diff --git a/backend/tests/test_public_demo_zip.py b/backend/tests/test_public_demo_zip.py
index 5f1178d..a176699 100644
--- a/backend/tests/test_public_demo_zip.py
+++ b/backend/tests/test_public_demo_zip.py
@@ -40,10 +40,49 @@ def test_generator_is_reproducible_and_contains_searchable_source(self):
                 self.assertIn("src/catalog.py", paths)
                 self.assertIn("src/api.py", paths)
                 self.assertIn("find_title", archive.read("src/catalog.py").decode("utf-8"))
+                self.assertTrue(all(
+                    member.date_time == (1980, 1, 1, 0, 0, 0)
+                    and member.create_system == 3
+                    and member.external_attr >> 16 == 0o100644
+                    for member in archive.infolist()
+                ))
                 self.assertTrue(
                     all(not p.startswith("/") and ".." not in Path(p).parts for p in paths)
                 )
 
+    def test_generator_does_not_overwrite_existing_output(self):
+        with tempfile.TemporaryDirectory() as directory:
+            destination = Path(directory) / "existing.zip"
+            sentinel = b"EXISTING_DOCUMENT_DO_NOT_TOUCH"
+            destination.write_bytes(sentinel)
+            result = subprocess.run(
+                [sys.executable, str(GENERATOR), "--output", str(destination)],
+                cwd=ROOT, capture_output=True, text=True, timeout=20
+            )
+            self.assertNotEqual(result.returncode, 0)
+            self.assertIn("already exists", result.stderr.lower())
+            self.assertEqual(destination.read_bytes(), sentinel)
+
+
+    @unittest.skipIf(sys.platform == "win32", "Windows symlink setup needs special privileges")
+    def test_generator_does_not_follow_existing_symlink(self):
+        with tempfile.TemporaryDirectory() as directory:
+            root = Path(directory)
+            target = root / "important.txt"
+            sentinel = b"DO_NOT_REPLACE_SYMLINK_TARGET"
+            target.write_bytes(sentinel)
+            link = root / "fictional-catalog.zip"
+            link.symlink_to(target)
+            result = subprocess.run(
+                [sys.executable, str(GENERATOR), "--output", str(link)],
+                cwd=ROOT, capture_output=True, text=True, timeout=20
+            )
+            self.assertNotEqual(result.returncode, 0)
+            self.assertIn("already exists", result.stderr.lower())
+            self.assertTrue(link.is_symlink())
+            self.assertEqual(target.read_bytes(), sentinel)
+
+
 
 class PublishedDemoHttpTests(unittest.IsolatedAsyncioTestCase):
     async def test_published_zip_imports_searches_and_deletes_without_ollama(self):
diff --git a/docs/first-run.md b/docs/first-run.md
index df268e9..74c9e98 100644
--- a/docs/first-run.md
+++ b/docs/first-run.md
@@ -79,6 +79,8 @@ Import **fictional-catalog.zip** through the UI, then search for **find_title**.
 The search result should show **src/catalog.py**, line **2**. These are real
 source lines from a fictional fixture, **not** an AI model answer.
 
+The generator refuses to overwrite an existing output path, including a symlink. If you run the command again, choose a different filename or manually remove only your disposable sample ZIP first.
+
 CI checks this exact public ZIP generator and its real FastAPI ZIP-import/search/
 deletion journey, without starting Ollama:
 
diff --git a/examples/create_demo_zip.py b/examples/create_demo_zip.py
index d2a8b59..9bf8060 100644
--- a/examples/create_demo_zip.py
+++ b/examples/create_demo_zip.py
@@ -13,13 +13,25 @@
 def create_demo_zip(destination: Path) -> Path:
     destination = Path(destination)
     destination.parent.mkdir(parents=True, exist_ok=True)
-    with zipfile.ZipFile(destination, "w", compression=zipfile.ZIP_DEFLATED) as archive:
-        for path, contents in DEMO_FILES.items():
-            archive.writestr(path, contents)
+    # Exclusive creation refuses existing files and symlinks; a first-run demo
+    # should never replace another file at a caller-selected output path.
+    with destination.open("xb") as output:
+        with zipfile.ZipFile(output, "w", compression=zipfile.ZIP_DEFLATED) as archive:
+            for path, contents in DEMO_FILES.items():
+                # An explicit DOS timestamp and file mode keep ZIP members
+                # reproducible across seconds and independent of host OS.
+                member = zipfile.ZipInfo(path, date_time=(1980, 1, 1, 0, 0, 0))
+                member.compress_type = zipfile.ZIP_DEFLATED
+                member.create_system = 3
+                member.external_attr = 0o100644 << 16
+                archive.writestr(member, contents)
     return destination
 
 if __name__ == "__main__":
     parser = argparse.ArgumentParser()
     parser.add_argument("--output", required=True, type=Path)
     args = parser.parse_args()
-    print(create_demo_zip(args.output))
+    try:
+        print(create_demo_zip(args.output))
+    except FileExistsError:
+        parser.error(f"Output already exists: {args.output}. Choose another --output path.")