Let Compact continue on another model or vendor

internal/agents/compact.go

@@ -24,15 +24,38 @@ const leadHandoffPrompt = `Write a handoff for a fresh lead agent that will take
 	`questions you have surfaced or answered and why; what you last pointed the human's attention at; and anything you promised to do or check. ` +
 	`Be complete: the next agent sees nothing else. Your final message is the handoff, and nothing else.`
 
-func (m *Manager) Compact(ctx context.Context, sessionID string) (api.AgentSession, error) {
+func (m *Manager) Compact(ctx context.Context, sessionID, model string) (api.AgentSession, error) {
 	sess, err := m.store.GetSession(ctx, sessionID)
 	if err != nil {
 		return api.AgentSession{}, err
 	}
-	if sess.Owner != "" {
-		return m.Send(ctx, sessionID, "byre", leadHandoffPrompt, "")
+	task, err := m.taskFor(ctx, sess)
+	if err != nil {
+		return api.AgentSession{}, err
 	}
-	return m.Send(ctx, sessionID, "byre", handoffPrompt, "")
+	prompt := handoffPrompt
+	if sess.Owner != "" {
+		prompt = leadHandoffPrompt
+	}
+	err = transition(ctx, m.store, sess.Id, api.AgentSessionStateIdle, api.AgentSessionStateRunning, "byre")
+	if err != nil {
+		return api.AgentSession{}, err
+	}
+	m.mu.Lock()
+	m.handoffModel[sessionID] = model
+	m.mu.Unlock()
+	return m.launch(ctx, sess, task, "byre", prompt, nil)
+}
+
+func (m *Manager) continuationModel(sess api.AgentSession) string {
+	m.mu.Lock()
+	model := m.handoffModel[sess.Id]
+	delete(m.handoffModel, sess.Id)
+	m.mu.Unlock()
+	if model == "" {
+		return sess.Model
+	}
+	return model
 }
 
 func isHandoffPrompt(actor, text string) bool {
@@ -60,13 +83,14 @@ func (m *Manager) handoff(ctx context.Context, sessionID, handoff string) error
 	if err != nil {
 		return err
 	}
+	model := m.continuationModel(sess)
 
 	if sess.Owner != "" {
 		err = m.retire(ctx, sess, "byre")
 		if err != nil {
 			return err
 		}
-		_, err = m.createLead(ctx, creator, leadHandoffPreamble+handoff, sess.Model, "byre", nil)
+		_, err = m.createLead(ctx, creator, leadHandoffPreamble+handoff, model, "byre", nil)
 		return err
 	}
 
@@ -88,7 +112,7 @@ func (m *Manager) handoff(ctx context.Context, sessionID, handoff string) error
 	if err != nil {
 		return err
 	}
-	_, err = m.create(ctx, task, creator, handoffPreamble+handoff, sess.Model, "byre", nil)
+	_, err = m.create(ctx, task, creator, handoffPreamble+handoff, model, "byre", nil)
 	if errors.Is(err, ErrConflict) {
 		m.startDiscard(task.Id)
 		return nil

internal/agents/lead.go

@@ -293,7 +293,7 @@ func (m *Manager) leadTurnEnded(ctx context.Context, l *live, res endResult) err
 		go m.continueFromHandoff(l.id, res.result)
 	case wantCompact && finished && !compacting:
 		go func() {
-			_, err := m.Compact(context.Background(), l.id)
+			_, err := m.Compact(context.Background(), l.id, "")
 			if err != nil {
 				slog.Error("compact lead", "session", l.id, "err", err)
 			}

internal/agents/leadtools.go

@@ -325,13 +325,24 @@ func (m *Manager) leadTools(l *live) []mcp.Tool {
 				return "Interrupted.", nil
 			}),
 
-		tool("compact_task", "Have a task's idle agent write a handoff and continue in a fresh session.", taskOnly,
+		tool("compact_task", "Have a task's idle agent write a handoff and continue in a fresh session.",
+			`{"type": "object", "properties": {
+				"task_id": {"type": "string"},
+				"model": {"type": "string", "description": "Start the fresh session on this model, on either vendor; the current agent still writes the handoff. Defaults to the current model."}
+			}, "required": ["task_id"]}`,
 			func(ctx context.Context, user api.User, raw json.RawMessage) (string, error) {
+				args := struct {
+					Model string `json:"model"`
+				}{}
+				err := json.Unmarshal(raw, &args)
+				if err != nil {
+					return "", err
+				}
 				sess, err := m.taskSessionArg(ctx, raw)
 				if err != nil {
 					return "", err
 				}
-				_, err = m.Compact(ctx, sess.Id)
+				_, err = m.Compact(ctx, sess.Id, args.Model)
 				if err != nil {
 					return "", err
 				}

internal/agents/manager.go

@@ -56,6 +56,7 @@ type Manager struct {
 	startedTodo  map[string]time.Time
 	startRefused map[string]bool
 	finishing    map[string]bool
+	handoffModel map[string]string
 
 	deliverMu    sync.Mutex
 	deliverLocks map[string]*sessionLock
@@ -141,6 +142,7 @@ func New(st *store.Store, worktreeRoot, leadRoot, codexHome string, status func(
 		startedTodo:  map[string]time.Time{},
 		startRefused: map[string]bool{},
 		finishing:    map[string]bool{},
+		handoffModel: map[string]string{},
 		signals:      make(chan []api.Event, 1024),
 		snaps:        map[string]*snapState{},
 		merges:       map[string]*projectMerge{},