Add session forking capability

[wpfleger96]

This PR adds session forking (duplication) across CLI, backend API, and desktop UI with a unified, simplified approach

Based on feedback this PR was simplified from the original implementation

I often do something like this in Claude Code by running claude --resume <id> --fork-session:

                                                                        /-> (still original): User2 ("write tests for x")
Original: User1 ("implement x for me") -> LLM1 ("ok implemented x") ---|
                                                                        \-> forked: User3 ("write docs for x")

After the change in this PR, now I can do in goose by goose --resume --session-id <id> --fork.

Unified Fork API

• Renamed edit_message endpoint → fork and added boolean truncate and copy parameters
• Single endpoint handles all use cases: full copy, edit-in-place, and branch-at-point
• Removed redundant fork_session() wrapper (thin wrapper around copy_session()) from original PR implementation
• Removed over-engineered UI code: useForkSession hook, ChatInput fork button from original PR implementation

Backend Changes

• POST /sessions/{id}/fork with ForkRequest { truncate: bool, copy: bool, timestamp?: i64 }
• Validation: truncate=true requires timestamp, returns structured ErrorResponse
• Uses existing copy_session() and truncate_conversation() primitives
• Fixed copy_session() to preserve provider_name and model_config

CLI Implementation

• --fork flag (requires --resume) duplicates sessions
• Calls copy_session() directly (no HTTP endpoint needed)
• Simplified session resolution to eliminate duplication

Desktop UI

• Added "Duplicate" button in session list using forkSession({ truncate: false, copy: true })
• Updated existing edit message fork to use new unified API
• Removed fork button from ChatInput and useForkSession hook

Use Case Matrix

truncate	copy	Result	Use Case
false	true	Full session copy	Duplicate for parallel work (my use case for this PR)
true	false	Edit in place	Truncate current session (existing Edit in Place UI option)
true	true	Branch at point	Copy + truncate at timestamp (exiting Fork Session UI option)

Addresses #5865

[Copilot]

Pull request overview

This PR implements session forking functionality across the full stack (CLI, backend API, and desktop UI), allowing users to create independent copies of sessions with preserved conversation history. This enables exploring different conversation paths without losing the original thread, similar to how Claude Code handles session forking.

Key changes:

• Added POST /sessions/{id}/fork endpoint that leverages copy_session() with provider and model preservation
• Implemented --fork CLI flag (requires --resume) supporting session ID, name, or path identifiers
• Created fork buttons in SessionListView and ChatInput that open forked sessions in new windows via useForkSession hook

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
crates/goose/src/session/session_manager.rs	Added fork_session() method and fixed copy_session() to preserve provider_name and model_config; includes test for conversation preservation
crates/goose-server/src/routes/session.rs	Added fork_session API endpoint handler that returns forked session
crates/goose-server/src/openapi.rs	Registered fork_session endpoint in OpenAPI schema
crates/goose-cli/src/cli.rs	Added --fork flag with --resume requirement; updated get_or_create_session_id to handle fork logic for ID/name/path identifiers
crates/goose-cli/src/session/builder.rs	Added fork field to SessionBuilderConfig struct and updated default/test values
crates/goose-cli/src/commands/bench.rs	Set fork: false in benchmark session configuration
ui/desktop/src/hooks/useForkSession.ts	Created hook that calls fork API, handles errors/loading, and opens forked session in new window
ui/desktop/src/components/sessions/SessionListView.tsx	Added fork button with GitFork icon and handleForkSession callback that reloads sessions on success
ui/desktop/src/components/ChatInput.tsx	Added fork button in bottom toolbar with disabled state during forking
ui/desktop/src/api/types.gen.ts	Generated TypeScript types for ForkSession API including data, errors, and responses
ui/desktop/src/api/sdk.gen.ts	Generated forkSession SDK function as POST to /sessions/{session_id}/fork
ui/desktop/openapi.json	Added OpenAPI specification for fork endpoint with 200/404/500 responses

[Copilot]

Pull request overview

Copilot reviewed 12 out of 12 changed files in this pull request and generated no new comments.

[DOsinga]

We have the same functionality in edit message, this duplicates this with an extra method and an extra route; either remove the old way or use it I would say.

[zanesq]

Just reiterating what @DOsinga said the desktop now has fork session support in chat

[Copilot]

Pull request overview

Copilot reviewed 18 out of 18 changed files in this pull request and generated 1 comment.

[wpfleger96]

@DOsinga @zanesq just simplified the PR based your feedback, what do y'all think now? I named this "fork" since that's what Claude Code calls the equivalent functionality I'm trying to implement, but I think I made it confusing since there's already a "Fork Session" in goose UI. what I want here is really "duplicate" instead of "fork"

truncate	copy	Result	Use Case
false	true	Full session copy	Duplicate for parallel work (my use case for this PR)
true	false	Edit in place	Truncate current session (existing Edit in Place UI option)
true	true	Branch at point	Copy + truncate at timestamp (exiting Fork Session UI option)

[Copilot]

Pull request overview

Copilot reviewed 18 out of 18 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 20 out of 20 changed files in this pull request and generated 1 comment.

[zanesq]

LGTM Front end changes look good also tested locally in the desktop app and everything is working. I'll defer to @DOsinga for the cli changes

[Copilot]

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

crates/goose-cli/src/cli.rs:742

• The --history flag at line 741 requires resume, but when --fork is used, the user might also want to see the history. The condition at line 1131 correctly handles (resume || fork) && history, but the CLI argument at line 741 only requires resume. This means users must specify both --resume --fork --history instead of being able to use --fork --history. Consider updating the requires constraint to allow --history with either --resume or --fork.

        /// Show message history when resuming
        #[arg(
            long,
            help = "Show previous messages when resuming a session",
            requires = "resume"
        )]

[Copilot]

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated no new comments.

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