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ValidationPostToolUse· Write|Edit
Biome format + lint after write
Lint errors fixed in the same loop, not in CI
Formats the file with biome check --write and returns the remaining lint errors to the agent so it fixes them immediately within the same loop.
What does the Biome format + lint after write hook do?
Biome format + lint after write is a Claude Code PostToolUse hook matching Write|Edit. It fires automatically at that lifecycle event — outside the model, so it can't be skipped or forgotten. Lint errors fixed in the same loop, not in CI.
As a PostToolUse hook it runs after the action, reacting to what just happened rather than blocking it. Because it is a deterministic Node.js script, it executes on every matching event without relying on the model to remember — the guarantee that makes agentic workflows safe to automate.
Use cases
- Continuous quality
- Auto-fix loop
- Lint standards
Tags
#validation#biome#lint#format#quality
settings.json fragment
{
"hooks": {
"PostToolUse": [
{
"hooks": [
{
"command": "node $CLAUDE_PROJECT_DIR/.claude/hooks/post-write-biome.mjs",
"type": "command"
}
],
"matcher": "Write|Edit"
}
]
}
}Script · .claude/hooks/post-write-biome.mjs
#!/usr/bin/env node
// @hookstack post-write-biome
// Formate et vérifie le fichier avec Biome après écriture (PostToolUse Write|Edit)
import { execSync } from "node:child_process";
import { readFileSync } from "node:fs";
import { fileURLToPath } from "node:url";
import { hookSpawnEnv } from "./lib/spawn-env.mjs";
function defaultExec(cmd) {
return execSync(cmd, {
stdio: "pipe",
timeout: 15_000,
env: hookSpawnEnv(process.env.CLAUDE_PROJECT_DIR ?? process.cwd()),
});
}
export function run(input, { exec = defaultExec } = {}) {
const filePath = input.tool_input?.file_path ?? "";
if (!filePath || !/\.([cm]?[jt]sx?|jsonc?)$/.test(filePath)) return null;
try {
// `biome check --write --error-on-warnings` : formate (--write) ET remonte
// les erreurs restantes — fusion de l'ancien post-write-autoformat (format)
// et post-write-biome (lint). `biome check` (et non `biome lint`) pour matcher
// le CI qui lance `biome check .` (lint + format + organize imports).
exec(`npx --no-install biome check --write --error-on-warnings "${filePath}"`);
return null;
} catch (err) {
const output = err.stdout?.toString() ?? "";
return output ? { message: `Biome: ${output.trim()}\n` } : null;
}
}
/* v8 ignore next 5 */
if (process.argv[1] === fileURLToPath(import.meta.url)) {
const input = JSON.parse(readFileSync(0, "utf8"));
const result = run(input);
if (result?.message) process.stderr.write(result.message);
}