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ValidationPreToolUse· Write

Block oversized file writes

Stops a 500 KB dump from bloating your repo

Blocks a single Write whose content exceeds ~500 KB — almost always an accidental dump, a generated blob, or a runaway paste that balloons the repo and the diff. Pure byte-length check, no file I/O, so it is effectively free.

What does the Block oversized file writes hook do?

Block oversized file writes is a Claude Code PreToolUse hook matching Write. It fires automatically at that lifecycle event — outside the model, so it can't be skipped or forgotten. Stops a 500 KB dump from bloating your repo.

As a PreToolUse hook it runs before the action completes, so it can block or adjust what Claude is about to do. 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

  • Repo hygiene
  • Reviewable diffs
  • Catch accidental dumps

Tags

#validation#files#repo-hygiene#size-limit#guardrail

settings.json fragment

{
  "hooks": {
    "PreToolUse": [
      {
        "hooks": [
          {
            "command": "node $CLAUDE_PROJECT_DIR/.claude/hooks/block-huge-write.mjs",
            "type": "command"
          }
        ],
        "matcher": "Write"
      }
    ]
  }
}

Script · .claude/hooks/block-huge-write.mjs

#!/usr/bin/env node
// @hookstack pre-edit-block-huge-write
// Bloque l'écriture d'un fichier anormalement volumineux (PreToolUse Write)
import { readFileSync } from "node:fs";
import { fileURLToPath } from "node:url";

// 500 Ko : au-delà, c'est presque toujours un dump, un blob généré ou un collage accidentel.
const MAX_BYTES = 500_000;

export function run(input, { maxBytes = MAX_BYTES } = {}) {
	// Seul Write fournit le contenu complet ; Edit est un patch ciblé, on l'ignore.
	if (input.tool_name && input.tool_name !== "Write") return null;
	const content = input.tool_input?.content;
	if (typeof content !== "string") return null;

	const bytes = Buffer.byteLength(content, "utf8");
	if (bytes <= maxBytes) return null;

	const filePath = input.tool_input?.file_path ?? "le fichier";
	const kb = Math.round(bytes / 1024);
	return {
		decision: "block",
		reason:
			`Écriture de ${filePath} bloquée : ${kb} Ko (> ${Math.round(maxBytes / 1024)} Ko). ` +
			"Un fichier de cette taille est généralement un dump ou un blob généré qui gonfle le repo et le diff. " +
			"Vérifiez l'intention : générez-le à la volée, gitignorez-le, ou découpez-le.",
	};
}

/* 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) process.stdout.write(JSON.stringify(result));
}

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