diff --git a/.github/workflows/verify.yml b/.github/workflows/verify.yml
index 01de1fba..0ef4a472 100644
--- a/.github/workflows/verify.yml
+++ b/.github/workflows/verify.yml
@@ -11,6 +11,8 @@ jobs:
       - uses: actions/setup-node@v4
         with:
           node-version: 20
+      - name: Hook installer regressions
+        run: node --test tests/install-hook.test.mjs
       - name: References, gates, fixtures, versions and links agree
         run: node skills/hallmark/scripts/refcheck.mjs
       - name: Every theme validates
diff --git a/skills/hallmark/scripts/install-hook.mjs b/skills/hallmark/scripts/install-hook.mjs
index 4b0a6cfd..5c94d72d 100644
--- a/skills/hallmark/scripts/install-hook.mjs
+++ b/skills/hallmark/scripts/install-hook.mjs
@@ -49,6 +49,7 @@ if (has("--remove") && !existsSync(settingsPath)) {
 const settings = readSettings();
 if (!settings || typeof settings !== "object" || Array.isArray(settings)) { console.error(`refusing to rewrite ${settingsPath}: it is not a JSON object`); process.exit(1); }
 settings.hooks ??= {};
+if (typeof settings.hooks !== "object" || Array.isArray(settings.hooks)) { console.error(`refusing to rewrite ${settingsPath}: hooks is not a JSON object`); process.exit(1); }
 settings.hooks.PostToolUse ??= [];
 if (!Array.isArray(settings.hooks.PostToolUse)) { console.error(`refusing to rewrite ${settingsPath}: hooks.PostToolUse is not a list`); process.exit(1); }
 // Write beside the file and rename over it, so a crash never leaves settings.json half written.
diff --git a/tests/install-hook.test.mjs b/tests/install-hook.test.mjs
new file mode 100644
index 00000000..40519ec6
--- /dev/null
+++ b/tests/install-hook.test.mjs
@@ -0,0 +1,82 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { mkdtempSync, mkdirSync, readFileSync, writeFileSync, rmSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { dirname, join, resolve } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { spawnSync } from 'node:child_process';
+
+const script = resolve(dirname(fileURLToPath(import.meta.url)), '../skills/hallmark/scripts/install-hook.mjs');
+const command = `node "${resolve(dirname(script), 'lint-hook.mjs')}"`;
+function fixture(t, settings) {
+  const dir = mkdtempSync(join(tmpdir(), 'hallmark-hook-test-'));
+  t.after(() => rmSync(dir, { recursive: true, force: true }));
+  mkdirSync(join(dir, '.claude'));
+  const file = join(dir, '.claude/settings.json');
+  const original = JSON.stringify(settings, null, 4) + '\n';
+  writeFileSync(file, original);
+  return {
+    original,
+    read: () => readFileSync(file, 'utf8'),
+    run: (...args) => spawnSync(process.execPath, [script, ...args], { cwd: dir, encoding: 'utf8' }),
+  };
+}
+
+for (const hooks of [[], [{ matcher: '*', hooks: [] }], 'invalid', 42, false]) {
+  for (const mode of [[], ['--remove']]) {
+    test(`refuses hooks ${JSON.stringify(hooks)} during ${mode.length ? 'removal' : 'installation'}`, (t) => {
+      const f = fixture(t, { hooks, permissions: { deny: ['Read(private.txt)'] } });
+      const result = f.run(...mode);
+      assert.equal(result.status, 1);
+      assert.match(result.stderr, /hooks is not a JSON object/);
+      assert.doesNotMatch(result.stdout, /installed|removed/);
+      assert.equal(f.read(), f.original, 'invalid settings must remain byte-for-byte unchanged');
+    });
+  }
+}
+
+for (const settings of [{}, { hooks: null }, { hooks: {} }]) {
+  test(`initializes missing hooks for ${JSON.stringify(settings)}`, (t) => {
+    const f = fixture(t, settings);
+    assert.equal(f.run().status, 0);
+    const result = JSON.parse(f.read());
+    assert.equal(result.hooks.PostToolUse.length, 1);
+    assert.equal(result.hooks.PostToolUse[0].hooks[0].command, command);
+  });
+}
+
+test('installation is idempotent and preserves unrelated configuration and shared hooks', (t) => {
+  const other = { type: 'command', command: 'echo other' };
+  const f = fixture(t, {
+    permissions: { deny: ['Read(private.txt)'] },
+    hooks: {
+      PreToolUse: [{ matcher: '*', hooks: [other] }],
+      PostToolUse: [{ matcher: 'Write', hooks: [other, { type: 'command', command }] }],
+    },
+  });
+  assert.equal(f.run().status, 0);
+  const once = f.read();
+  assert.equal(f.run().status, 0);
+  assert.equal(f.read(), once);
+  const result = JSON.parse(once);
+  assert.deepEqual(result.permissions, { deny: ['Read(private.txt)'] });
+  assert.deepEqual(result.hooks.PreToolUse, [{ matcher: '*', hooks: [other] }]);
+  assert.deepEqual(result.hooks.PostToolUse[0], { matcher: 'Write', hooks: [other] });
+  assert.equal(result.hooks.PostToolUse.length, 2);
+  assert.equal(f.run('--remove').status, 0);
+  const removed = JSON.parse(f.read());
+  assert.deepEqual(removed.hooks.PostToolUse, [{ matcher: 'Write', hooks: [other] }]);
+  assert.deepEqual(removed.permissions, result.permissions);
+});
+
+test('refuses invalid PostToolUse without rewriting the settings', (t) => {
+  const f = fixture(t, { hooks: { PostToolUse: {} } });
+  assert.equal(f.run().status, 1);
+  assert.equal(f.read(), f.original);
+});
+
+test('--print does not validate or modify existing settings', (t) => {
+  const f = fixture(t, { hooks: [] });
+  assert.equal(f.run('--print').status, 0);
+  assert.equal(f.read(), f.original);
+});