What is an AI agent harness?

The harness is the scaffolding around a model — its tools, memory, permission system, and evaluation loop. Anthropic's Jess Yan describes it as what elevates a model from random token generation to an actionable product.

Should the model and the harness be developed together?

Yes, according to Jess Yan: it is impossible to get maximum performance without tying the harness and model together, and Anthropic always tests models in conjunction with a harness rather than in isolation.

Why do AI agents need granular evaluation criteria?

The Claude Agent SDK team found that vague criteria produce vague critiques, so the agent just shrugs and does things; granular, specific criteria tell the agent exactly which line to fix.

The AI Agent Harness: What Builders Say

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