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