refactor: consolidate failover logic into FailoverState

- Merge FailoverRetry/FailoverSwitch into single FailoverContinue action
- Extract HandleSelectionExhausted into FailoverState (was duplicated 3×)
- Move helper functions (needForceCacheBilling, sleepWithContext) into failover_loop.go
- Inline sleepFailoverDelay, replace sleepAntigravitySingleAccountBackoff with constant
- Delete gateway_handler_single_account_retry_test.go (tested removed function)
- Add 6 test cases for HandleSelectionExhausted

backend/internal/handler/failover_loop.go

@@ -3,6 +3,7 @@ package handler
 import (
 	"context"
 	"log"
+	"net/http"
 	"time"
 
 	"github.com/Wei-Shaw/sub2api/internal/service"
@@ -18,10 +19,8 @@ type TempUnscheduler interface {
 type FailoverAction int
 
 const (
-	// FailoverRetry 同账号重试（调用方应 continue 重新进入循环，不更换账号）
-	FailoverRetry FailoverAction = iota
-	// FailoverSwitch 切换账号（调用方应 continue 重新选择账号）
-	FailoverSwitch
+	// FailoverContinue 继续循环（同账号重试或切换账号，调用方统一 continue）
+	FailoverContinue FailoverAction = iota
 	// FailoverExhausted 切换次数耗尽（调用方应返回错误响应）
 	FailoverExhausted
 	// FailoverCanceled context 已取消（调用方应直接 return）
@@ -33,6 +32,10 @@ const (
 	maxSameAccountRetries = 2
 	// sameAccountRetryDelay 同账号重试间隔
 	sameAccountRetryDelay = 500 * time.Millisecond
+	// singleAccountBackoffDelay 单账号分组 503 退避重试固定延时。
+	// Service 层在 SingleAccountRetry 模式下已做充分原地重试（最多 3 次、总等待 30s），
+	// Handler 层只需短暂间隔后重新进入 Service 层即可。
+	singleAccountBackoffDelay = 2 * time.Second
 )
 
 // FailoverState 跨循环迭代共享的 failover 状态
@@ -80,7 +83,7 @@ func (s *FailoverState) HandleFailoverError(
 		if !sleepWithContext(ctx, sameAccountRetryDelay) {
 			return FailoverCanceled
 		}
-		return FailoverRetry
+		return FailoverContinue
 	}
 
 	// 同账号重试用尽，执行临时封禁
@@ -103,12 +106,44 @@ func (s *FailoverState) HandleFailoverError(
 
 	// Antigravity 平台换号线性递增延时
 	if platform == service.PlatformAntigravity {
-		if !sleepFailoverDelay(ctx, s.SwitchCount) {
+		delay := time.Duration(s.SwitchCount-1) * time.Second
+		if !sleepWithContext(ctx, delay) {
 			return FailoverCanceled
 		}
 	}
 
-	return FailoverSwitch
+	return FailoverContinue
+}
+
+// HandleSelectionExhausted 处理选号失败（所有候选账号都在排除列表中）时的退避重试决策。
+// 针对 Antigravity 单账号分组的 503 (MODEL_CAPACITY_EXHAUSTED) 场景：
+// 清除排除列表、等待退避后重新选号。
+//
+// 返回 FailoverContinue 时，调用方应设置 SingleAccountRetry context 并 continue。
+// 返回 FailoverExhausted 时，调用方应返回错误响应。
+// 返回 FailoverCanceled 时，调用方应直接 return。
+func (s *FailoverState) HandleSelectionExhausted(ctx context.Context) FailoverAction {
+	if s.LastFailoverErr != nil &&
+		s.LastFailoverErr.StatusCode == http.StatusServiceUnavailable &&
+		s.SwitchCount <= s.MaxSwitches {
+
+		log.Printf("Antigravity single-account 503 backoff: waiting %v before retry (attempt %d)",
+			singleAccountBackoffDelay, s.SwitchCount)
+		if !sleepWithContext(ctx, singleAccountBackoffDelay) {
+			return FailoverCanceled
+		}
+		log.Printf("Antigravity single-account 503 retry: clearing failed accounts, retry %d/%d",
+			s.SwitchCount, s.MaxSwitches)
+		s.FailedAccountIDs = make(map[int64]struct{})
+		return FailoverContinue
+	}
+	return FailoverExhausted
+}
+
+// needForceCacheBilling 判断 failover 时是否需要强制缓存计费。
+// 粘性会话切换账号、或上游明确标记时，将 input_tokens 转为 cache_read 计费。
+func needForceCacheBilling(hasBoundSession bool, failoverErr *service.UpstreamFailoverError) bool {
+	return hasBoundSession || (failoverErr != nil && failoverErr.ForceCacheBilling)
 }
 
 // sleepWithContext 等待指定时长，返回 false 表示 context 已取消。

backend/internal/handler/failover_loop_test.go

@@ -135,7 +135,7 @@ func TestHandleFailoverError_BasicSwitch(t *testing.T) {
 
 		action := fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
 
-		require.Equal(t, FailoverSwitch, action)
+		require.Equal(t, FailoverContinue, action)
 		require.Equal(t, 1, fs.SwitchCount)
 		require.Contains(t, fs.FailedAccountIDs, int64(100))
 		require.Equal(t, err, fs.LastFailoverErr)
@@ -153,7 +153,7 @@ func TestHandleFailoverError_BasicSwitch(t *testing.T) {
 		action := fs.HandleFailoverError(context.Background(), mock, 100, service.PlatformAntigravity, err)
 		elapsed := time.Since(start)
 
-		require.Equal(t, FailoverSwitch, action)
+		require.Equal(t, FailoverContinue, action)
 		require.Equal(t, 1, fs.SwitchCount)
 		require.Less(t, elapsed, 200*time.Millisecond, "第一次切换延迟应为 0")
 	})
@@ -169,7 +169,7 @@ func TestHandleFailoverError_BasicSwitch(t *testing.T) {
 		action := fs.HandleFailoverError(context.Background(), mock, 200, service.PlatformAntigravity, err)
 		elapsed := time.Since(start)
 
-		require.Equal(t, FailoverSwitch, action)
+		require.Equal(t, FailoverContinue, action)
 		require.Equal(t, 2, fs.SwitchCount)
 		require.GreaterOrEqual(t, elapsed, 800*time.Millisecond, "第二次切换延迟应约 1s")
 		require.Less(t, elapsed, 3*time.Second)
@@ -182,13 +182,13 @@ func TestHandleFailoverError_BasicSwitch(t *testing.T) {
 		// 第一次切换：0→1
 		err1 := newTestFailoverErr(500, false, false)
 		action := fs.HandleFailoverError(context.Background(), mock, 100, "openai", err1)
-		require.Equal(t, FailoverSwitch, action)
+		require.Equal(t, FailoverContinue, action)
 		require.Equal(t, 1, fs.SwitchCount)
 
 		// 第二次切换：1→2
 		err2 := newTestFailoverErr(502, false, false)
 		action = fs.HandleFailoverError(context.Background(), mock, 200, "openai", err2)
-		require.Equal(t, FailoverSwitch, action)
+		require.Equal(t, FailoverContinue, action)
 		require.Equal(t, 2, fs.SwitchCount)
 
 		// 第三次已耗尽：SwitchCount(2) >= MaxSwitches(2)
@@ -272,7 +272,7 @@ func TestHandleFailoverError_CacheBilling(t *testing.T) {
 // ---------------------------------------------------------------------------
 
 func TestHandleFailoverError_SameAccountRetry(t *testing.T) {
-	t.Run("第一次重试返回FailoverRetry", func(t *testing.T) {
+	t.Run("第一次重试返回FailoverContinue", func(t *testing.T) {
 		mock := &mockTempUnscheduler{}
 		fs := NewFailoverState(3, false)
 		err := newTestFailoverErr(400, true, false)
@@ -281,7 +281,7 @@ func TestHandleFailoverError_SameAccountRetry(t *testing.T) {
 		action := fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
 		elapsed := time.Since(start)
 
-		require.Equal(t, FailoverRetry, action)
+		require.Equal(t, FailoverContinue, action)
 		require.Equal(t, 1, fs.SameAccountRetryCount[100])
 		require.Equal(t, 0, fs.SwitchCount, "同账号重试不应增加切换计数")
 		require.NotContains(t, fs.FailedAccountIDs, int64(100), "同账号重试不应加入失败列表")
@@ -291,19 +291,19 @@ func TestHandleFailoverError_SameAccountRetry(t *testing.T) {
 		require.Less(t, elapsed, 2*time.Second)
 	})
 
-	t.Run("第二次重试仍返回FailoverRetry", func(t *testing.T) {
+	t.Run("第二次重试仍返回FailoverContinue", func(t *testing.T) {
 		mock := &mockTempUnscheduler{}
 		fs := NewFailoverState(3, false)
 		err := newTestFailoverErr(400, true, false)
 
 		// 第一次
 		action := fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
-		require.Equal(t, FailoverRetry, action)
+		require.Equal(t, FailoverContinue, action)
 		require.Equal(t, 1, fs.SameAccountRetryCount[100])
 
 		// 第二次
 		action = fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
-		require.Equal(t, FailoverRetry, action)
+		require.Equal(t, FailoverContinue, action)
 		require.Equal(t, 2, fs.SameAccountRetryCount[100])
 
 		require.Empty(t, mock.calls, "两次重试期间均不应调用 TempUnschedule")
@@ -321,7 +321,7 @@ func TestHandleFailoverError_SameAccountRetry(t *testing.T) {
 
 		// 第三次：重试已达到 maxSameAccountRetries(2)，应切换账号
 		action := fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
-		require.Equal(t, FailoverSwitch, action)
+		require.Equal(t, FailoverContinue, action)
 		require.Equal(t, 1, fs.SwitchCount)
 		require.Contains(t, fs.FailedAccountIDs, int64(100))
 
@@ -338,12 +338,12 @@ func TestHandleFailoverError_SameAccountRetry(t *testing.T) {
 
 		// 账号 100 第一次重试
 		action := fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
-		require.Equal(t, FailoverRetry, action)
+		require.Equal(t, FailoverContinue, action)
 		require.Equal(t, 1, fs.SameAccountRetryCount[100])
 
 		// 账号 200 第一次重试（独立计数）
 		action = fs.HandleFailoverError(context.Background(), mock, 200, "openai", err)
-		require.Equal(t, FailoverRetry, action)
+		require.Equal(t, FailoverContinue, action)
 		require.Equal(t, 1, fs.SameAccountRetryCount[200])
 		require.Equal(t, 1, fs.SameAccountRetryCount[100], "账号 100 的计数不应受影响")
 	})
@@ -358,11 +358,11 @@ func TestHandleFailoverError_SameAccountRetry(t *testing.T) {
 		fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
 		// 第三次: 重试耗尽 → 切换
 		action := fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
-		require.Equal(t, FailoverSwitch, action)
+		require.Equal(t, FailoverContinue, action)
 
 		// 再次遇到账号 100，计数仍为 2，条件不满足 → 直接切换
 		action = fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
-		require.Equal(t, FailoverSwitch, action)
+		require.Equal(t, FailoverContinue, action)
 		require.Len(t, mock.calls, 2, "第二次耗尽也应调用 TempUnschedule")
 	})
 }
@@ -470,7 +470,7 @@ func TestHandleFailoverError_FailedAccountIDs(t *testing.T) {
 		fs := NewFailoverState(3, false)
 
 		action := fs.HandleFailoverError(context.Background(), mock, 100, "openai", newTestFailoverErr(400, true, false))
-		require.Equal(t, FailoverRetry, action)
+		require.Equal(t, FailoverContinue, action)
 		require.NotContains(t, fs.FailedAccountIDs, int64(100))
 	})
 
@@ -524,27 +524,27 @@ func TestHandleFailoverError_IntegrationScenario(t *testing.T) {
 		// 1. 账号 100 遇到可重试错误，同账号重试 2 次
 		retryErr := newTestFailoverErr(400, true, false)
 		action := fs.HandleFailoverError(context.Background(), mock, 100, "openai", retryErr)
-		require.Equal(t, FailoverRetry, action)
+		require.Equal(t, FailoverContinue, action)
 		require.True(t, fs.ForceCacheBilling, "hasBoundSession=true 应设置 ForceCacheBilling")
 
 		action = fs.HandleFailoverError(context.Background(), mock, 100, "openai", retryErr)
-		require.Equal(t, FailoverRetry, action)
+		require.Equal(t, FailoverContinue, action)
 
 		// 2. 账号 100 重试耗尽 → TempUnschedule + 切换
 		action = fs.HandleFailoverError(context.Background(), mock, 100, "openai", retryErr)
-		require.Equal(t, FailoverSwitch, action)
+		require.Equal(t, FailoverContinue, action)
 		require.Equal(t, 1, fs.SwitchCount)
 		require.Len(t, mock.calls, 1)
 
 		// 3. 账号 200 遇到不可重试错误 → 直接切换
 		switchErr := newTestFailoverErr(500, false, false)
 		action = fs.HandleFailoverError(context.Background(), mock, 200, "openai", switchErr)
-		require.Equal(t, FailoverSwitch, action)
+		require.Equal(t, FailoverContinue, action)
 		require.Equal(t, 2, fs.SwitchCount)
 
 		// 4. 账号 300 遇到不可重试错误 → 再切换
 		action = fs.HandleFailoverError(context.Background(), mock, 300, "openai", switchErr)
-		require.Equal(t, FailoverSwitch, action)
+		require.Equal(t, FailoverContinue, action)
 		require.Equal(t, 3, fs.SwitchCount)
 
 		// 5. 账号 400 → 已耗尽 (SwitchCount=3 >= MaxSwitches=3)
@@ -568,14 +568,14 @@ func TestHandleFailoverError_IntegrationScenario(t *testing.T) {
 		start := time.Now()
 		action := fs.HandleFailoverError(context.Background(), mock, 100, service.PlatformAntigravity, err)
 		elapsed := time.Since(start)
-		require.Equal(t, FailoverSwitch, action)
+		require.Equal(t, FailoverContinue, action)
 		require.Less(t, elapsed, 200*time.Millisecond, "第一次切换延迟为 0")
 
 		// 第二次切换：delay = 1s
 		start = time.Now()
 		action = fs.HandleFailoverError(context.Background(), mock, 200, service.PlatformAntigravity, err)
 		elapsed = time.Since(start)
-		require.Equal(t, FailoverSwitch, action)
+		require.Equal(t, FailoverContinue, action)
 		require.GreaterOrEqual(t, elapsed, 800*time.Millisecond, "第二次切换延迟约 1s")
 
 		// 第三次：耗尽（无延迟，因为在检查延迟之前就返回了）
@@ -618,7 +618,7 @@ func TestHandleFailoverError_EdgeCases(t *testing.T) {
 		err := newTestFailoverErr(0, false, false)
 
 		action := fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
-		require.Equal(t, FailoverSwitch, action)
+		require.Equal(t, FailoverContinue, action)
 	})
 
 	t.Run("AccountID为0也能正常跟踪", func(t *testing.T) {
@@ -627,7 +627,7 @@ func TestHandleFailoverError_EdgeCases(t *testing.T) {
 		err := newTestFailoverErr(500, true, false)
 
 		action := fs.HandleFailoverError(context.Background(), mock, 0, "openai", err)
-		require.Equal(t, FailoverRetry, action)
+		require.Equal(t, FailoverContinue, action)
 		require.Equal(t, 1, fs.SameAccountRetryCount[0])
 	})
 
@@ -637,7 +637,7 @@ func TestHandleFailoverError_EdgeCases(t *testing.T) {
 		err := newTestFailoverErr(500, true, false)
 
 		action := fs.HandleFailoverError(context.Background(), mock, -1, "openai", err)
-		require.Equal(t, FailoverRetry, action)
+		require.Equal(t, FailoverContinue, action)
 		require.Equal(t, 1, fs.SameAccountRetryCount[-1])
 	})
 
@@ -651,7 +651,82 @@ func TestHandleFailoverError_EdgeCases(t *testing.T) {
 		action := fs.HandleFailoverError(context.Background(), mock, 100, "", err)
 		elapsed := time.Since(start)
 
-		require.Equal(t, FailoverSwitch, action)
+		require.Equal(t, FailoverContinue, action)
 		require.Less(t, elapsed, 200*time.Millisecond, "空平台不应触发 Antigravity 延迟")
 	})
 }
+
+// ---------------------------------------------------------------------------
+// HandleSelectionExhausted 测试
+// ---------------------------------------------------------------------------
+
+func TestHandleSelectionExhausted(t *testing.T) {
+	t.Run("无LastFailoverErr时返回Exhausted", func(t *testing.T) {
+		fs := NewFailoverState(3, false)
+		// LastFailoverErr 为 nil
+
+		action := fs.HandleSelectionExhausted(context.Background())
+		require.Equal(t, FailoverExhausted, action)
+	})
+
+	t.Run("非503错误返回Exhausted", func(t *testing.T) {
+		fs := NewFailoverState(3, false)
+		fs.LastFailoverErr = newTestFailoverErr(500, false, false)
+
+		action := fs.HandleSelectionExhausted(context.Background())
+		require.Equal(t, FailoverExhausted, action)
+	})
+
+	t.Run("503且未耗尽_等待后返回Continue并清除失败列表", func(t *testing.T) {
+		fs := NewFailoverState(3, false)
+		fs.LastFailoverErr = newTestFailoverErr(503, false, false)
+		fs.FailedAccountIDs[100] = struct{}{}
+		fs.SwitchCount = 1
+
+		start := time.Now()
+		action := fs.HandleSelectionExhausted(context.Background())
+		elapsed := time.Since(start)
+
+		require.Equal(t, FailoverContinue, action)
+		require.Empty(t, fs.FailedAccountIDs, "应清除失败账号列表")
+		require.GreaterOrEqual(t, elapsed, 1500*time.Millisecond, "应等待约 2s")
+		require.Less(t, elapsed, 5*time.Second)
+	})
+
+	t.Run("503但SwitchCount已超过MaxSwitches_返回Exhausted", func(t *testing.T) {
+		fs := NewFailoverState(2, false)
+		fs.LastFailoverErr = newTestFailoverErr(503, false, false)
+		fs.SwitchCount = 3 // > MaxSwitches(2)
+
+		start := time.Now()
+		action := fs.HandleSelectionExhausted(context.Background())
+		elapsed := time.Since(start)
+
+		require.Equal(t, FailoverExhausted, action)
+		require.Less(t, elapsed, 100*time.Millisecond, "不应等待")
+	})
+
+	t.Run("503但context已取消_返回Canceled", func(t *testing.T) {
+		fs := NewFailoverState(3, false)
+		fs.LastFailoverErr = newTestFailoverErr(503, false, false)
+
+		ctx, cancel := context.WithCancel(context.Background())
+		cancel()
+
+		start := time.Now()
+		action := fs.HandleSelectionExhausted(ctx)
+		elapsed := time.Since(start)
+
+		require.Equal(t, FailoverCanceled, action)
+		require.Less(t, elapsed, 100*time.Millisecond, "应立即返回")
+	})
+
+	t.Run("503且SwitchCount等于MaxSwitches_仍可重试", func(t *testing.T) {
+		fs := NewFailoverState(2, false)
+		fs.LastFailoverErr = newTestFailoverErr(503, false, false)
+		fs.SwitchCount = 2 // == MaxSwitches，条件是 <=，仍可重试
+
+		action := fs.HandleSelectionExhausted(context.Background())
+		require.Equal(t, FailoverContinue, action)
+	})
+}

backend/internal/handler/gateway_handler.go

@@ -248,25 +248,22 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 					h.handleStreamingAwareError(c, http.StatusServiceUnavailable, "api_error", "No available accounts: "+err.Error(), streamStarted)
 					return
 				}
-				// Antigravity 单账号退避重试：分组内没有其他可用账号时，
-				// 对 503 错误不直接返回，而是清除排除列表、等待退避后重试同一个账号。
-				// 谷歌上游 503 (MODEL_CAPACITY_EXHAUSTED) 通常是暂时性的，等几秒就能恢复。
-				if fs.LastFailoverErr != nil && fs.LastFailoverErr.StatusCode == http.StatusServiceUnavailable && fs.SwitchCount <= fs.MaxSwitches {
-					if sleepAntigravitySingleAccountBackoff(c.Request.Context(), fs.SwitchCount) {
-						log.Printf("Antigravity single-account 503 retry: clearing failed accounts, retry %d/%d", fs.SwitchCount, fs.MaxSwitches)
-						fs.FailedAccountIDs = make(map[int64]struct{})
-						// 设置 context 标记，让 Service 层预检查等待限流过期而非直接切换
-						ctx := context.WithValue(c.Request.Context(), ctxkey.SingleAccountRetry, true)
-						c.Request = c.Request.WithContext(ctx)
-						continue
+				action := fs.HandleSelectionExhausted(c.Request.Context())
+				switch action {
+				case FailoverContinue:
+					ctx := context.WithValue(c.Request.Context(), ctxkey.SingleAccountRetry, true)
+					c.Request = c.Request.WithContext(ctx)
+					continue
+				case FailoverCanceled:
+					return
+				default: // FailoverExhausted
+					if fs.LastFailoverErr != nil {
+						h.handleFailoverExhausted(c, fs.LastFailoverErr, service.PlatformGemini, streamStarted)
+					} else {
+						h.handleFailoverExhaustedSimple(c, 502, streamStarted)
 					}
+					return
 				}
-				if fs.LastFailoverErr != nil {
-					h.handleFailoverExhausted(c, fs.LastFailoverErr, service.PlatformGemini, streamStarted)
-				} else {
-					h.handleFailoverExhaustedSimple(c, 502, streamStarted)
-				}
-				return
 			}
 			account := selection.Account
 			setOpsSelectedAccount(c, account.ID)
@@ -357,7 +354,7 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 				if errors.As(err, &failoverErr) {
 					action := fs.HandleFailoverError(c.Request.Context(), h.gatewayService, account.ID, account.Platform, failoverErr)
 					switch action {
-					case FailoverRetry, FailoverSwitch:
+					case FailoverContinue:
 						continue
 					case FailoverExhausted:
 						h.handleFailoverExhausted(c, fs.LastFailoverErr, service.PlatformGemini, streamStarted)
@@ -424,25 +421,22 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 					h.handleStreamingAwareError(c, http.StatusServiceUnavailable, "api_error", "No available accounts: "+err.Error(), streamStarted)
 					return
 				}
-				// Antigravity 单账号退避重试：分组内没有其他可用账号时，
-				// 对 503 错误不直接返回，而是清除排除列表、等待退避后重试同一个账号。
-				// 谷歌上游 503 (MODEL_CAPACITY_EXHAUSTED) 通常是暂时性的，等几秒就能恢复。
-				if fs.LastFailoverErr != nil && fs.LastFailoverErr.StatusCode == http.StatusServiceUnavailable && fs.SwitchCount <= fs.MaxSwitches {
-					if sleepAntigravitySingleAccountBackoff(c.Request.Context(), fs.SwitchCount) {
-						log.Printf("Antigravity single-account 503 retry: clearing failed accounts, retry %d/%d", fs.SwitchCount, fs.MaxSwitches)
-						fs.FailedAccountIDs = make(map[int64]struct{})
-						// 设置 context 标记，让 Service 层预检查等待限流过期而非直接切换
-						ctx := context.WithValue(c.Request.Context(), ctxkey.SingleAccountRetry, true)
-						c.Request = c.Request.WithContext(ctx)
-						continue
+				action := fs.HandleSelectionExhausted(c.Request.Context())
+				switch action {
+				case FailoverContinue:
+					ctx := context.WithValue(c.Request.Context(), ctxkey.SingleAccountRetry, true)
+					c.Request = c.Request.WithContext(ctx)
+					continue
+				case FailoverCanceled:
+					return
+				default: // FailoverExhausted
+					if fs.LastFailoverErr != nil {
+						h.handleFailoverExhausted(c, fs.LastFailoverErr, platform, streamStarted)
+					} else {
+						h.handleFailoverExhaustedSimple(c, 502, streamStarted)
 					}
+					return
 				}
-				if fs.LastFailoverErr != nil {
-					h.handleFailoverExhausted(c, fs.LastFailoverErr, platform, streamStarted)
-				} else {
-					h.handleFailoverExhaustedSimple(c, 502, streamStarted)
-				}
-				return
 			}
 			account := selection.Account
 			setOpsSelectedAccount(c, account.ID)
@@ -566,7 +560,7 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 				if errors.As(err, &failoverErr) {
 					action := fs.HandleFailoverError(c.Request.Context(), h.gatewayService, account.ID, account.Platform, failoverErr)
 					switch action {
-					case FailoverRetry, FailoverSwitch:
+					case FailoverContinue:
 						continue
 					case FailoverExhausted:
 						h.handleFailoverExhausted(c, fs.LastFailoverErr, account.Platform, streamStarted)
@@ -835,48 +829,6 @@ func (h *GatewayHandler) handleConcurrencyError(c *gin.Context, err error, slotT
 		fmt.Sprintf("Concurrency limit exceeded for %s, please retry later", slotType), streamStarted)
 }
 
-// needForceCacheBilling 判断 failover 时是否需要强制缓存计费
-// 粘性会话切换账号、或上游明确标记时，将 input_tokens 转为 cache_read 计费
-func needForceCacheBilling(hasBoundSession bool, failoverErr *service.UpstreamFailoverError) bool {
-	return hasBoundSession || (failoverErr != nil && failoverErr.ForceCacheBilling)
-}
-
-// sleepFailoverDelay 账号切换线性递增延时：第1次0s、第2次1s、第3次2s…
-// 返回 false 表示 context 已取消。
-func sleepFailoverDelay(ctx context.Context, switchCount int) bool {
-	delay := time.Duration(switchCount-1) * time.Second
-	if delay <= 0 {
-		return true
-	}
-	select {
-	case <-ctx.Done():
-		return false
-	case <-time.After(delay):
-		return true
-	}
-}
-
-// sleepAntigravitySingleAccountBackoff Antigravity 平台单账号分组的 503 退避重试延时。
-// 当分组内只有一个可用账号且上游返回 503（MODEL_CAPACITY_EXHAUSTED）时使用，
-// 采用短固定延时策略。Service 层在 SingleAccountRetry 模式下已经做了充分的原地重试
-// （最多 3 次、总等待 30s），所以 Handler 层的退避只需短暂等待即可。
-// 返回 false 表示 context 已取消。
-func sleepAntigravitySingleAccountBackoff(ctx context.Context, retryCount int) bool {
-	// 固定短延时：2s
-	// Service 层已经在原地等待了足够长的时间（retryDelay × 重试次数），
-	// Handler 层只需短暂间隔后重新进入 Service 层即可。
-	const delay = 2 * time.Second
-
-	log.Printf("Antigravity single-account 503 backoff: waiting %v before retry (attempt %d)", delay, retryCount)
-
-	select {
-	case <-ctx.Done():
-		return false
-	case <-time.After(delay):
-		return true
-	}
-}
-
 func (h *GatewayHandler) handleFailoverExhausted(c *gin.Context, failoverErr *service.UpstreamFailoverError, platform string, streamStarted bool) {
 	statusCode := failoverErr.StatusCode
 	responseBody := failoverErr.ResponseBody

backend/internal/handler/gateway_handler_single_account_retry_test.go

@@ -1,51 +0,0 @@
-package handler
-
-import (
-	"context"
-	"testing"
-	"time"
-
-	"github.com/stretchr/testify/require"
-)
-
-// ---------------------------------------------------------------------------
-// sleepAntigravitySingleAccountBackoff 测试
-// ---------------------------------------------------------------------------
-
-func TestSleepAntigravitySingleAccountBackoff_ReturnsTrue(t *testing.T) {
-	ctx := context.Background()
-	start := time.Now()
-	ok := sleepAntigravitySingleAccountBackoff(ctx, 1)
-	elapsed := time.Since(start)
-
-	require.True(t, ok, "should return true when context is not canceled")
-	// 固定延迟 2s
-	require.GreaterOrEqual(t, elapsed, 1500*time.Millisecond, "should wait approximately 2s")
-	require.Less(t, elapsed, 5*time.Second, "should not wait too long")
-}
-
-func TestSleepAntigravitySingleAccountBackoff_ContextCanceled(t *testing.T) {
-	ctx, cancel := context.WithCancel(context.Background())
-	cancel() // 立即取消
-
-	start := time.Now()
-	ok := sleepAntigravitySingleAccountBackoff(ctx, 1)
-	elapsed := time.Since(start)
-
-	require.False(t, ok, "should return false when context is canceled")
-	require.Less(t, elapsed, 500*time.Millisecond, "should return immediately on cancel")
-}
-
-func TestSleepAntigravitySingleAccountBackoff_FixedDelay(t *testing.T) {
-	// 验证不同 retryCount 都使用固定 2s 延迟
-	ctx := context.Background()
-
-	start := time.Now()
-	ok := sleepAntigravitySingleAccountBackoff(ctx, 5)
-	elapsed := time.Since(start)
-
-	require.True(t, ok)
-	// 即使 retryCount=5，延迟仍然是固定的 2s
-	require.GreaterOrEqual(t, elapsed, 1500*time.Millisecond)
-	require.Less(t, elapsed, 5*time.Second)
-}

backend/internal/handler/gemini_v1beta_handler.go

@@ -337,21 +337,18 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 				googleError(c, http.StatusServiceUnavailable, "No available Gemini accounts: "+err.Error())
 				return
 			}
-			// Antigravity 单账号退避重试：分组内没有其他可用账号时，
-			// 对 503 错误不直接返回，而是清除排除列表、等待退避后重试同一个账号。
-			// 谷歌上游 503 (MODEL_CAPACITY_EXHAUSTED) 通常是暂时性的，等几秒就能恢复。
-			if fs.LastFailoverErr != nil && fs.LastFailoverErr.StatusCode == http.StatusServiceUnavailable && fs.SwitchCount <= fs.MaxSwitches {
-				if sleepAntigravitySingleAccountBackoff(c.Request.Context(), fs.SwitchCount) {
-					log.Printf("Antigravity single-account 503 retry: clearing failed accounts, retry %d/%d", fs.SwitchCount, fs.MaxSwitches)
-					fs.FailedAccountIDs = make(map[int64]struct{})
-					// 设置 context 标记，让 Service 层预检查等待限流过期而非直接切换
-					ctx := context.WithValue(c.Request.Context(), ctxkey.SingleAccountRetry, true)
-					c.Request = c.Request.WithContext(ctx)
-					continue
-				}
+			action := fs.HandleSelectionExhausted(c.Request.Context())
+			switch action {
+			case FailoverContinue:
+				ctx := context.WithValue(c.Request.Context(), ctxkey.SingleAccountRetry, true)
+				c.Request = c.Request.WithContext(ctx)
+				continue
+			case FailoverCanceled:
+				return
+			default: // FailoverExhausted
+				h.handleGeminiFailoverExhausted(c, fs.LastFailoverErr)
+				return
 			}
-			h.handleGeminiFailoverExhausted(c, fs.LastFailoverErr)
-			return
 		}
 		account := selection.Account
 		setOpsSelectedAccount(c, account.ID)
@@ -441,7 +438,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 			if errors.As(err, &failoverErr) {
 				action := fs.HandleFailoverError(c.Request.Context(), h.gatewayService, account.ID, account.Platform, failoverErr)
 				switch action {
-				case FailoverRetry, FailoverSwitch:
+				case FailoverContinue:
 					continue
 				case FailoverExhausted:
 					h.handleGeminiFailoverExhausted(c, fs.LastFailoverErr)