diff --git a/backend/internal/repository/concurrency_cache.go b/backend/internal/repository/concurrency_cache.go
index b5f500a3..072dfe69 100644
--- a/backend/internal/repository/concurrency_cache.go
+++ b/backend/internal/repository/concurrency_cache.go
@@ -11,54 +11,89 @@ import (
 )
 
 const (
-	accountConcurrencyKeyPrefix = "concurrency:account:"
-	userConcurrencyKeyPrefix    = "concurrency:user:"
-	waitQueueKeyPrefix          = "concurrency:wait:"
-	concurrencyTTL              = 5 * time.Minute
+	// Key prefixes for independent slot keys
+	// Format: concurrency:account:{accountID}:{requestID}
+	accountSlotKeyPrefix = "concurrency:account:"
+	// Format: concurrency:user:{userID}:{requestID}
+	userSlotKeyPrefix = "concurrency:user:"
+	// Wait queue keeps counter format: concurrency:wait:{userID}
+	waitQueueKeyPrefix = "concurrency:wait:"
+
+	// Slot TTL - each slot expires independently
+	slotTTL = 5 * time.Minute
 )
 
 var (
+	// acquireScript uses SCAN to count existing slots and creates new slot if under limit
+	// KEYS[1] = pattern for SCAN (e.g., "concurrency:account:2:*")
+	// KEYS[2] = full slot key (e.g., "concurrency:account:2:req_xxx")
+	// ARGV[1] = maxConcurrency
+	// ARGV[2] = TTL in seconds
 	acquireScript = redis.NewScript(`
-		local current = redis.call('GET', KEYS[1])
-		if current == false then
-			current = 0
-		else
-			current = tonumber(current)
-		end
-		if current < tonumber(ARGV[1]) then
-			redis.call('INCR', KEYS[1])
-			redis.call('EXPIRE', KEYS[1], ARGV[2])
+		local pattern = KEYS[1]
+		local slotKey = KEYS[2]
+		local maxConcurrency = tonumber(ARGV[1])
+		local ttl = tonumber(ARGV[2])
+
+		-- Count existing slots using SCAN
+		local cursor = "0"
+		local count = 0
+		repeat
+			local result = redis.call('SCAN', cursor, 'MATCH', pattern, 'COUNT', 100)
+			cursor = result[1]
+			count = count + #result[2]
+		until cursor == "0"
+
+		-- Check if we can acquire a slot
+		if count < maxConcurrency then
+			redis.call('SET', slotKey, '1', 'EX', ttl)
 			return 1
 		end
+
 		return 0
 	`)
 
-	releaseScript = redis.NewScript(`
-		local current = redis.call('GET', KEYS[1])
-		if current ~= false and tonumber(current) > 0 then
-			redis.call('DECR', KEYS[1])
-		end
-		return 1
+	// getCountScript counts slots using SCAN
+	// KEYS[1] = pattern for SCAN
+	getCountScript = redis.NewScript(`
+		local pattern = KEYS[1]
+		local cursor = "0"
+		local count = 0
+		repeat
+			local result = redis.call('SCAN', cursor, 'MATCH', pattern, 'COUNT', 100)
+			cursor = result[1]
+			count = count + #result[2]
+		until cursor == "0"
+		return count
 	`)
 
+	// incrementWaitScript - only sets TTL on first creation to avoid refreshing
+	// KEYS[1] = wait queue key
+	// ARGV[1] = maxWait
+	// ARGV[2] = TTL in seconds
 	incrementWaitScript = redis.NewScript(`
-		local waitKey = KEYS[1]
-		local maxWait = tonumber(ARGV[1])
-		local ttl = tonumber(ARGV[2])
-		local current = redis.call('GET', waitKey)
+		local current = redis.call('GET', KEYS[1])
 		if current == false then
 			current = 0
 		else
 			current = tonumber(current)
 		end
-		if current >= maxWait then
+
+		if current >= tonumber(ARGV[1]) then
 			return 0
 		end
-		redis.call('INCR', waitKey)
-		redis.call('EXPIRE', waitKey, ttl)
+
+		local newVal = redis.call('INCR', KEYS[1])
+
+		-- Only set TTL on first creation to avoid refreshing zombie data
+		if newVal == 1 then
+			redis.call('EXPIRE', KEYS[1], ARGV[2])
+		end
+
 		return 1
 	`)
 
+	// decrementWaitScript - same as before
 	decrementWaitScript = redis.NewScript(`
 		local current = redis.call('GET', KEYS[1])
 		if current ~= false and tonumber(current) > 0 then
@@ -76,49 +111,86 @@ func NewConcurrencyCache(rdb *redis.Client) ports.ConcurrencyCache {
 	return &concurrencyCache{rdb: rdb}
 }
 
-func (c *concurrencyCache) AcquireAccountSlot(ctx context.Context, accountID int64, maxConcurrency int) (bool, error) {
-	key := fmt.Sprintf("%s%d", accountConcurrencyKeyPrefix, accountID)
-	result, err := acquireScript.Run(ctx, c.rdb, []string{key}, maxConcurrency, int(concurrencyTTL.Seconds())).Int()
+// Helper functions for key generation
+func accountSlotKey(accountID int64, requestID string) string {
+	return fmt.Sprintf("%s%d:%s", accountSlotKeyPrefix, accountID, requestID)
+}
+
+func accountSlotPattern(accountID int64) string {
+	return fmt.Sprintf("%s%d:*", accountSlotKeyPrefix, accountID)
+}
+
+func userSlotKey(userID int64, requestID string) string {
+	return fmt.Sprintf("%s%d:%s", userSlotKeyPrefix, userID, requestID)
+}
+
+func userSlotPattern(userID int64) string {
+	return fmt.Sprintf("%s%d:*", userSlotKeyPrefix, userID)
+}
+
+func waitQueueKey(userID int64) string {
+	return fmt.Sprintf("%s%d", waitQueueKeyPrefix, userID)
+}
+
+// Account slot operations
+
+func (c *concurrencyCache) AcquireAccountSlot(ctx context.Context, accountID int64, maxConcurrency int, requestID string) (bool, error) {
+	pattern := accountSlotPattern(accountID)
+	slotKey := accountSlotKey(accountID, requestID)
+
+	result, err := acquireScript.Run(ctx, c.rdb, []string{pattern, slotKey}, maxConcurrency, int(slotTTL.Seconds())).Int()
 	if err != nil {
 		return false, err
 	}
 	return result == 1, nil
 }
 
-func (c *concurrencyCache) ReleaseAccountSlot(ctx context.Context, accountID int64) error {
-	key := fmt.Sprintf("%s%d", accountConcurrencyKeyPrefix, accountID)
-	_, err := releaseScript.Run(ctx, c.rdb, []string{key}).Result()
-	return err
+func (c *concurrencyCache) ReleaseAccountSlot(ctx context.Context, accountID int64, requestID string) error {
+	slotKey := accountSlotKey(accountID, requestID)
+	return c.rdb.Del(ctx, slotKey).Err()
 }
 
 func (c *concurrencyCache) GetAccountConcurrency(ctx context.Context, accountID int64) (int, error) {
-	key := fmt.Sprintf("%s%d", accountConcurrencyKeyPrefix, accountID)
-	return c.rdb.Get(ctx, key).Int()
+	pattern := accountSlotPattern(accountID)
+	result, err := getCountScript.Run(ctx, c.rdb, []string{pattern}).Int()
+	if err != nil {
+		return 0, err
+	}
+	return result, nil
 }
 
-func (c *concurrencyCache) AcquireUserSlot(ctx context.Context, userID int64, maxConcurrency int) (bool, error) {
-	key := fmt.Sprintf("%s%d", userConcurrencyKeyPrefix, userID)
-	result, err := acquireScript.Run(ctx, c.rdb, []string{key}, maxConcurrency, int(concurrencyTTL.Seconds())).Int()
+// User slot operations
+
+func (c *concurrencyCache) AcquireUserSlot(ctx context.Context, userID int64, maxConcurrency int, requestID string) (bool, error) {
+	pattern := userSlotPattern(userID)
+	slotKey := userSlotKey(userID, requestID)
+
+	result, err := acquireScript.Run(ctx, c.rdb, []string{pattern, slotKey}, maxConcurrency, int(slotTTL.Seconds())).Int()
 	if err != nil {
 		return false, err
 	}
 	return result == 1, nil
 }
 
-func (c *concurrencyCache) ReleaseUserSlot(ctx context.Context, userID int64) error {
-	key := fmt.Sprintf("%s%d", userConcurrencyKeyPrefix, userID)
-	_, err := releaseScript.Run(ctx, c.rdb, []string{key}).Result()
-	return err
+func (c *concurrencyCache) ReleaseUserSlot(ctx context.Context, userID int64, requestID string) error {
+	slotKey := userSlotKey(userID, requestID)
+	return c.rdb.Del(ctx, slotKey).Err()
 }
 
 func (c *concurrencyCache) GetUserConcurrency(ctx context.Context, userID int64) (int, error) {
-	key := fmt.Sprintf("%s%d", userConcurrencyKeyPrefix, userID)
-	return c.rdb.Get(ctx, key).Int()
+	pattern := userSlotPattern(userID)
+	result, err := getCountScript.Run(ctx, c.rdb, []string{pattern}).Int()
+	if err != nil {
+		return 0, err
+	}
+	return result, nil
 }
 
+// Wait queue operations
+
 func (c *concurrencyCache) IncrementWaitCount(ctx context.Context, userID int64, maxWait int) (bool, error) {
-	key := fmt.Sprintf("%s%d", waitQueueKeyPrefix, userID)
-	result, err := incrementWaitScript.Run(ctx, c.rdb, []string{key}, maxWait, int(concurrencyTTL.Seconds())).Int()
+	key := waitQueueKey(userID)
+	result, err := incrementWaitScript.Run(ctx, c.rdb, []string{key}, maxWait, int(slotTTL.Seconds())).Int()
 	if err != nil {
 		return false, err
 	}
@@ -126,7 +198,7 @@ func (c *concurrencyCache) IncrementWaitCount(ctx context.Context, userID int64,
 }
 
 func (c *concurrencyCache) DecrementWaitCount(ctx context.Context, userID int64) error {
-	key := fmt.Sprintf("%s%d", waitQueueKeyPrefix, userID)
+	key := waitQueueKey(userID)
 	_, err := decrementWaitScript.Run(ctx, c.rdb, []string{key}).Result()
 	return err
 }
diff --git a/backend/internal/service/concurrency_service.go b/backend/internal/service/concurrency_service.go
index c554624c..cec2aab8 100644
--- a/backend/internal/service/concurrency_service.go
+++ b/backend/internal/service/concurrency_service.go
@@ -2,12 +2,26 @@ package service
 
 import (
 	"context"
+	"crypto/rand"
+	"encoding/hex"
+	"fmt"
 	"log"
 	"time"
 
 	"sub2api/internal/service/ports"
 )
 
+// generateRequestID generates a unique request ID for concurrency slot tracking
+// Uses 8 random bytes (16 hex chars) for uniqueness
+func generateRequestID() string {
+	b := make([]byte, 8)
+	if _, err := rand.Read(b); err != nil {
+		// Fallback to nanosecond timestamp (extremely rare case)
+		return fmt.Sprintf("%x", time.Now().UnixNano())
+	}
+	return hex.EncodeToString(b)
+}
+
 const (
 	// Default extra wait slots beyond concurrency limit
 	defaultExtraWaitSlots = 20
@@ -41,7 +55,10 @@ func (s *ConcurrencyService) AcquireAccountSlot(ctx context.Context, accountID i
 		}, nil
 	}
 
-	acquired, err := s.cache.AcquireAccountSlot(ctx, accountID, maxConcurrency)
+	// Generate unique request ID for this slot
+	requestID := generateRequestID()
+
+	acquired, err := s.cache.AcquireAccountSlot(ctx, accountID, maxConcurrency, requestID)
 	if err != nil {
 		return nil, err
 	}
@@ -52,8 +69,8 @@ func (s *ConcurrencyService) AcquireAccountSlot(ctx context.Context, accountID i
 			ReleaseFunc: func() {
 				bgCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 				defer cancel()
-				if err := s.cache.ReleaseAccountSlot(bgCtx, accountID); err != nil {
-					log.Printf("Warning: failed to release account slot for %d: %v", accountID, err)
+				if err := s.cache.ReleaseAccountSlot(bgCtx, accountID, requestID); err != nil {
+					log.Printf("Warning: failed to release account slot for %d (req=%s): %v", accountID, requestID, err)
 				}
 			},
 		}, nil
@@ -77,7 +94,10 @@ func (s *ConcurrencyService) AcquireUserSlot(ctx context.Context, userID int64,
 		}, nil
 	}
 
-	acquired, err := s.cache.AcquireUserSlot(ctx, userID, maxConcurrency)
+	// Generate unique request ID for this slot
+	requestID := generateRequestID()
+
+	acquired, err := s.cache.AcquireUserSlot(ctx, userID, maxConcurrency, requestID)
 	if err != nil {
 		return nil, err
 	}
@@ -88,8 +108,8 @@ func (s *ConcurrencyService) AcquireUserSlot(ctx context.Context, userID int64,
 			ReleaseFunc: func() {
 				bgCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 				defer cancel()
-				if err := s.cache.ReleaseUserSlot(bgCtx, userID); err != nil {
-					log.Printf("Warning: failed to release user slot for %d: %v", userID, err)
+				if err := s.cache.ReleaseUserSlot(bgCtx, userID, requestID); err != nil {
+					log.Printf("Warning: failed to release user slot for %d (req=%s): %v", userID, requestID, err)
 				}
 			},
 		}, nil
diff --git a/backend/internal/service/ports/concurrency_cache.go b/backend/internal/service/ports/concurrency_cache.go
index 313737b7..2344fe62 100644
--- a/backend/internal/service/ports/concurrency_cache.go
+++ b/backend/internal/service/ports/concurrency_cache.go
@@ -3,17 +3,21 @@ package ports
 import "context"
 
 // ConcurrencyCache defines cache operations for concurrency service
+// Uses independent keys per request slot with native Redis TTL for automatic cleanup
 type ConcurrencyCache interface {
-	// Slot management
-	AcquireAccountSlot(ctx context.Context, accountID int64, maxConcurrency int) (bool, error)
-	ReleaseAccountSlot(ctx context.Context, accountID int64) error
+	// Account slot management - each slot is a separate key with independent TTL
+	// Key format: concurrency:account:{accountID}:{requestID}
+	AcquireAccountSlot(ctx context.Context, accountID int64, maxConcurrency int, requestID string) (bool, error)
+	ReleaseAccountSlot(ctx context.Context, accountID int64, requestID string) error
 	GetAccountConcurrency(ctx context.Context, accountID int64) (int, error)
 
-	AcquireUserSlot(ctx context.Context, userID int64, maxConcurrency int) (bool, error)
-	ReleaseUserSlot(ctx context.Context, userID int64) error
+	// User slot management - each slot is a separate key with independent TTL
+	// Key format: concurrency:user:{userID}:{requestID}
+	AcquireUserSlot(ctx context.Context, userID int64, maxConcurrency int, requestID string) (bool, error)
+	ReleaseUserSlot(ctx context.Context, userID int64, requestID string) error
 	GetUserConcurrency(ctx context.Context, userID int64) (int, error)
 
-	// Wait queue
+	// Wait queue - uses counter with TTL set only on creation
 	IncrementWaitCount(ctx context.Context, userID int64, maxWait int) (bool, error)
 	DecrementWaitCount(ctx context.Context, userID int64) error
 }