Checks

A check is a post-action validator. It runs after a tool completes — typically Edit/Write/MultiEdit — and inspects the resulting state. A failure message surfaces to Claude, which then iterates. This is the post-action half of the harness. See How it works for where checks sit in the agentic loop.

When to author one

Write a check for anything you can fix on the next iteration: lint and type errors, failing tests, naming/convention drift, contract violations. If the cost of a single slip-through is high and irreversible, use a guard instead.

Minimal example

// harness/checks/lint-edited-file.ts
import { defineCheck, checkFail, checkOk, projectDir, Tools } from "agent-harness-sdk";
import { execSync } from "node:child_process";

export const lintEditedFile = defineCheck({
  name: "lint-edited-file",
  tools: [Tools.Edit, Tools.Write, Tools.MultiEdit],
  files: ["**/*.ts", "**/*.tsx"],
  run: async (filePath) => {
    try {
      execSync(`npx --no-install eslint "${filePath}"`, {
        stdio: "pipe",
        cwd: projectDir(),
      });
    } catch (err) {
      const out = (err as { stdout?: Buffer }).stdout?.toString() ?? "";
      return checkFail(`lint-edited-file failed:\n${out}`);
    }
    return checkOk();
  },
});

Register it in harness/harness.config.ts under checks: [...].

Activation conditions

Checks use the same conditions as guards — tools, files, when (see the Guards guide). Scope a check to the edits it cares about with files so it doesn't run on unrelated calls.

run(filePath, input) receives the edited file path directly. It can shell out to a linter or test runner, or do pure-JS inspection. Use projectDir() to resolve paths and run commands from the project root.

The contract

Helper	Produces
checkOk()	the state is valid
checkFail(message)	report a problem; message is shown to Claude

• Failure messages must be actionable — include enough context for Claude to fix it, not just "validation failed". Paste the linter/test output.

Other events

A check fires on post-tool-use by default. Set on for end-of-turn audits:

defineCheck({ name: "end-of-turn-audit", on: "stop", run: async () => checkOk() });