What
When a request carrying the message-threads header gets back an HTTP 400 error that is not one of the known thread-related error codes, Claude Code now resends the turn statelessly, without the message-threads header, and marks that thread type as dropped for the rest of the session. This path is tracked by a new telemetry event, tengu_tether_unrecognised_400, added alongside the existing tengu_tether_decision, tengu_tether_live_outcome, and tengu_tether_echo_audit events.
Why This lets a conversation keep going after an unexpected 400 error tied to the message-threads feature, instead of failing outright, while still recording that the fallback happened.