diff --git a/backend/internal/service/openai_gateway_scheduling.go b/backend/internal/service/openai_gateway_scheduling.go
index 7ab83a3de8..a90bad3e25 100644
--- a/backend/internal/service/openai_gateway_scheduling.go
+++ b/backend/internal/service/openai_gateway_scheduling.go
@@ -1174,6 +1174,11 @@ func (s *OpenAIGatewayService) selectAccountWithLoadAwareness(ctx context.Contex
 	}
 
 	// ============ Layer 1: Sticky session ============
+	// A healthy sticky account whose bounded wait queue is full may be used as a
+	// one-request capacity spillover in Layer 2. Keep that spillover temporary:
+	// rewriting the durable binding here would make a short burst migrate the
+	// whole conversation to a cache-cold account.
+	stickySpillover := false
 	if sessionHash != "" {
 		accountID := stickyAccountID
 		if accountID > 0 && !isExcluded(accountID) {
@@ -1215,6 +1220,7 @@ func (s *OpenAIGatewayService) selectAccountWithLoadAwareness(ctx context.Contex
 								MaxWaiting:     cfg.StickySessionMaxWaiting,
 							})
 						}
+						stickySpillover = true
 					}
 				}
 			}
@@ -1366,7 +1372,7 @@ func (s *OpenAIGatewayService) selectAccountWithLoadAwareness(ctx context.Contex
 				if selectErr != nil {
 					return nil, true, selectErr
 				}
-				if sessionHash != "" && !gatewayProfitControlGateActive(ctx) {
+				if sessionHash != "" && !stickySpillover && !gatewayProfitControlGateActive(ctx) {
 					_ = s.setStickySessionAccountID(ctx, groupID, sessionHash, fresh.ID, openaiStickySessionTTL)
 				}
 				return selection, true, nil
@@ -1405,7 +1411,7 @@ func (s *OpenAIGatewayService) selectAccountWithLoadAwareness(ctx context.Contex
 				if selectErr != nil {
 					return nil, selectErr
 				}
-				if sessionHash != "" && !gatewayProfitControlGateActive(ctx) {
+				if sessionHash != "" && !stickySpillover && !gatewayProfitControlGateActive(ctx) {
 					_ = s.setStickySessionAccountID(ctx, groupID, sessionHash, fresh.ID, openaiStickySessionTTL)
 				}
 				return selection, nil
diff --git a/backend/internal/service/openai_gateway_service_test.go b/backend/internal/service/openai_gateway_service_test.go
index 77f4ee33d6..568c948627 100644
--- a/backend/internal/service/openai_gateway_service_test.go
+++ b/backend/internal/service/openai_gateway_service_test.go
@@ -1207,6 +1207,49 @@ func TestOpenAISelectAccountWithLoadAwareness_StickyWaitPlan(t *testing.T) {
 	}
 }
 
+func TestOpenAISelectAccountWithLoadAwareness_StickyCapacitySpilloverKeepsBinding(t *testing.T) {
+	sessionHash := "sticky-spillover"
+	groupID := int64(1)
+	repo := stubOpenAIAccountRepo{
+		accounts: []Account{
+			{ID: 1, Platform: PlatformOpenAI, Status: StatusActive, Schedulable: true, Concurrency: 6, Priority: 1, GroupIDs: []int64{groupID}},
+			{ID: 2, Platform: PlatformOpenAI, Status: StatusActive, Schedulable: true, Concurrency: 6, Priority: 1, GroupIDs: []int64{groupID}},
+		},
+	}
+	cache := &stubGatewayCache{
+		sessionBindings: map[string]int64{"openai:" + sessionHash: 1},
+	}
+	concurrencyCache := stubConcurrencyCache{
+		acquireResults: map[int64]bool{1: false, 2: true},
+		waitCounts:     map[int64]int{1: 1},
+		loadMap: map[int64]*AccountLoadInfo{
+			1: {AccountID: 1, LoadRate: 100},
+			2: {AccountID: 2, LoadRate: 10},
+		},
+	}
+	cfg := &config.Config{RunMode: config.RunModeStandard}
+	cfg.Gateway.Scheduling.LoadBatchEnabled = true
+	cfg.Gateway.Scheduling.StickySessionMaxWaiting = 1
+
+	svc := &OpenAIGatewayService{
+		accountRepo:        repo,
+		cache:              cache,
+		cfg:                cfg,
+		concurrencyService: NewConcurrencyService(concurrencyCache),
+	}
+
+	selection, err := svc.SelectAccountWithLoadAwareness(context.Background(), &groupID, sessionHash, "gpt-4", nil)
+	require.NoError(t, err)
+	require.NotNil(t, selection)
+	require.NotNil(t, selection.Account)
+	require.Equal(t, int64(2), selection.Account.ID, "capacity spillover should use the other account for this request")
+	require.True(t, selection.Acquired)
+	require.Equal(t, int64(1), cache.sessionBindings["openai:"+sessionHash], "capacity spillover must not migrate the durable sticky binding")
+	if selection.ReleaseFunc != nil {
+		selection.ReleaseFunc()
+	}
+}
+
 func TestOpenAISelectAccountWithLoadAwareness_PrefersLowerLoad(t *testing.T) {
 	groupID := int64(1)
 	repo := stubOpenAIAccountRepo{