Check That Can Fail Question Explained
This worked Harness Engineering practice question tests Checks and Sensors. Try the answer first, then review why D is the best choice.
Short answer
The correct answer is D. A link-check script, plus one broken link on purpose to see it fail.
When the agent writes "all links verified," no program has necessarily tested the links. A check is a program that prints a failure when something is wrong. A script that reads every link on the page and lists each missing target shows you and the agent the same result. Break one link on purpose and confirm that the check fails. A check that you have only seen pass could be a check that passes on everything. The correct answer is D. A link-check script, plus one broken link on purpose to see it fail
The Question
Your agent reports "all links verified" after every edit. A visitor emails you about a broken menu link. What should you add?
Why D is correct
When the agent writes "all links verified," no program has necessarily tested the links. A check is a program that prints a failure when something is wrong. A script that reads every link on the page and lists each missing target shows you and the agent the same result. Break one link on purpose and confirm that the check fails. A check that you have only seen pass could be a check that passes on everything. The correct answer is D. A link-check script, plus one broken link on purpose to see it fail
Why the other options are wrong
The agent already says it checked. A second request produces a second report from the agent, and still no program runs that could report a failure.
A rule tells the agent what to do. It does not detect a broken link, so the agent can still break one and report success in good faith.
Any model can mistype an anchor. Without a check, the new model's report is as unverified as the old one's.
Key idea: Checks and Sensors
Every question here is a situation you meet with Claude Code or Codex: the agent gets something wrong, and you choose what to change. The best answer is usually a file, a check, or a hook that stops the mistake from repeating, not a louder prompt. If you miss a question, read the explanation and note which part of the harness it uses: the AGENTS.md map, a skill, a check, a visitor check, a hook, or a handoff note.
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