diff --git a/backend/internal/handler/admin/channel_handler.go b/backend/internal/handler/admin/channel_handler.go
index 08c6370c..6fdf0e5e 100644
--- a/backend/internal/handler/admin/channel_handler.go
+++ b/backend/internal/handler/admin/channel_handler.go
@@ -130,7 +130,7 @@ func channelToResponse(ch *service.Channel) *channelResponse {
 	}
 	resp.BillingModelSource = ch.BillingModelSource
 	if resp.BillingModelSource == "" {
-		resp.BillingModelSource = "channel_mapped"
+		resp.BillingModelSource = service.BillingModelSourceChannelMapped
 	}
 	if resp.GroupIDs == nil {
 		resp.GroupIDs = []int64{}
@@ -147,11 +147,11 @@ func channelToResponse(ch *service.Channel) *channelResponse {
 		}
 		billingMode := string(p.BillingMode)
 		if billingMode == "" {
-			billingMode = "token"
+			billingMode = string(service.BillingModeToken)
 		}
 		platform := p.Platform
 		if platform == "" {
-			platform = "anthropic"
+			platform = service.PlatformAnthropic
 		}
 		intervals := make([]pricingIntervalResponse, 0, len(p.Intervals))
 		for _, iv := range p.Intervals {
@@ -194,7 +194,7 @@ func pricingRequestToService(reqs []channelModelPricingRequest) []service.Channe
 		}
 		platform := r.Platform
 		if platform == "" {
-			platform = "anthropic"
+			platform = service.PlatformAnthropic
 		}
 		intervals := make([]service.PricingInterval, 0, len(r.Intervals))
 		for _, iv := range r.Intervals {
diff --git a/backend/internal/service/gateway_service.go b/backend/internal/service/gateway_service.go
index 7ae5cf54..2ace9f61 100644
--- a/backend/internal/service/gateway_service.go
+++ b/backend/internal/service/gateway_service.go
@@ -60,6 +60,19 @@ const (
 	claudeMimicDebugInfoKey = "claude_mimic_debug_info"
 )
 
+// MediaType 媒体类型常量
+const (
+	MediaTypeImage  = "image"
+	MediaTypeVideo  = "video"
+	MediaTypePrompt = "prompt"
+)
+
+const (
+	claudeMaxMessageOverheadTokens = 3
+	claudeMaxBlockOverheadTokens   = 1
+	claudeMaxUnknownContentTokens  = 4
+)
+
 // ForceCacheBillingContextKey 强制缓存计费上下文键
 // 用于粘性会话切换时，将 input_tokens 转为 cache_read_input_tokens 计费
 type forceCacheBillingKeyType struct{}
@@ -7744,7 +7757,7 @@ func (s *GatewayService) RecordUsage(ctx context.Context, input *RecordUsageInpu
 	}
 
 	// 根据请求类型选择计费方式
-	if result.MediaType == "image" || result.MediaType == "video" {
+	if result.MediaType == MediaTypeImage || result.MediaType == MediaTypeVideo {
 		var soraConfig *SoraPriceConfig
 		if apiKey.Group != nil {
 			soraConfig = &SoraPriceConfig{
@@ -7754,12 +7767,12 @@ func (s *GatewayService) RecordUsage(ctx context.Context, input *RecordUsageInpu
 				VideoPricePerRequestHD: apiKey.Group.SoraVideoPricePerRequestHD,
 			}
 		}
-		if result.MediaType == "image" {
+		if result.MediaType == MediaTypeImage {
 			cost = s.billingService.CalculateSoraImageCost(result.ImageSize, result.ImageCount, soraConfig, multiplier)
 		} else {
 			cost = s.billingService.CalculateSoraVideoCost(billingModel, soraConfig, multiplier)
 		}
-	} else if result.MediaType == "prompt" {
+	} else if result.MediaType == MediaTypePrompt {
 		cost = &CostBreakdown{}
 	} else if result.ImageCount > 0 {
 		// 图片生成计费：渠道级别定价优先，否则走按次计费（兼容旧版本）
@@ -7767,7 +7780,7 @@ func (s *GatewayService) RecordUsage(ctx context.Context, input *RecordUsageInpu
 		if s.resolver != nil && apiKey.Group != nil {
 			gid := apiKey.Group.ID
 			resolved := s.resolver.Resolve(ctx, PricingInput{Model: billingModel, GroupID: &gid})
-			if resolved.Source == "channel" {
+			if resolved.Source == PricingSourceChannel {
 				hasChannelPricing = true
 			}
 		}
@@ -7900,15 +7913,15 @@ func (s *GatewayService) RecordUsage(ctx context.Context, input *RecordUsageInpu
 	}
 
 	// 设置计费模式
-	if result.MediaType != "image" && result.MediaType != "video" && result.MediaType != "prompt" {
+	if result.MediaType != MediaTypeImage && result.MediaType != MediaTypeVideo && result.MediaType != MediaTypePrompt {
 		if cost != nil && cost.BillingMode != "" {
 			billingMode := cost.BillingMode
 			usageLog.BillingMode = &billingMode
 		} else if result.ImageCount > 0 {
-			billingMode := "image"
+			billingMode := string(BillingModeImage)
 			usageLog.BillingMode = &billingMode
 		} else {
-			billingMode := "token"
+			billingMode := string(BillingModeToken)
 			usageLog.BillingMode = &billingMode
 		}
 	}
@@ -8038,7 +8051,7 @@ func (s *GatewayService) RecordUsageWithLongContext(ctx context.Context, input *
 		if s.resolver != nil && apiKey.Group != nil {
 			gid := apiKey.Group.ID
 			resolved := s.resolver.Resolve(ctx, PricingInput{Model: billingModel, GroupID: &gid})
-			if resolved.Source == "channel" {
+			if resolved.Source == PricingSourceChannel {
 				hasChannelPricing = true
 			}
 		}
@@ -8094,7 +8107,7 @@ func (s *GatewayService) RecordUsageWithLongContext(ctx context.Context, input *
 				Model:   billingModel,
 				GroupID: &gid,
 			})
-			if resolved.Source == "channel" {
+			if resolved.Source == PricingSourceChannel {
 				// 有渠道定价，渠道区间已包含上下文分层
 				cost, err = s.billingService.CalculateCostUnified(CostInput{
 					Ctx:            ctx,
@@ -8179,10 +8192,10 @@ func (s *GatewayService) RecordUsageWithLongContext(ctx context.Context, input *
 		billingMode := cost.BillingMode
 		usageLog.BillingMode = &billingMode
 	} else if result.ImageCount > 0 {
-		billingMode := "image"
+		billingMode := string(BillingModeImage)
 		usageLog.BillingMode = &billingMode
 	} else {
-		billingMode := "token"
+		billingMode := string(BillingModeToken)
 		usageLog.BillingMode = &billingMode
 	}
 
diff --git a/backend/internal/service/model_pricing_resolver.go b/backend/internal/service/model_pricing_resolver.go
index ffa03111..ad31008f 100644
--- a/backend/internal/service/model_pricing_resolver.go
+++ b/backend/internal/service/model_pricing_resolver.go
@@ -5,6 +5,13 @@ import (
 	"log/slog"
 )
 
+// PricingSource 定价来源标识
+const (
+	PricingSourceChannel  = "channel"
+	PricingSourceLiteLLM  = "litellm"
+	PricingSourceFallback = "fallback"
+)
+
 // ResolvedPricing 统一定价解析结果
 type ResolvedPricing struct {
 	// Mode 计费模式
@@ -78,9 +85,9 @@ func (r *ModelPricingResolver) resolveBasePricing(model string) (*ModelPricing,
 	if err != nil {
 		slog.Debug("failed to get model pricing from LiteLLM, using fallback",
 			"model", model, "error", err)
-		return nil, "fallback"
+		return nil, PricingSourceFallback
 	}
-	return pricing, "litellm"
+	return pricing, PricingSourceLiteLLM
 }
 
 // applyChannelOverrides 应用渠道定价覆盖
@@ -90,7 +97,7 @@ func (r *ModelPricingResolver) applyChannelOverrides(ctx context.Context, groupI
 		return
 	}
 
-	resolved.Source = "channel"
+	resolved.Source = PricingSourceChannel
 	resolved.Mode = chPricing.BillingMode
 	if resolved.Mode == "" {
 		resolved.Mode = BillingModeToken
diff --git a/backend/internal/service/openai_gateway_service.go b/backend/internal/service/openai_gateway_service.go
index beff1879..628370e3 100644
--- a/backend/internal/service/openai_gateway_service.go
+++ b/backend/internal/service/openai_gateway_service.go
@@ -4290,7 +4290,7 @@ func (s *OpenAIGatewayService) RecordUsage(ctx context.Context, input *OpenAIRec
 		billingMode := cost.BillingMode
 		usageLog.BillingMode = &billingMode
 	} else {
-		billingMode := "token"
+		billingMode := string(BillingModeToken)
 		usageLog.BillingMode = &billingMode
 	}
 	// 添加 UserAgent