diff --git a/backend/internal/handler/openai_chat_completions.go b/backend/internal/handler/openai_chat_completions.go
new file mode 100644
index 000000000..30e2c6b9c
--- /dev/null
+++ b/backend/internal/handler/openai_chat_completions.go
@@ -0,0 +1,530 @@
+package handler
+
+import (
+	"bytes"
+	"crypto/rand"
+	"encoding/hex"
+	"encoding/json"
+	"io"
+	"net/http"
+	"strings"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/gin-gonic/gin"
+)
+
+// ChatCompletions handles OpenAI Chat Completions API compatibility.
+// POST /v1/chat/completions
+func (h *OpenAIGatewayHandler) ChatCompletions(c *gin.Context) {
+	body, err := io.ReadAll(c.Request.Body)
+	if err != nil {
+		if maxErr, ok := extractMaxBytesError(err); ok {
+			h.errorResponse(c, http.StatusRequestEntityTooLarge, "invalid_request_error", buildBodyTooLargeMessage(maxErr.Limit))
+			return
+		}
+		h.errorResponse(c, http.StatusBadRequest, "invalid_request_error", "Failed to read request body")
+		return
+	}
+	if len(body) == 0 {
+		h.errorResponse(c, http.StatusBadRequest, "invalid_request_error", "Request body is empty")
+		return
+	}
+
+	// Preserve original chat-completions request for upstream passthrough when needed.
+	c.Set(service.OpenAIChatCompletionsBodyKey, body)
+
+	var chatReq map[string]any
+	if err := json.Unmarshal(body, &chatReq); err != nil {
+		h.errorResponse(c, http.StatusBadRequest, "invalid_request_error", "Failed to parse request body")
+		return
+	}
+
+	includeUsage := false
+	if streamOptions, ok := chatReq["stream_options"].(map[string]any); ok {
+		if v, ok := streamOptions["include_usage"].(bool); ok {
+			includeUsage = v
+		}
+	}
+	c.Set(service.OpenAIChatCompletionsIncludeUsageKey, includeUsage)
+
+	converted, err := service.ConvertChatCompletionsToResponses(chatReq)
+	if err != nil {
+		h.errorResponse(c, http.StatusBadRequest, "invalid_request_error", err.Error())
+		return
+	}
+
+	convertedBody, err := json.Marshal(converted)
+	if err != nil {
+		h.errorResponse(c, http.StatusInternalServerError, "api_error", "Failed to process request")
+		return
+	}
+
+	stream, _ := converted["stream"].(bool)
+	model, _ := converted["model"].(string)
+	writer := newChatCompletionsResponseWriter(c.Writer, stream, includeUsage, model)
+	c.Writer = writer
+	c.Request.Body = io.NopCloser(bytes.NewReader(convertedBody))
+	c.Request.ContentLength = int64(len(convertedBody))
+
+	h.Responses(c)
+	writer.Finalize()
+}
+
+type chatCompletionsResponseWriter struct {
+	gin.ResponseWriter
+	stream       bool
+	includeUsage bool
+	buffer       bytes.Buffer
+	streamBuf    bytes.Buffer
+	state        *chatCompletionStreamState
+	corrector    *service.CodexToolCorrector
+	finalized    bool
+	passthrough  bool
+}
+
+type chatCompletionStreamState struct {
+	id            string
+	model         string
+	created       int64
+	sentRole      bool
+	sawToolCall   bool
+	sawText       bool
+	toolCallIndex map[string]int
+	usage         map[string]any
+}
+
+func newChatCompletionsResponseWriter(w gin.ResponseWriter, stream bool, includeUsage bool, model string) *chatCompletionsResponseWriter {
+	return &chatCompletionsResponseWriter{
+		ResponseWriter: w,
+		stream:         stream,
+		includeUsage:   includeUsage,
+		state: &chatCompletionStreamState{
+			model:         strings.TrimSpace(model),
+			toolCallIndex: make(map[string]int),
+		},
+		corrector: service.NewCodexToolCorrector(),
+	}
+}
+
+func (w *chatCompletionsResponseWriter) Write(data []byte) (int, error) {
+	if w.passthrough {
+		return w.ResponseWriter.Write(data)
+	}
+	if w.stream {
+		n, err := w.streamBuf.Write(data)
+		if err != nil {
+			return n, err
+		}
+		w.flushStreamBuffer()
+		return n, nil
+	}
+
+	if w.finalized {
+		return len(data), nil
+	}
+	return w.buffer.Write(data)
+}
+
+func (w *chatCompletionsResponseWriter) WriteString(s string) (int, error) {
+	return w.Write([]byte(s))
+}
+
+func (w *chatCompletionsResponseWriter) Finalize() {
+	if w.finalized {
+		return
+	}
+	w.finalized = true
+	if w.passthrough {
+		return
+	}
+	if w.stream {
+		return
+	}
+
+	body := w.buffer.Bytes()
+	if len(body) == 0 {
+		return
+	}
+
+	w.ResponseWriter.Header().Del("Content-Length")
+
+	converted, err := service.ConvertResponsesToChatCompletion(body)
+	if err != nil {
+		_, _ = w.ResponseWriter.Write(body)
+		return
+	}
+
+	corrected := converted
+	if correctedStr, ok := w.corrector.CorrectToolCallsInSSEData(string(converted)); ok {
+		corrected = []byte(correctedStr)
+	}
+
+	_, _ = w.ResponseWriter.Write(corrected)
+}
+
+func (w *chatCompletionsResponseWriter) SetPassthrough() {
+	w.passthrough = true
+}
+
+func (w *chatCompletionsResponseWriter) flushStreamBuffer() {
+	for {
+		buf := w.streamBuf.Bytes()
+		idx := bytes.IndexByte(buf, '\n')
+		if idx == -1 {
+			return
+		}
+		lineBytes := w.streamBuf.Next(idx + 1)
+		line := strings.TrimRight(string(lineBytes), "\r\n")
+		w.handleStreamLine(line)
+	}
+}
+
+func (w *chatCompletionsResponseWriter) handleStreamLine(line string) {
+	if line == "" {
+		return
+	}
+	if strings.HasPrefix(line, ":") {
+		_, _ = w.ResponseWriter.Write([]byte(line + "\n\n"))
+		return
+	}
+	if !strings.HasPrefix(line, "data:") {
+		return
+	}
+
+	data := strings.TrimSpace(strings.TrimPrefix(line, "data:"))
+	for _, chunk := range w.convertResponseDataToChatChunks(data) {
+		if chunk == "" {
+			continue
+		}
+		if chunk == "[DONE]" {
+			_, _ = w.ResponseWriter.Write([]byte("data: [DONE]\n\n"))
+			continue
+		}
+		_, _ = w.ResponseWriter.Write([]byte("data: " + chunk + "\n\n"))
+	}
+}
+
+func (w *chatCompletionsResponseWriter) convertResponseDataToChatChunks(data string) []string {
+	if data == "" {
+		return nil
+	}
+	if data == "[DONE]" {
+		return []string{"[DONE]"}
+	}
+
+	var payload map[string]any
+	if err := json.Unmarshal([]byte(data), &payload); err != nil {
+		return []string{data}
+	}
+
+	if _, ok := payload["error"]; ok {
+		return []string{data}
+	}
+
+	eventType := strings.TrimSpace(getString(payload["type"]))
+	if eventType == "" {
+		return []string{data}
+	}
+
+	w.state.applyMetadata(payload)
+
+	switch eventType {
+	case "response.created":
+		return nil
+	case "response.output_text.delta":
+		delta := getString(payload["delta"])
+		if delta == "" {
+			return nil
+		}
+		w.state.sawText = true
+		return []string{w.buildTextDeltaChunk(delta)}
+	case "response.output_text.done":
+		if w.state.sawText {
+			return nil
+		}
+		text := getString(payload["text"])
+		if text == "" {
+			return nil
+		}
+		w.state.sawText = true
+		return []string{w.buildTextDeltaChunk(text)}
+	case "response.output_item.added", "response.output_item.delta":
+		if item, ok := payload["item"].(map[string]any); ok {
+			if callID, name, args, ok := extractToolCallFromItem(item); ok {
+				w.state.sawToolCall = true
+				return []string{w.buildToolCallChunk(callID, name, args)}
+			}
+		}
+	case "response.completed", "response.done":
+		if responseObj, ok := payload["response"].(map[string]any); ok {
+			w.state.applyResponseUsage(responseObj)
+		}
+		return []string{w.buildFinalChunk()}
+	}
+
+	if strings.Contains(eventType, "tool_call") || strings.Contains(eventType, "function_call") {
+		callID := strings.TrimSpace(getString(payload["call_id"]))
+		if callID == "" {
+			callID = strings.TrimSpace(getString(payload["tool_call_id"]))
+		}
+		if callID == "" {
+			callID = strings.TrimSpace(getString(payload["id"]))
+		}
+		args := getString(payload["delta"])
+		name := strings.TrimSpace(getString(payload["name"]))
+		if callID != "" && (args != "" || name != "") {
+			w.state.sawToolCall = true
+			return []string{w.buildToolCallChunk(callID, name, args)}
+		}
+	}
+
+	return nil
+}
+
+func (w *chatCompletionsResponseWriter) buildTextDeltaChunk(delta string) string {
+	w.state.ensureDefaults()
+	payload := map[string]any{
+		"content": delta,
+	}
+	if !w.state.sentRole {
+		payload["role"] = "assistant"
+		w.state.sentRole = true
+	}
+	return w.buildChunk(payload, nil, nil)
+}
+
+func (w *chatCompletionsResponseWriter) buildToolCallChunk(callID, name, args string) string {
+	w.state.ensureDefaults()
+	index := w.state.toolCallIndexFor(callID)
+	function := map[string]any{}
+	if name != "" {
+		function["name"] = name
+	}
+	if args != "" {
+		function["arguments"] = args
+	}
+	toolCall := map[string]any{
+		"index":    index,
+		"id":       callID,
+		"type":     "function",
+		"function": function,
+	}
+
+	delta := map[string]any{
+		"tool_calls": []any{toolCall},
+	}
+	if !w.state.sentRole {
+		delta["role"] = "assistant"
+		w.state.sentRole = true
+	}
+
+	return w.buildChunk(delta, nil, nil)
+}
+
+func (w *chatCompletionsResponseWriter) buildFinalChunk() string {
+	w.state.ensureDefaults()
+	finishReason := "stop"
+	if w.state.sawToolCall {
+		finishReason = "tool_calls"
+	}
+	usage := map[string]any(nil)
+	if w.includeUsage && w.state.usage != nil {
+		usage = w.state.usage
+	}
+	return w.buildChunk(map[string]any{}, finishReason, usage)
+}
+
+func (w *chatCompletionsResponseWriter) buildChunk(delta map[string]any, finishReason any, usage map[string]any) string {
+	w.state.ensureDefaults()
+	chunk := map[string]any{
+		"id":      w.state.id,
+		"object":  "chat.completion.chunk",
+		"created": w.state.created,
+		"model":   w.state.model,
+		"choices": []any{
+			map[string]any{
+				"index":         0,
+				"delta":         delta,
+				"finish_reason": finishReason,
+			},
+		},
+	}
+	if usage != nil {
+		chunk["usage"] = usage
+	}
+
+	data, _ := json.Marshal(chunk)
+	if corrected, ok := w.corrector.CorrectToolCallsInSSEData(string(data)); ok {
+		return corrected
+	}
+	return string(data)
+}
+
+func (s *chatCompletionStreamState) ensureDefaults() {
+	if s.id == "" {
+		s.id = "chatcmpl-" + randomHexUnsafe(12)
+	}
+	if s.model == "" {
+		s.model = "unknown"
+	}
+	if s.created == 0 {
+		s.created = time.Now().Unix()
+	}
+}
+
+func (s *chatCompletionStreamState) toolCallIndexFor(callID string) int {
+	if idx, ok := s.toolCallIndex[callID]; ok {
+		return idx
+	}
+	idx := len(s.toolCallIndex)
+	s.toolCallIndex[callID] = idx
+	return idx
+}
+
+func (s *chatCompletionStreamState) applyMetadata(payload map[string]any) {
+	if responseObj, ok := payload["response"].(map[string]any); ok {
+		s.applyResponseMetadata(responseObj)
+	}
+
+	if s.id == "" {
+		if id := strings.TrimSpace(getString(payload["response_id"])); id != "" {
+			s.id = id
+		} else if id := strings.TrimSpace(getString(payload["id"])); id != "" {
+			s.id = id
+		}
+	}
+	if s.model == "" {
+		if model := strings.TrimSpace(getString(payload["model"])); model != "" {
+			s.model = model
+		}
+	}
+	if s.created == 0 {
+		if created := getInt64(payload["created_at"]); created != 0 {
+			s.created = created
+		} else if created := getInt64(payload["created"]); created != 0 {
+			s.created = created
+		}
+	}
+}
+
+func (s *chatCompletionStreamState) applyResponseMetadata(responseObj map[string]any) {
+	if s.id == "" {
+		if id := strings.TrimSpace(getString(responseObj["id"])); id != "" {
+			s.id = id
+		}
+	}
+	if s.model == "" {
+		if model := strings.TrimSpace(getString(responseObj["model"])); model != "" {
+			s.model = model
+		}
+	}
+	if s.created == 0 {
+		if created := getInt64(responseObj["created_at"]); created != 0 {
+			s.created = created
+		}
+	}
+}
+
+func (s *chatCompletionStreamState) applyResponseUsage(responseObj map[string]any) {
+	usage, ok := responseObj["usage"].(map[string]any)
+	if !ok {
+		return
+	}
+	promptTokens := int(getNumber(usage["input_tokens"]))
+	completionTokens := int(getNumber(usage["output_tokens"]))
+	if promptTokens == 0 && completionTokens == 0 {
+		return
+	}
+	s.usage = map[string]any{
+		"prompt_tokens":     promptTokens,
+		"completion_tokens": completionTokens,
+		"total_tokens":      promptTokens + completionTokens,
+	}
+}
+
+func extractToolCallFromItem(item map[string]any) (string, string, string, bool) {
+	itemType := strings.TrimSpace(getString(item["type"]))
+	if itemType != "tool_call" && itemType != "function_call" {
+		return "", "", "", false
+	}
+	callID := strings.TrimSpace(getString(item["call_id"]))
+	if callID == "" {
+		callID = strings.TrimSpace(getString(item["id"]))
+	}
+	name := strings.TrimSpace(getString(item["name"]))
+	args := getString(item["arguments"])
+	if fn, ok := item["function"].(map[string]any); ok {
+		if name == "" {
+			name = strings.TrimSpace(getString(fn["name"]))
+		}
+		if args == "" {
+			args = getString(fn["arguments"])
+		}
+	}
+	if callID == "" && name == "" && args == "" {
+		return "", "", "", false
+	}
+	if callID == "" {
+		callID = "call_" + randomHexUnsafe(6)
+	}
+	return callID, name, args, true
+}
+
+func getString(value any) string {
+	switch v := value.(type) {
+	case string:
+		return v
+	case []byte:
+		return string(v)
+	case json.Number:
+		return v.String()
+	default:
+		return ""
+	}
+}
+
+func getNumber(value any) float64 {
+	switch v := value.(type) {
+	case float64:
+		return v
+	case float32:
+		return float64(v)
+	case int:
+		return float64(v)
+	case int64:
+		return float64(v)
+	case json.Number:
+		f, _ := v.Float64()
+		return f
+	default:
+		return 0
+	}
+}
+
+func getInt64(value any) int64 {
+	switch v := value.(type) {
+	case int64:
+		return v
+	case int:
+		return int64(v)
+	case float64:
+		return int64(v)
+	case json.Number:
+		i, _ := v.Int64()
+		return i
+	default:
+		return 0
+	}
+}
+
+func randomHexUnsafe(byteLength int) string {
+	if byteLength <= 0 {
+		byteLength = 8
+	}
+	buf := make([]byte, byteLength)
+	if _, err := rand.Read(buf); err != nil {
+		return "000000"
+	}
+	return hex.EncodeToString(buf)
+}
diff --git a/backend/internal/server/routes/gateway.go b/backend/internal/server/routes/gateway.go
index 930c8b9ee..613943287 100644
--- a/backend/internal/server/routes/gateway.go
+++ b/backend/internal/server/routes/gateway.go
@@ -1,8 +1,6 @@
 package routes
 
 import (
-	"net/http"
-
 	"github.com/Wei-Shaw/sub2api/internal/config"
 	"github.com/Wei-Shaw/sub2api/internal/handler"
 	"github.com/Wei-Shaw/sub2api/internal/server/middleware"
@@ -43,15 +41,8 @@ func RegisterGatewayRoutes(
 		gateway.GET("/usage", h.Gateway.Usage)
 		// OpenAI Responses API
 		gateway.POST("/responses", h.OpenAIGateway.Responses)
-		// 明确阻止旧协议入口：OpenAI 仅支持 Responses API，避免客户端误解为会自动路由到其它平台。
-		gateway.POST("/chat/completions", func(c *gin.Context) {
-			c.JSON(http.StatusBadRequest, gin.H{
-				"error": gin.H{
-					"type":    "invalid_request_error",
-					"message": "Unsupported legacy protocol: /v1/chat/completions is not supported. Please use /v1/responses.",
-				},
-			})
-		})
+		// OpenAI Chat Completions API
+		gateway.POST("/chat/completions", h.OpenAIGateway.ChatCompletions)
 	}
 
 	// Gemini 原生 API 兼容层（Gemini SDK/CLI 直连）
@@ -69,6 +60,8 @@ func RegisterGatewayRoutes(
 
 	// OpenAI Responses API（不带v1前缀的别名）
 	r.POST("/responses", bodyLimit, clientRequestID, opsErrorLogger, gin.HandlerFunc(apiKeyAuth), h.OpenAIGateway.Responses)
+	// OpenAI Chat Completions API（不带v1前缀的别名）
+	r.POST("/chat/completions", bodyLimit, clientRequestID, opsErrorLogger, gin.HandlerFunc(apiKeyAuth), h.OpenAIGateway.ChatCompletions)
 
 	// Antigravity 模型列表
 	r.GET("/antigravity/models", gin.HandlerFunc(apiKeyAuth), h.Gateway.AntigravityModels)
diff --git a/backend/internal/service/openai_chat_completions.go b/backend/internal/service/openai_chat_completions.go
new file mode 100644
index 000000000..c4c95ff22
--- /dev/null
+++ b/backend/internal/service/openai_chat_completions.go
@@ -0,0 +1,513 @@
+package service
+
+import (
+	"encoding/json"
+	"errors"
+	"strings"
+	"time"
+)
+
+// ConvertChatCompletionsToResponses converts an OpenAI Chat Completions request to a Responses request.
+func ConvertChatCompletionsToResponses(req map[string]any) (map[string]any, error) {
+	if req == nil {
+		return nil, errors.New("request is nil")
+	}
+
+	model := strings.TrimSpace(getString(req["model"]))
+	if model == "" {
+		return nil, errors.New("model is required")
+	}
+
+	messagesRaw, ok := req["messages"]
+	if !ok {
+		return nil, errors.New("messages is required")
+	}
+	messages, ok := messagesRaw.([]any)
+	if !ok {
+		return nil, errors.New("messages must be an array")
+	}
+
+	input, err := convertChatMessagesToResponsesInput(messages)
+	if err != nil {
+		return nil, err
+	}
+
+	out := make(map[string]any, len(req)+1)
+	for key, value := range req {
+		switch key {
+		case "messages", "max_tokens", "max_completion_tokens", "stream_options", "functions", "function_call":
+			continue
+		default:
+			out[key] = value
+		}
+	}
+
+	out["model"] = model
+	out["input"] = input
+
+	if _, ok := out["max_output_tokens"]; !ok {
+		if v, ok := req["max_tokens"]; ok {
+			out["max_output_tokens"] = v
+		} else if v, ok := req["max_completion_tokens"]; ok {
+			out["max_output_tokens"] = v
+		}
+	}
+
+	if _, ok := out["tools"]; !ok {
+		if functions, ok := req["functions"].([]any); ok && len(functions) > 0 {
+			tools := make([]any, 0, len(functions))
+			for _, fn := range functions {
+				if fnMap, ok := fn.(map[string]any); ok {
+					tools = append(tools, map[string]any{
+						"type":     "function",
+						"function": fnMap,
+					})
+				}
+			}
+			if len(tools) > 0 {
+				out["tools"] = tools
+			}
+		}
+	}
+
+	if _, ok := out["tool_choice"]; !ok {
+		if functionCall, ok := req["function_call"]; ok {
+			out["tool_choice"] = functionCall
+		}
+	}
+
+	return out, nil
+}
+
+// ConvertResponsesToChatCompletion converts an OpenAI Responses response body to Chat Completions format.
+func ConvertResponsesToChatCompletion(body []byte) ([]byte, error) {
+	var resp map[string]any
+	if err := json.Unmarshal(body, &resp); err != nil {
+		return nil, err
+	}
+
+	id := strings.TrimSpace(getString(resp["id"]))
+	if id == "" {
+		id = "chatcmpl-" + safeRandomHex(12)
+	}
+	model := strings.TrimSpace(getString(resp["model"]))
+
+	created := getInt64(resp["created_at"])
+	if created == 0 {
+		created = getInt64(resp["created"])
+	}
+	if created == 0 {
+		created = time.Now().Unix()
+	}
+
+	text, toolCalls := extractResponseTextAndToolCalls(resp)
+	finishReason := "stop"
+	if len(toolCalls) > 0 {
+		finishReason = "tool_calls"
+	}
+
+	message := map[string]any{
+		"role":    "assistant",
+		"content": text,
+	}
+	if len(toolCalls) > 0 {
+		message["tool_calls"] = toolCalls
+	}
+
+	chatResp := map[string]any{
+		"id":      id,
+		"object":  "chat.completion",
+		"created": created,
+		"model":   model,
+		"choices": []any{
+			map[string]any{
+				"index":         0,
+				"message":       message,
+				"finish_reason": finishReason,
+			},
+		},
+	}
+
+	if usage := extractResponseUsage(resp); usage != nil {
+		chatResp["usage"] = usage
+	}
+	if fingerprint := strings.TrimSpace(getString(resp["system_fingerprint"])); fingerprint != "" {
+		chatResp["system_fingerprint"] = fingerprint
+	}
+
+	return json.Marshal(chatResp)
+}
+
+func convertChatMessagesToResponsesInput(messages []any) ([]any, error) {
+	input := make([]any, 0, len(messages))
+	for _, msg := range messages {
+		msgMap, ok := msg.(map[string]any)
+		if !ok {
+			return nil, errors.New("message must be an object")
+		}
+		role := strings.TrimSpace(getString(msgMap["role"]))
+		if role == "" {
+			return nil, errors.New("message role is required")
+		}
+
+		switch role {
+		case "tool":
+			callID := strings.TrimSpace(getString(msgMap["tool_call_id"]))
+			if callID == "" {
+				callID = strings.TrimSpace(getString(msgMap["id"]))
+			}
+			output := extractMessageContentText(msgMap["content"])
+			input = append(input, map[string]any{
+				"type":    "function_call_output",
+				"call_id": callID,
+				"output":  output,
+			})
+		case "function":
+			callID := strings.TrimSpace(getString(msgMap["name"]))
+			output := extractMessageContentText(msgMap["content"])
+			input = append(input, map[string]any{
+				"type":    "function_call_output",
+				"call_id": callID,
+				"output":  output,
+			})
+		default:
+			convertedContent := convertChatContent(msgMap["content"])
+			toolCalls := []any(nil)
+			if role == "assistant" {
+				toolCalls = extractToolCallsFromMessage(msgMap)
+			}
+			skipAssistantMessage := role == "assistant" && len(toolCalls) > 0 && isEmptyContent(convertedContent)
+			if !skipAssistantMessage {
+				msgItem := map[string]any{
+					"role":    role,
+					"content": convertedContent,
+				}
+				if name := strings.TrimSpace(getString(msgMap["name"])); name != "" {
+					msgItem["name"] = name
+				}
+				input = append(input, msgItem)
+			}
+			if role == "assistant" && len(toolCalls) > 0 {
+				input = append(input, toolCalls...)
+			}
+		}
+	}
+	return input, nil
+}
+
+func convertChatContent(content any) any {
+	switch v := content.(type) {
+	case nil:
+		return ""
+	case string:
+		return v
+	case []any:
+		converted := make([]any, 0, len(v))
+		for _, part := range v {
+			partMap, ok := part.(map[string]any)
+			if !ok {
+				converted = append(converted, part)
+				continue
+			}
+			partType := strings.TrimSpace(getString(partMap["type"]))
+			switch partType {
+			case "text":
+				text := getString(partMap["text"])
+				if text != "" {
+					converted = append(converted, map[string]any{
+						"type": "input_text",
+						"text": text,
+					})
+					continue
+				}
+			case "image_url":
+				imageURL := ""
+				if imageObj, ok := partMap["image_url"].(map[string]any); ok {
+					imageURL = getString(imageObj["url"])
+				} else {
+					imageURL = getString(partMap["image_url"])
+				}
+				if imageURL != "" {
+					converted = append(converted, map[string]any{
+						"type":      "input_image",
+						"image_url": imageURL,
+					})
+					continue
+				}
+			case "input_text", "input_image":
+				converted = append(converted, partMap)
+				continue
+			}
+			converted = append(converted, partMap)
+		}
+		return converted
+	default:
+		return v
+	}
+}
+
+func extractToolCallsFromMessage(msg map[string]any) []any {
+	var out []any
+	if toolCalls, ok := msg["tool_calls"].([]any); ok {
+		for _, call := range toolCalls {
+			callMap, ok := call.(map[string]any)
+			if !ok {
+				continue
+			}
+			callID := strings.TrimSpace(getString(callMap["id"]))
+			if callID == "" {
+				callID = strings.TrimSpace(getString(callMap["call_id"]))
+			}
+			name := ""
+			args := ""
+			if fn, ok := callMap["function"].(map[string]any); ok {
+				name = strings.TrimSpace(getString(fn["name"]))
+				args = getString(fn["arguments"])
+			}
+			if name == "" && args == "" {
+				continue
+			}
+			item := map[string]any{
+				"type": "tool_call",
+			}
+			if callID != "" {
+				item["call_id"] = callID
+			}
+			if name != "" {
+				item["name"] = name
+			}
+			if args != "" {
+				item["arguments"] = args
+			}
+			out = append(out, item)
+		}
+	}
+
+	if fnCall, ok := msg["function_call"].(map[string]any); ok {
+		name := strings.TrimSpace(getString(fnCall["name"]))
+		args := getString(fnCall["arguments"])
+		if name != "" || args != "" {
+			callID := strings.TrimSpace(getString(msg["tool_call_id"]))
+			if callID == "" {
+				callID = name
+			}
+			item := map[string]any{
+				"type": "function_call",
+			}
+			if callID != "" {
+				item["call_id"] = callID
+			}
+			if name != "" {
+				item["name"] = name
+			}
+			if args != "" {
+				item["arguments"] = args
+			}
+			out = append(out, item)
+		}
+	}
+
+	return out
+}
+
+func extractMessageContentText(content any) string {
+	switch v := content.(type) {
+	case string:
+		return v
+	case []any:
+		parts := make([]string, 0, len(v))
+		for _, part := range v {
+			partMap, ok := part.(map[string]any)
+			if !ok {
+				continue
+			}
+			partType := strings.TrimSpace(getString(partMap["type"]))
+			if partType == "" || partType == "text" || partType == "output_text" || partType == "input_text" {
+				text := getString(partMap["text"])
+				if text != "" {
+					parts = append(parts, text)
+				}
+			}
+		}
+		return strings.Join(parts, "")
+	default:
+		return ""
+	}
+}
+
+func isEmptyContent(content any) bool {
+	switch v := content.(type) {
+	case nil:
+		return true
+	case string:
+		return strings.TrimSpace(v) == ""
+	case []any:
+		return len(v) == 0
+	default:
+		return false
+	}
+}
+
+func extractResponseTextAndToolCalls(resp map[string]any) (string, []any) {
+	output, ok := resp["output"].([]any)
+	if !ok {
+		if text, ok := resp["output_text"].(string); ok {
+			return text, nil
+		}
+		return "", nil
+	}
+
+	textParts := make([]string, 0)
+	toolCalls := make([]any, 0)
+
+	for _, item := range output {
+		itemMap, ok := item.(map[string]any)
+		if !ok {
+			continue
+		}
+		itemType := strings.TrimSpace(getString(itemMap["type"]))
+
+		if itemType == "tool_call" || itemType == "function_call" {
+			if tc := responseItemToChatToolCall(itemMap); tc != nil {
+				toolCalls = append(toolCalls, tc)
+			}
+			continue
+		}
+
+		content := itemMap["content"]
+		switch v := content.(type) {
+		case string:
+			if v != "" {
+				textParts = append(textParts, v)
+			}
+		case []any:
+			for _, part := range v {
+				partMap, ok := part.(map[string]any)
+				if !ok {
+					continue
+				}
+				partType := strings.TrimSpace(getString(partMap["type"]))
+				switch partType {
+				case "output_text", "text", "input_text":
+					text := getString(partMap["text"])
+					if text != "" {
+						textParts = append(textParts, text)
+					}
+				case "tool_call", "function_call":
+					if tc := responseItemToChatToolCall(partMap); tc != nil {
+						toolCalls = append(toolCalls, tc)
+					}
+				}
+			}
+		}
+	}
+
+	return strings.Join(textParts, ""), toolCalls
+}
+
+func responseItemToChatToolCall(item map[string]any) map[string]any {
+	callID := strings.TrimSpace(getString(item["call_id"]))
+	if callID == "" {
+		callID = strings.TrimSpace(getString(item["id"]))
+	}
+	name := strings.TrimSpace(getString(item["name"]))
+	arguments := getString(item["arguments"])
+	if fn, ok := item["function"].(map[string]any); ok {
+		if name == "" {
+			name = strings.TrimSpace(getString(fn["name"]))
+		}
+		if arguments == "" {
+			arguments = getString(fn["arguments"])
+		}
+	}
+
+	if name == "" && arguments == "" && callID == "" {
+		return nil
+	}
+
+	if callID == "" {
+		callID = "call_" + safeRandomHex(6)
+	}
+
+	return map[string]any{
+		"id":   callID,
+		"type": "function",
+		"function": map[string]any{
+			"name":      name,
+			"arguments": arguments,
+		},
+	}
+}
+
+func extractResponseUsage(resp map[string]any) map[string]any {
+	usage, ok := resp["usage"].(map[string]any)
+	if !ok {
+		return nil
+	}
+	promptTokens := int(getNumber(usage["input_tokens"]))
+	completionTokens := int(getNumber(usage["output_tokens"]))
+	if promptTokens == 0 && completionTokens == 0 {
+		return nil
+	}
+
+	return map[string]any{
+		"prompt_tokens":     promptTokens,
+		"completion_tokens": completionTokens,
+		"total_tokens":      promptTokens + completionTokens,
+	}
+}
+
+func getString(value any) string {
+	switch v := value.(type) {
+	case string:
+		return v
+	case []byte:
+		return string(v)
+	case json.Number:
+		return v.String()
+	default:
+		return ""
+	}
+}
+
+func getNumber(value any) float64 {
+	switch v := value.(type) {
+	case float64:
+		return v
+	case float32:
+		return float64(v)
+	case int:
+		return float64(v)
+	case int64:
+		return float64(v)
+	case json.Number:
+		f, _ := v.Float64()
+		return f
+	default:
+		return 0
+	}
+}
+
+func getInt64(value any) int64 {
+	switch v := value.(type) {
+	case int64:
+		return v
+	case int:
+		return int64(v)
+	case float64:
+		return int64(v)
+	case json.Number:
+		i, _ := v.Int64()
+		return i
+	default:
+		return 0
+	}
+}
+
+func safeRandomHex(byteLength int) string {
+	value, err := randomHexString(byteLength)
+	if err != nil || value == "" {
+		return "000000"
+	}
+	return value
+}
diff --git a/backend/internal/service/openai_chat_completions_forward.go b/backend/internal/service/openai_chat_completions_forward.go
new file mode 100644
index 000000000..703f3af17
--- /dev/null
+++ b/backend/internal/service/openai_chat_completions_forward.go
@@ -0,0 +1,486 @@
+package service
+
+import (
+	"bufio"
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"strings"
+	"sync/atomic"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/util/responseheaders"
+	"github.com/gin-gonic/gin"
+)
+
+type chatStreamingResult struct {
+	usage        *OpenAIUsage
+	firstTokenMs *int
+}
+
+func (s *OpenAIGatewayService) forwardChatCompletions(ctx context.Context, c *gin.Context, account *Account, body []byte, includeUsage bool, startTime time.Time) (*OpenAIForwardResult, error) {
+	// Parse request body once (avoid multiple parse/serialize cycles)
+	var reqBody map[string]any
+	if err := json.Unmarshal(body, &reqBody); err != nil {
+		return nil, fmt.Errorf("parse request: %w", err)
+	}
+
+	reqModel, _ := reqBody["model"].(string)
+	reqStream, _ := reqBody["stream"].(bool)
+	originalModel := reqModel
+
+	bodyModified := false
+	mappedModel := account.GetMappedModel(reqModel)
+	if mappedModel != reqModel {
+		log.Printf("[OpenAI Chat] Model mapping applied: %s -> %s (account: %s)", reqModel, mappedModel, account.Name)
+		reqBody["model"] = mappedModel
+		bodyModified = true
+	}
+
+	if reqStream && includeUsage {
+		streamOptions, _ := reqBody["stream_options"].(map[string]any)
+		if streamOptions == nil {
+			streamOptions = map[string]any{}
+		}
+		if _, ok := streamOptions["include_usage"]; !ok {
+			streamOptions["include_usage"] = true
+			reqBody["stream_options"] = streamOptions
+			bodyModified = true
+		}
+	}
+
+	if bodyModified {
+		var err error
+		body, err = json.Marshal(reqBody)
+		if err != nil {
+			return nil, fmt.Errorf("serialize request body: %w", err)
+		}
+	}
+
+	// Get access token
+	token, _, err := s.GetAccessToken(ctx, account)
+	if err != nil {
+		return nil, err
+	}
+
+	upstreamReq, err := s.buildChatCompletionsRequest(ctx, c, account, body, token)
+	if err != nil {
+		return nil, err
+	}
+
+	proxyURL := ""
+	if account.ProxyID != nil && account.Proxy != nil {
+		proxyURL = account.Proxy.URL()
+	}
+
+	if c != nil {
+		c.Set(OpsUpstreamRequestBodyKey, string(body))
+	}
+
+	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,
+		})
+		c.JSON(http.StatusBadGateway, gin.H{
+			"error": gin.H{
+				"type":    "upstream_error",
+				"message": "Upstream request failed",
+			},
+		})
+		return nil, fmt.Errorf("upstream request failed: %s", safeErr)
+	}
+	defer func() { _ = resp.Body.Close() }()
+
+	if resp.StatusCode >= 400 {
+		if s.shouldFailoverUpstreamError(resp.StatusCode) {
+			respBody, _ := io.ReadAll(io.LimitReader(resp.Body, 2<<20))
+			_ = resp.Body.Close()
+			resp.Body = io.NopCloser(bytes.NewReader(respBody))
+
+			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.handleFailoverSideEffects(ctx, resp, account)
+			return nil, &UpstreamFailoverError{StatusCode: resp.StatusCode}
+		}
+		return s.handleErrorResponse(ctx, resp, c, account, body)
+	}
+
+	var usage *OpenAIUsage
+	var firstTokenMs *int
+	if reqStream {
+		streamResult, err := s.handleChatCompletionsStreamingResponse(ctx, resp, c, account, startTime, originalModel, mappedModel)
+		if err != nil {
+			return nil, err
+		}
+		usage = streamResult.usage
+		firstTokenMs = streamResult.firstTokenMs
+	} else {
+		usage, err = s.handleChatCompletionsNonStreamingResponse(resp, c, originalModel, mappedModel)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	if usage == nil {
+		usage = &OpenAIUsage{}
+	}
+
+	return &OpenAIForwardResult{
+		RequestID:    resp.Header.Get("x-request-id"),
+		Usage:        *usage,
+		Model:        originalModel,
+		Stream:       reqStream,
+		Duration:     time.Since(startTime),
+		FirstTokenMs: firstTokenMs,
+	}, nil
+}
+
+func (s *OpenAIGatewayService) buildChatCompletionsRequest(ctx context.Context, c *gin.Context, account *Account, body []byte, token string) (*http.Request, error) {
+	var targetURL string
+	baseURL := account.GetOpenAIBaseURL()
+	if baseURL == "" {
+		targetURL = openaiChatAPIURL
+	} else {
+		validatedURL, err := s.validateUpstreamBaseURL(baseURL)
+		if err != nil {
+			return nil, err
+		}
+		targetURL = validatedURL + "/chat/completions"
+	}
+
+	req, err := http.NewRequestWithContext(ctx, "POST", targetURL, bytes.NewReader(body))
+	if err != nil {
+		return nil, err
+	}
+
+	req.Header.Set("authorization", "Bearer "+token)
+
+	for key, values := range c.Request.Header {
+		lowerKey := strings.ToLower(key)
+		if openaiChatAllowedHeaders[lowerKey] {
+			for _, v := range values {
+				req.Header.Add(key, v)
+			}
+		}
+	}
+
+	customUA := account.GetOpenAIUserAgent()
+	if customUA != "" {
+		req.Header.Set("user-agent", customUA)
+	}
+
+	if req.Header.Get("content-type") == "" {
+		req.Header.Set("content-type", "application/json")
+	}
+
+	return req, nil
+}
+
+func (s *OpenAIGatewayService) handleChatCompletionsStreamingResponse(ctx context.Context, resp *http.Response, c *gin.Context, account *Account, startTime time.Time, originalModel, mappedModel string) (*chatStreamingResult, error) {
+	if s.cfg != nil {
+		responseheaders.WriteFilteredHeaders(c.Writer.Header(), resp.Header, s.cfg.Security.ResponseHeaders)
+	}
+
+	c.Header("Content-Type", "text/event-stream")
+	c.Header("Cache-Control", "no-cache")
+	c.Header("Connection", "keep-alive")
+	c.Header("X-Accel-Buffering", "no")
+
+	if v := resp.Header.Get("x-request-id"); v != "" {
+		c.Header("x-request-id", v)
+	}
+
+	w := c.Writer
+	flusher, ok := w.(http.Flusher)
+	if !ok {
+		return nil, errors.New("streaming not supported")
+	}
+
+	usage := &OpenAIUsage{}
+	var firstTokenMs *int
+
+	scanner := bufio.NewScanner(resp.Body)
+	maxLineSize := defaultMaxLineSize
+	if s.cfg != nil && s.cfg.Gateway.MaxLineSize > 0 {
+		maxLineSize = s.cfg.Gateway.MaxLineSize
+	}
+	scanner.Buffer(make([]byte, 64*1024), maxLineSize)
+
+	type scanEvent struct {
+		line string
+		err  error
+	}
+	events := make(chan scanEvent, 16)
+	done := make(chan struct{})
+	sendEvent := func(ev scanEvent) bool {
+		select {
+		case events <- ev:
+			return true
+		case <-done:
+			return false
+		}
+	}
+	var lastReadAt int64
+	atomic.StoreInt64(&lastReadAt, time.Now().UnixNano())
+	go func() {
+		defer close(events)
+		for scanner.Scan() {
+			atomic.StoreInt64(&lastReadAt, time.Now().UnixNano())
+			if !sendEvent(scanEvent{line: scanner.Text()}) {
+				return
+			}
+		}
+		if err := scanner.Err(); err != nil {
+			_ = sendEvent(scanEvent{err: err})
+		}
+	}()
+	defer close(done)
+
+	streamInterval := time.Duration(0)
+	if s.cfg != nil && s.cfg.Gateway.StreamDataIntervalTimeout > 0 {
+		streamInterval = time.Duration(s.cfg.Gateway.StreamDataIntervalTimeout) * time.Second
+	}
+	var intervalTicker *time.Ticker
+	if streamInterval > 0 {
+		intervalTicker = time.NewTicker(streamInterval)
+		defer intervalTicker.Stop()
+	}
+	var intervalCh <-chan time.Time
+	if intervalTicker != nil {
+		intervalCh = intervalTicker.C
+	}
+
+	keepaliveInterval := time.Duration(0)
+	if s.cfg != nil && s.cfg.Gateway.StreamKeepaliveInterval > 0 {
+		keepaliveInterval = time.Duration(s.cfg.Gateway.StreamKeepaliveInterval) * time.Second
+	}
+	var keepaliveTicker *time.Ticker
+	if keepaliveInterval > 0 {
+		keepaliveTicker = time.NewTicker(keepaliveInterval)
+		defer keepaliveTicker.Stop()
+	}
+	var keepaliveCh <-chan time.Time
+	if keepaliveTicker != nil {
+		keepaliveCh = keepaliveTicker.C
+	}
+	lastDataAt := time.Now()
+
+	errorEventSent := false
+	sendErrorEvent := func(reason string) {
+		if errorEventSent {
+			return
+		}
+		errorEventSent = true
+		_, _ = fmt.Fprintf(w, "event: error\ndata: {\"error\":\"%s\"}\n\n", reason)
+		flusher.Flush()
+	}
+
+	needModelReplace := originalModel != mappedModel
+
+	for {
+		select {
+		case ev, ok := <-events:
+			if !ok {
+				return &chatStreamingResult{usage: usage, firstTokenMs: firstTokenMs}, nil
+			}
+			if ev.err != nil {
+				if errors.Is(ev.err, bufio.ErrTooLong) {
+					log.Printf("SSE line too long: account=%d max_size=%d error=%v", account.ID, maxLineSize, ev.err)
+					sendErrorEvent("response_too_large")
+					return &chatStreamingResult{usage: usage, firstTokenMs: firstTokenMs}, ev.err
+				}
+				sendErrorEvent("stream_read_error")
+				return &chatStreamingResult{usage: usage, firstTokenMs: firstTokenMs}, fmt.Errorf("stream read error: %w", ev.err)
+			}
+
+			line := ev.line
+			lastDataAt = time.Now()
+
+			if openaiSSEDataRe.MatchString(line) {
+				data := openaiSSEDataRe.ReplaceAllString(line, "")
+
+				if needModelReplace {
+					line = s.replaceModelInSSELine(line, mappedModel, originalModel)
+				}
+
+				if correctedData, corrected := s.toolCorrector.CorrectToolCallsInSSEData(data); corrected {
+					line = "data: " + correctedData
+				}
+
+				if _, err := fmt.Fprintf(w, "%s\n", line); err != nil {
+					sendErrorEvent("write_failed")
+					return &chatStreamingResult{usage: usage, firstTokenMs: firstTokenMs}, err
+				}
+				flusher.Flush()
+
+				if firstTokenMs == nil {
+					if event := parseChatStreamEvent(data); event != nil {
+						if chatChunkHasDelta(event) {
+							ms := int(time.Since(startTime).Milliseconds())
+							firstTokenMs = &ms
+						}
+						applyChatUsageFromEvent(event, usage)
+					}
+				} else {
+					if event := parseChatStreamEvent(data); event != nil {
+						applyChatUsageFromEvent(event, usage)
+					}
+				}
+			} else {
+				if _, err := fmt.Fprintf(w, "%s\n", line); err != nil {
+					sendErrorEvent("write_failed")
+					return &chatStreamingResult{usage: usage, firstTokenMs: firstTokenMs}, err
+				}
+				flusher.Flush()
+			}
+
+		case <-intervalCh:
+			lastRead := time.Unix(0, atomic.LoadInt64(&lastReadAt))
+			if time.Since(lastRead) < streamInterval {
+				continue
+			}
+			log.Printf("Stream data interval timeout: account=%d model=%s interval=%s", account.ID, originalModel, streamInterval)
+			if s.rateLimitService != nil {
+				s.rateLimitService.HandleStreamTimeout(ctx, account, originalModel)
+			}
+			sendErrorEvent("stream_timeout")
+			return &chatStreamingResult{usage: usage, firstTokenMs: firstTokenMs}, fmt.Errorf("stream data interval timeout")
+
+		case <-keepaliveCh:
+			if time.Since(lastDataAt) < keepaliveInterval {
+				continue
+			}
+			if _, err := fmt.Fprint(w, ":\n\n"); err != nil {
+				return &chatStreamingResult{usage: usage, firstTokenMs: firstTokenMs}, err
+			}
+			flusher.Flush()
+		}
+	}
+}
+
+func (s *OpenAIGatewayService) handleChatCompletionsNonStreamingResponse(resp *http.Response, c *gin.Context, originalModel, mappedModel string) (*OpenAIUsage, error) {
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, err
+	}
+
+	usage := &OpenAIUsage{}
+	var parsed map[string]any
+	if json.Unmarshal(body, &parsed) == nil {
+		if usageMap, ok := parsed["usage"].(map[string]any); ok {
+			applyChatUsageFromMap(usageMap, usage)
+		}
+	}
+
+	if originalModel != mappedModel {
+		body = s.replaceModelInResponseBody(body, mappedModel, originalModel)
+	}
+	body = s.correctToolCallsInResponseBody(body)
+
+	responseheaders.WriteFilteredHeaders(c.Writer.Header(), resp.Header, s.cfg.Security.ResponseHeaders)
+
+	contentType := "application/json"
+	if s.cfg != nil && !s.cfg.Security.ResponseHeaders.Enabled {
+		if upstreamType := resp.Header.Get("Content-Type"); upstreamType != "" {
+			contentType = upstreamType
+		}
+	}
+
+	c.Data(resp.StatusCode, contentType, body)
+	return usage, nil
+}
+
+func parseChatStreamEvent(data string) map[string]any {
+	if data == "" || data == "[DONE]" {
+		return nil
+	}
+	var event map[string]any
+	if json.Unmarshal([]byte(data), &event) != nil {
+		return nil
+	}
+	return event
+}
+
+func chatChunkHasDelta(event map[string]any) bool {
+	choices, ok := event["choices"].([]any)
+	if !ok {
+		return false
+	}
+	for _, choice := range choices {
+		choiceMap, ok := choice.(map[string]any)
+		if !ok {
+			continue
+		}
+		delta, ok := choiceMap["delta"].(map[string]any)
+		if !ok {
+			continue
+		}
+		if content, ok := delta["content"].(string); ok && strings.TrimSpace(content) != "" {
+			return true
+		}
+		if toolCalls, ok := delta["tool_calls"].([]any); ok && len(toolCalls) > 0 {
+			return true
+		}
+		if functionCall, ok := delta["function_call"].(map[string]any); ok && len(functionCall) > 0 {
+			return true
+		}
+	}
+	return false
+}
+
+func applyChatUsageFromEvent(event map[string]any, usage *OpenAIUsage) {
+	if event == nil || usage == nil {
+		return
+	}
+	usageMap, ok := event["usage"].(map[string]any)
+	if !ok {
+		return
+	}
+	applyChatUsageFromMap(usageMap, usage)
+}
+
+func applyChatUsageFromMap(usageMap map[string]any, usage *OpenAIUsage) {
+	if usageMap == nil || usage == nil {
+		return
+	}
+	promptTokens := int(getNumber(usageMap["prompt_tokens"]))
+	completionTokens := int(getNumber(usageMap["completion_tokens"]))
+	if promptTokens > 0 {
+		usage.InputTokens = promptTokens
+	}
+	if completionTokens > 0 {
+		usage.OutputTokens = completionTokens
+	}
+}
diff --git a/backend/internal/service/openai_chat_completions_test.go b/backend/internal/service/openai_chat_completions_test.go
new file mode 100644
index 000000000..635bda236
--- /dev/null
+++ b/backend/internal/service/openai_chat_completions_test.go
@@ -0,0 +1,132 @@
+package service
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestConvertChatCompletionsToResponses(t *testing.T) {
+	req := map[string]any{
+		"model": "gpt-4o",
+		"messages": []any{
+			map[string]any{
+				"role":    "user",
+				"content": "hello",
+			},
+			map[string]any{
+				"role": "assistant",
+				"tool_calls": []any{
+					map[string]any{
+						"id":   "call_1",
+						"type": "function",
+						"function": map[string]any{
+							"name":      "ping",
+							"arguments": "{}",
+						},
+					},
+				},
+			},
+			map[string]any{
+				"role":          "tool",
+				"tool_call_id":  "call_1",
+				"content":       "ok",
+				"response":      "ignored",
+				"response_time": 1,
+			},
+		},
+		"functions": []any{
+			map[string]any{
+				"name":        "ping",
+				"description": "ping tool",
+				"parameters":  map[string]any{"type": "object"},
+			},
+		},
+		"function_call": map[string]any{"name": "ping"},
+	}
+
+	converted, err := ConvertChatCompletionsToResponses(req)
+	require.NoError(t, err)
+	require.Equal(t, "gpt-4o", converted["model"])
+
+	input, ok := converted["input"].([]any)
+	require.True(t, ok)
+	require.Len(t, input, 3)
+
+	toolCall := findInputItemByType(input, "tool_call")
+	require.NotNil(t, toolCall)
+	require.Equal(t, "call_1", toolCall["call_id"])
+
+	toolOutput := findInputItemByType(input, "function_call_output")
+	require.NotNil(t, toolOutput)
+	require.Equal(t, "call_1", toolOutput["call_id"])
+
+	tools, ok := converted["tools"].([]any)
+	require.True(t, ok)
+	require.Len(t, tools, 1)
+
+	require.Equal(t, map[string]any{"name": "ping"}, converted["tool_choice"])
+}
+
+func TestConvertResponsesToChatCompletion(t *testing.T) {
+	resp := map[string]any{
+		"id":         "resp_123",
+		"model":      "gpt-4o",
+		"created_at": 1700000000,
+		"output": []any{
+			map[string]any{
+				"type": "message",
+				"role": "assistant",
+				"content": []any{
+					map[string]any{
+						"type": "output_text",
+						"text": "hi",
+					},
+				},
+			},
+		},
+		"usage": map[string]any{
+			"input_tokens":  2,
+			"output_tokens": 3,
+		},
+	}
+	body, err := json.Marshal(resp)
+	require.NoError(t, err)
+
+	converted, err := ConvertResponsesToChatCompletion(body)
+	require.NoError(t, err)
+
+	var chat map[string]any
+	require.NoError(t, json.Unmarshal(converted, &chat))
+	require.Equal(t, "chat.completion", chat["object"])
+
+	choices, ok := chat["choices"].([]any)
+	require.True(t, ok)
+	require.Len(t, choices, 1)
+
+	choice, ok := choices[0].(map[string]any)
+	require.True(t, ok)
+	message, ok := choice["message"].(map[string]any)
+	require.True(t, ok)
+	require.Equal(t, "hi", message["content"])
+
+	usage, ok := chat["usage"].(map[string]any)
+	require.True(t, ok)
+	require.Equal(t, float64(2), usage["prompt_tokens"])
+	require.Equal(t, float64(3), usage["completion_tokens"])
+	require.Equal(t, float64(5), usage["total_tokens"])
+}
+
+func findInputItemByType(items []any, itemType string) map[string]any {
+	for _, item := range items {
+		itemMap, ok := item.(map[string]any)
+		if !ok {
+			continue
+		}
+		if itemMap["type"] == itemType {
+			return itemMap
+		}
+	}
+	return nil
+}
diff --git a/backend/internal/service/openai_gateway_service.go b/backend/internal/service/openai_gateway_service.go
index f26ce03f0..b47d2c4dd 100644
--- a/backend/internal/service/openai_gateway_service.go
+++ b/backend/internal/service/openai_gateway_service.go
@@ -11,6 +11,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"regexp"
 	"sort"
 	"strconv"
 	"strings"
@@ -33,6 +34,7 @@ const (
 	chatgptCodexURL = "https://chatgpt.com/backend-api/codex/responses"
 	// OpenAI Platform API for API Key accounts (fallback)
 	openaiPlatformAPIURL   = "https://api.openai.com/v1/responses"
+	openaiChatAPIURL       = "https://api.openai.com/v1/chat/completions"
 	openaiStickySessionTTL = time.Hour // 粘性会话TTL
 	codexCLIUserAgent      = "codex_cli_rs/0.98.0"
 	// codex_cli_only 拒绝时单个请求头日志长度上限（字符）
@@ -42,6 +44,16 @@ const (
 	OpenAIParsedRequestBodyKey = "openai_parsed_request_body"
 )
 
+// OpenAIChatCompletionsBodyKey stores the original chat-completions payload in gin.Context.
+const OpenAIChatCompletionsBodyKey = "openai_chat_completions_body"
+
+// OpenAIChatCompletionsIncludeUsageKey stores stream_options.include_usage in gin.Context.
+const OpenAIChatCompletionsIncludeUsageKey = "openai_chat_completions_include_usage"
+
+// openaiSSEDataRe matches SSE data lines with optional whitespace after colon.
+// Some upstream APIs return non-standard "data:" without space (should be "data: ").
+var openaiSSEDataRe = regexp.MustCompile(`^data:\s*`)
+
 // OpenAI allowed headers whitelist (for non-passthrough).
 var openaiAllowedHeaders = map[string]bool{
 	"accept-language": true,
@@ -81,6 +93,19 @@ var codexCLIOnlyDebugHeaderWhitelist = []string{
 	"X-Real-IP",
 }
 
+// OpenAI chat-completions allowed headers (extend responses whitelist).
+var openaiChatAllowedHeaders = map[string]bool{
+	"accept-language":     true,
+	"content-type":        true,
+	"conversation_id":     true,
+	"user-agent":          true,
+	"originator":          true,
+	"session_id":          true,
+	"openai-organization": true,
+	"openai-project":      true,
+	"openai-beta":         true,
+}
+
 // OpenAICodexUsageSnapshot represents Codex API usage limits from response headers
 type OpenAICodexUsageSnapshot struct {
 	PrimaryUsedPercent          *float64 `json:"primary_used_percent,omitempty"`
@@ -1005,6 +1030,23 @@ func (s *OpenAIGatewayService) Forward(ctx context.Context, c *gin.Context, acco
 		return nil, errors.New("codex_cli_only restriction: only codex official clients are allowed")
 	}
 
+	if c != nil && account != nil && account.Type == AccountTypeAPIKey {
+		if raw, ok := c.Get(OpenAIChatCompletionsBodyKey); ok {
+			if rawBody, ok := raw.([]byte); ok && len(rawBody) > 0 {
+				includeUsage := false
+				if v, ok := c.Get(OpenAIChatCompletionsIncludeUsageKey); ok {
+					if flag, ok := v.(bool); ok {
+						includeUsage = flag
+					}
+				}
+				if passthroughWriter, ok := c.Writer.(interface{ SetPassthrough() }); ok {
+					passthroughWriter.SetPassthrough()
+				}
+				return s.forwardChatCompletions(ctx, c, account, rawBody, includeUsage, startTime)
+			}
+		}
+	}
+
 	originalBody := body
 	reqModel, reqStream, promptCacheKey := extractOpenAIRequestMetaFromBody(body)
 	originalModel := reqModel