README.md

metadata

license: cc-by-4.0
pretty_name: 'Receipts: do the tests in a fix catch the bug?'
language:
  - en
tags:
  - code
  - testing
  - software-engineering
  - ai-agents
  - coding-agents
  - claude
  - claude-code
  - agent-skills
  - pull-requests
  - red-green
  - tdd
size_categories:
  - n<1K
task_categories:
  - text-classification
configs:
  - config_name: changes
    data_files: data/changes.jsonl
    default: true
  - config_name: tests
    data_files: data/tests.jsonl

Receipts study: do the tests in a fix catch the bug?

181 real code changes from 17 open-source repositories. For each one, every test the change added or edited was run twice: with the change, and with the changed source files reverted to the parent commit or the pull request's merge base. A test for a fix should fail without it.

Made with Receipts, which does this check for any pull request (CLI, GitHub Action, and a skill for Claude Code and other coding agents).

Try it on your own branch, nothing to install: npx github:syntaxixr/receipts check

Results

	Changes	Judged	Proven	Mixed	Unproven	Weak only
Maintainer fix commits	81	71	64 (90%)	2 (3%)	5 (7%)	0 (0%)
Agent pull requests	100	91	75 (82%)	5 (5%)	2 (2%)	9 (10%)

Percentages are of judged changes. The gap that shows up only in agent work is weak only: every test fails on the old code just because it imports a name the change adds, so no test ever runs against the old behavior. No maintainer commit had that shape.

Configurations

• changes (default): one row per change. group is maintainer (fix commits on the default branch) or agent (pull requests whose commits carry a coding agent's fingerprint: Claude Code, Codex, Cursor, Copilot). outcome is proven, mixed, unproven, weak, or env when the tests could not be judged in this environment. The verdict columns count the change's tests per verdict.
• tests: one row per judged test, with its verdict and the raw outcomes with and without the change (with_change, without_change; failures were re-run to spot flakes).

Verdicts

Verdict	With the change	Without it	Meaning
PROVEN	passes	fails	The test catches what the change fixes
GUARD	passes	passes	Guards neighboring behavior next to a PROVEN test
THEATER	passes	passes	No test proves the change
WEAK	passes	fails on import	The code it calls did not exist yet
BROKEN	fails		The change does not pass its own test
FLAKY	mixed	mixed	Different results on the same code
SKIPPED	skipped		The test did not run
PRESERVED / CHANGED			For behavior-preserving changes: passes on both sides / fails on the old code

Method and caveats

Maintainer sample: recent single-parent commits on the default branch whose message mentions a fix, a bug or an issue, and that change both code and tests (click, itsdangerous, markupsafe, sqlparse, humanize, marshmallow, more-itertools, tomlkit, dateutil, dayjs, ufo, defu). Agent sample: up to 20 pull requests per repository, newest first, open and closed alike (anthropics/claude-agent-sdk-python, openai/openai-agents-python, PrefectHQ/fastmcp, modelcontextprotocol/python-sdk, simonw/llm).

Each project was installed once at the tip of its default branch, so older changes can fail for environmental reasons; those are env and excluded from the percentages. The samples are small and not random: the numbers describe these projects, not the ecosystem. "Agent-authored" means an agent's fingerprint appears in the PR's commits; humans steer those agents. Full method: docs/study.md. Scripts to reproduce: study/.

License

The data (verdicts, test ids, commit subjects and links) is released under CC BY 4.0. The code it describes belongs to its projects under their own licenses.