diff --git a/internal/ctxmgr/chat_turns.go b/internal/ctxmgr/chat_turns.go
index 2d2c7c84..b31d2527 100644
--- a/internal/ctxmgr/chat_turns.go
+++ b/internal/ctxmgr/chat_turns.go
@@ -6,7 +6,6 @@ import (
 	"encoding/hex"
 	"errors"
 	"fmt"
-	"sync"
 	"time"
 )
 
@@ -165,8 +164,25 @@ func (m *ContextManager) acquireChatTurnDB(
 	return &AcquireTurnResult{Record: rec, WonLease: true}, nil
 }
 
-var memTurnsMu sync.Mutex
-var memTurns = map[string]*ChatTurnRecord{} // sessionID|clientTurnID -> record
+
+const maxMemTurns = 2000
+const memTurnTTL = 2 * time.Hour
+
+// pruneStaleTurnsLocked 调用方需持有 m.mu 写锁
+func (m *ContextManager) pruneStaleTurnsLocked(ttl time.Duration) int {
+	if ttl <= 0 {
+		ttl = memTurnTTL
+	}
+	now := time.Now()
+	cleaned := 0
+	for k, rec := range m.turns {
+		if rec == nil || now.Sub(rec.UpdatedAt) > ttl {
+			delete(m.turns, k)
+			cleaned++
+		}
+	}
+	return cleaned
+}
 
 func turnMemKey(sessionID, clientTurnID string) string {
 	return sessionID + "|" + clientTurnID
@@ -176,11 +192,11 @@ func (m *ContextManager) acquireChatTurnMemory(
 	sessionID, clientTurnID, bodyHash, requestID string,
 	userID, projectID int64,
 ) (*AcquireTurnResult, error) {
-	memTurnsMu.Lock()
-	defer memTurnsMu.Unlock()
+	m.mu.Lock()
+	defer m.mu.Unlock()
 
 	key := turnMemKey(sessionID, clientTurnID)
-	if existing, ok := memTurns[key]; ok {
+	if existing, ok := m.turns[key]; ok {
 		if existing.RequestBodySHA256 != bodyHash {
 			return nil, ErrIdempotencyKeyReused
 		}
@@ -205,7 +221,7 @@ func (m *ContextManager) acquireChatTurnMemory(
 
 	now := time.Now()
 	leaseUntil := now.Add(5 * time.Minute)
-	if existing, ok := memTurns[key]; ok {
+	if existing, ok := m.turns[key]; ok {
 		existing.Status = TurnStatusProcessing
 		existing.OwnerToken = fmt.Sprintf("mem_%d", now.UnixNano())
 		existing.LastRequestID = requestID
@@ -215,8 +231,12 @@ func (m *ContextManager) acquireChatTurnMemory(
 		return &AcquireTurnResult{Record: existing, WonLease: true}, nil
 	}
 
+	if len(m.turns) > maxMemTurns {
+		m.pruneStaleTurnsLocked(memTurnTTL)
+	}
+
 	rec := &ChatTurnRecord{
-		ID:                int64(len(memTurns) + 1),
+		ID:                int64(len(m.turns) + 1),
 		SessionID:         sessionID,
 		ClientTurnID:      clientTurnID,
 		UserID:            userID,
@@ -231,7 +251,7 @@ func (m *ContextManager) acquireChatTurnMemory(
 		UpdatedAt:         now,
 		LeaseExpiresAt:    &leaseUntil,
 	}
-	memTurns[key] = rec
+	m.turns[key] = rec
 	return &AcquireTurnResult{Record: rec, WonLease: true}, nil
 }
 
@@ -244,9 +264,9 @@ func (m *ContextManager) UpdateChatTurnUserMessage(ctx context.Context, sessionI
 		`, userMessageID, sessionID, clientTurnID)
 		return err
 	}
-	memTurnsMu.Lock()
-	defer memTurnsMu.Unlock()
-	if rec, ok := memTurns[turnMemKey(sessionID, clientTurnID)]; ok {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if rec, ok := m.turns[turnMemKey(sessionID, clientTurnID)]; ok {
 		rec.UserMessageID = userMessageID
 		rec.UpdatedAt = time.Now()
 	}
@@ -264,9 +284,9 @@ func (m *ContextManager) CompleteChatTurn(ctx context.Context, sessionID, client
 		`, TurnStatusCompleted, assistantMessageID, now, sessionID, clientTurnID)
 		return err
 	}
-	memTurnsMu.Lock()
-	defer memTurnsMu.Unlock()
-	if rec, ok := memTurns[turnMemKey(sessionID, clientTurnID)]; ok {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if rec, ok := m.turns[turnMemKey(sessionID, clientTurnID)]; ok {
 		rec.Status = TurnStatusCompleted
 		rec.AssistantMessageID = assistantMessageID
 		rec.CompletedAt = &now
@@ -284,9 +304,9 @@ func (m *ContextManager) FailChatTurn(ctx context.Context, sessionID, clientTurn
 		`, TurnStatusFailed, sessionID, clientTurnID)
 		return err
 	}
-	memTurnsMu.Lock()
-	defer memTurnsMu.Unlock()
-	if rec, ok := memTurns[turnMemKey(sessionID, clientTurnID)]; ok {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if rec, ok := m.turns[turnMemKey(sessionID, clientTurnID)]; ok {
 		rec.Status = TurnStatusFailed
 		rec.UpdatedAt = time.Now()
 	}
@@ -323,9 +343,9 @@ func (m *ContextManager) GetChatTurn(ctx context.Context, sessionID, clientTurnI
 		rec.LeaseExpiresAt = leaseExpiresAt
 		return &rec, nil
 	}
-	memTurnsMu.Lock()
-	defer memTurnsMu.Unlock()
-	rec, ok := memTurns[turnMemKey(sessionID, clientTurnID)]
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	rec, ok := m.turns[turnMemKey(sessionID, clientTurnID)]
 	if !ok {
 		return nil, ErrChatTurnNotFound
 	}
diff --git a/internal/ctxmgr/chat_turns_test.go b/internal/ctxmgr/chat_turns_test.go
index 354ad713..8e0dae68 100644
--- a/internal/ctxmgr/chat_turns_test.go
+++ b/internal/ctxmgr/chat_turns_test.go
@@ -2,12 +2,13 @@ package ctxmgr
 
 import (
 	"context"
+	"strconv"
 	"sync"
 	"testing"
+	"time"
 )
 
 func TestAcquireChatTurn_DuplicateSameBody(t *testing.T) {
-	memTurns = map[string]*ChatTurnRecord{}
 	mgr := NewContextManager(nil, nil, DefaultTimeDecayConfig())
 	ctx := context.Background()
 	sessionID := "sess-dup"
@@ -32,7 +33,6 @@ func TestAcquireChatTurn_DuplicateSameBody(t *testing.T) {
 }
 
 func TestAcquireChatTurn_ConcurrentDuplicate(t *testing.T) {
-	memTurns = map[string]*ChatTurnRecord{}
 	mgr := NewContextManager(nil, nil, DefaultTimeDecayConfig())
 	ctx := context.Background()
 	sessionID := "sess-conc"
@@ -65,7 +65,6 @@ func TestAcquireChatTurn_ConcurrentDuplicate(t *testing.T) {
 }
 
 func TestAcquireChatTurn_DifferentBodyRejected(t *testing.T) {
-	memTurns = map[string]*ChatTurnRecord{}
 	mgr := NewContextManager(nil, nil, DefaultTimeDecayConfig())
 	ctx := context.Background()
 	sessionID := "sess-reuse"
@@ -80,3 +79,32 @@ func TestAcquireChatTurn_DifferentBodyRejected(t *testing.T) {
 		t.Fatalf("expected ErrIdempotencyKeyReused, got %v", err)
 	}
 }
+
+func TestAcquireChatTurn_StaleEntriesPruned(t *testing.T) {
+	mgr := NewContextManager(nil, nil, DefaultTimeDecayConfig())
+	ctx := context.Background()
+	mgr.mu.Lock()
+	now := time.Now()
+	for i := 0; i < maxMemTurns+50; i++ {
+		key := turnMemKey("sess-prune", "old-"+strconv.Itoa(i))
+		mgr.turns[key] = &ChatTurnRecord{
+			ID:           int64(i + 1),
+			SessionID:    "sess-prune",
+			ClientTurnID: "old-" + strconv.Itoa(i),
+			Status:       TurnStatusCompleted,
+			UpdatedAt:    now.Add(-3 * time.Hour),
+		}
+	}
+	mgr.mu.Unlock()
+	bodyHash := HashTurnBody("sess-prune", "fresh")
+	_, err := mgr.AcquireChatTurn(ctx, "sess-prune", "fresh-turn", bodyHash, "req-fresh", 1, 1)
+	if err != nil {
+		t.Fatalf("acquire: %v", err)
+	}
+	mgr.mu.RLock()
+	n := len(mgr.turns)
+	mgr.mu.RUnlock()
+	if n > maxMemTurns {
+		t.Fatalf("expected turns pruned under capacity, got %d", n)
+	}
+}
diff --git a/internal/ctxmgr/manager.go b/internal/ctxmgr/manager.go
index 4e8d79b8..864ec7a3 100644
--- a/internal/ctxmgr/manager.go
+++ b/internal/ctxmgr/manager.go
@@ -33,6 +33,7 @@ type ContextManager struct {
 	summaries          map[string]*ChatSummary            // session_id -> rolling summary
 	digests            map[string]map[string]*DailyDigest // session_id -> day_key -> DailyDigest
 	readPositions      map[string]*ChatReadPosition       // session_id:user_id -> read position
+	turns              map[string]*ChatTurnRecord         // sessionID|clientTurnID -> 幂等回合（仅内存降级）
 	cache              *MultiLevelCache                   // 多级统一缓存系统 (C1-C5)
 	semanticSummarizer SemanticSummarizer                 // no-tool summarizer; nil → heuristic default
 	nextActivityID     int64
@@ -60,6 +61,7 @@ func NewContextManager(pool *pgxpool.Pool, cas *storage.CAS, decayCfg TimeDecayC
 		summaries:      make(map[string]*ChatSummary),
 		digests:        make(map[string]map[string]*DailyDigest),
 		readPositions:  make(map[string]*ChatReadPosition),
+		turns:          make(map[string]*ChatTurnRecord),
 		cache:          NewMultiLevelCache(),
 		nextActivityID: 1,
 		nextMessageID:  1,