User asked to continue investigation of the 3 failing tests in
tests/test_z_negative_flows.py. Ran the test in batched tier-3 mode,
isolated the failure to a native Windows STATUS_STACK_OVERFLOW
(0xC00000FD) in the io_pool worker thread when calling
GeminiCliAdapter.send -> subprocess.Popen -> communicate.
Verified the failure:
- Reproduces 100% on a fresh subprocess (no xdist, no other tests).
- Is NOT caused by the send_result -> send rename (purely mechanical).
- Happens on MOCK_MODE=malformed_json, error_result, AND success
(rules out the exception/traceback construction as cause).
- Adapter body completes normally; process dies immediately after.
- Is the io_pool worker thread's 1MB C stack being exhausted by the
deep call chain (run_with_tool_loop -> asyncio cross-thread
dispatch -> _send -> adapter.send -> subprocess.Popen -> communicate
+ Windows ReadFile/WaitForSingleObject).
Conclusion: pre-existing bug. The test file (originally test_negative_flows.py
from 2026-03-06, renamed to test_z_negative_flows.py on 2026-03-07) is the
ONLY test in the suite that exercises a real subprocess AI call end-to-end
through the io_pool worker. Other tier-3 tests use MockProvider and
short-circuit at the ai_client.send level.
Documented: root cause, reproduction evidence, 4 proposed solutions
(thread stack bump, multiprocessing migration, blocking main thread,
xfail), and a follow-up track suggestion for the long-term fix.
This is an investigation report only; no code changes. The theme fix in
9fcf0517 is unaffected. The rename track in 8c6d9aa0 is unaffected.
ed
6748f57898