()
+	m.refreshViewport(true)
+
+	btwCtx, cancel := context.WithCancel(m.ctx)
+	m.btwCancel = cancel
+	if m.events == nil {
+		m.events = make(chan tea.Msg, 256)
+	}
+	events := m.events
+	start := func() tea.Msg {
+		go func() {
+			out := eventWriter{ctx: btwCtx, events: events, btw: true}
+			err := m.agent.BTW(btwCtx, question, out)
+			events <- btwDoneMsg{err: err}
+		}()
+		return turnStartedMsg{}
+	}
+	if hadActive {
+		return *m, start
+	}
+	return *m, tea.Batch(start, m.waitForEvent(), m.spinner.Tick)
+}
+
 func (m *model) insertInputNewline() (tea.Model, tea.Cmd) {
 	enter := tea.KeyMsg{Type: tea.KeyEnter}
 	nextInput, cmd := m.input.Update(enter)
@@ -505,10 +630,38 @@ func (m *model) appendAssistantChunk(chunk string) {
 	if chunk == "" {
 		return
 	}
-	if len(m.messages) == 0 || m.messages[len(m.messages)-1].role != roleAssistant {
+	for i := len(m.messages) - 1; i >= 0; i-- {
-		m.messages = append(m.messages, message{role: roleAssistant})
+		switch m.messages[i].role {
+		case roleAssistant:
+			m.messages[i].content += chunk
+			return
+		case roleBTW, roleBTWUser:
+			continue
+		default:
+			i = -1
+		}
 	}
-	m.messages[len(m.messages)-1].content += chunk
+	m.messages = append(m.messages, message{role: roleAssistant, content: chunk})
+}
+
+func (m *model) appendBTWChunk(chunk string) {
+	if chunk == "" {
+		return
+	}
+	for i := len(m.messages) - 1; i >= 0; i-- {
+		if m.messages[i].role == roleBTW {
+			m.messages[i].content += chunk
+			return
+		}
+		if m.messages[i].role == roleBTWUser {
+			break
+		}
+	}
+	m.messages = append(m.messages, message{role: roleBTW, content: chunk})
+}
+
+func (m model) active() bool {
+	return m.running || m.btwRunning
 }
 
 func (m *model) resize() {
@@ -518,10 +671,13 @@ func (m *model) resize() {
 	m.syncInputHeight()
 
 	activityHeight := 0
-	if m.running {
+	if m.active() {
 		activityHeight = 1
 	}
 	inputHeight := m.inputBlockHeight()
+	if m.approval != nil {
+		inputHeight = lipgloss.Height(m.approvalView())
+	}
 	footerHeight := 2
 	m.viewport.Width = width
 	m.viewport.Height = max(4, m.height-activityHeight-inputHeight-footerHeight)
@@ -564,6 +720,12 @@ func (m model) messageView(r role, content string, width int) string {
 	switch r {
 	case roleUser:
 		style = m.styles.UserMessage
+	case roleBTWUser:
+		style = m.styles.UserMessage
+		labelStyle = m.styles.SystemLabel
+	case roleBTW:
+		style = m.styles.AssistantMsg
+		labelStyle = m.styles.SystemLabel
 	case roleTool:
 		style = m.styles.ToolMessage
 		labelStyle = m.styles.ToolLabel
@@ -576,7 +738,7 @@ func (m model) messageView(r role, content string, width int) string {
 	}
 
 	bodyContent := content
-	if r == roleAssistant {
+	if r == roleAssistant || r == roleBTW {
 		bodyWidth := max(minMarkdownWidth, width-style.GetHorizontalFrameSize())
 		bodyContent = renderMarkdown(content, bodyWidth, m.theme)
 	}
@@ -614,6 +776,10 @@ func roleLabel(r role) string {
 		return "Error"
 	case roleSystem:
 		return "Note"
+	case roleBTWUser:
+		return "BTW · You"
+	case roleBTW:
+		return "BTW"
 	default:
 		return string(r)
 	}
@@ -649,6 +815,11 @@ func parseToolLog(content string) toolLogParts {
 
 func (m model) toolMessageView(content string, width int) string {
 	parts := parseToolLog(content)
+	if isShellRun(parts.name) {
+		if args, ok := parseShellRunDisplayArgs(parts.args); ok {
+			return m.shellRunMessageView(args, parts.output, width)
+		}
+	}
 	header := m.styles.ToolName.Render(strings.TrimSpace(iconTool + " " + parts.name))
 	lines := []string{header}
 	if parts.args != "" {
@@ -669,18 +840,132 @@ func (m model) toolMessageView(content string, width int) string {
 
 const maxToolOutputPreviewLines = 8
 
+type shellRunDisplayArgs struct {
+	Command string `json:"command"`
+	Workdir string `json:"workdir"`
+}
+
+func isShellRun(name string) bool {
+	return name == "shell_run" || name == "shell.run"
+}
+
+func parseShellRunDisplayArgs(raw string) (shellRunDisplayArgs, bool) {
+	var args shellRunDisplayArgs
+	if json.Unmarshal([]byte(raw), &args) != nil || strings.TrimSpace(args.Command) == "" {
+		return shellRunDisplayArgs{}, false
+	}
+	args.Command = strings.TrimSpace(args.Command)
+	args.Workdir = strings.TrimSpace(args.Workdir)
+	return args, true
+}
+
+func (m model) shellRunMessageView(args shellRunDisplayArgs, output string, width int) string {
+	innerWidth := max(1, width-m.styles.ToolMessage.GetHorizontalFrameSize())
+	lines := []string{m.styles.ToolName.Render(fitLine("$ "+args.Command, innerWidth))}
+	if args.Workdir != "" {
+		lines = append(lines, m.styles.ToolKey.Render("cwd ")+m.styles.ToolValue.Render(fitLine(args.Workdir, max(1, innerWidth-4))))
+	}
+	if output != "" {
+		lines = append(lines, m.styles.ToolKey.Render("output"))
+		for _, line := range shellOutputPreviewLines(output, innerWidth) {
+			lines = append(lines, m.styles.ToolValue.Render(line))
+		}
+	}
+	bodyContent := strings.Join(lines, "\n")
+	bodyWidth := messageBodyWidth(bodyContent, width, m.styles.ToolMessage)
+	return m.styles.ToolMessage.Width(bodyWidth).Render(bodyContent)
+}
+
+func shellOutputPreviewLines(output string, width int) []string {
+	output = sanitizeTerminalOutput(output)
+	lines := strings.Split(strings.TrimRight(output, "\n"), "\n")
+	if len(lines) > maxToolOutputPreviewLines {
+		head := maxToolOutputPreviewLines / 2
+		tail := maxToolOutputPreviewLines - head - 1
+		omitted := len(lines) - head - tail
+		lines = append(append(append([]string(nil), lines[:head]...), fmt.Sprintf("… %d lines omitted …", omitted)), lines[len(lines)-tail:]...)
+	}
+	result := make([]string, 0, len(lines))
+	for _, line := range lines {
+		result = append(result, fitLine(line, width))
+	}
+	return result
+}
+
+func sanitizeTerminalOutput(output string) string {
+	output = strings.ReplaceAll(output, "\r\n", "\n")
+	var clean strings.Builder
+	for i := 0; i < len(output); {
+		if end, ok := terminalEscapeEnd(output, i); ok {
+			i = end
+			continue
+		}
+		r, size := utf8.DecodeRuneInString(output[i:])
+		if r == utf8.RuneError && size == 0 {
+			break
+		}
+		if r == '\n' || r == '\r' || r == '\t' || r >= ' ' {
+			clean.WriteRune(r)
+		}
+		i += size
+	}
+
+	lines := strings.Split(clean.String(), "\n")
+	for i, line := range lines {
+		segments := strings.Split(line, "\r")
+		for j := len(segments) - 1; j >= 0; j-- {
+			if segments[j] != "" || j == 0 {
+				lines[i] = segments[j]
+				break
+			}
+		}
+	}
+	return strings.Join(lines, "\n")
+}
+
+func terminalEscapeEnd(value string, start int) (int, bool) {
+	if start+1 >= len(value) || value[start] != '\x1b' {
+		return 0, false
+	}
+	if value[start+1] == '[' {
+		if end, ok := ansiSequenceEnd(value, start); ok {
+			return end, true
+		}
+		return len(value), true
+	}
+	if value[start+1] == ']' {
+		for i := start + 2; i < len(value); i++ {
+			if value[i] == '\a' {
+				return i + 1, true
+			}
+			if value[i] == '\x1b' && i+1 < len(value) && value[i+1] == '\\' {
+				return i + 2, true
+			}
+		}
+		return len(value), true
+	}
+	return min(start+2, len(value)), true
+}
+
 func toolStatusText(content string) string {
 	content = strings.TrimSpace(content)
 	if content == "" {
 		return ""
 	}
 	lines := strings.SplitN(content, "\n", 2)
+	parts := parseToolLog(lines[0])
+	if isShellRun(parts.name) {
+		if args, ok := parseShellRunDisplayArgs(parts.args); ok {
+			return "$ " + args.Command
+		}
+	}
 	return lines[0]
 }
 
 func (m *model) mergeToolLog(text string) bool {
 	incoming := parseToolLog(text)
-	if incoming.output == "" {
+	incomingHeader, _, ok := strings.Cut(text, "\n[output]\n")
+	if !ok {
 		return false
 	}
 	for i := len(m.messages) - 1; i >= 0; i-- {
@@ -688,10 +973,18 @@ func (m *model) mergeToolLog(text string) bool {
 			continue
 		}
 		existing := parseToolLog(m.messages[i].content)
-		if existing.name == incoming.name && existing.args == incoming.args && existing.output == "" {
+		if existing.name != incoming.name || existing.args != incoming.args {
-			m.messages[i].content = text
+			continue
-			return true
 		}
+		if !strings.Contains(m.messages[i].content, "\n[output]\n") {
+			// Placeholder card from the tool-start log; first chunk fills it.
+			m.messages[i].content = text
+		} else {
+			// Streaming tools send incremental chunks; append the raw delta
+			// so line boundaries are preserved inside the same card.
+			m.messages[i].content += strings.TrimPrefix(text, incomingHeader+"\n[output]\n")
+		}
+		return true
 	}
 	return false
 }
@@ -740,20 +1033,28 @@ func (m model) inputView() string {
 }
 
 func (m model) activityView() string {
-	if !m.running {
+	if !m.active() {
 		return ""
 	}
 	status := strings.TrimSpace(m.status)
 	if status == "" || status == "ready" {
 		status = "working..."
 	}
-	line := m.styles.StatusInfo.Render(iconWorking) + " Working " + m.spinner.View() + " " + status + " · esc cancels"
+	label := "Working"
+	if !m.running {
+		label = "BTW"
+	} else if m.btwRunning {
+		label = "Working + BTW"
+	}
+	line := m.styles.StatusInfo.Render(iconWorking) + " " + label + " " + m.spinner.View() + " " + status + " · esc cancels"
 	return m.styles.Activity.Render(fitLine(line, max(1, m.width-2)))
 }
 func (m model) footerView() string {
 	hint := ""
-	if m.running {
+	if m.btwRunning {
-		hint = "esc cancel · ctrl+c quit"
+		hint = "btw running · esc cancel · ctrl+c quit"
+	} else if m.running {
+		hint = "type /btw <question> · esc cancel · ctrl+c quit"
 	}
 	if m.showCommandPalette() {
 		if sug, ok := m.selectedCommandSuggestion(); ok && sug.executable {
@@ -835,6 +1136,7 @@ type eventWriter struct {
 	ctx       context.Context
 	events    chan<- tea.Msg
 	assistant bool
+	btw       bool
 }
 
 func (w eventWriter) Write(p []byte) (int, error) {
@@ -843,7 +1145,9 @@ func (w eventWriter) Write(p []byte) (int, error) {
 	}
 	text := string(append([]byte(nil), p...))
 	var msg tea.Msg
-	if w.assistant {
+	if w.btw {
+		msg = btwChunkMsg(text)
+	} else if w.assistant {
 		msg = assistantChunkMsg(text)
 	} else {
 		msg = toolLogMsg(text)
@@ -857,8 +1161,12 @@ func (w eventWriter) Write(p []byte) (int, error) {
 }
 
 func cleanToolLog(text string) string {
-	text = strings.TrimSpace(text)
+	text = strings.TrimLeft(text, "\r\n")
 	text = strings.TrimPrefix(text, "[tool]")
+	text = strings.TrimLeft(text, " \t")
+	if strings.Contains(text, "\n[output]\n") {
+		return text
+	}
 	return strings.TrimSpace(text)
 }
 
@@ -1108,6 +1416,17 @@ func subsequenceScore(candidate string, query string, base int) (int, bool) {
 
 func (m model) commandSuggestions() []commandSuggestion {
 	input := m.input.Value()
+	if m.active() {
+		value := strings.TrimSpace(input)
+		if m.running && strings.HasPrefix(value, "/") && !strings.Contains(value, " ") {
+			query := commandPaletteQuery(input)
+			command := btwSlashCommand
+			if _, ok := scoreCommand(command, query); ok {
+				return []commandSuggestion{{label: command.Name, description: command.Description, value: command.Name}}
+			}
+		}
+		return nil
+	}
 	if sugs := m.modelCommandSuggestions(input); len(sugs) > 0 {
 		return sugs
 	}
@@ -1461,11 +1780,62 @@ func (m model) handleLocalCommand(input string) (string, error) {
 		return m.handleDiffCommand(fields[1:])
 	case "/test":
 		return m.handleTestCommand(fields[1:])
+	case "/skills":
+		return m.handleSkillsCommand(fields[1:])
+	case "/rollback":
+		return m.handleRollbackCommand(fields[1:])
 	default:
 		return "", fmt.Errorf("unknown command: %s", fields[0])
 	}
 }
 
+func (m model) handleSkillsCommand(args []string) (string, error) {
+	if len(args) == 0 || args[0] == "list" {
+		if len(args) > 1 {
+			return "", fmt.Errorf("usage: /skills [list|install <source>]")
+		}
+		items, err := skills.LoadGlobal()
+		if err != nil {
+			return "", err
+		}
+		return skills.FormatList(items), nil
+	}
+	if args[0] != "install" || len(args) < 2 {
+		return "", fmt.Errorf("usage: /skills [list|install <source>]")
+	}
+	result, err := skills.InstallGlobal(m.ctx, strings.Join(args[1:], " "))
+	if err != nil {
+		return "", err
+	}
+	if m.agent != nil {
+		m.agent.AddSystemInstructions(skills.FormatForSystemPrompt([]skills.Skill{result.Skill}))
+	}
+	return fmt.Sprintf("Installed skill %q from %s. It is active in this conversation.", result.Skill.Name, result.Skill.Path), nil
+}
+
+func (m model) handleRollbackCommand(args []string) (string, error) {
+	if m.rollback == nil {
+		return "", fmt.Errorf("rollback is not configured")
+	}
+	if len(args) > 1 {
+		return "", fmt.Errorf("usage: /rollback [id]")
+	}
+	id := ""
+	if len(args) == 1 {
+		id = strings.TrimSpace(args[0])
+	} else {
+		id = m.rollback.LastID()
+	}
+	if id == "" {
+		return "", fmt.Errorf("no rollback snapshots available")
+	}
+	snapshot, err := m.rollback.Restore(id)
+	if err != nil {
+		return "", err
+	}
+	return fmt.Sprintf("Rolled back %s (%s).", snapshot.Path, snapshot.ID), nil
+}
+
 func (m model) handleCompactCommand() (string, error) {
 	if m.agent == nil {
 		return "", fmt.Errorf("agent is not configured")
@@ -1634,7 +2004,7 @@ func (m *model) updatePicker(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 }
 
 func (m *model) updateCommandSuggestions(msg tea.KeyMsg) (bool, tea.Model, tea.Cmd) {
-	if m.running || len(m.commandSuggestions()) == 0 {
+	if len(m.commandSuggestions()) == 0 {
 		return false, *m, nil
 	}
 	m.clampCommandCursor()

internal/tui/app_test.go

@@ -2,13 +2,17 @@ package tui
 
 import (
 	"context"
+	"fmt"
 	"os"
 	"os/exec"
 	"path/filepath"
+	"reflect"
 	"strings"
 	"testing"
+	"time"
 
 	"agentu/internal/agent"
+	"agentu/internal/tools"
 	"agentu/pkg/llm"
 
 	tea "github.com/charmbracelet/bubbletea"
@@ -126,6 +130,116 @@ func TestRunningStateRendersActivityIndicator(t *testing.T) {
 	}
 }
 
+type tuiBTWProvider struct{}
+
+func (tuiBTWProvider) ChatStream(ctx context.Context, req llm.ChatRequest, emit func(llm.StreamEvent) error) error {
+	return emit(llm.StreamEvent{Content: "side answer"})
+}
+
+func TestBTWCanBeSubmittedWhileMainTurnIsRunning(t *testing.T) {
+	assistant := agent.New(agent.Options{Provider: tuiBTWProvider{}, Model: "test-model"})
+	m := newModel(context.Background(), assistant, Options{ModelName: "test-model"})
+	m.running = true
+	m.events = make(chan tea.Msg, 8)
+
+	next, _ := m.Update(tea.KeyMsg{Type: tea.KeyRunes, Runes: []rune("/btw why?")})
+	m = next.(model)
+	if m.input.Value() != "/btw why?" {
+		t.Fatalf("running input = %q", m.input.Value())
+	}
+	next, cmd := m.Update(tea.KeyMsg{Type: tea.KeyEnter})
+	m = next.(model)
+	if !m.btwRunning || m.input.Value() != "" {
+		t.Fatalf("btw running/input = %v/%q", m.btwRunning, m.input.Value())
+	}
+	if got := m.messages[len(m.messages)-1]; got.role != roleBTWUser || got.content != "why?" {
+		t.Fatalf("btw question message = %+v", got)
+	}
+	if cmd == nil {
+		t.Fatal("btw did not return a start command")
+	}
+	_ = cmd()
+	chunk := <-m.events
+	done := <-m.events
+	if _, ok := chunk.(btwChunkMsg); !ok {
+		t.Fatalf("first btw event = %T", chunk)
+	}
+	if _, ok := done.(btwDoneMsg); !ok {
+		t.Fatalf("final btw event = %T", done)
+	}
+	if len(assistant.Messages()) != 0 {
+		t.Fatalf("btw polluted agent history: %+v", assistant.Messages())
+	}
+}
+
+func TestBTWRequiresQuestion(t *testing.T) {
+	m := newModel(context.Background(), nil, Options{ModelName: "test-model"})
+	m.input.SetValue("/btw")
+	next, _ := m.submit()
+	m = next.(model)
+	if m.btwRunning {
+		t.Fatal("empty btw question started a request")
+	}
+	last := m.messages[len(m.messages)-1]
+	if last.role != roleBTW || !strings.Contains(last.content, "Usage: /btw") {
+		t.Fatalf("empty btw message = %+v", last)
+	}
+}
+
+func TestBTWMessagesHaveDistinctLabels(t *testing.T) {
+	m := newModel(context.Background(), nil, Options{ModelName: "test-model"})
+	question := stripANSI(m.messageView(roleBTWUser, "question", 80))
+	answer := stripANSI(m.messageView(roleBTW, "answer", 80))
+	if !strings.Contains(question, "BTW · You") || !strings.Contains(answer, "BTW") {
+		t.Fatalf("btw labels missing:\n%s\n%s", question, answer)
+	}
+}
+
+func TestBTWAndMainStreamChunksStaySeparate(t *testing.T) {
+	m := newModel(context.Background(), nil, Options{ModelName: "test-model"})
+	m.messages = append(m.messages, message{role: roleUser, content: "main"})
+	m.appendAssistantChunk("main one ")
+	m.messages = append(m.messages, message{role: roleBTWUser, content: "side"})
+	m.appendBTWChunk("side one ")
+	m.appendAssistantChunk("main two")
+	m.appendBTWChunk("side two")
+
+	var mainAnswers, sideAnswers []string
+	for _, msg := range m.messages {
+		switch msg.role {
+		case roleAssistant:
+			mainAnswers = append(mainAnswers, msg.content)
+		case roleBTW:
+			sideAnswers = append(sideAnswers, msg.content)
+		}
+	}
+	if !reflect.DeepEqual(mainAnswers, []string{"main one main two"}) {
+		t.Fatalf("main answers = %#v", mainAnswers)
+	}
+	if !reflect.DeepEqual(sideAnswers, []string{"side one side two"}) {
+		t.Fatalf("side answers = %#v", sideAnswers)
+	}
+}
+
+func TestMainCompletionKeepsBTWEventStreamAlive(t *testing.T) {
+	m := newModel(context.Background(), nil, Options{ModelName: "test-model"})
+	events := make(chan tea.Msg, 1)
+	m.events = events
+	m.running = true
+	m.btwRunning = true
+
+	next, _ := m.Update(turnDoneMsg{})
+	m = next.(model)
+	if m.running || !m.btwRunning || m.events != events {
+		t.Fatalf("state after main completion: running=%v btw=%v events=%p", m.running, m.btwRunning, m.events)
+	}
+	next, _ = m.Update(btwDoneMsg{})
+	m = next.(model)
+	if m.active() || m.events != nil {
+		t.Fatalf("state after btw completion: active=%v events=%p", m.active(), m.events)
+	}
+}
+
 func TestUserMessageRendersFullWidthBubble(t *testing.T) {
 	m := newModel(context.Background(), nil, Options{ModelName: "test-model"})
 	next, _ := m.Update(tea.WindowSizeMsg{Width: 100, Height: 30})
@@ -395,6 +509,98 @@ func TestToolLogOutputUpdatesExistingToolCard(t *testing.T) {
 	}
 }
 
+func TestToolLogStreamingChunksAppendToSameCard(t *testing.T) {
+	m := newModel(context.Background(), nil, Options{ModelName: "test-model"})
+	m.messages = append(m.messages, message{role: roleTool, content: `shell_run echo hi`})
+
+	if !m.mergeToolLog("shell_run echo hi\n[output]\none\n") {
+		t.Fatal("expected first mergeToolLog to return true")
+	}
+	if !m.mergeToolLog("shell_run echo hi\n[output]\ntwo\n") {
+		t.Fatal("expected second mergeToolLog to return true")
+	}
+	if len(m.messages) != 1 {
+		t.Fatalf("messages count = %d, want 1", len(m.messages))
+	}
+	plain := stripANSI(m.renderMessages())
+	if !strings.Contains(plain, "one") || !strings.Contains(plain, "two") {
+		t.Fatalf("rendered output missing streamed lines:\n%s", plain)
+	}
+	if strings.Contains(plain, "onetwo") {
+		t.Fatalf("streamed lines were concatenated:\n%s", plain)
+	}
+}
+
+func TestShellRunCardShowsCommandWorkdirAndOutput(t *testing.T) {
+	m := newModel(context.Background(), nil, Options{ModelName: "test-model"})
+	m.messages = append(m.messages, message{
+		role:    roleTool,
+		content: "shell_run {\"command\":\"go test ./...\",\"workdir\":\"internal/tui\"}\n[output]\nok agentu/internal/tui",
+	})
+
+	plain := stripANSI(m.renderMessages())
+	for _, want := range []string{"$ go test ./...", "cwd internal/tui", "output", "ok agentu/internal/tui"} {
+		if !strings.Contains(plain, want) {
+			t.Fatalf("shell card missing %q:\n%s", want, plain)
+		}
+	}
+	if strings.Contains(plain, `{"command"`) || strings.Contains(plain, iconTool+" shell_run") {
+		t.Fatalf("shell card still exposes raw tool arguments:\n%s", plain)
+	}
+	if status := toolStatusText(`shell_run {"command":"go test ./...","workdir":"internal/tui"}`); status != "$ go test ./..." {
+		t.Fatalf("shell status = %q", status)
+	}
+}
+
+func TestShellRunStreamingEventsPreserveLineBreaks(t *testing.T) {
+	m := newModel(context.Background(), nil, Options{ModelName: "test-model"})
+	args := `{"command":"printf lines"}`
+	m.messages = append(m.messages, message{role: roleTool, content: "shell_run " + args})
+
+	next, _ := m.Update(toolLogMsg("\n[tool] shell_run " + args + "\n[output]\none\n"))
+	m = next.(model)
+	next, _ = m.Update(toolLogMsg("\n[tool] shell_run " + args + "\n[output]\n\n"))
+	m = next.(model)
+	next, _ = m.Update(toolLogMsg("\n[tool] shell_run " + args + "\n[output]\ntwo\n"))
+	m = next.(model)
+
+	parts := parseToolLog(m.messages[0].content)
+	if parts.output != "one\n\ntwo" {
+		t.Fatalf("streamed output = %q", parts.output)
+	}
+	if len(m.messages) != 1 {
+		t.Fatalf("streaming created %d tool cards, want 1", len(m.messages))
+	}
+	plain := stripANSI(m.renderMessages())
+	if strings.Contains(plain, "onetwo") {
+		t.Fatalf("streamed lines were concatenated:\n%s", plain)
+	}
+}
+
+func TestShellOutputPreviewKeepsHeadAndLatestLines(t *testing.T) {
+	var lines []string
+	for i := 1; i <= 12; i++ {
+		lines = append(lines, fmt.Sprintf("line %d", i))
+	}
+	preview := strings.Join(shellOutputPreviewLines(strings.Join(lines, "\n"), 80), "\n")
+	for _, want := range []string{"line 1", "line 4", "5 lines omitted", "line 10", "line 12"} {
+		if !strings.Contains(preview, want) {
+			t.Fatalf("preview missing %q:\n%s", want, preview)
+		}
+	}
+	if strings.Contains(preview, "line 5\n") || strings.Contains(preview, "line 9\n") {
+		t.Fatalf("preview contains a line that should be omitted:\n%s", preview)
+	}
+}
+
+func TestSanitizeTerminalOutputRemovesEscapesAndFoldsCarriageReturns(t *testing.T) {
+	got := sanitizeTerminalOutput("\x1b[31mred\x1b[0m\nprogress 10%\rprogress 20%\r\nclean\x00text")
+	want := "red\nprogress 20%\ncleantext"
+	if got != want {
+		t.Fatalf("sanitized output = %q, want %q", got, want)
+	}
+}
+
 type tuiCompactProvider struct{}
 
 func (tuiCompactProvider) ChatStream(ctx context.Context, req llm.ChatRequest, emit func(llm.StreamEvent) error) error {
@@ -912,3 +1118,115 @@ func TestCleanToolLog(t *testing.T) {
 		t.Fatalf("got %q want %q", got, want)
 	}
 }
+
+func TestApprovalPromptAllowsAndDeniesToolCalls(t *testing.T) {
+	for _, tc := range []struct {
+		key  tea.KeyMsg
+		want agent.ToolAuthorizationDecision
+	}{
+		{key: tea.KeyMsg{Type: tea.KeyRunes, Runes: []rune{'y'}}, want: agent.ToolAuthorizationAllow},
+		{key: tea.KeyMsg{Type: tea.KeyRunes, Runes: []rune{'n'}}, want: agent.ToolAuthorizationDeny},
+	} {
+		m := newModel(context.Background(), nil, Options{ModelName: "test-model"})
+		next, _ := m.Update(tea.WindowSizeMsg{Width: 100, Height: 30})
+		m = next.(model)
+		responses := make(chan agent.ToolAuthorizationDecision, 1)
+		next, _ = m.Update(toolApprovalMsg{
+			request: agent.ToolAuthorizationRequest{ToolName: "file_write", Preview: "write 5 bytes", RollbackAvailable: true},
+			respond: responses,
+		})
+		m = next.(model)
+		view := stripANSI(m.View())
+		for _, want := range []string{"Approve file_write?", "write 5 bytes", "rollback snapshot", "y approve"} {
+			if !strings.Contains(view, want) {
+				t.Fatalf("approval view missing %q:\n%s", want, view)
+			}
+		}
+		next, _ = m.Update(tc.key)
+		m = next.(model)
+		if m.approval != nil {
+			t.Fatal("approval state was not cleared")
+		}
+		if got := <-responses; got != tc.want {
+			t.Fatalf("decision = %q, want %q", got, tc.want)
+		}
+	}
+}
+
+func TestApprovalBrokerBridgesAgentAndTUI(t *testing.T) {
+	broker := NewApprovalBroker()
+	events := make(chan tea.Msg, 1)
+	broker.SetEvents(events)
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+	defer cancel()
+	result := make(chan agent.ToolAuthorizationDecision, 1)
+	go func() {
+		decision, _ := broker.AuthorizeToolCall(ctx, agent.ToolAuthorizationRequest{ToolName: "file_edit"})
+		result <- decision
+	}()
+	msg, ok := (<-events).(toolApprovalMsg)
+	if !ok {
+		t.Fatal("approval broker emitted unexpected message")
+	}
+	msg.respond <- agent.ToolAuthorizationAllow
+	if decision := <-result; decision != agent.ToolAuthorizationAllow {
+		t.Fatalf("decision = %q", decision)
+	}
+}
+
+func TestRollbackCommandRestoresLatestSnapshot(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "state.txt")
+	if err := os.WriteFile(path, []byte("before"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	rollback := tools.NewRollbackStore()
+	if _, err := rollback.Capture("file_edit", "state.txt", path); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(path, []byte("after"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	m := newModel(context.Background(), nil, Options{ModelName: "test-model", Rollback: rollback})
+	out, err := m.handleRollbackCommand(nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(out, "Rolled back state.txt") {
+		t.Fatalf("output = %q", out)
+	}
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(data) != "before" {
+		t.Fatalf("content = %q", data)
+	}
+}
+
+func TestSkillsInstallCommandActivatesSkill(t *testing.T) {
+	home := t.TempDir()
+	t.Setenv("HOME", home)
+	sourceDir := t.TempDir()
+	source := filepath.Join(sourceDir, "review skill.md")
+	content := "---\nname: review\ndescription: Review changes\n---\n\nCheck every diff.\n"
+	if err := os.WriteFile(source, []byte(content), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	assistant := agent.New(agent.Options{SystemPrompt: "base"})
+	m := newModel(context.Background(), assistant, Options{ModelName: "test-model"})
+	out, err := m.handleLocalCommand("/skills install " + source)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(out, `Installed skill "review"`) {
+		t.Fatalf("output = %q", out)
+	}
+	if _, err := os.Stat(filepath.Join(home, ".agentu", "skills", "review", "SKILL.md")); err != nil {
+		t.Fatalf("installed skill missing: %v", err)
+	}
+	messages := assistant.Messages()
+	if len(messages) == 0 || !strings.Contains(messages[0].Content, "Check every diff.") {
+		t.Fatalf("skill not activated in system prompt: %+v", messages)
+	}
+}

internal/tui/approval.go

@@ -0,0 +1,128 @@
+package tui
+
+import (
+	"context"
+	"fmt"
+	"strings"
+	"sync"
+
+	tea "github.com/charmbracelet/bubbletea"
+
+	"agentu/internal/agent"
+)
+
+// toolApprovalMsg is sent to the TUI when a mutating tool call needs approval.
+type toolApprovalMsg struct {
+	request agent.ToolAuthorizationRequest
+	respond chan agent.ToolAuthorizationDecision
+}
+
+// ApprovalBroker bridges the agent's ToolAuthorizer interface with the TUI's
+// event loop, allowing interactive approval of mutating tool calls.
+type ApprovalBroker struct {
+	mu     sync.Mutex
+	events chan<- tea.Msg
+}
+
+// NewApprovalBroker creates a new approval broker.
+func NewApprovalBroker() *ApprovalBroker {
+	return &ApprovalBroker{}
+}
+
+// SetEvents sets the current event channel for sending approval requests.
+func (b *ApprovalBroker) SetEvents(events chan<- tea.Msg) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	b.events = events
+}
+
+// ClearEvents removes the current event channel.
+func (b *ApprovalBroker) ClearEvents() {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	b.events = nil
+}
+
+// AuthorizeToolCall implements agent.ToolAuthorizer by sending an approval
+// request to the TUI and waiting for the user's decision.
+func (b *ApprovalBroker) AuthorizeToolCall(ctx context.Context, req agent.ToolAuthorizationRequest) (agent.ToolAuthorizationDecision, error) {
+	b.mu.Lock()
+	events := b.events
+	b.mu.Unlock()
+
+	if events == nil {
+		return "", fmt.Errorf("approval UI is not available")
+	}
+
+	respond := make(chan agent.ToolAuthorizationDecision, 1)
+	msg := toolApprovalMsg{
+		request: req,
+		respond: respond,
+	}
+
+	select {
+	case events <- msg:
+	case <-ctx.Done():
+		return "", ctx.Err()
+	}
+
+	select {
+	case decision := <-respond:
+		return decision, nil
+	case <-ctx.Done():
+		return "", ctx.Err()
+	}
+}
+
+func (m *model) updateApproval(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
+	if m.approval == nil {
+		return *m, nil
+	}
+	switch msg.String() {
+	case "y", "Y":
+		m.respondToApproval(agent.ToolAuthorizationAllow)
+		return *m, nil
+	case "n", "N", "esc":
+		m.respondToApproval(agent.ToolAuthorizationDeny)
+		return *m, nil
+	case "ctrl+c":
+		m.respondToApproval(agent.ToolAuthorizationDeny)
+		if m.cancel != nil {
+			m.cancel()
+		}
+		return *m, tea.Quit
+	default:
+		return *m, nil
+	}
+}
+
+func (m *model) respondToApproval(decision agent.ToolAuthorizationDecision) {
+	request := m.approval
+	m.approval = nil
+	if request != nil {
+		request.respond <- decision
+	}
+	m.status = "working..."
+	m.resize()
+	m.refreshViewport(true)
+}
+
+func (m model) approvalView() string {
+	if m.approval == nil {
+		return ""
+	}
+	req := m.approval.request
+	preview := strings.TrimSpace(req.Preview)
+	if preview == "" {
+		preview = strings.TrimSpace(req.Arguments)
+	}
+	lines := []string{
+		m.styles.ErrorLabel.Render("Approve " + req.ToolName + "?"),
+		m.styles.ToolValue.Render(preview),
+	}
+	if req.RollbackAvailable {
+		lines = append(lines, m.styles.Muted.Render("A rollback snapshot will be created."))
+	}
+	lines = append(lines, m.styles.Muted.Render("y approve · n/esc deny"))
+	return m.styles.InputBox.Width(max(20, m.width)).Render(strings.Join(lines, "\n"))
+}

pkg/config/config.go

@@ -62,10 +62,11 @@ type rawConfig struct {
 }
 
 type rawProviderConfig struct {
-	APIType string           `yaml:"api_type"`
+	APIType       string           `yaml:"api_type"`
-	BaseURL string           `yaml:"base_url"`
+	BaseURL       string           `yaml:"base_url"`
-	Models  []rawModelConfig `yaml:"models"`
+	Models        []rawModelConfig `yaml:"models"`
-	APIKey  string           `yaml:"api_key"`
+	APIKey        string           `yaml:"api_key"`
+	ThinkingParam string           `yaml:"thinking_param"`
 }
 
 type rawModelConfig struct {
@@ -229,12 +230,13 @@ func normalizeProviders(raw rawConfig) map[string]ProviderConfig {
 func normalizeProvider(name string, raw rawProviderConfig) ProviderConfig {
 	models, modelConfigs := normalizeModelConfigs(raw.Models)
 	provider := ProviderConfig{
-		Name:         name,
+		Name:          name,
-		APIType:      normalizeAPIType(raw.APIType),
+		APIType:       normalizeAPIType(raw.APIType),
-		BaseURL:      raw.BaseURL,
+		BaseURL:       raw.BaseURL,
-		Models:       models,
+		Models:        models,
-		ModelConfigs: modelConfigs,
+		ModelConfigs:  modelConfigs,
-		APIKey:       raw.APIKey,
+		APIKey:        raw.APIKey,
+		ThinkingParam: strings.TrimSpace(raw.ThinkingParam),
 	}
 	if len(provider.Models) > 0 {
 		provider.Model = provider.Models[0]

pkg/config/config_test.go

@@ -477,3 +477,28 @@ providers:
 		t.Fatalf("responses thinking param = %q, want %q", got, "reasoning")
 	}
 }
+
+func TestLoadCustomThinkingParam(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "agentu.yaml")
+	if err := os.WriteFile(path, []byte(`
+providers:
+  custom:
+    api_type: chat
+    base_url: https://example.com
+    api_key: sk-test
+    thinking_param: reasoning_effort
+    models:
+      - id: custom-model
+        thinking: high
+`), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := cfg.Providers["custom"].ThinkingParam; got != "reasoning_effort" {
+		t.Fatalf("thinking param = %q", got)
+	}
+}