The API request builder now supports a per_turn_effort capability: when the server reports it supports per-turn effort, Claude sends an outputConfig.effort field on system-turn messages. If the server rejects this (400 with "effort parameter" and "not support"), the effort field is stripped and the model is added to a permanent blocklist for the session.
Evidence
"rejected output_config.effort; latching unsupported and retrying without it." (search for "per_turn_effort")
Strings lifted out of the shipped bundle, so the claim above can be checked against them.