ceilidhboy Bug Fix — Red-Green, No Exceptions
A bug report means our tests are incomplete. The bug is a gap in test coverage, not just a gap in production code. Fix the test first; the production fix follows.
Hard gate
Do not write or modify production code until a failing test exists and is confirmed failing. This is non-negotiable. If you catch yourself reaching for a production file, stop and ask: "Do I have a failing test yet?"
Step 1 — Expose the bug (red)
Run the existing test suite. If a test already fails and matches the reported bug, step 1 is done — the test already exposes the gap. Move to step 2.
If all tests pass, write a new test that reproduces the bug:
- Place it in the appropriate test file, next to related tests.
- Use the simplest setup that triggers the failure.
- The test must assert the correct behaviour (what the user expects), so it fails against the current (broken) code.
Run the test and confirm it fails. Show the failure output. This is proof that the test catches the bug. Do not proceed until you see red.
Step 2 — Fix the bug (green)
- Modify production code to make the failing test pass.
- Run the full test suite — all tests must be green.
- If your fix breaks other tests, investigate: either the fix is wrong, or the other tests were relying on the buggy behaviour (update them too, with justification).
What not to do
- Do not fix the bug and then write a test that passes. That test proves nothing — it would have passed before the fix too.
- Do not skip step 1 because "the fix is obvious." Obvious fixes still need a guard against regression.
- Do not batch multiple bug fixes into one step-1/step-2 cycle. Each bug gets its own failing test, its own fix. Isolation is the point.
Example flow
User: "Tiles overlap when I drag them."
Agent:
1. Run tests → all green. No existing test covers drag overlap.
2. Write test: "drag into obstacle → actual position must not overlap obstacle."
3. Run test → FAILS (red). Bug confirmed.
4. Fix production code.
5. Run test → PASSES (green).
6. Run full suite → all green.