bahadirkisbet Bugfix TDD
A bug fix without a failing test that reproduces it is a guess. Reproduce first — a red test that captures the bug — then fix until it is green. The test is the proof the bug existed and the guard that it stays fixed.
Announce at start: "Reproducing the bug with a failing test before fixing."
Iron Law
NO BUG FIX WITHOUT A TEST THAT REPRODUCES THE BUG AND FAILS FIRST.
When to Use
- Any bug, regression, or "it does the wrong thing" report.
- Not for: net-new features (that's ordinary TDD), pure refactors with behavior unchanged.
Step 1: Reproduce with a Failing Test
Write the smallest test that exercises the buggy path and asserts the correct behavior. Run it. It MUST fail.
If you can't write a test that fails, you don't yet understand the bug. Keep investigating; don't fix blind.
Step 2: Confirm the Failure Is the Bug
Read the failure. Is it failing because of the actual bug, or because the test is wrong? A test that fails for the wrong reason proves nothing — fix the test until it fails for the bug's reason.
Step 3: Fix
Make the minimal change that turns the test green. Don't expand scope; don't fix unrelated things in the same step.
Step 4: Confirm Green + No Regressions
- The new test passes.
- The surrounding suite still passes — the fix didn't break a neighbor.
- Re-read the diff: did you fix the cause, or paper over the symptom just enough to make the test pass?
Red Flags — STOP
| Thought | Reality |
|---|---|
| "I see the bug — I'll just fix it" | Then you have no proof it was the bug, or that it stays fixed. Red test first. |
| "I'll add the test after the fix" | After means never, and a test written green never proves it catches the bug. |
| "It's too hard to write a test for this" | Then you don't understand the bug yet. Keep digging. |
| "The fix is obvious" | Obvious fixes paper over symptoms. The failing test keeps you honest. |
Common Mistakes
- Fixing first, adding a test after (or never) — you lose the proof the test catches the bug.
- A test that passes even before the fix — it doesn't exercise the bug.
- A test that fails for the wrong reason — verify the failure mode first.
- Expanding the fix beyond what the test demands.
- Skipping the full-suite run — confirm the fix didn't regress a neighbor.
Required next: bardak:no-silent-errors (don't let the fix add a silent failure) and bardak:pipeline-verification (verify the fix end to end, not just the new unit test).