diff --git a/backend/internal/service/gateway_service.go b/backend/internal/service/gateway_service.go
index af8838dc..49591ecd 100644
--- a/backend/internal/service/gateway_service.go
+++ b/backend/internal/service/gateway_service.go
@@ -1218,6 +1218,7 @@ func (s *GatewayService) SelectAccountWithLoadAwareness(ctx context.Context, gro
 						return a.account.LastUsedAt.Before(*b.account.LastUsedAt)
 					}
 				})
+				shuffleWithinSortGroups(routingAvailable)
 
 				// 4. 尝试获取槽位
 				for _, item := range routingAvailable {
@@ -2027,6 +2028,79 @@ func sortAccountsByPriorityAndLastUsed(accounts []*Account, preferOAuth bool) {
 			return a.LastUsedAt.Before(*b.LastUsedAt)
 		}
 	})
+	shuffleWithinPriorityAndLastUsed(accounts)
+}
+
+// shuffleWithinSortGroups 对排序后的 accountWithLoad 切片，按 (Priority, LoadRate, LastUsedAt) 分组后组内随机打乱。
+// 防止并发请求读取同一快照时，确定性排序导致所有请求命中相同账号。
+func shuffleWithinSortGroups(accounts []accountWithLoad) {
+	if len(accounts) <= 1 {
+		return
+	}
+	i := 0
+	for i < len(accounts) {
+		j := i + 1
+		for j < len(accounts) && sameAccountWithLoadGroup(accounts[i], accounts[j]) {
+			j++
+		}
+		if j-i > 1 {
+			mathrand.Shuffle(j-i, func(a, b int) {
+				accounts[i+a], accounts[i+b] = accounts[i+b], accounts[i+a]
+			})
+		}
+		i = j
+	}
+}
+
+// sameAccountWithLoadGroup 判断两个 accountWithLoad 是否属于同一排序组
+func sameAccountWithLoadGroup(a, b accountWithLoad) bool {
+	if a.account.Priority != b.account.Priority {
+		return false
+	}
+	if a.loadInfo.LoadRate != b.loadInfo.LoadRate {
+		return false
+	}
+	return sameLastUsedAt(a.account.LastUsedAt, b.account.LastUsedAt)
+}
+
+// shuffleWithinPriorityAndLastUsed 对排序后的 []*Account 切片，按 (Priority, LastUsedAt) 分组后组内随机打乱。
+func shuffleWithinPriorityAndLastUsed(accounts []*Account) {
+	if len(accounts) <= 1 {
+		return
+	}
+	i := 0
+	for i < len(accounts) {
+		j := i + 1
+		for j < len(accounts) && sameAccountGroup(accounts[i], accounts[j]) {
+			j++
+		}
+		if j-i > 1 {
+			mathrand.Shuffle(j-i, func(a, b int) {
+				accounts[i+a], accounts[i+b] = accounts[i+b], accounts[i+a]
+			})
+		}
+		i = j
+	}
+}
+
+// sameAccountGroup 判断两个 Account 是否属于同一排序组（Priority + LastUsedAt）
+func sameAccountGroup(a, b *Account) bool {
+	if a.Priority != b.Priority {
+		return false
+	}
+	return sameLastUsedAt(a.LastUsedAt, b.LastUsedAt)
+}
+
+// sameLastUsedAt 判断两个 LastUsedAt 是否相同（精度到秒）
+func sameLastUsedAt(a, b *time.Time) bool {
+	switch {
+	case a == nil && b == nil:
+		return true
+	case a == nil || b == nil:
+		return false
+	default:
+		return a.Unix() == b.Unix()
+	}
 }
 
 // selectByCallCount 从候选账号中选择调用次数最少的账号（Antigravity 专用）
diff --git a/backend/internal/service/openai_gateway_service.go b/backend/internal/service/openai_gateway_service.go
index fbe81cb4..6c4fe256 100644
--- a/backend/internal/service/openai_gateway_service.go
+++ b/backend/internal/service/openai_gateway_service.go
@@ -580,10 +580,6 @@ func (s *OpenAIGatewayService) SelectAccountWithLoadAwareness(ctx context.Contex
 			}
 		}
 	} else {
-		type accountWithLoad struct {
-			account  *Account
-			loadInfo *AccountLoadInfo
-		}
 		var available []accountWithLoad
 		for _, acc := range candidates {
 			loadInfo := loadMap[acc.ID]
@@ -618,6 +614,7 @@ func (s *OpenAIGatewayService) SelectAccountWithLoadAwareness(ctx context.Contex
 					return a.account.LastUsedAt.Before(*b.account.LastUsedAt)
 				}
 			})
+			shuffleWithinSortGroups(available)
 
 			for _, item := range available {
 				result, err := s.tryAcquireAccountSlot(ctx, item.account.ID, item.account.Concurrency)
diff --git a/backend/internal/service/scheduler_shuffle_test.go b/backend/internal/service/scheduler_shuffle_test.go
new file mode 100644
index 00000000..78ac5f57
--- /dev/null
+++ b/backend/internal/service/scheduler_shuffle_test.go
@@ -0,0 +1,318 @@
+//go:build unit
+
+package service
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+// ============ shuffleWithinSortGroups 测试 ============
+
+func TestShuffleWithinSortGroups_Empty(t *testing.T) {
+	shuffleWithinSortGroups(nil)
+	shuffleWithinSortGroups([]accountWithLoad{})
+}
+
+func TestShuffleWithinSortGroups_SingleElement(t *testing.T) {
+	accounts := []accountWithLoad{
+		{account: &Account{ID: 1, Priority: 1}, loadInfo: &AccountLoadInfo{LoadRate: 10}},
+	}
+	shuffleWithinSortGroups(accounts)
+	require.Equal(t, int64(1), accounts[0].account.ID)
+}
+
+func TestShuffleWithinSortGroups_DifferentGroups_OrderPreserved(t *testing.T) {
+	now := time.Now()
+	earlier := now.Add(-1 * time.Hour)
+
+	accounts := []accountWithLoad{
+		{account: &Account{ID: 1, Priority: 1, LastUsedAt: &earlier}, loadInfo: &AccountLoadInfo{LoadRate: 10}},
+		{account: &Account{ID: 2, Priority: 1, LastUsedAt: &now}, loadInfo: &AccountLoadInfo{LoadRate: 20}},
+		{account: &Account{ID: 3, Priority: 2, LastUsedAt: &earlier}, loadInfo: &AccountLoadInfo{LoadRate: 10}},
+	}
+
+	// 每个元素都属于不同组（Priority 或 LoadRate 或 LastUsedAt 不同），顺序不变
+	for i := 0; i < 20; i++ {
+		cpy := make([]accountWithLoad, len(accounts))
+		copy(cpy, accounts)
+		shuffleWithinSortGroups(cpy)
+		require.Equal(t, int64(1), cpy[0].account.ID)
+		require.Equal(t, int64(2), cpy[1].account.ID)
+		require.Equal(t, int64(3), cpy[2].account.ID)
+	}
+}
+
+func TestShuffleWithinSortGroups_SameGroup_Shuffled(t *testing.T) {
+	now := time.Now()
+	// 同一秒的时间戳视为同一组
+	sameSecond := time.Unix(now.Unix(), 0)
+	sameSecond2 := time.Unix(now.Unix(), 500_000_000) // 同一秒但不同纳秒
+
+	accounts := []accountWithLoad{
+		{account: &Account{ID: 1, Priority: 1, LastUsedAt: &sameSecond}, loadInfo: &AccountLoadInfo{LoadRate: 10}},
+		{account: &Account{ID: 2, Priority: 1, LastUsedAt: &sameSecond2}, loadInfo: &AccountLoadInfo{LoadRate: 10}},
+		{account: &Account{ID: 3, Priority: 1, LastUsedAt: &sameSecond}, loadInfo: &AccountLoadInfo{LoadRate: 10}},
+	}
+
+	// 多次执行，验证所有 ID 都出现在第一个位置（说明确实被打乱了）
+	seen := map[int64]bool{}
+	for i := 0; i < 100; i++ {
+		cpy := make([]accountWithLoad, len(accounts))
+		copy(cpy, accounts)
+		shuffleWithinSortGroups(cpy)
+		seen[cpy[0].account.ID] = true
+		// 无论怎么打乱，所有 ID 都应在候选中
+		ids := map[int64]bool{}
+		for _, a := range cpy {
+			ids[a.account.ID] = true
+		}
+		require.True(t, ids[1] && ids[2] && ids[3])
+	}
+	// 至少 2 个不同的 ID 出现在首位（随机性验证）
+	require.GreaterOrEqual(t, len(seen), 2, "shuffle should produce different orderings")
+}
+
+func TestShuffleWithinSortGroups_NilLastUsedAt_SameGroup(t *testing.T) {
+	accounts := []accountWithLoad{
+		{account: &Account{ID: 1, Priority: 1, LastUsedAt: nil}, loadInfo: &AccountLoadInfo{LoadRate: 0}},
+		{account: &Account{ID: 2, Priority: 1, LastUsedAt: nil}, loadInfo: &AccountLoadInfo{LoadRate: 0}},
+		{account: &Account{ID: 3, Priority: 1, LastUsedAt: nil}, loadInfo: &AccountLoadInfo{LoadRate: 0}},
+	}
+
+	seen := map[int64]bool{}
+	for i := 0; i < 100; i++ {
+		cpy := make([]accountWithLoad, len(accounts))
+		copy(cpy, accounts)
+		shuffleWithinSortGroups(cpy)
+		seen[cpy[0].account.ID] = true
+	}
+	require.GreaterOrEqual(t, len(seen), 2, "nil LastUsedAt accounts should be shuffled")
+}
+
+func TestShuffleWithinSortGroups_MixedGroups(t *testing.T) {
+	now := time.Now()
+	earlier := now.Add(-1 * time.Hour)
+	sameAsNow := time.Unix(now.Unix(), 0)
+
+	// 组1: Priority=1, LoadRate=10, LastUsedAt=earlier (ID 1)  — 单元素组
+	// 组2: Priority=1, LoadRate=20, LastUsedAt=now (ID 2, 3)   — 双元素组
+	// 组3: Priority=2, LoadRate=10, LastUsedAt=earlier (ID 4)  — 单元素组
+	accounts := []accountWithLoad{
+		{account: &Account{ID: 1, Priority: 1, LastUsedAt: &earlier}, loadInfo: &AccountLoadInfo{LoadRate: 10}},
+		{account: &Account{ID: 2, Priority: 1, LastUsedAt: &now}, loadInfo: &AccountLoadInfo{LoadRate: 20}},
+		{account: &Account{ID: 3, Priority: 1, LastUsedAt: &sameAsNow}, loadInfo: &AccountLoadInfo{LoadRate: 20}},
+		{account: &Account{ID: 4, Priority: 2, LastUsedAt: &earlier}, loadInfo: &AccountLoadInfo{LoadRate: 10}},
+	}
+
+	for i := 0; i < 20; i++ {
+		cpy := make([]accountWithLoad, len(accounts))
+		copy(cpy, accounts)
+		shuffleWithinSortGroups(cpy)
+
+		// 组间顺序不变
+		require.Equal(t, int64(1), cpy[0].account.ID, "group 1 position fixed")
+		require.Equal(t, int64(4), cpy[3].account.ID, "group 3 position fixed")
+
+		// 组2 内部可以打乱，但仍在位置 1 和 2
+		mid := map[int64]bool{cpy[1].account.ID: true, cpy[2].account.ID: true}
+		require.True(t, mid[2] && mid[3], "group 2 elements should stay in positions 1-2")
+	}
+}
+
+// ============ shuffleWithinPriorityAndLastUsed 测试 ============
+
+func TestShuffleWithinPriorityAndLastUsed_Empty(t *testing.T) {
+	shuffleWithinPriorityAndLastUsed(nil)
+	shuffleWithinPriorityAndLastUsed([]*Account{})
+}
+
+func TestShuffleWithinPriorityAndLastUsed_SingleElement(t *testing.T) {
+	accounts := []*Account{{ID: 1, Priority: 1}}
+	shuffleWithinPriorityAndLastUsed(accounts)
+	require.Equal(t, int64(1), accounts[0].ID)
+}
+
+func TestShuffleWithinPriorityAndLastUsed_SameGroup_Shuffled(t *testing.T) {
+	accounts := []*Account{
+		{ID: 1, Priority: 1, LastUsedAt: nil},
+		{ID: 2, Priority: 1, LastUsedAt: nil},
+		{ID: 3, Priority: 1, LastUsedAt: nil},
+	}
+
+	seen := map[int64]bool{}
+	for i := 0; i < 100; i++ {
+		cpy := make([]*Account, len(accounts))
+		copy(cpy, accounts)
+		shuffleWithinPriorityAndLastUsed(cpy)
+		seen[cpy[0].ID] = true
+	}
+	require.GreaterOrEqual(t, len(seen), 2, "same group should be shuffled")
+}
+
+func TestShuffleWithinPriorityAndLastUsed_DifferentPriority_OrderPreserved(t *testing.T) {
+	accounts := []*Account{
+		{ID: 1, Priority: 1, LastUsedAt: nil},
+		{ID: 2, Priority: 2, LastUsedAt: nil},
+		{ID: 3, Priority: 3, LastUsedAt: nil},
+	}
+
+	for i := 0; i < 20; i++ {
+		cpy := make([]*Account, len(accounts))
+		copy(cpy, accounts)
+		shuffleWithinPriorityAndLastUsed(cpy)
+		require.Equal(t, int64(1), cpy[0].ID)
+		require.Equal(t, int64(2), cpy[1].ID)
+		require.Equal(t, int64(3), cpy[2].ID)
+	}
+}
+
+func TestShuffleWithinPriorityAndLastUsed_DifferentLastUsedAt_OrderPreserved(t *testing.T) {
+	now := time.Now()
+	earlier := now.Add(-1 * time.Hour)
+
+	accounts := []*Account{
+		{ID: 1, Priority: 1, LastUsedAt: nil},
+		{ID: 2, Priority: 1, LastUsedAt: &earlier},
+		{ID: 3, Priority: 1, LastUsedAt: &now},
+	}
+
+	for i := 0; i < 20; i++ {
+		cpy := make([]*Account, len(accounts))
+		copy(cpy, accounts)
+		shuffleWithinPriorityAndLastUsed(cpy)
+		require.Equal(t, int64(1), cpy[0].ID)
+		require.Equal(t, int64(2), cpy[1].ID)
+		require.Equal(t, int64(3), cpy[2].ID)
+	}
+}
+
+// ============ sameLastUsedAt 测试 ============
+
+func TestSameLastUsedAt(t *testing.T) {
+	now := time.Now()
+	sameSecond := time.Unix(now.Unix(), 0)
+	sameSecondDiffNano := time.Unix(now.Unix(), 999_999_999)
+	differentSecond := now.Add(1 * time.Second)
+
+	t.Run("both nil", func(t *testing.T) {
+		require.True(t, sameLastUsedAt(nil, nil))
+	})
+
+	t.Run("one nil one not", func(t *testing.T) {
+		require.False(t, sameLastUsedAt(nil, &now))
+		require.False(t, sameLastUsedAt(&now, nil))
+	})
+
+	t.Run("same second different nanoseconds", func(t *testing.T) {
+		require.True(t, sameLastUsedAt(&sameSecond, &sameSecondDiffNano))
+	})
+
+	t.Run("different seconds", func(t *testing.T) {
+		require.False(t, sameLastUsedAt(&now, &differentSecond))
+	})
+
+	t.Run("exact same time", func(t *testing.T) {
+		require.True(t, sameLastUsedAt(&now, &now))
+	})
+}
+
+// ============ sameAccountWithLoadGroup 测试 ============
+
+func TestSameAccountWithLoadGroup(t *testing.T) {
+	now := time.Now()
+	sameSecond := time.Unix(now.Unix(), 0)
+
+	t.Run("same group", func(t *testing.T) {
+		a := accountWithLoad{account: &Account{Priority: 1, LastUsedAt: &now}, loadInfo: &AccountLoadInfo{LoadRate: 10}}
+		b := accountWithLoad{account: &Account{Priority: 1, LastUsedAt: &sameSecond}, loadInfo: &AccountLoadInfo{LoadRate: 10}}
+		require.True(t, sameAccountWithLoadGroup(a, b))
+	})
+
+	t.Run("different priority", func(t *testing.T) {
+		a := accountWithLoad{account: &Account{Priority: 1, LastUsedAt: &now}, loadInfo: &AccountLoadInfo{LoadRate: 10}}
+		b := accountWithLoad{account: &Account{Priority: 2, LastUsedAt: &now}, loadInfo: &AccountLoadInfo{LoadRate: 10}}
+		require.False(t, sameAccountWithLoadGroup(a, b))
+	})
+
+	t.Run("different load rate", func(t *testing.T) {
+		a := accountWithLoad{account: &Account{Priority: 1, LastUsedAt: &now}, loadInfo: &AccountLoadInfo{LoadRate: 10}}
+		b := accountWithLoad{account: &Account{Priority: 1, LastUsedAt: &now}, loadInfo: &AccountLoadInfo{LoadRate: 20}}
+		require.False(t, sameAccountWithLoadGroup(a, b))
+	})
+
+	t.Run("different last used at", func(t *testing.T) {
+		later := now.Add(1 * time.Second)
+		a := accountWithLoad{account: &Account{Priority: 1, LastUsedAt: &now}, loadInfo: &AccountLoadInfo{LoadRate: 10}}
+		b := accountWithLoad{account: &Account{Priority: 1, LastUsedAt: &later}, loadInfo: &AccountLoadInfo{LoadRate: 10}}
+		require.False(t, sameAccountWithLoadGroup(a, b))
+	})
+
+	t.Run("both nil LastUsedAt", func(t *testing.T) {
+		a := accountWithLoad{account: &Account{Priority: 1, LastUsedAt: nil}, loadInfo: &AccountLoadInfo{LoadRate: 0}}
+		b := accountWithLoad{account: &Account{Priority: 1, LastUsedAt: nil}, loadInfo: &AccountLoadInfo{LoadRate: 0}}
+		require.True(t, sameAccountWithLoadGroup(a, b))
+	})
+}
+
+// ============ sameAccountGroup 测试 ============
+
+func TestSameAccountGroup(t *testing.T) {
+	now := time.Now()
+
+	t.Run("same group", func(t *testing.T) {
+		a := &Account{Priority: 1, LastUsedAt: nil}
+		b := &Account{Priority: 1, LastUsedAt: nil}
+		require.True(t, sameAccountGroup(a, b))
+	})
+
+	t.Run("different priority", func(t *testing.T) {
+		a := &Account{Priority: 1, LastUsedAt: nil}
+		b := &Account{Priority: 2, LastUsedAt: nil}
+		require.False(t, sameAccountGroup(a, b))
+	})
+
+	t.Run("different LastUsedAt", func(t *testing.T) {
+		later := now.Add(1 * time.Second)
+		a := &Account{Priority: 1, LastUsedAt: &now}
+		b := &Account{Priority: 1, LastUsedAt: &later}
+		require.False(t, sameAccountGroup(a, b))
+	})
+}
+
+// ============ sortAccountsByPriorityAndLastUsed 集成随机化测试 ============
+
+func TestSortAccountsByPriorityAndLastUsed_WithShuffle(t *testing.T) {
+	t.Run("same priority and nil LastUsedAt are shuffled", func(t *testing.T) {
+		accounts := []*Account{
+			{ID: 1, Priority: 1, LastUsedAt: nil},
+			{ID: 2, Priority: 1, LastUsedAt: nil},
+			{ID: 3, Priority: 1, LastUsedAt: nil},
+		}
+
+		seen := map[int64]bool{}
+		for i := 0; i < 100; i++ {
+			cpy := make([]*Account, len(accounts))
+			copy(cpy, accounts)
+			sortAccountsByPriorityAndLastUsed(cpy, false)
+			seen[cpy[0].ID] = true
+		}
+		require.GreaterOrEqual(t, len(seen), 2, "identical sort keys should produce different orderings after shuffle")
+	})
+
+	t.Run("different priorities still sorted correctly", func(t *testing.T) {
+		now := time.Now()
+		accounts := []*Account{
+			{ID: 3, Priority: 3, LastUsedAt: &now},
+			{ID: 1, Priority: 1, LastUsedAt: &now},
+			{ID: 2, Priority: 2, LastUsedAt: &now},
+		}
+
+		sortAccountsByPriorityAndLastUsed(accounts, false)
+		require.Equal(t, int64(1), accounts[0].ID)
+		require.Equal(t, int64(2), accounts[1].ID)
+		require.Equal(t, int64(3), accounts[2].ID)
+	})
+}