Error Message Fix Question Explained
This worked Harness Engineering practice question tests Checks and Sensors. Try the answer first, then review why A is the best choice.
Short answer
The correct answer is A. Make the error name the item, the expected value, and the fix.
The check's output goes into the agent's context, so the error message works as an instruction. "Tea: page shows $8, shop-details.md says $3. Fix index.html." tells the agent what to change and where. OpenAI's Codex team writes its error messages to include remediation instructions for the agent. A failure that names nothing leaves the agent guessing. The correct answer is A. Make the error name the item, the expected value, and the fix
The Question
Your price check now fails when the page and the source file disagree. Its only output is "Error: price mismatch." The agent reads it, changes three unrelated lines, and asks you what is wrong. What should you change?
Why A is correct
The check's output goes into the agent's context, so the error message works as an instruction. "Tea: page shows $8, shop-details.md says $3. Fix index.html." tells the agent what to change and where. OpenAI's Codex team writes its error messages to include remediation instructions for the agent. A failure that names nothing leaves the agent guessing. The correct answer is A. Make the error name the item, the expected value, and the fix
Why the other options are wrong
A stack trace points at lines inside the checker, not at the price on the page. The agent is more likely to edit the checker than the page.
The agent reads the check's output, not a log file that nobody told it to open. The details have to appear in the output of the check.
The sentence names no item, no value, and no file. It gives the agent the same amount to work with as "price mismatch" did.
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.
Ready to test the full skill?
Take the free Harness Engineering quiz and find the exact basics to review next. No signup required.
Practice related questions
Same skill area or adjacent concept. Use these after reviewing the explanation.