The first turn of an auto-threaded @mention already points its source at the new thread, but it read chat_topic from the text channel the mention was posted in. The thread's next message reads the thread's own topic (none, outside forums), so the pinned session-context prompt was re-rendered on turn 2 of every auto-threaded conversation in a channel with a topic: a prompt-cache miss, and a Channel Topic line the model saw once and then lost.
(cherry picked from commit d9fedccacd120ca1833269ed0608c3c89c4c1015)
d63cdd32a4