The chat the caller should edit this app through.

For an EDITOR this is their DEFAULT open work branch's chat if they have one, else main's. For a VIEWER it is always main's, read-only.

It creates no branch. It used to auto-fork one, which meant merely OPENING the builder as an editor spawned a branch — the same complaint as the chat-message path, one step earlier: nothing has been edited yet, so there is nothing for a branch to hold. A branch is now created only when a turn actually changes the app's source (see api/app_chat.py).

"Default" is exact and not a hedge: an editor can hold several open work branches, and this route takes no branch id, so it answers "which chat do I open first", not "the caller's chat". A client that has a specific branch in hand already holds its chat_id from GET /branches and should use that. Unlike regenerate, guessing here costs a wrong default view rather than a run dispatched onto the wrong branch, so it stays a default.