| name | add-regression-test |
| description | After a bug fix lands (or is staged), generate a regression test that fails on the pre-fix code and passes on the post-fix code — pinning the bug so it can't silently return. Walks the agent through reproducing the failure first (against the pre-fix state via `git stash` or temporary revert), writing the assertion that captures it, then verifying the test goes red without the fix and green with it. Trigger phrases — "add a regression test", "pin this bug", "write a test for the fix", "/add-regression-test", "lock in this bug fix", "make sure this doesn't come back", or invoke automatically after a /fix-bug session that produced a code fix. Skip for — fixes whose behavior change is already covered by existing passing tests, pure refactors with no behavior delta, fixes to test code itself, and unreproducible heisenbugs. |
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Add Regression Test
A regression test is only valuable if it would have failed before the fix and passes after. Anything else is a confidence test pretending to be a regression test. The whole skill is built around proving both halves.
Phase 1 — Identify the Fix
Determine what changed:
- If the fix is staged or committed but not pushed:
git diff <base>..HEAD -- <files> and git log <base>..HEAD --oneline.
- If the fix is in the working tree:
git diff (unstaged) plus git diff --cached (staged).
- If the user names a commit:
git show <sha>.
Read the changed code. Identify:
- The function / route / component the fix lives in
- The input shape that triggers the bug (from the diff context, the bug report, or the user)
- The expected externally-observable outcome (return value, DB row, HTTP response, emitted event, rendered DOM)
If the fix is purely structural (renames, refactor, reformatting) with no behavior delta — stop. Tell the user there is nothing to regression-test.
Exit: the bug's input shape and expected outcome are written down.
Phase 2 — Detect Test Harness
Apply the harness-detection rules from code-write-tests (or use that skill's references file directly): config files, lockfile, scripts. If no harness exists, hand off to code-write-tests Phase 3 to install one — do not introduce a runner here.
Exit: the runner command and the test-file convention are known.
Phase 3 — Reproduce the Failure
This is the load-bearing step. MANDATORY — READ references/reproduction-modes.md before choosing a mode.
Pick one mode:
- A. Fix is unstaged in working tree → write the test,
git stash only the fix files, run the test (must fail), git stash pop, run the test (must pass).
- B. Fix is staged but not committed → write the test,
git stash --keep-index to set aside the test, then git stash pop-trick won't work cleanly; instead use mode C.
- C. Fix is committed locally → write the test on top, then
git revert --no-commit <fix-sha> in a temporary worktree (or via git stash of the fix files), confirm the test fails, restore.
- D. Fix is squashed into a larger commit and can't be cleanly isolated → revert just the offending hunks via
git checkout <pre-fix-sha> -- <file> in a worktree, confirm test fails, restore.
In every mode, the gate is the same: the test must fail without the fix code present. A test that passes both before and after is broken — go back and tighten the assertion.
Exit: the test failed against the pre-fix code with output that names the actual bug behavior (not "expected X, got undefined" — the failure message should resemble the original bug report).
Phase 4 — Verify the Pass
Restore the fix, run the test. It must pass.
If it doesn't pass: either the assertion is wrong (too strict, asserting unrelated behavior) or the fix is incomplete. Surface to the user — do not loosen the assertion to make it green.
Exit: test passes with the fix in place.
Phase 5 — Wire It In
- Place the test in the project's convention (colocated
<name>.test.ts, tests/, __tests__/, etc. — match the nearest sibling).
- Name it after the bug's symptom, not the fix:
it("returns 404 when slug contains uppercase letters"), not it("normalizes slug case"). The next reader needs to recognize the bug from the test name.
- If a bug tracker reference exists (issue #, ticket id), include it in a one-line comment above the test.
Run the full suite — the new test plus its neighbors must all pass.
Exit: test is in place; full suite is green.
Phase 6 — Report
Regression test added: <path>:<line>
Pinned: <one-line bug description>
Verified failed against: <pre-fix sha or "stashed working tree">
Verified passes against: HEAD
Run: <project test command>
NEVER
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NEVER skip the failure verification.
Instead: stash, revert, or use a temporary worktree to confirm the test fails without the fix, every time.
Why: a test written against a fix that's already in the file passes by construction. Without the failing-state proof, you have a confidence test, not a regression test — it cannot detect the bug coming back.
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NEVER assert on internal call sequences or implementation details.
Instead: assert on the externally-observable outcome — return value, DB row state, HTTP response, rendered DOM, emitted event.
Why: implementation-coupled tests pass for the wrong reasons and break on every refactor. The bug is observable; the test should be too.
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NEVER loosen an assertion to make a failing test pass.
Instead: if the test fails after the fix, surface the discrepancy to the user. Either the fix is incomplete or the assertion is wrong — the user decides.
Why: silently weakening the assertion is the exact failure mode that lets the bug return without anyone noticing. The test stops protecting against the very thing it was written for.
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NEVER name the test after the fix.
Instead: name it after the bug's symptom (the wrong behavior, not the corrected one).
Why: "normalizes slug case" tells the next reader nothing about what bug it pins; "returns 404 when slug contains uppercase letters" makes the regression obvious if the test ever fails again.
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NEVER write a regression test for an unreproducible heisenbug.
Instead: stop and tell the user the bug needs a deterministic reproduction first; suggest adding logging or a feature flag to capture the failing input.
Why: a flaky test pinning a flaky bug is twice the noise — the test will fail intermittently on CI for unrelated reasons and get muted, defeating its purpose.
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NEVER bundle unrelated cleanup or "while I'm here" assertions into the regression test.
Instead: one test, one assertion, one bug. Other tests go in their own block.
Why: if the file is later changed and the regression assertion fails, the developer needs to recognize exactly which behavior regressed. Mixed assertions make that ambiguous and erode trust in the test name.
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NEVER mock the layer where the bug lived.
Instead: test the real code path that produced the bug. If the bug was in the data-access layer, hit the test DB; if it was in a handler, call the handler.
mocking the buggy layer means the test asserts your idea of how it works, not how it works. The original bug came from a mismatch between the two — the test must exercise the real thing.
