fix(tests): MCP stdio harness subprocess PYTHONPATH + startup diagnostics (#227) - #228
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…ONPATH (#227) _spawn_server() ran the child from a tmp cwd outside the checkout, so `python -m ledger_agent.mcp_server` died with ModuleNotFoundError when the package was not pip-installed into that interpreter; the failure surfaced as a misleading KeyError: 1 from _exchange(). Prepend the repo root to PYTHONPATH for the child, and turn blank-stdout EOF into an AssertionError carrying the child return code and stderr. Verified: 8/8 MCP tests; full suite 697 passed / 0 failed / 0 skipped.
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Closes #227.
Root cause
tests/test_mcp.py::_spawn_server()runs the child from a tmp cwd outsidethe checkout (
cwd=tmp_path.parent), sopython -m ledger_agent.mcp_servercould not import
ledger_agentunless the package happened to bepip-installed into that interpreter. The child exited before initialization
and
_exchange()surfaced the misleadingKeyError: 1.Reproduced locally with a bare interpreter:
cd /tmp && python3 -m ledger_agent.mcp_server --db /tmp/x.db→ModuleNotFoundError: No module named 'ledger_agent'(exit 1)PYTHONPATH=<repo root>prepended → clean start (exit 0 on EOF)Fix
_spawn_server(): prepend the repository root to the child'sPYTHONPATH._exchange(): blank stdout now raises anAssertionErrorcontaining thechild return code and stderr (plus a distinct message if the child is
still running) instead of returning a partial response dict.
Validation
python3 -m pytest tests/test_mcp.py -q: 8 passed