What Is A Harness Question Explained
This worked Harness Engineering practice question tests Harness Basics. Try the answer first, then review why B is the best choice.
Short answer
The correct answer is B. The files, tools, checks, and notes around the model.
The harness is everything in the agent except the model. It includes the instruction files the agent reads, the tools it can call, the checks that test its output, and any notes kept between sessions. Claude Code and Codex each come with a harness that their makers wrote. Harness engineering means you add your own files and checks to that harness, so the agent makes fewer mistakes on your project. The correct answer is B. The files, tools, checks, and notes around the model
The Question
A teammate says Claude Code and Codex are "just a model in a terminal." You answer with the definition from harness engineering: an agent is a model plus a harness. What is the harness?
Why B is correct
The harness is everything in the agent except the model. It includes the instruction files the agent reads, the tools it can call, the checks that test its output, and any notes kept between sessions. Claude Code and Codex each come with a harness that their makers wrote. Harness engineering means you add your own files and checks to that harness, so the agent makes fewer mistakes on your project. The correct answer is B. The files, tools, checks, and notes around the model
Why the other options are wrong
The model is the part that writes text. In the definition Agent = Model + Harness, the model is on the other side of the plus sign.
Context window size is a property of the model. The harness is the files, tools, and checks around the model, and you can add to it without an upgrade.
Claude Code and Codex already contain a harness. Harness engineering adds files and checks to the tool you already use, such as an AGENTS.md, a check script, or a hook.
Key idea: Harness Basics
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.