diff --git a/packages/rstack/src/fmt/cli.ts b/packages/rstack/src/fmt/cli.ts
index 0dc1347..7cada94 100644
--- a/packages/rstack/src/fmt/cli.ts
+++ b/packages/rstack/src/fmt/cli.ts
@@ -14,6 +14,7 @@ interface ParsedFmtCLIArgs {
   ignorePaths: string[];
   ignoreUnknown: boolean;
   noErrorOnUnmatchedPattern: boolean;
+  withNodeModules: boolean;
   maxWorkers?: number;
   help: boolean;
   /** Path the stdin content is formatted as; it need not exist on disk. */
@@ -34,6 +35,7 @@ ${color.cyan('Options')}:
   --ignore-path <path>             Path to an additional ignore file (repeatable)
   -u, --ignore-unknown             Ignore unknown files
   --no-error-on-unmatched-pattern  Do not error when no files match
+  --with-node-modules              Process files inside node_modules
   --parallel-workers <count>       Number of parallel workers
   --stdin-filepath <path>          Format stdin as if it were saved at <path>
   -h, --help                       Display this help message`;
@@ -61,6 +63,7 @@ const parseFmtCLIArgs = (args: string[]): ParsedFmtCLIArgs => {
       'ignore-path': { type: 'string', multiple: true },
       'ignore-unknown': { type: 'boolean', short: 'u' },
       'no-error-on-unmatched-pattern': { type: 'boolean' },
+      'with-node-modules': { type: 'boolean' },
       'parallel-workers': { type: 'string' },
       'stdin-filepath': { type: 'string' },
       help: { type: 'boolean', short: 'h' },
@@ -81,6 +84,7 @@ const parseFmtCLIArgs = (args: string[]): ParsedFmtCLIArgs => {
   const ignorePaths = values.ignorePath ?? [];
   const ignoreUnknown = values.ignoreUnknown ?? false;
   const noErrorOnUnmatchedPattern = values.noErrorOnUnmatchedPattern ?? false;
+  const withNodeModules = values.withNodeModules ?? false;
   const parallelWorkers = values.parallelWorkers;
   const maxWorkers = parseMaxWorkers(parallelWorkers);
   const help = values.help ?? false;
@@ -104,6 +108,7 @@ const parseFmtCLIArgs = (args: string[]): ParsedFmtCLIArgs => {
     ignorePaths,
     ignoreUnknown,
     noErrorOnUnmatchedPattern,
+    withNodeModules,
     maxWorkers,
     help,
     stdinFilepath,
@@ -239,6 +244,7 @@ const runFmtCLI = async (args: string[]): Promise<void> => {
       noErrorOnUnmatchedPattern,
       patterns,
       stdinFilepath,
+      withNodeModules,
     } = parseFmtCLIArgs(args);
     if (help) {
       logger.log(fmtHelpMessage);
@@ -266,6 +272,7 @@ const runFmtCLI = async (args: string[]): Promise<void> => {
       patterns,
       config,
       ignorePaths,
+      withNodeModules,
     });
 
     if (files.length === 0) {
diff --git a/packages/rstack/src/fmt/discoverPaths.ts b/packages/rstack/src/fmt/discoverPaths.ts
index f0e16f4..acc013d 100644
--- a/packages/rstack/src/fmt/discoverPaths.ts
+++ b/packages/rstack/src/fmt/discoverPaths.ts
@@ -5,12 +5,14 @@ import isBinaryPath from 'is-binary-path';
 import micromatch from 'micromatch';
 import readdir, { type Dirent } from 'tiny-readdir';
 
-const alwaysIgnoredNames = new Set(['.git', '.sl', '.svn', '.hg', '.jj', 'node_modules']);
+const defaultIgnoredDirNames = new Set(['.git', '.sl', '.svn', '.hg', '.jj', 'node_modules']);
 
 interface DiscoverFmtPathsOptions {
   /** Absolute directory used to resolve input paths. */
   cwd: string;
   patterns?: string[];
+  /** Whether files inside node_modules may be discovered. */
+  withNodeModules?: boolean;
   /** Returns whether a scanned directory can be pruned before traversal. */
   isDirectoryIgnored?: (directoryPath: string) => boolean;
 }
@@ -47,11 +49,15 @@ const getDirentParentPath = (dirent: Dirent): string =>
 const getDirentPath = (dirent: Dirent, parentPath: string): string =>
   `${parentPath}${parentPath === path.sep ? '' : path.sep}${dirent.name}`;
 
-const hasAlwaysIgnoredSegment = (cwd: string, filePath: string): boolean =>
+const hasBuiltInIgnoredSegment = (
+  cwd: string,
+  filePath: string,
+  ignoredDirNames: ReadonlySet<string>,
+): boolean =>
   path
     .relative(cwd, filePath)
     .split(path.sep)
-    .some((segment) => alwaysIgnoredNames.has(segment));
+    .some((segment) => ignoredDirNames.has(segment));
 
 const findGitRoot = async (cwd: string): Promise<string> => {
   let directoryPath = cwd;
@@ -202,6 +208,7 @@ class GitIgnoreMatcher {
 
 const createTraversalOptions = (
   gitIgnore: GitIgnoreMatcher,
+  ignoredDirNames: ReadonlySet<string>,
   isIncluded?: (filePath: string) => boolean,
   isDirectoryIgnored?: (directoryPath: string) => boolean,
 ) => {
@@ -212,7 +219,7 @@ const createTraversalOptions = (
     followSymlinks: false,
     ignore: (targetPath: string) => {
       const isDirectory = directories.delete(targetPath);
-      if (alwaysIgnoredNames.has(path.basename(targetPath))) {
+      if (ignoredDirNames.has(path.basename(targetPath))) {
         return true;
       }
 
@@ -265,7 +272,11 @@ type ClassifiedPatterns = {
   negativeGlobs: string[];
 };
 
-const classifyPatterns = async (cwd: string, patterns: string[]): Promise<ClassifiedPatterns> => {
+const classifyPatterns = async (
+  cwd: string,
+  patterns: string[],
+  ignoredDirNames: ReadonlySet<string>,
+): Promise<ClassifiedPatterns> => {
   const entries = await Promise.all(
     patterns.map(async (pattern): Promise<PatternEntry | undefined> => {
       if (pattern.startsWith('!')) {
@@ -273,7 +284,7 @@ const classifyPatterns = async (cwd: string, patterns: string[]): Promise<Classi
       }
 
       const filePath = path.resolve(cwd, pattern);
-      if (hasAlwaysIgnoredSegment(cwd, filePath)) {
+      if (hasBuiltInIgnoredSegment(cwd, filePath, ignoredDirNames)) {
         return;
       }
 
@@ -346,15 +357,24 @@ const getTraversalRoots = (cwd: string, directories: string[], globs: string[]):
 const discoverFmtPaths = async ({
   cwd,
   patterns: inputPatterns,
+  withNodeModules = false,
   isDirectoryIgnored,
 }: DiscoverFmtPathsOptions): Promise<string[]> => {
   const patterns = inputPatterns?.length ? inputPatterns : ['.'];
+  const ignoredDirNames = withNodeModules
+    ? new Set(defaultIgnoredDirNames)
+    : defaultIgnoredDirNames;
+
+  if (withNodeModules) {
+    ignoredDirNames.delete('node_modules');
+  }
+
   const {
     files: explicitFiles,
     directories,
     globs,
     negativeGlobs,
-  } = await classifyPatterns(cwd, patterns);
+  } = await classifyPatterns(cwd, patterns, ignoredDirNames);
   const directoryRoots = getOutermostPaths(directories);
   const globMatchers = globs.map((pattern) => micromatch.matcher(pattern, { dot: true }));
   const candidates = new Set(explicitFiles);
@@ -389,7 +409,10 @@ const discoverFmtPaths = async ({
             };
 
         return (
-          await readdir(rootPath, createTraversalOptions(gitIgnore, isIncluded, isDirectoryIgnored))
+          await readdir(
+            rootPath,
+            createTraversalOptions(gitIgnore, ignoredDirNames, isIncluded, isDirectoryIgnored),
+          )
         ).files;
       }),
     );
diff --git a/packages/rstack/src/fmt/discovery.ts b/packages/rstack/src/fmt/discovery.ts
index 47dc157..532859b 100644
--- a/packages/rstack/src/fmt/discovery.ts
+++ b/packages/rstack/src/fmt/discovery.ts
@@ -13,12 +13,14 @@ const discoverFmtFiles = async ({
   cwd,
   patterns,
   ignorePaths,
+  withNodeModules,
   config,
 }: DiscoverFmtFilesOptions): Promise<FmtFileRequest[]> => {
   const isIgnored = await createIgnoreMatcher({ config, cwd, ignorePaths });
   const candidates = await discoverFmtPaths({
     cwd,
     patterns,
+    withNodeModules,
     isDirectoryIgnored: (directoryPath) => isIgnored(directoryPath, true),
   });
   if (candidates.length === 0) {
diff --git a/packages/rstack/src/fmt/types.ts b/packages/rstack/src/fmt/types.ts
index db9b2d6..e93a4bd 100644
--- a/packages/rstack/src/fmt/types.ts
+++ b/packages/rstack/src/fmt/types.ts
@@ -58,6 +58,8 @@ interface DiscoverFmtFilesOptions {
   patterns?: string[];
   /** Ignore files resolved from `cwd`; each file's patterns are relative to its own directory. */
   ignorePaths?: string[];
+  /** Whether files inside node_modules may be discovered. */
+  withNodeModules?: boolean;
   /** Resolved project config applied to discovered files. */
   config: ResolvedFmtConfig;
 }
diff --git a/packages/rstack/tests/cli/fmt/index.test.ts b/packages/rstack/tests/cli/fmt/index.test.ts
index 7f568af..cdd41b7 100644
--- a/packages/rstack/tests/cli/fmt/index.test.ts
+++ b/packages/rstack/tests/cli/fmt/index.test.ts
@@ -128,6 +128,21 @@ test('formats the current directory with Prettier defaults', () => {
   expect(readProjectFile('index.ts')).toBe('const message = "hello";\n');
 });
 
+test('formats files in node_modules with --with-node-modules', () => {
+  const source = 'const message="hello"';
+  writeProjectFile('node_modules/example/index.ts', source);
+
+  const skipped = runFmt(['node_modules/example']);
+  expect(skipped.status).toBe(2);
+  expect(readProjectFile('node_modules/example/index.ts')).toBe(source);
+
+  const result = runFmt(['--with-node-modules', 'node_modules/example']);
+  expect(result.status).toBe(0);
+  expectWriteSummary(result.stdout, 1, 1);
+  expect(result.stderr).toBe('');
+  expect(readProjectFile('node_modules/example/index.ts')).toBe('const message = "hello";\n');
+});
+
 test('summarizes write mode when no files change', () => {
   writeProjectFile('index.ts', 'const message = "hello";\n');
 
diff --git a/packages/rstack/tests/fmt/__snapshots__/cli.test.ts.snap b/packages/rstack/tests/fmt/__snapshots__/cli.test.ts.snap
index 2df10bd..9d69d08 100644
--- a/packages/rstack/tests/fmt/__snapshots__/cli.test.ts.snap
+++ b/packages/rstack/tests/fmt/__snapshots__/cli.test.ts.snap
@@ -13,6 +13,7 @@ Options:
   --ignore-path <path>             Path to an additional ignore file (repeatable)
   -u, --ignore-unknown             Ignore unknown files
   --no-error-on-unmatched-pattern  Do not error when no files match
+  --with-node-modules              Process files inside node_modules
   --parallel-workers <count>       Number of parallel workers
   --stdin-filepath <path>          Format stdin as if it were saved at <path>
   -h, --help                       Display this help message"
diff --git a/packages/rstack/tests/fmt/cli.test.ts b/packages/rstack/tests/fmt/cli.test.ts
index 9521ccb..e687031 100644
--- a/packages/rstack/tests/fmt/cli.test.ts
+++ b/packages/rstack/tests/fmt/cli.test.ts
@@ -24,6 +24,7 @@ test('uses write mode by default', () => {
     ignorePaths: [],
     ignoreUnknown: false,
     noErrorOnUnmatchedPattern: false,
+    withNodeModules: false,
     maxWorkers: undefined,
     help: false,
   });
@@ -40,6 +41,7 @@ test.each([
     ignorePaths: [],
     ignoreUnknown: false,
     noErrorOnUnmatchedPattern: false,
+    withNodeModules: false,
     maxWorkers: undefined,
     help: false,
   });
@@ -52,6 +54,7 @@ test('configures parallel worker count', () => {
     ignorePaths: [],
     ignoreUnknown: false,
     noErrorOnUnmatchedPattern: false,
+    withNodeModules: false,
     maxWorkers: 3,
     help: false,
   });
@@ -75,6 +78,7 @@ test('preserves file paths and globs', () => {
     ignorePaths: [],
     ignoreUnknown: false,
     noErrorOnUnmatchedPattern: false,
+    withNodeModules: false,
     maxWorkers: undefined,
     help: false,
   });
@@ -87,6 +91,7 @@ test('treats arguments after the terminator as paths', () => {
     ignorePaths: [],
     ignoreUnknown: false,
     noErrorOnUnmatchedPattern: false,
+    withNodeModules: false,
     maxWorkers: undefined,
     help: false,
   });
@@ -111,6 +116,10 @@ test.each(['-u', '--ignore-unknown', '--ignoreUnknown'])('parses %s', (option) =
   expect(parseFmtCLIArgs([option]).ignoreUnknown).toBe(true);
 });
 
+test('parses --with-node-modules', () => {
+  expect(parseFmtCLIArgs(['--with-node-modules']).withNodeModules).toBe(true);
+});
+
 test('parses --stdin-filepath', () => {
   expect(parseFmtCLIArgs(['--stdin-filepath', 'src/index.ts'])).toEqual({
     mode: 'write',
@@ -118,6 +127,7 @@ test('parses --stdin-filepath', () => {
     ignorePaths: [],
     ignoreUnknown: false,
     noErrorOnUnmatchedPattern: false,
+    withNodeModules: false,
     maxWorkers: undefined,
     help: false,
     stdinFilepath: 'src/index.ts',
@@ -131,6 +141,7 @@ test('accepts a worker count with --stdin-filepath', () => {
     ignorePaths: [],
     ignoreUnknown: false,
     noErrorOnUnmatchedPattern: false,
+    withNodeModules: false,
     maxWorkers: 2,
     help: false,
     stdinFilepath: 'index.ts',
diff --git a/packages/rstack/tests/fmt/discoverPaths.test.ts b/packages/rstack/tests/fmt/discoverPaths.test.ts
index a712f61..02bfbd2 100644
--- a/packages/rstack/tests/fmt/discoverPaths.test.ts
+++ b/packages/rstack/tests/fmt/discoverPaths.test.ts
@@ -19,6 +19,7 @@ test('discovers non-binary files in stable order and skips hard-ignored paths',
     writeProjectFile(rootPath, '.jj/internal.js');
 
     const files = await discoverFmtPaths({ cwd: rootPath });
+    const filesWithNodeModules = await discoverFmtPaths({ cwd: rootPath, withNodeModules: true });
 
     expect(relativePaths(rootPath, files)).toEqual([
       'a.js',
@@ -26,9 +27,35 @@ test('discovers non-binary files in stable order and skips hard-ignored paths',
       path.join('folder with spaces', 'c.ts'),
       'unknown.extension',
     ]);
+    expect(relativePaths(rootPath, filesWithNodeModules)).toEqual([
+      'a.js',
+      'b.ts',
+      path.join('folder with spaces', 'c.ts'),
+      path.join('node_modules', 'package', 'index.js'),
+      'unknown.extension',
+    ]);
     await expect(
       discoverFmtPaths({ cwd: rootPath, patterns: ['node_modules/package/index.js'] }),
     ).resolves.toEqual([]);
+    await expect(
+      discoverFmtPaths({
+        cwd: rootPath,
+        patterns: ['node_modules/package/index.js'],
+        withNodeModules: true,
+      }),
+    ).resolves.toEqual([path.join(rootPath, 'node_modules/package/index.js')]);
+  });
+});
+
+test('keeps node_modules excluded by gitignore when built-in exclusion is disabled', async () => {
+  await withTempProject(async (rootPath) => {
+    writeProjectFile(rootPath, '.gitignore', 'node_modules/\n');
+    writeProjectFile(rootPath, 'node_modules/package/index.js');
+    writeProjectFile(rootPath, 'index.js');
+
+    const files = await discoverFmtPaths({ cwd: rootPath, withNodeModules: true });
+
+    expect(relativePaths(rootPath, files)).toEqual(['.gitignore', 'index.js']);
   });
 });
 
diff --git a/website/docs/en/guide/cli/fmt.mdx b/website/docs/en/guide/cli/fmt.mdx
index b92ac26..9e4f09e 100644
--- a/website/docs/en/guide/cli/fmt.mdx
+++ b/website/docs/en/guide/cli/fmt.mdx
@@ -149,6 +149,16 @@ Formatted output is written to stdout and diagnostics to stderr. If the input pa
 
 > `--stdin-filepath` cannot be combined with file arguments or with `--write`, `--check`, or `--list-different`.
 
+### `--with-node-modules`
+
+Process files inside `node_modules`, which `rs fmt` excludes by default:
+
+```bash
+rs fmt --with-node-modules node_modules/example/index.js
+```
+
+This option only disables the built-in `node_modules` exclusion. Directory and glob scans still follow `.gitignore`, while `ignorePatterns` and `--ignore-path` continue to apply to every input.
+
 ### `--write`
 
 Write formatted files in place. This is the default mode, so specifying `--write` is optional:
diff --git a/website/docs/zh/guide/cli/fmt.mdx b/website/docs/zh/guide/cli/fmt.mdx
index 78fd64f..a425863 100644
--- a/website/docs/zh/guide/cli/fmt.mdx
+++ b/website/docs/zh/guide/cli/fmt.mdx
@@ -149,6 +149,16 @@ cat src/index.ts | rs fmt --stdin-filepath src/index.ts
 
 > `--stdin-filepath` 不能与文件参数或 `--write`、`--check`、`--list-different` 同时使用。
 
+### `--with-node-modules`
+
+处理 `node_modules` 中的文件。默认情况下，`rs fmt` 会排除这些文件：
+
+```bash
+rs fmt --with-node-modules node_modules/example/index.js
+```
+
+此选项只会关闭内置的 `node_modules` 排除规则。目录和 glob 扫描仍然遵循 `.gitignore`，`ignorePatterns` 和 `--ignore-path` 也会继续作用于所有输入。
+
 ### `--write`
 
 将格式化结果写回文件。这是默认模式，因此可以省略 `--write`：