diff --git a/backend/internal/config/config.go b/backend/internal/config/config.go
index 18baa34881..b8b880eab0 100644
--- a/backend/internal/config/config.go
+++ b/backend/internal/config/config.go
@@ -1945,7 +1945,10 @@ func setDefaults() {
 	viper.SetDefault("gateway.usage_record.worker_count", 128)
 	viper.SetDefault("gateway.usage_record.queue_size", 16384)
 	viper.SetDefault("gateway.usage_record.task_timeout_seconds", 5)
-	viper.SetDefault("gateway.usage_record.overflow_policy", UsageRecordOverflowPolicySample)
+	// 默认 sync：队列满时由提交方内联执行（提交点在响应写出之后，不阻塞客户端）。
+	// sample/drop 会在溢出时静默丢弃计费任务，造成扣费与 usage_logs 对账缺口（issue #3656），
+	// 仅供显式配置的运维场景使用。
+	viper.SetDefault("gateway.usage_record.overflow_policy", UsageRecordOverflowPolicySync)
 	viper.SetDefault("gateway.usage_record.overflow_sample_percent", 10)
 	viper.SetDefault("gateway.usage_record.auto_scale_enabled", true)
 	viper.SetDefault("gateway.usage_record.auto_scale_min_workers", 128)
diff --git a/backend/internal/config/config_test.go b/backend/internal/config/config_test.go
index bf7a327563..804155d1a9 100644
--- a/backend/internal/config/config_test.go
+++ b/backend/internal/config/config_test.go
@@ -1903,8 +1903,8 @@ func TestLoad_DefaultGatewayUsageRecordConfig(t *testing.T) {
 	if cfg.Gateway.UsageRecord.TaskTimeoutSeconds != 5 {
 		t.Fatalf("task_timeout_seconds = %d, want 5", cfg.Gateway.UsageRecord.TaskTimeoutSeconds)
 	}
-	if cfg.Gateway.UsageRecord.OverflowPolicy != UsageRecordOverflowPolicySample {
-		t.Fatalf("overflow_policy = %s, want %s", cfg.Gateway.UsageRecord.OverflowPolicy, UsageRecordOverflowPolicySample)
+	if cfg.Gateway.UsageRecord.OverflowPolicy != UsageRecordOverflowPolicySync {
+		t.Fatalf("overflow_policy = %s, want %s", cfg.Gateway.UsageRecord.OverflowPolicy, UsageRecordOverflowPolicySync)
 	}
 	if cfg.Gateway.UsageRecord.OverflowSamplePercent != 10 {
 		t.Fatalf("overflow_sample_percent = %d, want 10", cfg.Gateway.UsageRecord.OverflowSamplePercent)
diff --git a/backend/internal/repository/usage_log_repo.go b/backend/internal/repository/usage_log_repo.go
index 885e63f9fd..24c648b0a5 100644
--- a/backend/internal/repository/usage_log_repo.go
+++ b/backend/internal/repository/usage_log_repo.go
@@ -372,12 +372,13 @@ func (r *usageLogRepository) CreateBestEffort(ctx context.Context, log *service.
 		}
 	}
 
+	// 队列满时阻塞等待而非立即丢弃：批处理器持续排空队列，短暂等待即可入队。
+	// 立即丢弃会造成“已扣费但无 usage_log”的永久数据缺口（issue #3656）；
+	// 阻塞上限由调用方 ctx 期限约束，超时后由上层同步兜底。
 	select {
 	case r.bestEffortBatchCh <- req:
 	case <-ctx.Done():
 		return service.MarkUsageLogCreateDropped(ctx.Err())
-	default:
-		return service.MarkUsageLogCreateDropped(errors.New("usage log best-effort queue full"))
 	}
 
 	select {
@@ -493,12 +494,12 @@ func (r *usageLogRepository) createBatched(ctx context.Context, log *service.Usa
 		resultCh: make(chan usageLogCreateResult, 1),
 	}
 
+	// 队列满时阻塞等待而非立即报错：本路径是 best-effort 丢弃后的最后兜底，
+	// 立即失败会让日志永久丢失；阻塞上限由调用方 ctx 期限约束。
 	select {
 	case r.createBatchCh <- req:
 	case <-ctx.Done():
 		return false, service.MarkUsageLogCreateNotPersisted(ctx.Err())
-	default:
-		return false, service.MarkUsageLogCreateNotPersisted(errors.New("usage log create batch queue full"))
 	}
 
 	select {
@@ -520,22 +521,28 @@ func (r *usageLogRepository) createBatched(ctx context.Context, log *service.Usa
 }
 
 func (r *usageLogRepository) ensureCreateBatcher() {
-	if r == nil || r.db == nil || r.createBatchCh != nil {
+	if r == nil || r.db == nil {
 		return
 	}
+	// nil 检查必须在 Once 内部：在外层做无同步快路径读会与 Once 内的写构成数据竞争。
 	r.createBatchOnce.Do(func() {
-		r.createBatchCh = make(chan usageLogCreateRequest, usageLogCreateBatchQueueCap)
-		go r.runCreateBatcher(r.db)
+		if r.createBatchCh == nil {
+			r.createBatchCh = make(chan usageLogCreateRequest, usageLogCreateBatchQueueCap)
+			go r.runCreateBatcher(r.db)
+		}
 	})
 }
 
 func (r *usageLogRepository) ensureBestEffortBatcher() {
-	if r == nil || r.db == nil || r.bestEffortBatchCh != nil {
+	if r == nil || r.db == nil {
 		return
 	}
+	// 同 ensureCreateBatcher：nil 检查放在 Once 内部以避免数据竞争。
 	r.bestEffortBatchOnce.Do(func() {
-		r.bestEffortBatchCh = make(chan usageLogBestEffortRequest, usageLogBestEffortBatchQueueCap)
-		go r.runBestEffortBatcher(r.db)
+		if r.bestEffortBatchCh == nil {
+			r.bestEffortBatchCh = make(chan usageLogBestEffortRequest, usageLogBestEffortBatchQueueCap)
+			go r.runBestEffortBatcher(r.db)
+		}
 	})
 }
 
diff --git a/backend/internal/repository/usage_log_repo_integration_test.go b/backend/internal/repository/usage_log_repo_integration_test.go
index ed3050d89c..b43c6d56ef 100644
--- a/backend/internal/repository/usage_log_repo_integration_test.go
+++ b/backend/internal/repository/usage_log_repo_integration_test.go
@@ -288,21 +288,21 @@ func TestUsageLogRepositoryCreateBestEffort_BatchPathDuplicateRequestID(t *testi
 	}, 3*time.Second, 20*time.Millisecond)
 }
 
-func TestUsageLogRepositoryCreateBestEffort_QueueFullReturnsDropped(t *testing.T) {
-	ctx := context.Background()
+func TestUsageLogRepositoryCreateBestEffort_QueueFullBlocksUntilCtxDeadline(t *testing.T) {
+	// 队列满时不再立即丢弃：阻塞等待入队，直到调用方 ctx 到期才标记 dropped（issue #3656）。
 	client := testEntClient(t)
 	repo := newUsageLogRepositoryWithSQL(client, integrationDB)
 	repo.bestEffortBatchCh = make(chan usageLogBestEffortRequest, 1)
 	repo.bestEffortBatchCh <- usageLogBestEffortRequest{}
 
-	user := mustCreateUser(t, client, &service.User{Email: fmt.Sprintf("usage-best-effort-full-%d@example.com", time.Now().UnixNano())})
-	apiKey := mustCreateApiKey(t, client, &service.APIKey{UserID: user.ID, Key: "sk-usage-best-effort-full-" + uuid.NewString(), Name: "k"})
-	account := mustCreateAccount(t, client, &service.Account{Name: "acc-usage-best-effort-full-" + uuid.NewString()})
+	ctx, cancel := context.WithTimeout(context.Background(), 200*time.Millisecond)
+	defer cancel()
 
+	start := time.Now()
 	err := repo.CreateBestEffort(ctx, &service.UsageLog{
-		UserID:       user.ID,
-		APIKeyID:     apiKey.ID,
-		AccountID:    account.ID,
+		UserID:       1,
+		APIKeyID:     2,
+		AccountID:    3,
 		RequestID:    uuid.NewString(),
 		Model:        "claude-3",
 		InputTokens:  10,
@@ -314,6 +314,40 @@ func TestUsageLogRepositoryCreateBestEffort_QueueFullReturnsDropped(t *testing.T
 
 	require.Error(t, err)
 	require.True(t, service.IsUsageLogCreateDropped(err))
+	require.GreaterOrEqual(t, time.Since(start), 150*time.Millisecond)
+}
+
+func TestUsageLogRepositoryCreateBestEffort_QueueFullWaitsForDrain(t *testing.T) {
+	// 队列满但批处理器随后排空时，阻塞的入队应成功完成而非丢弃。
+	client := testEntClient(t)
+	repo := newUsageLogRepositoryWithSQL(client, integrationDB)
+	repo.bestEffortBatchCh = make(chan usageLogBestEffortRequest, 1)
+	repo.bestEffortBatchCh <- usageLogBestEffortRequest{}
+
+	go func() {
+		time.Sleep(100 * time.Millisecond)
+		<-repo.bestEffortBatchCh // 排空占位请求，为阻塞中的入队腾出空间
+		req := <-repo.bestEffortBatchCh
+		sendUsageLogBestEffortResult(req.resultCh, nil)
+	}()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	defer cancel()
+
+	err := repo.CreateBestEffort(ctx, &service.UsageLog{
+		UserID:       1,
+		APIKeyID:     2,
+		AccountID:    3,
+		RequestID:    uuid.NewString(),
+		Model:        "claude-3",
+		InputTokens:  10,
+		OutputTokens: 20,
+		TotalCost:    0.5,
+		ActualCost:   0.5,
+		CreatedAt:    time.Now().UTC(),
+	})
+
+	require.NoError(t, err)
 }
 
 func TestUsageLogRepositoryCreate_BatchPathCanceledContextMarksNotPersisted(t *testing.T) {
@@ -346,7 +380,7 @@ func TestUsageLogRepositoryCreate_BatchPathCanceledContextMarksNotPersisted(t *t
 }
 
 func TestUsageLogRepositoryCreate_BatchPathQueueFullMarksNotPersisted(t *testing.T) {
-	ctx := context.Background()
+	// 队列满时阻塞等待入队，直到调用方 ctx 到期才标记 not persisted（issue #3656）。
 	client := testEntClient(t)
 	repo := newUsageLogRepositoryWithSQL(client, integrationDB)
 	repo.createBatchCh = make(chan usageLogCreateRequest, 1)
@@ -356,6 +390,10 @@ func TestUsageLogRepositoryCreate_BatchPathQueueFullMarksNotPersisted(t *testing
 	apiKey := mustCreateApiKey(t, client, &service.APIKey{UserID: user.ID, Key: "sk-usage-create-full-" + uuid.NewString(), Name: "k"})
 	account := mustCreateAccount(t, client, &service.Account{Name: "acc-usage-create-full-" + uuid.NewString()})
 
+	ctx, cancel := context.WithTimeout(context.Background(), 200*time.Millisecond)
+	defer cancel()
+
+	start := time.Now()
 	inserted, err := repo.Create(ctx, &service.UsageLog{
 		UserID:       user.ID,
 		APIKeyID:     apiKey.ID,
@@ -372,6 +410,7 @@ func TestUsageLogRepositoryCreate_BatchPathQueueFullMarksNotPersisted(t *testing
 	require.False(t, inserted)
 	require.Error(t, err)
 	require.True(t, service.IsUsageLogCreateNotPersisted(err))
+	require.GreaterOrEqual(t, time.Since(start), 150*time.Millisecond)
 }
 
 func TestUsageLogRepositoryCreate_BatchPathCanceledAfterQueueMarksNotPersisted(t *testing.T) {
diff --git a/backend/internal/service/gateway_record_usage_test.go b/backend/internal/service/gateway_record_usage_test.go
index c819eeca6e..2769251820 100644
--- a/backend/internal/service/gateway_record_usage_test.go
+++ b/backend/internal/service/gateway_record_usage_test.go
@@ -440,7 +440,9 @@ func TestGatewayServiceRecordUsage_GeneratesRequestIDWhenAllSourcesMissing(t *te
 	require.Equal(t, billingRepo.lastCmd.RequestID, usageRepo.lastLog.RequestID)
 }
 
-func TestGatewayServiceRecordUsage_DroppedUsageLogDoesNotSyncFallback(t *testing.T) {
+func TestGatewayServiceRecordUsage_DroppedUsageLogFallsBackToSyncCreate(t *testing.T) {
+	// 计费成功后 best-effort 写入被丢弃（队列超时）时必须同步兜底，
+	// 否则出现“已扣费但无 usage_log”的对账缺口（issue #3656）。
 	usageRepo := &openAIRecordUsageBestEffortLogRepoStub{
 		bestEffortErr: MarkUsageLogCreateDropped(errors.New("usage log best-effort queue full")),
 	}
@@ -464,7 +466,9 @@ func TestGatewayServiceRecordUsage_DroppedUsageLogDoesNotSyncFallback(t *testing
 
 	require.NoError(t, err)
 	require.Equal(t, 1, usageRepo.bestEffortCalls)
-	require.Equal(t, 0, usageRepo.createCalls)
+	require.Equal(t, 1, usageRepo.createCalls)
+	// 兜底调用使用的 ctx 必须仍然存活，不能带着已死的 ctx 走过场。
+	require.NoError(t, usageRepo.lastCtxErr)
 }
 
 func TestGatewayServiceRecordUsage_BillingErrorSkipsUsageLogWrite(t *testing.T) {
diff --git a/backend/internal/service/gateway_service.go b/backend/internal/service/gateway_service.go
index 160a92a8e1..54035345d9 100644
--- a/backend/internal/service/gateway_service.go
+++ b/backend/internal/service/gateway_service.go
@@ -9473,10 +9473,17 @@ func writeUsageLogBestEffort(ctx context.Context, repo UsageLogRepository, usage
 	if writer, ok := repo.(usageLogBestEffortWriter); ok {
 		if err := writer.CreateBestEffort(usageCtx, usageLog); err != nil {
 			logger.LegacyPrintf(logKey, "Create usage log failed: %v", err)
-			if IsUsageLogCreateDropped(err) {
-				return
+			// 计费已在此前完成，日志必须落库：dropped（批处理队列超时）同样走同步兜底，
+			// 否则会出现“已扣费但无 usage_log”的对账缺口（issue #3656）。
+			// 重复写入由 usage_logs 的 ON CONFLICT (request_id, api_key_id) DO NOTHING 防护。
+			fallbackCtx := usageCtx
+			if usageCtx.Err() != nil {
+				// usageCtx 已耗尽（best-effort 入队阻塞到期限）：换新的 detached 窗口，避免兜底必然失败。
+				var fallbackCancel context.CancelFunc
+				fallbackCtx, fallbackCancel = detachedBillingContext(context.Background())
+				defer fallbackCancel()
 			}
-			if _, syncErr := repo.Create(usageCtx, usageLog); syncErr != nil {
+			if _, syncErr := repo.Create(fallbackCtx, usageLog); syncErr != nil {
 				logger.LegacyPrintf(logKey, "Create usage log sync fallback failed: %v", syncErr)
 			}
 		}
diff --git a/backend/internal/service/usage_record_worker_pool.go b/backend/internal/service/usage_record_worker_pool.go
index 5da0b89023..bb5ae452c8 100644
--- a/backend/internal/service/usage_record_worker_pool.go
+++ b/backend/internal/service/usage_record_worker_pool.go
@@ -15,10 +15,11 @@ import (
 )
 
 const (
-	defaultUsageRecordWorkerCount          = 128
-	defaultUsageRecordQueueSize            = 16384
-	defaultUsageRecordTaskTimeoutSeconds   = 5
-	defaultUsageRecordOverflowPolicy       = config.UsageRecordOverflowPolicySample
+	defaultUsageRecordWorkerCount        = 128
+	defaultUsageRecordQueueSize          = 16384
+	defaultUsageRecordTaskTimeoutSeconds = 5
+	// 默认 sync：溢出时提交方内联执行，保证计费任务不被静默丢弃（issue #3656）。
+	defaultUsageRecordOverflowPolicy       = config.UsageRecordOverflowPolicySync
 	defaultUsageRecordOverflowSampleRatio  = 10
 	defaultUsageRecordAutoScaleEnabled     = true
 	defaultUsageRecordAutoScaleMinWorkers  = 128