diff --git a/src/plugins-builtin.ts b/src/plugins-builtin.ts
index 672ce7a..63c0d58 100644
--- a/src/plugins-builtin.ts
+++ b/src/plugins-builtin.ts
@@ -1,5 +1,6 @@
 import { tool } from 'ai';
 import { z } from 'zod';
+import { posix } from './ignore';
 import type { Plugin } from './plugins';
 
 /**
@@ -87,7 +88,7 @@ const SECRET_PATHS: { re: RegExp; why: string }[] = [
   { re: /(^|[\\/])\.gnupg[\\/]/i, why: 'a GPG directory' },
 ];
 
-/** Every path a write tool might carry, including a patch's markers. */
+/** Every path a write tool might carry, including a patch's markers and a move's ends. */
 function writtenPaths(toolName: string, input: unknown): string[] {
   const o = (input ?? {}) as Record<string, unknown>;
 
@@ -99,9 +100,16 @@ function writtenPaths(toolName: string, input: unknown): string[] {
     ];
   }
 
+  if (toolName === 'move_file') {
+    return [o['from'], o['to']].filter((p): p is string => typeof p === 'string');
+  }
+
   return typeof o['path'] === 'string' ? [o['path']] : [];
 }
 
+/** Write tools a path-based guard has to cover. Missing one is a silent bypass. */
+const WRITE_TOOLS = ['write_file', 'edit_file', 'multi_edit', 'apply_patch', 'move_file', 'delete_file'];
+
 export const secretsPlugin: Plugin = {
   name: 'secrets',
   description: 'refuses to write credential files',
@@ -110,7 +118,7 @@ export const secretsPlugin: Plugin = {
     'user which file and which key, and let them write it themselves. Do not work around the refusal by writing ' +
     'the same content somewhere else.',
   beforeToolCall: ({ toolName, input }) => {
-    if (!['write_file', 'edit_file', 'multi_edit', 'apply_patch'].includes(toolName)) return undefined;
+    if (!WRITE_TOOLS.includes(toolName)) return undefined;
 
     for (const path of writtenPaths(toolName, input)) {
       for (const { re, why } of SECRET_PATHS) {
@@ -123,6 +131,49 @@ export const secretsPlugin: Plugin = {
   },
 };
 
+/**
+ * Paths a write must not touch, for reasons other than secrecy.
+ *
+ * These are not credentials, so the secrets plugin has nothing to say about them.
+ * They are files whose contents are owned by a tool rather than by anyone editing
+ * them by hand: git's own object store, a resolver's lockfile, an installed
+ * dependency tree, a build directory. A model editing one of these produces a
+ * repository that looks fine and behaves wrongly, and the failure surfaces
+ * somewhere else entirely.
+ */
+const PROTECTED_PATHS: { re: RegExp; why: string }[] = [
+  { re: /(^|[\\/])\.git[\\/]/i, why: "git's own object store" },
+  {
+    re: /(^|[\\/])(bun\.lock|bun\.lockb|package-lock\.json|pnpm-lock\.yaml|yarn\.lock|Cargo\.lock|poetry\.lock|uv\.lock|composer\.lock|go\.sum|Gemfile\.lock)$/i,
+    why: 'a lockfile the package manager owns',
+  },
+  { re: /(^|[\\/])node_modules[\\/]/i, why: 'an installed dependency' },
+  { re: /(^|[\\/])(vendor|target[\\/]debug|target[\\/]release)[\\/]/i, why: 'a vendored or build directory' },
+  { re: /(^|[\\/])(dist|build|out|\.next|\.nuxt|\.svelte-kit|coverage)[\\/]/i, why: 'generated build output' },
+  { re: /(^|[\\/])\.(venv|tox|mypy_cache|pytest_cache|ruff_cache|turbo|parcel-cache)[\\/]/i, why: 'a tool cache' },
+];
+
+export const protectPlugin: Plugin = {
+  name: 'protect',
+  description: 'refuses writes to lockfiles, .git, dependencies, and build output',
+  appendix:
+    'The protect plugin refuses writes to .git, lockfiles, node_modules, vendored code, and build output. A ' +
+    'lockfile is regenerated by its package manager: run the install or update command through bash instead of ' +
+    'editing the file. Generated output is regenerated by its build. Do not route around the refusal.',
+  beforeToolCall: ({ toolName, input }) => {
+    if (!WRITE_TOOLS.includes(toolName)) return undefined;
+
+    for (const path of writtenPaths(toolName, input)) {
+      for (const { re, why } of PROTECTED_PATHS) {
+        if (re.test(posix(path))) {
+          return `refusing to write ${path} (${why}). Regenerate it with the tool that owns it rather than editing it.`;
+        }
+      }
+    }
+    return undefined;
+  },
+};
+
 const FORMATTERS: { file: string; script: string; command: string[] }[] = [
   { file: 'package.json', script: 'format', command: ['bun', 'run', 'format'] },
   { file: 'Cargo.toml', script: '', command: ['cargo', 'fmt'] },
@@ -167,15 +218,22 @@ export const formatPlugin: Plugin = {
   },
 };
 
-export const BUILTIN_PLUGINS: Plugin[] = [guardPlugin, secretsPlugin, bellPlugin, timePlugin, formatPlugin];
+export const BUILTIN_PLUGINS: Plugin[] = [
+  guardPlugin,
+  secretsPlugin,
+  protectPlugin,
+  bellPlugin,
+  timePlugin,
+  formatPlugin,
+];
 
 /**
  * Enabled unless the config turns them off.
  *
- * `guard` and `secrets` are refusals, so they are on: a user who has to opt into a
- * safety check does not have it. `bell` and `format` both act on their own — one
- * makes noise, the other writes files — so they are opt-in.
+ * `guard`, `secrets`, and `protect` are refusals, so they are on: a user who has to
+ * opt into a safety check does not have it. `bell` and `format` both act on their
+ * own — one makes noise, the other writes files — so they are opt-in.
  */
-export const DEFAULT_ENABLED = ['guard', 'secrets', 'time'];
+export const DEFAULT_ENABLED = ['guard', 'secrets', 'protect', 'time'];
 
-export { DESTRUCTIVE, SECRET_PATHS };
+export { DESTRUCTIVE, SECRET_PATHS, PROTECTED_PATHS };
diff --git a/test/plugins-builtin.test.ts b/test/plugins-builtin.test.ts
index 8f69399..59d61ab 100644
--- a/test/plugins-builtin.test.ts
+++ b/test/plugins-builtin.test.ts
@@ -3,14 +3,16 @@ import { mkdtempSync, rmSync } from 'node:fs';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
 import { createHost } from '../src/plugins';
-import { BUILTIN_PLUGINS, DEFAULT_ENABLED, formatPlugin, secretsPlugin } from '../src/plugins-builtin';
+import { BUILTIN_PLUGINS, DEFAULT_ENABLED, formatPlugin, protectPlugin, secretsPlugin } from '../src/plugins-builtin';
 
 const cwd = process.cwd();
 const check = (toolName: string, input: unknown) => secretsPlugin.beforeToolCall!({ toolName, input, cwd });
+const guarded = (toolName: string, input: unknown) => protectPlugin.beforeToolCall!({ toolName, input, cwd });
 
 test('secrets is on by default, because an opt-in safety check is not one', () => {
   expect(DEFAULT_ENABLED).toContain('secrets');
   expect(DEFAULT_ENABLED).toContain('guard');
+  expect(DEFAULT_ENABLED).toContain('protect');
   // Both of these act on their own rather than refusing, so they are opt-in.
   expect(DEFAULT_ENABLED).not.toContain('bell');
   expect(DEFAULT_ENABLED).not.toContain('format');
@@ -41,7 +43,7 @@ test('writing a registry or credential file is refused', async () => {
 });
 
 test('every write tool is covered, including apply_patch', async () => {
-  for (const tool of ['write_file', 'edit_file', 'multi_edit']) {
+  for (const tool of ['write_file', 'edit_file', 'multi_edit', 'delete_file']) {
     expect(await check(tool, { path: '.env' }), tool).toContain('refusing to write');
   }
 
@@ -50,6 +52,59 @@ test('every write tool is covered, including apply_patch', async () => {
   const patch = '*** Update File: src/app.ts\n-a\n+b\n*** Add File: .env\n+SECRET=x';
   expect(await check('apply_patch', { patch })).toContain('refusing to write');
   expect(await check('apply_patch', { patch: '*** Move to: .env.production\n' })).toContain('refusing to write');
+
+  // A move carries two paths and neither is called `path`.
+  expect(await check('move_file', { from: 'src/app.ts', to: '.env' })).toContain('refusing to write');
+  expect(await check('move_file', { from: '.env', to: 'src/app.ts' })).toContain('refusing to write');
+});
+
+test('protect refuses git internals, lockfiles, dependencies, and build output', async () => {
+  for (const path of [
+    '.git/config',
+    '.git/hooks/pre-commit',
+    'bun.lock',
+    'package-lock.json',
+    'Cargo.lock',
+    'go.sum',
+    'node_modules/react/index.js',
+    'vendor/github.com/pkg/errors.go',
+    'dist/bundle.js',
+    'coverage/lcov.info',
+    '.next/build-manifest.json',
+  ]) {
+    expect(await guarded('write_file', { path }), path).toContain('refusing to write');
+  }
+});
+
+test('protect covers every write tool and both ends of a move', async () => {
+  for (const tool of ['write_file', 'edit_file', 'multi_edit', 'delete_file']) {
+    expect(await guarded(tool, { path: 'bun.lock' }), tool).toContain('refusing to write');
+  }
+  expect(await guarded('apply_patch', { patch: '*** Update File: bun.lock\n-a\n+b' })).toContain('refusing to write');
+  expect(await guarded('move_file', { from: 'src/a.ts', to: 'node_modules/a.ts' })).toContain('refusing to write');
+});
+
+test('protect matches a Windows path separator as well', async () => {
+  expect(await guarded('write_file', { path: 'node_modules\\react\\index.js' })).toContain('refusing to write');
+  expect(await guarded('write_file', { path: '.git\\config' })).toContain('refusing to write');
+});
+
+test('protect leaves ordinary files and lookalike names alone', async () => {
+  for (const path of [
+    'src/app.ts',
+    'docs/dist-layout.md',
+    'src/gitignore-parser.ts',
+    'test/node_modules-resolution.test.ts',
+    'package.json',
+    'distributed/queue.ts',
+  ]) {
+    expect(await guarded('write_file', { path }), path).toBeUndefined();
+  }
+});
+
+test('protect says what to do instead of editing the file', async () => {
+  expect(protectPlugin.appendix).toContain('package manager');
+  expect(String(await guarded('write_file', { path: 'bun.lock' }))).toContain('Regenerate it');
 });
 
 test('ordinary source files are untouched', async () => {