diff --git a/backend/internal/service/antigravity_gateway_gemini.go b/backend/internal/service/antigravity_gateway_gemini.go
index 2b622afd37..77a7be3df7 100644
--- a/backend/internal/service/antigravity_gateway_gemini.go
+++ b/backend/internal/service/antigravity_gateway_gemini.go
@@ -91,7 +91,7 @@ func (s *AntigravityGatewayService) ForwardGemini(ctx context.Context, c *gin.Co
 		MarkOpsClientBusinessLimited(c, OpsClientBusinessLimitedReasonLocalFeatureGate)
 		return nil, s.writeGoogleError(c, http.StatusForbidden, fmt.Sprintf("model %s not in whitelist", originalModel))
 	}
-	billingModel := mappedModel
+	forwardedModel := mappedModel
 
 	// 获取 access_token
 	if s.tokenProvider == nil {
@@ -204,7 +204,7 @@ func (s *AntigravityGatewayService) ForwardGemini(ctx context.Context, c *gin.Co
 						if err == nil && fallbackResp.StatusCode < 400 {
 							_ = resp.Body.Close()
 							resp = fallbackResp
-							billingModel = fallbackModel
+							forwardedModel = fallbackModel
 						} else if fallbackResp != nil {
 							_ = fallbackResp.Body.Close()
 						}
@@ -436,7 +436,7 @@ handleSuccess:
 		RequestID:                     requestID,
 		Usage:                         *usage,
 		Model:                         originalModel,
-		UpstreamModel:                 billingModel,
+		UpstreamModel:                 forwardedModel,
 		UpstreamResponseModel:         observedUpstreamResponseModel(c),
 		UpstreamResponseModelConflict: observedUpstreamResponseModelConflict(c),
 		Stream:                        stream,
diff --git a/backend/internal/service/antigravity_gateway_service_test.go b/backend/internal/service/antigravity_gateway_service_test.go
index 02c2d85e36..00fee73a11 100644
--- a/backend/internal/service/antigravity_gateway_service_test.go
+++ b/backend/internal/service/antigravity_gateway_service_test.go
@@ -200,13 +200,18 @@ func (c *recordingInternal500CounterCache) ResetInternal500Count(_ context.Conte
 	return nil
 }
 
-type antigravitySettingRepoStub struct{}
+type antigravitySettingRepoStub struct {
+	values map[string]string
+}
 
 func (s *antigravitySettingRepoStub) Get(ctx context.Context, key string) (*Setting, error) {
 	panic("unexpected Get call")
 }
 
 func (s *antigravitySettingRepoStub) GetValue(ctx context.Context, key string) (string, error) {
+	if value, ok := s.values[key]; ok {
+		return value, nil
+	}
 	return "", ErrSettingNotFound
 }
 
@@ -862,6 +867,73 @@ func TestAntigravityGatewayService_ForwardGemini_BillsWithMappedModel(t *testing
 	require.Equal(t, mappedModel, result.UpstreamModel)
 }
 
+func TestAntigravityGatewayService_ForwardGemini_FallbackReportsActualUpstreamModel(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	writer := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(writer)
+
+	body := []byte(`{"contents":[{"role":"user","parts":[{"text":"hello"}]}]}`)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1beta/models/gemini-primary:generateContent", bytes.NewReader(body))
+
+	const (
+		originalModel = "gemini-primary"
+		mappedModel   = "gemini-primary-upstream"
+		fallbackModel = "gemini-fallback-upstream"
+	)
+	upstream := &queuedHTTPUpstreamStub{responses: []*http.Response{
+		{
+			StatusCode: http.StatusNotFound,
+			Header:     http.Header{"Content-Type": []string{"application/json"}},
+			Body:       io.NopCloser(strings.NewReader(`{"error":{"code":404,"message":"model not found"}}`)),
+		},
+		{
+			StatusCode: http.StatusOK,
+			Header:     http.Header{"Content-Type": []string{"text/event-stream"}},
+			Body: io.NopCloser(strings.NewReader(
+				`data: {"response":{"modelVersion":"gemini-fallback-upstream","candidates":[{"content":{"parts":[{"text":"ok"}]},"finishReason":"STOP"}],"usageMetadata":{"promptTokenCount":8,"candidatesTokenCount":3}}}` + "\n\n",
+			)),
+		},
+	}}
+	settings := &antigravitySettingRepoStub{values: map[string]string{
+		SettingKeyEnableModelFallback:      "true",
+		SettingKeyFallbackModelAntigravity: fallbackModel,
+	}}
+	svc := &AntigravityGatewayService{
+		settingService: NewSettingService(settings, &config.Config{Gateway: config.GatewayConfig{MaxLineSize: defaultMaxLineSize}}),
+		tokenProvider:  &AntigravityTokenProvider{},
+		httpUpstream:   upstream,
+	}
+	account := &Account{
+		ID:          9,
+		Name:        "acc-gemini-fallback",
+		Platform:    PlatformAntigravity,
+		Type:        AccountTypeOAuth,
+		Status:      StatusActive,
+		Concurrency: 1,
+		Credentials: map[string]any{
+			"access_token": "token",
+			"project_id":   "proj",
+			"model_mapping": map[string]any{
+				originalModel: mappedModel,
+			},
+		},
+	}
+
+	result, err := svc.ForwardGemini(context.Background(), c, account, originalModel, "generateContent", true, body, false)
+	require.NoError(t, err)
+	require.NotNil(t, result)
+	require.Equal(t, originalModel, result.Model)
+	require.Equal(t, fallbackModel, result.UpstreamModel)
+	require.Equal(t, fallbackModel, result.UpstreamResponseModel)
+	require.False(t, result.UpstreamResponseModelConflict)
+	mismatch := upstreamModelMismatch(result.UpstreamModel, result.UpstreamResponseModel)
+	require.NotNil(t, mismatch)
+	require.False(t, *mismatch)
+	require.Len(t, upstream.requestBodies, 2)
+	require.Contains(t, string(upstream.requestBodies[0]), `"model":"`+mappedModel+`"`)
+	require.Contains(t, string(upstream.requestBodies[1]), `"model":"`+fallbackModel+`"`)
+}
+
 func TestAntigravityGatewayService_ForwardGemini_RetriesCorruptedThoughtSignature(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	writer := httptest.NewRecorder()
diff --git a/backend/internal/service/antigravity_gateway_streaming.go b/backend/internal/service/antigravity_gateway_streaming.go
index 0b3a672853..2b68fc115a 100644
--- a/backend/internal/service/antigravity_gateway_streaming.go
+++ b/backend/internal/service/antigravity_gateway_streaming.go
@@ -35,11 +35,11 @@ func (s *AntigravityGatewayService) observeAntigravityGeminiSSELine(c *gin.Conte
 	if payload == "" || payload == "[DONE]" {
 		return
 	}
-	raw := []byte(payload)
-	if inner, err := s.unwrapV1InternalResponse(raw); err == nil && len(inner) > 0 {
-		raw = inner
-	}
-	observer.ObserveGemini(raw)
+	// Observe the original payload: ObserveGemini supports both the v1internal
+	// wrapper and direct Gemini response shapes. The main stream handler will
+	// unwrap the same line for business processing, so unwrapping here would be
+	// duplicate work on every SSE event.
+	observer.ObserveGemini([]byte(payload))
 }
 
 // antigravityClientWriter 封装流式响应的客户端写入，自动检测断开并标记。
diff --git a/backend/internal/service/upstream_response_model.go b/backend/internal/service/upstream_response_model.go
index 2452c382d0..cc282c228f 100644
--- a/backend/internal/service/upstream_response_model.go
+++ b/backend/internal/service/upstream_response_model.go
@@ -53,34 +53,21 @@ func normalizeObservedUpstreamResponseModel(model string) string {
 }
 
 func (o *upstreamResponseModelObserver) ObserveOpenAI(payload []byte, eventType string) {
-	if len(payload) == 0 || !gjson.ValidBytes(payload) {
-		return
-	}
-	model := firstTrimmedGJSONModel(
-		gjson.GetBytes(payload, "response.model"),
-		gjson.GetBytes(payload, "model"),
-	)
+	model := firstValidTrimmedGJSONModel(payload, "response.model", "model")
 	o.Observe(model, isUpstreamResponseModelTerminalEvent(eventType))
 }
 
 func (o *upstreamResponseModelObserver) ObserveAnthropic(payload []byte) {
-	if len(payload) == 0 || !gjson.ValidBytes(payload) {
-		return
-	}
-	model := firstTrimmedGJSONModel(
-		gjson.GetBytes(payload, "message.model"),
-		gjson.GetBytes(payload, "model"),
-	)
+	model := firstValidTrimmedGJSONModel(payload, "message.model", "model")
 	o.Observe(model, false)
 }
 
 func (o *upstreamResponseModelObserver) ObserveGemini(payload []byte) {
-	if len(payload) == 0 || !gjson.ValidBytes(payload) {
-		return
-	}
-	model := firstTrimmedGJSONModel(
-		gjson.GetBytes(payload, "modelVersion"),
-		gjson.GetBytes(payload, "response.modelVersion"),
+	model := firstValidTrimmedGJSONModel(
+		payload,
+		"modelVersion",
+		"response.modelVersion",
+		"response.response.modelVersion",
 	)
 	// Gemini streaming has no universal terminal event carrying modelVersion;
 	// treating each declaration as terminal retains the latest chunk.
@@ -139,12 +126,22 @@ func observeOpenAISSEBody(observer *upstreamResponseModelObserver, body string)
 	})
 }
 
-func firstTrimmedGJSONModel(values ...gjson.Result) string {
-	for _, value := range values {
+func firstValidTrimmedGJSONModel(payload []byte, paths ...string) string {
+	if len(payload) == 0 {
+		return ""
+	}
+	for _, path := range paths {
+		value := gjson.GetBytes(payload, path)
 		if !value.Exists() || value.Type != gjson.String {
 			continue
 		}
 		if model := strings.TrimSpace(value.String()); model != "" {
+			// Validate only after finding a candidate. This avoids a full validation
+			// pass on the common model-free delta path while still rejecting malformed
+			// payloads that appear to declare a model.
+			if !gjson.ValidBytes(payload) {
+				return ""
+			}
 			return model
 		}
 	}
diff --git a/backend/internal/service/upstream_response_model_bench_test.go b/backend/internal/service/upstream_response_model_bench_test.go
new file mode 100644
index 0000000000..93d6854f72
--- /dev/null
+++ b/backend/internal/service/upstream_response_model_bench_test.go
@@ -0,0 +1,98 @@
+package service
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/tidwall/gjson"
+)
+
+var (
+	upstreamResponseModelBenchmarkSink string
+
+	upstreamResponseModelBenchmarkTerminal = []byte(`{"type":"response.completed","response":{"id":"resp_123","model":"gpt-5.5-2026-04-23"},"usage":{"input_tokens":128,"output_tokens":64}}`)
+	upstreamResponseModelBenchmarkDelta    = []byte(`{"type":"response.output_text.delta","delta":"hello"}`)
+	upstreamResponseModelBenchmarkWrapper  = []byte(`{"response":{"response":{"modelVersion":"gemini-3-pro","candidates":[]}}}`)
+)
+
+func BenchmarkUpstreamResponseModelOpenAI(b *testing.B) {
+	tests := []struct {
+		name    string
+		payload []byte
+	}{
+		{name: "terminal_with_model", payload: upstreamResponseModelBenchmarkTerminal},
+		{name: "delta_without_model", payload: upstreamResponseModelBenchmarkDelta},
+	}
+
+	for _, tt := range tests {
+		b.Run(tt.name, func(b *testing.B) {
+			b.Run("legacy", func(b *testing.B) {
+				b.ReportAllocs()
+				for b.Loop() {
+					upstreamResponseModelBenchmarkSink = benchmarkLegacyOpenAIModel(tt.payload)
+				}
+			})
+			b.Run("optimized", func(b *testing.B) {
+				b.ReportAllocs()
+				for b.Loop() {
+					upstreamResponseModelBenchmarkSink = firstValidTrimmedGJSONModel(tt.payload, "response.model", "model")
+				}
+			})
+		})
+	}
+}
+
+func BenchmarkUpstreamResponseModelAntigravityWrapper(b *testing.B) {
+	b.Run("legacy", func(b *testing.B) {
+		b.ReportAllocs()
+		for b.Loop() {
+			upstreamResponseModelBenchmarkSink = benchmarkLegacyWrappedGeminiModel(upstreamResponseModelBenchmarkWrapper)
+		}
+	})
+	b.Run("optimized", func(b *testing.B) {
+		b.ReportAllocs()
+		for b.Loop() {
+			upstreamResponseModelBenchmarkSink = firstValidTrimmedGJSONModel(
+				upstreamResponseModelBenchmarkWrapper,
+				"modelVersion",
+				"response.modelVersion",
+				"response.response.modelVersion",
+			)
+		}
+	})
+}
+
+func benchmarkLegacyOpenAIModel(payload []byte) string {
+	if len(payload) == 0 || !gjson.ValidBytes(payload) {
+		return ""
+	}
+	return benchmarkFirstTrimmedGJSONModel(
+		gjson.GetBytes(payload, "response.model"),
+		gjson.GetBytes(payload, "model"),
+	)
+}
+
+func benchmarkLegacyWrappedGeminiModel(payload []byte) string {
+	if inner := gjson.GetBytes(payload, "response"); inner.Exists() {
+		payload = []byte(inner.Raw)
+	}
+	if len(payload) == 0 || !gjson.ValidBytes(payload) {
+		return ""
+	}
+	return benchmarkFirstTrimmedGJSONModel(
+		gjson.GetBytes(payload, "modelVersion"),
+		gjson.GetBytes(payload, "response.modelVersion"),
+	)
+}
+
+func benchmarkFirstTrimmedGJSONModel(values ...gjson.Result) string {
+	for _, value := range values {
+		if !value.Exists() || value.Type != gjson.String {
+			continue
+		}
+		if model := strings.TrimSpace(value.String()); model != "" {
+			return model
+		}
+	}
+	return ""
+}
diff --git a/backend/internal/service/upstream_response_model_test.go b/backend/internal/service/upstream_response_model_test.go
index 37d382e851..a831e74e65 100644
--- a/backend/internal/service/upstream_response_model_test.go
+++ b/backend/internal/service/upstream_response_model_test.go
@@ -67,6 +67,56 @@ func TestObserveOpenAISSEBodyIgnoresMalformedPayload(t *testing.T) {
 	require.False(t, observer.Conflict())
 }
 
+func TestObserveAntigravityGeminiSSELineReadsWrapperModelWithoutUnwrap(t *testing.T) {
+	tests := []struct {
+		name    string
+		payload string
+		want    string
+	}{
+		{
+			name:    "top-level sibling",
+			payload: `{"modelVersion":"gemini-3-pro","response":{"candidates":[]}}`,
+			want:    "gemini-3-pro",
+		},
+		{
+			name:    "single wrapper",
+			payload: `{"response":{"modelVersion":"gemini-3-pro","candidates":[]}}`,
+			want:    "gemini-3-pro",
+		},
+		{
+			name:    "nested response after one wrapper",
+			payload: `{"response":{"response":{"modelVersion":"gemini-3-pro","candidates":[]}}}`,
+			want:    "gemini-3-pro",
+		},
+		{
+			name:    "outer declaration takes precedence",
+			payload: `{"modelVersion":"gemini-outer","response":{"modelVersion":"gemini-inner","candidates":[]}}`,
+			want:    "gemini-outer",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			gin.SetMode(gin.TestMode)
+			c, _ := gin.CreateTestContext(nil)
+			beginUpstreamResponseModelObservation(c)
+
+			svc := &AntigravityGatewayService{}
+			svc.observeAntigravityGeminiSSELine(c, "data: "+tt.payload)
+
+			require.Equal(t, tt.want, observedUpstreamResponseModel(c))
+			require.False(t, observedUpstreamResponseModelConflict(c))
+		})
+	}
+}
+
+func TestUpstreamResponseModelObserverRejectsMalformedJSONWithModelField(t *testing.T) {
+	observer := &upstreamResponseModelObserver{}
+	observer.ObserveOpenAI([]byte(`{"response":{"model":"gpt-5.4"}`), "response.completed")
+
+	require.Empty(t, observer.Model())
+}
+
 func TestUpstreamResponseModelObserverBoundsUntrustedModelName(t *testing.T) {
 	observer := &upstreamResponseModelObserver{}
 	observer.Observe("  "+strings.Repeat("模", upstreamResponseModelMaxLength+1)+"  ", false)