feat(openai): add /v1/messages endpoint and API compatibility layer

Add Anthropic Messages API support for OpenAI platform groups, enabling
clients using Claude-style /v1/messages format to access OpenAI accounts
through automatic protocol conversion.

- Add apicompat package with type definitions and bidirectional converters
  (Anthropic ↔ Chat, Chat ↔ Responses, Anthropic ↔ Responses)
- Implement /v1/messages endpoint for OpenAI gateway with streaming support
- Add model mapping UI for OpenAI OAuth accounts (whitelist + mapping modes)
- Support prompt caching fields and codex OAuth transforms
- Fix tool call ID conversion for Responses API (fc_ prefix)
- Ensure function_call_output has non-empty output field

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/handler/openai_gateway_handler.go

@@ -424,6 +424,262 @@ func (h *OpenAIGatewayHandler) logOpenAIRemoteCompactOutcome(c *gin.Context, sta
 	log.Warn("codex.remote_compact.failed")
 }
 
+// Messages handles Anthropic Messages API requests routed to OpenAI platform.
+// POST /v1/messages (when group platform is OpenAI)
+func (h *OpenAIGatewayHandler) Messages(c *gin.Context) {
+	streamStarted := false
+	defer h.recoverResponsesPanic(c, &streamStarted)
+
+	requestStart := time.Now()
+
+	apiKey, ok := middleware2.GetAPIKeyFromContext(c)
+	if !ok {
+		h.anthropicErrorResponse(c, http.StatusUnauthorized, "authentication_error", "Invalid API key")
+		return
+	}
+
+	subject, ok := middleware2.GetAuthSubjectFromContext(c)
+	if !ok {
+		h.anthropicErrorResponse(c, http.StatusInternalServerError, "api_error", "User context not found")
+		return
+	}
+	reqLog := requestLogger(
+		c,
+		"handler.openai_gateway.messages",
+		zap.Int64("user_id", subject.UserID),
+		zap.Int64("api_key_id", apiKey.ID),
+		zap.Any("group_id", apiKey.GroupID),
+	)
+	if !h.ensureResponsesDependencies(c, reqLog) {
+		return
+	}
+
+	body, err := pkghttputil.ReadRequestBodyWithPrealloc(c.Request)
+	if err != nil {
+		if maxErr, ok := extractMaxBytesError(err); ok {
+			h.anthropicErrorResponse(c, http.StatusRequestEntityTooLarge, "invalid_request_error", buildBodyTooLargeMessage(maxErr.Limit))
+			return
+		}
+		h.anthropicErrorResponse(c, http.StatusBadRequest, "invalid_request_error", "Failed to read request body")
+		return
+	}
+	if len(body) == 0 {
+		h.anthropicErrorResponse(c, http.StatusBadRequest, "invalid_request_error", "Request body is empty")
+		return
+	}
+
+	if !gjson.ValidBytes(body) {
+		h.anthropicErrorResponse(c, http.StatusBadRequest, "invalid_request_error", "Failed to parse request body")
+		return
+	}
+
+	modelResult := gjson.GetBytes(body, "model")
+	if !modelResult.Exists() || modelResult.Type != gjson.String || modelResult.String() == "" {
+		h.anthropicErrorResponse(c, http.StatusBadRequest, "invalid_request_error", "model is required")
+		return
+	}
+	reqModel := modelResult.String()
+	reqStream := gjson.GetBytes(body, "stream").Bool()
+
+	reqLog = reqLog.With(zap.String("model", reqModel), zap.Bool("stream", reqStream))
+
+	setOpsRequestContext(c, reqModel, reqStream, body)
+
+	subscription, _ := middleware2.GetSubscriptionFromContext(c)
+
+	service.SetOpsLatencyMs(c, service.OpsAuthLatencyMsKey, time.Since(requestStart).Milliseconds())
+	routingStart := time.Now()
+
+	userReleaseFunc, acquired := h.acquireResponsesUserSlot(c, subject.UserID, subject.Concurrency, reqStream, &streamStarted, reqLog)
+	if !acquired {
+		return
+	}
+	if userReleaseFunc != nil {
+		defer userReleaseFunc()
+	}
+
+	if err := h.billingCacheService.CheckBillingEligibility(c.Request.Context(), apiKey.User, apiKey, apiKey.Group, subscription); err != nil {
+		reqLog.Info("openai_messages.billing_eligibility_check_failed", zap.Error(err))
+		status, code, message := billingErrorDetails(err)
+		h.anthropicStreamingAwareError(c, status, code, message, streamStarted)
+		return
+	}
+
+	sessionHash := h.gatewayService.GenerateSessionHash(c, body)
+	promptCacheKey := h.gatewayService.ExtractSessionID(c, body)
+
+	maxAccountSwitches := h.maxAccountSwitches
+	switchCount := 0
+	failedAccountIDs := make(map[int64]struct{})
+	var lastFailoverErr *service.UpstreamFailoverError
+
+	for {
+		reqLog.Debug("openai_messages.account_selecting", zap.Int("excluded_account_count", len(failedAccountIDs)))
+		selection, scheduleDecision, err := h.gatewayService.SelectAccountWithScheduler(
+			c.Request.Context(),
+			apiKey.GroupID,
+			"", // no previous_response_id
+			sessionHash,
+			reqModel,
+			failedAccountIDs,
+			service.OpenAIUpstreamTransportAny,
+		)
+		if err != nil {
+			reqLog.Warn("openai_messages.account_select_failed",
+				zap.Error(err),
+				zap.Int("excluded_account_count", len(failedAccountIDs)),
+			)
+			if len(failedAccountIDs) == 0 {
+				h.anthropicStreamingAwareError(c, http.StatusServiceUnavailable, "api_error", "Service temporarily unavailable", streamStarted)
+				return
+			}
+			if lastFailoverErr != nil {
+				h.handleAnthropicFailoverExhausted(c, lastFailoverErr, streamStarted)
+			} else {
+				h.anthropicStreamingAwareError(c, http.StatusBadGateway, "api_error", "Upstream request failed", streamStarted)
+			}
+			return
+		}
+		if selection == nil || selection.Account == nil {
+			h.anthropicStreamingAwareError(c, http.StatusServiceUnavailable, "api_error", "No available accounts", streamStarted)
+			return
+		}
+		account := selection.Account
+		reqLog.Debug("openai_messages.account_selected", zap.Int64("account_id", account.ID), zap.String("account_name", account.Name))
+		_ = scheduleDecision
+		setOpsSelectedAccount(c, account.ID, account.Platform)
+
+		accountReleaseFunc, acquired := h.acquireResponsesAccountSlot(c, apiKey.GroupID, sessionHash, selection, reqStream, &streamStarted, reqLog)
+		if !acquired {
+			return
+		}
+
+		service.SetOpsLatencyMs(c, service.OpsRoutingLatencyMsKey, time.Since(routingStart).Milliseconds())
+		forwardStart := time.Now()
+
+		result, err := h.gatewayService.ForwardAsAnthropic(c.Request.Context(), c, account, body, promptCacheKey)
+
+		forwardDurationMs := time.Since(forwardStart).Milliseconds()
+		if accountReleaseFunc != nil {
+			accountReleaseFunc()
+		}
+		upstreamLatencyMs, _ := getContextInt64(c, service.OpsUpstreamLatencyMsKey)
+		responseLatencyMs := forwardDurationMs
+		if upstreamLatencyMs > 0 && forwardDurationMs > upstreamLatencyMs {
+			responseLatencyMs = forwardDurationMs - upstreamLatencyMs
+		}
+		service.SetOpsLatencyMs(c, service.OpsResponseLatencyMsKey, responseLatencyMs)
+		if err == nil && result != nil && result.FirstTokenMs != nil {
+			service.SetOpsLatencyMs(c, service.OpsTimeToFirstTokenMsKey, int64(*result.FirstTokenMs))
+		}
+		if err != nil {
+			var failoverErr *service.UpstreamFailoverError
+			if errors.As(err, &failoverErr) {
+				h.gatewayService.ReportOpenAIAccountScheduleResult(account.ID, false, nil)
+				h.gatewayService.RecordOpenAIAccountSwitch()
+				failedAccountIDs[account.ID] = struct{}{}
+				lastFailoverErr = failoverErr
+				if switchCount >= maxAccountSwitches {
+					h.handleAnthropicFailoverExhausted(c, failoverErr, streamStarted)
+					return
+				}
+				switchCount++
+				reqLog.Warn("openai_messages.upstream_failover_switching",
+					zap.Int64("account_id", account.ID),
+					zap.Int("upstream_status", failoverErr.StatusCode),
+					zap.Int("switch_count", switchCount),
+					zap.Int("max_switches", maxAccountSwitches),
+				)
+				continue
+			}
+			h.gatewayService.ReportOpenAIAccountScheduleResult(account.ID, false, nil)
+			wroteFallback := h.ensureAnthropicErrorResponse(c, streamStarted)
+			reqLog.Warn("openai_messages.forward_failed",
+				zap.Int64("account_id", account.ID),
+				zap.Bool("fallback_error_response_written", wroteFallback),
+				zap.Error(err),
+			)
+			return
+		}
+		if result != nil {
+			h.gatewayService.ReportOpenAIAccountScheduleResult(account.ID, true, result.FirstTokenMs)
+		} else {
+			h.gatewayService.ReportOpenAIAccountScheduleResult(account.ID, true, nil)
+		}
+
+		userAgent := c.GetHeader("User-Agent")
+		clientIP := ip.GetClientIP(c)
+
+		h.submitUsageRecordTask(func(ctx context.Context) {
+			if err := h.gatewayService.RecordUsage(ctx, &service.OpenAIRecordUsageInput{
+				Result:        result,
+				APIKey:        apiKey,
+				User:          apiKey.User,
+				Account:       account,
+				Subscription:  subscription,
+				UserAgent:     userAgent,
+				IPAddress:     clientIP,
+				APIKeyService: h.apiKeyService,
+			}); err != nil {
+				logger.L().With(
+					zap.String("component", "handler.openai_gateway.messages"),
+					zap.Int64("user_id", subject.UserID),
+					zap.Int64("api_key_id", apiKey.ID),
+					zap.Any("group_id", apiKey.GroupID),
+					zap.String("model", reqModel),
+					zap.Int64("account_id", account.ID),
+				).Error("openai_messages.record_usage_failed", zap.Error(err))
+			}
+		})
+		reqLog.Debug("openai_messages.request_completed",
+			zap.Int64("account_id", account.ID),
+			zap.Int("switch_count", switchCount),
+		)
+		return
+	}
+}
+
+// anthropicErrorResponse writes an error in Anthropic Messages API format.
+func (h *OpenAIGatewayHandler) anthropicErrorResponse(c *gin.Context, status int, errType, message string) {
+	c.JSON(status, gin.H{
+		"type": "error",
+		"error": gin.H{
+			"type":    errType,
+			"message": message,
+		},
+	})
+}
+
+// anthropicStreamingAwareError handles errors that may occur during streaming,
+// using Anthropic SSE error format.
+func (h *OpenAIGatewayHandler) anthropicStreamingAwareError(c *gin.Context, status int, errType, message string, streamStarted bool) {
+	if streamStarted {
+		flusher, ok := c.Writer.(http.Flusher)
+		if ok {
+			errorEvent := "event: error\ndata: " + `{"type":"error","error":{"type":` + strconv.Quote(errType) + `,"message":` + strconv.Quote(message) + `}}` + "\n\n"
+			fmt.Fprint(c.Writer, errorEvent) //nolint:errcheck
+			flusher.Flush()
+		}
+		return
+	}
+	h.anthropicErrorResponse(c, status, errType, message)
+}
+
+// handleAnthropicFailoverExhausted maps upstream failover errors to Anthropic format.
+func (h *OpenAIGatewayHandler) handleAnthropicFailoverExhausted(c *gin.Context, failoverErr *service.UpstreamFailoverError, streamStarted bool) {
+	status, errType, errMsg := h.mapUpstreamError(failoverErr.StatusCode)
+	h.anthropicStreamingAwareError(c, status, errType, errMsg, streamStarted)
+}
+
+// ensureAnthropicErrorResponse writes a fallback Anthropic error if no response was written.
+func (h *OpenAIGatewayHandler) ensureAnthropicErrorResponse(c *gin.Context, streamStarted bool) bool {
+	if c == nil || c.Writer == nil || c.Writer.Written() {
+		return false
+	}
+	h.anthropicStreamingAwareError(c, http.StatusBadGateway, "api_error", "Upstream request failed", streamStarted)
+	return true
+}
+
 func (h *OpenAIGatewayHandler) validateFunctionCallOutputRequest(c *gin.Context, body []byte, reqLog *zap.Logger) bool {
 	if !gjson.GetBytes(body, `input.#(type=="function_call_output")`).Exists() {
 		return true

backend/internal/pkg/apicompat/anthropic_responses_test.go

@@ -0,0 +1,534 @@
+package apicompat
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// ---------------------------------------------------------------------------
+// AnthropicToResponses tests
+// ---------------------------------------------------------------------------
+
+func TestAnthropicToResponses_BasicText(t *testing.T) {
+	req := &AnthropicRequest{
+		Model:     "gpt-5.2",
+		MaxTokens: 1024,
+		Stream:    true,
+		Messages: []AnthropicMessage{
+			{Role: "user", Content: json.RawMessage(`"Hello"`)},
+		},
+	}
+
+	resp, err := AnthropicToResponses(req)
+	require.NoError(t, err)
+	assert.Equal(t, "gpt-5.2", resp.Model)
+	assert.True(t, resp.Stream)
+	assert.Equal(t, 1024, *resp.MaxOutputTokens)
+	assert.False(t, *resp.Store)
+
+	var items []ResponsesInputItem
+	require.NoError(t, json.Unmarshal(resp.Input, &items))
+	require.Len(t, items, 1)
+	assert.Equal(t, "user", items[0].Role)
+}
+
+func TestAnthropicToResponses_SystemPrompt(t *testing.T) {
+	t.Run("string", func(t *testing.T) {
+		req := &AnthropicRequest{
+			Model:     "gpt-5.2",
+			MaxTokens: 100,
+			System:    json.RawMessage(`"You are helpful."`),
+			Messages:  []AnthropicMessage{{Role: "user", Content: json.RawMessage(`"Hi"`)}},
+		}
+		resp, err := AnthropicToResponses(req)
+		require.NoError(t, err)
+
+		var items []ResponsesInputItem
+		require.NoError(t, json.Unmarshal(resp.Input, &items))
+		require.Len(t, items, 2)
+		assert.Equal(t, "system", items[0].Role)
+	})
+
+	t.Run("array", func(t *testing.T) {
+		req := &AnthropicRequest{
+			Model:     "gpt-5.2",
+			MaxTokens: 100,
+			System:    json.RawMessage(`[{"type":"text","text":"Part 1"},{"type":"text","text":"Part 2"}]`),
+			Messages:  []AnthropicMessage{{Role: "user", Content: json.RawMessage(`"Hi"`)}},
+		}
+		resp, err := AnthropicToResponses(req)
+		require.NoError(t, err)
+
+		var items []ResponsesInputItem
+		require.NoError(t, json.Unmarshal(resp.Input, &items))
+		require.Len(t, items, 2)
+		assert.Equal(t, "system", items[0].Role)
+		// System text should be joined with double newline.
+		var text string
+		require.NoError(t, json.Unmarshal(items[0].Content, &text))
+		assert.Equal(t, "Part 1\n\nPart 2", text)
+	})
+}
+
+func TestAnthropicToResponses_ToolUse(t *testing.T) {
+	req := &AnthropicRequest{
+		Model:     "gpt-5.2",
+		MaxTokens: 1024,
+		Messages: []AnthropicMessage{
+			{Role: "user", Content: json.RawMessage(`"What is the weather?"`)},
+			{Role: "assistant", Content: json.RawMessage(`[{"type":"text","text":"Let me check."},{"type":"tool_use","id":"call_1","name":"get_weather","input":{"city":"NYC"}}]`)},
+			{Role: "user", Content: json.RawMessage(`[{"type":"tool_result","tool_use_id":"call_1","content":"Sunny, 72°F"}]`)},
+		},
+		Tools: []AnthropicTool{
+			{Name: "get_weather", Description: "Get weather", InputSchema: json.RawMessage(`{"type":"object","properties":{"city":{"type":"string"}}}`)},
+		},
+	}
+
+	resp, err := AnthropicToResponses(req)
+	require.NoError(t, err)
+
+	// Check tools
+	require.Len(t, resp.Tools, 1)
+	assert.Equal(t, "function", resp.Tools[0].Type)
+	assert.Equal(t, "get_weather", resp.Tools[0].Name)
+
+	// Check input items
+	var items []ResponsesInputItem
+	require.NoError(t, json.Unmarshal(resp.Input, &items))
+	// user + assistant + function_call + function_call_output = 4
+	require.Len(t, items, 4)
+
+	assert.Equal(t, "user", items[0].Role)
+	assert.Equal(t, "assistant", items[1].Role)
+	assert.Equal(t, "function_call", items[2].Type)
+	assert.Equal(t, "fc_call_1", items[2].CallID)
+	assert.Equal(t, "function_call_output", items[3].Type)
+	assert.Equal(t, "fc_call_1", items[3].CallID)
+	assert.Equal(t, "Sunny, 72°F", items[3].Output)
+}
+
+func TestAnthropicToResponses_ThinkingIgnored(t *testing.T) {
+	req := &AnthropicRequest{
+		Model:     "gpt-5.2",
+		MaxTokens: 1024,
+		Messages: []AnthropicMessage{
+			{Role: "user", Content: json.RawMessage(`"Hello"`)},
+			{Role: "assistant", Content: json.RawMessage(`[{"type":"thinking","thinking":"deep thought"},{"type":"text","text":"Hi!"}]`)},
+			{Role: "user", Content: json.RawMessage(`"More"`)},
+		},
+	}
+
+	resp, err := AnthropicToResponses(req)
+	require.NoError(t, err)
+
+	var items []ResponsesInputItem
+	require.NoError(t, json.Unmarshal(resp.Input, &items))
+	// user + assistant(text only, thinking ignored) + user = 3
+	require.Len(t, items, 3)
+	assert.Equal(t, "assistant", items[1].Role)
+	// Assistant content should only have text, not thinking.
+	var parts []ResponsesContentPart
+	require.NoError(t, json.Unmarshal(items[1].Content, &parts))
+	require.Len(t, parts, 1)
+	assert.Equal(t, "output_text", parts[0].Type)
+	assert.Equal(t, "Hi!", parts[0].Text)
+}
+
+func TestAnthropicToResponses_MaxTokensFloor(t *testing.T) {
+	req := &AnthropicRequest{
+		Model:     "gpt-5.2",
+		MaxTokens: 10, // below minMaxOutputTokens (128)
+		Messages:  []AnthropicMessage{{Role: "user", Content: json.RawMessage(`"Hi"`)}},
+	}
+
+	resp, err := AnthropicToResponses(req)
+	require.NoError(t, err)
+	assert.Equal(t, 128, *resp.MaxOutputTokens)
+}
+
+// ---------------------------------------------------------------------------
+// ResponsesToAnthropic (non-streaming) tests
+// ---------------------------------------------------------------------------
+
+func TestResponsesToAnthropic_TextOnly(t *testing.T) {
+	resp := &ResponsesResponse{
+		ID:     "resp_123",
+		Model:  "gpt-5.2",
+		Status: "completed",
+		Output: []ResponsesOutput{
+			{
+				Type: "message",
+				Content: []ResponsesContentPart{
+					{Type: "output_text", Text: "Hello there!"},
+				},
+			},
+		},
+		Usage: &ResponsesUsage{InputTokens: 10, OutputTokens: 5, TotalTokens: 15},
+	}
+
+	anth := ResponsesToAnthropic(resp, "claude-opus-4-6")
+	assert.Equal(t, "resp_123", anth.ID)
+	assert.Equal(t, "claude-opus-4-6", anth.Model)
+	assert.Equal(t, "end_turn", anth.StopReason)
+	require.Len(t, anth.Content, 1)
+	assert.Equal(t, "text", anth.Content[0].Type)
+	assert.Equal(t, "Hello there!", anth.Content[0].Text)
+	assert.Equal(t, 10, anth.Usage.InputTokens)
+	assert.Equal(t, 5, anth.Usage.OutputTokens)
+}
+
+func TestResponsesToAnthropic_ToolUse(t *testing.T) {
+	resp := &ResponsesResponse{
+		ID:     "resp_456",
+		Model:  "gpt-5.2",
+		Status: "completed",
+		Output: []ResponsesOutput{
+			{
+				Type: "message",
+				Content: []ResponsesContentPart{
+					{Type: "output_text", Text: "Let me check."},
+				},
+			},
+			{
+				Type:      "function_call",
+				CallID:    "call_1",
+				Name:      "get_weather",
+				Arguments: `{"city":"NYC"}`,
+			},
+		},
+	}
+
+	anth := ResponsesToAnthropic(resp, "claude-opus-4-6")
+	assert.Equal(t, "tool_use", anth.StopReason)
+	require.Len(t, anth.Content, 2)
+	assert.Equal(t, "text", anth.Content[0].Type)
+	assert.Equal(t, "tool_use", anth.Content[1].Type)
+	assert.Equal(t, "call_1", anth.Content[1].ID)
+	assert.Equal(t, "get_weather", anth.Content[1].Name)
+}
+
+func TestResponsesToAnthropic_Reasoning(t *testing.T) {
+	resp := &ResponsesResponse{
+		ID:     "resp_789",
+		Model:  "gpt-5.2",
+		Status: "completed",
+		Output: []ResponsesOutput{
+			{
+				Type: "reasoning",
+				Summary: []ResponsesSummary{
+					{Type: "summary_text", Text: "Thinking about the answer..."},
+				},
+			},
+			{
+				Type: "message",
+				Content: []ResponsesContentPart{
+					{Type: "output_text", Text: "42"},
+				},
+			},
+		},
+	}
+
+	anth := ResponsesToAnthropic(resp, "claude-opus-4-6")
+	require.Len(t, anth.Content, 2)
+	assert.Equal(t, "thinking", anth.Content[0].Type)
+	assert.Equal(t, "Thinking about the answer...", anth.Content[0].Thinking)
+	assert.Equal(t, "text", anth.Content[1].Type)
+	assert.Equal(t, "42", anth.Content[1].Text)
+}
+
+func TestResponsesToAnthropic_Incomplete(t *testing.T) {
+	resp := &ResponsesResponse{
+		ID:     "resp_inc",
+		Model:  "gpt-5.2",
+		Status: "incomplete",
+		IncompleteDetails: &ResponsesIncompleteDetails{
+			Reason: "max_output_tokens",
+		},
+		Output: []ResponsesOutput{
+			{
+				Type:    "message",
+				Content: []ResponsesContentPart{{Type: "output_text", Text: "Partial..."}},
+			},
+		},
+	}
+
+	anth := ResponsesToAnthropic(resp, "claude-opus-4-6")
+	assert.Equal(t, "max_tokens", anth.StopReason)
+}
+
+func TestResponsesToAnthropic_EmptyOutput(t *testing.T) {
+	resp := &ResponsesResponse{
+		ID:     "resp_empty",
+		Model:  "gpt-5.2",
+		Status: "completed",
+		Output: []ResponsesOutput{},
+	}
+
+	anth := ResponsesToAnthropic(resp, "claude-opus-4-6")
+	require.Len(t, anth.Content, 1)
+	assert.Equal(t, "text", anth.Content[0].Type)
+	assert.Equal(t, "", anth.Content[0].Text)
+}
+
+// ---------------------------------------------------------------------------
+// Streaming: ResponsesEventToAnthropicEvents tests
+// ---------------------------------------------------------------------------
+
+func TestStreamingTextOnly(t *testing.T) {
+	state := NewResponsesEventToAnthropicState()
+
+	// 1. response.created
+	events := ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type: "response.created",
+		Response: &ResponsesResponse{
+			ID:    "resp_1",
+			Model: "gpt-5.2",
+		},
+	}, state)
+	require.Len(t, events, 1)
+	assert.Equal(t, "message_start", events[0].Type)
+
+	// 2. output_item.added (message)
+	events = ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type:        "response.output_item.added",
+		OutputIndex: 0,
+		Item:        &ResponsesOutput{Type: "message"},
+	}, state)
+	assert.Len(t, events, 0) // message item doesn't emit events
+
+	// 3. text delta
+	events = ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type:  "response.output_text.delta",
+		Delta: "Hello",
+	}, state)
+	require.Len(t, events, 2) // content_block_start + content_block_delta
+	assert.Equal(t, "content_block_start", events[0].Type)
+	assert.Equal(t, "text", events[0].ContentBlock.Type)
+	assert.Equal(t, "content_block_delta", events[1].Type)
+	assert.Equal(t, "text_delta", events[1].Delta.Type)
+	assert.Equal(t, "Hello", events[1].Delta.Text)
+
+	// 4. more text
+	events = ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type:  "response.output_text.delta",
+		Delta: " world",
+	}, state)
+	require.Len(t, events, 1) // only delta, no new block start
+	assert.Equal(t, "content_block_delta", events[0].Type)
+
+	// 5. text done
+	events = ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type: "response.output_text.done",
+	}, state)
+	require.Len(t, events, 1)
+	assert.Equal(t, "content_block_stop", events[0].Type)
+
+	// 6. completed
+	events = ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type: "response.completed",
+		Response: &ResponsesResponse{
+			Status: "completed",
+			Usage:  &ResponsesUsage{InputTokens: 10, OutputTokens: 5},
+		},
+	}, state)
+	require.Len(t, events, 2) // message_delta + message_stop
+	assert.Equal(t, "message_delta", events[0].Type)
+	assert.Equal(t, "end_turn", events[0].Delta.StopReason)
+	assert.Equal(t, 10, events[0].Usage.InputTokens)
+	assert.Equal(t, 5, events[0].Usage.OutputTokens)
+	assert.Equal(t, "message_stop", events[1].Type)
+}
+
+func TestStreamingToolCall(t *testing.T) {
+	state := NewResponsesEventToAnthropicState()
+
+	// 1. response.created
+	ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type:     "response.created",
+		Response: &ResponsesResponse{ID: "resp_2", Model: "gpt-5.2"},
+	}, state)
+
+	// 2. function_call added
+	events := ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type:        "response.output_item.added",
+		OutputIndex: 0,
+		Item:        &ResponsesOutput{Type: "function_call", CallID: "call_1", Name: "get_weather"},
+	}, state)
+	require.Len(t, events, 1)
+	assert.Equal(t, "content_block_start", events[0].Type)
+	assert.Equal(t, "tool_use", events[0].ContentBlock.Type)
+	assert.Equal(t, "call_1", events[0].ContentBlock.ID)
+
+	// 3. arguments delta
+	events = ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type:        "response.function_call_arguments.delta",
+		OutputIndex: 0,
+		Delta:       `{"city":`,
+	}, state)
+	require.Len(t, events, 1)
+	assert.Equal(t, "content_block_delta", events[0].Type)
+	assert.Equal(t, "input_json_delta", events[0].Delta.Type)
+	assert.Equal(t, `{"city":`, events[0].Delta.PartialJSON)
+
+	// 4. arguments done
+	events = ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type: "response.function_call_arguments.done",
+	}, state)
+	require.Len(t, events, 1)
+	assert.Equal(t, "content_block_stop", events[0].Type)
+
+	// 5. completed with tool_calls
+	events = ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type: "response.completed",
+		Response: &ResponsesResponse{
+			Status: "completed",
+			Usage:  &ResponsesUsage{InputTokens: 20, OutputTokens: 10},
+		},
+	}, state)
+	require.Len(t, events, 2)
+	assert.Equal(t, "tool_use", events[0].Delta.StopReason)
+}
+
+func TestStreamingReasoning(t *testing.T) {
+	state := NewResponsesEventToAnthropicState()
+
+	ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type:     "response.created",
+		Response: &ResponsesResponse{ID: "resp_3", Model: "gpt-5.2"},
+	}, state)
+
+	// reasoning item added
+	events := ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type:        "response.output_item.added",
+		OutputIndex: 0,
+		Item:        &ResponsesOutput{Type: "reasoning"},
+	}, state)
+	require.Len(t, events, 1)
+	assert.Equal(t, "content_block_start", events[0].Type)
+	assert.Equal(t, "thinking", events[0].ContentBlock.Type)
+
+	// reasoning text delta
+	events = ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type:        "response.reasoning_summary_text.delta",
+		OutputIndex: 0,
+		Delta:       "Let me think...",
+	}, state)
+	require.Len(t, events, 1)
+	assert.Equal(t, "content_block_delta", events[0].Type)
+	assert.Equal(t, "thinking_delta", events[0].Delta.Type)
+	assert.Equal(t, "Let me think...", events[0].Delta.Thinking)
+
+	// reasoning done
+	events = ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type: "response.reasoning_summary_text.done",
+	}, state)
+	require.Len(t, events, 1)
+	assert.Equal(t, "content_block_stop", events[0].Type)
+}
+
+func TestStreamingIncomplete(t *testing.T) {
+	state := NewResponsesEventToAnthropicState()
+
+	ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type:     "response.created",
+		Response: &ResponsesResponse{ID: "resp_4", Model: "gpt-5.2"},
+	}, state)
+
+	ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type:  "response.output_text.delta",
+		Delta: "Partial output...",
+	}, state)
+
+	events := ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type: "response.incomplete",
+		Response: &ResponsesResponse{
+			Status:            "incomplete",
+			IncompleteDetails: &ResponsesIncompleteDetails{Reason: "max_output_tokens"},
+			Usage:             &ResponsesUsage{InputTokens: 100, OutputTokens: 4096},
+		},
+	}, state)
+
+	// Should close the text block + message_delta + message_stop
+	require.Len(t, events, 3)
+	assert.Equal(t, "content_block_stop", events[0].Type)
+	assert.Equal(t, "message_delta", events[1].Type)
+	assert.Equal(t, "max_tokens", events[1].Delta.StopReason)
+	assert.Equal(t, "message_stop", events[2].Type)
+}
+
+func TestFinalizeStream_NeverStarted(t *testing.T) {
+	state := NewResponsesEventToAnthropicState()
+	events := FinalizeResponsesAnthropicStream(state)
+	assert.Nil(t, events)
+}
+
+func TestFinalizeStream_AlreadyCompleted(t *testing.T) {
+	state := NewResponsesEventToAnthropicState()
+	state.MessageStartSent = true
+	state.MessageStopSent = true
+	events := FinalizeResponsesAnthropicStream(state)
+	assert.Nil(t, events)
+}
+
+func TestFinalizeStream_AbnormalTermination(t *testing.T) {
+	state := NewResponsesEventToAnthropicState()
+
+	// Simulate a stream that started but never completed
+	ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type:     "response.created",
+		Response: &ResponsesResponse{ID: "resp_5", Model: "gpt-5.2"},
+	}, state)
+
+	ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type:  "response.output_text.delta",
+		Delta: "Interrupted...",
+	}, state)
+
+	// Stream ends without response.completed
+	events := FinalizeResponsesAnthropicStream(state)
+	require.Len(t, events, 3) // content_block_stop + message_delta + message_stop
+	assert.Equal(t, "content_block_stop", events[0].Type)
+	assert.Equal(t, "message_delta", events[1].Type)
+	assert.Equal(t, "end_turn", events[1].Delta.StopReason)
+	assert.Equal(t, "message_stop", events[2].Type)
+}
+
+func TestStreamingEmptyResponse(t *testing.T) {
+	state := NewResponsesEventToAnthropicState()
+
+	ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type:     "response.created",
+		Response: &ResponsesResponse{ID: "resp_6", Model: "gpt-5.2"},
+	}, state)
+
+	events := ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type: "response.completed",
+		Response: &ResponsesResponse{
+			Status: "completed",
+			Usage:  &ResponsesUsage{InputTokens: 5, OutputTokens: 0},
+		},
+	}, state)
+
+	require.Len(t, events, 2) // message_delta + message_stop
+	assert.Equal(t, "message_delta", events[0].Type)
+	assert.Equal(t, "end_turn", events[0].Delta.StopReason)
+}
+
+func TestResponsesAnthropicEventToSSE(t *testing.T) {
+	evt := AnthropicStreamEvent{
+		Type: "message_start",
+		Message: &AnthropicResponse{
+			ID:   "resp_1",
+			Type: "message",
+			Role: "assistant",
+		},
+	}
+	sse, err := ResponsesAnthropicEventToSSE(evt)
+	require.NoError(t, err)
+	assert.Contains(t, sse, "event: message_start\n")
+	assert.Contains(t, sse, "data: ")
+	assert.Contains(t, sse, `"resp_1"`)
+}

backend/internal/pkg/apicompat/anthropic_to_responses.go

@@ -0,0 +1,283 @@
+package apicompat
+
+import (
+	"encoding/json"
+	"strings"
+)
+
+// AnthropicToResponses converts an Anthropic Messages request directly into
+// a Responses API request. This preserves fields that would be lost in a
+// Chat Completions intermediary round-trip (e.g. thinking, cache_control,
+// structured system prompts).
+func AnthropicToResponses(req *AnthropicRequest) (*ResponsesRequest, error) {
+	input, err := convertAnthropicToResponsesInput(req.System, req.Messages)
+	if err != nil {
+		return nil, err
+	}
+
+	inputJSON, err := json.Marshal(input)
+	if err != nil {
+		return nil, err
+	}
+
+	out := &ResponsesRequest{
+		Model:       req.Model,
+		Input:       inputJSON,
+		Temperature: req.Temperature,
+		TopP:        req.TopP,
+		Stream:      req.Stream,
+		Include:     []string{"reasoning.encrypted_content"},
+	}
+
+	storeFalse := false
+	out.Store = &storeFalse
+
+	if req.MaxTokens > 0 {
+		v := req.MaxTokens
+		if v < minMaxOutputTokens {
+			v = minMaxOutputTokens
+		}
+		out.MaxOutputTokens = &v
+	}
+
+	if len(req.Tools) > 0 {
+		out.Tools = convertAnthropicToolsToResponses(req.Tools)
+	}
+
+	return out, nil
+}
+
+// convertAnthropicToResponsesInput builds the Responses API input items array
+// from the Anthropic system field and message list.
+func convertAnthropicToResponsesInput(system json.RawMessage, msgs []AnthropicMessage) ([]ResponsesInputItem, error) {
+	var out []ResponsesInputItem
+
+	// System prompt → system role input item.
+	if len(system) > 0 {
+		sysText, err := parseAnthropicSystemPrompt(system)
+		if err != nil {
+			return nil, err
+		}
+		if sysText != "" {
+			content, _ := json.Marshal(sysText)
+			out = append(out, ResponsesInputItem{
+				Role:    "system",
+				Content: content,
+			})
+		}
+	}
+
+	for _, m := range msgs {
+		items, err := anthropicMsgToResponsesItems(m)
+		if err != nil {
+			return nil, err
+		}
+		out = append(out, items...)
+	}
+	return out, nil
+}
+
+// parseAnthropicSystemPrompt handles the Anthropic system field which can be
+// a plain string or an array of text blocks.
+func parseAnthropicSystemPrompt(raw json.RawMessage) (string, error) {
+	var s string
+	if err := json.Unmarshal(raw, &s); err == nil {
+		return s, nil
+	}
+	var blocks []AnthropicContentBlock
+	if err := json.Unmarshal(raw, &blocks); err != nil {
+		return "", err
+	}
+	var parts []string
+	for _, b := range blocks {
+		if b.Type == "text" && b.Text != "" {
+			parts = append(parts, b.Text)
+		}
+	}
+	return strings.Join(parts, "\n\n"), nil
+}
+
+// anthropicMsgToResponsesItems converts a single Anthropic message into one
+// or more Responses API input items.
+func anthropicMsgToResponsesItems(m AnthropicMessage) ([]ResponsesInputItem, error) {
+	switch m.Role {
+	case "user":
+		return anthropicUserToResponses(m.Content)
+	case "assistant":
+		return anthropicAssistantToResponses(m.Content)
+	default:
+		return anthropicUserToResponses(m.Content)
+	}
+}
+
+// anthropicUserToResponses handles an Anthropic user message. Content can be a
+// plain string or an array of blocks. tool_result blocks are extracted into
+// function_call_output items.
+func anthropicUserToResponses(raw json.RawMessage) ([]ResponsesInputItem, error) {
+	// Try plain string.
+	var s string
+	if err := json.Unmarshal(raw, &s); err == nil {
+		content, _ := json.Marshal(s)
+		return []ResponsesInputItem{{Role: "user", Content: content}}, nil
+	}
+
+	var blocks []AnthropicContentBlock
+	if err := json.Unmarshal(raw, &blocks); err != nil {
+		return nil, err
+	}
+
+	var out []ResponsesInputItem
+
+	// Extract tool_result blocks → function_call_output items.
+	for _, b := range blocks {
+		if b.Type != "tool_result" {
+			continue
+		}
+		text := extractAnthropicToolResultText(b)
+		if text == "" {
+			// OpenAI Responses API requires "output" field; use placeholder for empty results.
+			text = "(empty)"
+		}
+		out = append(out, ResponsesInputItem{
+			Type:   "function_call_output",
+			CallID: toResponsesCallID(b.ToolUseID),
+			Output: text,
+		})
+	}
+
+	// Remaining text blocks → user message.
+	text := extractAnthropicTextFromBlocks(blocks)
+	if text != "" {
+		content, _ := json.Marshal(text)
+		out = append(out, ResponsesInputItem{Role: "user", Content: content})
+	}
+
+	return out, nil
+}
+
+// anthropicAssistantToResponses handles an Anthropic assistant message.
+// Text content → assistant message with output_text parts.
+// tool_use blocks → function_call items.
+// thinking blocks → ignored (OpenAI doesn't accept them as input).
+func anthropicAssistantToResponses(raw json.RawMessage) ([]ResponsesInputItem, error) {
+	// Try plain string.
+	var s string
+	if err := json.Unmarshal(raw, &s); err == nil {
+		parts := []ResponsesContentPart{{Type: "output_text", Text: s}}
+		partsJSON, err := json.Marshal(parts)
+		if err != nil {
+			return nil, err
+		}
+		return []ResponsesInputItem{{Role: "assistant", Content: partsJSON}}, nil
+	}
+
+	var blocks []AnthropicContentBlock
+	if err := json.Unmarshal(raw, &blocks); err != nil {
+		return nil, err
+	}
+
+	var items []ResponsesInputItem
+
+	// Text content → assistant message with output_text content parts.
+	text := extractAnthropicTextFromBlocks(blocks)
+	if text != "" {
+		parts := []ResponsesContentPart{{Type: "output_text", Text: text}}
+		partsJSON, err := json.Marshal(parts)
+		if err != nil {
+			return nil, err
+		}
+		items = append(items, ResponsesInputItem{Role: "assistant", Content: partsJSON})
+	}
+
+	// tool_use → function_call items.
+	for _, b := range blocks {
+		if b.Type != "tool_use" {
+			continue
+		}
+		args := "{}"
+		if len(b.Input) > 0 {
+			args = string(b.Input)
+		}
+		fcID := toResponsesCallID(b.ID)
+		items = append(items, ResponsesInputItem{
+			Type:      "function_call",
+			CallID:    fcID,
+			Name:      b.Name,
+			Arguments: args,
+			ID:        fcID,
+		})
+	}
+
+	return items, nil
+}
+
+// toResponsesCallID converts an Anthropic tool ID (toolu_xxx / call_xxx) to a
+// Responses API function_call ID that starts with "fc_".
+func toResponsesCallID(id string) string {
+	if strings.HasPrefix(id, "fc_") {
+		return id
+	}
+	return "fc_" + id
+}
+
+// fromResponsesCallID reverses toResponsesCallID, stripping the "fc_" prefix
+// that was added during request conversion.
+func fromResponsesCallID(id string) string {
+	if after, ok := strings.CutPrefix(id, "fc_"); ok {
+		// Only strip if the remainder doesn't look like it was already "fc_" prefixed.
+		// E.g. "fc_toolu_xxx" → "toolu_xxx", "fc_call_xxx" → "call_xxx"
+		if strings.HasPrefix(after, "toolu_") || strings.HasPrefix(after, "call_") {
+			return after
+		}
+	}
+	return id
+}
+
+// extractAnthropicToolResultText gets the text content from a tool_result block.
+func extractAnthropicToolResultText(b AnthropicContentBlock) string {
+	if len(b.Content) == 0 {
+		return ""
+	}
+	var s string
+	if err := json.Unmarshal(b.Content, &s); err == nil {
+		return s
+	}
+	var inner []AnthropicContentBlock
+	if err := json.Unmarshal(b.Content, &inner); err == nil {
+		var parts []string
+		for _, ib := range inner {
+			if ib.Type == "text" && ib.Text != "" {
+				parts = append(parts, ib.Text)
+			}
+		}
+		return strings.Join(parts, "\n\n")
+	}
+	return ""
+}
+
+// extractAnthropicTextFromBlocks joins all text blocks, ignoring thinking/
+// tool_use/tool_result blocks.
+func extractAnthropicTextFromBlocks(blocks []AnthropicContentBlock) string {
+	var parts []string
+	for _, b := range blocks {
+		if b.Type == "text" && b.Text != "" {
+			parts = append(parts, b.Text)
+		}
+	}
+	return strings.Join(parts, "\n\n")
+}
+
+// convertAnthropicToolsToResponses maps Anthropic tool definitions to
+// Responses API function tools (input_schema → parameters).
+func convertAnthropicToolsToResponses(tools []AnthropicTool) []ResponsesTool {
+	out := make([]ResponsesTool, len(tools))
+	for i, t := range tools {
+		out[i] = ResponsesTool{
+			Type:        "function",
+			Name:        t.Name,
+			Description: t.Description,
+			Parameters:  t.InputSchema,
+		}
+	}
+	return out
+}

backend/internal/pkg/apicompat/responses_to_anthropic.go

@@ -0,0 +1,436 @@
+package apicompat
+
+import (
+	"encoding/json"
+	"fmt"
+	"time"
+)
+
+// ---------------------------------------------------------------------------
+// Non-streaming: ResponsesResponse → AnthropicResponse
+// ---------------------------------------------------------------------------
+
+// ResponsesToAnthropic converts a Responses API response directly into an
+// Anthropic Messages response. Reasoning output items are mapped to thinking
+// blocks; function_call items become tool_use blocks.
+func ResponsesToAnthropic(resp *ResponsesResponse, model string) *AnthropicResponse {
+	out := &AnthropicResponse{
+		ID:    resp.ID,
+		Type:  "message",
+		Role:  "assistant",
+		Model: model,
+	}
+
+	var blocks []AnthropicContentBlock
+
+	for _, item := range resp.Output {
+		switch item.Type {
+		case "reasoning":
+			summaryText := ""
+			for _, s := range item.Summary {
+				if s.Type == "summary_text" && s.Text != "" {
+					summaryText += s.Text
+				}
+			}
+			if summaryText != "" {
+				blocks = append(blocks, AnthropicContentBlock{
+					Type:     "thinking",
+					Thinking: summaryText,
+				})
+			}
+		case "message":
+			for _, part := range item.Content {
+				if part.Type == "output_text" && part.Text != "" {
+					blocks = append(blocks, AnthropicContentBlock{
+						Type: "text",
+						Text: part.Text,
+					})
+				}
+			}
+		case "function_call":
+			blocks = append(blocks, AnthropicContentBlock{
+				Type:  "tool_use",
+				ID:    fromResponsesCallID(item.CallID),
+				Name:  item.Name,
+				Input: json.RawMessage(item.Arguments),
+			})
+		}
+	}
+
+	if len(blocks) == 0 {
+		blocks = append(blocks, AnthropicContentBlock{Type: "text", Text: ""})
+	}
+	out.Content = blocks
+
+	out.StopReason = responsesStatusToAnthropicStopReason(resp.Status, resp.IncompleteDetails, blocks)
+
+	if resp.Usage != nil {
+		out.Usage = AnthropicUsage{
+			InputTokens:  resp.Usage.InputTokens,
+			OutputTokens: resp.Usage.OutputTokens,
+		}
+		if resp.Usage.InputTokensDetails != nil {
+			out.Usage.CacheReadInputTokens = resp.Usage.InputTokensDetails.CachedTokens
+		}
+	}
+
+	return out
+}
+
+func responsesStatusToAnthropicStopReason(status string, details *ResponsesIncompleteDetails, blocks []AnthropicContentBlock) string {
+	switch status {
+	case "incomplete":
+		if details != nil && details.Reason == "max_output_tokens" {
+			return "max_tokens"
+		}
+		return "end_turn"
+	case "completed":
+		if len(blocks) > 0 && blocks[len(blocks)-1].Type == "tool_use" {
+			return "tool_use"
+		}
+		return "end_turn"
+	default:
+		return "end_turn"
+	}
+}
+
+// ---------------------------------------------------------------------------
+// Streaming: ResponsesStreamEvent → []AnthropicStreamEvent (stateful converter)
+// ---------------------------------------------------------------------------
+
+// ResponsesEventToAnthropicState tracks state for converting a sequence of
+// Responses SSE events directly into Anthropic SSE events.
+type ResponsesEventToAnthropicState struct {
+	MessageStartSent bool
+	MessageStopSent  bool
+
+	ContentBlockIndex int
+	ContentBlockOpen  bool
+	CurrentBlockType  string // "text" | "thinking" | "tool_use"
+
+	// OutputIndexToBlockIdx maps Responses output_index → Anthropic content block index.
+	OutputIndexToBlockIdx map[int]int
+
+	InputTokens          int
+	OutputTokens         int
+	CacheReadInputTokens int
+
+	ResponseID string
+	Model      string
+	Created    int64
+}
+
+// NewResponsesEventToAnthropicState returns an initialised stream state.
+func NewResponsesEventToAnthropicState() *ResponsesEventToAnthropicState {
+	return &ResponsesEventToAnthropicState{
+		OutputIndexToBlockIdx: make(map[int]int),
+		Created:               time.Now().Unix(),
+	}
+}
+
+// ResponsesEventToAnthropicEvents converts a single Responses SSE event into
+// zero or more Anthropic SSE events, updating state as it goes.
+func ResponsesEventToAnthropicEvents(
+	evt *ResponsesStreamEvent,
+	state *ResponsesEventToAnthropicState,
+) []AnthropicStreamEvent {
+	switch evt.Type {
+	case "response.created":
+		return resToAnthHandleCreated(evt, state)
+	case "response.output_item.added":
+		return resToAnthHandleOutputItemAdded(evt, state)
+	case "response.output_text.delta":
+		return resToAnthHandleTextDelta(evt, state)
+	case "response.output_text.done":
+		return resToAnthHandleBlockDone(state)
+	case "response.function_call_arguments.delta":
+		return resToAnthHandleFuncArgsDelta(evt, state)
+	case "response.function_call_arguments.done":
+		return resToAnthHandleBlockDone(state)
+	case "response.output_item.done":
+		return resToAnthHandleOutputItemDone(evt, state)
+	case "response.reasoning_summary_text.delta":
+		return resToAnthHandleReasoningDelta(evt, state)
+	case "response.reasoning_summary_text.done":
+		return resToAnthHandleBlockDone(state)
+	case "response.completed", "response.incomplete":
+		return resToAnthHandleCompleted(evt, state)
+	default:
+		return nil
+	}
+}
+
+// FinalizeResponsesAnthropicStream emits synthetic termination events if the
+// stream ended without a proper completion event.
+func FinalizeResponsesAnthropicStream(state *ResponsesEventToAnthropicState) []AnthropicStreamEvent {
+	if !state.MessageStartSent || state.MessageStopSent {
+		return nil
+	}
+
+	var events []AnthropicStreamEvent
+	events = append(events, closeCurrentBlock(state)...)
+
+	events = append(events,
+		AnthropicStreamEvent{
+			Type: "message_delta",
+			Delta: &AnthropicDelta{
+				StopReason: "end_turn",
+			},
+			Usage: &AnthropicUsage{
+				InputTokens:          state.InputTokens,
+				OutputTokens:         state.OutputTokens,
+				CacheReadInputTokens: state.CacheReadInputTokens,
+			},
+		},
+		AnthropicStreamEvent{Type: "message_stop"},
+	)
+	state.MessageStopSent = true
+	return events
+}
+
+// ResponsesAnthropicEventToSSE formats an AnthropicStreamEvent as an SSE line pair.
+func ResponsesAnthropicEventToSSE(evt AnthropicStreamEvent) (string, error) {
+	data, err := json.Marshal(evt)
+	if err != nil {
+		return "", err
+	}
+	return fmt.Sprintf("event: %s\ndata: %s\n\n", evt.Type, data), nil
+}
+
+// --- internal handlers ---
+
+func resToAnthHandleCreated(evt *ResponsesStreamEvent, state *ResponsesEventToAnthropicState) []AnthropicStreamEvent {
+	if evt.Response != nil {
+		state.ResponseID = evt.Response.ID
+		// Only use upstream model if no override was set (e.g. originalModel)
+		if state.Model == "" {
+			state.Model = evt.Response.Model
+		}
+	}
+
+	if state.MessageStartSent {
+		return nil
+	}
+	state.MessageStartSent = true
+
+	return []AnthropicStreamEvent{{
+		Type: "message_start",
+		Message: &AnthropicResponse{
+			ID:      state.ResponseID,
+			Type:    "message",
+			Role:    "assistant",
+			Content: []AnthropicContentBlock{},
+			Model:   state.Model,
+			Usage: AnthropicUsage{
+				InputTokens:  0,
+				OutputTokens: 0,
+			},
+		},
+	}}
+}
+
+func resToAnthHandleOutputItemAdded(evt *ResponsesStreamEvent, state *ResponsesEventToAnthropicState) []AnthropicStreamEvent {
+	if evt.Item == nil {
+		return nil
+	}
+
+	switch evt.Item.Type {
+	case "function_call":
+		var events []AnthropicStreamEvent
+		events = append(events, closeCurrentBlock(state)...)
+
+		idx := state.ContentBlockIndex
+		state.OutputIndexToBlockIdx[evt.OutputIndex] = idx
+		state.ContentBlockOpen = true
+		state.CurrentBlockType = "tool_use"
+
+		events = append(events, AnthropicStreamEvent{
+			Type:  "content_block_start",
+			Index: &idx,
+			ContentBlock: &AnthropicContentBlock{
+				Type:  "tool_use",
+				ID:    fromResponsesCallID(evt.Item.CallID),
+				Name:  evt.Item.Name,
+				Input: json.RawMessage("{}"),
+			},
+		})
+		return events
+
+	case "reasoning":
+		var events []AnthropicStreamEvent
+		events = append(events, closeCurrentBlock(state)...)
+
+		idx := state.ContentBlockIndex
+		state.OutputIndexToBlockIdx[evt.OutputIndex] = idx
+		state.ContentBlockOpen = true
+		state.CurrentBlockType = "thinking"
+
+		events = append(events, AnthropicStreamEvent{
+			Type:  "content_block_start",
+			Index: &idx,
+			ContentBlock: &AnthropicContentBlock{
+				Type:     "thinking",
+				Thinking: "",
+			},
+		})
+		return events
+
+	case "message":
+		return nil
+	}
+
+	return nil
+}
+
+func resToAnthHandleTextDelta(evt *ResponsesStreamEvent, state *ResponsesEventToAnthropicState) []AnthropicStreamEvent {
+	if evt.Delta == "" {
+		return nil
+	}
+
+	var events []AnthropicStreamEvent
+
+	if !state.ContentBlockOpen || state.CurrentBlockType != "text" {
+		events = append(events, closeCurrentBlock(state)...)
+
+		idx := state.ContentBlockIndex
+		state.ContentBlockOpen = true
+		state.CurrentBlockType = "text"
+
+		events = append(events, AnthropicStreamEvent{
+			Type:  "content_block_start",
+			Index: &idx,
+			ContentBlock: &AnthropicContentBlock{
+				Type: "text",
+				Text: "",
+			},
+		})
+	}
+
+	idx := state.ContentBlockIndex
+	events = append(events, AnthropicStreamEvent{
+		Type:  "content_block_delta",
+		Index: &idx,
+		Delta: &AnthropicDelta{
+			Type: "text_delta",
+			Text: evt.Delta,
+		},
+	})
+	return events
+}
+
+func resToAnthHandleFuncArgsDelta(evt *ResponsesStreamEvent, state *ResponsesEventToAnthropicState) []AnthropicStreamEvent {
+	if evt.Delta == "" {
+		return nil
+	}
+
+	blockIdx, ok := state.OutputIndexToBlockIdx[evt.OutputIndex]
+	if !ok {
+		return nil
+	}
+
+	return []AnthropicStreamEvent{{
+		Type:  "content_block_delta",
+		Index: &blockIdx,
+		Delta: &AnthropicDelta{
+			Type:        "input_json_delta",
+			PartialJSON: evt.Delta,
+		},
+	}}
+}
+
+func resToAnthHandleReasoningDelta(evt *ResponsesStreamEvent, state *ResponsesEventToAnthropicState) []AnthropicStreamEvent {
+	if evt.Delta == "" {
+		return nil
+	}
+
+	blockIdx, ok := state.OutputIndexToBlockIdx[evt.OutputIndex]
+	if !ok {
+		return nil
+	}
+
+	return []AnthropicStreamEvent{{
+		Type:  "content_block_delta",
+		Index: &blockIdx,
+		Delta: &AnthropicDelta{
+			Type:     "thinking_delta",
+			Thinking: evt.Delta,
+		},
+	}}
+}
+
+func resToAnthHandleBlockDone(state *ResponsesEventToAnthropicState) []AnthropicStreamEvent {
+	if !state.ContentBlockOpen {
+		return nil
+	}
+	return closeCurrentBlock(state)
+}
+
+func resToAnthHandleOutputItemDone(evt *ResponsesStreamEvent, state *ResponsesEventToAnthropicState) []AnthropicStreamEvent {
+	if evt.Item == nil {
+		return nil
+	}
+	if state.ContentBlockOpen {
+		return closeCurrentBlock(state)
+	}
+	return nil
+}
+
+func resToAnthHandleCompleted(evt *ResponsesStreamEvent, state *ResponsesEventToAnthropicState) []AnthropicStreamEvent {
+	if state.MessageStopSent {
+		return nil
+	}
+
+	var events []AnthropicStreamEvent
+	events = append(events, closeCurrentBlock(state)...)
+
+	stopReason := "end_turn"
+	if evt.Response != nil {
+		if evt.Response.Usage != nil {
+			state.InputTokens = evt.Response.Usage.InputTokens
+			state.OutputTokens = evt.Response.Usage.OutputTokens
+			if evt.Response.Usage.InputTokensDetails != nil {
+				state.CacheReadInputTokens = evt.Response.Usage.InputTokensDetails.CachedTokens
+			}
+		}
+		switch evt.Response.Status {
+		case "incomplete":
+			if evt.Response.IncompleteDetails != nil && evt.Response.IncompleteDetails.Reason == "max_output_tokens" {
+				stopReason = "max_tokens"
+			}
+		case "completed":
+			if state.ContentBlockIndex > 0 && state.CurrentBlockType == "tool_use" {
+				stopReason = "tool_use"
+			}
+		}
+	}
+
+	events = append(events,
+		AnthropicStreamEvent{
+			Type: "message_delta",
+			Delta: &AnthropicDelta{
+				StopReason: stopReason,
+			},
+			Usage: &AnthropicUsage{
+				InputTokens:          state.InputTokens,
+				OutputTokens:         state.OutputTokens,
+				CacheReadInputTokens: state.CacheReadInputTokens,
+			},
+		},
+		AnthropicStreamEvent{Type: "message_stop"},
+	)
+	state.MessageStopSent = true
+	return events
+}
+
+func closeCurrentBlock(state *ResponsesEventToAnthropicState) []AnthropicStreamEvent {
+	if !state.ContentBlockOpen {
+		return nil
+	}
+	idx := state.ContentBlockIndex
+	state.ContentBlockOpen = false
+	state.ContentBlockIndex++
+	return []AnthropicStreamEvent{{
+		Type:  "content_block_stop",
+		Index: &idx,
+	}}
+}

backend/internal/pkg/apicompat/types.go

@@ -0,0 +1,544 @@
+// Package apicompat provides type definitions and conversion utilities for
+// translating between Anthropic Messages, OpenAI Chat Completions, and OpenAI
+// Responses API formats. It enables multi-protocol support so that clients
+// using different API formats can be served through a unified gateway.
+package apicompat
+
+import "encoding/json"
+
+// ---------------------------------------------------------------------------
+// Anthropic Messages API types
+// ---------------------------------------------------------------------------
+
+// AnthropicRequest is the request body for POST /v1/messages.
+type AnthropicRequest struct {
+	Model       string             `json:"model"`
+	MaxTokens   int                `json:"max_tokens"`
+	System      json.RawMessage    `json:"system,omitempty"` // string or []AnthropicContentBlock
+	Messages    []AnthropicMessage `json:"messages"`
+	Tools       []AnthropicTool    `json:"tools,omitempty"`
+	Stream      bool               `json:"stream,omitempty"`
+	Temperature *float64           `json:"temperature,omitempty"`
+	TopP        *float64           `json:"top_p,omitempty"`
+	StopSeqs    []string           `json:"stop_sequences,omitempty"`
+}
+
+// AnthropicMessage is a single message in the Anthropic conversation.
+type AnthropicMessage struct {
+	Role    string          `json:"role"` // "user" | "assistant"
+	Content json.RawMessage `json:"content"`
+}
+
+// AnthropicContentBlock is one block inside a message's content array.
+type AnthropicContentBlock struct {
+	Type string `json:"type"`
+
+	// type=text
+	Text string `json:"text,omitempty"`
+
+	// type=thinking
+	Thinking string `json:"thinking,omitempty"`
+
+	// type=tool_use
+	ID    string          `json:"id,omitempty"`
+	Name  string          `json:"name,omitempty"`
+	Input json.RawMessage `json:"input,omitempty"`
+
+	// type=tool_result
+	ToolUseID string          `json:"tool_use_id,omitempty"`
+	Content   json.RawMessage `json:"content,omitempty"` // string or []AnthropicContentBlock
+	IsError   bool            `json:"is_error,omitempty"`
+}
+
+// AnthropicTool describes a tool available to the model.
+type AnthropicTool struct {
+	Name        string          `json:"name"`
+	Description string          `json:"description,omitempty"`
+	InputSchema json.RawMessage `json:"input_schema"` // JSON Schema object
+}
+
+// AnthropicResponse is the non-streaming response from POST /v1/messages.
+type AnthropicResponse struct {
+	ID           string                  `json:"id"`
+	Type         string                  `json:"type"` // "message"
+	Role         string                  `json:"role"` // "assistant"
+	Content      []AnthropicContentBlock `json:"content"`
+	Model        string                  `json:"model"`
+	StopReason   string                  `json:"stop_reason"`
+	StopSequence *string                 `json:"stop_sequence,omitempty"`
+	Usage        AnthropicUsage          `json:"usage"`
+}
+
+// AnthropicUsage holds token counts in Anthropic format.
+type AnthropicUsage struct {
+	InputTokens              int `json:"input_tokens"`
+	OutputTokens             int `json:"output_tokens"`
+	CacheCreationInputTokens int `json:"cache_creation_input_tokens"`
+	CacheReadInputTokens     int `json:"cache_read_input_tokens"`
+}
+
+// ---------------------------------------------------------------------------
+// Anthropic SSE event types
+// ---------------------------------------------------------------------------
+
+// AnthropicStreamEvent is a single SSE event in the Anthropic streaming protocol.
+type AnthropicStreamEvent struct {
+	Type string `json:"type"`
+
+	// message_start
+	Message *AnthropicResponse `json:"message,omitempty"`
+
+	// content_block_start
+	Index        *int                   `json:"index,omitempty"`
+	ContentBlock *AnthropicContentBlock `json:"content_block,omitempty"`
+
+	// content_block_delta
+	Delta *AnthropicDelta `json:"delta,omitempty"`
+
+	// message_delta
+	Usage *AnthropicUsage `json:"usage,omitempty"`
+}
+
+// AnthropicDelta carries incremental content in streaming events.
+type AnthropicDelta struct {
+	Type string `json:"type,omitempty"` // "text_delta" | "input_json_delta" | "thinking_delta" | "signature_delta"
+
+	// text_delta
+	Text string `json:"text,omitempty"`
+
+	// input_json_delta
+	PartialJSON string `json:"partial_json,omitempty"`
+
+	// thinking_delta
+	Thinking string `json:"thinking,omitempty"`
+
+	// signature_delta
+	Signature string `json:"signature,omitempty"`
+
+	// message_delta fields
+	StopReason   string  `json:"stop_reason,omitempty"`
+	StopSequence *string `json:"stop_sequence,omitempty"`
+}
+
+// ---------------------------------------------------------------------------
+// OpenAI Chat Completions API types
+// ---------------------------------------------------------------------------
+
+// ChatRequest is the request body for POST /v1/chat/completions.
+type ChatRequest struct {
+	Model       string          `json:"model"`
+	Messages    []ChatMessage   `json:"messages"`
+	MaxTokens   *int            `json:"max_tokens,omitempty"`
+	Temperature *float64        `json:"temperature,omitempty"`
+	TopP        *float64        `json:"top_p,omitempty"`
+	Stream      bool            `json:"stream,omitempty"`
+	Tools       []ChatTool      `json:"tools,omitempty"`
+	Stop        json.RawMessage `json:"stop,omitempty"` // string or []string
+}
+
+// ChatMessage is a single message in the Chat Completions conversation.
+type ChatMessage struct {
+	Role    string          `json:"role"`              // "system" | "user" | "assistant" | "tool"
+	Content json.RawMessage `json:"content,omitempty"` // string or []ChatContentPart
+
+	// assistant fields
+	ToolCalls []ChatToolCall `json:"tool_calls,omitempty"`
+
+	// tool fields
+	ToolCallID string `json:"tool_call_id,omitempty"`
+
+	// Copilot-specific reasoning passthrough
+	ReasoningText   string `json:"reasoning_text,omitempty"`
+	ReasoningOpaque string `json:"reasoning_opaque,omitempty"`
+}
+
+// ChatContentPart is a typed content part in a multi-part message.
+type ChatContentPart struct {
+	Type string `json:"type"` // "text" | "image_url"
+	Text string `json:"text,omitempty"`
+}
+
+// ChatToolCall represents a tool invocation in an assistant message.
+// In streaming deltas, Index identifies which tool call is being updated.
+type ChatToolCall struct {
+	Index    int              `json:"index"`
+	ID       string           `json:"id,omitempty"`
+	Type     string           `json:"type,omitempty"` // "function"
+	Function ChatFunctionCall `json:"function"`
+}
+
+// ChatFunctionCall holds the function name and arguments.
+type ChatFunctionCall struct {
+	Name      string `json:"name"`
+	Arguments string `json:"arguments"`
+}
+
+// ChatTool describes a tool available to the model.
+type ChatTool struct {
+	Type     string       `json:"type"` // "function"
+	Function ChatFunction `json:"function"`
+}
+
+// ChatFunction is the function definition inside a ChatTool.
+type ChatFunction struct {
+	Name        string          `json:"name"`
+	Description string          `json:"description,omitempty"`
+	Parameters  json.RawMessage `json:"parameters,omitempty"` // JSON Schema
+}
+
+// ChatResponse is the non-streaming response from POST /v1/chat/completions.
+type ChatResponse struct {
+	ID      string       `json:"id"`
+	Object  string       `json:"object"` // "chat.completion"
+	Created int64        `json:"created"`
+	Model   string       `json:"model"`
+	Choices []ChatChoice `json:"choices"`
+	Usage   *ChatUsage   `json:"usage,omitempty"`
+}
+
+// ChatChoice is one completion choice.
+type ChatChoice struct {
+	Index        int         `json:"index"`
+	Message      ChatMessage `json:"message"`
+	FinishReason string      `json:"finish_reason"`
+}
+
+// ChatUsage holds token counts in Chat Completions format.
+type ChatUsage struct {
+	PromptTokens     int `json:"prompt_tokens"`
+	CompletionTokens int `json:"completion_tokens"`
+	TotalTokens      int `json:"total_tokens"`
+}
+
+// ---------------------------------------------------------------------------
+// Chat Completions SSE types
+// ---------------------------------------------------------------------------
+
+// ChatStreamChunk is a single SSE chunk in the Chat Completions streaming protocol.
+type ChatStreamChunk struct {
+	ID      string             `json:"id"`
+	Object  string             `json:"object"` // "chat.completion.chunk"
+	Created int64              `json:"created"`
+	Model   string             `json:"model"`
+	Choices []ChatStreamChoice `json:"choices"`
+	Usage   *ChatUsage         `json:"usage,omitempty"`
+}
+
+// ChatStreamChoice is one choice inside a streaming chunk.
+type ChatStreamChoice struct {
+	Index        int             `json:"index"`
+	Delta        ChatStreamDelta `json:"delta"`
+	FinishReason *string         `json:"finish_reason"`
+}
+
+// ChatStreamDelta carries incremental content in a streaming chunk.
+type ChatStreamDelta struct {
+	Role      string         `json:"role,omitempty"`
+	Content   string         `json:"content,omitempty"`
+	ToolCalls []ChatToolCall `json:"tool_calls,omitempty"`
+
+	// Copilot-specific reasoning passthrough (streaming)
+	ReasoningText   string `json:"reasoning_text,omitempty"`
+	ReasoningOpaque string `json:"reasoning_opaque,omitempty"`
+}
+
+// ---------------------------------------------------------------------------
+// OpenAI Responses API types
+// ---------------------------------------------------------------------------
+
+// ResponsesRequest is the request body for POST /v1/responses.
+type ResponsesRequest struct {
+	Model           string          `json:"model"`
+	Input           json.RawMessage `json:"input"` // string or []ResponsesInputItem
+	MaxOutputTokens *int            `json:"max_output_tokens,omitempty"`
+	Temperature     *float64        `json:"temperature,omitempty"`
+	TopP            *float64        `json:"top_p,omitempty"`
+	Stream          bool            `json:"stream,omitempty"`
+	Tools           []ResponsesTool `json:"tools,omitempty"`
+	Include         []string        `json:"include,omitempty"`
+	Store           *bool           `json:"store,omitempty"`
+}
+
+// ResponsesInputItem is one item in the Responses API input array.
+// The Type field determines which other fields are populated.
+type ResponsesInputItem struct {
+	// Common
+	Type string `json:"type,omitempty"` // "" for role-based messages
+
+	// Role-based messages (system/user/assistant)
+	Role    string          `json:"role,omitempty"`
+	Content json.RawMessage `json:"content,omitempty"` // string or []ResponsesContentPart
+
+	// type=function_call
+	CallID    string `json:"call_id,omitempty"`
+	Name      string `json:"name,omitempty"`
+	Arguments string `json:"arguments,omitempty"`
+	ID        string `json:"id,omitempty"`
+
+	// type=function_call_output
+	Output string `json:"output,omitempty"`
+}
+
+// ResponsesContentPart is a typed content part in a Responses message.
+type ResponsesContentPart struct {
+	Type string `json:"type"` // "input_text" | "output_text" | "input_image"
+	Text string `json:"text,omitempty"`
+}
+
+// ResponsesTool describes a tool in the Responses API.
+type ResponsesTool struct {
+	Type        string          `json:"type"` // "function" | "web_search" | "local_shell" etc.
+	Name        string          `json:"name,omitempty"`
+	Description string          `json:"description,omitempty"`
+	Parameters  json.RawMessage `json:"parameters,omitempty"`
+	Strict      *bool           `json:"strict,omitempty"`
+}
+
+// ResponsesResponse is the non-streaming response from POST /v1/responses.
+type ResponsesResponse struct {
+	ID     string            `json:"id"`
+	Object string            `json:"object"` // "response"
+	Model  string            `json:"model"`
+	Status string            `json:"status"` // "completed" | "incomplete" | "failed"
+	Output []ResponsesOutput `json:"output"`
+	Usage  *ResponsesUsage   `json:"usage,omitempty"`
+
+	// incomplete_details is present when status="incomplete"
+	IncompleteDetails *ResponsesIncompleteDetails `json:"incomplete_details,omitempty"`
+}
+
+// ResponsesIncompleteDetails explains why a response is incomplete.
+type ResponsesIncompleteDetails struct {
+	Reason string `json:"reason"` // "max_output_tokens" | "content_filter"
+}
+
+// ResponsesOutput is one output item in a Responses API response.
+type ResponsesOutput struct {
+	Type string `json:"type"` // "message" | "reasoning" | "function_call"
+
+	// type=message
+	ID      string                 `json:"id,omitempty"`
+	Role    string                 `json:"role,omitempty"`
+	Content []ResponsesContentPart `json:"content,omitempty"`
+	Status  string                 `json:"status,omitempty"`
+
+	// type=reasoning
+	EncryptedContent string             `json:"encrypted_content,omitempty"`
+	Summary          []ResponsesSummary `json:"summary,omitempty"`
+
+	// type=function_call
+	CallID    string `json:"call_id,omitempty"`
+	Name      string `json:"name,omitempty"`
+	Arguments string `json:"arguments,omitempty"`
+}
+
+// ResponsesSummary is a summary text block inside a reasoning output.
+type ResponsesSummary struct {
+	Type string `json:"type"` // "summary_text"
+	Text string `json:"text"`
+}
+
+// ResponsesUsage holds token counts in Responses API format.
+type ResponsesUsage struct {
+	InputTokens  int `json:"input_tokens"`
+	OutputTokens int `json:"output_tokens"`
+	TotalTokens  int `json:"total_tokens"`
+
+	// Optional detailed breakdown
+	InputTokensDetails  *ResponsesInputTokensDetails  `json:"input_tokens_details,omitempty"`
+	OutputTokensDetails *ResponsesOutputTokensDetails `json:"output_tokens_details,omitempty"`
+}
+
+// ---------------------------------------------------------------------------
+// Responses SSE event types
+// ---------------------------------------------------------------------------
+
+// ResponsesStreamEvent is a single SSE event in the Responses streaming protocol.
+// The Type field corresponds to the "type" in the JSON payload.
+type ResponsesStreamEvent struct {
+	Type string `json:"type"`
+
+	// response.created / response.completed / response.failed / response.incomplete
+	Response *ResponsesResponse `json:"response,omitempty"`
+
+	// response.output_item.added / response.output_item.done
+	Item *ResponsesOutput `json:"item,omitempty"`
+
+	// response.output_text.delta / response.output_text.done
+	OutputIndex  int    `json:"output_index,omitempty"`
+	ContentIndex int    `json:"content_index,omitempty"`
+	Delta        string `json:"delta,omitempty"`
+	Text         string `json:"text,omitempty"`
+	ItemID       string `json:"item_id,omitempty"`
+
+	// response.function_call_arguments.delta / done
+	CallID    string `json:"call_id,omitempty"`
+	Name      string `json:"name,omitempty"`
+	Arguments string `json:"arguments,omitempty"`
+
+	// response.reasoning_summary_text.delta / done
+	// Reuses Text/Delta fields above, SummaryIndex identifies which summary part
+	SummaryIndex int `json:"summary_index,omitempty"`
+
+	// error event fields
+	Code  string `json:"code,omitempty"`
+	Param string `json:"param,omitempty"`
+
+	// Sequence number for ordering events
+	SequenceNumber int `json:"sequence_number,omitempty"`
+}
+
+// ResponsesOutputReasoning is a reasoning output item in the Responses API.
+// This type represents the "type":"reasoning" output item that contains
+// extended thinking from the model.
+type ResponsesOutputReasoning struct {
+	ID               string                      `json:"id,omitempty"`
+	Type             string                      `json:"type"`             // "reasoning"
+	Status           string                      `json:"status,omitempty"` // "in_progress" | "completed" | "incomplete"
+	EncryptedContent string                      `json:"encrypted_content,omitempty"`
+	Summary          []ResponsesReasoningSummary `json:"summary,omitempty"`
+}
+
+// ResponsesReasoningSummary is a summary text block inside a reasoning output.
+type ResponsesReasoningSummary struct {
+	Type string `json:"type"` // "summary_text"
+	Text string `json:"text"`
+}
+
+// ResponsesStreamState maintains the state for converting Responses streaming
+// events to Chat Completions format. It tracks content blocks, tool calls,
+// reasoning blocks, and other streaming artifacts.
+type ResponsesStreamState struct {
+	// Response metadata
+	ID      string
+	Model   string
+	Created int64
+
+	// Content tracking
+	ContentIndex  int
+	CurrentText   string
+	CurrentItemID string
+	PendingText   []string // Text to accumulate before emitting
+
+	// Tool call tracking
+	ToolCalls       []ResponsesToolCallState
+	CurrentToolCall *ResponsesToolCallState
+
+	// Reasoning tracking
+	ReasoningBlocks  []ResponsesReasoningState
+	CurrentReasoning *ResponsesReasoningState
+
+	// Usage tracking
+	InputTokens  int
+	OutputTokens int
+
+	// Status tracking
+	Status       string
+	FinishReason string
+}
+
+// ResponsesToolCallState tracks a single tool call during streaming.
+type ResponsesToolCallState struct {
+	Index      int
+	ItemID     string
+	CallID     string
+	Name       string
+	Arguments  string
+	Status     string
+	IsComplete bool
+}
+
+// ResponsesReasoningState tracks a reasoning block during streaming.
+type ResponsesReasoningState struct {
+	ItemID       string
+	SummaryIndex int
+	SummaryText  string
+	Status       string
+	IsComplete   bool
+}
+
+// ResponsesUsageDetail provides additional token usage details in Responses format.
+type ResponsesUsageDetail struct {
+	InputTokens  int `json:"input_tokens"`
+	OutputTokens int `json:"output_tokens"`
+	TotalTokens  int `json:"total_tokens"`
+
+	// Optional detailed breakdown
+	InputTokensDetails  *ResponsesInputTokensDetails  `json:"input_tokens_details,omitempty"`
+	OutputTokensDetails *ResponsesOutputTokensDetails `json:"output_tokens_details,omitempty"`
+}
+
+// ResponsesInputTokensDetails breaks down input token usage.
+type ResponsesInputTokensDetails struct {
+	CachedTokens int `json:"cached_tokens,omitempty"`
+}
+
+// ResponsesOutputTokensDetails breaks down output token usage.
+type ResponsesOutputTokensDetails struct {
+	ReasoningTokens int `json:"reasoning_tokens,omitempty"`
+}
+
+// ---------------------------------------------------------------------------
+// Finish reason mapping helpers
+// ---------------------------------------------------------------------------
+
+// ChatFinishToAnthropic maps a Chat Completions finish_reason to an Anthropic stop_reason.
+func ChatFinishToAnthropic(reason string) string {
+	switch reason {
+	case "stop":
+		return "end_turn"
+	case "tool_calls":
+		return "tool_use"
+	case "length":
+		return "max_tokens"
+	default:
+		return "end_turn"
+	}
+}
+
+// AnthropicStopToChat maps an Anthropic stop_reason to a Chat Completions finish_reason.
+func AnthropicStopToChat(reason string) string {
+	switch reason {
+	case "end_turn":
+		return "stop"
+	case "tool_use":
+		return "tool_calls"
+	case "max_tokens":
+		return "length"
+	default:
+		return "stop"
+	}
+}
+
+// ResponsesStatusToChat maps a Responses API status to a Chat Completions finish_reason.
+func ResponsesStatusToChat(status string, details *ResponsesIncompleteDetails) string {
+	switch status {
+	case "completed":
+		return "stop"
+	case "incomplete":
+		if details != nil && details.Reason == "max_output_tokens" {
+			return "length"
+		}
+		return "stop"
+	default:
+		return "stop"
+	}
+}
+
+// ChatFinishToResponsesStatus maps a Chat Completions finish_reason to a Responses status.
+func ChatFinishToResponsesStatus(reason string) string {
+	switch reason {
+	case "length":
+		return "incomplete"
+	default:
+		return "completed"
+	}
+}
+
+// ---------------------------------------------------------------------------
+// Shared constants
+// ---------------------------------------------------------------------------
+
+// minMaxOutputTokens is the floor for max_output_tokens in a Responses request.
+// Very small values may cause upstream API errors, so we enforce a minimum.
+const minMaxOutputTokens = 128

backend/internal/server/routes/gateway.go

@@ -43,8 +43,28 @@ func RegisterGatewayRoutes(
 	gateway.Use(gin.HandlerFunc(apiKeyAuth))
 	gateway.Use(requireGroupAnthropic)
 	{
-		gateway.POST("/messages", h.Gateway.Messages)
+		// /v1/messages: auto-route based on group platform
-		gateway.POST("/messages/count_tokens", h.Gateway.CountTokens)
+		gateway.POST("/messages", func(c *gin.Context) {
+			if getGroupPlatform(c) == service.PlatformOpenAI {
+				h.OpenAIGateway.Messages(c)
+				return
+			}
+			h.Gateway.Messages(c)
+		})
+		// /v1/messages/count_tokens: OpenAI groups get 404
+		gateway.POST("/messages/count_tokens", func(c *gin.Context) {
+			if getGroupPlatform(c) == service.PlatformOpenAI {
+				c.JSON(http.StatusNotFound, gin.H{
+					"type": "error",
+					"error": gin.H{
+						"type":    "not_found_error",
+						"message": "Token counting is not supported for this platform",
+					},
+				})
+				return
+			}
+			h.Gateway.CountTokens(c)
+		})
 		gateway.GET("/models", h.Gateway.Models)
 		gateway.GET("/usage", h.Gateway.Usage)
 		// OpenAI Responses API
@@ -132,3 +152,12 @@ func RegisterGatewayRoutes(
 	// Sora 媒体代理（签名 URL，无需 API Key）
 	r.GET("/sora/media-signed/*filepath", h.SoraGateway.MediaProxySigned)
 }
+
+// getGroupPlatform extracts the group platform from the API Key stored in context.
+func getGroupPlatform(c *gin.Context) string {
+	apiKey, ok := middleware.GetAPIKeyFromContext(c)
+	if !ok || apiKey.Group == nil {
+		return ""
+	}
+	return apiKey.Group.Platform
+}

backend/internal/service/gateway_service.go

@@ -5421,6 +5421,11 @@ func extractUpstreamErrorMessage(body []byte) string {
 		return m
 	}
 
+	// ChatGPT 内部 API 风格：{"detail":"..."}
+	if d := gjson.GetBytes(body, "detail").String(); strings.TrimSpace(d) != "" {
+		return d
+	}
+
 	// 兜底：尝试顶层 message
 	return gjson.GetBytes(body, "message").String()
 }

backend/internal/service/openai_gateway_messages.go

@@ -0,0 +1,376 @@
+package service
+
+import (
+	"bufio"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/pkg/apicompat"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/logger"
+	"github.com/Wei-Shaw/sub2api/internal/util/responseheaders"
+	"github.com/gin-gonic/gin"
+	"go.uber.org/zap"
+)
+
+// ForwardAsAnthropic accepts an Anthropic Messages request body, converts it
+// to OpenAI Responses API format, forwards to the OpenAI upstream, and converts
+// the response back to Anthropic Messages format. This enables Claude Code
+// clients to access OpenAI models through the standard /v1/messages endpoint.
+func (s *OpenAIGatewayService) ForwardAsAnthropic(
+	ctx context.Context,
+	c *gin.Context,
+	account *Account,
+	body []byte,
+	promptCacheKey string,
+) (*OpenAIForwardResult, error) {
+	startTime := time.Now()
+
+	// 1. Parse Anthropic request
+	var anthropicReq apicompat.AnthropicRequest
+	if err := json.Unmarshal(body, &anthropicReq); err != nil {
+		return nil, fmt.Errorf("parse anthropic request: %w", err)
+	}
+	originalModel := anthropicReq.Model
+	isStream := anthropicReq.Stream
+
+	// 2. Convert Anthropic → Responses
+	responsesReq, err := apicompat.AnthropicToResponses(&anthropicReq)
+	if err != nil {
+		return nil, fmt.Errorf("convert anthropic to responses: %w", err)
+	}
+
+	// 3. Model mapping
+	mappedModel := account.GetMappedModel(originalModel)
+	responsesReq.Model = mappedModel
+
+	logger.L().Info("openai messages: model mapping applied",
+		zap.Int64("account_id", account.ID),
+		zap.String("original_model", originalModel),
+		zap.String("mapped_model", mappedModel),
+		zap.Bool("stream", isStream),
+	)
+
+	// 4. Marshal Responses request body, then apply OAuth codex transform
+	responsesBody, err := json.Marshal(responsesReq)
+	if err != nil {
+		return nil, fmt.Errorf("marshal responses request: %w", err)
+	}
+
+	if account.Type == AccountTypeOAuth {
+		var reqBody map[string]any
+		if err := json.Unmarshal(responsesBody, &reqBody); err != nil {
+			return nil, fmt.Errorf("unmarshal for codex transform: %w", err)
+		}
+		applyCodexOAuthTransform(reqBody, false)
+		// OAuth codex transform forces stream=true upstream, so always use
+		// the streaming response handler regardless of what the client asked.
+		isStream = true
+		responsesBody, err = json.Marshal(reqBody)
+		if err != nil {
+			return nil, fmt.Errorf("remarshal after codex transform: %w", err)
+		}
+	}
+
+	// 5. Get access token
+	token, _, err := s.GetAccessToken(ctx, account)
+	if err != nil {
+		return nil, fmt.Errorf("get access token: %w", err)
+	}
+
+	// 6. Build upstream request
+	upstreamReq, err := s.buildUpstreamRequest(ctx, c, account, responsesBody, token, isStream, promptCacheKey, false)
+	if err != nil {
+		return nil, fmt.Errorf("build upstream request: %w", err)
+	}
+
+	// 7. Send request
+	proxyURL := ""
+	if account.Proxy != nil {
+		proxyURL = account.Proxy.URL()
+	}
+	resp, err := s.httpUpstream.Do(upstreamReq, proxyURL, account.ID, account.Concurrency)
+	if err != nil {
+		safeErr := sanitizeUpstreamErrorMessage(err.Error())
+		setOpsUpstreamError(c, 0, safeErr, "")
+		appendOpsUpstreamError(c, OpsUpstreamErrorEvent{
+			Platform:           account.Platform,
+			AccountID:          account.ID,
+			AccountName:        account.Name,
+			UpstreamStatusCode: 0,
+			Kind:               "request_error",
+			Message:            safeErr,
+		})
+		writeAnthropicError(c, http.StatusBadGateway, "api_error", "Upstream request failed")
+		return nil, fmt.Errorf("upstream request failed: %s", safeErr)
+	}
+	defer func() { _ = resp.Body.Close() }()
+
+	// 8. Handle error response with failover
+	if resp.StatusCode >= 400 {
+		if s.shouldFailoverUpstreamError(resp.StatusCode) {
+			respBody, _ := io.ReadAll(io.LimitReader(resp.Body, 2<<20))
+			_ = resp.Body.Close()
+
+			upstreamMsg := strings.TrimSpace(extractUpstreamErrorMessage(respBody))
+			upstreamMsg = sanitizeUpstreamErrorMessage(upstreamMsg)
+			upstreamDetail := ""
+			if s.cfg != nil && s.cfg.Gateway.LogUpstreamErrorBody {
+				maxBytes := s.cfg.Gateway.LogUpstreamErrorBodyMaxBytes
+				if maxBytes <= 0 {
+					maxBytes = 2048
+				}
+				upstreamDetail = truncateString(string(respBody), maxBytes)
+			}
+			appendOpsUpstreamError(c, OpsUpstreamErrorEvent{
+				Platform:           account.Platform,
+				AccountID:          account.ID,
+				AccountName:        account.Name,
+				UpstreamStatusCode: resp.StatusCode,
+				UpstreamRequestID:  resp.Header.Get("x-request-id"),
+				Kind:               "failover",
+				Message:            upstreamMsg,
+				Detail:             upstreamDetail,
+			})
+			s.rateLimitService.HandleUpstreamError(ctx, account, resp.StatusCode, resp.Header, respBody)
+			return nil, &UpstreamFailoverError{StatusCode: resp.StatusCode, ResponseBody: respBody}
+		}
+		// Non-failover error: return Anthropic-formatted error to client
+		return s.handleAnthropicErrorResponse(resp, c)
+	}
+
+	// 9. Handle normal response
+	if isStream {
+		return s.handleAnthropicStreamingResponse(resp, c, originalModel, startTime)
+	}
+	return s.handleAnthropicNonStreamingResponse(resp, c, originalModel, startTime)
+}
+
+// handleAnthropicErrorResponse reads an upstream error and returns it in
+// Anthropic error format.
+func (s *OpenAIGatewayService) handleAnthropicErrorResponse(
+	resp *http.Response,
+	c *gin.Context,
+) (*OpenAIForwardResult, error) {
+	body, _ := io.ReadAll(io.LimitReader(resp.Body, 2<<20))
+
+	upstreamMsg := strings.TrimSpace(extractUpstreamErrorMessage(body))
+	if upstreamMsg == "" {
+		upstreamMsg = fmt.Sprintf("Upstream error: %d", resp.StatusCode)
+	}
+	upstreamMsg = sanitizeUpstreamErrorMessage(upstreamMsg)
+
+	// Record upstream error details for ops logging
+	upstreamDetail := ""
+	if s.cfg != nil && s.cfg.Gateway.LogUpstreamErrorBody {
+		maxBytes := s.cfg.Gateway.LogUpstreamErrorBodyMaxBytes
+		if maxBytes <= 0 {
+			maxBytes = 2048
+		}
+		upstreamDetail = truncateString(string(body), maxBytes)
+	}
+	setOpsUpstreamError(c, resp.StatusCode, upstreamMsg, upstreamDetail)
+
+	errType := "api_error"
+	switch {
+	case resp.StatusCode == 400:
+		errType = "invalid_request_error"
+	case resp.StatusCode == 404:
+		errType = "not_found_error"
+	case resp.StatusCode == 429:
+		errType = "rate_limit_error"
+	case resp.StatusCode >= 500:
+		errType = "api_error"
+	}
+
+	writeAnthropicError(c, resp.StatusCode, errType, upstreamMsg)
+	return nil, fmt.Errorf("upstream error: %d %s", resp.StatusCode, upstreamMsg)
+}
+
+// handleAnthropicNonStreamingResponse reads a Responses API JSON response,
+// converts it to Anthropic Messages format, and writes it to the client.
+func (s *OpenAIGatewayService) handleAnthropicNonStreamingResponse(
+	resp *http.Response,
+	c *gin.Context,
+	originalModel string,
+	startTime time.Time,
+) (*OpenAIForwardResult, error) {
+	requestID := resp.Header.Get("x-request-id")
+
+	respBody, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("read upstream response: %w", err)
+	}
+
+	var responsesResp apicompat.ResponsesResponse
+	if err := json.Unmarshal(respBody, &responsesResp); err != nil {
+		return nil, fmt.Errorf("parse responses response: %w", err)
+	}
+
+	anthropicResp := apicompat.ResponsesToAnthropic(&responsesResp, originalModel)
+
+	var usage OpenAIUsage
+	if responsesResp.Usage != nil {
+		usage = OpenAIUsage{
+			InputTokens:  responsesResp.Usage.InputTokens,
+			OutputTokens: responsesResp.Usage.OutputTokens,
+		}
+		if responsesResp.Usage.InputTokensDetails != nil {
+			usage.CacheReadInputTokens = responsesResp.Usage.InputTokensDetails.CachedTokens
+		}
+	}
+
+	if s.responseHeaderFilter != nil {
+		responseheaders.WriteFilteredHeaders(c.Writer.Header(), resp.Header, s.responseHeaderFilter)
+	}
+	c.JSON(http.StatusOK, anthropicResp)
+
+	return &OpenAIForwardResult{
+		RequestID: requestID,
+		Usage:     usage,
+		Model:     originalModel,
+		Stream:    false,
+		Duration:  time.Since(startTime),
+	}, nil
+}
+
+// handleAnthropicStreamingResponse reads Responses SSE events from upstream,
+// converts each to Anthropic SSE events, and writes them to the client.
+func (s *OpenAIGatewayService) handleAnthropicStreamingResponse(
+	resp *http.Response,
+	c *gin.Context,
+	originalModel string,
+	startTime time.Time,
+) (*OpenAIForwardResult, error) {
+	requestID := resp.Header.Get("x-request-id")
+
+	if s.responseHeaderFilter != nil {
+		responseheaders.WriteFilteredHeaders(c.Writer.Header(), resp.Header, s.responseHeaderFilter)
+	}
+	c.Writer.Header().Set("Content-Type", "text/event-stream")
+	c.Writer.Header().Set("Cache-Control", "no-cache")
+	c.Writer.Header().Set("Connection", "keep-alive")
+	c.Writer.WriteHeader(http.StatusOK)
+
+	state := apicompat.NewResponsesEventToAnthropicState()
+	state.Model = originalModel
+	var usage OpenAIUsage
+	var firstTokenMs *int
+	firstChunk := true
+
+	scanner := bufio.NewScanner(resp.Body)
+	scanner.Buffer(make([]byte, 0, 64*1024), 1024*1024)
+
+	for scanner.Scan() {
+		line := scanner.Text()
+
+		if !strings.HasPrefix(line, "data: ") || line == "data: [DONE]" {
+			continue
+		}
+		payload := line[6:]
+
+		if firstChunk {
+			firstChunk = false
+			ms := int(time.Since(startTime).Milliseconds())
+			firstTokenMs = &ms
+		}
+
+		// Parse the Responses SSE event
+		var event apicompat.ResponsesStreamEvent
+		if err := json.Unmarshal([]byte(payload), &event); err != nil {
+			logger.L().Warn("openai messages stream: failed to parse event",
+				zap.Error(err),
+				zap.String("request_id", requestID),
+			)
+			continue
+		}
+
+		// Extract usage from completion events
+		if (event.Type == "response.completed" || event.Type == "response.incomplete") &&
+			event.Response != nil && event.Response.Usage != nil {
+			usage = OpenAIUsage{
+				InputTokens:  event.Response.Usage.InputTokens,
+				OutputTokens: event.Response.Usage.OutputTokens,
+			}
+			if event.Response.Usage.InputTokensDetails != nil {
+				usage.CacheReadInputTokens = event.Response.Usage.InputTokensDetails.CachedTokens
+			}
+		}
+
+		// Convert to Anthropic events
+		events := apicompat.ResponsesEventToAnthropicEvents(&event, state)
+		for _, evt := range events {
+			sse, err := apicompat.ResponsesAnthropicEventToSSE(evt)
+			if err != nil {
+				logger.L().Warn("openai messages stream: failed to marshal event",
+					zap.Error(err),
+					zap.String("request_id", requestID),
+				)
+				continue
+			}
+			if _, err := fmt.Fprint(c.Writer, sse); err != nil {
+				// Client disconnected — return collected usage
+				logger.L().Info("openai messages stream: client disconnected",
+					zap.String("request_id", requestID),
+				)
+				return &OpenAIForwardResult{
+					RequestID:    requestID,
+					Usage:        usage,
+					Model:        originalModel,
+					Stream:       true,
+					Duration:     time.Since(startTime),
+					FirstTokenMs: firstTokenMs,
+				}, nil
+			}
+		}
+		if len(events) > 0 {
+			c.Writer.Flush()
+		}
+	}
+
+	if err := scanner.Err(); err != nil {
+		if !errors.Is(err, context.Canceled) && !errors.Is(err, context.DeadlineExceeded) {
+			logger.L().Warn("openai messages stream: read error",
+				zap.Error(err),
+				zap.String("request_id", requestID),
+			)
+		}
+	}
+
+	// Ensure the Anthropic stream is properly terminated
+	if finalEvents := apicompat.FinalizeResponsesAnthropicStream(state); len(finalEvents) > 0 {
+		for _, evt := range finalEvents {
+			sse, err := apicompat.ResponsesAnthropicEventToSSE(evt)
+			if err != nil {
+				continue
+			}
+			fmt.Fprint(c.Writer, sse) //nolint:errcheck
+		}
+		c.Writer.Flush()
+	}
+
+	return &OpenAIForwardResult{
+		RequestID:    requestID,
+		Usage:        usage,
+		Model:        originalModel,
+		Stream:       true,
+		Duration:     time.Since(startTime),
+		FirstTokenMs: firstTokenMs,
+	}, nil
+}
+
+// writeAnthropicError writes an error response in Anthropic Messages API format.
+func writeAnthropicError(c *gin.Context, statusCode int, errType, message string) {
+	c.JSON(statusCode, gin.H{
+		"type": "error",
+		"error": gin.H{
+			"type":    errType,
+			"message": message,
+		},
+	})
+}

backend/internal/service/openai_gateway_service.go

@@ -888,6 +888,22 @@ func isOpenAIInstructionsRequiredError(upstreamStatusCode int, upstreamMsg strin
 	return false
 }
 
+// ExtractSessionID extracts the raw session ID from headers or body without hashing.
+// Used by ForwardAsAnthropic to pass as prompt_cache_key for upstream cache.
+func (s *OpenAIGatewayService) ExtractSessionID(c *gin.Context, body []byte) string {
+	if c == nil {
+		return ""
+	}
+	sessionID := strings.TrimSpace(c.GetHeader("session_id"))
+	if sessionID == "" {
+		sessionID = strings.TrimSpace(c.GetHeader("conversation_id"))
+	}
+	if sessionID == "" && len(body) > 0 {
+		sessionID = strings.TrimSpace(gjson.GetBytes(body, "prompt_cache_key").String())
+	}
+	return sessionID
+}
+
 // GenerateSessionHash generates a sticky-session hash for OpenAI requests.
 //
 // Priority:

frontend/src/components/account/CreateAccountModal.vue

@@ -1230,6 +1230,142 @@
       <!-- API Key 账号配额限制 -->
       <QuotaLimitCard v-if="form.type === 'apikey'" v-model="editQuotaLimit" />
 
+      <!-- OpenAI OAuth Model Mapping (OAuth 类型没有 apikey 容器，需要独立的模型映射区域) -->
+      <div
+        v-if="form.platform === 'openai' && accountCategory === 'oauth-based'"
+        class="border-t border-gray-200 pt-4 dark:border-dark-600"
+      >
+        <label class="input-label">{{ t('admin.accounts.modelRestriction') }}</label>
+
+        <div
+          v-if="isOpenAIModelRestrictionDisabled"
+          class="mb-3 rounded-lg bg-amber-50 p-3 dark:bg-amber-900/20"
+        >
+          <p class="text-xs text-amber-700 dark:text-amber-400">
+            {{ t('admin.accounts.openai.modelRestrictionDisabledByPassthrough') }}
+          </p>
+        </div>
+
+        <template v-else>
+          <!-- Mode Toggle -->
+          <div class="mb-4 flex gap-2">
+            <button
+              type="button"
+              @click="modelRestrictionMode = 'whitelist'"
+              :class="[
+                'flex-1 rounded-lg px-4 py-2 text-sm font-medium transition-all',
+                modelRestrictionMode === 'whitelist'
+                  ? 'bg-primary-100 text-primary-700 dark:bg-primary-900/30 dark:text-primary-400'
+                  : 'bg-gray-100 text-gray-600 hover:bg-gray-200 dark:bg-dark-600 dark:text-gray-400 dark:hover:bg-dark-500'
+              ]"
+            >
+              {{ t('admin.accounts.modelWhitelist') }}
+            </button>
+            <button
+              type="button"
+              @click="modelRestrictionMode = 'mapping'"
+              :class="[
+                'flex-1 rounded-lg px-4 py-2 text-sm font-medium transition-all',
+                modelRestrictionMode === 'mapping'
+                  ? 'bg-purple-100 text-purple-700 dark:bg-purple-900/30 dark:text-purple-400'
+                  : 'bg-gray-100 text-gray-600 hover:bg-gray-200 dark:bg-dark-600 dark:text-gray-400 dark:hover:bg-dark-500'
+              ]"
+            >
+              {{ t('admin.accounts.modelMapping') }}
+            </button>
+          </div>
+
+          <!-- Whitelist Mode -->
+          <div v-if="modelRestrictionMode === 'whitelist'">
+            <ModelWhitelistSelector v-model="allowedModels" :platform="form.platform" />
+            <p class="text-xs text-gray-500 dark:text-gray-400">
+              {{ t('admin.accounts.selectedModels', { count: allowedModels.length }) }}
+              <span v-if="allowedModels.length === 0">{{
+                t('admin.accounts.supportsAllModels')
+              }}</span>
+            </p>
+          </div>
+
+          <!-- Mapping Mode -->
+          <div v-else>
+            <div class="mb-3 rounded-lg bg-purple-50 p-3 dark:bg-purple-900/20">
+              <p class="text-xs text-purple-700 dark:text-purple-400">
+                {{ t('admin.accounts.mapRequestModels') }}
+              </p>
+            </div>
+
+            <div v-if="modelMappings.length > 0" class="mb-3 space-y-2">
+              <div
+                v-for="(mapping, index) in modelMappings"
+                :key="'oauth-' + getModelMappingKey(mapping)"
+                class="flex items-center gap-2"
+              >
+                <input
+                  v-model="mapping.from"
+                  type="text"
+                  class="input flex-1"
+                  :placeholder="t('admin.accounts.requestModel')"
+                />
+                <svg
+                  class="h-4 w-4 flex-shrink-0 text-gray-400"
+                  fill="none"
+                  viewBox="0 0 24 24"
+                  stroke="currentColor"
+                >
+                  <path
+                    stroke-linecap="round"
+                    stroke-linejoin="round"
+                    stroke-width="2"
+                    d="M14 5l7 7m0 0l-7 7m7-7H3"
+                  />
+                </svg>
+                <input
+                  v-model="mapping.to"
+                  type="text"
+                  class="input flex-1"
+                  :placeholder="t('admin.accounts.actualModel')"
+                />
+                <button
+                  type="button"
+                  @click="removeModelMapping(index)"
+                  class="rounded-lg p-2 text-red-500 transition-colors hover:bg-red-50 hover:text-red-600 dark:hover:bg-red-900/20"
+                >
+                  <svg class="h-4 w-4" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+                    <path
+                      stroke-linecap="round"
+                      stroke-linejoin="round"
+                      stroke-width="2"
+                      d="M19 7l-.867 12.142A2 2 0 0116.138 21H7.862a2 2 0 01-1.995-1.858L5 7m5 4v6m4-6v6m1-10V4a1 1 0 00-1-1h-4a1 1 0 00-1 1v3M4 7h16"
+                    />
+                  </svg>
+                </button>
+              </div>
+            </div>
+
+            <button
+              type="button"
+              @click="addModelMapping"
+              class="mb-3 w-full rounded-lg border-2 border-dashed border-gray-300 px-4 py-2 text-gray-600 transition-colors hover:border-gray-400 hover:text-gray-700 dark:border-dark-500 dark:text-gray-400 dark:hover:border-dark-400 dark:hover:text-gray-300"
+            >
+              + {{ t('admin.accounts.addMapping') }}
+            </button>
+
+            <!-- Quick Add Buttons -->
+            <div class="flex flex-wrap gap-2">
+              <button
+                v-for="preset in presetMappings"
+                :key="'oauth-' + preset.label"
+                type="button"
+                @click="addPresetMapping(preset.from, preset.to)"
+                :class="['rounded-lg px-3 py-1 text-xs transition-colors', preset.color]"
+              >
+                + {{ preset.label }}
+              </button>
+            </div>
+          </div>
+        </template>
+      </div>
+
       <!-- Temp Unschedulable Rules -->
       <div class="border-t border-gray-200 pt-4 dark:border-dark-600 space-y-4">
         <div class="mb-3 flex items-center justify-between">
@@ -3603,6 +3739,14 @@ const handleOpenAIExchange = async (authCode: string) => {
     const shouldCreateOpenAI = form.platform === 'openai'
     const shouldCreateSora = form.platform === 'sora'
 
+    // Add model mapping for OpenAI OAuth accounts（透传模式下不应用）
+    if (shouldCreateOpenAI && !isOpenAIModelRestrictionDisabled.value) {
+      const modelMapping = buildModelMappingObject(modelRestrictionMode.value, allowedModels.value, modelMappings.value)
+      if (modelMapping) {
+        credentials.model_mapping = modelMapping
+      }
+    }
+
     // 应用临时不可调度配置
     if (!applyTempUnschedConfig(credentials)) {
       return
@@ -3713,6 +3857,14 @@ const handleOpenAIValidateRT = async (refreshTokenInput: string) => {
         const oauthExtra = oauthClient.buildExtraInfo(tokenInfo) as Record<string, unknown> | undefined
         const extra = buildOpenAIExtra(oauthExtra)
 
+        // Add model mapping for OpenAI OAuth accounts（透传模式下不应用）
+        if (shouldCreateOpenAI && !isOpenAIModelRestrictionDisabled.value) {
+          const modelMapping = buildModelMappingObject(modelRestrictionMode.value, allowedModels.value, modelMappings.value)
+          if (modelMapping) {
+            credentials.model_mapping = modelMapping
+          }
+        }
+
         // Generate account name with index for batch
         const accountName = refreshTokens.length > 1 ? `${form.name} #${i + 1}` : form.name
 

frontend/src/components/account/EditAccountModal.vue

@@ -351,6 +351,142 @@
 
       </div>
 
+      <!-- OpenAI OAuth Model Mapping (OAuth 类型没有 apikey 容器，需要独立的模型映射区域) -->
+      <div
+        v-if="account.platform === 'openai' && account.type === 'oauth'"
+        class="border-t border-gray-200 pt-4 dark:border-dark-600"
+      >
+        <label class="input-label">{{ t('admin.accounts.modelRestriction') }}</label>
+
+        <div
+          v-if="isOpenAIModelRestrictionDisabled"
+          class="mb-3 rounded-lg bg-amber-50 p-3 dark:bg-amber-900/20"
+        >
+          <p class="text-xs text-amber-700 dark:text-amber-400">
+            {{ t('admin.accounts.openai.modelRestrictionDisabledByPassthrough') }}
+          </p>
+        </div>
+
+        <template v-else>
+          <!-- Mode Toggle -->
+          <div class="mb-4 flex gap-2">
+            <button
+              type="button"
+              @click="modelRestrictionMode = 'whitelist'"
+              :class="[
+                'flex-1 rounded-lg px-4 py-2 text-sm font-medium transition-all',
+                modelRestrictionMode === 'whitelist'
+                  ? 'bg-primary-100 text-primary-700 dark:bg-primary-900/30 dark:text-primary-400'
+                  : 'bg-gray-100 text-gray-600 hover:bg-gray-200 dark:bg-dark-600 dark:text-gray-400 dark:hover:bg-dark-500'
+              ]"
+            >
+              {{ t('admin.accounts.modelWhitelist') }}
+            </button>
+            <button
+              type="button"
+              @click="modelRestrictionMode = 'mapping'"
+              :class="[
+                'flex-1 rounded-lg px-4 py-2 text-sm font-medium transition-all',
+                modelRestrictionMode === 'mapping'
+                  ? 'bg-purple-100 text-purple-700 dark:bg-purple-900/30 dark:text-purple-400'
+                  : 'bg-gray-100 text-gray-600 hover:bg-gray-200 dark:bg-dark-600 dark:text-gray-400 dark:hover:bg-dark-500'
+              ]"
+            >
+              {{ t('admin.accounts.modelMapping') }}
+            </button>
+          </div>
+
+          <!-- Whitelist Mode -->
+          <div v-if="modelRestrictionMode === 'whitelist'">
+            <ModelWhitelistSelector v-model="allowedModels" :platform="account?.platform || 'anthropic'" />
+            <p class="text-xs text-gray-500 dark:text-gray-400">
+              {{ t('admin.accounts.selectedModels', { count: allowedModels.length }) }}
+              <span v-if="allowedModels.length === 0">{{
+                t('admin.accounts.supportsAllModels')
+              }}</span>
+            </p>
+          </div>
+
+          <!-- Mapping Mode -->
+          <div v-else>
+            <div class="mb-3 rounded-lg bg-purple-50 p-3 dark:bg-purple-900/20">
+              <p class="text-xs text-purple-700 dark:text-purple-400">
+                {{ t('admin.accounts.mapRequestModels') }}
+              </p>
+            </div>
+
+            <div v-if="modelMappings.length > 0" class="mb-3 space-y-2">
+              <div
+                v-for="(mapping, index) in modelMappings"
+                :key="'oauth-' + getModelMappingKey(mapping)"
+                class="flex items-center gap-2"
+              >
+                <input
+                  v-model="mapping.from"
+                  type="text"
+                  class="input flex-1"
+                  :placeholder="t('admin.accounts.requestModel')"
+                />
+                <svg
+                  class="h-4 w-4 flex-shrink-0 text-gray-400"
+                  fill="none"
+                  viewBox="0 0 24 24"
+                  stroke="currentColor"
+                >
+                  <path
+                    stroke-linecap="round"
+                    stroke-linejoin="round"
+                    stroke-width="2"
+                    d="M14 5l7 7m0 0l-7 7m7-7H3"
+                  />
+                </svg>
+                <input
+                  v-model="mapping.to"
+                  type="text"
+                  class="input flex-1"
+                  :placeholder="t('admin.accounts.actualModel')"
+                />
+                <button
+                  type="button"
+                  @click="removeModelMapping(index)"
+                  class="rounded-lg p-2 text-red-500 transition-colors hover:bg-red-50 hover:text-red-600 dark:hover:bg-red-900/20"
+                >
+                  <svg class="h-4 w-4" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+                    <path
+                      stroke-linecap="round"
+                      stroke-linejoin="round"
+                      stroke-width="2"
+                      d="M19 7l-.867 12.142A2 2 0 0116.138 21H7.862a2 2 0 01-1.995-1.858L5 7m5 4v6m4-6v6m1-10V4a1 1 0 00-1-1h-4a1 1 0 00-1 1v3M4 7h16"
+                    />
+                  </svg>
+                </button>
+              </div>
+            </div>
+
+            <button
+              type="button"
+              @click="addModelMapping"
+              class="mb-3 w-full rounded-lg border-2 border-dashed border-gray-300 px-4 py-2 text-gray-600 transition-colors hover:border-gray-400 hover:text-gray-700 dark:border-dark-500 dark:text-gray-400 dark:hover:border-dark-400 dark:hover:text-gray-300"
+            >
+              + {{ t('admin.accounts.addMapping') }}
+            </button>
+
+            <!-- Quick Add Buttons -->
+            <div class="flex flex-wrap gap-2">
+              <button
+                v-for="preset in presetMappings"
+                :key="'oauth-' + preset.label"
+                type="button"
+                @click="addPresetMapping(preset.from, preset.to)"
+                :class="['rounded-lg px-3 py-1 text-xs transition-colors', preset.color]"
+              >
+                + {{ preset.label }}
+              </button>
+            </div>
+          </div>
+        </template>
+      </div>
+
       <!-- Upstream fields (only for upstream type) -->
       <div v-if="account.type === 'upstream'" class="space-y-4">
         <div>
@@ -1659,9 +1795,33 @@ watch(
               ? 'https://generativelanguage.googleapis.com'
               : 'https://api.anthropic.com'
         editBaseUrl.value = platformDefaultUrl
-        modelRestrictionMode.value = 'whitelist'
+
-        modelMappings.value = []
+        // Load model mappings for OpenAI OAuth accounts
-        allowedModels.value = []
+        if (newAccount.platform === 'openai' && newAccount.credentials) {
+          const oauthCredentials = newAccount.credentials as Record<string, unknown>
+          const existingMappings = oauthCredentials.model_mapping as Record<string, string> | undefined
+          if (existingMappings && typeof existingMappings === 'object') {
+            const entries = Object.entries(existingMappings)
+            const isWhitelistMode = entries.length > 0 && entries.every(([from, to]) => from === to)
+            if (isWhitelistMode) {
+              modelRestrictionMode.value = 'whitelist'
+              allowedModels.value = entries.map(([from]) => from)
+              modelMappings.value = []
+            } else {
+              modelRestrictionMode.value = 'mapping'
+              modelMappings.value = entries.map(([from, to]) => ({ from, to }))
+              allowedModels.value = []
+            }
+          } else {
+            modelRestrictionMode.value = 'whitelist'
+            modelMappings.value = []
+            allowedModels.value = []
+          }
+        } else {
+          modelRestrictionMode.value = 'whitelist'
+          modelMappings.value = []
+          allowedModels.value = []
+        }
         customErrorCodesEnabled.value = false
         selectedErrorCodes.value = []
       }
@@ -2163,6 +2323,28 @@ const handleSubmit = async () => {
       updatePayload.credentials = newCredentials
     }
 
+    // OpenAI OAuth: persist model mapping to credentials
+    if (props.account.platform === 'openai' && props.account.type === 'oauth') {
+      const currentCredentials = (updatePayload.credentials as Record<string, unknown>) ||
+        ((props.account.credentials as Record<string, unknown>) || {})
+      const newCredentials: Record<string, unknown> = { ...currentCredentials }
+      const shouldApplyModelMapping = !openaiPassthroughEnabled.value
+
+      if (shouldApplyModelMapping) {
+        const modelMapping = buildModelMappingObject(modelRestrictionMode.value, allowedModels.value, modelMappings.value)
+        if (modelMapping) {
+          newCredentials.model_mapping = modelMapping
+        } else {
+          delete newCredentials.model_mapping
+        }
+      } else if (currentCredentials.model_mapping) {
+        // 透传模式保留现有映射
+        newCredentials.model_mapping = currentCredentials.model_mapping
+      }
+
+      updatePayload.credentials = newCredentials
+    }
+
     // Antigravity: persist model mapping to credentials (applies to all antigravity types)
     // Antigravity 只支持映射模式
     if (props.account.platform === 'antigravity') {