diff --git a/packages/rstack/tests/cli/staged/fmt.test.ts b/packages/rstack/tests/cli/staged/fmt.test.ts
new file mode 100644
index 0000000..1ce8fa1
--- /dev/null
+++ b/packages/rstack/tests/cli/staged/fmt.test.ts
@@ -0,0 +1,93 @@
+import { spawnSync } from 'node:child_process';
+import { mkdtempSync, readFileSync, rmSync, writeFileSync } from 'node:fs';
+import path from 'node:path';
+import { afterEach, beforeEach, expect, test } from 'rstack/test';
+import { RSTACK_BIN_PATH } from '#test-helpers';
+
+let projectPath: string;
+let env: NodeJS.ProcessEnv;
+
+const writeProjectFile = (filePath: string, content: string): void =>
+  writeFileSync(path.join(projectPath, filePath), content);
+
+const readProjectFile = (filePath: string): string =>
+  readFileSync(path.join(projectPath, filePath), 'utf8');
+
+const git = (args: string[]): string => {
+  const result = spawnSync('git', args, {
+    cwd: projectPath,
+    encoding: 'utf8',
+    env,
+  });
+
+  if (result.status !== 0) {
+    throw new Error(result.stderr || `Git exited with status ${result.status}.`);
+  }
+
+  return result.stdout;
+};
+
+const runStaged = () =>
+  spawnSync(process.execPath, [RSTACK_BIN_PATH, 'staged', '--no-stash'], {
+    cwd: projectPath,
+    encoding: 'utf8',
+    env,
+  });
+
+beforeEach(() => {
+  projectPath = mkdtempSync(path.join(import.meta.dirname, 'staged-fmt-project-'));
+  env = {
+    ...process.env,
+    GIT_CONFIG_GLOBAL: path.join(projectPath, 'global.gitconfig'),
+    GIT_CONFIG_NOSYSTEM: '1',
+  };
+
+  git(['init', '--quiet']);
+  writeProjectFile(
+    'rstack.config.ts',
+    `import { define } from 'rstack';
+
+define.fmt({
+  ignorePatterns: ['ignored-by-fmt.ts'],
+});
+
+define.staged({
+  '*': 'rs fmt',
+});
+`,
+  );
+});
+
+afterEach(() => {
+  rmSync(projectPath, { force: true, recursive: true });
+});
+
+test('formats staged files with rs fmt and applies ignore rules', () => {
+  writeProjectFile('.gitignore', 'ignored-by-git.ts\n');
+  writeProjectFile('file with spaces.ts', 'const spaced="spaced"');
+  writeProjectFile('ignored-by-git.ts', 'const gitIgnored="git ignored"');
+  writeProjectFile('ignored-by-fmt.ts', 'const fmtIgnored="fmt ignored"');
+
+  git(['add', '--', 'file with spaces.ts', 'ignored-by-fmt.ts']);
+  git(['add', '--force', '--', 'ignored-by-git.ts']);
+
+  const result = runStaged();
+
+  expect(result.status).toBe(0);
+  expect(readProjectFile('file with spaces.ts')).toBe('const spaced = "spaced";\n');
+  expect(readProjectFile('ignored-by-git.ts')).toBe('const gitIgnored = "git ignored";\n');
+  expect(readProjectFile('ignored-by-fmt.ts')).toBe('const fmtIgnored="fmt ignored"');
+  expect(git(['show', ':file with spaces.ts'])).toBe('const spaced = "spaced";\n');
+  expect(git(['show', ':ignored-by-git.ts'])).toBe('const gitIgnored = "git ignored";\n');
+});
+
+test('propagates rs fmt failures', () => {
+  writeProjectFile('invalid.ts', 'const value = ;');
+  git(['add', '--', 'invalid.ts']);
+
+  const result = runStaged();
+
+  expect(result.status).toBe(1);
+  expect(`${result.stdout}\n${result.stderr}`).toContain('SyntaxError');
+  expect(readProjectFile('invalid.ts')).toBe('const value = ;');
+});