Add chat thread forking

docs/api/rest.md

@@ -89,6 +89,8 @@ Behavior notes:
       "type": "chat",
       "id": "chat-id",
       "title": "optional title",
+      "parentChatId": null,
+      "titleGenerationPending": false,
       "createdAt": "2026-02-14T00:00:00.000Z",
       "updatedAt": "2026-02-14T00:00:00.000Z",
       "starred": true,
@@ -117,7 +119,8 @@ Behavior notes:
 Behavior notes:
 - This endpoint is intended for combined conversation/search lists such as sidebars.
 - The legacy `GET /v1/chats` and `GET /v1/searches` endpoints remain available for clients that need separate collections.
-- The response currently combines up to 100 chats and up to 100 searches.
+- The response currently combines the 100 most recently updated chats, any additional root chats needed to group those forks, and up to 100 searches.
+- Root chats have `parentChatId: null`. Every fork points directly to its single root chat, including a fork created from another fork, so clients can group rows without traversing a fork chain.
 - `starred`/`starredAt` are backed by membership in a reserved `Project` with id `starred`; future project folders can reuse the same project item model.
 
 ## Chats
@@ -148,15 +151,32 @@ Behavior notes:
 
 Behavior notes:
 - `provider` and `model` must be supplied together when present.
+- Newly created non-fork chats have `parentChatId: null` and `titleGenerationPending: false`.
 - When `provider`/`model` are supplied, the new chat initializes `initiatedProvider`/`initiatedModel` and `lastUsedProvider`/`lastUsedModel`.
 - `additionalSystemPrompt` is trimmed and stored on the chat; blank values are stored as `null`.
 - `enabledTools` stores the enabled Sybil-managed tool names for future chat completions. Unknown tool names are ignored; omitted values default to all currently available tools.
 - Optional `messages` are inserted as the initial transcript. Attachment metadata uses the same schema and limits as chat completion messages.
 
+### `POST /v1/chats/:chatId/fork`
+- Body: `{ "messageId"?: string }`
+- Response: `{ "chat": ChatSummary }`
+- Source chat not found: `404 { "message": "chat not found" }`
+- Supplied message missing from the source chat: `404 { "message": "message not found in chat" }`
+- Supplied message is not an assistant response: `400 { "message": "fork message must be an assistant response" }`
+
+Behavior notes:
+- With no `messageId`, the server copies the complete source transcript. With an assistant response `messageId`, it copies the transcript through that response, inclusive.
+- The source chat is unchanged. Copied messages receive new ids while retaining their role, content, name, creation time, order, and metadata except for the source request's transport-only `clientRequestId`. Attachments and tool-call metadata are retained. LLM call logs, star/project memberships, and other child threads are not copied.
+- The fork inherits the source chat's user, initiated/last-used provider and model, additional system prompt, and enabled tool settings.
+- A fork of a root or another fork always sets `parentChatId` to the single root chat id. Root chats keep `parentChatId: null`.
+- A whole-chat fork starts with `Fork of <source title>` (or `Fork of Untitled chat`). A message fork starts with `Fork of '<message snippet>'`.
+- The placeholder title is returned with `titleGenerationPending: true`. After the first new prompt, call `POST /v1/chats/title/suggest`; the placeholder is eligible for replacement exactly once.
+
 ### `PATCH /v1/chats/:chatId`
 - Body: any subset of `{ "title": string, "additionalSystemPrompt": string|null, "enabledTools": string[] }`
 - Response: `{ "chat": ChatSummary }`
 - Blank titles are rejected. The server trims surrounding whitespace before storing the title.
+- Setting a title clears `titleGenerationPending`, preventing an in-flight or later automatic suggestion from replacing the manual title.
 - `additionalSystemPrompt: null` clears the stored prompt. Blank string values are also stored as `null`.
 - `enabledTools: []` disables Sybil-managed tools for this chat. Omitted settings are left unchanged.
 - Updating chat fields changes the returned chat's `updatedAt`.
@@ -183,14 +203,19 @@ Behavior notes:
 - Response: `{ "chat": ChatSummary }`
 
 Behavior notes:
-- If the chat already has a non-empty title, server returns the existing chat unchanged.
+- If the chat already has a non-empty title and `titleGenerationPending` is false, server returns the existing chat unchanged.
+- A fork placeholder with `titleGenerationPending: true` is eligible for the same title-generation flow as an untitled original chat. A successful or fallback suggestion clears the flag.
 - If a title is set while suggestion generation is in flight, server returns the current chat instead of overwriting that title.
-- When no title exists at write time, server uses OpenAI `gpt-4.1-mini` to generate a one-line title (up to ~4 words), updates the chat title, and returns the updated chat.
+- For an eligible untitled chat or pending fork placeholder, server uses OpenAI `gpt-4.1-mini` to generate a one-line title (up to ~4 words), updates the chat title, and returns the updated chat.
 - If the title provider is unavailable or rejects the request, server still persists a deterministic title derived from the first line of `content` instead of leaving the chat untitled.
 
 ### `DELETE /v1/chats/:chatId`
 - Response: `{ "deleted": true }`
 - Not found: `404 { "message": "chat not found" }`
+- Active chat or fork: `409 { "message": "chat or fork has an active stream" }`
+- Concurrent family deletion: `409 { "message": "chat family deletion already in progress" }`
+- Deleting a root chat also deletes its grouped forks. Deleting a fork leaves the root and sibling forks unchanged.
+- A root cannot be deleted while it or any grouped fork has an active completion stream. A fork cannot be deleted while its own completion stream is active.
 
 ### `GET /v1/chats/:chatId`
 - Response: `{ "chat": ChatDetail }`
@@ -429,6 +454,8 @@ Behavior notes:
 {
   "id": "...",
   "title": null,
+  "parentChatId": null,
+  "titleGenerationPending": false,
   "createdAt": "...",
   "updatedAt": "...",
   "starred": false,
@@ -481,6 +508,8 @@ Behavior notes:
 {
   "id": "...",
   "title": null,
+  "parentChatId": null,
+  "titleGenerationPending": false,
   "createdAt": "...",
   "updatedAt": "...",
   "starred": false,

docs/api/streaming-chat.md

@@ -73,8 +73,10 @@ Notes:
 
 Persisted chat streams with a `chatId` are backend-owned active runs:
 - Once started, the backend keeps the stream running even if the HTTP client disconnects or refreshes.
+- The backend reserves the active run before persisting submitted messages, preventing a root or fork deletion from interleaving with stream setup.
 - While running, `GET /v1/active-runs` includes the `chatId`.
 - Starting a second persisted stream for the same active `chatId` returns `409`, unless its `clientRequestId` matches the active submission, in which case the existing stream is replayed.
+- Starting a persisted stream while that chat family is being deleted returns `409 { "message": "chat family deletion already in progress" }`.
 - Clients can reattach with `POST /v1/chats/:chatId/stream/attach`.
 
 ## Attach Endpoint