diff --git a/core/engine.go b/core/engine.go
index 8344bd6..c1748e8 100644
--- a/core/engine.go
+++ b/core/engine.go
@@ -251,6 +251,10 @@ func NewEngine(name string, ag Agent, platforms []Platform, sessionStorePath str
 		eventIdleTimeout:  defaultEventIdleTimeout,
 	}
 
+	if ag != nil {
+		e.sessions.InvalidateForAgent(ag.Name())
+	}
+
 	if cp, ok := ag.(CommandProvider); ok {
 		e.commands.SetAgentDirs(cp.CommandDirs())
 	}
@@ -1427,7 +1431,7 @@ func (e *Engine) getOrCreateInteractiveStateWith(sessionKey string, p Platform,
 	}
 
 	if newID := agentSession.CurrentSessionID(); newID != "" {
-		session.CompareAndSetAgentSessionID(newID)
+		session.CompareAndSetAgentSessionID(newID, agent.Name())
 	}
 
 	state = &interactiveState{
@@ -1632,7 +1636,7 @@ func (e *Engine) processInteractiveEvents(state *interactiveState, session *Sess
 				}
 			}
 			if event.SessionID != "" {
-				if session.CompareAndSetAgentSessionID(event.SessionID) {
+				if session.CompareAndSetAgentSessionID(event.SessionID, e.agent.Name()) {
 					pendingName := session.GetName()
 					if pendingName != "" && pendingName != "session" && pendingName != "default" {
 						e.sessions.SetSessionName(event.SessionID, pendingName)
@@ -1721,7 +1725,7 @@ func (e *Engine) processInteractiveEvents(state *interactiveState, session *Sess
 
 		case EventResult:
 			if event.SessionID != "" {
-				session.SetAgentSessionID(event.SessionID)
+				session.SetAgentSessionID(event.SessionID, e.agent.Name())
 			}
 
 			fullResponse := event.Content
@@ -2292,7 +2296,7 @@ func (e *Engine) cmdSwitch(p Platform, msg *Message, args []string) {
 	slog.Info("cmdSwitch: cleanup done", "session_key", msg.SessionKey)
 
 	session := sessions.GetOrCreateActive(msg.SessionKey)
-	session.SetAgentInfo(matched.ID, matched.Summary)
+	session.SetAgentInfo(matched.ID, agent.Name(), matched.Summary)
 	session.ClearHistory()
 	sessions.Save()
 
@@ -3538,7 +3542,7 @@ func (e *Engine) cmdModel(p Platform, msg *Message, args []string) {
 	e.cleanupInteractiveState(msg.SessionKey)
 
 	s := e.sessions.GetOrCreateActive(msg.SessionKey)
-	s.SetAgentSessionID("")
+	s.SetAgentSessionID("", "")
 	s.ClearHistory()
 	e.sessions.Save()
 
@@ -3619,7 +3623,7 @@ func (e *Engine) cmdReasoning(p Platform, msg *Message, args []string) {
 	e.cleanupInteractiveState(msg.SessionKey)
 
 	s := e.sessions.GetOrCreateActive(msg.SessionKey)
-	s.SetAgentSessionID("")
+	s.SetAgentSessionID("", "")
 	s.ClearHistory()
 	e.sessions.Save()
 
@@ -4591,7 +4595,7 @@ func (e *Engine) executeCardAction(cmd, args, sessionKey string) {
 		switcher.SetModel(target)
 		e.cleanupInteractiveState(sessionKey)
 		s := e.sessions.GetOrCreateActive(sessionKey)
-		s.SetAgentSessionID("")
+		s.SetAgentSessionID("", "")
 		s.ClearHistory()
 		e.sessions.Save()
 
@@ -4613,7 +4617,7 @@ func (e *Engine) executeCardAction(cmd, args, sessionKey string) {
 				switcher.SetReasoningEffort(target)
 				e.cleanupInteractiveState(sessionKey)
 				s := e.sessions.GetOrCreateActive(sessionKey)
-				s.SetAgentSessionID("")
+				s.SetAgentSessionID("", "")
 				s.ClearHistory()
 				e.sessions.Save()
 				return
@@ -4709,7 +4713,7 @@ func (e *Engine) executeCardAction(cmd, args, sessionKey string) {
 		interactiveKey := e.interactiveKeyForSessionKey(sessionKey)
 		e.cleanupInteractiveState(interactiveKey)
 		session := sessions.GetOrCreateActive(sessionKey)
-		session.SetAgentInfo(matched.ID, matched.Summary)
+		session.SetAgentInfo(matched.ID, agent.Name(), matched.Summary)
 		session.ClearHistory()
 		sessions.Save()
 
@@ -7094,7 +7098,7 @@ func (e *Engine) HandleRelay(ctx context.Context, fromProject, chatID, message s
 	}
 	defer agentSession.Close()
 
-	if session.CompareAndSetAgentSessionID(agentSession.CurrentSessionID()) {
+	if session.CompareAndSetAgentSessionID(agentSession.CurrentSessionID(), e.agent.Name()) {
 		e.sessions.Save()
 	}
 
@@ -7113,13 +7117,13 @@ func (e *Engine) HandleRelay(ctx context.Context, fromProject, chatID, message s
 				textParts = append(textParts, event.Content)
 			}
 			if event.SessionID != "" {
-				if session.CompareAndSetAgentSessionID(event.SessionID) {
+				if session.CompareAndSetAgentSessionID(event.SessionID, e.agent.Name()) {
 					e.sessions.Save()
 				}
 			}
 		case EventResult:
 			if event.SessionID != "" {
-				session.SetAgentSessionID(event.SessionID)
+				session.SetAgentSessionID(event.SessionID, e.agent.Name())
 				e.sessions.Save()
 			}
 			resp := event.Content
diff --git a/core/engine_test.go b/core/engine_test.go
index e4bb8b8..f394677 100644
--- a/core/engine_test.go
+++ b/core/engine_test.go
@@ -908,7 +908,7 @@ func TestCmdCurrent_UsesLegacyTextOnPlatformWithoutCardSupport(t *testing.T) {
 	msg := &Message{SessionKey: "test:user1", ReplyCtx: "ctx"}
 	session := e.sessions.GetOrCreateActive(msg.SessionKey)
 	session.Name = "Focus"
-	session.SetAgentSessionID("session-123")
+	session.SetAgentSessionID("session-123", "test")
 	session.History = append(session.History, HistoryEntry{Role: "user", Content: "hello", Timestamp: time.Now()})
 
 	e.cmdCurrent(p, msg)
@@ -1243,7 +1243,7 @@ func TestDeleteMode_SubmitBlocksActiveSession(t *testing.T) {
 	}}}
 	e := NewEngine("test", agent, []Platform{p}, "", LangEnglish)
 	msg := &Message{SessionKey: "feishu:user1", ReplyCtx: "ctx"}
-	e.sessions.GetOrCreateActive(msg.SessionKey).SetAgentSessionID("session-1")
+	e.sessions.GetOrCreateActive(msg.SessionKey).SetAgentSessionID("session-1", "test")
 
 	e.cmdDelete(p, msg, nil)
 	_ = e.handleCardNav("act:/delete-mode toggle session-1", msg.SessionKey)
@@ -1267,7 +1267,7 @@ func TestDeleteMode_ActiveSessionMarkedWithArrowAndNotSelectable(t *testing.T) {
 	}}}
 	e := NewEngine("test", agent, []Platform{p}, "", LangEnglish)
 	msg := &Message{SessionKey: "feishu:user1", ReplyCtx: "ctx"}
-	e.sessions.GetOrCreateActive(msg.SessionKey).SetAgentSessionID("session-1")
+	e.sessions.GetOrCreateActive(msg.SessionKey).SetAgentSessionID("session-1", "test")
 
 	e.cmdDelete(p, msg, nil)
 	if len(p.repliedCards) != 1 {
@@ -1442,7 +1442,7 @@ func TestCmdReasoning_SwitchesEffortAndResetsSession(t *testing.T) {
 	msg := &Message{SessionKey: "test:user1", ReplyCtx: "ctx"}
 
 	s := e.sessions.GetOrCreateActive(msg.SessionKey)
-	s.SetAgentSessionID("existing-session")
+	s.SetAgentSessionID("existing-session", "test")
 	s.AddHistory("user", "hello")
 
 	e.cmdReasoning(p, msg, []string{"3"})
@@ -1769,7 +1769,7 @@ func TestRenderListCard_MakesEveryVisibleSessionClickable(t *testing.T) {
 	}
 
 	e := NewEngine("test", &stubListAgent{sessions: sessions}, []Platform{&stubPlatformEngine{n: "test"}}, "", LangEnglish)
-	e.sessions.GetOrCreateActive("test:user1").SetAgentSessionID(sessions[5].ID)
+	e.sessions.GetOrCreateActive("test:user1").SetAgentSessionID(sessions[5].ID, "test")
 
 	card, err := e.renderListCard("test:user1", 1)
 	if err != nil {
diff --git a/core/session.go b/core/session.go
index 4017fab..9323520 100644
--- a/core/session.go
+++ b/core/session.go
@@ -15,6 +15,7 @@ type Session struct {
 	ID             string         `json:"id"`
 	Name           string         `json:"name"`
 	AgentSessionID string         `json:"agent_session_id"`
+	AgentType      string         `json:"agent_type,omitempty"`
 	History        []HistoryEntry `json:"history"`
 	CreatedAt      time.Time      `json:"created_at"`
 	UpdatedAt      time.Time      `json:"updated_at"`
@@ -50,11 +51,12 @@ func (s *Session) AddHistory(role, content string) {
 	})
 }
 
-// SetAgentInfo atomically sets the agent session ID and name.
-func (s *Session) SetAgentInfo(agentSessionID, name string) {
+// SetAgentInfo atomically sets the agent session ID, agent type, and name.
+func (s *Session) SetAgentInfo(agentSessionID, agentType, name string) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 	s.AgentSessionID = agentSessionID
+	s.AgentType = agentType
 	s.Name = name
 }
 
@@ -72,22 +74,24 @@ func (s *Session) GetName() string {
 	return s.Name
 }
 
-// SetAgentSessionID atomically sets the agent session ID.
-func (s *Session) SetAgentSessionID(id string) {
+// SetAgentSessionID atomically sets the agent session ID and agent type.
+func (s *Session) SetAgentSessionID(id, agentType string) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 	s.AgentSessionID = id
+	s.AgentType = agentType
 }
 
 // CompareAndSetAgentSessionID sets the agent session ID only if it is currently empty.
 // Returns true if the value was set, false if it was already non-empty.
-func (s *Session) CompareAndSetAgentSessionID(id string) bool {
+func (s *Session) CompareAndSetAgentSessionID(id, agentType string) bool {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 	if s.AgentSessionID != "" {
 		return false
 	}
 	s.AgentSessionID = id
+	s.AgentType = agentType
 	return true
 }
 
@@ -305,6 +309,7 @@ func (sm *SessionManager) saveLocked() {
 			ID:             s.ID,
 			Name:           s.Name,
 			AgentSessionID: s.AgentSessionID,
+			AgentType:      s.AgentType,
 			History:        append([]HistoryEntry(nil), s.History...),
 			CreatedAt:      s.CreatedAt,
 			UpdatedAt:      s.UpdatedAt,
@@ -367,3 +372,32 @@ func (sm *SessionManager) load() {
 
 	slog.Info("session: loaded from disk", "path", sm.storePath, "sessions", len(sm.sessions))
 }
+
+// InvalidateForAgent clears AgentSessionID on all sessions whose AgentType
+// does not match the current agent. This handles the case where the user
+// switches agent types (e.g. opencode → pi) and stale session IDs from the
+// old agent would cause errors.
+func (sm *SessionManager) InvalidateForAgent(agentType string) {
+	sm.mu.Lock()
+	defer sm.mu.Unlock()
+
+	invalidated := 0
+	for _, s := range sm.sessions {
+		s.mu.Lock()
+		if s.AgentSessionID != "" && s.AgentType != "" && s.AgentType != agentType {
+			slog.Info("session: invalidating stale agent session",
+				"session", s.ID,
+				"old_agent", s.AgentType,
+				"new_agent", agentType,
+				"old_agent_session_id", s.AgentSessionID,
+			)
+			s.AgentSessionID = ""
+			s.AgentType = agentType
+			invalidated++
+		}
+		s.mu.Unlock()
+	}
+	if invalidated > 0 {
+		sm.saveLocked()
+	}
+}
diff --git a/core/session_test.go b/core/session_test.go
index bbd36c7..70b4868 100644
--- a/core/session_test.go
+++ b/core/session_test.go
@@ -198,7 +198,7 @@ func TestSession_GetAgentSessionID(t *testing.T) {
 	if got := s.GetAgentSessionID(); got != "" {
 		t.Errorf("initial GetAgentSessionID = %q, want empty", got)
 	}
-	s.SetAgentSessionID("sess-1")
+	s.SetAgentSessionID("sess-1", "test")
 	if got := s.GetAgentSessionID(); got != "sess-1" {
 		t.Errorf("GetAgentSessionID = %q, want %q", got, "sess-1")
 	}
@@ -211,6 +211,53 @@ func TestSession_GetName(t *testing.T) {
 	}
 }
 
+func TestSessionManager_InvalidateForAgent(t *testing.T) {
+	sm := NewSessionManager("")
+
+	// Create sessions with different agent types
+	s1 := sm.NewSession("user1", "sess1")
+	s1.SetAgentSessionID("old-id-1", "opencode")
+
+	s2 := sm.NewSession("user2", "sess2")
+	s2.SetAgentSessionID("old-id-2", "claudecode")
+
+	s3 := sm.NewSession("user3", "sess3")
+	s3.SetAgentSessionID("old-id-3", "") // pre-migration, no agent type
+
+	s4 := sm.NewSession("user4", "sess4") // no agent session ID at all
+
+	sm.InvalidateForAgent("claudecode")
+
+	// s1: opencode → should be invalidated
+	if got := s1.GetAgentSessionID(); got != "" {
+		t.Errorf("s1 (opencode) AgentSessionID = %q, want empty (should be invalidated)", got)
+	}
+	if s1.AgentType != "claudecode" {
+		t.Errorf("s1 AgentType = %q, want %q after invalidation", s1.AgentType, "claudecode")
+	}
+
+	// s2: claudecode → should be untouched
+	if got := s2.GetAgentSessionID(); got != "old-id-2" {
+		t.Errorf("s2 (claudecode) AgentSessionID = %q, want %q (should be preserved)", got, "old-id-2")
+	}
+	if s2.AgentType != "claudecode" {
+		t.Errorf("s2 AgentType = %q, want %q", s2.AgentType, "claudecode")
+	}
+
+	// s3: empty agent type → should be untouched (backward compat)
+	if got := s3.GetAgentSessionID(); got != "old-id-3" {
+		t.Errorf("s3 (empty type) AgentSessionID = %q, want %q (migration-safe)", got, "old-id-3")
+	}
+	if s3.AgentType != "" {
+		t.Errorf("s3 AgentType = %q, want empty (pre-migration should be untouched)", s3.AgentType)
+	}
+
+	// s4: no agent session ID → should be untouched
+	if got := s4.GetAgentSessionID(); got != "" {
+		t.Errorf("s4 (no session ID) AgentSessionID = %q, want empty", got)
+	}
+}
+
 func TestSession_ConcurrentGetSet(t *testing.T) {
 	s := &Session{}
 	var wg sync.WaitGroup
@@ -218,7 +265,7 @@ func TestSession_ConcurrentGetSet(t *testing.T) {
 		wg.Add(2)
 		go func() {
 			defer wg.Done()
-			s.SetAgentSessionID("id")
+			s.SetAgentSessionID("id", "test")
 		}()
 		go func() {
 			defer wg.Done()