Fork this chat — branch conversations on both backends, with full UI

[pufit]

What

A complete "fork this chat" feature: branch any conversation into a new independent chat — from its end or from any earlier message — on both agent backends (Claude and Codex).

The backend already had fork scaffolding (POST /api/sessions/fork, WS fork, codex thread/fork + lastTurnId), but Claude couldn't fork mid-conversation, forks started visually empty, invalid forks failed only on their first message, and nothing in the UI exposed any of it.

Backend

• Claude mid-point forks: ClaudeClient now records each turn's last main-chain transcript-entry uuid and emits it as TurnCompleted.native_turn_id (exact codex parity — same column, same mapping helper). Message-anchored forks pass it as resume_session_at alongside fork_session (gated on spec.fork so it can never truncate the original in place).
• Display parity: fork_session() copies the source's message rows up to the fork boundary into the fork (db.copy_messages_to_session) — the forked chat opens showing exactly the history the agent remembers. Copies keep created_at and native_turn_id (forks are re-forkable) and get external_id="forkcopy:<src id>" for provenance.
• Fail fast, honestly: fork requests are validated at the API call (404 unknown source; 409 no native conversation yet / no turn mapping at the anchor) instead of erroring on the fork's first message. The engine-side missing-mapping refusal is now backend-agnostic — a message fork must never silently carry the full context.
• Symmetric fork-window guard in PATCH /api/sessions/{id}: un-nesting a not-yet-started fork is refused (it would silently discard the pending fork context).

UI/UX

• Hover toolbar on every message (Slack-style, floats top-right of the reading column, zero layout shift): copy message + "fork from here". The fork action appears only where an anchor exists (native_turn_id present in the transcript), so it never promises a branch it can't make.
• Header "⑂ Fork" button + ⌘⇧F — fork the whole chat; shown once the session has a native conversation.
• Sidebar kebab → Fork on any loaded session row.
• Fork provenance banner in a forked chat — "Forked from {parent}", one click back to the source; forks also get a violet branch icon in the sidebar and nest under their source (existing parent_session_id nesting).
• Branch pills at divergence points: messages that have forks hanging off them show ⑂ <fork title> pills linking to each branch.
• Post-fork flow: instant fork → navigate into the new chat (push, so Back returns to the source) → copied history + banner orient you immediately.

Verified

• pytest tests/ — 3367 passed (new coverage: copy boundaries incl. user-message anchors, unmapped-anchor refusal, no-conversation refusal, claude uuid capture incl. sidechain exclusion, per-turn anchor reset, resume_session_at gating).
• npm run build + vitest (229 passed; the one failing suite is a pre-existing collection failure on main), eslint clean on touched files.
• Live E2E probe of the risky seam with the real Claude CLI: seeded two turns, forked at turn 1's live-captured uuid — the fork recalled turn 1's content and had genuinely dropped turn 2's, in a new session id, source untouched.
• Visual pass in the running UI (header button, hover toolbars, gating on sessions without anchors).

Notes

• Sessions predating per-turn anchor recording (all existing Claude chats) can be forked whole; per-message forks light up as new turns complete. Codex rows have carried native_turn_id since v039.
• No schema change — uses the existing native_turn_id column and v039 indexes.

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