When the Python interpreter begins teardown (user closes hermes, SIGTERM,
OOM-kill), every executor-backed operation raises 'cannot schedule new
futures after interpreter shutdown'. The outer except handler in
run_conversation caught this error but did not recognize it as fatal —
it kept retrying (API calls #4, #5, #6) until max_iterations, each time
hitting the same dead executor and printing another traceback.
The fix adds an early check: if sys.is_finalizing() or the error matches
the 'cannot schedule new futures' pattern, break immediately with a clean
interpreter_shutdown exit reason instead of retrying. The codebase already
had this pattern in cron/scheduler.py and agent/tool_executor.py — the
conversation loop just wasn't using it.
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