diff --git a/backend/internal/domain/constants.go b/backend/internal/domain/constants.go
index 3c84890c5b..3be604daff 100644
--- a/backend/internal/domain/constants.go
+++ b/backend/internal/domain/constants.go
@@ -145,6 +145,12 @@ var DefaultAntigravityModelMapping = map[string]string{
 	"gemini-3.6-flash-low":    "gemini-3.6-flash-low",
 	"gemini-3.6-flash-medium": "gemini-3.6-flash-medium",
 	"gemini-3.6-flash-tiered": "gemini-3.6-flash-tiered",
+	// Gemini 3.8 Flash tiered models
+	"gemini-3.8-flash":        "gemini-3.8-flash",
+	"gemini-3.8-flash-high":   "gemini-3.8-flash-high",
+	"gemini-3.8-flash-low":    "gemini-3.8-flash-low",
+	"gemini-3.8-flash-medium": "gemini-3.8-flash-medium",
+	"gemini-3.8-flash-tiered": "gemini-3.8-flash-tiered",
 	// Gemini 3 image 兼容映射（向 3.1 image 迁移）
 	"gemini-3-pro-image":         "gemini-3.1-flash-image",
 	"gemini-3-pro-image-preview": "gemini-3.1-flash-image",
diff --git a/backend/internal/domain/constants_test.go b/backend/internal/domain/constants_test.go
index 628489a620..7813cbbc7a 100644
--- a/backend/internal/domain/constants_test.go
+++ b/backend/internal/domain/constants_test.go
@@ -89,6 +89,14 @@ func TestDefaultAntigravityModelMapping_Gemini36FlashModels(t *testing.T) {
 	}
 }
 
+func TestDefaultAntigravityModelMapping_Gemini38FlashModels(t *testing.T) {
+	for _, model := range []string{"gemini-3.8-flash", "gemini-3.8-flash-high", "gemini-3.8-flash-low", "gemini-3.8-flash-medium", "gemini-3.8-flash-tiered"} {
+		if got := DefaultAntigravityModelMapping[model]; got != model {
+			t.Fatalf("expected %s to map to itself, got %q", model, got)
+		}
+	}
+}
+
 func TestDefaultBedrockModelMapping_ContainsNewClaudeModels(t *testing.T) {
 	t.Parallel()
 
diff --git a/backend/internal/pkg/antigravity/claude_types.go b/backend/internal/pkg/antigravity/claude_types.go
index a4ba557ef3..811c810546 100644
--- a/backend/internal/pkg/antigravity/claude_types.go
+++ b/backend/internal/pkg/antigravity/claude_types.go
@@ -184,6 +184,11 @@ var geminiModels = []modelDef{
 	{ID: "gemini-3.6-flash-low", DisplayName: "Gemini 3.6 Flash Low", CreatedAt: "2026-07-21T00:00:00Z", IsReasoning: true},
 	{ID: "gemini-3.6-flash-medium", DisplayName: "Gemini 3.6 Flash Medium", CreatedAt: "2026-07-21T00:00:00Z", IsReasoning: true},
 	{ID: "gemini-3.6-flash-tiered", DisplayName: "Gemini 3.6 Flash", CreatedAt: "2026-07-21T00:00:00Z", IsReasoning: true},
+	{ID: "gemini-3.8-flash", DisplayName: "Gemini 3.8 Flash", CreatedAt: "2026-09-02T00:00:00Z"},
+	{ID: "gemini-3.8-flash-high", DisplayName: "Gemini 3.8 Flash High", CreatedAt: "2026-09-02T00:00:00Z", IsReasoning: true},
+	{ID: "gemini-3.8-flash-low", DisplayName: "Gemini 3.8 Flash Low", CreatedAt: "2026-09-02T00:00:00Z", IsReasoning: true},
+	{ID: "gemini-3.8-flash-medium", DisplayName: "Gemini 3.8 Flash Medium", CreatedAt: "2026-09-02T00:00:00Z", IsReasoning: true},
+	{ID: "gemini-3.8-flash-tiered", DisplayName: "Gemini 3.8 Flash", CreatedAt: "2026-09-02T00:00:00Z", IsReasoning: true},
 	{ID: "gemini-3-pro-preview", DisplayName: "Gemini 3 Pro Preview", CreatedAt: "2025-06-01T00:00:00Z", IsReasoning: true},
 	{ID: "gemini-3-pro-image", DisplayName: "Gemini 3 Pro Image", CreatedAt: "2025-06-01T00:00:00Z"},
 }
diff --git a/backend/internal/pkg/antigravity/claude_types_test.go b/backend/internal/pkg/antigravity/claude_types_test.go
index 8b75f092b5..860a9db28c 100644
--- a/backend/internal/pkg/antigravity/claude_types_test.go
+++ b/backend/internal/pkg/antigravity/claude_types_test.go
@@ -26,6 +26,11 @@ func TestDefaultModels_ContainsNewAndLegacyImageModels(t *testing.T) {
 		"gemini-3.6-flash-low",
 		"gemini-3.6-flash-medium",
 		"gemini-3.6-flash-tiered",
+		"gemini-3.8-flash",
+		"gemini-3.8-flash-high",
+		"gemini-3.8-flash-low",
+		"gemini-3.8-flash-medium",
+		"gemini-3.8-flash-tiered",
 	}
 
 	for _, id := range requiredIDs {
diff --git a/backend/internal/service/account.go b/backend/internal/service/account.go
index ad1ad82a1b..a962d94159 100644
--- a/backend/internal/service/account.go
+++ b/backend/internal/service/account.go
@@ -662,6 +662,11 @@ func (a *Account) resolveModelMapping(rawMapping map[string]any) map[string]stri
 				"gemini-3.6-flash-low",
 				"gemini-3.6-flash-medium",
 				"gemini-3.6-flash-tiered",
+				"gemini-3.8-flash",
+				"gemini-3.8-flash-high",
+				"gemini-3.8-flash-low",
+				"gemini-3.8-flash-medium",
+				"gemini-3.8-flash-tiered",
 			})
 			applyAntigravityGemini31ProAliases(result)
 		}
diff --git a/backend/internal/service/billing_service.go b/backend/internal/service/billing_service.go
index 1f3a346f64..2fe2a913f4 100644
--- a/backend/internal/service/billing_service.go
+++ b/backend/internal/service/billing_service.go
@@ -400,6 +400,18 @@ func (s *BillingService) initFallbackPricing() {
 		SupportsCacheBreakdown: false,
 	}
 
+	// Gemini 3.8 Flash (Google AI pricing: $0.75 input / $3.75 output /
+	// $0.075 cached input per MTok, promotional through 2026-12-31; official
+	// rates double to $1.50/$7.50/$0.15 from 2027-01-01). Antigravity's
+	// -high/-low/-medium/-tiered aliases are matched below so unavailable
+	// remote pricing never records token-bearing requests at $0.
+	s.fallbackPrices["gemini-3.8-flash"] = &ModelPricing{
+		InputPricePerToken:     0.75e-6,
+		OutputPricePerToken:    3.75e-6,
+		CacheReadPricePerToken: 0.075e-6,
+		SupportsCacheBreakdown: false,
+	}
+
 	// OpenAI GPT-5.4（业务指定价格）
 	s.fallbackPrices["gpt-5.4"] = &ModelPricing{
 		InputPricePerToken:             2.5e-6,  // $2.5 per MTok
@@ -851,6 +863,9 @@ func (s *BillingService) getFallbackPricing(model string) *ModelPricing {
 	if strings.Contains(modelLower, "gemini-3.6-flash") || strings.Contains(modelLower, "gemini-3-6-flash") {
 		return s.fallbackPrices["gemini-3.6-flash"]
 	}
+	if strings.Contains(modelLower, "gemini-3.8-flash") || strings.Contains(modelLower, "gemini-3-8-flash") {
+		return s.fallbackPrices["gemini-3.8-flash"]
+	}
 
 	// DeepSeek 系列：官方模型 V4 Pro/Flash（含 vision-exp）按各自价卡；
 	// 其余 deepseek-*（含已停服的 deepseek-chat / deepseek-reasoner 与未知型号）
diff --git a/backend/internal/service/pricing_service.go b/backend/internal/service/pricing_service.go
index 826f6a7803..84b6d448e0 100644
--- a/backend/internal/service/pricing_service.go
+++ b/backend/internal/service/pricing_service.go
@@ -1111,15 +1111,16 @@ func normalizeModelNameForPricing(model string) string {
 	return normalizeGeminiThinkingTierAlias(model)
 }
 
-// normalizeGeminiThinkingTierAlias maps Antigravity's Gemini 3.6 Flash
+// normalizeGeminiThinkingTierAlias maps Antigravity's Gemini Flash
 // thinking-tier model IDs to the public base model. The tier controls reasoning
 // behavior, not the published token rate, so this keeps -high/-low/-medium and
-// -tiered requests on the same price card as gemini-3.6-flash.
+// -tiered requests on the same price card as their base model.
 func normalizeGeminiThinkingTierAlias(model string) string {
-	const baseModel = "gemini-3.6-flash"
-	for _, tier := range []string{"-high", "-low", "-medium", "-tiered"} {
-		if model == baseModel+tier {
-			return baseModel
+	for _, baseModel := range []string{"gemini-3.6-flash", "gemini-3.8-flash"} {
+		for _, tier := range []string{"-high", "-low", "-medium", "-tiered"} {
+			if model == baseModel+tier {
+				return baseModel
+			}
 		}
 	}
 	return model
diff --git a/backend/internal/service/pricing_service_test.go b/backend/internal/service/pricing_service_test.go
index a02c74fe72..dc7674ef5c 100644
--- a/backend/internal/service/pricing_service_test.go
+++ b/backend/internal/service/pricing_service_test.go
@@ -533,6 +533,51 @@ func TestBillingService_Gemini36FlashThinkingTierFallbacksAreBillable(t *testing
 	}
 }
 
+func TestPricingService_Gemini38FlashThinkingTiersUseBasePricing(t *testing.T) {
+	basePricing := &LiteLLMModelPricing{
+		InputCostPerToken:       0.75e-6,
+		OutputCostPerToken:      3.75e-6,
+		CacheReadInputTokenCost: 0.075e-6,
+	}
+	svc := &PricingService{pricingData: map[string]*LiteLLMModelPricing{
+		"gemini-3.8-flash": basePricing,
+	}}
+
+	for _, model := range []string{
+		"gemini-3.8-flash",
+		"gemini-3.8-flash-high",
+		"gemini-3.8-flash-low",
+		"gemini-3.8-flash-medium",
+		"gemini-3.8-flash-tiered",
+	} {
+		t.Run(model, func(t *testing.T) {
+			require.Same(t, basePricing, svc.GetModelPricing(model))
+		})
+	}
+}
+
+func TestBillingService_Gemini38FlashThinkingTierFallbacksAreBillable(t *testing.T) {
+	svc := NewBillingService(&config.Config{}, nil)
+	tokens := UsageTokens{InputTokens: 1_000_000, OutputTokens: 1_000_000, CacheReadTokens: 1_000_000}
+
+	for _, model := range []string{
+		"gemini-3.8-flash",
+		"gemini-3.8-flash-high",
+		"gemini-3.8-flash-low",
+		"gemini-3.8-flash-medium",
+		"gemini-3.8-flash-tiered",
+	} {
+		t.Run(model, func(t *testing.T) {
+			cost, err := svc.CalculateCost(model, tokens, 1)
+			require.NoError(t, err)
+			require.InDelta(t, 0.75, cost.InputCost, 1e-12)
+			require.InDelta(t, 3.75, cost.OutputCost, 1e-12)
+			require.InDelta(t, 0.075, cost.CacheReadCost, 1e-12)
+			require.InDelta(t, 4.575, cost.TotalCost, 1e-12)
+		})
+	}
+}
+
 func TestDefaultPricingIncludesGemini36FlashRates(t *testing.T) {
 	data, err := os.ReadFile(filepath.Join("..", "..", "resources", "model-pricing", "model_prices_and_context_window.json"))
 	require.NoError(t, err)