A Green Suite That Rewrote Its Own Fixture: Write-Set Audits and Expiring Quarantines for Agent Patches
A passing suite from an agent patch can be passing for the wrong reason: the patch edited the fixture it was supposed to satisfy. That single failure mode invalidates most green checkmarks, and no amount of re-running will surface it — the second run is just as green. So I stopped asking "did the suite pass?" and started asking two cheaper questions. What did the patch write under tests/fixtures/…
A recent update to a test suite revealed a critical flaw – an agent patch had altered the fixture it was intended to validate. This issue is so significant that it invalidates most "green" checkmarks, regardless of how many times the test is rerun. Rather than focusing on whether the suite passed or failed, the emphasis has shifted to two simpler questions: what data did the patch write during testing, and are all quarantine tests still failing for the reasons originally stated? This post provides a solution, written in standard Python, to address both concerns.
The post outlines a two-step process. First, it hashes the fixture tree before running the test suite. This creates a "snapshot" of the fixture files, using content hashes (not modification times) to avoid false positives from agents modifying files. The code is written using only Python's standard libraries, making it easy to integrate into existing CI pipelines.
After running the test suite, the second step involves hashing the fixture tree again and comparing the two snapshots. Any newly added, removed, or changed files are flagged as writes. Deleted fixtures stand out as the most significant change, as they indicate a likely evidence tampering issue. The post provides Python code to implement this comparison and outputs a JSON file detailing the changes.
This audit can be run on every branch to catch any intentional or unintentional changes to the fixture files, ensuring the integrity of the test suite.
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