msd-core/hooks/msd-read-guard.js

#!/usr/bin/env node
// msd-hook-version: {{MSD_VERSION}}
// MSD Read Guard — PreToolUse hook
// Injects advisory guidance when Write/Edit targets an existing file,
// reminding the model to Read the file first.
//
// Background: Non-Claude models (e.g. MiniMax M2.5 on OpenCode) don't
// natively follow the read-before-edit pattern. When they attempt to
// Write/Edit an existing file without reading it, the runtime rejects
// with "You must read file before overwriting it." The model retries
// without reading, creating an infinite loop that burns through usage.
//
// This hook prevents that loop by injecting clear guidance BEFORE the
// tool call reaches the runtime. The model sees the advisory and can
// issue a Read call on the next turn.
//
// Triggers on: Write and Edit tool calls
// Action: Advisory (does not block) — injects read-first guidance
// Only fires when the target file already exists on disk.

const fs = require('fs');
const path = require('path');
const { HOOK_ON_CRASH, allow, crash } = require('./lib/hook-exit.js');

// This guard is pure advisory UX — a reminder to Read before Write/Edit on
// non-Claude-Code runtimes. A crash here must not block a legitimate Write/
// Edit; the worst outcome of failing open is the model hitting the runtime's
// own read-before-edit rejection it was trying to help avoid (#3911).
const ON_CRASH = HOOK_ON_CRASH.ALLOW;

let input = '';
const stdinTimeout = setTimeout(() => allow(undefined), 3000);
process.stdin.setEncoding('utf8');
process.stdin.on('data', chunk => input += chunk);
process.stdin.on('end', () => {
  clearTimeout(stdinTimeout);
  try {
    const data = JSON.parse(input);
    const toolName = data.tool_name;

    // Only intercept Write and Edit tool calls
    if (toolName !== 'Write' && toolName !== 'Edit') {
      allow(undefined);
    }

    // Claude Code natively enforces read-before-edit — skip the advisory (#1984, #2344, #2520).
    //
    // Detection signals, in priority order:
    //   1. `data.session_id` on the hook's stdin payload — part of Claude
    //      Code's documented PreToolUse hook-input schema, always present.
    //      Reliable across Claude Code versions because it's schema, not env.
    //   2. `CLAUDE_CODE_ENTRYPOINT` / `CLAUDE_CODE_SSE_PORT` — env vars that
    //      Claude Code does propagate to hook subprocesses (verified on
    //      Claude Code CLI 2.1.116).
    //   3. `CLAUDE_SESSION_ID` / `CLAUDECODE` — kept for back-compat and in
    //      case future Claude Code versions propagate them to hook
    //      subprocesses. On 2.1.116 they reach Bash tool subprocesses but
    //      not hook subprocesses, which is why checking them alone is
    //      insufficient (regression of #2344 fixed here as #2520).
    const isClaudeCode =
      (typeof data.session_id === 'string' && data.session_id.length > 0) ||
      process.env.CLAUDE_CODE_ENTRYPOINT ||
      process.env.CLAUDE_CODE_SSE_PORT ||
      process.env.CLAUDE_SESSION_ID ||
      process.env.CLAUDECODE;
    if (isClaudeCode) {
      allow(undefined);
    }

    // #2595 (review Major 3, sibling sweep): typed read — same class as the
    // worktree guard's, advisory-only here (no exit(2) path in this hook).
    const filePath = typeof data.tool_input?.file_path === 'string'
      ? data.tool_input.file_path
      : '';
    if (!filePath) {
      allow(undefined);
    }

    // Only inject guidance when the file already exists.
    // New files don't need a prior Read — the runtime allows creating them directly.
    let fileExists = false;
    try {
      fs.accessSync(filePath, fs.constants.F_OK);
      fileExists = true;
    } catch {
      // File does not exist — no guidance needed
    }

    if (!fileExists) {
      allow(undefined);
    }

    const fileName = path.basename(filePath);

    // Advisory guidance — does not block the operation
    const output = {
      hookSpecificOutput: {
        hookEventName: 'PreToolUse',
        additionalContext:
          `READ-BEFORE-EDIT REMINDER: You are about to modify "${fileName}" which already exists. ` +
          'If you have not already used the Read tool to read this file in the current session, ' +
          'you MUST Read it first before editing. The runtime will reject edits to files that ' +
          'have not been read. Use the Read tool on this file path, then retry your edit.',
        code: 'READ_BEFORE_EDIT',
        fileName,
      },
    };

    process.stdout.write(JSON.stringify(output));
  } catch {
    // Silent fail — never block tool execution.
    // ON_CRASH is declared ALLOW at module top: this preserves today's
    // exit(0) fail-open behavior exactly (#3911).
    crash(ON_CRASH, undefined);
  }
});