What Is an AI Coding Agent Harness?
Short answer. An AI coding agent harness is software that sits around an AI coding agent — Claude Code, Cursor, Codex, or similar — and enforces a small set of controls mechanically, with hard evidence, instead of only asking the model nicely through a prompt. Where a prompt or a rules file can only advise, a harness can check a deterministic condition and refuse to proceed when it fails.
Why "advice" is not enough
Most AI coding tooling works by injecting instructions into the model's context: a system prompt, a rules file, a slash command. That is real and useful, but it has one structural limit — the model can narrate compliance and keep going anyway. Nothing mechanically stops a runaway retry loop, unbounded spend, or a destructive command if the only defense is a paragraph asking the agent to be careful.
A harness exists specifically to stop the failure modes that narration cannot: a loop that never stops failing, spend with no ceiling, an irreversible command, delivery nobody reviewed.
What Maistik Forge's harness actually enforces
Forge is mostly a configurator — it renders instructions the agent's model then carries out, which is the right shape for anything that needs judgment. Around that, it ships a small, named set of controls that enforce: they check evidence the tool can verify on its own and return a non-zero exit when a run has gone wrong, regardless of what the agent claims. See the enforcement doctrine for the full list — the ones referenced most often are the guardrails gate, the verify circuit-breaker, the spend budget, and the secret & PII scan.
Questions people actually ask
Is a harness the same thing as an agent?
No. A harness has no model and cannot write code on its own — it wraps and bounds an agent someone else supplies. Forge's bounded autonomous run makes this explicit: it orchestrates an executor you pass in via --cmd and enforces gates between iterations; it never generates code itself.
Is a rules file (like a .cursorrules file) a harness?
No, and this is the most common confusion. A rules file is advisory context injected into the model — useful, but the model can still ignore it under pressure. A harness additionally runs a real check outside the model and returns a non-zero exit when that check fails. See Forge vs Cursor rules for the full comparison.