4. Reproduce-Before-You-Fix

Reconstruction for teaching. Fictional org (GridDesk), synthetic data; the receipts are generated, not from a real run.

Pattern: reproduction-gate · Primitive: /goal · Domain: coding

Use when

A regression needs fixing and you want to forbid touching source until the bug is reliably reproduced — so the fix addresses a proven cause, not a guess.

The loop (copy-paste)

This is the library card for this example. Copy the contract and fill the brackets:

Goal:        Fix the regression in <repo> only after proving it is real.
Context:     <repo>; the suspected bad range; a way to run the failing scenario repeatedly.
Constraints: FORBIDDEN to edit source until a test is reliably red (e.g. 10/10 runs fail).
Done-when:   The reproducing test was red pre-fix and is green post-fix.
Evidence:    A repro-gate log (attempts until reliably red); a git-bisect trail; the diff.
If-blocked:  If the bug will not reproduce reliably, stop and report — do not guess a fix.

Verify

A separate check confirms the gate log shows a reliably-red test (10/10 runs fail) before the first source edit, and that the same test is green after the fix.

Steps

Try to reproduce until the test is reliably red; keep the gate closed until then.
Bisect the range to the culprit commit.
Apply the minimal fix; re-run to green.

What happened

It took 5 attempts to make the bug reliably red (the first four were flaky — the gate stayed closed). Only then did the loop bisect a 60-commit range to the culprit, and the real fix was a 1-line cache-TTL change. The loop "knew" a plausible fix early but refused to apply it until it could prove the bug was real. (Illustrative — as of June 2026, verify before relying.)

Reproduce-before-fix: cost by phase

The receipts

Reproduction gate log — 4 flaky attempts, then a reliably-red 5th.
git bisect trail — 60 → 1 commit, then the one-line diff.
Loop log (gate checks) · cost ledger · all artifacts.

Notes

The reproduction-gate is the whole trick: source edits are forbidden until the bug is reliably red. It trades a little time up front for a fix you can prove.