A Durable Object holds the mailbox. Work runs in a separate Worker invocation and reports back by posting. This page only appends and reads. Send as many messages as you like while it works: the composer does not lock, nothing is dropped, and jobs that finish close together arrive as one message.
One turn per burst, however many messages it holds. A message arriving cuts a reply short but never the work. Jobs that finish close together arrive as one message.
Every line is a row in the thread’s event log, the same log the agent reads. Nothing here is client state.