Error codes

A refusal carries a code. Branch on the code and show the message.

Code
bad-request, unknown-request	The frame, the name or the payload does not fit the protocol.
chat-not-found	Create or load the chat first.
chat-unreadable	Its record is from a version this one cannot read. It is left on disk.
chat-busy	A turn runs; wait, or ask for force.
request-not-found	The approval, question or queued message is no longer there: read the state again.
history-expired	The cursor is gone; attach again.
chat-unsupported	This host or this CLI does not do that.
project-config-unsupported	projectConfigFor turned the folder's configuration off and this CLI cannot ignore it; the turn does not start.
invalid-attachments	An attachment the host could not take.
account-unavailable, account-incompatible, invalid-account	The account is not there, cannot take the chat over, or is not valid.
secrets-unavailable	No keychain on this machine.
internal	Something else failed; the host logged it.

Others name what was not found (subagent-not-found, task-not-found, turn-not-found, item-not-found, bookmark-not-found), too-many-bookmarks, a visual that breaks a limit (visual-invalid, visual-too-large), and a CLI transcript that is missing or not readable (transcript-missing, transcript-format). Tools refuse in their own format.