Verification is part of the delivered project when it can prevent the same behavior from breaking again. Use the project's existing framework and conventions.
Keep the check with the project
For a bug fix or behavior change, add or update the smallest maintained test in the repository's normal test layout that exercises the changed behavior. If a specification makes a reusable suite or test category a deliverable, implement that suite instead of treating it as private verification.
An inline interpreter command, heredoc, scratch script, temporary-directory test, demo, or manual probe may help diagnose the problem, but it does not replace the maintained test. Keep a useful self-authored test unless the user explicitly asks for a disposable probe.
Do not add a new test framework solely to satisfy this skill. If the repository has no maintained harness and the request does not require one, disclose when durable coverage would be disproportionate or infeasible and use the narrowest honest verification instead.
On a later turn, update the maintained test even when the old suite remains green and the user does not repeat the test request. Cover the new behavior, not merely an unrelated cleanup in the test file.
Prove the maintained check
Observe the authentic failure before the implementation change when it is safe and runnable. Then make the smallest implementation change and run the focused maintained test after the final relevant source or test edit.
Do not weaken or delete a real failing test to obtain green. Fix the product, or explain why the test's expectation is wrong before changing that expectation. Do not repeat an unchanged check solely to collect evidence.
Run a broad or full suite only when the user, a repository gate, or proportionate risk requires it. A focused regression test comes first; broad proof does not substitute for missing focused coverage.
If the real runner is unavailable but the focused test can still be authored honestly, leave the focused test in the repository and say it was not run. Do not fabricate a passing result.