diff --git a/pkg/agent/loop.go b/pkg/agent/loop.go
index 406ba35..1544fe4 100644
--- a/pkg/agent/loop.go
+++ b/pkg/agent/loop.go
@@ -475,6 +475,7 @@ func (al *AgentLoop) processMessage(ctx context.Context, msg bus.InboundMessage)
 		Content: userContent,
 	})
 	al.sessions.SetLastLanguage(msg.SessionKey, responseLang)
+	al.compactSessionIfNeeded(msg.SessionKey)
 
 	al.sessions.Save(al.sessions.GetOrCreate(msg.SessionKey))
 
@@ -653,6 +654,7 @@ func (al *AgentLoop) processSystemMessage(ctx context.Context, msg bus.InboundMe
 		Content: finalContent,
 	})
 	al.sessions.SetLastLanguage(sessionKey, responseLang)
+	al.compactSessionIfNeeded(sessionKey)
 
 	al.sessions.Save(al.sessions.GetOrCreate(sessionKey))
 
@@ -680,6 +682,32 @@ func truncate(s string, maxLen int) string {
 }
 
 // GetStartupInfo returns information about loaded tools and skills for logging.
+func (al *AgentLoop) compactSessionIfNeeded(sessionKey string) {
+	if !al.compactionEnabled {
+		return
+	}
+	trigger := al.compactionTrigger
+	if trigger <= 0 {
+		trigger = 60
+	}
+	keepRecent := al.compactionKeepRecent
+	if keepRecent <= 0 || keepRecent >= trigger {
+		keepRecent = trigger / 2
+		if keepRecent < 10 {
+			keepRecent = 10
+		}
+	}
+	h := al.sessions.GetHistory(sessionKey)
+	if len(h) <= trigger {
+		return
+	}
+	removed := len(h) - keepRecent
+	note := fmt.Sprintf("[runtime-compaction] removed %d old messages, kept %d recent messages", removed, keepRecent)
+	if al.sessions.CompactSession(sessionKey, keepRecent, note) {
+		al.sessions.Save(al.sessions.GetOrCreate(sessionKey))
+	}
+}
+
 // RunStartupSelfCheckAllSessions runs startup compaction checks across loaded sessions.
 func (al *AgentLoop) RunStartupSelfCheckAllSessions(ctx context.Context) StartupCompactionReport {
 	report := StartupCompactionReport{TotalSessions: al.sessions.Count()}
@@ -712,17 +740,11 @@ func (al *AgentLoop) RunStartupSelfCheckAllSessions(ctx context.Context) Startup
 		}
 
 		removed := len(history) - keepRecent
-		summary := al.sessions.GetSummary(key)
 		note := fmt.Sprintf("[startup-compaction] removed %d old messages, kept %d recent messages", removed, keepRecent)
-		if strings.TrimSpace(summary) == "" {
-			al.sessions.SetSummary(key, note)
-		} else {
-			al.sessions.SetSummary(key, summary+"\n"+note)
+		if al.sessions.CompactSession(key, keepRecent, note) {
+			al.sessions.Save(al.sessions.GetOrCreate(key))
+			report.CompactedSessions++
 		}
-
-		al.sessions.TruncateHistory(key, keepRecent)
-		al.sessions.Save(al.sessions.GetOrCreate(key))
-		report.CompactedSessions++
 	}
 
 	return report
diff --git a/pkg/session/manager.go b/pkg/session/manager.go
index 671264e..eeb8606 100644
--- a/pkg/session/manager.go
+++ b/pkg/session/manager.go
@@ -20,6 +20,7 @@ type Session struct {
 	Kind              string              `json:"kind,omitempty"`
 	Messages          []providers.Message `json:"messages"`
 	Summary           string              `json:"summary,omitempty"`
+	CompactionCount   int                 `json:"compaction_count,omitempty"`
 	LastLanguage      string              `json:"last_language,omitempty"`
 	PreferredLanguage string              `json:"preferred_language,omitempty"`
 	Created           time.Time           `json:"created"`
@@ -180,6 +181,32 @@ func (sm *SessionManager) SetSummary(key string, summary string) {
 	}
 }
 
+func (sm *SessionManager) CompactSession(key string, keepLast int, note string) bool {
+	sm.mu.RLock()
+	session, ok := sm.sessions[key]
+	sm.mu.RUnlock()
+	if !ok {
+		return false
+	}
+
+	session.mu.Lock()
+	defer session.mu.Unlock()
+	if keepLast <= 0 || len(session.Messages) <= keepLast {
+		return false
+	}
+	session.Messages = session.Messages[len(session.Messages)-keepLast:]
+	session.CompactionCount++
+	if strings.TrimSpace(note) != "" {
+		if strings.TrimSpace(session.Summary) == "" {
+			session.Summary = note
+		} else {
+			session.Summary += "\n" + note
+		}
+	}
+	session.Updated = time.Now()
+	return true
+}
+
 func (sm *SessionManager) GetLanguagePreferences(key string) (preferred string, last string) {
 	sm.mu.RLock()
 	session, ok := sm.sessions[key]
@@ -271,6 +298,7 @@ func (sm *SessionManager) List(limit int) []Session {
 			SessionID:         s.SessionID,
 			Kind:              s.Kind,
 			Summary:           s.Summary,
+			CompactionCount:   s.CompactionCount,
 			LastLanguage:      s.LastLanguage,
 			PreferredLanguage: s.PreferredLanguage,
 			Created:           s.Created,
@@ -388,7 +416,7 @@ func (sm *SessionManager) writeOpenClawSessionsIndex() error {
 			"updatedAt":       s.Updated.UnixMilli(),
 			"systemSent":      true,
 			"abortedLastRun":  false,
-			"compactionCount": 0,
+			"compactionCount": s.CompactionCount,
 			"chatType":        mapKindToChatType(s.Kind),
 			"sessionFile":     sessionFile,
 			"kind":            s.Kind,
@@ -430,11 +458,12 @@ func (sm *SessionManager) loadSessions() error {
 	indexPath := filepath.Join(sm.storage, "sessions.json")
 	if data, err := os.ReadFile(indexPath); err == nil {
 		var index map[string]struct {
-			SessionID  string `json:"sessionId"`
-			SessionKey string `json:"sessionKey"`
-			UpdatedAt  int64  `json:"updatedAt"`
-			Kind       string `json:"kind"`
-			ChatType   string `json:"chatType"`
+			SessionID       string `json:"sessionId"`
+			SessionKey      string `json:"sessionKey"`
+			UpdatedAt       int64  `json:"updatedAt"`
+			Kind            string `json:"kind"`
+			ChatType        string `json:"chatType"`
+			CompactionCount int    `json:"compactionCount"`
 		}
 		if err := json.Unmarshal(data, &index); err == nil {
 			for key, row := range index {
@@ -451,6 +480,7 @@ func (sm *SessionManager) loadSessions() error {
 				if row.UpdatedAt > 0 {
 					session.Updated = time.UnixMilli(row.UpdatedAt)
 				}
+				session.CompactionCount = row.CompactionCount
 				session.mu.Unlock()
 			}
 		}