repo-mailbox/hooks/scripts/pre-state-line-guard.mjs

#!/usr/bin/env node
// Hook: pre-state-line-guard.mjs
// Event: PreToolUse (Write|Edit)
// Purpose: block a Write/Edit that would push a STATE.md past the documented
// ~60-line convention (global CLAUDE.md's Kontinuitets-system section).
//
// PreToolUse, not PostToolUse: org-ops' work order (20260814T144553Z) asked
// for a PostToolUse hook, but PostToolUse fires AFTER the tool already ran
// and cannot undo the write (confirmed against the official hooks docs,
// 2026-08-14: "Can block? No" for PostToolUse). PreToolUse is the only event
// that can deny before the file lands. The prose limit existed already and
// still drifted silently to 155-156 lines in a real STATE.md before anyone
// noticed via /insights - a hook is the mechanical backstop prose can't be.
//
// Blocking convention (stderr + exit 2) matches llm-security's
// pre-write-pathguard.mjs, the only other PreToolUse Write/Edit guard in
// this marketplace.
//
// Protocol:
//   - Read JSON from stdin: { tool_name, tool_input }
//   - Only Write/Edit targeting a file named exactly STATE.md (any
//     directory) are checked; everything else fails open immediately.
//   - Write: the projected content is tool_input.content.
//   - Edit: the projected content is the CURRENT on-disk file with
//     old_string replaced by new_string (every occurrence if
//     tool_input.replace_all is true, otherwise the first only) - the same
//     transform the real Edit tool applies. Anything this hook cannot
//     project confidently (file missing, old_string not found, fields of
//     the wrong type) is left to the real tool, which will give a clearer
//     error than a guess here would.
//   - RATCHET: denies only when the projected line count is BOTH over
//     MAX_LINES and larger than the file's CURRENT line count (0 for a file
//     that doesn't exist yet). A file already over the limit is the normal
//     starting point for a trim, not an edge case - measured on the real
//     tree 2026-08-14, 23 of the machine's STATE.md files were already over
//     60 lines, one at 1405. Comparing only against MAX_LINES (no ratchet)
//     would deny every incremental trim of those files that doesn't land at
//     <=60 in one shot - the opposite of what a guard meant to make trimming
//     possible should do. The ratchet still blocks what the guard exists to
//     block: a compliant file growing past the limit, or a brand-new file
//     being created oversized.
//   - Block: stderr + exit 2
//   - Allow: exit 0, no output

import { readFileSync } from 'node:fs';
import { basename } from 'node:path';

const MAX_LINES = 60;

function allow() {
  process.exit(0);
}

function countLines(text) {
  const matches = text.match(/\n/g);
  return matches ? matches.length : 0;
}

function currentLineCountOf(path) {
  try {
    return countLines(readFileSync(path, 'utf-8'));
  } catch {
    return 0;
  }
}

let input;
try {
  input = JSON.parse(readFileSync(0, 'utf-8'));
} catch {
  allow();
}

const toolName = input?.tool_name;
const toolInput = input?.tool_input ?? {};
const filePath = toolInput.file_path;

if (
  (toolName !== 'Write' && toolName !== 'Edit') ||
  typeof filePath !== 'string' ||
  basename(filePath) !== 'STATE.md'
) {
  allow();
}

let projected;
let currentLines;
if (toolName === 'Write') {
  if (typeof toolInput.content !== 'string') allow();
  projected = toolInput.content;
  currentLines = currentLineCountOf(filePath);
} else {
  let current;
  try {
    current = readFileSync(filePath, 'utf-8');
  } catch {
    allow();
  }
  const oldStr = toolInput.old_string;
  const newStr = toolInput.new_string;
  if (typeof oldStr !== 'string' || typeof newStr !== 'string' || !current.includes(oldStr)) {
    allow();
  }
  projected = toolInput.replace_all
    ? current.split(oldStr).join(newStr)
    : current.replace(oldStr, newStr);
  currentLines = countLines(current);
}

const lines = countLines(projected);
if (lines > MAX_LINES && lines > currentLines) {
  process.stderr.write(
    `\n[repo-mailbox] STATE LINE GUARD: ${toolName} blocked\n` +
    `  File: ${filePath}\n` +
    `  Projected: ${lines} lines (current: ${currentLines}, max ${MAX_LINES} per the STATE.md convention)\n\n` +
    `This would grow STATE.md further past the limit. Trim it instead -- ` +
    `any write that reduces the line count is allowed, even if still over ${MAX_LINES}.\n`
  );
  process.exit(2);
}

process.exit(0);