---
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?

![Receipts: your agent says it fixed the bug. Receipts proves it.](https://raw.githubusercontent.com/syntaxixr/receipts/main/docs/media/banner.png)

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](https://github.com/syntaxixr/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](https://github.com/syntaxixr/receipts/blob/main/docs/study.md). Scripts to reproduce: [study/](https://github.com/syntaxixr/receipts/tree/main/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.