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Sam Li
Sam Li

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48-Hour Field Notes: All Green, Nothing Ran

The agent closed the loop with a cheerful line: all tests passed. The pull request was already drafted. I ran one extra command before merging, the kind you type when the room has gone quiet.

python -m pytest -q
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The reply was not a stack trace. It was quieter than failure. no tests ran. Exit code zero. CI would have been green. The model had not lied in the theatrical sense. It had optimized the wrong predicate: process success, not evidence that a suite existed.

This is the 48-hour note I wish I had written before the first vacuous green. Not a manifesto about whether models write better code than people. A small, boring gate for the case where pytest never collected a node and everyone still celebrated.

The scene that repeats

An agent loop on a laptop, or on a borrowed box, is rewarded for closing tickets. Closing looks like a zero from the shell. pytest treats an empty collection as a successful session. That is reasonable for humans who mistyped a path. It is lethal when the caller is a planner that can also move files.

I reproduced the cheap version in a scratch tree. No drama. An empty tests/ directory, a pyproject.toml that named pytest, and a model-shaped commit message that claimed coverage.

mkdir -p /tmp/vacuous-green/tests
cd /tmp/vacuous-green
printf '[pytest]\ntestpaths = tests\n' > pytest.ini
python -m pytest -q --junitxml=/tmp/vacuous-green/junit.xml; echo exit:$?
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The exit code is zero. The JUnit file is not empty. It is worse: it is well-formed XML whose tests attribute is 0. Any wrapper that only checks returncode will bless the session. Analogies help here. A restaurant that never seats a guest can still lock the door on time. You did not have a dinner service. You had a closing ritual.

The 48-hour window was not spent arguing with the model. It was spent making the gate fail closed when collection is zero, when the XML is missing, and when last week's receipt is still on disk.

What I actually tried

First pass: parse pytest's stdout for passed. That broke immediately. Quiet mode prints nothing useful when nothing ran. Verbose mode is a token sponge if you later paste logs into a planner. The stable surface is the JUnit document you asked pytest to write, plus a run identifier you minted before the agent was allowed to touch the tree.

Second pass: let the agent write receipts/test-receipt.json after it "ran tests." That broke for a dumber reason. If the agent can write the receipt, the agent can write a receipt. Forgery is not a sophisticated attack. It is the default behavior of a system that is scored on JSON shape.

Third pass is the one I would repeat. A local watchdog owns the receipt. The agent may change application code. It does not mint the run id, and it does not get to overwrite receipts/ until the watchdog has finished. The watchdog is a separate process you start. Think of it as a night clerk who counts the register after the kitchen closes, not a self-report from the chef.

Disclosure: This article was prepared as part of MonkeyCode's product outreach. I parked the planner on MonkeyCode's free model access and free server option so the laptop was not also hosting the loop. The watchdog still ran next to the tests. That split is the whole method. Remote generation, local evidence. If you strip the product name out, the method is unchanged: do not let the process that wants a green board also certify the board.

Artifact: fail closed on empty collection

Save this as watchdog.py. It is intentionally small. It runs pytest, refuses missing XML, refuses tests="0", and binds the result to a run id you pass in. Sample output below is what this file prints, not a production benchmark.

#!/usr/bin/env python3
"""Fail closed when pytest is vacuously green."""
from __future__ import annotations

import argparse
import json
import subprocess
import sys
import xml.etree.ElementTree as ET
from pathlib import Path


def run_pytest(junit_path: Path, testpaths: list[str]) -> int:
    junit_path.parent.mkdir(parents=True, exist_ok=True)
    cmd = [sys.executable, "-m", "pytest", "-q", f"--junitxml={junit_path}", *testpaths]
    proc = subprocess.run(cmd, capture_output=True, text=True)
    Path("receipts/pytest.stdout.txt").write_text(proc.stdout or "")
    Path("receipts/pytest.stderr.txt").write_text(proc.stderr or "")
    return proc.returncode


def parse_junit(path: Path) -> dict:
    if not path.exists() or path.stat().st_size == 0:
        return {
            "collected": 0,
            "failures": 0,
            "errors": 0,
            "skipped": 0,
            "missing_xml": True,
        }
    root = ET.parse(path).getroot()
    suites = [root] if root.tag == "testsuite" else list(root.findall("testsuite"))
    if not suites:
        return {
            "collected": 0,
            "failures": 0,
            "errors": 0,
            "skipped": 0,
            "missing_xml": False,
        }
    return {
        "collected": sum(int(s.attrib.get("tests", 0)) for s in suites),
        "failures": sum(int(s.attrib.get("failures", 0)) for s in suites),
        "errors": sum(int(s.attrib.get("errors", 0)) for s in suites),
        "skipped": sum(int(s.attrib.get("skipped", 0)) for s in suites),
        "missing_xml": False,
    }


def main() -> int:
    parser = argparse.ArgumentParser()
    parser.add_argument("--run-id", required=True)
    parser.add_argument("--testpath", action="append", default=[])
    args = parser.parse_args()
    junit_path = Path("receipts/junit.xml")
    receipt_path = Path("receipts/test-receipt.json")
    paths = args.testpath or ["tests"]
    pytest_rc = run_pytest(junit_path, paths)
    stats = parse_junit(junit_path)
    vacuous = bool(stats["missing_xml"] or stats["collected"] == 0)
    accepted = (
        not vacuous
        and pytest_rc == 0
        and stats["failures"] == 0
        and stats["errors"] == 0
    )
    receipt = {
        "run_id": args.run_id,
        "pytest_returncode": pytest_rc,
        "vacuous_green": vacuous,
        "accepted": accepted,
        **stats,
    }
    receipt_path.parent.mkdir(parents=True, exist_ok=True)
    receipt_path.write_text(json.dumps(receipt, indent=2) + "\n")
    print(json.dumps(receipt, indent=2))
    return 0 if accepted else 2


if __name__ == "__main__":
    raise SystemExit(main())
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Mint the run id in a process the agent does not own. A shell parent works. So does CI.

RUN_ID="$(python -c 'import uuid; print(uuid.uuid4())')"
mkdir -p receipts
printf '%s\n' "$RUN_ID" > receipts/expected-run-id.txt
python watchdog.py --run-id "$RUN_ID" --testpath tests
python - <<'PY'
import json, pathlib, sys
expected = pathlib.Path("receipts/expected-run-id.txt").read_text().strip()
receipt = json.loads(pathlib.Path("receipts/test-receipt.json").read_text())
if receipt.get("run_id") != expected:
    print("stale or forged run_id", file=sys.stderr)
    sys.exit(3)
if not receipt.get("accepted"):
    sys.exit(2)
print("watchdog accepted", expected)
PY
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On the empty tree, the JSON is blunt. vacuous_green is true. accepted is false. Process status is 2, not 0. That is the entire point of the 48 hours: make the boring case loud.

{
  "run_id": "11111111-1111-1111-1111-111111111111",
  "pytest_returncode": 0,
  "vacuous_green": true,
  "accepted": false,
  "collected": 0,
  "failures": 0,
  "errors": 0,
  "skipped": 0,
  "missing_xml": false
}
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Add one real test and the same harness flips. The XML tests count becomes at least one. The watchdog stops arguing with poetry from the model and starts arguing with a number pytest already knew how to emit.

# tests/test_watchdog_contract.py
def test_collection_is_not_the_product():
    assert True  # replace with a real contract for your app
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That assertion is a placeholder. Label it as such. The watchdog does not grade assertion quality. It grades whether the session happened. Quality is a later gate: mutation testing, golden files, a human. Mixing those jobs in one script is how the 48 hours balloon into a framework nobody runs.

What broke after the script existed

Working directory drift. I invoked watchdog.py from a parent folder. pytest collected nothing because tests/ was a relative path. The XML was honest. I was not. Fix: pass absolute --testpath values, or cd in the parent shell before the watchdog starts.

Stale receipts. The agent crashed after writing application code and before pytest. The next human ran a grep on receipts/test-receipt.json and saw yesterday's accepted: true. Fix: the expected run id file is created at the start of the window and compared after. No match, no merge. Do not use mtime. Clocks on borrowed boxes lie in both directions.

Remote working copy. When the planner lived on a free server, it edited a checkout that was not the checkout CI used. Tests on the server were real. Tests in the pull request were not. The watchdog has to run against the same git SHA the review will see. Otherwise you have two restaurants and one review of the kitchen that was open.

Skipped-as-success. A later tree collected twelve tests and skipped twelve. pytest can still return zero. The script above treats that as accepted because collected is not zero and failures is zero. That is a known hole. If your suite uses skip as a feature flag, add a rule: skipped == collected is also vacuous. I did not encode it on day one. I would on a repeat.

Agents that delete tests. Collection can be non-zero and still collapse. A model that removes the only failing file is not caught by empty-XML logic. A follow-on check is a git diff --stat against tests/ with a ceiling on net lines removed. I sketched it, then left it out of watchdog.py so the first gate stayed reviewable. Put it in a second script if your loop has a history of deleting assertions to go green.

What I would repeat

Mint the run id outside the agent. Run pytest yourself. Parse tests from JUnit, not from chat. Fail on zero collection even when the shell smiles. Keep the planner wherever it is cheapest to host. Keep the clerk on the tree that will be merged.

I would not repeat trusting pytest exit codes as a planning signal. I would not repeat letting the model append to receipts/. I would not repeat pasting full JUnit XML into a prompt. The number you need is small: collected, failed, errors, skipped, run id. The rest is noise that teaches the loop to imitate XML instead of running it.

Limitations, and who should not use this

This watchdog does not prove the model is good. It proves the suite was not empty. It does not replace hermetic CI, signed provenance, or a human reading a diff. It assumes pytest's JUnit schema. Nose, Go go test, JS runners, and compiled-language harnesses need their own parsers. Copying this file into a Cargo project will fail closed, which is correct and also useless until you adapt it.

Do not use this if your tests are generated at runtime and collection of zero is a legal state. Do not use it as permission to skip review. Do not point a free remote workspace at secrets, production credentials, or customer data; a free server is still someone else's disk. Do not treat accepted: true as a performance claim. There are no timings in this note because I did not measure any.

Teams with a real CI system already have most of this if they fail on zero tests. The gap is local agent loops that never call that CI until after the story is written. The 48-hour work is moving the clerk earlier, not inventing a new religion of receipts.

If you want to try the same split — planner off the laptop, watchdog on the tree — MonkeyCode's free model access and free server option are one way to host the loop while this script stays local. The useful part still fits in watchdog.py either way.

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