diff --git a/backend/internal/handler/batch_image_handler.go b/backend/internal/handler/batch_image_handler.go
index b93c5a65ae..62990d251a 100644
--- a/backend/internal/handler/batch_image_handler.go
+++ b/backend/internal/handler/batch_image_handler.go
@@ -42,6 +42,9 @@ func (h *BatchImageHandler) Submit(c *gin.Context) {
 	if !h.checkSecurityAuditBeforeSubmit(c, &req) {
 		return
 	}
+	if sessionID := service.ExtractClientSessionID(c); sessionID != "" {
+		req.SessionID = &sessionID
+	}
 	got, err := h.service.Submit(c.Request.Context(), owner, req, c.GetHeader("Idempotency-Key"))
 	if err != nil {
 		batchImageError(c, err)
diff --git a/backend/internal/handler/dto/mappers.go b/backend/internal/handler/dto/mappers.go
index 1ae9a5e790..ec55117846 100644
--- a/backend/internal/handler/dto/mappers.go
+++ b/backend/internal/handler/dto/mappers.go
@@ -669,6 +669,7 @@ func usageLogFromServiceUser(l *service.UsageLog) UsageLog {
 		MediaType:                 l.MediaType,
 		UserAgent:                 l.UserAgent,
 		IPAddress:                 l.IPAddress,
+		SessionID:                 l.SessionID,
 		CacheTTLOverridden:        l.CacheTTLOverridden,
 		BillingMode:               l.BillingMode,
 		CreatedAt:                 l.CreatedAt,
diff --git a/backend/internal/handler/dto/types.go b/backend/internal/handler/dto/types.go
index 38133cff3b..34b631cd5c 100644
--- a/backend/internal/handler/dto/types.go
+++ b/backend/internal/handler/dto/types.go
@@ -521,6 +521,9 @@ type UsageLog struct {
 	UserAgent *string `json:"user_agent"`
 	// IPAddress is visible to the owner of the usage record.
 	IPAddress *string `json:"ip_address,omitempty"`
+	// SessionID is the explicit client-provided request correlation identifier
+	// (e.g. the session_id / X-Session-Id headers). Omitted when absent.
+	SessionID *string `json:"session_id,omitempty"`
 
 	// Cache TTL Override 标记
 	CacheTTLOverridden bool `json:"cache_ttl_overridden"`
diff --git a/backend/internal/handler/gateway_handler.go b/backend/internal/handler/gateway_handler.go
index 1ea81d1fb5..3d014f9f45 100644
--- a/backend/internal/handler/gateway_handler.go
+++ b/backend/internal/handler/gateway_handler.go
@@ -535,6 +535,7 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 			// ForceCacheBilling 提前拍成标量，避免 worker 闭包保活 failover 状态里的响应体。
 			forceCacheBilling := fs.ForceCacheBilling
 			quotaPlatform := service.QuotaPlatform(c.Request.Context(), apiKey)
+			sessionID := service.ExtractClientSessionID(c)
 			h.submitUsageRecordTask(c.Request.Context(), func(ctx context.Context) {
 				if err := h.gatewayService.RecordUsage(ctx, &service.RecordUsageInput{
 					Result:             result,
@@ -547,6 +548,7 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 					UpstreamEndpoint:   upstreamEndpoint,
 					UserAgent:          userAgent,
 					IPAddress:          clientIP,
+					SessionID:          sessionID,
 					RequestPayloadHash: requestPayloadHash,
 					ForceCacheBilling:  forceCacheBilling,
 					APIKeyService:      h.apiKeyService,
@@ -970,6 +972,7 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 			// ForceCacheBilling 提前拍成标量，避免 worker 闭包保活 failover 状态里的响应体。
 			forceCacheBilling := fs.ForceCacheBilling
 			quotaPlatform := service.QuotaPlatform(c.Request.Context(), currentAPIKey)
+			sessionID := service.ExtractClientSessionID(c)
 			h.submitUsageRecordTask(c.Request.Context(), func(ctx context.Context) {
 				if err := h.gatewayService.RecordUsage(ctx, &service.RecordUsageInput{
 					Result:             result,
@@ -982,6 +985,7 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 					UpstreamEndpoint:   upstreamEndpoint,
 					UserAgent:          userAgent,
 					IPAddress:          clientIP,
+					SessionID:          sessionID,
 					RequestPayloadHash: requestPayloadHash,
 					ForceCacheBilling:  forceCacheBilling,
 					APIKeyService:      h.apiKeyService,
diff --git a/backend/internal/handler/gateway_handler_chat_completions.go b/backend/internal/handler/gateway_handler_chat_completions.go
index 3cfb9026e2..7061325f8a 100644
--- a/backend/internal/handler/gateway_handler_chat_completions.go
+++ b/backend/internal/handler/gateway_handler_chat_completions.go
@@ -298,6 +298,7 @@ func (h *GatewayHandler) ChatCompletions(c *gin.Context) {
 		upstreamEndpoint := GetUpstreamEndpoint(c, account.Platform)
 
 		quotaPlatform := service.QuotaPlatform(c.Request.Context(), apiKey)
+		sessionID := service.ExtractClientSessionID(c)
 		h.submitUsageRecordTask(c.Request.Context(), func(ctx context.Context) {
 			if err := h.gatewayService.RecordUsage(ctx, &service.RecordUsageInput{
 				Result:             result,
@@ -312,6 +313,7 @@ func (h *GatewayHandler) ChatCompletions(c *gin.Context) {
 				IPAddress:          clientIP,
 				RequestPayloadHash: requestPayloadHash,
 				APIKeyService:      h.apiKeyService,
+				SessionID:          sessionID,
 				ChannelUsageFields: clientRequestedUsageFields(c, channelMapping, reqModel, result.UpstreamModel),
 			}); err != nil {
 				reqLog.Error("gateway.cc.record_usage_failed",
diff --git a/backend/internal/handler/gateway_handler_responses.go b/backend/internal/handler/gateway_handler_responses.go
index bd545d2cf6..8f5d8531bb 100644
--- a/backend/internal/handler/gateway_handler_responses.go
+++ b/backend/internal/handler/gateway_handler_responses.go
@@ -276,6 +276,7 @@ func (h *GatewayHandler) Responses(c *gin.Context) {
 		upstreamEndpoint := GetUpstreamEndpoint(c, account.Platform)
 
 		quotaPlatform := service.QuotaPlatform(c.Request.Context(), apiKey)
+		sessionID := service.ExtractClientSessionID(c)
 		h.submitUsageRecordTask(c.Request.Context(), func(ctx context.Context) {
 			if err := h.gatewayService.RecordUsage(ctx, &service.RecordUsageInput{
 				Result:             result,
@@ -290,6 +291,7 @@ func (h *GatewayHandler) Responses(c *gin.Context) {
 				IPAddress:          clientIP,
 				RequestPayloadHash: requestPayloadHash,
 				APIKeyService:      h.apiKeyService,
+				SessionID:          sessionID,
 				ChannelUsageFields: clientRequestedUsageFields(c, channelMapping, reqModel, result.UpstreamModel),
 			}); err != nil {
 				reqLog.Error("gateway.responses.record_usage_failed",
diff --git a/backend/internal/handler/gemini_v1beta_handler.go b/backend/internal/handler/gemini_v1beta_handler.go
index 7b349ae1db..40470ac418 100644
--- a/backend/internal/handler/gemini_v1beta_handler.go
+++ b/backend/internal/handler/gemini_v1beta_handler.go
@@ -532,6 +532,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 		// ForceCacheBilling 提前拍成标量，避免 worker 闭包保活 failover 状态里的响应体。
 		forceCacheBilling := fs.ForceCacheBilling
 		quotaPlatform := service.QuotaPlatform(c.Request.Context(), apiKey)
+		sessionID := service.ExtractClientSessionID(c)
 		h.submitUsageRecordTask(c.Request.Context(), func(ctx context.Context) {
 			if err := h.gatewayService.RecordUsageWithLongContext(ctx, &service.RecordUsageLongContextInput{
 				Result:                result,
@@ -549,6 +550,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 				LongContextMultiplier: 2.0,    // 超出部分双倍计费
 				ForceCacheBilling:     forceCacheBilling,
 				APIKeyService:         h.apiKeyService,
+				SessionID:             sessionID,
 				ChannelUsageFields:    clientRequestedUsageFields(c, channelMapping, reqModel, result.UpstreamModel),
 			}); err != nil {
 				logger.L().With(
diff --git a/backend/internal/handler/grok_media.go b/backend/internal/handler/grok_media.go
index 0522d689b1..fd0697884e 100644
--- a/backend/internal/handler/grok_media.go
+++ b/backend/internal/handler/grok_media.go
@@ -465,6 +465,7 @@ func recordGrokMediaUsage(
 ) {
 	userAgent := c.GetHeader("User-Agent")
 	clientIP := ip.GetClientIP(c)
+	sessionID := service.ExtractClientSessionID(c)
 	payloadForHash := body
 	if len(payloadForHash) == 0 && strings.TrimSpace(requestID) != "" {
 		payloadForHash = []byte(requestID)
@@ -493,6 +494,7 @@ func recordGrokMediaUsage(
 			RequestPayloadHash: service.HashUsageRequestPayload(payloadForHash),
 			APIKeyService:      h.apiKeyService,
 			QuotaPlatform:      quotaPlatform,
+			SessionID:          sessionID,
 			ChannelUsageFields: channelUsageFields,
 		}); err != nil {
 			logger.L().With(
diff --git a/backend/internal/handler/openai_alpha_search.go b/backend/internal/handler/openai_alpha_search.go
index eca8931e87..c76b7d9fa5 100644
--- a/backend/internal/handler/openai_alpha_search.go
+++ b/backend/internal/handler/openai_alpha_search.go
@@ -233,6 +233,7 @@ func (h *OpenAIGatewayHandler) recordAlphaSearchUsage(
 ) {
 	userAgent := c.GetHeader("User-Agent")
 	clientIP := ip.GetClientIP(c)
+	sessionID := service.ExtractClientSessionID(c)
 	requestPayloadHash := service.HashUsageRequestPayload(body)
 	inboundEndpoint := GetInboundEndpoint(c)
 	upstreamEndpoint := GetUpstreamEndpoint(c, account.Platform)
@@ -252,6 +253,7 @@ func (h *OpenAIGatewayHandler) recordAlphaSearchUsage(
 			RequestPayloadHash: requestPayloadHash,
 			APIKeyService:      h.apiKeyService,
 			QuotaPlatform:      quotaPlatform,
+			SessionID:          sessionID,
 			ChannelUsageFields: channelMapping.ToUsageFields(requestedModel, result.UpstreamModel),
 		}); err != nil {
 			logger.L().With(
diff --git a/backend/internal/handler/openai_chat_completions.go b/backend/internal/handler/openai_chat_completions.go
index 7477057113..39c12253b1 100644
--- a/backend/internal/handler/openai_chat_completions.go
+++ b/backend/internal/handler/openai_chat_completions.go
@@ -340,6 +340,7 @@ func (h *OpenAIGatewayHandler) ChatCompletions(c *gin.Context) {
 		inboundEndpoint := GetInboundEndpoint(c)
 		upstreamEndpoint := resolveOpenAIUpstreamEndpoint(c, account, result)
 		quotaPlatform := service.QuotaPlatform(c.Request.Context(), apiKey)
+		sessionID := service.ExtractClientSessionID(c)
 
 		cyberBlocked := service.GetOpsCyberPolicy(c) != nil
 		h.submitOpenAIUsageRecordTask(c.Request.Context(), result, func(ctx context.Context) {
@@ -355,6 +356,7 @@ func (h *OpenAIGatewayHandler) ChatCompletions(c *gin.Context) {
 				IPAddress:          clientIP,
 				APIKeyService:      h.apiKeyService,
 				QuotaPlatform:      quotaPlatform,
+				SessionID:          sessionID,
 				ChannelUsageFields: clientRequestedUsageFields(c, channelMapping, reqModel, result.UpstreamModel),
 				CyberBlocked:       cyberBlocked,
 			}); err != nil {
diff --git a/backend/internal/handler/openai_embeddings.go b/backend/internal/handler/openai_embeddings.go
index b7bf64ce0e..3ed44b12b8 100644
--- a/backend/internal/handler/openai_embeddings.go
+++ b/backend/internal/handler/openai_embeddings.go
@@ -243,6 +243,7 @@ func (h *OpenAIGatewayHandler) Embeddings(c *gin.Context) {
 		inboundEndpoint := GetInboundEndpoint(c)
 		upstreamEndpoint := GetUpstreamEndpoint(c, account.Platform)
 		quotaPlatform := service.QuotaPlatform(c.Request.Context(), apiKey)
+		sessionID := service.ExtractClientSessionID(c)
 
 		h.submitOpenAIUsageRecordTask(c.Request.Context(), result, func(ctx context.Context) {
 			if err := h.gatewayService.RecordUsage(ctx, &service.OpenAIRecordUsageInput{
@@ -257,6 +258,7 @@ func (h *OpenAIGatewayHandler) Embeddings(c *gin.Context) {
 				IPAddress:          clientIP,
 				APIKeyService:      h.apiKeyService,
 				QuotaPlatform:      quotaPlatform,
+				SessionID:          sessionID,
 				ChannelUsageFields: clientRequestedUsageFields(c, channelMapping, reqModel, result.UpstreamModel),
 			}); err != nil {
 				logger.L().With(
diff --git a/backend/internal/handler/openai_gateway_handler.go b/backend/internal/handler/openai_gateway_handler.go
index 1e5949f36f..761f291e90 100644
--- a/backend/internal/handler/openai_gateway_handler.go
+++ b/backend/internal/handler/openai_gateway_handler.go
@@ -627,6 +627,7 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 		inboundEndpoint := GetInboundEndpoint(c)
 		upstreamEndpoint := resolveOpenAIUpstreamEndpoint(c, account, result)
 		quotaPlatform := service.QuotaPlatform(c.Request.Context(), apiKey)
+		sessionID := service.ExtractClientSessionID(c)
 
 		// 使用量记录通过有界 worker 池提交，避免请求热路径创建无界 goroutine。
 		cyberBlocked := service.GetOpsCyberPolicy(c) != nil
@@ -644,6 +645,7 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 				RequestPayloadHash: requestPayloadHash,
 				APIKeyService:      h.apiKeyService,
 				QuotaPlatform:      quotaPlatform,
+				SessionID:          sessionID,
 				ChannelUsageFields: clientRequestedUsageFields(c, channelMapping, reqModel, result.UpstreamModel),
 				CyberBlocked:       cyberBlocked,
 			}); err != nil {
@@ -1138,6 +1140,7 @@ func (h *OpenAIGatewayHandler) Messages(c *gin.Context) {
 		inboundEndpoint := GetInboundEndpoint(c)
 		upstreamEndpoint := resolveOpenAIUpstreamEndpoint(c, account, result)
 		quotaPlatform := service.QuotaPlatform(c.Request.Context(), apiKey)
+		sessionID := service.ExtractClientSessionID(c)
 
 		cyberBlocked := service.GetOpsCyberPolicy(c) != nil
 		h.submitOpenAIUsageRecordTask(c.Request.Context(), result, func(ctx context.Context) {
@@ -1154,6 +1157,7 @@ func (h *OpenAIGatewayHandler) Messages(c *gin.Context) {
 				RequestPayloadHash: requestPayloadHash,
 				APIKeyService:      h.apiKeyService,
 				QuotaPlatform:      quotaPlatform,
+				SessionID:          sessionID,
 				ChannelUsageFields: clientRequestedUsageFields(c, channelMappingMsg, reqModel, result.UpstreamModel),
 				CyberBlocked:       cyberBlocked,
 			}); err != nil {
@@ -1862,6 +1866,7 @@ func (h *OpenAIGatewayHandler) ResponsesWebSocket(c *gin.Context) {
 				inboundEndpoint := GetInboundEndpoint(c)
 				upstreamEndpoint := resolveOpenAIUpstreamEndpoint(c, account, result)
 				quotaPlatform := service.QuotaPlatform(c.Request.Context(), apiKey)
+				sessionID := service.ExtractClientSessionID(c)
 				cyberBlocked := service.GetOpsCyberPolicy(c) != nil
 				h.submitOpenAIUsageRecordTask(ctx, result, func(taskCtx context.Context) {
 					if err := h.gatewayService.RecordUsage(taskCtx, &service.OpenAIRecordUsageInput{
@@ -1877,6 +1882,7 @@ func (h *OpenAIGatewayHandler) ResponsesWebSocket(c *gin.Context) {
 						RequestPayloadHash: requestPayloadHash,
 						APIKeyService:      h.apiKeyService,
 						QuotaPlatform:      quotaPlatform,
+						SessionID:          sessionID,
 						ChannelUsageFields: clientRequestedUsageFields(c, channelMappingWS, reqModel, result.UpstreamModel),
 						CyberBlocked:       cyberBlocked,
 					}); err != nil {
@@ -2884,6 +2890,8 @@ func (h *OpenAIGatewayHandler) recordCyberPolicyIfMarked(c *gin.Context, apiKey
 		userAgent = c.GetHeader("User-Agent")
 		clientIPStr = strings.TrimSpace(ip.GetClientIP(c))
 	}
+	// 提前拍成标量，避免在下方 goroutine 内访问 gin.Context。
+	sessionID := service.ExtractClientSessionID(c)
 	apiKeyPrefix := ""
 	if apiKey != nil {
 		apiKeyPrefix = keyPrefix(apiKey.Key, 8)
@@ -2940,6 +2948,7 @@ func (h *OpenAIGatewayHandler) recordCyberPolicyIfMarked(c *gin.Context, apiKey
 				UpstreamEndpoint:   upstreamEndpoint,
 				UserAgent:          userAgent,
 				IPAddress:          clientIPStr,
+				SessionID:          sessionID,
 				RequestPayloadHash: requestPayloadHash,
 				APIKeyService:      apiKeySvc,
 				ChannelUsageFields: channelFields,
diff --git a/backend/internal/handler/openai_images.go b/backend/internal/handler/openai_images.go
index 814a515a1e..72e2f462ad 100644
--- a/backend/internal/handler/openai_images.go
+++ b/backend/internal/handler/openai_images.go
@@ -376,6 +376,7 @@ func (h *OpenAIGatewayHandler) Images(c *gin.Context) {
 		if result != nil {
 			upstreamModel = result.UpstreamModel
 		}
+		sessionID := service.ExtractClientSessionID(c)
 		h.submitMandatoryUsageRecordTask(c.Request.Context(), func(ctx context.Context) {
 			if err := h.gatewayService.RecordUsage(ctx, &service.OpenAIRecordUsageInput{
 				Result:             result,
@@ -390,6 +391,7 @@ func (h *OpenAIGatewayHandler) Images(c *gin.Context) {
 				RequestPayloadHash: requestPayloadHash,
 				APIKeyService:      h.apiKeyService,
 				QuotaPlatform:      quotaPlatform,
+				SessionID:          sessionID,
 				ChannelUsageFields: clientRequestedUsageFields(c, channelMapping, requestModel, upstreamModel),
 			}); err != nil {
 				logger.L().With(
diff --git a/backend/internal/repository/batch_image_repo.go b/backend/internal/repository/batch_image_repo.go
index 6531f75fd5..7c8991c8d4 100644
--- a/backend/internal/repository/batch_image_repo.go
+++ b/backend/internal/repository/batch_image_repo.go
@@ -750,7 +750,7 @@ INSERT INTO batch_image_jobs (
     batch_discount_multiplier, hold_multiplier, billable_unit_price, hold_unit_price,
     pricing_snapshot_version,
     currency, hold_id,
-    idempotency_key, request_hash, manifest_hash, retry_count, output_expires_at
+    idempotency_key, request_hash, manifest_hash, retry_count, session_id, output_expires_at
 ) VALUES (
     $1, $2, $3, $4, $5, $6, $7, $8, $9,
     $10, $11, $12, $13, $14,
@@ -760,7 +760,7 @@ INSERT INTO batch_image_jobs (
     $25, $26, $27, $28,
     $29,
     $30, $31,
-    $32, $33, $34, $35, $36
+    $32, $33, $34, $35, $36, $37
 )
 RETURNING `+batchImageJobColumns,
 		params.BatchID, params.UserID, params.APIKeyID, params.AccountID, params.Provider, params.Model, params.TaskName, params.ParentBatchID, params.Status,
@@ -771,7 +771,7 @@ RETURNING `+batchImageJobColumns,
 		params.BatchDiscountMultiplier, params.HoldMultiplier, params.BillableUnitPrice, params.HoldUnitPrice,
 		params.PricingSnapshotVersion,
 		params.Currency, params.HoldID,
-		params.IdempotencyKey, params.RequestHash, params.ManifestHash, params.RetryCount, params.OutputExpiresAt,
+		params.IdempotencyKey, params.RequestHash, params.ManifestHash, params.RetryCount, params.SessionID, params.OutputExpiresAt,
 	))
 }
 
@@ -825,7 +825,7 @@ batch_discount_multiplier, hold_multiplier, billable_unit_price, hold_unit_price
 pricing_snapshot_version,
 currency, hold_id,
 idempotency_key, request_hash, manifest_hash,
-retry_count, version, output_expires_at, input_deleted_at, output_deleted_at, downloaded_at, user_deleted_at,
+retry_count, version, session_id, output_expires_at, input_deleted_at, output_deleted_at, downloaded_at, user_deleted_at,
 last_error_code, last_error_message,
 created_at, updated_at, submitted_at, started_at, finished_at, settled_at`
 
@@ -838,6 +838,7 @@ func scanBatchImageJob(row rowScanner) (*service.BatchImageJob, error) {
 	var parentBatchID sql.NullString
 	var holdAmount, actualCost sql.NullFloat64
 	var holdID, idempotencyKey, requestHash, manifestHash sql.NullString
+	var sessionID sql.NullString
 	var outputExpiresAt, inputDeletedAt, outputDeletedAt, downloadedAt, userDeletedAt sql.NullTime
 	var lastErrorCode, lastErrorMessage sql.NullString
 	var submittedAt, startedAt, finishedAt, settledAt sql.NullTime
@@ -852,7 +853,7 @@ func scanBatchImageJob(row rowScanner) (*service.BatchImageJob, error) {
 		&job.PricingSnapshotVersion,
 		&job.Currency, &holdID,
 		&idempotencyKey, &requestHash, &manifestHash,
-		&job.RetryCount, &job.Version, &outputExpiresAt, &inputDeletedAt, &outputDeletedAt, &downloadedAt, &userDeletedAt,
+		&job.RetryCount, &job.Version, &sessionID, &outputExpiresAt, &inputDeletedAt, &outputDeletedAt, &downloadedAt, &userDeletedAt,
 		&lastErrorCode, &lastErrorMessage,
 		&job.CreatedAt, &job.UpdatedAt, &submittedAt, &startedAt, &finishedAt, &settledAt,
 	)
@@ -874,6 +875,7 @@ func scanBatchImageJob(row rowScanner) (*service.BatchImageJob, error) {
 	job.IdempotencyKey = batchImageNullStringPtr(idempotencyKey)
 	job.RequestHash = batchImageNullStringPtr(requestHash)
 	job.ManifestHash = batchImageNullStringPtr(manifestHash)
+	job.SessionID = batchImageNullStringPtr(sessionID)
 	job.OutputExpiresAt = batchImageNullTimePtr(outputExpiresAt)
 	job.InputDeletedAt = batchImageNullTimePtr(inputDeletedAt)
 	job.OutputDeletedAt = batchImageNullTimePtr(outputDeletedAt)
diff --git a/backend/internal/repository/usage_log_repo_insert.go b/backend/internal/repository/usage_log_repo_insert.go
index 54d24b5957..809b0652b5 100644
--- a/backend/internal/repository/usage_log_repo_insert.go
+++ b/backend/internal/repository/usage_log_repo_insert.go
@@ -79,6 +79,7 @@ var usageLogInsertArgTypes = [...]string{
 	"text",        // billing_tier
 	"text",        // billing_mode
 	"numeric",     // account_stats_cost
+	"text",        // session_id
 	"timestamptz", // created_at
 }
 
@@ -274,6 +275,7 @@ func (r *usageLogRepository) createSingle(ctx context.Context, sqlq sqlExecutor,
 			billing_tier,
 			billing_mode,
 			account_stats_cost,
+			session_id,
 			created_at
 		) VALUES (
 			$1, $2, $3, $4, $5, $6, $7,
@@ -281,7 +283,7 @@ func (r *usageLogRepository) createSingle(ctx context.Context, sqlq sqlExecutor,
 			$10, $11, $12, $13,
 			$14, $15, $16, $17,
 			$18, $19, $20, $21, $22, $23,
-			$24, $25, $26, $27, $28, $29, $30, $31, $32, $33, $34, $35, $36, $37, $38, $39, $40, $41, $42, $43, $44, $45, $46, $47, $48, $49, $50, $51, $52, $53, $54, $55, $56
+			$24, $25, $26, $27, $28, $29, $30, $31, $32, $33, $34, $35, $36, $37, $38, $39, $40, $41, $42, $43, $44, $45, $46, $47, $48, $49, $50, $51, $52, $53, $54, $55, $56, $57
 		)
 		ON CONFLICT (request_id, api_key_id) DO NOTHING
 		RETURNING id, created_at
@@ -728,10 +730,13 @@ func buildUsageLogBatchInsertQuery(keys []string, preparedByKey map[string]usage
 			billing_tier,
 			billing_mode,
 			account_stats_cost,
+			session_id,
 			created_at
 		) AS (VALUES `)
 
-	args := make([]any, 0, len(keys)*56)
+	// Each batch row prepends the synthetic input_index before the 57
+	// usage-log column values.
+	args := make([]any, 0, len(keys)*58)
 	argPos := 1
 	for idx, key := range keys {
 		if idx > 0 {
@@ -815,6 +820,7 @@ func buildUsageLogBatchInsertQuery(keys []string, preparedByKey map[string]usage
 				billing_tier,
 				billing_mode,
 				account_stats_cost,
+				session_id,
 				created_at
 			)
 			SELECT
@@ -873,6 +879,7 @@ func buildUsageLogBatchInsertQuery(keys []string, preparedByKey map[string]usage
 				billing_tier,
 				billing_mode,
 				account_stats_cost,
+				session_id,
 				created_at
 			FROM input
 			ON CONFLICT (request_id, api_key_id) DO NOTHING
@@ -971,10 +978,11 @@ func buildUsageLogBestEffortInsertQuery(preparedList []usageLogInsertPrepared) (
 			billing_tier,
 			billing_mode,
 			account_stats_cost,
+			session_id,
 			created_at
 		) AS (VALUES `)
 
-	args := make([]any, 0, len(preparedList)*56)
+	args := make([]any, 0, len(preparedList)*57)
 	argPos := 1
 	for idx, prepared := range preparedList {
 		if idx > 0 {
@@ -1055,6 +1063,7 @@ func buildUsageLogBestEffortInsertQuery(preparedList []usageLogInsertPrepared) (
 			billing_tier,
 			billing_mode,
 			account_stats_cost,
+			session_id,
 			created_at
 		)
 		SELECT
@@ -1113,6 +1122,7 @@ func buildUsageLogBestEffortInsertQuery(preparedList []usageLogInsertPrepared) (
 			billing_tier,
 			billing_mode,
 			account_stats_cost,
+			session_id,
 			created_at
 		FROM input
 		ON CONFLICT (request_id, api_key_id) DO NOTHING
@@ -1179,6 +1189,7 @@ func execUsageLogInsertNoResult(ctx context.Context, sqlq sqlExecutor, prepared
 			billing_tier,
 			billing_mode,
 			account_stats_cost,
+			session_id,
 			created_at
 		) VALUES (
 			$1, $2, $3, $4, $5, $6, $7,
@@ -1186,7 +1197,7 @@ func execUsageLogInsertNoResult(ctx context.Context, sqlq sqlExecutor, prepared
 			$10, $11, $12, $13,
 			$14, $15, $16, $17,
 			$18, $19, $20, $21, $22, $23,
-			$24, $25, $26, $27, $28, $29, $30, $31, $32, $33, $34, $35, $36, $37, $38, $39, $40, $41, $42, $43, $44, $45, $46, $47, $48, $49, $50, $51, $52, $53, $54, $55, $56
+			$24, $25, $26, $27, $28, $29, $30, $31, $32, $33, $34, $35, $36, $37, $38, $39, $40, $41, $42, $43, $44, $45, $46, $47, $48, $49, $50, $51, $52, $53, $54, $55, $56, $57
 		)
 		ON CONFLICT (request_id, api_key_id) DO NOTHING
 	`, prepared.args...)
@@ -1227,6 +1238,7 @@ func prepareUsageLogInsert(log *service.UsageLog) usageLogInsertPrepared {
 	modelMappingChain := nullString(log.ModelMappingChain)
 	billingTier := nullString(log.BillingTier)
 	billingMode := nullString(log.BillingMode)
+	sessionID := nullString(log.SessionID)
 	requestedModel := strings.TrimSpace(log.RequestedModel)
 	if requestedModel == "" {
 		requestedModel = strings.TrimSpace(log.Model)
@@ -1299,6 +1311,7 @@ func prepareUsageLogInsert(log *service.UsageLog) usageLogInsertPrepared {
 			billingTier,
 			billingMode,
 			log.AccountStatsCost, // account_stats_cost
+			sessionID,            // session_id
 			createdAt,
 		},
 	}
diff --git a/backend/internal/repository/usage_log_repo_query.go b/backend/internal/repository/usage_log_repo_query.go
index fb0e03ed60..88b7eeaf35 100644
--- a/backend/internal/repository/usage_log_repo_query.go
+++ b/backend/internal/repository/usage_log_repo_query.go
@@ -19,7 +19,7 @@ import (
 	"github.com/Wei-Shaw/sub2api/internal/service"
 )
 
-const usageLogSelectColumns = "id, user_id, api_key_id, account_id, request_id, model, requested_model, upstream_model, group_id, subscription_id, input_tokens, output_tokens, cache_creation_tokens, cache_read_tokens, cache_creation_5m_tokens, cache_creation_1h_tokens, image_output_tokens, image_output_cost, image_input_tokens, image_input_cost, input_cost, output_cost, cache_creation_cost, cache_read_cost, total_cost, actual_cost, rate_multiplier, account_rate_multiplier, billing_type, request_type, stream, openai_ws_mode, duration_ms, first_token_ms, user_agent, ip_address, image_count, image_size, image_input_size, image_output_size, image_size_source, image_size_breakdown, video_count, video_resolution, video_duration_seconds, service_tier, reasoning_effort, inbound_endpoint, upstream_endpoint, cache_ttl_overridden, long_context_billing_applied, channel_id, model_mapping_chain, billing_tier, billing_mode, account_stats_cost, created_at"
+const usageLogSelectColumns = "id, user_id, api_key_id, account_id, request_id, model, requested_model, upstream_model, group_id, subscription_id, input_tokens, output_tokens, cache_creation_tokens, cache_read_tokens, cache_creation_5m_tokens, cache_creation_1h_tokens, image_output_tokens, image_output_cost, image_input_tokens, image_input_cost, input_cost, output_cost, cache_creation_cost, cache_read_cost, total_cost, actual_cost, rate_multiplier, account_rate_multiplier, billing_type, request_type, stream, openai_ws_mode, duration_ms, first_token_ms, user_agent, ip_address, image_count, image_size, image_input_size, image_output_size, image_size_source, image_size_breakdown, video_count, video_resolution, video_duration_seconds, service_tier, reasoning_effort, inbound_endpoint, upstream_endpoint, cache_ttl_overridden, long_context_billing_applied, channel_id, model_mapping_chain, billing_tier, billing_mode, account_stats_cost, session_id, created_at"
 
 func (r *usageLogRepository) GetByID(ctx context.Context, id int64) (log *service.UsageLog, err error) {
 	query := "SELECT " + usageLogSelectColumns + " FROM usage_logs WHERE id = $1"
@@ -481,6 +481,7 @@ func scanUsageLog(scanner interface{ Scan(...any) error }) (*service.UsageLog, e
 		billingTier               sql.NullString
 		billingMode               sql.NullString
 		accountStatsCost          sql.NullFloat64
+		sessionID                 sql.NullString
 		createdAt                 time.Time
 	)
 
@@ -541,6 +542,7 @@ func scanUsageLog(scanner interface{ Scan(...any) error }) (*service.UsageLog, e
 		&billingTier,
 		&billingMode,
 		&accountStatsCost,
+		&sessionID,
 		&createdAt,
 	); err != nil {
 		return nil, err
@@ -661,6 +663,9 @@ func scanUsageLog(scanner interface{ Scan(...any) error }) (*service.UsageLog, e
 	if accountStatsCost.Valid {
 		log.AccountStatsCost = &accountStatsCost.Float64
 	}
+	if sessionID.Valid {
+		log.SessionID = &sessionID.String
+	}
 
 	return log, nil
 }
diff --git a/backend/internal/repository/usage_log_repo_request_type_test.go b/backend/internal/repository/usage_log_repo_request_type_test.go
index f039029608..aff472b0cb 100644
--- a/backend/internal/repository/usage_log_repo_request_type_test.go
+++ b/backend/internal/repository/usage_log_repo_request_type_test.go
@@ -96,6 +96,7 @@ func TestUsageLogRepositoryCreateSyncRequestTypeAndLegacyFields(t *testing.T) {
 			sqlmock.AnyArg(), // billing_tier
 			sqlmock.AnyArg(), // billing_mode
 			sqlmock.AnyArg(), // account_stats_cost
+			sqlmock.AnyArg(), // session_id
 			createdAt,
 		).
 		WillReturnRows(sqlmock.NewRows([]string{"id", "created_at"}).AddRow(int64(99), createdAt))
@@ -185,6 +186,7 @@ func TestUsageLogRepositoryCreate_PersistsServiceTier(t *testing.T) {
 			sqlmock.AnyArg(), // billing_tier
 			sqlmock.AnyArg(), // billing_mode
 			sqlmock.AnyArg(), // account_stats_cost
+			sqlmock.AnyArg(), // session_id
 			createdAt,
 		).
 		WillReturnRows(sqlmock.NewRows([]string{"id", "created_at"}).AddRow(int64(100), createdAt))
@@ -826,6 +828,7 @@ func TestScanUsageLogRequestTypeAndLegacyFallback(t *testing.T) {
 			sql.NullString{},
 			sql.NullString{},
 			sql.NullFloat64{},
+			sql.NullString{},
 			now,
 		}})
 		require.NoError(t, err)
@@ -900,6 +903,7 @@ func TestScanUsageLogRequestTypeAndLegacyFallback(t *testing.T) {
 			sql.NullString{},  // billing_tier
 			sql.NullString{},  // billing_mode
 			sql.NullFloat64{}, // account_stats_cost
+			sql.NullString{},  // session_id
 			now,
 		}})
 		require.NoError(t, err)
@@ -957,6 +961,7 @@ func TestScanUsageLogRequestTypeAndLegacyFallback(t *testing.T) {
 			sql.NullString{},  // billing_tier
 			sql.NullString{},  // billing_mode
 			sql.NullFloat64{}, // account_stats_cost
+			sql.NullString{},  // session_id
 			now,
 		}})
 		require.NoError(t, err)
@@ -1014,6 +1019,7 @@ func TestScanUsageLogRequestTypeAndLegacyFallback(t *testing.T) {
 			sql.NullString{},  // billing_tier
 			sql.NullString{},  // billing_mode
 			sql.NullFloat64{}, // account_stats_cost
+			sql.NullString{},  // session_id
 			now,
 		}})
 		require.NoError(t, err)
diff --git a/backend/internal/repository/usage_log_session_id_integration_test.go b/backend/internal/repository/usage_log_session_id_integration_test.go
new file mode 100644
index 0000000000..76778b4162
--- /dev/null
+++ b/backend/internal/repository/usage_log_session_id_integration_test.go
@@ -0,0 +1,71 @@
+//go:build integration
+
+package repository
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/google/uuid"
+
+	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/stretchr/testify/require"
+)
+
+// TestUsageLog_SessionIDPersistence proves session_id round-trips from insert to
+// read and is omitted (NULL) when absent.
+func TestUsageLog_SessionIDPersistence(t *testing.T) {
+	ctx := context.Background()
+	client := testEntClient(t)
+	repo := newUsageLogRepositoryWithSQL(client, integrationDB)
+
+	user := mustCreateUser(t, client, &service.User{Email: "session-id-" + uuid.NewString() + "@example.com"})
+	apiKey := mustCreateApiKey(t, client, &service.APIKey{UserID: user.ID, Key: "sk-session-" + uuid.NewString(), Name: "k"})
+	account := mustCreateAccount(t, client, &service.Account{Name: "acc-session-" + uuid.NewString()})
+
+	sessionID := "sess-" + uuid.NewString()
+
+	withSession := &service.UsageLog{
+		UserID:       user.ID,
+		APIKeyID:     apiKey.ID,
+		AccountID:    account.ID,
+		RequestID:    uuid.NewString(),
+		Model:        "claude-3",
+		InputTokens:  10,
+		OutputTokens: 5,
+		TotalCost:    1.0,
+		ActualCost:   1.0,
+		SessionID:    &sessionID,
+		CreatedAt:    time.Now().UTC(),
+	}
+	_, err := repo.Create(ctx, withSession)
+	require.NoError(t, err)
+	require.NotZero(t, withSession.ID)
+
+	withoutSession := &service.UsageLog{
+		UserID:       user.ID,
+		APIKeyID:     apiKey.ID,
+		AccountID:    account.ID,
+		RequestID:    uuid.NewString(),
+		Model:        "claude-3",
+		InputTokens:  7,
+		OutputTokens: 3,
+		TotalCost:    0.5,
+		ActualCost:   0.5,
+		CreatedAt:    time.Now().UTC(),
+	}
+	_, err = repo.Create(ctx, withoutSession)
+	require.NoError(t, err)
+
+	// Round-trip: session id survives insert → read.
+	got, err := repo.GetByID(ctx, withSession.ID)
+	require.NoError(t, err)
+	require.NotNil(t, got.SessionID)
+	require.Equal(t, sessionID, *got.SessionID)
+
+	// Omission: absent session id reads back as nil (NULL), not empty string.
+	gotNone, err := repo.GetByID(ctx, withoutSession.ID)
+	require.NoError(t, err)
+	require.Nil(t, gotNone.SessionID)
+}
diff --git a/backend/internal/repository/usage_log_session_id_unit_test.go b/backend/internal/repository/usage_log_session_id_unit_test.go
new file mode 100644
index 0000000000..7804dce306
--- /dev/null
+++ b/backend/internal/repository/usage_log_session_id_unit_test.go
@@ -0,0 +1,91 @@
+//go:build unit
+
+package repository
+
+import (
+	"database/sql"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/stretchr/testify/require"
+)
+
+func newSessionIDUsageLog(sessionID *string) *service.UsageLog {
+	return &service.UsageLog{
+		UserID:       1,
+		APIKeyID:     2,
+		AccountID:    3,
+		RequestID:    "req-session-id",
+		Model:        "claude-3",
+		InputTokens:  10,
+		OutputTokens: 5,
+		TotalCost:    1.0,
+		ActualCost:   1.0,
+		SessionID:    sessionID,
+		CreatedAt:    time.Now().UTC(),
+	}
+}
+
+// TestPrepareUsageLogInsert_SessionIDArgWiring pins the session_id column to the
+// arg slice / arg-type table so the five INSERT column lists stay in sync. session_id
+// is the penultimate arg (created_at is always last).
+func TestPrepareUsageLogInsert_SessionIDArgWiring(t *testing.T) {
+	require.Len(t, usageLogInsertArgTypes, 57, "arg-type table must include session_id")
+
+	sessionID := "sess-persisted-123"
+	prepared := prepareUsageLogInsert(newSessionIDUsageLog(&sessionID))
+
+	require.Len(t, prepared.args, len(usageLogInsertArgTypes),
+		"prepared args must match the arg-type table length")
+
+	// created_at is last; session_id is the arg immediately before it.
+	sessionArg := prepared.args[len(prepared.args)-2]
+	ns, ok := sessionArg.(sql.NullString)
+	require.True(t, ok, "session_id arg should be a sql.NullString, got %T", sessionArg)
+	require.True(t, ns.Valid)
+	require.Equal(t, sessionID, ns.String)
+
+	require.Equal(t, "text", usageLogInsertArgTypes[len(usageLogInsertArgTypes)-2],
+		"session_id arg type must be text")
+}
+
+// TestPrepareUsageLogInsert_SessionIDNullWhenAbsent proves an absent session id is
+// persisted as SQL NULL rather than an empty string.
+func TestPrepareUsageLogInsert_SessionIDNullWhenAbsent(t *testing.T) {
+	prepared := prepareUsageLogInsert(newSessionIDUsageLog(nil))
+	sessionArg := prepared.args[len(prepared.args)-2]
+	ns, ok := sessionArg.(sql.NullString)
+	require.True(t, ok, "session_id arg should be a sql.NullString, got %T", sessionArg)
+	require.False(t, ns.Valid, "absent session id must be NULL, not empty string")
+
+	empty := ""
+	preparedEmpty := prepareUsageLogInsert(newSessionIDUsageLog(&empty))
+	nsEmpty := preparedEmpty.args[len(preparedEmpty.args)-2].(sql.NullString)
+	require.False(t, nsEmpty.Valid, "empty session id must also be NULL")
+}
+
+// TestUsageLogInsertQueries_IncludeSessionID guards that every generated INSERT path
+// and the SELECT column list reference session_id.
+func TestUsageLogInsertQueries_IncludeSessionID(t *testing.T) {
+	require.Contains(t, usageLogSelectColumns, "session_id",
+		"SELECT column list must include session_id")
+
+	sessionID := "sess-in-query"
+	log := newSessionIDUsageLog(&sessionID)
+	prepared := prepareUsageLogInsert(log)
+	key := usageLogBatchKey(log.RequestID, log.APIKeyID)
+
+	batchQuery, batchArgs := buildUsageLogBatchInsertQuery([]string{key},
+		map[string]usageLogInsertPrepared{key: prepared})
+	require.Contains(t, batchQuery, "session_id")
+	// Two column references (INSERT column list + SELECT ... FROM input) plus the CTE def.
+	require.GreaterOrEqual(t, strings.Count(batchQuery, "session_id"), 3)
+	require.Len(t, batchArgs, len(prepared.args)+1,
+		"batch args include the synthetic input_index before usage-log values")
+
+	bestEffortQuery, bestEffortArgs := buildUsageLogBestEffortInsertQuery([]usageLogInsertPrepared{prepared})
+	require.Contains(t, bestEffortQuery, "session_id")
+	require.Len(t, bestEffortArgs, len(prepared.args))
+}
diff --git a/backend/internal/service/batch_image.go b/backend/internal/service/batch_image.go
index af0206b8b0..130428f18e 100644
--- a/backend/internal/service/batch_image.go
+++ b/backend/internal/service/batch_image.go
@@ -137,6 +137,7 @@ type BatchImageJob struct {
 	IdempotencyKey *string
 	RequestHash    *string
 	ManifestHash   *string
+	SessionID      *string
 
 	RetryCount int
 	Version    int
@@ -196,6 +197,7 @@ type CreateBatchImageJobParams struct {
 	IdempotencyKey *string
 	RequestHash    *string
 	ManifestHash   *string
+	SessionID      *string
 
 	RetryCount int
 
diff --git a/backend/internal/service/batch_image_processor_test.go b/backend/internal/service/batch_image_processor_test.go
index f54a1a99ec..ca3a583c99 100644
--- a/backend/internal/service/batch_image_processor_test.go
+++ b/backend/internal/service/batch_image_processor_test.go
@@ -425,6 +425,7 @@ func (r *fakeBatchImageRepository) CreateBatchImageJob(_ context.Context, params
 		Currency:                params.Currency,
 		IdempotencyKey:          params.IdempotencyKey,
 		RequestHash:             params.RequestHash,
+		SessionID:               params.SessionID,
 		CreatedAt:               time.Now(),
 	}
 	r.jobs[job.BatchID] = job
diff --git a/backend/internal/service/batch_image_public.go b/backend/internal/service/batch_image_public.go
index 2960d4cd9f..580d1f4934 100644
--- a/backend/internal/service/batch_image_public.go
+++ b/backend/internal/service/batch_image_public.go
@@ -57,6 +57,7 @@ type BatchImageSubmitRequest struct {
 	AspectRatio      string                 `json:"aspect_ratio"`
 	ImageSize        string                 `json:"image_size"`
 	Metadata         map[string]string      `json:"metadata"`
+	SessionID        *string                `json:"-"`
 }
 
 type BatchImageSubmitItem struct {
@@ -281,6 +282,7 @@ func (s *BatchImagePublicService) Submit(ctx context.Context, owner BatchImageOw
 		HoldID:                  &holdID,
 		IdempotencyKey:          batchImageOptionalStringPtr(idempotencyKey),
 		RequestHash:             batchImageStringPtr(requestHash),
+		SessionID:               normalized.SessionID,
 	})
 	if err != nil {
 		return nil, err
diff --git a/backend/internal/service/batch_image_public_test.go b/backend/internal/service/batch_image_public_test.go
index 2e5b930a7b..a69468f846 100644
--- a/backend/internal/service/batch_image_public_test.go
+++ b/backend/internal/service/batch_image_public_test.go
@@ -27,8 +27,10 @@ func TestBatchImagePublicService_Submit(t *testing.T) {
 
 	t.Run("accepts valid request stores refs and enqueues once", func(t *testing.T) {
 		svc, repo, queue, gemini, _ := newTestBatchImagePublicService(true)
+		req := validBatchImageSubmitRequest()
+		req.SessionID = batchImageStringPtr("batch-session-123")
 
-		got, err := svc.Submit(ctx, testBatchImageOwner(), validBatchImageSubmitRequest(), "")
+		got, err := svc.Submit(ctx, testBatchImageOwner(), req, "")
 		require.NoError(t, err)
 		require.Equal(t, "image.batch", got.Object)
 		require.Equal(t, "queued", got.Status)
@@ -61,6 +63,7 @@ func TestBatchImagePublicService_Submit(t *testing.T) {
 		require.InDelta(t, 0.6, job.HoldMultiplier, 1e-12)
 		require.InDelta(t, 0.125, job.BillableUnitPrice, 1e-12)
 		require.InDelta(t, 0.15, job.HoldUnitPrice, 1e-12)
+		require.Equal(t, "batch-session-123", batchImageDerefString(job.SessionID))
 	})
 
 	t.Run("combines user group image rate account rate discount and hold margin", func(t *testing.T) {
@@ -389,15 +392,18 @@ func TestBatchImagePublicService_Submit(t *testing.T) {
 	})
 
 	t.Run("idempotency returns same batch without provider resubmit", func(t *testing.T) {
-		svc, _, queue, gemini, _ := newTestBatchImagePublicService(true)
+		svc, repo, queue, gemini, _ := newTestBatchImagePublicService(true)
 		req := validBatchImageSubmitRequest()
+		req.SessionID = batchImageStringPtr("original-session")
 
 		first, err := svc.Submit(ctx, testBatchImageOwner(), req, "client-key")
 		require.NoError(t, err)
+		req.SessionID = batchImageStringPtr("retry-session")
 		second, err := svc.Submit(ctx, testBatchImageOwner(), req, "client-key")
 		require.NoError(t, err)
 
 		require.Equal(t, first.ID, second.ID)
+		require.Equal(t, "original-session", batchImageDerefString(repo.jobs[first.ID].SessionID))
 		require.Len(t, gemini.submits, 1)
 		require.Equal(t, []string{first.ID}, queue.enqueued)
 	})
diff --git a/backend/internal/service/batch_image_settlement.go b/backend/internal/service/batch_image_settlement.go
index 26d2abb0fd..e9b8a3685e 100644
--- a/backend/internal/service/batch_image_settlement.go
+++ b/backend/internal/service/batch_image_settlement.go
@@ -277,6 +277,7 @@ func (s *BatchImageSettlementService) recordUsageLog(ctx context.Context, job *B
 		RequestType:           RequestTypeSync,
 		BillingMode:           &billingMode,
 		ImageSize:             &imageSize,
+		SessionID:             job.SessionID,
 		CreatedAt:             createdAt,
 	}
 	writeUsageLogBestEffort(ctx, s.UsageLogRepo, usageLog, "service.batch_image_settlement")
diff --git a/backend/internal/service/batch_image_settlement_test.go b/backend/internal/service/batch_image_settlement_test.go
index a1fe3e899d..f1fc3a15eb 100644
--- a/backend/internal/service/batch_image_settlement_test.go
+++ b/backend/internal/service/batch_image_settlement_test.go
@@ -18,9 +18,14 @@ func TestBatchImageSettlementService_SettlesAndChargesSuccessfulImagesOnly(t *te
 	job.SuccessCount = 3
 	job.FailCount = 2
 	job.ItemCount = 5
+	job.SessionID = batchImageStringPtr("batch-settlement-session")
 	repo.jobs[job.BatchID] = job
 	billing := &fakeBatchImageBillingRepo{}
-	svc := &BatchImageSettlementService{Repo: repo, BillingRepo: billing, Pricing: &fakeBatchImagePricingResolver{unitPrice: 0.25}}
+	usageLogs := &openAIRecordUsageLogRepoStub{}
+	svc := &BatchImageSettlementService{
+		Repo: repo, BillingRepo: billing, Pricing: &fakeBatchImagePricingResolver{unitPrice: 0.25},
+		UsageLogRepo: usageLogs,
+	}
 
 	result, err := svc.Settle(context.Background(), job.BatchID)
 	require.NoError(t, err)
@@ -32,6 +37,7 @@ func TestBatchImageSettlementService_SettlesAndChargesSuccessfulImagesOnly(t *te
 	require.Equal(t, 0.75, *repo.jobs[job.BatchID].ActualCost)
 	require.NotEmpty(t, batchImageDerefString(repo.jobs[job.BatchID].ManifestHash))
 	require.NotNil(t, repo.jobs[job.BatchID].SettledAt)
+	require.Equal(t, "batch-settlement-session", batchImageDerefString(usageLogs.lastLog.SessionID))
 	require.Len(t, billing.captures, 1)
 	require.Equal(t, int64(321), billing.captures[0].APIKeyID)
 	require.Equal(t, job.UserID, billing.captures[0].UserID)
diff --git a/backend/internal/service/gateway_usage_billing.go b/backend/internal/service/gateway_usage_billing.go
index 1606f0f3c1..f445814d01 100644
--- a/backend/internal/service/gateway_usage_billing.go
+++ b/backend/internal/service/gateway_usage_billing.go
@@ -46,6 +46,7 @@ type RecordUsageInput struct {
 	UpstreamEndpoint   string             // 上游端点（标准化后的上游路径）
 	UserAgent          string             // 请求的 User-Agent
 	IPAddress          string             // 请求的客户端 IP 地址
+	SessionID          string             // 客户端显式会话标识（session_id / X-Session-Id 等请求头），仅用于用量行会话关联
 	RequestPayloadHash string             // 请求体语义哈希，用于降低 request_id 误复用时的静默误去重风险
 	ForceCacheBilling  bool               // 强制缓存计费：将 input_tokens 转为 cache_read 计费（用于粘性会话切换）
 	APIKeyService      APIKeyQuotaUpdater // 可选：用于更新API Key配额
@@ -572,6 +573,7 @@ func (s *GatewayService) RecordUsage(ctx context.Context, input *RecordUsageInpu
 		UpstreamEndpoint:   input.UpstreamEndpoint,
 		UserAgent:          input.UserAgent,
 		IPAddress:          input.IPAddress,
+		SessionID:          input.SessionID,
 		RequestPayloadHash: input.RequestPayloadHash,
 		ForceCacheBilling:  input.ForceCacheBilling,
 		APIKeyService:      input.APIKeyService,
@@ -591,6 +593,7 @@ type RecordUsageLongContextInput struct {
 	UpstreamEndpoint      string             // 上游端点（标准化后的上游路径）
 	UserAgent             string             // 请求的 User-Agent
 	IPAddress             string             // 请求的客户端 IP 地址
+	SessionID             string             // 客户端显式会话标识（session_id / X-Session-Id 等请求头），仅用于用量行会话关联
 	RequestPayloadHash    string             // 请求体语义哈希，用于降低 request_id 误复用时的静默误去重风险
 	LongContextThreshold  int                // 长上下文阈值（如 200000）
 	LongContextMultiplier float64            // 超出阈值部分的倍率（如 2.0）
@@ -613,6 +616,7 @@ func (s *GatewayService) RecordUsageWithLongContext(ctx context.Context, input *
 		UpstreamEndpoint:   input.UpstreamEndpoint,
 		UserAgent:          input.UserAgent,
 		IPAddress:          input.IPAddress,
+		SessionID:          input.SessionID,
 		RequestPayloadHash: input.RequestPayloadHash,
 		ForceCacheBilling:  input.ForceCacheBilling,
 		APIKeyService:      input.APIKeyService,
@@ -635,6 +639,7 @@ type recordUsageCoreInput struct {
 	UpstreamEndpoint   string
 	UserAgent          string
 	IPAddress          string
+	SessionID          string
 	RequestPayloadHash string
 	ForceCacheBilling  bool
 	APIKeyService      APIKeyQuotaUpdater
@@ -1010,6 +1015,7 @@ func (s *GatewayService) buildRecordUsageLog(
 		ModelMappingChain:     optionalTrimmedStringPtr(input.ModelMappingChain),
 		UserAgent:             optionalTrimmedStringPtr(input.UserAgent),
 		IPAddress:             optionalTrimmedStringPtr(input.IPAddress),
+		SessionID:             optionalTrimmedStringPtr(input.SessionID),
 		GroupID:               apiKey.GroupID,
 		SubscriptionID:        optionalSubscriptionID(subscription),
 		CreatedAt:             time.Now(),
diff --git a/backend/internal/service/openai_gateway_grok_cache.go b/backend/internal/service/openai_gateway_grok_cache.go
index ca798d8141..170528ce76 100644
--- a/backend/internal/service/openai_gateway_grok_cache.go
+++ b/backend/internal/service/openai_gateway_grok_cache.go
@@ -125,6 +125,11 @@ func isGrokRequestContext(c *gin.Context) bool {
 	if c == nil {
 		return false
 	}
+	if c.Request != nil {
+		if platform, ok := ResolvedTargetPlatformFromContext(c.Request.Context()); ok {
+			return platform == PlatformGrok
+		}
+	}
 	v, exists := c.Get("api_key")
 	if !exists {
 		return false
diff --git a/backend/internal/service/openai_gateway_scheduling.go b/backend/internal/service/openai_gateway_scheduling.go
index a895b2ebf5..01316cb58a 100644
--- a/backend/internal/service/openai_gateway_scheduling.go
+++ b/backend/internal/service/openai_gateway_scheduling.go
@@ -26,6 +26,15 @@ const (
 	codeBuddyConversationHeader   = "X-Conversation-ID"
 )
 
+var explicitOpenAIHeaderSessionNames = []string{
+	"session_id",
+	"conversation_id",
+	openCodeSessionAffinityHeader,
+	openCodeSessionIDHeader,
+	openCodeNativeSessionHeader,
+	codeBuddyConversationHeader,
+}
+
 // explicitOpenAIHeaderSessionID resolves stable conversation identifiers sent
 // by OpenAI-compatible clients. Keep this list limited to session-scoped
 // fields: request/message IDs rotate every turn and would defeat sticky routing
@@ -35,14 +44,7 @@ func explicitOpenAIHeaderSessionID(c *gin.Context) string {
 		return ""
 	}
 
-	for _, header := range []string{
-		"session_id",
-		"conversation_id",
-		openCodeSessionAffinityHeader,
-		openCodeSessionIDHeader,
-		openCodeNativeSessionHeader,
-		codeBuddyConversationHeader,
-	} {
+	for _, header := range explicitOpenAIHeaderSessionNames {
 		if sessionID := strings.TrimSpace(c.GetHeader(header)); sessionID != "" {
 			return sessionID
 		}
diff --git a/backend/internal/service/openai_gateway_usage.go b/backend/internal/service/openai_gateway_usage.go
index 773b31d625..16429f7402 100644
--- a/backend/internal/service/openai_gateway_usage.go
+++ b/backend/internal/service/openai_gateway_usage.go
@@ -29,6 +29,7 @@ type OpenAIRecordUsageInput struct {
 	UpstreamEndpoint   string
 	UserAgent          string // 请求的 User-Agent
 	IPAddress          string // 请求的客户端 IP 地址
+	SessionID          string // 客户端显式会话标识（session_id / X-Session-Id 等请求头），仅用于用量行会话关联
 	RequestPayloadHash string
 	APIKeyService      APIKeyQuotaUpdater
 	QuotaPlatform      string // user×platform quota platform resolved by the handler before async billing.
@@ -55,6 +56,7 @@ type CyberPolicyUsageInput struct {
 	UpstreamEndpoint   string
 	UserAgent          string
 	IPAddress          string
+	SessionID          string
 	RequestPayloadHash string
 	APIKeyService      APIKeyQuotaUpdater
 	ChannelUsageFields
@@ -89,6 +91,7 @@ func (s *OpenAIGatewayService) RecordCyberPolicyUsageLog(ctx context.Context, in
 		UpstreamEndpoint:   in.UpstreamEndpoint,
 		UserAgent:          in.UserAgent,
 		IPAddress:          in.IPAddress,
+		SessionID:          in.SessionID,
 		RequestPayloadHash: in.RequestPayloadHash,
 		APIKeyService:      in.APIKeyService,
 		ChannelUsageFields: in.ChannelUsageFields,
@@ -333,6 +336,9 @@ func (s *OpenAIGatewayService) RecordUsage(ctx context.Context, input *OpenAIRec
 		usageLog.IPAddress = &input.IPAddress
 	}
 
+	// 添加 SessionID（客户端显式会话标识；缺失/无效时保持 nil）
+	usageLog.SessionID = optionalTrimmedStringPtr(input.SessionID)
+
 	if apiKey.GroupID != nil {
 		usageLog.GroupID = apiKey.GroupID
 	}
diff --git a/backend/internal/service/session_id.go b/backend/internal/service/session_id.go
new file mode 100644
index 0000000000..8cf39f056b
--- /dev/null
+++ b/backend/internal/service/session_id.go
@@ -0,0 +1,75 @@
+package service
+
+import (
+	"strings"
+	"unicode/utf8"
+
+	"github.com/gin-gonic/gin"
+)
+
+// maxPersistedSessionIDLength bounds the persisted client session identifier to the
+// usage_logs.session_id column width (VARCHAR(255)). Longer values are rejected so
+// distinct identifiers can never alias through truncation.
+const maxPersistedSessionIDLength = 255
+
+// clientSessionIDHeaders extends the OpenAI-compatible sticky-session signals with
+// native protocol identifiers that are safe to persist but must not alter OpenAI
+// scheduling behavior.
+var clientSessionIDHeaders = append(
+	append([]string(nil), explicitOpenAIHeaderSessionNames...),
+	claudeCodeSessionHeader,
+)
+
+// ExtractClientSessionID resolves the explicit client-provided session identifier from
+// request headers for usage-log correlation and returns it sanitized. It is
+// protocol-agnostic and shared by every gateway handler so all supported protocols
+// record session_id through one seam. Returns "" when no valid identifier is present.
+//
+// This value feeds only usage_logs.session_id persistence. It does NOT affect sticky
+// routing, account selection, request_id semantics, or upstream prompt caching, which
+// keep their own (intentionally broader) session-signal resolution.
+func ExtractClientSessionID(c *gin.Context) string {
+	if c == nil || c.Request == nil {
+		return ""
+	}
+	for _, header := range clientSessionIDHeaders {
+		if sessionID := sanitizeSessionID(c.GetHeader(header)); sessionID != "" {
+			return sessionID
+		}
+	}
+	if isGrokRequestContext(c) {
+		if sessionID := sanitizeSessionID(c.GetHeader(grokConversationIDHeader)); sessionID != "" {
+			return sessionID
+		}
+	}
+	return ""
+}
+
+// sanitizeSessionID normalizes a raw client-supplied session identifier for safe
+// persistence: it trims surrounding whitespace, rejects the value outright if it
+// contains any control character (CR/LF/tab/NUL/…) so a log- or header-injection style
+// payload cannot slip into stored correlation data, and rejects values longer than
+// the DB column bound. Absent or invalid input yields "".
+func sanitizeSessionID(raw string) string {
+	if !utf8.ValidString(raw) {
+		return ""
+	}
+	trimmed := strings.TrimSpace(raw)
+	if trimmed == "" {
+		return ""
+	}
+	count := 0
+	for _, r := range trimmed {
+		if r < 0x20 || r == 0x7f {
+			// An explicit correlation id never legitimately contains control
+			// characters; drop the whole value rather than persist a mangled or
+			// partially-injected identifier.
+			return ""
+		}
+		count++
+		if count > maxPersistedSessionIDLength {
+			return ""
+		}
+	}
+	return trimmed
+}
diff --git a/backend/internal/service/session_id_test.go b/backend/internal/service/session_id_test.go
new file mode 100644
index 0000000000..1eb746d6ef
--- /dev/null
+++ b/backend/internal/service/session_id_test.go
@@ -0,0 +1,155 @@
+//go:build unit
+
+package service
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/require"
+)
+
+func newSessionHeaderContext(t *testing.T, headers map[string]string) *gin.Context {
+	t.Helper()
+	gin.SetMode(gin.TestMode)
+	c, _ := gin.CreateTestContext(httptest.NewRecorder())
+	req := httptest.NewRequest(http.MethodPost, "/v1/messages", nil)
+	for k, v := range headers {
+		req.Header.Set(k, v)
+	}
+	c.Request = req
+	return c
+}
+
+func TestSanitizeSessionID(t *testing.T) {
+	longRunes := strings.Repeat("a", maxPersistedSessionIDLength+50)
+	multibyte := strings.Repeat("好", maxPersistedSessionIDLength+10)
+
+	tests := []struct {
+		name string
+		in   string
+		want string
+	}{
+		{"empty", "", ""},
+		{"whitespace only", "   \t  ", ""},
+		{"trims surrounding whitespace", "  sess-123  ", "sess-123"},
+		{"plain value", "conv_abc-123.XYZ", "conv_abc-123.XYZ"},
+		{"uuid", "550e8400-e29b-41d4-a716-446655440000", "550e8400-e29b-41d4-a716-446655440000"},
+		{"reject CR", "sess\r123", ""},
+		{"reject LF", "sess\n123", ""},
+		{"reject CRLF injection", "sess-1\r\nSet-Cookie: x=y", ""},
+		{"reject tab inside", "sess\t123", ""},
+		{"reject NUL", "sess\x00123", ""},
+		{"reject DEL", "sess\x7f123", ""},
+		{"reject invalid UTF-8", string([]byte{'s', 'e', 's', 's', '-', 0xff}), ""},
+		{"accepts value at column bound", strings.Repeat("b", maxPersistedSessionIDLength), strings.Repeat("b", maxPersistedSessionIDLength)},
+		{"rejects overlong value", longRunes, ""},
+		{"rejects overlong multibyte value", multibyte, ""},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := sanitizeSessionID(tc.in)
+			require.Equal(t, tc.want, got, "sanitizeSessionID(%q)", tc.in)
+			// Sanitized output must never exceed the DB column bound (rune-counted).
+			require.LessOrEqual(t, len([]rune(got)), maxPersistedSessionIDLength)
+		})
+	}
+}
+
+func TestExtractClientSessionID_NilContext(t *testing.T) {
+	require.Equal(t, "", ExtractClientSessionID(nil))
+}
+
+func TestExtractClientSessionID_NilRequest(t *testing.T) {
+	require.Equal(t, "", ExtractClientSessionID(&gin.Context{}))
+}
+
+func TestExtractClientSessionID_AbsentReturnsEmpty(t *testing.T) {
+	c := newSessionHeaderContext(t, nil)
+	require.Equal(t, "", ExtractClientSessionID(c))
+}
+
+func TestExtractClientSessionID_SupportedHeaders(t *testing.T) {
+	tests := []struct {
+		name   string
+		header string
+		value  string
+	}{
+		{"session_id", "session_id", "sess-A"},
+		{"conversation_id", "conversation_id", "conv-B"},
+		{"X-Session-Affinity", openCodeSessionAffinityHeader, "aff-C"},
+		{"X-Session-Id", openCodeSessionIDHeader, "sid-D"},
+		{"X-OpenCode-Session", openCodeNativeSessionHeader, "oc-E"},
+		{"X-Conversation-ID", codeBuddyConversationHeader, "cb-F"},
+		{"X-Claude-Code-Session-Id", claudeCodeSessionHeader, "cc-G"},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			c := newSessionHeaderContext(t, map[string]string{tc.header: tc.value})
+			require.Equal(t, tc.value, ExtractClientSessionID(c))
+		})
+	}
+}
+
+func TestExtractClientSessionID_HeaderPrecedence(t *testing.T) {
+	// session_id ranks ahead of conversation_id and the X-* variants.
+	c := newSessionHeaderContext(t, map[string]string{
+		"session_id":                "primary",
+		"conversation_id":           "secondary",
+		openCodeSessionIDHeader:     "tertiary",
+		codeBuddyConversationHeader: "quaternary",
+	})
+	require.Equal(t, "primary", ExtractClientSessionID(c))
+}
+
+func TestExtractClientSessionID_Sanitizes(t *testing.T) {
+	c := newSessionHeaderContext(t, map[string]string{openCodeSessionIDHeader: "  clean-123  "})
+	require.Equal(t, "clean-123", ExtractClientSessionID(c))
+}
+
+func TestExtractClientSessionID_IgnoresNonSessionHeaders(t *testing.T) {
+	// prompt_cache_key, request/message ids, and a Grok conversation header on a
+	// non-Grok request are NOT persisted as session_id.
+	c := newSessionHeaderContext(t, map[string]string{
+		"prompt_cache_key": "cache-key-should-not-persist",
+		"X-Request-Id":     "req-should-not-persist",
+		"x-grok-conv-id":   "grok-conv-should-not-persist",
+	})
+	require.Equal(t, "", ExtractClientSessionID(c))
+}
+
+func TestExtractClientSessionID_GrokConversationHeader(t *testing.T) {
+	c := newSessionHeaderContext(t, map[string]string{
+		grokConversationIDHeader: "grok-native-session",
+	})
+	c.Set("api_key", &APIKey{
+		ID:    42,
+		Group: &Group{Platform: PlatformGrok},
+	})
+
+	require.Equal(t, "grok-native-session", ExtractClientSessionID(c))
+}
+
+func TestExtractClientSessionID_GrokConversationHeaderForCompositeRoute(t *testing.T) {
+	c := newSessionHeaderContext(t, map[string]string{
+		grokConversationIDHeader: "grok-composite-session",
+	})
+	c.Set("api_key", &APIKey{
+		ID:    43,
+		Group: &Group{Platform: PlatformComposite},
+	})
+	c.Request = c.Request.WithContext(WithResolvedTargetPlatform(context.Background(), PlatformGrok))
+
+	require.Equal(t, "grok-composite-session", ExtractClientSessionID(c))
+}
+
+func TestExtractClientSessionID_InjectionHeaderDropped(t *testing.T) {
+	// A supported header carrying a CRLF payload is rejected, not persisted mangled.
+	c := newSessionHeaderContext(t, map[string]string{"session_id": "abc"})
+	c.Request.Header.Set("session_id", "abc\r\nX-Injected: 1")
+	require.Equal(t, "", ExtractClientSessionID(c))
+}
diff --git a/backend/internal/service/usage_log.go b/backend/internal/service/usage_log.go
index 4e8df04aaf..d10ca619bc 100644
--- a/backend/internal/service/usage_log.go
+++ b/backend/internal/service/usage_log.go
@@ -165,6 +165,10 @@ type UsageLog struct {
 	FirstTokenMs *int
 	UserAgent    *string
 	IPAddress    *string
+	// SessionID is the explicit client-provided request correlation identifier
+	// (e.g. the session_id / X-Session-Id headers). Nil when the client sent no
+	// valid session header. It is never derived from prompt_cache_key or content.
+	SessionID *string
 
 	// Cache TTL Override 标记（管理员强制替换了缓存 TTL 计费）
 	CacheTTLOverridden bool
diff --git a/backend/migrations/187_add_usage_log_session_id.sql b/backend/migrations/187_add_usage_log_session_id.sql
new file mode 100644
index 0000000000..7d6a4f6e21
--- /dev/null
+++ b/backend/migrations/187_add_usage_log_session_id.sql
@@ -0,0 +1,14 @@
+-- Persist the explicit client-provided request session/conversation identifier
+-- (e.g. the session_id / X-Session-Id / X-Conversation-ID request headers) so
+-- usage rows can be correlated by session for querying.
+--
+-- Nullable with no default: on PostgreSQL 11+ this is a metadata-only change, so
+-- it does NOT rewrite the (potentially large) usage_logs table. Absent/invalid
+-- session identifiers stay NULL. Only explicit client correlation identity is
+-- stored here; prompt_cache_key and content-derived sticky hashes are never
+-- persisted as session_id.
+ALTER TABLE usage_logs ADD COLUMN IF NOT EXISTS session_id VARCHAR(255);
+
+-- Batch image usage is recorded asynchronously after the originating HTTP
+-- request has finished, so retain the same sanitized identifier on the job.
+ALTER TABLE batch_image_jobs ADD COLUMN IF NOT EXISTS session_id VARCHAR(255);