# Message Editing & Forking — Implementation

**Version:** 0.7.x (Phase 2 per ARCHITECTURE.md §8)
**Depends on:** parent_id (006), channel_cursors (008) — both deployed

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## Status: Complete (backend + frontend)

### Backend (already existed)
- [x] Migration 013 (`deleted_at`, `sibling_index`)
- [x] `tree.go` — `getActivePath`, `getActiveLeaf`, `getPathToLeaf`, `getSiblings`,
      `findLeafFromMessage`, `nextSiblingIndex`, `updateCursor`
- [x] `completion.go` — `loadConversation` walks active path via recursive CTE,
      `persistMessage` is cursor-aware with sibling_index, returns new message ID
- [x] `messages.go` — `GetActivePath`, `EditMessage`, `Regenerate` (streaming + sibling),
      `UpdateCursor`, `ListSiblings`
- [x] Routes wired in `main.go`

### Frontend (new work)
- [x] `api.js` — `getActivePath`, `editMessage`, `streamRegenerate`, `updateCursor`, `listSiblings`
- [x] `app.js` — tree-aware message flow:
    - `selectChat` uses `/path` endpoint
    - `sendMessage` reloads path after streaming for server-authoritative IDs
    - `regenerateMessage(id)` creates sibling via message-specific endpoint
    - `editMessage(id)` → inline edit → `submitEdit` → sibling + completion
    - `switchSibling(id, direction)` navigates branches via cursor update
    - `reloadActivePath()` refreshes from server after every mutation
    - Legacy `regenerate()` button delegates to `regenerateMessage(lastAssistantId)`
- [x] `ui.js`:
    - `_messageHTML` renders ‹ 1/3 › branch indicators at fork points
    - Edit button on user messages, Regen button on assistant messages
    - `showEditInline` — textarea with Ctrl+Enter submit, Escape cancel
- [x] `styles.css` — `.branch-nav`, `.branch-arrow`, `.branch-pos`,
      `.msg-edit-input`, `.msg-edit-actions`, `.msg-edit-cancel`, `.msg-edit-submit`

---

## API Endpoints

| Method | Path | Purpose |
|--------|------|---------|
| `GET` | `/channels/:id/path` | Active path with sibling metadata |
| `POST` | `/channels/:id/messages/:msgId/edit` | Create sibling user message |
| `POST` | `/channels/:id/messages/:msgId/regenerate` | Create sibling assistant response (SSE) |
| `PUT` | `/channels/:id/cursor` | Switch active branch |
| `GET` | `/channels/:id/messages/:msgId/siblings` | List siblings at a fork point |

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## Design Decisions

1. **`sibling_index` column** — explicit integer, not derived from `created_at`. Tree is self-describing.
2. **New `/path` endpoint** — `ListMessages` retained for admin/debug. Frontend uses `/path` exclusively.
3. **Two-step edit** — API: edit creates sibling, completion is separate. Frontend chains them.
4. **Uniform ‹ › arrows** — same UI regardless of how fork was created (edit, regen, explicit).
5. **Message-specific regenerate** — replaces old channel-level stub. Bottom-bar button delegates.
6. **Branch switch → leaf** — clicking mid-tree sibling walks to deepest descendant via `findLeafFromMessage`.
7. **Path reload after every mutation** — `reloadActivePath()` called post-send, edit, regen, abort.

---

## User Flows

### Edit a message
1. Hover user message → click **Edit**
2. Textarea replaces message text (Ctrl+Enter to submit, Escape to cancel)
3. Submit → `POST /messages/:id/edit` (creates sibling) → `POST /chat/completions` (new response)
4. Branch indicator appears: ‹ 1/2 ›
5. Old branch preserved, navigable via arrows

### Regenerate a response
1. Hover assistant message → click **Regen** (or bottom-bar ↻ button)
2. `POST /messages/:id/regenerate` streams new response as sibling
3. Branch indicator appears: ‹ 1/2 ›
4. Original response preserved, navigable via arrows

### Navigate branches
1. Click ‹ or › on any fork point
2. `GET /messages/:id/siblings` → pick adjacent → `PUT /cursor` with sibling ID
3. Backend walks to leaf, returns new path
4. Entire conversation below fork point updates to show selected branch

---

## Testing Checklist

- [ ] Existing linear conversations load and render correctly (backward compat)
- [ ] New messages in linear conversation work as before
- [ ] Regen creates sibling, shows ‹ 1/2 ›, original preserved
- [ ] Multiple regens show ‹ 1/3 ›, all navigable
- [ ] Edit creates sibling user message + triggers new completion
- [ ] Branch arrows navigate correctly (← older, → newer)
- [ ] Context sent to LLM is active path only (no cross-branch pollution)
- [ ] Cursor persists across page reload (reopening chat shows last-viewed branch)
- [ ] Mobile: edit textarea usable, branch arrows tappable