diff --git a/backend/internal/server/middleware/api_key_auth.go b/backend/internal/server/middleware/api_key_auth.go
index bcec39760..a595fbdb3 100644
--- a/backend/internal/server/middleware/api_key_auth.go
+++ b/backend/internal/server/middleware/api_key_auth.go
@@ -217,7 +217,7 @@ func apiKeyAuthWithSubscription(apiKeyService *service.APIKeyService, subscripti
 			// Key 状态检查
 			switch apiKey.Status {
 			case service.StatusAPIKeyQuotaExhausted:
-				AbortWithError(c, 429, "API_KEY_QUOTA_EXHAUSTED", "API key 额度已用完")
+				abortWithAPIKeyQuotaError(c)
 				return
 			case service.StatusAPIKeyExpired:
 				AbortWithError(c, 403, "API_KEY_EXPIRED", "API key 已过期")
@@ -230,7 +230,7 @@ func apiKeyAuthWithSubscription(apiKeyService *service.APIKeyService, subscripti
 				return
 			}
 			if apiKey.IsQuotaExhausted() {
-				AbortWithError(c, 429, "API_KEY_QUOTA_EXHAUSTED", "API key 额度已用完")
+				abortWithAPIKeyQuotaError(c)
 				return
 			}
 
@@ -305,6 +305,34 @@ func hasAPIKeyCredentialInput(c *gin.Context) bool {
 		c.GetHeader("x-goog-api-key") != ""
 }
 
+func abortWithAPIKeyQuotaError(c *gin.Context) {
+	const message = "API key 额度已用完"
+	if isOpenAICompatibleAPIKeyRequest(c) {
+		abortWithOpenAIQuotaError(c, http.StatusTooManyRequests, message)
+		return
+	}
+	AbortWithError(c, http.StatusTooManyRequests, "API_KEY_QUOTA_EXHAUSTED", message)
+}
+
+func isOpenAICompatibleAPIKeyRequest(c *gin.Context) bool {
+	if c == nil || c.Request == nil || c.Request.URL == nil {
+		return false
+	}
+
+	path := strings.TrimRight(c.Request.URL.Path, "/")
+	for _, root := range []string{
+		"/v1/responses",
+		"/openai/v1/responses",
+		"/responses",
+		"/backend-api/codex/responses",
+	} {
+		if path == root || strings.HasPrefix(path, root+"/") {
+			return true
+		}
+	}
+	return false
+}
+
 func isAsyncImageTaskRead(method, path string) bool {
 	if method != http.MethodGet {
 		return false
diff --git a/backend/internal/server/middleware/api_key_auth_test.go b/backend/internal/server/middleware/api_key_auth_test.go
index 4d6d9b524..b6bad315e 100644
--- a/backend/internal/server/middleware/api_key_auth_test.go
+++ b/backend/internal/server/middleware/api_key_auth_test.go
@@ -1427,6 +1427,78 @@ func TestAPIKeyAuthRejectsExhaustedBalance(t *testing.T) {
 	requireAPIKeyAuthError(t, w, "INSUFFICIENT_BALANCE", "Insufficient account balance")
 }
 
+func TestAPIKeyAuthOpenAIQuotaErrorFormat(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	user := &service.User{ID: 11, Role: service.RoleUser, Status: service.StatusActive, Balance: 10}
+	group := &service.Group{ID: 8, Platform: service.PlatformOpenAI, Status: service.StatusActive}
+	apiKey := &service.APIKey{
+		ID: 105, UserID: user.ID, Key: "openai-quota-exhausted", Status: service.StatusAPIKeyQuotaExhausted,
+		User: user, Group: group, GroupID: &group.ID,
+	}
+	apiKeyRepo := &stubApiKeyRepo{getByKey: func(ctx context.Context, key string) (*service.APIKey, error) {
+		if key != apiKey.Key {
+			return nil, service.ErrAPIKeyNotFound
+		}
+		clone := *apiKey
+		userClone := *user
+		clone.User = &userClone
+		return &clone, nil
+	}}
+
+	cfg := &config.Config{RunMode: config.RunModeStandard}
+	router := newAuthTestRouter(service.NewAPIKeyService(apiKeyRepo, nil, nil, nil, nil, nil, cfg), nil, cfg)
+	w := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodPost, "/v1/responses", nil)
+	req.Header.Set("x-api-key", apiKey.Key)
+	router.ServeHTTP(w, req)
+
+	require.Equal(t, http.StatusTooManyRequests, w.Code)
+	var response struct {
+		Error struct {
+			Message string  `json:"message"`
+			Type    string  `json:"type"`
+			Param   *string `json:"param"`
+			Code    string  `json:"code"`
+		} `json:"error"`
+	}
+	require.NoError(t, json.Unmarshal(w.Body.Bytes(), &response))
+	require.Equal(t, "API key 额度已用完", response.Error.Message)
+	require.Equal(t, "insufficient_quota", response.Error.Type)
+	require.Nil(t, response.Error.Param)
+	require.Equal(t, "insufficient_quota", response.Error.Code)
+}
+
+func TestAPIKeyAuthQuotaErrorKeepsLegacyFormatOutsideResponses(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	user := &service.User{ID: 11, Role: service.RoleUser, Status: service.StatusActive, Balance: 10}
+	group := &service.Group{ID: 8, Platform: service.PlatformOpenAI, Status: service.StatusActive}
+	apiKey := &service.APIKey{
+		ID: 105, UserID: user.ID, Key: "openai-quota-exhausted", Status: service.StatusAPIKeyQuotaExhausted,
+		User: user, Group: group, GroupID: &group.ID,
+	}
+	apiKeyRepo := &stubApiKeyRepo{getByKey: func(ctx context.Context, key string) (*service.APIKey, error) {
+		if key != apiKey.Key {
+			return nil, service.ErrAPIKeyNotFound
+		}
+		clone := *apiKey
+		userClone := *user
+		clone.User = &userClone
+		return &clone, nil
+	}}
+
+	cfg := &config.Config{RunMode: config.RunModeStandard}
+	router := newAuthTestRouter(service.NewAPIKeyService(apiKeyRepo, nil, nil, nil, nil, nil, cfg), nil, cfg)
+	w := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodPost, "/v1/messages", nil)
+	req.Header.Set("x-api-key", apiKey.Key)
+	router.ServeHTTP(w, req)
+
+	require.Equal(t, http.StatusTooManyRequests, w.Code)
+	requireAPIKeyAuthError(t, w, "API_KEY_QUOTA_EXHAUSTED", "API key 额度已用完")
+}
+
 func newAuthTestRouter(apiKeyService *service.APIKeyService, subscriptionService *service.SubscriptionService, cfg *config.Config) *gin.Engine {
 	router := gin.New()
 	router.Use(gin.HandlerFunc(NewAPIKeyAuthMiddleware(apiKeyService, subscriptionService, cfg)))
@@ -1434,6 +1506,8 @@ func newAuthTestRouter(apiKeyService *service.APIKeyService, subscriptionService
 		c.JSON(http.StatusOK, gin.H{"ok": true})
 	}
 	router.GET("/t", ok)
+	router.POST("/v1/responses", ok)
+	router.POST("/v1/messages", ok)
 	router.GET("/v1/usage", ok)
 	router.GET("/v1/sub2api/billing", ok)
 	return router
diff --git a/backend/internal/server/middleware/middleware.go b/backend/internal/server/middleware/middleware.go
index 0556be2a5..bb89c819f 100644
--- a/backend/internal/server/middleware/middleware.go
+++ b/backend/internal/server/middleware/middleware.go
@@ -80,6 +80,19 @@ func AbortWithError(c *gin.Context, statusCode int, code, message string) {
 	c.Abort()
 }
 
+// abortWithOpenAIQuotaError writes the OpenAI-compatible insufficient quota response.
+func abortWithOpenAIQuotaError(c *gin.Context, statusCode int, message string) {
+	c.JSON(statusCode, gin.H{
+		"error": gin.H{
+			"message": message,
+			"type":    "insufficient_quota",
+			"param":   nil,
+			"code":    "insufficient_quota",
+		},
+	})
+	c.Abort()
+}
+
 // ──────────────────────────────────────────────────────────
 // RequireGroupAssignment — 未分组 Key 拦截中间件
 // ──────────────────────────────────────────────────────────