Add /fork slash command to duplicate current session into a new tab

Author: shaj13

pkg/session/branch.go

@@ -16,7 +16,7 @@ func BranchSession(parent *Session, branchAtPosition int) (*Session, error) {
 	if parent == nil {
 		return nil, errors.New("parent session is nil")
 	}
-	if branchAtPosition < 0 || branchAtPosition >= len(parent.Messages) {
+	if branchAtPosition < 0 || branchAtPosition > len(parent.Messages) {
 		return nil, fmt.Errorf("branch position %d out of range", branchAtPosition)
 	}
 

pkg/tui/commands/commands.go

@@ -117,6 +117,17 @@ func builtInSessionCommands() []Item {
 				return core.CmdHandler(messages.EvalSessionMsg{Filename: arg})
 			},
 		},
+		{
+			ID:           "session.fork",
+			Label:        "Fork",
+			SlashCommand: "/fork",
+			Description:  "Fork the current session into a new tab",
+			Category:     "Session",
+			Immediate:    true,
+			Execute: func(string) tea.Cmd {
+				return core.CmdHandler(messages.ForkSessionMsg{})
+			},
+		},
 		{
 			ID:           "session.exit",
 			Label:        "Exit",

pkg/tui/handlers.go

@@ -97,6 +97,51 @@ func (m *appModel) handleBranchFromEdit(msg messages.BranchFromEditMsg) (tea.Mod
 	)
 }
 
+func (m *appModel) handleForkSession() (tea.Model, tea.Cmd) {
+	currentSession := m.application.Session()
+	if currentSession == nil {
+		return m, notification.ErrorCmd("No active session to fork")
+	}
+
+	store := m.application.SessionStore()
+	if store == nil {
+		return m, notification.ErrorCmd("No session store configured")
+	}
+
+	spawner := m.supervisor.Spawner()
+	if spawner == nil {
+		return m, notification.ErrorCmd("Session spawning not available")
+	}
+
+	ctx := context.Background()
+
+	// Fork the session and clone all messages.
+	forkedSession, err := session.BranchSession(currentSession, len(currentSession.Messages))
+	if err != nil {
+		return m, notification.ErrorCmd(fmt.Sprintf("Failed to fork session: %v", err))
+	}
+
+	if err := store.AddSession(ctx, forkedSession); err != nil {
+		return m, notification.ErrorCmd(fmt.Sprintf("Failed to save forked session: %v", err))
+	}
+
+	a, _, cleanup, err := spawner(ctx, forkedSession.WorkingDir)
+	if err != nil {
+		return m, notification.ErrorCmd(fmt.Sprintf("Failed to create runtime for fork: %v", err))
+	}
+
+	a.ReplaceSession(ctx, forkedSession)
+	m.supervisor.AddSession(ctx, a, forkedSession, forkedSession.WorkingDir, cleanup)
+
+	if m.tuiStore != nil {
+		if err := m.tuiStore.AddTab(ctx, forkedSession.ID, forkedSession.WorkingDir); err != nil {
+			slog.Warn("Failed to persist forked tab", "error", err)
+		}
+	}
+
+	return m.handleSwitchTab(forkedSession.ID)
+}
+
 func (m *appModel) handleToggleSessionStar(sessionID string) (tea.Model, tea.Cmd) {
 	store := m.application.SessionStore()
 	if store == nil {

pkg/tui/messages/session.go

@@ -68,6 +68,9 @@ type (
 	// RegenerateTitleMsg regenerates the session title using the AI.
 	RegenerateTitleMsg struct{}
 
+	// ForkSessionMsg requests forking the current session into a new tab.
+	ForkSessionMsg struct{}
+
 	// StreamCancelledMsg notifies components that the stream has been cancelled.
 	StreamCancelledMsg struct{ ShowMessage bool }
 

pkg/tui/tui.go

@@ -855,6 +855,9 @@ func (m *appModel) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 	case messages.BranchFromEditMsg:
 		return m.handleBranchFromEdit(msg)
 
+	case messages.ForkSessionMsg:
+		return m.handleForkSession()
+
 	// --- Session commands (slash commands, command palette) ---
 
 	case messages.ToggleYoloMsg: