diff --git a/backend/internal/service/antigravity_internal500_penalty_test.go b/backend/internal/service/antigravity_internal500_penalty_test.go
new file mode 100644
index 00000000..03831839
--- /dev/null
+++ b/backend/internal/service/antigravity_internal500_penalty_test.go
@@ -0,0 +1,321 @@
+//go:build unit
+
+package service
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+// --- mock: Internal500CounterCache ---
+
+type mockInternal500Cache struct {
+	incrementCount int64
+	incrementErr   error
+	resetErr       error
+
+	incrementCalls []int64 // 记录 IncrementInternal500Count 被调用时的 accountID
+	resetCalls     []int64 // 记录 ResetInternal500Count 被调用时的 accountID
+}
+
+func (m *mockInternal500Cache) IncrementInternal500Count(_ context.Context, accountID int64) (int64, error) {
+	m.incrementCalls = append(m.incrementCalls, accountID)
+	return m.incrementCount, m.incrementErr
+}
+
+func (m *mockInternal500Cache) ResetInternal500Count(_ context.Context, accountID int64) error {
+	m.resetCalls = append(m.resetCalls, accountID)
+	return m.resetErr
+}
+
+// --- mock: 专用于 internal500 惩罚测试的 AccountRepository ---
+
+type internal500AccountRepoStub struct {
+	AccountRepository // 嵌入接口，未实现的方法会 panic（不应被调用）
+
+	tempUnschedCalls []tempUnschedCall
+	setErrorCalls    []setErrorCall
+}
+
+type tempUnschedCall struct {
+	accountID int64
+	until     time.Time
+	reason    string
+}
+
+type setErrorCall struct {
+	accountID int64
+	reason    string
+}
+
+func (r *internal500AccountRepoStub) SetTempUnschedulable(_ context.Context, id int64, until time.Time, reason string) error {
+	r.tempUnschedCalls = append(r.tempUnschedCalls, tempUnschedCall{accountID: id, until: until, reason: reason})
+	return nil
+}
+
+func (r *internal500AccountRepoStub) SetError(_ context.Context, id int64, errorMsg string) error {
+	r.setErrorCalls = append(r.setErrorCalls, setErrorCall{accountID: id, reason: errorMsg})
+	return nil
+}
+
+// =============================================================================
+// TestIsAntigravityInternalServerError
+// =============================================================================
+
+func TestIsAntigravityInternalServerError(t *testing.T) {
+	t.Run("匹配完整的 INTERNAL 500 body", func(t *testing.T) {
+		body := []byte(`{"error":{"code":500,"message":"Internal error encountered.","status":"INTERNAL"}}`)
+		require.True(t, isAntigravityInternalServerError(500, body))
+	})
+
+	t.Run("statusCode 不是 500", func(t *testing.T) {
+		body := []byte(`{"error":{"code":500,"message":"Internal error encountered.","status":"INTERNAL"}}`)
+		require.False(t, isAntigravityInternalServerError(429, body))
+		require.False(t, isAntigravityInternalServerError(503, body))
+		require.False(t, isAntigravityInternalServerError(200, body))
+	})
+
+	t.Run("body 中 message 不匹配", func(t *testing.T) {
+		body := []byte(`{"error":{"code":500,"message":"Some other error","status":"INTERNAL"}}`)
+		require.False(t, isAntigravityInternalServerError(500, body))
+	})
+
+	t.Run("body 中 status 不匹配", func(t *testing.T) {
+		body := []byte(`{"error":{"code":500,"message":"Internal error encountered.","status":"UNAVAILABLE"}}`)
+		require.False(t, isAntigravityInternalServerError(500, body))
+	})
+
+	t.Run("body 中 code 不匹配", func(t *testing.T) {
+		body := []byte(`{"error":{"code":503,"message":"Internal error encountered.","status":"INTERNAL"}}`)
+		require.False(t, isAntigravityInternalServerError(500, body))
+	})
+
+	t.Run("空 body", func(t *testing.T) {
+		require.False(t, isAntigravityInternalServerError(500, []byte{}))
+		require.False(t, isAntigravityInternalServerError(500, nil))
+	})
+
+	t.Run("其他 500 错误格式（纯文本）", func(t *testing.T) {
+		body := []byte(`Internal Server Error`)
+		require.False(t, isAntigravityInternalServerError(500, body))
+	})
+
+	t.Run("其他 500 错误格式（不同 JSON 结构）", func(t *testing.T) {
+		body := []byte(`{"message":"Internal Server Error","statusCode":500}`)
+		require.False(t, isAntigravityInternalServerError(500, body))
+	})
+}
+
+// =============================================================================
+// TestApplyInternal500Penalty
+// =============================================================================
+
+func TestApplyInternal500Penalty(t *testing.T) {
+	t.Run("count=1 → SetTempUnschedulable 10 分钟", func(t *testing.T) {
+		repo := &internal500AccountRepoStub{}
+		svc := &AntigravityGatewayService{accountRepo: repo}
+		account := &Account{ID: 1, Name: "acc-1"}
+
+		before := time.Now()
+		svc.applyInternal500Penalty(context.Background(), "[test]", account, 1)
+		after := time.Now()
+
+		require.Len(t, repo.tempUnschedCalls, 1)
+		require.Empty(t, repo.setErrorCalls)
+
+		call := repo.tempUnschedCalls[0]
+		require.Equal(t, int64(1), call.accountID)
+		require.Contains(t, call.reason, "INTERNAL 500")
+		// until 应在 [before+10m, after+10m] 范围内
+		require.True(t, call.until.After(before.Add(internal500PenaltyTier1Duration).Add(-time.Second)))
+		require.True(t, call.until.Before(after.Add(internal500PenaltyTier1Duration).Add(time.Second)))
+	})
+
+	t.Run("count=2 → SetTempUnschedulable 10 小时", func(t *testing.T) {
+		repo := &internal500AccountRepoStub{}
+		svc := &AntigravityGatewayService{accountRepo: repo}
+		account := &Account{ID: 2, Name: "acc-2"}
+
+		before := time.Now()
+		svc.applyInternal500Penalty(context.Background(), "[test]", account, 2)
+		after := time.Now()
+
+		require.Len(t, repo.tempUnschedCalls, 1)
+		require.Empty(t, repo.setErrorCalls)
+
+		call := repo.tempUnschedCalls[0]
+		require.Equal(t, int64(2), call.accountID)
+		require.Contains(t, call.reason, "INTERNAL 500")
+		require.True(t, call.until.After(before.Add(internal500PenaltyTier2Duration).Add(-time.Second)))
+		require.True(t, call.until.Before(after.Add(internal500PenaltyTier2Duration).Add(time.Second)))
+	})
+
+	t.Run("count=3 → SetError 永久禁用", func(t *testing.T) {
+		repo := &internal500AccountRepoStub{}
+		svc := &AntigravityGatewayService{accountRepo: repo}
+		account := &Account{ID: 3, Name: "acc-3"}
+
+		svc.applyInternal500Penalty(context.Background(), "[test]", account, 3)
+
+		require.Empty(t, repo.tempUnschedCalls)
+		require.Len(t, repo.setErrorCalls, 1)
+
+		call := repo.setErrorCalls[0]
+		require.Equal(t, int64(3), call.accountID)
+		require.Contains(t, call.reason, "INTERNAL 500 consecutive failures: 3")
+	})
+
+	t.Run("count=5 → SetError 永久禁用（>=3 都走永久禁用）", func(t *testing.T) {
+		repo := &internal500AccountRepoStub{}
+		svc := &AntigravityGatewayService{accountRepo: repo}
+		account := &Account{ID: 5, Name: "acc-5"}
+
+		svc.applyInternal500Penalty(context.Background(), "[test]", account, 5)
+
+		require.Empty(t, repo.tempUnschedCalls)
+		require.Len(t, repo.setErrorCalls, 1)
+
+		call := repo.setErrorCalls[0]
+		require.Equal(t, int64(5), call.accountID)
+		require.Contains(t, call.reason, "INTERNAL 500 consecutive failures: 5")
+	})
+
+	t.Run("count=0 → 不调用任何方法", func(t *testing.T) {
+		repo := &internal500AccountRepoStub{}
+		svc := &AntigravityGatewayService{accountRepo: repo}
+		account := &Account{ID: 10, Name: "acc-10"}
+
+		svc.applyInternal500Penalty(context.Background(), "[test]", account, 0)
+
+		require.Empty(t, repo.tempUnschedCalls)
+		require.Empty(t, repo.setErrorCalls)
+	})
+}
+
+// =============================================================================
+// TestHandleInternal500RetryExhausted
+// =============================================================================
+
+func TestHandleInternal500RetryExhausted(t *testing.T) {
+	t.Run("internal500Cache 为 nil → 不 panic，不调用任何方法", func(t *testing.T) {
+		repo := &internal500AccountRepoStub{}
+		svc := &AntigravityGatewayService{
+			accountRepo:      repo,
+			internal500Cache: nil,
+		}
+		account := &Account{ID: 1, Name: "acc-1"}
+
+		// 不应 panic
+		require.NotPanics(t, func() {
+			svc.handleInternal500RetryExhausted(context.Background(), "[test]", account)
+		})
+		require.Empty(t, repo.tempUnschedCalls)
+		require.Empty(t, repo.setErrorCalls)
+	})
+
+	t.Run("IncrementInternal500Count 返回 error → 不调用惩罚方法", func(t *testing.T) {
+		repo := &internal500AccountRepoStub{}
+		cache := &mockInternal500Cache{
+			incrementErr: errors.New("redis connection error"),
+		}
+		svc := &AntigravityGatewayService{
+			accountRepo:      repo,
+			internal500Cache: cache,
+		}
+		account := &Account{ID: 2, Name: "acc-2"}
+
+		svc.handleInternal500RetryExhausted(context.Background(), "[test]", account)
+
+		require.Len(t, cache.incrementCalls, 1)
+		require.Equal(t, int64(2), cache.incrementCalls[0])
+		require.Empty(t, repo.tempUnschedCalls)
+		require.Empty(t, repo.setErrorCalls)
+	})
+
+	t.Run("IncrementInternal500Count 返回 count=1 → 触发 tier1 惩罚", func(t *testing.T) {
+		repo := &internal500AccountRepoStub{}
+		cache := &mockInternal500Cache{
+			incrementCount: 1,
+		}
+		svc := &AntigravityGatewayService{
+			accountRepo:      repo,
+			internal500Cache: cache,
+		}
+		account := &Account{ID: 3, Name: "acc-3"}
+
+		svc.handleInternal500RetryExhausted(context.Background(), "[test]", account)
+
+		require.Len(t, cache.incrementCalls, 1)
+		require.Equal(t, int64(3), cache.incrementCalls[0])
+		// tier1: SetTempUnschedulable
+		require.Len(t, repo.tempUnschedCalls, 1)
+		require.Equal(t, int64(3), repo.tempUnschedCalls[0].accountID)
+		require.Empty(t, repo.setErrorCalls)
+	})
+
+	t.Run("IncrementInternal500Count 返回 count=3 → 触发 tier3 永久禁用", func(t *testing.T) {
+		repo := &internal500AccountRepoStub{}
+		cache := &mockInternal500Cache{
+			incrementCount: 3,
+		}
+		svc := &AntigravityGatewayService{
+			accountRepo:      repo,
+			internal500Cache: cache,
+		}
+		account := &Account{ID: 4, Name: "acc-4"}
+
+		svc.handleInternal500RetryExhausted(context.Background(), "[test]", account)
+
+		require.Len(t, cache.incrementCalls, 1)
+		require.Empty(t, repo.tempUnschedCalls)
+		require.Len(t, repo.setErrorCalls, 1)
+		require.Equal(t, int64(4), repo.setErrorCalls[0].accountID)
+	})
+}
+
+// =============================================================================
+// TestResetInternal500Counter
+// =============================================================================
+
+func TestResetInternal500Counter(t *testing.T) {
+	t.Run("internal500Cache 为 nil → 不 panic", func(t *testing.T) {
+		svc := &AntigravityGatewayService{
+			internal500Cache: nil,
+		}
+
+		require.NotPanics(t, func() {
+			svc.resetInternal500Counter(context.Background(), "[test]", 1)
+		})
+	})
+
+	t.Run("ResetInternal500Count 返回 error → 不 panic（仅日志）", func(t *testing.T) {
+		cache := &mockInternal500Cache{
+			resetErr: errors.New("redis timeout"),
+		}
+		svc := &AntigravityGatewayService{
+			internal500Cache: cache,
+		}
+
+		require.NotPanics(t, func() {
+			svc.resetInternal500Counter(context.Background(), "[test]", 42)
+		})
+		require.Len(t, cache.resetCalls, 1)
+		require.Equal(t, int64(42), cache.resetCalls[0])
+	})
+
+	t.Run("正常调用 → 调用 ResetInternal500Count", func(t *testing.T) {
+		cache := &mockInternal500Cache{}
+		svc := &AntigravityGatewayService{
+			internal500Cache: cache,
+		}
+
+		svc.resetInternal500Counter(context.Background(), "[test]", 99)
+
+		require.Len(t, cache.resetCalls, 1)
+		require.Equal(t, int64(99), cache.resetCalls[0])
+	})
+}