diff --git a/backend/internal/domain/constants.go b/backend/internal/domain/constants.go
index cd821165a4..1ed90ee940 100644
--- a/backend/internal/domain/constants.go
+++ b/backend/internal/domain/constants.go
@@ -105,11 +105,11 @@ var DefaultAntigravityModelMapping = map[string]string{
 	"claude-opus-4-6":            "claude-opus-4-6-thinking", // 简称映射
 	"claude-opus-4-5-thinking":   "claude-opus-4-6-thinking", // 迁移旧模型
 	"claude-sonnet-4-6":          "claude-sonnet-4-6",
-	"claude-sonnet-4-5":          "claude-sonnet-4-5",
-	"claude-sonnet-4-5-thinking": "claude-sonnet-4-5-thinking",
+	"claude-sonnet-4-5":          "claude-sonnet-4-5", // 显式 canonical 选择透传
+	"claude-sonnet-4-5-thinking": "claude-sonnet-4-6", // 迁移旧兼容别名
 	// Claude 详细版本 ID 映射
 	"claude-opus-4-5-20251101":   "claude-opus-4-6-thinking", // 迁移旧模型
-	"claude-sonnet-4-5-20250929": "claude-sonnet-4-5",
+	"claude-sonnet-4-5-20250929": "claude-sonnet-4-6",        // 迁移旧模型
 	// Claude Haiku → Sonnet（无 Haiku 支持）
 	"claude-haiku-4-5":          "claude-sonnet-4-6",
 	"claude-haiku-4-5-20251001": "claude-sonnet-4-6",
diff --git a/backend/internal/domain/constants_test.go b/backend/internal/domain/constants_test.go
index e847e22f30..acac038ef2 100644
--- a/backend/internal/domain/constants_test.go
+++ b/backend/internal/domain/constants_test.go
@@ -43,6 +43,21 @@ func TestDefaultAntigravityModelMapping_ContainsNewClaudeModels(t *testing.T) {
 	}
 }
 
+func TestDefaultAntigravityModelMapping_PreservesExplicitSonnet45AndMigratesLegacyAliases(t *testing.T) {
+	t.Parallel()
+
+	cases := map[string]string{
+		"claude-sonnet-4-5":          "claude-sonnet-4-5",
+		"claude-sonnet-4-5-thinking": "claude-sonnet-4-6",
+		"claude-sonnet-4-5-20250929": "claude-sonnet-4-6",
+	}
+	for model, want := range cases {
+		if got := DefaultAntigravityModelMapping[model]; got != want {
+			t.Fatalf("expected model %q to map to %q, got %q", model, want, got)
+		}
+	}
+}
+
 func TestDefaultAntigravityModelMapping_Gemini31ProAliases(t *testing.T) {
 	t.Parallel()
 
diff --git a/backend/internal/service/account_test_service.go b/backend/internal/service/account_test_service.go
index 451f90db16..e7ce083ff5 100644
--- a/backend/internal/service/account_test_service.go
+++ b/backend/internal/service/account_test_service.go
@@ -43,8 +43,9 @@ import (
 var sseDataPrefix = regexp.MustCompile(`^data:\s*`)
 
 const (
-	testClaudeAPIURL   = "https://api.anthropic.com/v1/messages?beta=true"
-	chatgptCodexAPIURL = "https://chatgpt.com/backend-api/codex/responses"
+	testClaudeAPIURL            = "https://api.anthropic.com/v1/messages?beta=true"
+	chatgptCodexAPIURL          = "https://chatgpt.com/backend-api/codex/responses"
+	defaultAntigravityTestModel = "claude-sonnet-4-6"
 )
 
 // TestEvent represents a SSE event for account testing
@@ -2311,11 +2312,7 @@ func (s *AccountTestService) routeAntigravityTest(c *gin.Context, account *Accou
 func (s *AccountTestService) testAntigravityAccountConnection(c *gin.Context, account *Account, modelID string) error {
 	ctx := c.Request.Context()
 
-	// 默认模型：Claude 使用 claude-sonnet-4-5，Gemini 使用 gemini-3-pro-preview
-	testModelID := modelID
-	if testModelID == "" {
-		testModelID = "claude-sonnet-4-5"
-	}
+	testModelID := antigravityConnectionTestModel(modelID)
 
 	if s.antigravityGatewayService == nil {
 		return s.sendErrorAndEnd(c, "Antigravity gateway service not configured")
@@ -2346,6 +2343,13 @@ func (s *AccountTestService) testAntigravityAccountConnection(c *gin.Context, ac
 	return nil
 }
 
+func antigravityConnectionTestModel(modelID string) string {
+	if modelID == "" {
+		return defaultAntigravityTestModel
+	}
+	return modelID
+}
+
 // buildGeminiAPIKeyRequest builds request for Gemini API Key accounts
 func (s *AccountTestService) buildGeminiAPIKeyRequest(ctx context.Context, account *Account, modelID string, payload []byte) (*http.Request, error) {
 	apiKey := account.GetCredential("api_key")
diff --git a/backend/internal/service/account_test_service_antigravity_test.go b/backend/internal/service/account_test_service_antigravity_test.go
new file mode 100644
index 0000000000..b3362f3f69
--- /dev/null
+++ b/backend/internal/service/account_test_service_antigravity_test.go
@@ -0,0 +1,16 @@
+//go:build unit
+
+package service
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestAntigravityConnectionTestModel(t *testing.T) {
+	t.Parallel()
+
+	require.Equal(t, "claude-sonnet-4-6", antigravityConnectionTestModel(""))
+	require.Equal(t, "gemini-3.1-pro-preview", antigravityConnectionTestModel("gemini-3.1-pro-preview"))
+}
diff --git a/backend/internal/service/antigravity_model_mapping_test.go b/backend/internal/service/antigravity_model_mapping_test.go
index 62ff2e295d..61cc7e31d6 100644
--- a/backend/internal/service/antigravity_model_mapping_test.go
+++ b/backend/internal/service/antigravity_model_mapping_test.go
@@ -69,10 +69,10 @@ func TestAntigravityGatewayService_GetMappedModel(t *testing.T) {
 			expected:       "claude-sonnet-4-6",
 		},
 		{
-			name:           "默认映射 - claude-sonnet-4-5-20250929 → claude-sonnet-4-5",
+			name:           "默认映射 - claude-sonnet-4-5-20250929 → claude-sonnet-4-6",
 			requestedModel: "claude-sonnet-4-5-20250929",
 			accountMapping: nil,
-			expected:       "claude-sonnet-4-5",
+			expected:       "claude-sonnet-4-6",
 		},
 
 		// 3. 默认映射中的透传（映射到自己）
@@ -89,7 +89,7 @@ func TestAntigravityGatewayService_GetMappedModel(t *testing.T) {
 			expected:       "claude-sonnet-4-6",
 		},
 		{
-			name:           "默认映射透传 - claude-sonnet-4-5",
+			name:           "显式 canonical 选择 - claude-sonnet-4-5 透传",
 			requestedModel: "claude-sonnet-4-5",
 			accountMapping: nil,
 			expected:       "claude-sonnet-4-5",
@@ -113,10 +113,19 @@ func TestAntigravityGatewayService_GetMappedModel(t *testing.T) {
 			expected:       "claude-opus-4-6-thinking",
 		},
 		{
-			name:           "默认映射透传 - claude-sonnet-4-5-thinking",
+			name:           "默认映射 - claude-sonnet-4-5-thinking → claude-sonnet-4-6",
 			requestedModel: "claude-sonnet-4-5-thinking",
 			accountMapping: nil,
-			expected:       "claude-sonnet-4-5-thinking",
+			expected:       "claude-sonnet-4-6",
+		},
+		{
+			name:           "账户显式目标只映射一步 - custom-sonnet → claude-sonnet-4-5",
+			requestedModel: "custom-sonnet",
+			accountMapping: map[string]string{
+				"custom-sonnet":     "claude-sonnet-4-5",
+				"claude-sonnet-4-5": "claude-sonnet-4-6",
+			},
+			expected: "claude-sonnet-4-5",
 		},
 		{
 			name:           "默认映射透传 - gemini-2.5-flash",
diff --git a/backend/internal/service/antigravity_rate_limit_test.go b/backend/internal/service/antigravity_rate_limit_test.go
index c9b4298291..d632f14bda 100644
--- a/backend/internal/service/antigravity_rate_limit_test.go
+++ b/backend/internal/service/antigravity_rate_limit_test.go
@@ -844,17 +844,28 @@ func TestSetAntigravityModelRateLimits_GeminiWritesFamilyScope(t *testing.T) {
 	require.Equal(t, antigravityGeminiModelRateLimitKey, repo.modelRateLimitCalls[1].modelKey)
 }
 
-func TestSetAntigravityModelRateLimits_ClaudeDoesNotWriteGeminiScope(t *testing.T) {
+func TestSetAntigravityModelRateLimits_DoesNotDoubleMapCustomChain(t *testing.T) {
 	repo := &stubAntigravityAccountRepo{}
 	svc := &AntigravityGatewayService{}
-	account := &Account{ID: 790, Platform: PlatformAntigravity}
+	account := &Account{
+		ID:       790,
+		Platform: PlatformAntigravity,
+		Credentials: map[string]any{
+			"model_mapping": map[string]any{
+				"custom-sonnet":     "claude-sonnet-4-5",
+				"claude-sonnet-4-5": "claude-sonnet-4-6",
+			},
+		},
+	}
 	resetAt := time.Now().Add(30 * time.Second)
+	canonicalModel := resolveFinalAntigravityModelKey(context.Background(), account, "custom-sonnet")
+	require.Equal(t, "claude-sonnet-4-5", canonicalModel)
 
 	success := svc.setAntigravityModelRateLimits(
 		context.Background(),
 		repo,
 		account,
-		"claude-sonnet-4-5",
+		canonicalModel,
 		"[test]",
 		429,
 		resetAt,
@@ -866,6 +877,33 @@ func TestSetAntigravityModelRateLimits_ClaudeDoesNotWriteGeminiScope(t *testing.
 	require.Equal(t, "claude-sonnet-4-5", repo.modelRateLimitCalls[0].modelKey)
 }
 
+func TestSetModelRateLimitAndClearSession_UsesUpstreamReportedModelMetadata(t *testing.T) {
+	repo := &stubAntigravityAccountRepo{}
+	svc := &AntigravityGatewayService{accountRepo: repo}
+	account := &Account{
+		ID:       791,
+		Platform: PlatformAntigravity,
+		Credentials: map[string]any{
+			"model_mapping": map[string]any{
+				"claude-sonnet-4-5": "claude-sonnet-4-6",
+			},
+		},
+	}
+
+	svc.setModelRateLimitAndClearSession(&handleModelRateLimitParams{
+		ctx:        context.Background(),
+		prefix:     "[test]",
+		account:    account,
+		statusCode: 429,
+	}, &antigravitySmartRetryInfo{
+		RetryDelay: 30 * time.Second,
+		ModelName:  "claude-sonnet-4-5",
+	})
+
+	require.Len(t, repo.modelRateLimitCalls, 1)
+	require.Equal(t, "claude-sonnet-4-5", repo.modelRateLimitCalls[0].modelKey)
+}
+
 func TestAntigravityRetryLoop_PreCheck_SwitchesWhenRateLimited(t *testing.T) {
 	upstream := &recordingOKUpstream{}
 	account := &Account{