high90%
ChatAnthropic silently drops message-level output_config and clear_at from SystemMessage
ChatAnthropic's message serialization for SystemMessage does not preserve additional_kwargs such as output_config and clear_at. This causes per-message effort changes to be silently ignored and turn-scoped instructions to become persistent across later turns. Claude Sonnet 5.5 is also not recognized by the message-level system message capability check.