diff --git a/scripts/hooks/stop-format-typecheck.js b/scripts/hooks/stop-format-typecheck.js
index a7bc0b784..8ae580db5 100644
--- a/scripts/hooks/stop-format-typecheck.js
+++ b/scripts/hooks/stop-format-typecheck.js
@@ -37,6 +37,29 @@ function parseAccumulator(raw) {
   return [...new Set(raw.split('\n').map(l => l.trim()).filter(Boolean))];
 }
 
+/**
+ * Is this file part of an installed plugin or marketplace clone?
+ *
+ * Those trees are third-party checkouts we merely read. Formatting them writes
+ * to code the user does not own, and when a repo's committed code has drifted
+ * from its own formatter config the rewrite is large: an unrelated bugfix ends
+ * up carrying hundreds of reformatted lines it never touched, which is enough
+ * to sink the contribution it was meant to support.
+ *
+ * Checks both a project-local install root and the user-level one, mirroring
+ * the lookup in scripts/harness-audit.js.
+ */
+function isPluginClonePath(filePath, cwd = process.cwd(), homeDir = os.homedir()) {
+  const resolved = path.resolve(filePath);
+  const roots = [path.join(cwd, '.claude', 'plugins')];
+  if (homeDir) roots.push(path.join(homeDir, '.claude', 'plugins'));
+
+  return roots.some(root => {
+    const rel = path.relative(root, resolved);
+    return rel !== '' && !rel.startsWith('..') && !path.isAbsolute(rel);
+  });
+}
+
 function getAccumFile() {
   const raw =
     process.env.CLAUDE_SESSION_ID ||
@@ -151,6 +174,7 @@ function main() {
   const byProjectRoot = new Map();
   for (const filePath of files) {
     if (!/\.(ts|tsx|js|jsx)$/.test(filePath)) continue;
+    if (isPluginClonePath(filePath)) continue;
     const resolved = path.resolve(filePath);
     if (!fs.existsSync(resolved)) continue;
     const root = findProjectRoot(path.dirname(resolved));
@@ -161,6 +185,7 @@ function main() {
   const byTsConfigDir = new Map();
   for (const filePath of files) {
     if (!/\.(ts|tsx)$/.test(filePath)) continue;
+    if (isPluginClonePath(filePath)) continue;
     const resolved = path.resolve(filePath);
     if (!fs.existsSync(resolved)) continue;
     const tsDir = findTsConfigDir(resolved);
@@ -223,4 +248,4 @@ if (require.main === module) {
   });
 }
 
-module.exports = { run, parseAccumulator };
+module.exports = { run, parseAccumulator, isPluginClonePath };
diff --git a/tests/hooks/stop-format-typecheck.test.js b/tests/hooks/stop-format-typecheck.test.js
index b509e9497..564bae693 100644
--- a/tests/hooks/stop-format-typecheck.test.js
+++ b/tests/hooks/stop-format-typecheck.test.js
@@ -13,7 +13,7 @@ const os = require('os');
 const path = require('path');
 
 const accumulator = require('../../scripts/hooks/post-edit-accumulator');
-const { parseAccumulator } = require('../../scripts/hooks/stop-format-typecheck');
+const { parseAccumulator, isPluginClonePath } = require('../../scripts/hooks/stop-format-typecheck');
 
 function test(name, fn) {
   try {
@@ -233,6 +233,46 @@ if (test('stop hook passes stdin through unchanged', () => {
   assert.strictEqual(result.toString(), input);
 })) passed++; else failed++;
 
+// --- Plugin and marketplace clones are read, not owned: never format them ---
+
+const FAKE_HOME = path.join(path.sep, 'home', 'someone');
+const FAKE_CWD = path.join(path.sep, 'work', 'project');
+
+if (test('skips a file inside the user-level plugin install root', () => {
+  const p = path.join(FAKE_HOME, '.claude', 'plugins', 'cache', 'some-plugin', 'scripts', 'tool.js');
+  assert.strictEqual(isPluginClonePath(p, FAKE_CWD, FAKE_HOME), true);
+})) passed++; else failed++;
+
+if (test('skips a file inside a marketplace clone', () => {
+  const p = path.join(FAKE_HOME, '.claude', 'plugins', 'marketplaces', 'some-market', 'tests', 'a.test.js');
+  assert.strictEqual(isPluginClonePath(p, FAKE_CWD, FAKE_HOME), true);
+})) passed++; else failed++;
+
+if (test('skips a file inside a project-local plugin install root', () => {
+  const p = path.join(FAKE_CWD, '.claude', 'plugins', 'local-plugin', 'index.js');
+  assert.strictEqual(isPluginClonePath(p, FAKE_CWD, FAKE_HOME), true);
+})) passed++; else failed++;
+
+if (test('still formats ordinary project files', () => {
+  const p = path.join(FAKE_CWD, 'src', 'app.ts');
+  assert.strictEqual(isPluginClonePath(p, FAKE_CWD, FAKE_HOME), false);
+})) passed++; else failed++;
+
+if (test('still formats the user own .claude config outside plugins', () => {
+  const p = path.join(FAKE_HOME, '.claude', 'scripts', 'hooks', 'mine.js');
+  assert.strictEqual(isPluginClonePath(p, FAKE_CWD, FAKE_HOME), false);
+})) passed++; else failed++;
+
+if (test('does not match a sibling directory sharing the prefix', () => {
+  const p = path.join(FAKE_HOME, '.claude', 'plugins-backup', 'thing.js');
+  assert.strictEqual(isPluginClonePath(p, FAKE_CWD, FAKE_HOME), false);
+})) passed++; else failed++;
+
+if (test('resolves traversal before deciding', () => {
+  const p = path.join(FAKE_CWD, 'src', '..', '.claude', 'plugins', 'p', 'x.js');
+  assert.strictEqual(isPluginClonePath(p, FAKE_CWD, FAKE_HOME), true);
+})) passed++; else failed++;
+
 // Restore env
 if (origSessionId === undefined) {
   delete process.env.CLAUDE_SESSION_ID;