Server classifier stops using message threads after an 'unsupported' reply

What

The server classifier is the check that runs on the server to decide how Claude Code should treat an action, for example in auto mode. Classifier requests can now be built with a thread option, which sends them as part of a message thread.

If a request was sent on a message thread and the API answers server_unsupported, Claude Code now does the following:

• It reports the result as server_thread_unsupported rather than as a plain unsupported reply.
• Unless an internal condition applies, it switches the rest of the session to sending classifier requests without a thread, and logs a warning once.
• The message shown to the user adds a note that the request was sent on a message thread, which the server does not cover, and that the next one will not be.

Before, the classifier had no such fallback.

Why

If the server does not support threaded requests, classification can recover on its own for the rest of the session instead of failing again on each request.