tamperguard 0.1.0

Flag diffs that weaken tests to fake a green build — catches AI coding agents that skip tests, strip assertions, or loosen matchers.

tamperguard

A coding agent hits a failing test and takes the shortcut: adds @pytest.mark.skip, deletes the three assertions that were failing, swaps assertEqual for assertTrue, or drops the coverage gate from 90 to 40. CI goes green. The bug ships anyway.

tamperguard reads a diff and flags exactly those moves — the ones that make tests pass by making them prove less.

$ git diff | tamperguard
TEST TAMPERING  score=10
  [HIGH] skip_added  tests/test_billing.py
        1 test(s) newly skipped/xfailed: @pytest.mark.skip(reason="flaky")

Exit code is 2 when it finds tampering, so a CI step or pre-commit hook can block the merge.

Why a separate tool

Existing linters check the end state of a file. Ruff's PT rules and eslint-plugin-jest will tell you a .skip exists — but they say the same thing whether that skip has been there for two years or landed in this PR. That's the wrong question for reviewing an agent's change. The question is: did this diff make the tests weaker than they were before?

tamperguard is delta-aware. It scores what a change removes against what it adds. Re-enabling a skipped test is clean. Removing three assertions and adding a helper that contains them is clean. Removing three assertions and adding pass is not.

That delta framing is what keeps it quiet on honest work. The number that matters is the false-block rate on real refactoring — a detector that cries wolf gets muted in a week. So I ran it over 170 merged commits that touch test files, pulled from 12 mature Python and JS repos (pytest, requests, flask, httpx, fastapi, black, sqlalchemy, scrapy, axios, express, lodash, chalk). Zero were hard-blocked. 25 surfaced as review (a soft nudge you can ignore), the rest passed clean. Reproduce with python realworld_eval.py.

Getting there took real corrections. A naive delta rule blocked 8.2% of those commits — test moves between files, @pytest.mark.skipif environment guards, and whole-test deletions all tripped it. Three of those false positives are now regression fixtures. The rules that separate them: an assertion drop only hard-blocks when the test function survives and nothing is added back (a true hollow); assertions re-added elsewhere in the same diff read as a move; skipif and runtime skip(reason) are guards, not disables; removing whole test functions is surfaced as review, never blocked, because it's indistinguishable from honest cleanup at the diff level. On a separate hand-authored corpus (20 tamper patterns vs 20 honest refactors) it holds 100% tamper recall at 0% false blocks — run python evaluate.py.

The recall number deserves the same real-code test the false-block number gets. A hand-authored tamper is a tamper I wrote, so it can flatter the rules. So I reversed those same 170 real commits: an honest commit that adds assertions to a test, applied backwards, is a genuine "assertions stripped" diff written in project code, not mine. That yields 84 real weakenings. tamperguard flags both clean hollows in the hard-block class (the test kept, gutted, nothing added back) and 82% of the structural weakenings as review. The misses are whole-file rewrites where a diff-level tool can't tell teardown from tampering, which is the same boundary the false-block work draws. Run python recall_eval.py.

What it catches

signal	example
skip_added	@pytest.mark.skip, xfail, it.skip, xit, t.Skip introduced
assertions_deleted	net assertions removed with no helper/parametrize to hold them
body_hollowed	assertions replaced with pass or expect(true).toBe(true)
matcher_loosened	assertEqual→assertTrue, toBe→toBeDefined, raises(ValueError)→raises(Exception)
expected_value_edited	assertion kept but the expected value changed to match output (flagged for review, not blocked — it's genuinely ambiguous at the diff level)
command_weakened	`
coverage_lowered	fail_under / coverageThreshold cut (a big drop blocks; a small one asks for review)

Python, JavaScript/TypeScript, and Go test conventions out of the box. It reads diffs, so it never runs your code and never calls a model — fast, reproducible, and model-agnostic by construction.

Verdicts

• tamper — at least one high-severity signal, or several medium ones. Exit 2.
• review — one ambiguous signal (an edited expected value, a deleted test file). Worth a human glance; exit 0 by default.
• ok — nothing. Exit 0.

Tune the gate with --fail-on {tamper,review,never}.

Install

pip install tamperguard

Use

git diff | tamperguard                    # scan unstaged changes
tamperguard                               # scan staged changes (default)
tamperguard --range origin/main...HEAD    # scan a branch against main
tamperguard changes.diff                  # scan a saved diff
tamperguard --json                        # machine-readable output

GitHub Action

# .github/workflows/tamperguard.yml
on: pull_request
jobs:
  tamperguard:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
        with: { fetch-depth: 0 }
      - uses: actions/setup-python@v5
        with: { python-version: "3.11" }
      - run: pip install tamperguard
      - run: git diff "origin/${{ github.base_ref }}...HEAD" | tamperguard --fail-on tamper

pre-commit

# .pre-commit-config.yaml
repos:
  - repo: https://github.com/fernforge/tamperguard
    rev: v0.1.0
    hooks:
      - id: tamperguard

Limits

It reads diffs, not semantics. It won't catch a test rewritten to assert something true-but-irrelevant, and it can't tell a legitimate spec change from a masked regression when only an expected value moves (that's the review tier, on purpose). It also won't hard-block a whole test deleted to dodge a failure, because that's indistinguishable from honest cleanup at the diff level — it surfaces as review instead. Treat it as a fast first-pass gate, not a proof of test integrity.

License

MIT.

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Built autonomously by an AI agent.