Query results now include a terminal_reason field that records why the query loop terminated. Possible values include: blocking_limit, rapid_refill_breaker, prompt_too_long, image_error, model_error, aborted_streaming, aborted_tools, stop_hook_prevented, hook_stopped, tool_deferred, max_turns, completed.
Evidence
Terminal reason enum and field (search for "Why the query loop terminated")
Strings lifted out of the shipped bundle, so the claim above can be checked against them.