diff --git a/backend/internal/middleware/rate_limiter.go b/backend/internal/middleware/rate_limiter.go
index 9526f071..13b71683 100644
--- a/backend/internal/middleware/rate_limiter.go
+++ b/backend/internal/middleware/rate_limiter.go
@@ -1,6 +1,7 @@
 package middleware
 
 import (
+	"context"
 	"net/http"
 	"time"
 
@@ -8,6 +9,33 @@ import (
 	"github.com/redis/go-redis/v9"
 )
 
+// RateLimitFailureMode Redis 故障策略
+type RateLimitFailureMode int
+
+const (
+	RateLimitFailOpen RateLimitFailureMode = iota
+	RateLimitFailClose
+)
+
+// RateLimitOptions 限流可选配置
+type RateLimitOptions struct {
+	FailureMode RateLimitFailureMode
+}
+
+var rateLimitScript = redis.NewScript(`
+local current = redis.call('INCR', KEYS[1])
+local ttl = redis.call('PTTL', KEYS[1])
+if current == 1 or ttl == -1 then
+  redis.call('PEXPIRE', KEYS[1], ARGV[1])
+end
+return current
+`)
+
+// rateLimitRun 允许测试覆写脚本执行逻辑
+var rateLimitRun = func(ctx context.Context, client *redis.Client, key string, windowMillis int64) (int64, error) {
+	return rateLimitScript.Run(ctx, client, []string{key}, windowMillis).Int64()
+}
+
 // RateLimiter Redis 速率限制器
 type RateLimiter struct {
 	redis  *redis.Client
@@ -27,34 +55,57 @@ func NewRateLimiter(redisClient *redis.Client) *RateLimiter {
 // limit: 时间窗口内最大请求数
 // window: 时间窗口
 func (r *RateLimiter) Limit(key string, limit int, window time.Duration) gin.HandlerFunc {
+	return r.LimitWithOptions(key, limit, window, RateLimitOptions{})
+}
+
+// LimitWithOptions 返回速率限制中间件（带可选配置）
+func (r *RateLimiter) LimitWithOptions(key string, limit int, window time.Duration, opts RateLimitOptions) gin.HandlerFunc {
+	failureMode := opts.FailureMode
+	if failureMode != RateLimitFailClose {
+		failureMode = RateLimitFailOpen
+	}
+
 	return func(c *gin.Context) {
 		ip := c.ClientIP()
 		redisKey := r.prefix + key + ":" + ip
 
 		ctx := c.Request.Context()
 
-		// 使用 INCR 原子操作增加计数
-		count, err := r.redis.Incr(ctx, redisKey).Result()
+		windowMillis := windowTTLMillis(window)
+
+		// 使用 Lua 脚本原子操作增加计数并设置过期
+		count, err := rateLimitRun(ctx, r.redis, redisKey, windowMillis)
 		if err != nil {
+			if failureMode == RateLimitFailClose {
+				abortRateLimit(c)
+				return
+			}
 			// Redis 错误时放行，避免影响正常服务
 			c.Next()
 			return
 		}
 
-		// 首次访问时设置过期时间
-		if count == 1 {
-			r.redis.Expire(ctx, redisKey, window)
-		}
-
 		// 超过限制
 		if count > int64(limit) {
-			c.AbortWithStatusJSON(http.StatusTooManyRequests, gin.H{
-				"error":   "rate limit exceeded",
-				"message": "Too many requests, please try again later",
-			})
+			abortRateLimit(c)
 			return
 		}
 
 		c.Next()
 	}
 }
+
+func windowTTLMillis(window time.Duration) int64 {
+	ttl := window.Milliseconds()
+	if ttl < 1 {
+		return 1
+	}
+	return ttl
+}
+
+func abortRateLimit(c *gin.Context) {
+	c.AbortWithStatusJSON(http.StatusTooManyRequests, gin.H{
+		"error":   "rate limit exceeded",
+		"message": "Too many requests, please try again later",
+	})
+}
diff --git a/backend/internal/middleware/rate_limiter_integration_test.go b/backend/internal/middleware/rate_limiter_integration_test.go
new file mode 100644
index 00000000..4759a988
--- /dev/null
+++ b/backend/internal/middleware/rate_limiter_integration_test.go
@@ -0,0 +1,114 @@
+//go:build integration
+
+package middleware
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"github.com/redis/go-redis/v9"
+	"github.com/stretchr/testify/require"
+	tcredis "github.com/testcontainers/testcontainers-go/modules/redis"
+)
+
+const redisImageTag = "redis:8.4-alpine"
+
+func TestRateLimiterSetsTTLAndDoesNotRefresh(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	ctx := context.Background()
+	rdb := startRedis(t, ctx)
+	limiter := NewRateLimiter(rdb)
+
+	router := gin.New()
+	router.Use(limiter.Limit("ttl-test", 10, 2*time.Second))
+	router.GET("/test", func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{"ok": true})
+	})
+
+	recorder := performRequest(router)
+	require.Equal(t, http.StatusOK, recorder.Code)
+
+	redisKey := limiter.prefix + "ttl-test:127.0.0.1"
+	ttlBefore, err := rdb.PTTL(ctx, redisKey).Result()
+	require.NoError(t, err)
+	require.Greater(t, ttlBefore, time.Duration(0))
+	require.LessOrEqual(t, ttlBefore, 2*time.Second)
+
+	time.Sleep(50 * time.Millisecond)
+
+	recorder = performRequest(router)
+	require.Equal(t, http.StatusOK, recorder.Code)
+
+	ttlAfter, err := rdb.PTTL(ctx, redisKey).Result()
+	require.NoError(t, err)
+	require.Less(t, ttlAfter, ttlBefore)
+}
+
+func TestRateLimiterFixesMissingTTL(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	ctx := context.Background()
+	rdb := startRedis(t, ctx)
+	limiter := NewRateLimiter(rdb)
+
+	router := gin.New()
+	router.Use(limiter.Limit("ttl-missing", 10, 2*time.Second))
+	router.GET("/test", func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{"ok": true})
+	})
+
+	redisKey := limiter.prefix + "ttl-missing:127.0.0.1"
+	require.NoError(t, rdb.Set(ctx, redisKey, 5, 0).Err())
+
+	ttlBefore, err := rdb.PTTL(ctx, redisKey).Result()
+	require.NoError(t, err)
+	require.Less(t, ttlBefore, time.Duration(0))
+
+	recorder := performRequest(router)
+	require.Equal(t, http.StatusOK, recorder.Code)
+
+	ttlAfter, err := rdb.PTTL(ctx, redisKey).Result()
+	require.NoError(t, err)
+	require.Greater(t, ttlAfter, time.Duration(0))
+}
+
+func performRequest(router *gin.Engine) *httptest.ResponseRecorder {
+	req := httptest.NewRequest(http.MethodGet, "/test", nil)
+	req.RemoteAddr = "127.0.0.1:1234"
+	recorder := httptest.NewRecorder()
+	router.ServeHTTP(recorder, req)
+	return recorder
+}
+
+func startRedis(t *testing.T, ctx context.Context) *redis.Client {
+	t.Helper()
+
+	redisContainer, err := tcredis.Run(ctx, redisImageTag)
+	require.NoError(t, err)
+	t.Cleanup(func() {
+		_ = redisContainer.Terminate(ctx)
+	})
+
+	redisHost, err := redisContainer.Host(ctx)
+	require.NoError(t, err)
+	redisPort, err := redisContainer.MappedPort(ctx, "6379/tcp")
+	require.NoError(t, err)
+
+	rdb := redis.NewClient(&redis.Options{
+		Addr: fmt.Sprintf("%s:%d", redisHost, redisPort.Int()),
+		DB:   0,
+	})
+	require.NoError(t, rdb.Ping(ctx).Err())
+
+	t.Cleanup(func() {
+		_ = rdb.Close()
+	})
+
+	return rdb
+}
diff --git a/backend/internal/middleware/rate_limiter_test.go b/backend/internal/middleware/rate_limiter_test.go
new file mode 100644
index 00000000..7c72e5be
--- /dev/null
+++ b/backend/internal/middleware/rate_limiter_test.go
@@ -0,0 +1,100 @@
+package middleware
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"github.com/redis/go-redis/v9"
+	"github.com/stretchr/testify/require"
+)
+
+func TestWindowTTLMillis(t *testing.T) {
+	require.Equal(t, int64(1), windowTTLMillis(500*time.Microsecond))
+	require.Equal(t, int64(1), windowTTLMillis(1500*time.Microsecond))
+	require.Equal(t, int64(2), windowTTLMillis(2500*time.Microsecond))
+}
+
+func TestRateLimiterFailureModes(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	rdb := redis.NewClient(&redis.Options{
+		Addr:         "127.0.0.1:1",
+		DialTimeout:  50 * time.Millisecond,
+		ReadTimeout:  50 * time.Millisecond,
+		WriteTimeout: 50 * time.Millisecond,
+	})
+	t.Cleanup(func() {
+		_ = rdb.Close()
+	})
+
+	limiter := NewRateLimiter(rdb)
+
+	failOpenRouter := gin.New()
+	failOpenRouter.Use(limiter.Limit("test", 1, time.Second))
+	failOpenRouter.GET("/test", func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{"ok": true})
+	})
+
+	req := httptest.NewRequest(http.MethodGet, "/test", nil)
+	req.RemoteAddr = "127.0.0.1:1234"
+	recorder := httptest.NewRecorder()
+	failOpenRouter.ServeHTTP(recorder, req)
+	require.Equal(t, http.StatusOK, recorder.Code)
+
+	failCloseRouter := gin.New()
+	failCloseRouter.Use(limiter.LimitWithOptions("test", 1, time.Second, RateLimitOptions{
+		FailureMode: RateLimitFailClose,
+	}))
+	failCloseRouter.GET("/test", func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{"ok": true})
+	})
+
+	req = httptest.NewRequest(http.MethodGet, "/test", nil)
+	req.RemoteAddr = "127.0.0.1:1234"
+	recorder = httptest.NewRecorder()
+	failCloseRouter.ServeHTTP(recorder, req)
+	require.Equal(t, http.StatusTooManyRequests, recorder.Code)
+}
+
+func TestRateLimiterSuccessAndLimit(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	originalRun := rateLimitRun
+	counts := []int64{1, 2}
+	callIndex := 0
+	rateLimitRun = func(ctx context.Context, client *redis.Client, key string, windowMillis int64) (int64, error) {
+		if callIndex >= len(counts) {
+			return counts[len(counts)-1], nil
+		}
+		value := counts[callIndex]
+		callIndex++
+		return value, nil
+	}
+	t.Cleanup(func() {
+		rateLimitRun = originalRun
+	})
+
+	limiter := NewRateLimiter(redis.NewClient(&redis.Options{Addr: "127.0.0.1:1"}))
+
+	router := gin.New()
+	router.Use(limiter.Limit("test", 1, time.Second))
+	router.GET("/test", func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{"ok": true})
+	})
+
+	req := httptest.NewRequest(http.MethodGet, "/test", nil)
+	req.RemoteAddr = "127.0.0.1:1234"
+	recorder := httptest.NewRecorder()
+	router.ServeHTTP(recorder, req)
+	require.Equal(t, http.StatusOK, recorder.Code)
+
+	req = httptest.NewRequest(http.MethodGet, "/test", nil)
+	req.RemoteAddr = "127.0.0.1:1234"
+	recorder = httptest.NewRecorder()
+	router.ServeHTTP(recorder, req)
+	require.Equal(t, http.StatusTooManyRequests, recorder.Code)
+}
diff --git a/backend/internal/server/routes/auth.go b/backend/internal/server/routes/auth.go
index 7d8a79e9..aa691eba 100644
--- a/backend/internal/server/routes/auth.go
+++ b/backend/internal/server/routes/auth.go
@@ -27,8 +27,10 @@ func RegisterAuthRoutes(
 		auth.POST("/register", h.Auth.Register)
 		auth.POST("/login", h.Auth.Login)
 		auth.POST("/send-verify-code", h.Auth.SendVerifyCode)
-		// 优惠码验证接口添加速率限制：每分钟最多 10 次
-		auth.POST("/validate-promo-code", rateLimiter.Limit("validate-promo", 10, time.Minute), h.Auth.ValidatePromoCode)
+		// 优惠码验证接口添加速率限制：每分钟最多 10 次（Redis 故障时 fail-close）
+		auth.POST("/validate-promo-code", rateLimiter.LimitWithOptions("validate-promo", 10, time.Minute, middleware.RateLimitOptions{
+			FailureMode: middleware.RateLimitFailClose,
+		}), h.Auth.ValidatePromoCode)
 		auth.GET("/oauth/linuxdo/start", h.Auth.LinuxDoOAuthStart)
 		auth.GET("/oauth/linuxdo/callback", h.Auth.LinuxDoOAuthCallback)
 	}
diff --git a/openspec/changes/update-rate-limit-ttl-atomic/design.md b/openspec/changes/update-rate-limit-ttl-atomic/design.md
new file mode 100644
index 00000000..55be897a
--- /dev/null
+++ b/openspec/changes/update-rate-limit-ttl-atomic/design.md
@@ -0,0 +1,37 @@
+## Context
+限流中间件当前采用 `INCR` 后 `EXPIRE` 的两步操作，且未处理 `EXPIRE` 失败，导致计数 key 可能没有过期时间。该情况一旦发生，计数会持续累加，触发长期限流并造成 Redis key 膨胀。
+
+## Goals / Non-Goals
+- Goals:
+  - 原子化 Redis 计数与过期设置
+  - 修复 TTL 缺失的历史 key
+  - 支持按接口配置 Redis 故障策略（fail-open/fail-close）
+  - 为需要强制保护的接口启用 fail-close
+- Non-Goals:
+  - 改变现有固定窗口限流算法
+  - 调整限流 key 格式或前缀
+  - 引入新的外部依赖
+
+## Decisions
+- 使用 Lua 脚本在 Redis 内部原子执行 `INCR`、`TTL` 与 `PEXPIRE`
+- 过期时间统一采用毫秒精度窗口（`window.Milliseconds()` 向下取整）以保持精度一致
+- 当毫秒窗口小于 1 时，按 1ms 设置过期，避免 0 导致立即过期
+- 当 `count == 1` 或 `TTL == -1` 时设置过期，避免刷新已有 TTL
+- 新增 `RateLimitOptions` 并提供 `LimitWithOptions`，由调用方显式配置故障策略
+- `Limit` 默认使用 fail-open 以保持兼容
+- 当 fail-close 生效时，Redis 执行失败直接返回 429
+
+## Alternatives considered
+- 使用 `MULTI/EXEC` 事务封装 `INCR` + `EXPIRE`：原子性可保证，但无法在同一事务内便捷修复 `TTL == -1`，且仍需额外判断逻辑
+- 使用 `SET` + `EX`/`NX` 组合：无法保留计数累加语义
+
+## Risks / Trade-offs
+- Lua 脚本会带来轻微 CPU 开销，但可接受
+- TTL 修复会在首次访问时设定过期，可能缩短历史脏 key 的“无限期”状态，这是期望的修复效果
+
+## Migration Plan
+- 上线后脚本在请求路径上自动修复 TTL 缺失的 key
+- 如需回滚，恢复原有两步命令即可
+
+## Open Questions
+- 无
diff --git a/openspec/changes/update-rate-limit-ttl-atomic/proposal.md b/openspec/changes/update-rate-limit-ttl-atomic/proposal.md
new file mode 100644
index 00000000..ba19528d
--- /dev/null
+++ b/openspec/changes/update-rate-limit-ttl-atomic/proposal.md
@@ -0,0 +1,14 @@
+# Change: 原子化 Redis 限流 TTL 设置
+
+## Why
+当前限流逻辑使用 `INCR` 后再 `EXPIRE`，非原子且未处理 `EXPIRE` 失败，会导致 key 可能永久存在，引发长期限流或 Redis 内存增长。
+
+## What Changes
+- 使用 Lua 脚本原子化 `INCR` + 过期设置
+- 当检测到 TTL 缺失时补设过期，修复历史脏数据
+- 支持 Redis 故障策略配置（默认放行，特定接口可 fail-close）
+- 新增 `limit-requests` capability，用于描述限流行为与故障策略
+
+## Impact
+- Affected specs: 新增 `specs/limit-requests/spec.md`
+- Affected code: `backend/internal/middleware/rate_limiter.go`
diff --git a/openspec/changes/update-rate-limit-ttl-atomic/specs/limit-requests/spec.md b/openspec/changes/update-rate-limit-ttl-atomic/specs/limit-requests/spec.md
new file mode 100644
index 00000000..e6f9b73c
--- /dev/null
+++ b/openspec/changes/update-rate-limit-ttl-atomic/specs/limit-requests/spec.md
@@ -0,0 +1,41 @@
+## ADDED Requirements
+### Requirement: 原子化限流计数与过期
+限流中间件 SHALL 在单个原子操作中完成 Redis 计数增量与过期设置，并且仅在首次创建或 TTL 缺失时设置过期，避免刷新已有 TTL；过期时间以毫秒为单位向下取整，最小为 1ms。
+
+#### Scenario: 首次请求创建计数器
+- **WHEN** 第一次请求命中该限流 key
+- **THEN** 计数增量为 1 且 key 过期时间设置为窗口值
+
+#### Scenario: 窗口小于 1ms
+- **WHEN** 限流窗口小于 1ms
+- **THEN** 过期时间按 1ms 设置
+
+#### Scenario: 窗口包含非整数毫秒
+- **WHEN** 限流窗口包含非整数毫秒
+- **THEN** 过期时间按毫秒向下取整
+
+#### Scenario: 已有 TTL 的计数器继续计数
+- **WHEN** 计数器已存在且 TTL 正常
+- **THEN** 计数递增且 TTL 不被刷新
+
+#### Scenario: 计数器缺失 TTL
+- **WHEN** 计数器存在但 TTL 为 -1
+- **THEN** 系统为该 key 补设窗口过期时间
+
+### Requirement: Redis 故障策略可配置
+限流中间件 SHALL 支持为每个限流 key 配置 Redis 故障策略，支持 fail-open 与 fail-close，默认 fail-open，配置由调用方在注册限流时提供。
+
+#### Scenario: fail-open 策略
+- **WHEN** 配置为 fail-open 且 Redis 脚本执行返回错误或连接不可用
+- **THEN** 请求继续处理且不执行限流阻断
+
+#### Scenario: fail-close 策略
+- **WHEN** 配置为 fail-close 且 Redis 脚本执行返回错误或连接不可用
+- **THEN** 请求被限流阻断并返回 429
+
+### Requirement: 优惠码验证接口 fail-close
+系统 SHALL 对 `/auth/validate-promo-code` 的限流在 Redis 故障时采用 fail-close。
+
+#### Scenario: 验证优惠码时 Redis 不可用
+- **WHEN** 请求 `/auth/validate-promo-code` 且 Redis 不可用
+- **THEN** 请求返回 429
diff --git a/openspec/changes/update-rate-limit-ttl-atomic/tasks.md b/openspec/changes/update-rate-limit-ttl-atomic/tasks.md
new file mode 100644
index 00000000..c033ca95
--- /dev/null
+++ b/openspec/changes/update-rate-limit-ttl-atomic/tasks.md
@@ -0,0 +1,6 @@
+## 1. Implementation
+- [x] 1.1 在限流中间件中引入 Lua 脚本原子化计数与过期设置（使用 PEXPIRE 毫秒窗口）
+- [x] 1.2 脚本内检测 `TTL == -1` 时补设过期，修复历史脏 key
+- [x] 1.3 引入 `RateLimitOptions` 与 `LimitWithOptions`，`Limit` 保持默认 fail-open
+- [x] 1.4 为 `/auth/validate-promo-code` 配置 fail-close 策略
+- [x] 1.5 添加测试覆盖首次请求、已有 TTL、TTL 缺失、非整数毫秒窗口与故障策略（使用 Redis 集成测试/testcontainers 方案）