feat(llm): support OpenAI Responses API via per-provider api_type

README.md

@@ -8,6 +8,7 @@ model/provider switching, and local tools.
 
 - 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.
@@ -48,6 +49,7 @@ Use `--config <path>` to load another file.
 ```yaml
 providers:
   loveuer:
+    api_type: chat
     base_url: https://ai.loveuer.com
     api_key: ${AGENTU_API_KEY}
     models:
@@ -64,6 +66,19 @@ Provider names are the keys under `providers`. At startup, agentu selects the
 first provider by name. Use `/model provider <name>` to switch providers for the
 current session.
 
+`api_type` selects which API protocol the provider speaks:
+
+```text
+chat      OpenAI-compatible /v1/chat/completions (default)
+responses OpenAI Responses API /v1/responses
+```
+
+The Responses API client converts the conversation history to Responses input
+items (`user`/`assistant`/`system` messages, `function_call`, and
+`function_call_output`), streams `response.output_text.delta` events, and
+supports function calling through `response.output_item.added`,
+`response.function_call_arguments.delta`, and `response.output_item.done`.
+
 `models` is required. `/model model <name>` is restricted to that list. Each
 entry is an object with:
 
@@ -83,7 +98,10 @@ none, middle, high, xhigh, max
 ```
 
 When configured or changed with `/model thinking ...`, the value is sent as a
-top-level OpenAI-compatible request field named `thinking`.
+top-level request field named `thinking`. For OpenAI reasoning models on the
+Responses API, set `thinking_param: reasoning` so the level is sent as the
+official `reasoning: {"effort": ...}` field; agentu maps its `middle` level to
+the official `medium` value and passes `none`, `high`, `xhigh`, `max` through.
 
 ## CLI Flags
 
@@ -174,5 +192,8 @@ Local config and build outputs are intentionally ignored by git.
 
 - Provider/model/thinking changes are session-only; agentu does not write them
   back to `~/.agentu/config.yaml`.
+- `api_type` is per provider and cannot be changed at runtime; use
+  `/model provider <name>` to switch to a provider configured with a different
+  API type.
 - The config schema is intentionally strict during MVP development. Deprecated
   fields such as `provider` or `active_provider` are rejected.

agentu.example.yaml

@@ -1,5 +1,8 @@
 providers:
   loveuer:
+    # API protocol: "chat" (OpenAI-compatible /v1/chat/completions, default)
+    # or "responses" (OpenAI Responses API /v1/responses).
+    api_type: chat
     base_url: https://ai.loveuer.com
     api_key: ${AGENTU_API_KEY}
     models:

internal/app/app.go

@@ -91,10 +91,17 @@ func loadConfig(configPath string) (*config.Config, error) {
 func defaultProvider(cfg *config.Config, httpClient *http.Client) (string, config.ProviderConfig, llm.Provider) {
 	providerName := cfg.DefaultProviderName()
 	providerConfig := cfg.Providers[providerName]
-	provider := llm.NewOpenAICompatibleClient(providerConfig.BaseURL, providerConfig.APIKey, httpClient)
+	provider := newProviderClient(providerConfig, httpClient)
 	return providerName, providerConfig, provider
 }
 
+func newProviderClient(providerConfig config.ProviderConfig, httpClient *http.Client) llm.Provider {
+	if providerConfig.APIType == config.APITypeResponses {
+		return llm.NewOpenAIResponsesClient(providerConfig.BaseURL, providerConfig.APIKey, httpClient)
+	}
+	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 {
 	return agent.New(agent.Options{
 		Provider:         provider,

internal/session/manager.go

@@ -204,10 +204,18 @@ func (m *Manager) CurrentThinking() string {
 	return m.provider.Thinking
 }
 
+func (m *Manager) CurrentAPIType() string {
+	if m.provider.APIType == "" {
+		return config.APITypeChat
+	}
+	return m.provider.APIType
+}
+
 func (m *Manager) Info() string {
 	var b strings.Builder
 	fmt.Fprintf(&b, "provider: %s\n", m.CurrentProvider())
 	fmt.Fprintf(&b, "model: %s\n", m.CurrentModel())
+	fmt.Fprintf(&b, "api: %s\n", m.CurrentAPIType())
 	fmt.Fprintf(&b, "thinking: %s\n", m.CurrentThinking())
 	fmt.Fprintf(&b, "providers: %s\n", strings.Join(m.providerNames(), ", "))
 	fmt.Fprintf(&b, "models: %s\n", strings.Join(m.provider.Models, ", "))
@@ -223,7 +231,7 @@ func (m *Manager) SetProvider(name string) (string, error) {
 	}
 	m.providerName = name
 	m.provider = provider
-	m.syncAgent(llm.NewOpenAICompatibleClient(provider.BaseURL, provider.APIKey, m.httpClient))
+	m.syncAgent(providerClient(provider, m.httpClient))
 	return "Switched provider.\n" + m.summary(), nil
 }
 
@@ -257,7 +265,14 @@ func (m *Manager) SetThinking(level string) (string, error) {
 }
 
 func (m *Manager) summary() string {
-	return fmt.Sprintf("provider: %s\nmodel: %s\nthinking: %s", m.CurrentProvider(), m.CurrentModel(), m.CurrentThinking())
+	return fmt.Sprintf("provider: %s\nmodel: %s\napi: %s\nthinking: %s", m.CurrentProvider(), m.CurrentModel(), m.CurrentAPIType(), m.CurrentThinking())
+}
+
+func providerClient(provider config.ProviderConfig, httpClient *http.Client) llm.Provider {
+	if provider.APIType == config.APITypeResponses {
+		return llm.NewOpenAIResponsesClient(provider.BaseURL, provider.APIKey, httpClient)
+	}
+	return llm.NewOpenAICompatibleClient(provider.BaseURL, provider.APIKey, httpClient)
 }
 
 func (m *Manager) maxContextTokens() int {

internal/session/manager_test.go

@@ -2,6 +2,8 @@ package session
 
 import (
 	"context"
+	"net/http"
+	"net/http/httptest"
 	"path/filepath"
 	"strings"
 	"testing"
@@ -109,6 +111,64 @@ func TestManagerRejectsUnknownModelAndThinking(t *testing.T) {
 	}
 }
 
+func TestManagerSwitchesToResponsesProvider(t *testing.T) {
+	var hitPath string
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		hitPath = r.URL.Path
+		w.Header().Set("Content-Type", "text/event-stream")
+		_, _ = w.Write([]byte(`data: {"type":"response.output_text.delta","item_id":"msg_1","output_index":0,"content_index":0,"delta":"ok"}` + "\n\n"))
+		_, _ = w.Write([]byte(`data: {"type":"response.completed","response":{"id":"resp_1","status":"completed"}}` + "\n\n"))
+	}))
+	defer server.Close()
+
+	cfg := &config.Config{
+		Providers: map[string]config.ProviderConfig{
+			"chat": {
+				Name:    "chat",
+				APIType: config.APITypeChat,
+				BaseURL: "https://chat.example.com",
+				APIKey:  "sk-test",
+				Model:   "model-a",
+				Models:  []string{"model-a"},
+			},
+			"responses": {
+				Name:    "responses",
+				APIType: config.APITypeResponses,
+				BaseURL: server.URL,
+				APIKey:  "sk-test",
+				Model:   "model-b",
+				Models:  []string{"model-b"},
+			},
+		},
+	}
+	assistant := agent.New(agent.Options{Provider: &recordingProvider{}, Model: "model-a"})
+	store, err := NewStore(filepath.Join(t.TempDir(), "sessions"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	manager, err := NewManager(cfg, assistant, server.Client(), store)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err := manager.SetProvider("responses"); err != nil {
+		t.Fatal(err)
+	}
+	if manager.CurrentAPIType() != config.APITypeResponses {
+		t.Fatalf("api type = %q", manager.CurrentAPIType())
+	}
+	var out strings.Builder
+	if err := assistant.RunTurn(context.Background(), "hi", &out, nil); err != nil {
+		t.Fatal(err)
+	}
+	if hitPath != "/v1/responses" {
+		t.Fatalf("path = %q, want /v1/responses", hitPath)
+	}
+	if out.String() != "ok" {
+		t.Fatalf("output = %q", out.String())
+	}
+}
+
 func TestManagerExposesModelCommandCandidates(t *testing.T) {
 	manager, _, _ := testManager(t)
 	models := manager.AvailableModels()

pkg/config/config.go

@@ -14,6 +14,11 @@ import (
 
 const DefaultPath = "~/.agentu/config.yaml"
 
+const (
+	APITypeChat      = "chat"
+	APITypeResponses = "responses"
+)
+
 type Config struct {
 	Providers map[string]ProviderConfig `yaml:"providers"`
 	Agent     AgentConfig               `yaml:"agent"`
@@ -21,6 +26,7 @@ type Config struct {
 
 type ProviderConfig struct {
 	Name               string
+	APIType            string
 	BaseURL            string
 	Model              string
 	Models             []string
@@ -56,6 +62,7 @@ type rawConfig struct {
 }
 
 type rawProviderConfig struct {
+	APIType string           `yaml:"api_type"`
 	BaseURL string           `yaml:"base_url"`
 	Models  []rawModelConfig `yaml:"models"`
 	APIKey  string           `yaml:"api_key"`
@@ -153,6 +160,9 @@ func (c *Config) Validate() error {
 	for name, provider := range c.Providers {
 		var missing []string
 		prefix := "providers." + name
+		if !IsAPIType(provider.APIType) {
+			return fmt.Errorf("%s.api_type must be one of: %s", prefix, strings.Join(APITypes(), ", "))
+		}
 		if strings.TrimSpace(provider.BaseURL) == "" {
 			missing = append(missing, prefix+".base_url")
 		}
@@ -220,6 +230,7 @@ func normalizeProvider(name string, raw rawProviderConfig) ProviderConfig {
 	models, modelConfigs := normalizeModelConfigs(raw.Models)
 	provider := ProviderConfig{
 		Name:         name,
+		APIType:      normalizeAPIType(raw.APIType),
 		BaseURL:      raw.BaseURL,
 		Models:       models,
 		ModelConfigs: modelConfigs,
@@ -235,6 +246,28 @@ func normalizeProvider(name string, raw rawProviderConfig) ProviderConfig {
 	return provider
 }
 
+func normalizeAPIType(value string) string {
+	value = strings.ToLower(strings.TrimSpace(value))
+	if value == "" {
+		return APITypeChat
+	}
+	return value
+}
+
+func APITypes() []string {
+	return []string{APITypeChat, APITypeResponses}
+}
+
+func IsAPIType(value string) bool {
+	value = strings.ToLower(strings.TrimSpace(value))
+	for _, apiType := range APITypes() {
+		if value == apiType {
+			return true
+		}
+	}
+	return false
+}
+
 func normalizeModelConfigs(rawModels []rawModelConfig) ([]string, map[string]ModelConfig) {
 	models := make([]string, 0, len(rawModels))
 	configs := make(map[string]ModelConfig, len(rawModels))

pkg/config/config_test.go

@@ -370,3 +370,74 @@ providers:
 		t.Fatalf("err = %v", err)
 	}
 }
+
+func TestLoadDefaultsAPITypeToChat(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("AGENTU_TEST_KEY", "sk-test")
+	path := filepath.Join(dir, "agentu.yaml")
+	if err := os.WriteFile(path, []byte(`
+providers:
+  test:
+    base_url: https://example.com
+    api_key: ${AGENTU_TEST_KEY}
+    models:
+      - id: test-model
+`), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := cfg.Providers["test"].APIType; got != APITypeChat {
+		t.Fatalf("api type = %q, want %q", got, APITypeChat)
+	}
+}
+
+func TestLoadParsesResponsesAPIType(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("AGENTU_TEST_KEY", "sk-test")
+	path := filepath.Join(dir, "agentu.yaml")
+	if err := os.WriteFile(path, []byte(`
+providers:
+  openai:
+    api_type: RESPONSES
+    base_url: https://api.openai.com
+    api_key: ${AGENTU_TEST_KEY}
+    models:
+      - id: gpt-test
+`), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := cfg.Providers["openai"].APIType; got != APITypeResponses {
+		t.Fatalf("api type = %q, want %q", got, APITypeResponses)
+	}
+}
+
+func TestLoadRejectsInvalidAPIType(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("AGENTU_TEST_KEY", "sk-test")
+	path := filepath.Join(dir, "agentu.yaml")
+	if err := os.WriteFile(path, []byte(`
+providers:
+  test:
+    api_type: completions
+    base_url: https://example.com
+    api_key: ${AGENTU_TEST_KEY}
+    models:
+      - id: test-model
+`), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	_, err := Load(path)
+	if err == nil || !strings.Contains(err.Error(), "api_type") {
+		t.Fatalf("err = %v", err)
+	}
+}

pkg/llm/responses.go

@@ -0,0 +1,460 @@
+package llm
+
+import (
+	"bufio"
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"sort"
+	"strings"
+)
+
+const responsesEndpoint = "/v1/responses"
+
+// OpenAIResponsesClient implements llm.Provider against the OpenAI Responses
+// API (/v1/responses). It maps the agent's chat-style conversation history to
+// Responses input items (messages, function_call, function_call_output) and
+// converts Responses streaming events back into the shared StreamEvent shape.
+type OpenAIResponsesClient struct {
+	baseURL    string
+	apiKey     string
+	httpClient *http.Client
+}
+
+func NewOpenAIResponsesClient(baseURL, apiKey string, httpClient *http.Client) *OpenAIResponsesClient {
+	if httpClient == nil {
+		httpClient = http.DefaultClient
+	}
+	return &OpenAIResponsesClient{
+		baseURL:    strings.TrimRight(baseURL, "/"),
+		apiKey:     apiKey,
+		httpClient: httpClient,
+	}
+}
+
+func (c *OpenAIResponsesClient) ChatStream(ctx context.Context, req ChatRequest, emit func(StreamEvent) error) error {
+	body, err := json.Marshal(buildResponsesRequest(req))
+	if err != nil {
+		return fmt.Errorf("marshal responses request: %w", err)
+	}
+	state := &responsesStreamState{}
+	return postResponsesSSE(ctx, c.baseURL, c.apiKey, c.httpClient, body, func(payload string) error {
+		return state.handle(payload, emit)
+	})
+}
+
+func postResponsesSSE(ctx context.Context, baseURL, apiKey string, httpClient *http.Client, body []byte, handle func(payload string) error) error {
+	var lastErr error
+	for attempt := 1; attempt <= maxChatAttempts; attempt++ {
+		httpReq, err := http.NewRequestWithContext(ctx, http.MethodPost, baseURL+responsesEndpoint, bytes.NewReader(body))
+		if err != nil {
+			return fmt.Errorf("create responses request: %w", err)
+		}
+		httpReq.Header.Set("Content-Type", "application/json")
+		httpReq.Header.Set("Accept", "text/event-stream")
+		if apiKey != "" {
+			httpReq.Header.Set("Authorization", "Bearer "+apiKey)
+		}
+
+		resp, err := httpClient.Do(httpReq)
+		if err != nil {
+			lastErr = fmt.Errorf("send responses request: %w", err)
+			if !shouldRetryRequest(ctx, attempt, 0) {
+				return lastErr
+			}
+			if waitErr := waitBeforeRetry(ctx); waitErr != nil {
+				return waitErr
+			}
+			continue
+		}
+
+		if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+			limit := io.LimitReader(resp.Body, 4096)
+			data, _ := io.ReadAll(limit)
+			_ = resp.Body.Close()
+			lastErr = fmt.Errorf("responses request failed: status=%d body=%s", resp.StatusCode, strings.TrimSpace(string(data)))
+			if !shouldRetryRequest(ctx, attempt, resp.StatusCode) {
+				return lastErr
+			}
+			if waitErr := waitBeforeRetry(ctx); waitErr != nil {
+				return waitErr
+			}
+			continue
+		}
+
+		defer resp.Body.Close()
+		return readResponsesSSE(resp.Body, handle)
+	}
+	return lastErr
+}
+
+func readResponsesSSE(r io.Reader, handle func(payload string) error) error {
+	reader := bufio.NewReader(r)
+	for {
+		line, err := reader.ReadString('\n')
+		if err != nil && !errors.Is(err, io.EOF) {
+			return fmt.Errorf("read responses stream: %w", err)
+		}
+
+		line = strings.TrimSpace(line)
+		if strings.HasPrefix(line, "data:") {
+			payload := strings.TrimSpace(strings.TrimPrefix(line, "data:"))
+			if payload == "[DONE]" {
+				return nil
+			}
+			if handleErr := handle(payload); handleErr != nil {
+				return handleErr
+			}
+		}
+
+		if errors.Is(err, io.EOF) {
+			return nil
+		}
+	}
+}
+
+// buildResponsesRequest converts a ChatRequest into the /v1/responses body.
+func buildResponsesRequest(req ChatRequest) map[string]any {
+	payload := map[string]any{
+		"model":  req.Model,
+		"input":  buildResponsesInput(req.Messages),
+		"stream": true,
+	}
+	if len(req.Tools) > 0 {
+		payload["tools"] = buildResponsesTools(req.Tools)
+	}
+	if req.ToolChoice != "" {
+		payload["tool_choice"] = req.ToolChoice
+	}
+	for key, value := range req.Extra {
+		if _, exists := payload[key]; exists {
+			continue
+		}
+		if key == "reasoning" {
+			if level, ok := value.(string); ok {
+				payload[key] = map[string]any{"effort": mapReasoningEffort(level)}
+				continue
+			}
+		}
+		payload[key] = value
+	}
+	return payload
+}
+
+// buildResponsesInput maps chat-style messages to Responses input items.
+// Assistant tool calls become function_call items and tool results become
+// function_call_output items, preserving their position in the conversation.
+func buildResponsesInput(messages []Message) []any {
+	items := make([]any, 0, len(messages)+4)
+	for _, msg := range messages {
+		switch msg.Role {
+		case RoleSystem:
+			items = append(items, responsesMessageInput{Role: "system", Content: msg.Content})
+		case RoleUser:
+			items = append(items, responsesMessageInput{Role: "user", Content: msg.Content})
+		case RoleAssistant:
+			if msg.Content != "" {
+				items = append(items, responsesMessageInput{Role: "assistant", Content: msg.Content})
+			}
+			for _, call := range msg.ToolCalls {
+				items = append(items, responsesFunctionCallInput{
+					Type:      "function_call",
+					CallID:    call.ID,
+					Name:      call.Function.Name,
+					Arguments: call.Function.Arguments,
+				})
+			}
+		case RoleTool:
+			items = append(items, responsesFunctionCallOutput{
+				Type:   "function_call_output",
+				CallID: msg.ToolCallID,
+				Output: msg.Content,
+			})
+		}
+	}
+	return items
+}
+
+func buildResponsesTools(tools []Tool) []responsesTool {
+	out := make([]responsesTool, 0, len(tools))
+	for _, tool := range tools {
+		out = append(out, responsesTool{
+			Type:        "function",
+			Name:        tool.Function.Name,
+			Description: tool.Function.Description,
+			Parameters:  tool.Function.Parameters,
+		})
+	}
+	return out
+}
+
+func mapReasoningEffort(level string) string {
+	level = strings.ToLower(strings.TrimSpace(level))
+	if level == "middle" {
+		return "medium"
+	}
+	return level
+}
+
+type responsesMessageInput struct {
+	Role    string `json:"role"`
+	Content string `json:"content"`
+}
+
+type responsesFunctionCallInput struct {
+	Type      string `json:"type"`
+	CallID    string `json:"call_id"`
+	Name      string `json:"name"`
+	Arguments string `json:"arguments"`
+}
+
+type responsesFunctionCallOutput struct {
+	Type   string `json:"type"`
+	CallID string `json:"call_id"`
+	Output string `json:"output"`
+}
+
+type responsesTool struct {
+	Type        string          `json:"type"`
+	Name        string          `json:"name"`
+	Description string          `json:"description,omitempty"`
+	Parameters  json.RawMessage `json:"parameters,omitempty"`
+}
+
+// responsesStreamState tracks in-flight output items while parsing SSE events.
+type responsesStreamState struct {
+	calls map[int]*responsesCallBuilder
+	done  bool
+}
+
+type responsesCallBuilder struct {
+	index     int
+	id        string
+	callType  string
+	name      string
+	arguments strings.Builder
+	finalized bool
+}
+
+func (s *responsesStreamState) handle(payload string, emit func(StreamEvent) error) error {
+	if s.done {
+		return nil
+	}
+	var event responsesStreamEvent
+	if err := json.Unmarshal([]byte(payload), &event); err != nil {
+		return fmt.Errorf("parse responses stream payload: %w", err)
+	}
+
+	switch event.Type {
+	case "error":
+		s.done = true
+		return event.err()
+	case "response.failed":
+		s.done = true
+		if event.Response != nil && event.Response.Error != nil {
+			return fmt.Errorf("provider error: %s: %s", event.Response.Error.Code, event.Response.Error.Message)
+		}
+		return fmt.Errorf("provider error: response failed")
+	case "response.incomplete":
+		s.done = true
+		return s.finish(emit, "length", nil)
+	case "response.completed":
+		s.done = true
+		var usage *Usage
+		if event.Response != nil && event.Response.Usage != nil {
+			usage = &Usage{
+				PromptTokens:     event.Response.Usage.InputTokens,
+				CompletionTokens: event.Response.Usage.OutputTokens,
+				TotalTokens:      event.Response.Usage.TotalTokens,
+			}
+		}
+		return s.finish(emit, "stop", usage)
+	case "response.output_text.delta":
+		if event.Delta != "" {
+			return emit(StreamEvent{Content: event.Delta})
+		}
+	case "response.output_item.added":
+		if event.Item != nil && event.Item.Type == "function_call" {
+			s.builder(event.OutputIndex).applyAdded(*event.Item)
+		}
+	case "response.function_call_arguments.delta":
+		if event.Delta != "" {
+			s.builder(event.OutputIndex).arguments.WriteString(event.Delta)
+		}
+	case "response.function_call_arguments.done":
+		builder := s.builder(event.OutputIndex)
+		if event.Name != "" {
+			builder.name = event.Name
+		}
+		if event.Arguments != "" {
+			builder.arguments.Reset()
+			builder.arguments.WriteString(event.Arguments)
+		}
+	case "response.output_item.done":
+		if event.Item != nil && event.Item.Type == "function_call" {
+			builder := s.builder(event.OutputIndex)
+			builder.applyDone(*event.Item)
+			return s.emitCall(builder, emit)
+		}
+	}
+	return nil
+}
+
+// finish flushes any not-yet-finalized function calls and reports the
+// terminal finish reason.
+func (s *responsesStreamState) finish(emit func(StreamEvent) error, finishReason string, usage *Usage) error {
+	if len(s.calls) == 0 {
+		return emit(StreamEvent{FinishReason: finishReason, Usage: usage})
+	}
+	indexes := make([]int, 0, len(s.calls))
+	for index := range s.calls {
+		indexes = append(indexes, index)
+	}
+	sort.Ints(indexes)
+	var deltas []ToolCallDelta
+	for _, index := range indexes {
+		builder := s.calls[index]
+		if builder.finalized {
+			continue
+		}
+		builder.finalized = true
+		deltas = append(deltas, builder.delta())
+	}
+	if len(deltas) > 0 {
+		if err := emit(StreamEvent{ToolCalls: deltas}); err != nil {
+			return err
+		}
+	}
+	return emit(StreamEvent{FinishReason: finishReason, Usage: usage})
+}
+
+func (s *responsesStreamState) emitCall(builder *responsesCallBuilder, emit func(StreamEvent) error) error {
+	if builder.finalized {
+		return nil
+	}
+	builder.finalized = true
+	return emit(StreamEvent{ToolCalls: []ToolCallDelta{builder.delta()}})
+}
+
+func (s *responsesStreamState) builder(index int) *responsesCallBuilder {
+	if s.calls == nil {
+		s.calls = make(map[int]*responsesCallBuilder)
+	}
+	builder := s.calls[index]
+	if builder == nil {
+		builder = &responsesCallBuilder{index: index}
+		s.calls[index] = builder
+	}
+	return builder
+}
+
+func (b *responsesCallBuilder) applyAdded(item responsesStreamItem) {
+	if b.id == "" {
+		b.id = item.CallID
+		if b.id == "" {
+			b.id = item.ID
+		}
+	}
+	if b.name == "" {
+		b.name = item.Name
+	}
+	if item.Arguments != "" {
+		b.arguments.Reset()
+		b.arguments.WriteString(item.Arguments)
+	}
+}
+
+func (b *responsesCallBuilder) applyDone(item responsesStreamItem) {
+	if item.CallID != "" {
+		b.id = item.CallID
+	} else if item.ID != "" {
+		b.id = item.ID
+	}
+	if item.Name != "" {
+		b.name = item.Name
+	}
+	if item.Arguments != "" {
+		b.arguments.Reset()
+		b.arguments.WriteString(item.Arguments)
+	}
+}
+
+func (b *responsesCallBuilder) delta() ToolCallDelta {
+	id := b.id
+	if id == "" {
+		id = fmt.Sprintf("call_%d", b.index)
+	}
+	callType := b.callType
+	if callType == "" {
+		callType = "function"
+	}
+	return ToolCallDelta{
+		Index:     b.index,
+		ID:        id,
+		Type:      callType,
+		Name:      b.name,
+		Arguments: b.arguments.String(),
+	}
+}
+
+type responsesStreamEvent struct {
+	Type        string                   `json:"type"`
+	OutputIndex int                      `json:"output_index"`
+	Delta       string                   `json:"delta"`
+	Arguments   string                   `json:"arguments"`
+	Name        string                   `json:"name"`
+	Code        string                   `json:"code"`
+	Message     string                   `json:"message"`
+	Param       string                   `json:"param"`
+	Item        *responsesStreamItem     `json:"item"`
+	Response    *responsesStreamResponse `json:"response"`
+	Error       *responsesStreamError    `json:"error"`
+}
+
+type responsesStreamItem struct {
+	Type      string `json:"type"`
+	ID        string `json:"id"`
+	CallID    string `json:"call_id"`
+	Name      string `json:"name"`
+	Arguments string `json:"arguments"`
+}
+
+type responsesStreamResponse struct {
+	Status string                `json:"status"`
+	Error  *responsesStreamError `json:"error"`
+	Usage  *responsesStreamUsage `json:"usage"`
+}
+
+type responsesStreamUsage struct {
+	InputTokens  int `json:"input_tokens"`
+	OutputTokens int `json:"output_tokens"`
+	TotalTokens  int `json:"total_tokens"`
+}
+
+type responsesStreamError struct {
+	Type    string `json:"type"`
+	Code    string `json:"code"`
+	Message string `json:"message"`
+	Param   string `json:"param"`
+}
+
+func (e responsesStreamEvent) err() error {
+	code, message, param := e.Code, e.Message, e.Param
+	if code == "" && message == "" && e.Error != nil {
+		code, message, param = e.Error.Code, e.Error.Message, e.Error.Param
+	}
+	if message == "" {
+		if param != "" {
+			return fmt.Errorf("provider error: %s", param)
+		}
+		return fmt.Errorf("provider error")
+	}
+	if code != "" {
+		return fmt.Errorf("provider error: %s: %s", code, message)
+	}
+	return fmt.Errorf("provider error: %s", message)
+}

pkg/llm/responses_test.go

@@ -0,0 +1,305 @@
+package llm
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func TestBuildResponsesRequest(t *testing.T) {
+	req := ChatRequest{
+		Model: "gpt-test",
+		Messages: []Message{
+			{Role: RoleSystem, Content: "be concise"},
+			{Role: RoleUser, Content: "hi"},
+			{
+				Role:    RoleAssistant,
+				Content: "let me check",
+				ToolCalls: []ToolCall{{
+					ID:   "call_1",
+					Type: "function",
+					Function: FunctionCall{
+						Name:      "file_read",
+						Arguments: `{"path":"README.md"}`,
+					},
+				}},
+			},
+			{Role: RoleTool, ToolCallID: "call_1", Content: "contents"},
+			{Role: RoleAssistant, Content: "done"},
+		},
+		Tools: []Tool{{
+			Type: "function",
+			Function: ToolFunction{
+				Name:        "file_read",
+				Description: "Read a local file",
+				Parameters:  json.RawMessage(`{"type":"object","properties":{}}`),
+			},
+		}},
+		ToolChoice: "auto",
+		Extra: map[string]any{
+			"reasoning":   "middle",
+			"thinking":    "high",
+			"temperature": 0.2,
+		},
+	}
+
+	payload := buildResponsesRequest(req)
+	data, err := json.Marshal(payload)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var raw map[string]any
+	if err := json.Unmarshal(data, &raw); err != nil {
+		t.Fatal(err)
+	}
+
+	if raw["model"] != "gpt-test" || raw["stream"] != true {
+		t.Fatalf("model/stream = %#v / %#v", raw["model"], raw["stream"])
+	}
+	if raw["tool_choice"] != "auto" {
+		t.Fatalf("tool_choice = %#v", raw["tool_choice"])
+	}
+
+	input, ok := raw["input"].([]any)
+	if !ok || len(input) != 6 {
+		t.Fatalf("input = %#v", raw["input"])
+	}
+	checkInputItem := func(index int, wantType, wantKey, wantValue string) {
+		t.Helper()
+		item, ok := input[index].(map[string]any)
+		if !ok {
+			t.Fatalf("input[%d] = %#v", index, input[index])
+		}
+		if item[wantKey] != wantValue {
+			t.Fatalf("input[%d].%s = %#v, want %q", index, wantKey, item[wantKey], wantValue)
+		}
+		if wantType != "" && item["type"] != wantType {
+			t.Fatalf("input[%d].type = %#v, want %q", index, item["type"], wantType)
+		}
+	}
+	checkInputItem(0, "", "role", "system")
+	checkInputItem(1, "", "role", "user")
+	checkInputItem(2, "", "role", "assistant")
+	checkInputItem(3, "function_call", "call_id", "call_1")
+	checkInputItem(4, "function_call_output", "call_id", "call_1")
+	checkInputItem(5, "", "role", "assistant")
+	if item := input[3].(map[string]any); item["name"] != "file_read" || item["arguments"] != `{"path":"README.md"}` {
+		t.Fatalf("function_call item = %#v", item)
+	}
+	if item := input[4].(map[string]any); item["output"] != "contents" {
+		t.Fatalf("function_call_output item = %#v", item)
+	}
+
+	tools, ok := raw["tools"].([]any)
+	if !ok || len(tools) != 1 {
+		t.Fatalf("tools = %#v", raw["tools"])
+	}
+	tool := tools[0].(map[string]any)
+	if tool["type"] != "function" || tool["name"] != "file_read" || tool["description"] != "Read a local file" {
+		t.Fatalf("tool = %#v", tool)
+	}
+	if _, ok := tool["parameters"].(map[string]any); !ok {
+		t.Fatalf("tool parameters = %#v", tool["parameters"])
+	}
+
+	reasoning, ok := raw["reasoning"].(map[string]any)
+	if !ok || reasoning["effort"] != "medium" {
+		t.Fatalf("reasoning = %#v", raw["reasoning"])
+	}
+	if raw["thinking"] != "high" {
+		t.Fatalf("thinking = %#v", raw["thinking"])
+	}
+	if raw["temperature"] != 0.2 {
+		t.Fatalf("temperature = %#v", raw["temperature"])
+	}
+}
+
+func TestOpenAIResponsesClientStreamsContentAndToolCalls(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/v1/responses" {
+			t.Fatalf("path = %s", r.URL.Path)
+		}
+		if got := r.Header.Get("Authorization"); got != "Bearer sk-test" {
+			t.Fatalf("authorization = %q", got)
+		}
+		body := mustReadBody(t, r)
+		var raw map[string]any
+		if err := json.NewDecoder(strings.NewReader(body)).Decode(&raw); err != nil {
+			t.Fatal(err)
+		}
+		if raw["stream"] != true {
+			t.Fatal("request did not enable stream")
+		}
+		if raw["tool_choice"] != "auto" {
+			t.Fatalf("tool_choice = %#v", raw["tool_choice"])
+		}
+		input, ok := raw["input"].([]any)
+		if !ok || len(input) == 0 {
+			t.Fatalf("input = %#v", raw["input"])
+		}
+
+		w.Header().Set("Content-Type", "text/event-stream")
+		events := []string{
+			`{"type":"response.output_item.added","output_index":0,"item":{"type":"message","id":"msg_1","role":"assistant","content":[]}}`,
+			`{"type":"response.output_text.delta","item_id":"msg_1","output_index":0,"content_index":0,"delta":"Hel"}`,
+			`{"type":"response.output_text.delta","item_id":"msg_1","output_index":0,"content_index":0,"delta":"lo"}`,
+			`{"type":"response.output_item.added","output_index":1,"item":{"type":"function_call","id":"fc_1","call_id":"call_1","name":"file_read","arguments":""}}`,
+			`{"type":"response.function_call_arguments.delta","item_id":"fc_1","output_index":1,"delta":"{\"path\""}`,
+			`{"type":"response.function_call_arguments.delta","item_id":"fc_1","output_index":1,"delta":":\"README.md\"}"}`,
+			`{"type":"response.function_call_arguments.done","item_id":"fc_1","output_index":1,"name":"file_read","arguments":"{\"path\":\"README.md\"}"}`,
+			`{"type":"response.output_item.done","output_index":1,"item":{"type":"function_call","id":"fc_1","call_id":"call_1","name":"file_read","arguments":"{\"path\":\"README.md\"}"}}`,
+			`{"type":"response.output_item.done","output_index":0,"item":{"type":"message","id":"msg_1","role":"assistant","content":[{"type":"output_text","text":"Hello"}]}}`,
+			`{"type":"response.completed","response":{"id":"resp_1","status":"completed","usage":{"input_tokens":10,"output_tokens":20,"total_tokens":30}}}`,
+		}
+		for _, event := range events {
+			_, _ = w.Write([]byte("data: " + event + "\n\n"))
+		}
+	}))
+	defer server.Close()
+
+	client := NewOpenAIResponsesClient(server.URL, "sk-test", server.Client())
+	var content strings.Builder
+	var calls []ToolCallDelta
+	var finishes []string
+	var usage *Usage
+	err := client.ChatStream(context.Background(), ChatRequest{
+		Model:      "gpt-test",
+		Messages:   []Message{{Role: RoleUser, Content: "hi"}},
+		ToolChoice: "auto",
+		Tools: []Tool{{
+			Type: "function",
+			Function: ToolFunction{
+				Name:        "file_read",
+				Description: "Read a local file",
+				Parameters:  json.RawMessage(`{"type":"object"}`),
+			},
+		}},
+	}, func(event StreamEvent) error {
+		content.WriteString(event.Content)
+		calls = append(calls, event.ToolCalls...)
+		if event.Usage != nil {
+			usage = event.Usage
+		}
+		if event.FinishReason != "" {
+			finishes = append(finishes, event.FinishReason)
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if content.String() != "Hello" {
+		t.Fatalf("content = %q", content.String())
+	}
+	if len(calls) != 1 {
+		t.Fatalf("calls = %#v", calls)
+	}
+	call := calls[0]
+	if call.Index != 1 || call.ID != "call_1" || call.Name != "file_read" {
+		t.Fatalf("call = %#v", call)
+	}
+	if call.Arguments != `{"path":"README.md"}` {
+		t.Fatalf("arguments = %q", call.Arguments)
+	}
+	if len(finishes) != 1 || finishes[0] != "stop" {
+		t.Fatalf("finishes = %#v", finishes)
+	}
+	if usage == nil || usage.PromptTokens != 10 || usage.CompletionTokens != 20 || usage.TotalTokens != 30 {
+		t.Fatalf("usage = %#v", usage)
+	}
+}
+
+func TestOpenAIResponsesClientErrorEvent(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "text/event-stream")
+		_, _ = w.Write([]byte(`data: {"type":"error","code":"invalid_request_error","message":"bad request"}` + "\n\n"))
+	}))
+	defer server.Close()
+
+	client := NewOpenAIResponsesClient(server.URL, "sk-test", server.Client())
+	err := client.ChatStream(context.Background(), ChatRequest{Model: "test"}, func(StreamEvent) error {
+		return nil
+	})
+	if err == nil || !strings.Contains(err.Error(), "bad request") {
+		t.Fatalf("err = %v", err)
+	}
+}
+
+func TestOpenAIResponsesClientFailedEvent(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "text/event-stream")
+		_, _ = w.Write([]byte(`data: {"type":"response.failed","response":{"id":"resp_1","status":"failed","error":{"code":"server_error","message":"boom"}}}` + "\n\n"))
+	}))
+	defer server.Close()
+
+	client := NewOpenAIResponsesClient(server.URL, "sk-test", server.Client())
+	err := client.ChatStream(context.Background(), ChatRequest{Model: "test"}, func(StreamEvent) error {
+		return nil
+	})
+	if err == nil || !strings.Contains(err.Error(), "boom") {
+		t.Fatalf("err = %v", err)
+	}
+}
+
+func TestOpenAIResponsesClientIncomplete(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "text/event-stream")
+		_, _ = w.Write([]byte(`data: {"type":"response.output_text.delta","item_id":"msg_1","output_index":0,"content_index":0,"delta":"partial"}` + "\n\n"))
+		_, _ = w.Write([]byte(`data: {"type":"response.incomplete","response":{"id":"resp_1","status":"incomplete","incomplete_details":{"reason":"max_tokens"}}}` + "\n\n"))
+	}))
+	defer server.Close()
+
+	client := NewOpenAIResponsesClient(server.URL, "sk-test", server.Client())
+	var content strings.Builder
+	var finishes []string
+	err := client.ChatStream(context.Background(), ChatRequest{Model: "test"}, func(event StreamEvent) error {
+		content.WriteString(event.Content)
+		if event.FinishReason != "" {
+			finishes = append(finishes, event.FinishReason)
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if content.String() != "partial" {
+		t.Fatalf("content = %q", content.String())
+	}
+	if len(finishes) != 1 || finishes[0] != "length" {
+		t.Fatalf("finishes = %#v", finishes)
+	}
+}
+
+func TestOpenAIResponsesClientRetriesTransientHTTPError(t *testing.T) {
+	attempts := 0
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		attempts++
+		if attempts == 1 {
+			http.Error(w, "temporary", http.StatusBadGateway)
+			return
+		}
+		w.Header().Set("Content-Type", "text/event-stream")
+		_, _ = w.Write([]byte(`data: {"type":"response.output_text.delta","item_id":"msg_1","output_index":0,"content_index":0,"delta":"ok"}` + "\n\n"))
+		_, _ = w.Write([]byte(`data: {"type":"response.completed","response":{"id":"resp_1","status":"completed"}}` + "\n\n"))
+	}))
+	defer server.Close()
+
+	client := NewOpenAIResponsesClient(server.URL, "sk-test", server.Client())
+	var content strings.Builder
+	err := client.ChatStream(context.Background(), ChatRequest{Model: "test"}, func(event StreamEvent) error {
+		content.WriteString(event.Content)
+		return nil
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if attempts != 2 {
+		t.Fatalf("attempts = %d, want 2", attempts)
+	}
+	if content.String() != "ok" {
+		t.Fatalf("content = %q", content.String())
+	}
+}