feat(thread): fork and open side chats without disrupting the current thread

[coygeek]

Summary

Add explicit fork and ephemeral side-chat actions so users can explore an alternative approach while preserving the original thread context.

Problem to solve

Long-running agent work often needs a separate investigation or alternative implementation, but T3 Code currently requires users to create a new thread manually and reconstruct enough context to continue safely.

Proposed behavior

Provide Fork to copy the current thread into a new durable thread or managed worktree, and Side chat to open a temporary focused conversation linked to the parent. Show the relationship and provide a return action without mutating the parent transcript.

Acceptance criteria

• Fork creates a new thread with an explicit source reference and selected Local or Worktree target.
• Side chat has isolated transcript and agent state, with a visible parent link and discard action.
• Parent and child can be navigated without losing drafts, running-state indicators, or unread state.
• Git branch and worktree identity are shown before a forked turn can write.
• Remote and mobile clients receive the same parent-child relationship and terminal state.

Affected area

Thread creation contracts, project/worktree preparation, navigation and sidebar state, and shared client runtime for web, desktop, and mobile.

Non-goals

This does not merge transcripts or automatically apply child changes to the parent branch.

Alternatives considered

Starting a blank thread preserves isolation but loses the selected context and makes the relationship invisible.

Supporting context

Official OpenAI documentation defines /fork for copying a local chat into a new chat or worktree and /side for an ephemeral side chat: https://learn.chatgpt.com/docs/reference/slash-commands#available-slash-commands. T3 Code already has thread routes and worktree setup contracts (packages/contracts/src/worktreeSetup.ts), so this request targets the missing user-visible fork and side-chat lifecycle.

Related upstream work: conversation branching discussion #6707, /side discussion #7311, server PR #9436, and client PR #9437. This proposal overlaps those efforts and records the intended user workflow.

Analysis basis: source inspection of T3 Code revision 9d4bb550a658. Statements about missing UI or contracts refer to that revision, not every provider-native capability or unmerged branch. This is a feature proposal based on source review, not a runtime bug reproduction.

[juliusmarminge]

Duplicate of the existing Ideas threads for this feature:

• [Feature]: conversation branching (fork thread from a message) #6707 — conversation branching / fork from a message
• [Feature]: Add a Codex-style /side command #7311 — Codex-style /side ephemeral side chat

This proposal overlaps those and records the same intended workflow (fork + side chat without disrupting the parent). Please +1 / fold any extra acceptance criteria into those threads.

Related implementation in progress: #9436 and #9437.