Fix composite model defaults for linked platforms

backend/internal/handler/gateway_handler.go

@@ -1085,8 +1085,15 @@ func (h *GatewayHandler) compositeAvailableModels(ctx context.Context, groupID *
 	}
 	seen := make(map[string]struct{})
 	models := make([]string, 0)
+	schedulablePlatforms := h.gatewayService.GetSchedulablePlatforms(ctx, groupID)
 	for _, platform := range []string{service.PlatformAnthropic, service.PlatformGemini, service.PlatformOpenAI, service.PlatformAntigravity, service.PlatformGrok} {
-		for _, model := range h.gatewayService.GetAvailableModels(ctx, groupID, platform) {
+		platformModels := h.gatewayService.GetAvailableModels(ctx, groupID, platform)
+		if len(platformModels) == 0 {
+			if _, ok := schedulablePlatforms[platform]; ok {
+				platformModels = defaultModelIDsForPlatform(platform)
+			}
+		}
+		for _, model := range platformModels {
 			model = strings.TrimSpace(model)
 			if model == "" {
 				continue

backend/internal/handler/gateway_models_test.go

@@ -349,6 +349,42 @@ func TestGatewayModels_CompositeCustomModelsListFiltersAcrossConcretePlatforms(t
 	require.Equal(t, []string{"gemini-2.5-flash", "ag-custom-model", "gpt-5.5"}, modelIDsForTest(got.Data))
 }
 
+func TestGatewayModels_CompositeUnmappedAccountsFallbackToLinkedPlatformsOnly(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	groupID := int64(34)
+	h := newGatewayModelsHandlerForTest(
+		&gatewayModelsAccountRepoStub{
+			byGroup: map[int64][]service.Account{
+				groupID: {
+					{ID: 1, Platform: service.PlatformOpenAI},
+					{ID: 2, Platform: service.PlatformGrok},
+				},
+			},
+		},
+	)
+
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodGet, "/v1/models", nil)
+	c.Set(string(middleware2.ContextKeyAPIKey), &service.APIKey{
+		Group: &service.Group{ID: groupID, Platform: service.PlatformComposite},
+	})
+
+	h.Models(c)
+
+	require.Equal(t, http.StatusOK, rec.Code)
+
+	var got gatewayModelsResponseForTest
+	require.NoError(t, json.Unmarshal(rec.Body.Bytes(), &got))
+
+	ids := modelIDsForTest(got.Data)
+	require.Contains(t, ids, "gpt-5.5")
+	require.Contains(t, ids, "grok-4.3")
+	require.NotContains(t, ids, "claude-sonnet-4-6")
+	require.NotContains(t, ids, "gemini-2.5-flash")
+}
+
 func TestGatewayModels_CustomModelsListKeepsConcreteModelAllowedByWildcardMapping(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 

backend/internal/service/gateway_service.go

@@ -1244,6 +1244,34 @@ func (s *GatewayService) GetAvailableModels(ctx context.Context, groupID *int64,
 	return cloneStringSlice(models)
 }
 
+// GetSchedulablePlatforms returns the concrete platforms that currently have
+// schedulable accounts in the target group.
+func (s *GatewayService) GetSchedulablePlatforms(ctx context.Context, groupID *int64) map[string]struct{} {
+	platforms := make(map[string]struct{})
+	if s == nil || s.accountRepo == nil {
+		return platforms
+	}
+
+	var accounts []Account
+	var err error
+	if groupID != nil {
+		accounts, err = s.accountRepo.ListSchedulableByGroupID(ctx, *groupID)
+	} else {
+		accounts, err = s.accountRepo.ListSchedulable(ctx)
+	}
+	if err != nil {
+		return platforms
+	}
+
+	for _, acc := range accounts {
+		platform := strings.TrimSpace(acc.Platform)
+		if platform != "" {
+			platforms[platform] = struct{}{}
+		}
+	}
+	return platforms
+}
+
 func (s *GatewayService) InvalidateAvailableModelsCache(groupID *int64, platform string) {
 	if s == nil || s.modelsListCache == nil {
 		return