refactor: extract failover error handling into FailoverState

- Extract duplicated failover logic from gateway_handler.go (3 places)
  and gemini_v1beta_handler.go into shared failover_loop.go
- Introduce FailoverState with HandleFailoverError and HandleSelectionExhausted
- Move helper functions (needForceCacheBilling, sleepWithContext) into failover_loop.go
- Add comprehensive unit tests (32+ test cases)
- Delete redundant gateway_handler_single_account_retry_test.go

backend/internal/handler/failover_loop.go

+package handler
+
+import (
+	"context"
+	"log"
+	"net/http"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/service"
+)
+
+// TempUnscheduler 用于 HandleFailoverError 中同账号重试耗尽后的临时封禁。
+// GatewayService 隐式实现此接口。
+type TempUnscheduler interface {
+	TempUnscheduleRetryableError(ctx context.Context, accountID int64, failoverErr *service.UpstreamFailoverError)
+}
+
+// FailoverAction 表示 failover 错误处理后的下一步动作
+type FailoverAction int
+
+const (
+	// 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）
+	FailoverCanceled
+)
+
+const (
+	// maxSameAccountRetries 同账号重试次数上限（针对 RetryableOnSameAccount 错误）
+	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
+type FailoverState struct {
+	SwitchCount           int
+	MaxSwitches           int
+	FailedAccountIDs      map[int64]struct{}
+	SameAccountRetryCount map[int64]int
+	LastFailoverErr       *service.UpstreamFailoverError
+	ForceCacheBilling     bool
+	hasBoundSession       bool
+}
+
+// NewFailoverState 创建 failover 状态
+func NewFailoverState(maxSwitches int, hasBoundSession bool) *FailoverState {
+	return &FailoverState{
+		MaxSwitches:           maxSwitches,
+		FailedAccountIDs:      make(map[int64]struct{}),
+		SameAccountRetryCount: make(map[int64]int),
+		hasBoundSession:       hasBoundSession,
+	}
+}
+
+// HandleFailoverError 处理 UpstreamFailoverError，返回下一步动作。
+// 包含：缓存计费判断、同账号重试、临时封禁、切换计数、Antigravity 延时。
+func (s *FailoverState) HandleFailoverError(
+	ctx context.Context,
+	gatewayService TempUnscheduler,
+	accountID int64,
+	platform string,
+	failoverErr *service.UpstreamFailoverError,
+) FailoverAction {
+	s.LastFailoverErr = failoverErr
+
+	// 缓存计费判断
+	if needForceCacheBilling(s.hasBoundSession, failoverErr) {
+		s.ForceCacheBilling = true
+	}
+
+	// 同账号重试：对 RetryableOnSameAccount 的临时性错误，先在同一账号上重试
+	if failoverErr.RetryableOnSameAccount && s.SameAccountRetryCount[accountID] < maxSameAccountRetries {
+		s.SameAccountRetryCount[accountID]++
+		log.Printf("Account %d: retryable error %d, same-account retry %d/%d",
+			accountID, failoverErr.StatusCode, s.SameAccountRetryCount[accountID], maxSameAccountRetries)
+		if !sleepWithContext(ctx, sameAccountRetryDelay) {
+			return FailoverCanceled
+		}
+		return FailoverContinue
+	}
+
+	// 同账号重试用尽，执行临时封禁
+	if failoverErr.RetryableOnSameAccount {
+		gatewayService.TempUnscheduleRetryableError(ctx, accountID, failoverErr)
+	}
+
+	// 加入失败列表
+	s.FailedAccountIDs[accountID] = struct{}{}
+
+	// 检查是否耗尽
+	if s.SwitchCount >= s.MaxSwitches {
+		return FailoverExhausted
+	}
+
+	// 递增切换计数
+	s.SwitchCount++
+	log.Printf("Account %d: upstream error %d, switching account %d/%d",
+		accountID, failoverErr.StatusCode, s.SwitchCount, s.MaxSwitches)
+
+	// Antigravity 平台换号线性递增延时
+	if platform == service.PlatformAntigravity {
+		delay := time.Duration(s.SwitchCount-1) * time.Second
+		if !sleepWithContext(ctx, delay) {
+			return FailoverCanceled
+		}
+	}
+
+	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。
+func sleepWithContext(ctx context.Context, d time.Duration) bool {
+	if d <= 0 {
+		return true
+	}
+	select {
+	case <-ctx.Done():
+		return false
+	case <-time.After(d):
+		return true
+	}
+}

backend/internal/handler/failover_loop_test.go

+package handler
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/service"
+
+	"github.com/stretchr/testify/require"
+)
+
+// ---------------------------------------------------------------------------
+// Mock
+// ---------------------------------------------------------------------------
+
+// mockTempUnscheduler 记录 TempUnscheduleRetryableError 的调用信息。
+type mockTempUnscheduler struct {
+	calls []tempUnscheduleCall
+}
+
+type tempUnscheduleCall struct {
+	accountID   int64
+	failoverErr *service.UpstreamFailoverError
+}
+
+func (m *mockTempUnscheduler) TempUnscheduleRetryableError(_ context.Context, accountID int64, failoverErr *service.UpstreamFailoverError) {
+	m.calls = append(m.calls, tempUnscheduleCall{accountID: accountID, failoverErr: failoverErr})
+}
+
+// ---------------------------------------------------------------------------
+// Helper
+// ---------------------------------------------------------------------------
+
+func newTestFailoverErr(statusCode int, retryable, forceBilling bool) *service.UpstreamFailoverError {
+	return &service.UpstreamFailoverError{
+		StatusCode:             statusCode,
+		RetryableOnSameAccount: retryable,
+		ForceCacheBilling:      forceBilling,
+	}
+}
+
+// ---------------------------------------------------------------------------
+// NewFailoverState 测试
+// ---------------------------------------------------------------------------
+
+func TestNewFailoverState(t *testing.T) {
+	t.Run("初始化字段正确", func(t *testing.T) {
+		fs := NewFailoverState(5, true)
+		require.Equal(t, 5, fs.MaxSwitches)
+		require.Equal(t, 0, fs.SwitchCount)
+		require.NotNil(t, fs.FailedAccountIDs)
+		require.Empty(t, fs.FailedAccountIDs)
+		require.NotNil(t, fs.SameAccountRetryCount)
+		require.Empty(t, fs.SameAccountRetryCount)
+		require.Nil(t, fs.LastFailoverErr)
+		require.False(t, fs.ForceCacheBilling)
+		require.True(t, fs.hasBoundSession)
+	})
+
+	t.Run("无绑定会话", func(t *testing.T) {
+		fs := NewFailoverState(3, false)
+		require.Equal(t, 3, fs.MaxSwitches)
+		require.False(t, fs.hasBoundSession)
+	})
+
+	t.Run("零最大切换次数", func(t *testing.T) {
+		fs := NewFailoverState(0, false)
+		require.Equal(t, 0, fs.MaxSwitches)
+	})
+}
+
+// ---------------------------------------------------------------------------
+// sleepWithContext 测试
+// ---------------------------------------------------------------------------
+
+func TestSleepWithContext(t *testing.T) {
+	t.Run("零时长立即返回true", func(t *testing.T) {
+		start := time.Now()
+		ok := sleepWithContext(context.Background(), 0)
+		require.True(t, ok)
+		require.Less(t, time.Since(start), 50*time.Millisecond)
+	})
+
+	t.Run("负时长立即返回true", func(t *testing.T) {
+		start := time.Now()
+		ok := sleepWithContext(context.Background(), -1*time.Second)
+		require.True(t, ok)
+		require.Less(t, time.Since(start), 50*time.Millisecond)
+	})
+
+	t.Run("正常等待后返回true", func(t *testing.T) {
+		start := time.Now()
+		ok := sleepWithContext(context.Background(), 50*time.Millisecond)
+		elapsed := time.Since(start)
+		require.True(t, ok)
+		require.GreaterOrEqual(t, elapsed, 40*time.Millisecond)
+		require.Less(t, elapsed, 500*time.Millisecond)
+	})
+
+	t.Run("已取消context立即返回false", func(t *testing.T) {
+		ctx, cancel := context.WithCancel(context.Background())
+		cancel()
+
+		start := time.Now()
+		ok := sleepWithContext(ctx, 5*time.Second)
+		require.False(t, ok)
+		require.Less(t, time.Since(start), 50*time.Millisecond)
+	})
+
+	t.Run("等待期间context取消返回false", func(t *testing.T) {
+		ctx, cancel := context.WithCancel(context.Background())
+		go func() {
+			time.Sleep(30 * time.Millisecond)
+			cancel()
+		}()
+
+		start := time.Now()
+		ok := sleepWithContext(ctx, 5*time.Second)
+		elapsed := time.Since(start)
+		require.False(t, ok)
+		require.Less(t, elapsed, 500*time.Millisecond)
+	})
+}
+
+// ---------------------------------------------------------------------------
+// HandleFailoverError — 基本切换流程
+// ---------------------------------------------------------------------------
+
+func TestHandleFailoverError_BasicSwitch(t *testing.T) {
+	t.Run("非重试错误_非Antigravity_直接切换", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false)
+		err := newTestFailoverErr(500, false, false)
+
+		action := fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
+
+		require.Equal(t, FailoverContinue, action)
+		require.Equal(t, 1, fs.SwitchCount)
+		require.Contains(t, fs.FailedAccountIDs, int64(100))
+		require.Equal(t, err, fs.LastFailoverErr)
+		require.False(t, fs.ForceCacheBilling)
+		require.Empty(t, mock.calls, "不应调用 TempUnschedule")
+	})
+
+	t.Run("非重试错误_Antigravity_第一次切换无延迟", func(t *testing.T) {
+		// switchCount 从 0→1 时，sleepFailoverDelay(ctx, 1) 的延时 = (1-1)*1s = 0
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false)
+		err := newTestFailoverErr(500, false, false)
+
+		start := time.Now()
+		action := fs.HandleFailoverError(context.Background(), mock, 100, service.PlatformAntigravity, err)
+		elapsed := time.Since(start)
+
+		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")
+	})
+
+	t.Run("非重试错误_Antigravity_第二次切换有1秒延迟", func(t *testing.T) {
+		// switchCount 从 1→2 时，sleepFailoverDelay(ctx, 2) 的延时 = (2-1)*1s = 1s
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false)
+		fs.SwitchCount = 1 // 模拟已切换一次
+
+		err := newTestFailoverErr(500, false, false)
+		start := time.Now()
+		action := fs.HandleFailoverError(context.Background(), mock, 200, service.PlatformAntigravity, err)
+		elapsed := time.Since(start)
+
+		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)
+	})
+
+	t.Run("连续切换直到耗尽", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(2, false)
+
+		// 第一次切换：0→1
+		err1 := newTestFailoverErr(500, false, false)
+		action := fs.HandleFailoverError(context.Background(), mock, 100, "openai", err1)
+		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, FailoverContinue, action)
+		require.Equal(t, 2, fs.SwitchCount)
+
+		// 第三次已耗尽：SwitchCount(2) >= MaxSwitches(2)
+		err3 := newTestFailoverErr(503, false, false)
+		action = fs.HandleFailoverError(context.Background(), mock, 300, "openai", err3)
+		require.Equal(t, FailoverExhausted, action)
+		require.Equal(t, 2, fs.SwitchCount, "耗尽时不应继续递增")
+
+		// 验证失败账号列表
+		require.Len(t, fs.FailedAccountIDs, 3)
+		require.Contains(t, fs.FailedAccountIDs, int64(100))
+		require.Contains(t, fs.FailedAccountIDs, int64(200))
+		require.Contains(t, fs.FailedAccountIDs, int64(300))
+
+		// LastFailoverErr 应为最后一次的错误
+		require.Equal(t, err3, fs.LastFailoverErr)
+	})
+
+	t.Run("MaxSwitches为0时首次即耗尽", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(0, false)
+		err := newTestFailoverErr(500, false, false)
+
+		action := fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
+		require.Equal(t, FailoverExhausted, action)
+		require.Equal(t, 0, fs.SwitchCount)
+		require.Contains(t, fs.FailedAccountIDs, int64(100))
+	})
+}
+
+// ---------------------------------------------------------------------------
+// HandleFailoverError — 缓存计费 (ForceCacheBilling)
+// ---------------------------------------------------------------------------
+
+func TestHandleFailoverError_CacheBilling(t *testing.T) {
+	t.Run("hasBoundSession为true时设置ForceCacheBilling", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, true) // hasBoundSession=true
+		err := newTestFailoverErr(500, false, false)
+
+		fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
+		require.True(t, fs.ForceCacheBilling)
+	})
+
+	t.Run("failoverErr.ForceCacheBilling为true时设置", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false)
+		err := newTestFailoverErr(500, false, true) // ForceCacheBilling=true
+
+		fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
+		require.True(t, fs.ForceCacheBilling)
+	})
+
+	t.Run("两者均为false时不设置", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false)
+		err := newTestFailoverErr(500, false, false)
+
+		fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
+		require.False(t, fs.ForceCacheBilling)
+	})
+
+	t.Run("一旦设置不会被后续错误重置", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false)
+
+		// 第一次：ForceCacheBilling=true → 设置
+		err1 := newTestFailoverErr(500, false, true)
+		fs.HandleFailoverError(context.Background(), mock, 100, "openai", err1)
+		require.True(t, fs.ForceCacheBilling)
+
+		// 第二次：ForceCacheBilling=false → 仍然保持 true
+		err2 := newTestFailoverErr(502, false, false)
+		fs.HandleFailoverError(context.Background(), mock, 200, "openai", err2)
+		require.True(t, fs.ForceCacheBilling, "ForceCacheBilling 一旦设置不应被重置")
+	})
+}
+
+// ---------------------------------------------------------------------------
+// HandleFailoverError — 同账号重试 (RetryableOnSameAccount)
+// ---------------------------------------------------------------------------
+
+func TestHandleFailoverError_SameAccountRetry(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)
+
+		start := time.Now()
+		action := fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
+		elapsed := time.Since(start)
+
+		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")
+		require.Empty(t, mock.calls, "同账号重试期间不应调用 TempUnschedule")
+		// 验证等待了 sameAccountRetryDelay (500ms)
+		require.GreaterOrEqual(t, elapsed, 400*time.Millisecond)
+		require.Less(t, elapsed, 2*time.Second)
+	})
+
+	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, FailoverContinue, action)
+		require.Equal(t, 1, fs.SameAccountRetryCount[100])
+
+		// 第二次
+		action = fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
+		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")
+	})
+
+	t.Run("第三次重试耗尽_触发TempUnschedule并切换", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false)
+		err := newTestFailoverErr(400, true, false)
+
+		// 第一次、第二次重试
+		fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
+		fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
+		require.Equal(t, 2, fs.SameAccountRetryCount[100])
+
+		// 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, FailoverContinue, action)
+		require.Equal(t, 1, fs.SwitchCount)
+		require.Contains(t, fs.FailedAccountIDs, int64(100))
+
+		// 验证 TempUnschedule 被调用
+		require.Len(t, mock.calls, 1)
+		require.Equal(t, int64(100), mock.calls[0].accountID)
+		require.Equal(t, err, mock.calls[0].failoverErr)
+	})
+
+	t.Run("不同账号独立跟踪重试次数", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(5, false)
+		err := newTestFailoverErr(400, true, false)
+
+		// 账号 100 第一次重试
+		action := fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
+		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, 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")
+	})
+
+	t.Run("重试耗尽后再次遇到同账号_直接切换", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(5, false)
+		err := newTestFailoverErr(400, true, false)
+
+		// 耗尽账号 100 的重试
+		fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
+		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, 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, FailoverContinue, action)
+		require.Len(t, mock.calls, 2, "第二次耗尽也应调用 TempUnschedule")
+	})
+}
+
+// ---------------------------------------------------------------------------
+// HandleFailoverError — TempUnschedule 调用验证
+// ---------------------------------------------------------------------------
+
+func TestHandleFailoverError_TempUnschedule(t *testing.T) {
+	t.Run("非重试错误不调用TempUnschedule", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false)
+		err := newTestFailoverErr(500, false, false) // RetryableOnSameAccount=false
+
+		fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
+		require.Empty(t, mock.calls)
+	})
+
+	t.Run("重试错误耗尽后调用TempUnschedule_传入正确参数", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false)
+		err := newTestFailoverErr(502, true, false)
+
+		// 耗尽重试
+		fs.HandleFailoverError(context.Background(), mock, 42, "openai", err)
+		fs.HandleFailoverError(context.Background(), mock, 42, "openai", err)
+		fs.HandleFailoverError(context.Background(), mock, 42, "openai", err)
+
+		require.Len(t, mock.calls, 1)
+		require.Equal(t, int64(42), mock.calls[0].accountID)
+		require.Equal(t, 502, mock.calls[0].failoverErr.StatusCode)
+		require.True(t, mock.calls[0].failoverErr.RetryableOnSameAccount)
+	})
+}
+
+// ---------------------------------------------------------------------------
+// HandleFailoverError — Context 取消
+// ---------------------------------------------------------------------------
+
+func TestHandleFailoverError_ContextCanceled(t *testing.T) {
+	t.Run("同账号重试sleep期间context取消", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false)
+		err := newTestFailoverErr(400, true, false)
+
+		ctx, cancel := context.WithCancel(context.Background())
+		cancel() // 立即取消
+
+		start := time.Now()
+		action := fs.HandleFailoverError(ctx, mock, 100, "openai", err)
+		elapsed := time.Since(start)
+
+		require.Equal(t, FailoverCanceled, action)
+		require.Less(t, elapsed, 100*time.Millisecond, "应立即返回")
+		// 重试计数仍应递增
+		require.Equal(t, 1, fs.SameAccountRetryCount[100])
+	})
+
+	t.Run("Antigravity延迟期间context取消", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false)
+		fs.SwitchCount = 1 // 下一次 switchCount=2 → delay = 1s
+		err := newTestFailoverErr(500, false, false)
+
+		ctx, cancel := context.WithCancel(context.Background())
+		cancel() // 立即取消
+
+		start := time.Now()
+		action := fs.HandleFailoverError(ctx, mock, 100, service.PlatformAntigravity, err)
+		elapsed := time.Since(start)
+
+		require.Equal(t, FailoverCanceled, action)
+		require.Less(t, elapsed, 100*time.Millisecond, "应立即返回而非等待 1s")
+	})
+}
+
+// ---------------------------------------------------------------------------
+// HandleFailoverError — FailedAccountIDs 跟踪
+// ---------------------------------------------------------------------------
+
+func TestHandleFailoverError_FailedAccountIDs(t *testing.T) {
+	t.Run("切换时添加到失败列表", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false)
+
+		fs.HandleFailoverError(context.Background(), mock, 100, "openai", newTestFailoverErr(500, false, false))
+		require.Contains(t, fs.FailedAccountIDs, int64(100))
+
+		fs.HandleFailoverError(context.Background(), mock, 200, "openai", newTestFailoverErr(502, false, false))
+		require.Contains(t, fs.FailedAccountIDs, int64(200))
+		require.Len(t, fs.FailedAccountIDs, 2)
+	})
+
+	t.Run("耗尽时也添加到失败列表", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(0, false)
+
+		action := fs.HandleFailoverError(context.Background(), mock, 100, "openai", newTestFailoverErr(500, false, false))
+		require.Equal(t, FailoverExhausted, action)
+		require.Contains(t, fs.FailedAccountIDs, int64(100))
+	})
+
+	t.Run("同账号重试期间不添加到失败列表", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false)
+
+		action := fs.HandleFailoverError(context.Background(), mock, 100, "openai", newTestFailoverErr(400, true, false))
+		require.Equal(t, FailoverContinue, action)
+		require.NotContains(t, fs.FailedAccountIDs, int64(100))
+	})
+
+	t.Run("同一账号多次切换不重复添加", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(5, false)
+
+		fs.HandleFailoverError(context.Background(), mock, 100, "openai", newTestFailoverErr(500, false, false))
+		fs.HandleFailoverError(context.Background(), mock, 100, "openai", newTestFailoverErr(500, false, false))
+		require.Len(t, fs.FailedAccountIDs, 1, "map 天然去重")
+	})
+}
+
+// ---------------------------------------------------------------------------
+// HandleFailoverError — LastFailoverErr 更新
+// ---------------------------------------------------------------------------
+
+func TestHandleFailoverError_LastFailoverErr(t *testing.T) {
+	t.Run("每次调用都更新LastFailoverErr", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false)
+
+		err1 := newTestFailoverErr(500, false, false)
+		fs.HandleFailoverError(context.Background(), mock, 100, "openai", err1)
+		require.Equal(t, err1, fs.LastFailoverErr)
+
+		err2 := newTestFailoverErr(502, false, false)
+		fs.HandleFailoverError(context.Background(), mock, 200, "openai", err2)
+		require.Equal(t, err2, fs.LastFailoverErr)
+	})
+
+	t.Run("同账号重试时也更新LastFailoverErr", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false)
+
+		err := newTestFailoverErr(400, true, false)
+		fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
+		require.Equal(t, err, fs.LastFailoverErr)
+	})
+}
+
+// ---------------------------------------------------------------------------
+// HandleFailoverError — 综合集成场景
+// ---------------------------------------------------------------------------
+
+func TestHandleFailoverError_IntegrationScenario(t *testing.T) {
+	t.Run("模拟完整failover流程_多账号混合重试与切换", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, true) // hasBoundSession=true
+
+		// 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, 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, 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, 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, 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, FailoverContinue, action)
+		require.Equal(t, 3, fs.SwitchCount)
+
+		// 5. 账号 400 → 已耗尽 (SwitchCount=3 >= MaxSwitches=3)
+		action = fs.HandleFailoverError(context.Background(), mock, 400, "openai", switchErr)
+		require.Equal(t, FailoverExhausted, action)
+
+		// 最终状态验证
+		require.Equal(t, 3, fs.SwitchCount, "耗尽时不再递增")
+		require.Len(t, fs.FailedAccountIDs, 4, "4个不同账号都在失败列表中")
+		require.True(t, fs.ForceCacheBilling)
+		require.Len(t, mock.calls, 1, "只有账号 100 触发了 TempUnschedule")
+	})
+
+	t.Run("模拟Antigravity平台完整流程", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(2, false)
+
+		err := newTestFailoverErr(500, false, false)
+
+		// 第一次切换：delay = 0s
+		start := time.Now()
+		action := fs.HandleFailoverError(context.Background(), mock, 100, service.PlatformAntigravity, err)
+		elapsed := time.Since(start)
+		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, 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）
+		start = time.Now()
+		action = fs.HandleFailoverError(context.Background(), mock, 300, service.PlatformAntigravity, err)
+		elapsed = time.Since(start)
+		require.Equal(t, FailoverExhausted, action)
+		require.Less(t, elapsed, 200*time.Millisecond, "耗尽时不应有延迟")
+	})
+
+	t.Run("ForceCacheBilling通过错误标志设置", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false) // hasBoundSession=false
+
+		// 第一次：ForceCacheBilling=false
+		err1 := newTestFailoverErr(500, false, false)
+		fs.HandleFailoverError(context.Background(), mock, 100, "openai", err1)
+		require.False(t, fs.ForceCacheBilling)
+
+		// 第二次：ForceCacheBilling=true（Antigravity 粘性会话切换）
+		err2 := newTestFailoverErr(500, false, true)
+		fs.HandleFailoverError(context.Background(), mock, 200, "openai", err2)
+		require.True(t, fs.ForceCacheBilling, "错误标志应触发 ForceCacheBilling")
+
+		// 第三次：ForceCacheBilling=false，但状态仍保持 true
+		err3 := newTestFailoverErr(500, false, false)
+		fs.HandleFailoverError(context.Background(), mock, 300, "openai", err3)
+		require.True(t, fs.ForceCacheBilling, "不应重置")
+	})
+}
+
+// ---------------------------------------------------------------------------
+// HandleFailoverError — 边界条件
+// ---------------------------------------------------------------------------
+
+func TestHandleFailoverError_EdgeCases(t *testing.T) {
+	t.Run("StatusCode为0的错误也能正常处理", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false)
+		err := newTestFailoverErr(0, false, false)
+
+		action := fs.HandleFailoverError(context.Background(), mock, 100, "openai", err)
+		require.Equal(t, FailoverContinue, action)
+	})
+
+	t.Run("AccountID为0也能正常跟踪", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false)
+		err := newTestFailoverErr(500, true, false)
+
+		action := fs.HandleFailoverError(context.Background(), mock, 0, "openai", err)
+		require.Equal(t, FailoverContinue, action)
+		require.Equal(t, 1, fs.SameAccountRetryCount[0])
+	})
+
+	t.Run("负AccountID也能正常跟踪", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false)
+		err := newTestFailoverErr(500, true, false)
+
+		action := fs.HandleFailoverError(context.Background(), mock, -1, "openai", err)
+		require.Equal(t, FailoverContinue, action)
+		require.Equal(t, 1, fs.SameAccountRetryCount[-1])
+	})
+
+	t.Run("空平台名称不触发Antigravity延迟", func(t *testing.T) {
+		mock := &mockTempUnscheduler{}
+		fs := NewFailoverState(3, false)
+		fs.SwitchCount = 1
+		err := newTestFailoverErr(500, false, false)
+
+		start := time.Now()
+		action := fs.HandleFailoverError(context.Background(), mock, 100, "", err)
+		elapsed := time.Since(start)
+
+		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

@@ -7,7 +7,6 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"log"
 	"net/http"
 	"strings"
 	"time"
@@ -257,12 +256,7 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 	hasBoundSession := sessionKey != "" && sessionBoundAccountID > 0
 
 	if platform == service.PlatformGemini {
-		maxAccountSwitches := h.maxAccountSwitchesGemini
-		switchCount := 0
-		failedAccountIDs := make(map[int64]struct{})
-		sameAccountRetryCount := make(map[int64]int) // 同账号重试计数
-		var lastFailoverErr *service.UpstreamFailoverError
-		var forceCacheBilling bool // 粘性会话切换时的缓存计费标记
+		fs := NewFailoverState(h.maxAccountSwitchesGemini, hasBoundSession)
 
 		// 单账号分组提前设置 SingleAccountRetry 标记，让 Service 层首次 503 就不设模型限流标记。
 		// 避免单账号分组收到 503 (MODEL_CAPACITY_EXHAUSTED) 时设 29s 限流，导致后续请求连续快速失败。
@@ -272,36 +266,29 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 		}
 
 		for {
-			selection, err := h.gatewayService.SelectAccountWithLoadAwareness(c.Request.Context(), apiKey.GroupID, sessionKey, reqModel, failedAccountIDs, "") // Gemini 不使用会话限制
+			selection, err := h.gatewayService.SelectAccountWithLoadAwareness(c.Request.Context(), apiKey.GroupID, sessionKey, reqModel, fs.FailedAccountIDs, "") // Gemini 不使用会话限制
 			if err != nil {
-				if len(failedAccountIDs) == 0 {
-					reqLog.Warn("gateway.account_select_failed", zap.Error(err), zap.Int("excluded_account_count", len(failedAccountIDs)))
-					h.handleStreamingAwareError(c, http.StatusServiceUnavailable, "api_error", "Service temporarily unavailable", streamStarted)
+				if len(fs.FailedAccountIDs) == 0 {
+					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 lastFailoverErr != nil && lastFailoverErr.StatusCode == http.StatusServiceUnavailable && switchCount <= maxAccountSwitches {
-					if sleepAntigravitySingleAccountBackoff(c.Request.Context(), switchCount) {
-						reqLog.Warn("gateway.single_account_retrying",
-							zap.Int("retry_count", switchCount),
-							zap.Int("max_retries", maxAccountSwitches),
-						)
-						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
+				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
-					}
-				}
-				if lastFailoverErr != nil {
-					h.handleFailoverExhausted(c, lastFailoverErr, service.PlatformGemini, streamStarted)
+				case FailoverCanceled:
+					return
+				default: // FailoverExhausted
+					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, account.Platform)
 
@@ -376,8 +363,8 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 			// 转发请求 - 根据账号平台分流
 			var result *service.ForwardResult
 			requestCtx := c.Request.Context()
-			if switchCount > 0 {
-				requestCtx = context.WithValue(requestCtx, ctxkey.AccountSwitchCount, switchCount)
+			if fs.SwitchCount > 0 {
+				requestCtx = context.WithValue(requestCtx, ctxkey.AccountSwitchCount, fs.SwitchCount)
 			}
 			if account.Platform == service.PlatformAntigravity {
 				result, err = h.antigravityGatewayService.ForwardGemini(requestCtx, c, account, reqModel, "generateContent", reqStream, body, hasBoundSession)
@@ -390,46 +377,17 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 			if err != nil {
 				var failoverErr *service.UpstreamFailoverError
 				if errors.As(err, &failoverErr) {
-					lastFailoverErr = failoverErr
-					if needForceCacheBilling(hasBoundSession, failoverErr) {
-						forceCacheBilling = true
-					}
-
-					// 同账号重试：对 RetryableOnSameAccount 的临时性错误，先在同一账号上重试
-					if failoverErr.RetryableOnSameAccount && sameAccountRetryCount[account.ID] < maxSameAccountRetries {
-						sameAccountRetryCount[account.ID]++
-						log.Printf("Account %d: retryable error %d, same-account retry %d/%d",
-							account.ID, failoverErr.StatusCode, sameAccountRetryCount[account.ID], maxSameAccountRetries)
-						if !sleepSameAccountRetryDelay(c.Request.Context()) {
-							return
-						}
+					action := fs.HandleFailoverError(c.Request.Context(), h.gatewayService, account.ID, account.Platform, failoverErr)
+					switch action {
+					case FailoverContinue:
 						continue
-					}
-
-					// 同账号重试用尽，执行临时封禁并切换账号
-					if failoverErr.RetryableOnSameAccount {
-						h.gatewayService.TempUnscheduleRetryableError(c.Request.Context(), account.ID, failoverErr)
-					}
-
-					failedAccountIDs[account.ID] = struct{}{}
-					if switchCount >= maxAccountSwitches {
-						h.handleFailoverExhausted(c, failoverErr, service.PlatformGemini, streamStarted)
+					case FailoverExhausted:
+						h.handleFailoverExhausted(c, fs.LastFailoverErr, service.PlatformGemini, streamStarted)
 						return
-					}
-					switchCount++
-					reqLog.Warn("gateway.upstream_failover_switching",
-						zap.Int64("account_id", account.ID),
-						zap.Int("upstream_status", failoverErr.StatusCode),
-						zap.Int("switch_count", switchCount),
-						zap.Int("max_switches", maxAccountSwitches),
-					)
-					if account.Platform == service.PlatformAntigravity {
-						if !sleepFailoverDelay(c.Request.Context(), switchCount) {
+					case FailoverCanceled:
 						return
 					}
 				}
-					continue
-				}
 				wroteFallback := h.ensureForwardErrorResponse(c, streamStarted)
 				reqLog.Error("gateway.forward_failed",
 					zap.Int64("account_id", account.ID),
@@ -453,7 +411,7 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 					Subscription:      subscription,
 					UserAgent:         userAgent,
 					IPAddress:         clientIP,
-					ForceCacheBilling: forceCacheBilling,
+					ForceCacheBilling: fs.ForceCacheBilling,
 					APIKeyService:     h.apiKeyService,
 				}); err != nil {
 					logger.L().With(
@@ -486,46 +444,34 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 	}
 
 	for {
-		maxAccountSwitches := h.maxAccountSwitches
-		switchCount := 0
-		failedAccountIDs := make(map[int64]struct{})
-		sameAccountRetryCount := make(map[int64]int) // 同账号重试计数
-		var lastFailoverErr *service.UpstreamFailoverError
+		fs := NewFailoverState(h.maxAccountSwitches, hasBoundSession)
 		retryWithFallback := false
-		var forceCacheBilling bool // 粘性会话切换时的缓存计费标记
 
 		for {
 			// 选择支持该模型的账号
-			selection, err := h.gatewayService.SelectAccountWithLoadAwareness(c.Request.Context(), currentAPIKey.GroupID, sessionKey, reqModel, failedAccountIDs, parsedReq.MetadataUserID)
+			selection, err := h.gatewayService.SelectAccountWithLoadAwareness(c.Request.Context(), currentAPIKey.GroupID, sessionKey, reqModel, fs.FailedAccountIDs, parsedReq.MetadataUserID)
 			if err != nil {
-				if len(failedAccountIDs) == 0 {
-					reqLog.Warn("gateway.account_select_failed", zap.Error(err), zap.Int("excluded_account_count", len(failedAccountIDs)))
-					h.handleStreamingAwareError(c, http.StatusServiceUnavailable, "api_error", "Service temporarily unavailable", streamStarted)
+				if len(fs.FailedAccountIDs) == 0 {
+					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 lastFailoverErr != nil && lastFailoverErr.StatusCode == http.StatusServiceUnavailable && switchCount <= maxAccountSwitches {
-					if sleepAntigravitySingleAccountBackoff(c.Request.Context(), switchCount) {
-						reqLog.Warn("gateway.single_account_retrying",
-							zap.Int("retry_count", switchCount),
-							zap.Int("max_retries", maxAccountSwitches),
-						)
-						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
+				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
-					}
-				}
-				if lastFailoverErr != nil {
-					h.handleFailoverExhausted(c, lastFailoverErr, platform, streamStarted)
+				case FailoverCanceled:
+					return
+				default: // FailoverExhausted
+					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, account.Platform)
 
@@ -600,8 +546,8 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 			// 转发请求 - 根据账号平台分流
 			var result *service.ForwardResult
 			requestCtx := c.Request.Context()
-			if switchCount > 0 {
-				requestCtx = context.WithValue(requestCtx, ctxkey.AccountSwitchCount, switchCount)
+			if fs.SwitchCount > 0 {
+				requestCtx = context.WithValue(requestCtx, ctxkey.AccountSwitchCount, fs.SwitchCount)
 			}
 			if account.Platform == service.PlatformAntigravity && account.Type != service.AccountTypeAPIKey {
 				result, err = h.antigravityGatewayService.Forward(requestCtx, c, account, body, hasBoundSession)
@@ -657,46 +603,17 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 				}
 				var failoverErr *service.UpstreamFailoverError
 				if errors.As(err, &failoverErr) {
-					lastFailoverErr = failoverErr
-					if needForceCacheBilling(hasBoundSession, failoverErr) {
-						forceCacheBilling = true
-					}
-
-					// 同账号重试：对 RetryableOnSameAccount 的临时性错误，先在同一账号上重试
-					if failoverErr.RetryableOnSameAccount && sameAccountRetryCount[account.ID] < maxSameAccountRetries {
-						sameAccountRetryCount[account.ID]++
-						log.Printf("Account %d: retryable error %d, same-account retry %d/%d",
-							account.ID, failoverErr.StatusCode, sameAccountRetryCount[account.ID], maxSameAccountRetries)
-						if !sleepSameAccountRetryDelay(c.Request.Context()) {
-							return
-						}
+					action := fs.HandleFailoverError(c.Request.Context(), h.gatewayService, account.ID, account.Platform, failoverErr)
+					switch action {
+					case FailoverContinue:
 						continue
-					}
-
-					// 同账号重试用尽，执行临时封禁并切换账号
-					if failoverErr.RetryableOnSameAccount {
-						h.gatewayService.TempUnscheduleRetryableError(c.Request.Context(), account.ID, failoverErr)
-					}
-
-					failedAccountIDs[account.ID] = struct{}{}
-					if switchCount >= maxAccountSwitches {
-						h.handleFailoverExhausted(c, failoverErr, account.Platform, streamStarted)
+					case FailoverExhausted:
+						h.handleFailoverExhausted(c, fs.LastFailoverErr, account.Platform, streamStarted)
 						return
-					}
-					switchCount++
-					reqLog.Warn("gateway.upstream_failover_switching",
-						zap.Int64("account_id", account.ID),
-						zap.Int("upstream_status", failoverErr.StatusCode),
-						zap.Int("switch_count", switchCount),
-						zap.Int("max_switches", maxAccountSwitches),
-					)
-					if account.Platform == service.PlatformAntigravity {
-						if !sleepFailoverDelay(c.Request.Context(), switchCount) {
+					case FailoverCanceled:
 						return
 					}
 				}
-					continue
-				}
 				wroteFallback := h.ensureForwardErrorResponse(c, streamStarted)
 				reqLog.Error("gateway.forward_failed",
 					zap.Int64("account_id", account.ID),
@@ -720,7 +637,7 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 					Subscription:      currentSubscription,
 					UserAgent:         userAgent,
 					IPAddress:         clientIP,
-					ForceCacheBilling: forceCacheBilling,
+					ForceCacheBilling: fs.ForceCacheBilling,
 					APIKeyService:     h.apiKeyService,
 				}); err != nil {
 					logger.L().With(
@@ -735,7 +652,7 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 			})
 			reqLog.Debug("gateway.request_completed",
 				zap.Int64("account_id", account.ID),
-				zap.Int("switch_count", switchCount),
+				zap.Int("switch_count", fs.SwitchCount),
 				zap.Bool("fallback_used", fallbackUsed),
 			)
 			return
@@ -982,69 +899,6 @@ 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)
-}
-
-const (
-	// maxSameAccountRetries 同账号重试次数上限（针对 RetryableOnSameAccount 错误）
-	maxSameAccountRetries = 2
-	// sameAccountRetryDelay 同账号重试间隔
-	sameAccountRetryDelay = 500 * time.Millisecond
-)
-
-// sleepSameAccountRetryDelay 同账号重试固定延时，返回 false 表示 context 已取消。
-func sleepSameAccountRetryDelay(ctx context.Context) bool {
-	select {
-	case <-ctx.Done():
-		return false
-	case <-time.After(sameAccountRetryDelay):
-		return true
-	}
-}
-
-// 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
-
-	logger.L().With(
-		zap.String("component", "handler.gateway.failover"),
-		zap.Duration("delay", delay),
-		zap.Int("retry_count", retryCount),
-	).Info("gateway.single_account_backoff_waiting")
-
-	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

-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

@@ -344,11 +344,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 	hasBoundSession := sessionKey != "" && sessionBoundAccountID > 0
 	cleanedForUnknownBinding := false
 
-	maxAccountSwitches := h.maxAccountSwitchesGemini
-	switchCount := 0
-	failedAccountIDs := make(map[int64]struct{})
-	var lastFailoverErr *service.UpstreamFailoverError
-	var forceCacheBilling bool // 粘性会话切换时的缓存计费标记
+	fs := NewFailoverState(h.maxAccountSwitchesGemini, hasBoundSession)
 
 	// 单账号分组提前设置 SingleAccountRetry 标记，让 Service 层首次 503 就不设模型限流标记。
 	// 避免单账号分组收到 503 (MODEL_CAPACITY_EXHAUSTED) 时设 29s 限流，导致后续请求连续快速失败。
@@ -358,30 +354,24 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 	}
 
 	for {
-		selection, err := h.gatewayService.SelectAccountWithLoadAwareness(c.Request.Context(), apiKey.GroupID, sessionKey, modelName, failedAccountIDs, "") // Gemini 不使用会话限制
+		selection, err := h.gatewayService.SelectAccountWithLoadAwareness(c.Request.Context(), apiKey.GroupID, sessionKey, modelName, fs.FailedAccountIDs, "") // Gemini 不使用会话限制
 		if err != nil {
-			if len(failedAccountIDs) == 0 {
+			if len(fs.FailedAccountIDs) == 0 {
 				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 lastFailoverErr != nil && lastFailoverErr.StatusCode == http.StatusServiceUnavailable && switchCount <= maxAccountSwitches {
-				if sleepAntigravitySingleAccountBackoff(c.Request.Context(), switchCount) {
-					reqLog.Warn("gemini.single_account_retrying",
-						zap.Int("retry_count", switchCount),
-						zap.Int("max_retries", maxAccountSwitches),
-					)
-					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
+			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
-				}
-			}
-			h.handleGeminiFailoverExhausted(c, lastFailoverErr)
+			case FailoverCanceled:
 				return
+			default: // FailoverExhausted
+				h.handleGeminiFailoverExhausted(c, fs.LastFailoverErr)
+				return
+			}
 		}
 		account := selection.Account
 		setOpsSelectedAccount(c, account.ID, account.Platform)
@@ -465,8 +455,8 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 		// 5) forward (根据平台分流)
 		var result *service.ForwardResult
 		requestCtx := c.Request.Context()
-		if switchCount > 0 {
-			requestCtx = context.WithValue(requestCtx, ctxkey.AccountSwitchCount, switchCount)
+		if fs.SwitchCount > 0 {
+			requestCtx = context.WithValue(requestCtx, ctxkey.AccountSwitchCount, fs.SwitchCount)
 		}
 		if account.Platform == service.PlatformAntigravity && account.Type != service.AccountTypeAPIKey {
 			result, err = h.antigravityGatewayService.ForwardGemini(requestCtx, c, account, modelName, action, stream, body, hasBoundSession)
@@ -479,30 +469,17 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 		if err != nil {
 			var failoverErr *service.UpstreamFailoverError
 			if errors.As(err, &failoverErr) {
-				failedAccountIDs[account.ID] = struct{}{}
-				if needForceCacheBilling(hasBoundSession, failoverErr) {
-					forceCacheBilling = true
-				}
-				if switchCount >= maxAccountSwitches {
-					lastFailoverErr = failoverErr
-					h.handleGeminiFailoverExhausted(c, lastFailoverErr)
+				failoverAction := fs.HandleFailoverError(c.Request.Context(), h.gatewayService, account.ID, account.Platform, failoverErr)
+				switch failoverAction {
+				case FailoverContinue:
+					continue
+				case FailoverExhausted:
+					h.handleGeminiFailoverExhausted(c, fs.LastFailoverErr)
 					return
-				}
-				lastFailoverErr = failoverErr
-				switchCount++
-				reqLog.Warn("gemini.upstream_failover_switching",
-					zap.Int64("account_id", account.ID),
-					zap.Int("upstream_status", failoverErr.StatusCode),
-					zap.Int("switch_count", switchCount),
-					zap.Int("max_switches", maxAccountSwitches),
-				)
-				if account.Platform == service.PlatformAntigravity {
-					if !sleepFailoverDelay(c.Request.Context(), switchCount) {
+				case FailoverCanceled:
 					return
 				}
 			}
-				continue
-			}
 			// ForwardNative already wrote the response
 			reqLog.Error("gemini.forward_failed", zap.Int64("account_id", account.ID), zap.Error(err))
 			return
@@ -539,7 +516,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 				IPAddress:             clientIP,
 				LongContextThreshold:  200000, // Gemini 200K 阈值
 				LongContextMultiplier: 2.0,    // 超出部分双倍计费
-				ForceCacheBilling:     forceCacheBilling,
+				ForceCacheBilling:     fs.ForceCacheBilling,
 				APIKeyService:         h.apiKeyService,
 			}); err != nil {
 				logger.L().With(
@@ -554,7 +531,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 		})
 		reqLog.Debug("gemini.request_completed",
 			zap.Int64("account_id", account.ID),
-			zap.Int("switch_count", switchCount),
+			zap.Int("switch_count", fs.SwitchCount),
 		)
 		return
 	}