diff --git a/tests/abstract/test_abstract_blocks.py b/tests/abstract/test_abstract_blocks.py
index c4770ed..795b70f 100644
--- a/tests/abstract/test_abstract_blocks.py
+++ b/tests/abstract/test_abstract_blocks.py
@@ -307,6 +307,8 @@ class TestImagePIL(unittest.TestCase):
 
         if FLASK_AVAILABLE and hasattr(cls, 'flask_thread'):
             cls.flask_server_running = False
+            cls.flask_server.shutdown()
+            cls.flask_server.server_close()
             cls.flask_thread.join(timeout=2)
 
     @classmethod
@@ -350,10 +352,9 @@ class TestImagePIL(unittest.TestCase):
         """Start a Flask server for URL testing."""
         import urllib.request
         import urllib.error
+        from werkzeug.serving import make_server
 
         cls.flask_app = Flask(__name__)
-        cls.flask_port = 5555  # Use a specific port for testing
-        cls.flask_server_running = True
 
         @cls.flask_app.route('/test.jpg')
         def serve_test_image():
@@ -363,11 +364,12 @@ class TestImagePIL(unittest.TestCase):
         def health_check():
             return 'OK', 200
 
-        def run_flask():
-            cls.flask_app.run(host='127.0.0.1', port=cls.flask_port, debug=False,
-                              use_reloader=False, threaded=True)
+        # Bind to an ephemeral port so concurrent/leftover test runs can't clash
+        cls.flask_server = make_server('127.0.0.1', 0, cls.flask_app, threaded=True)
+        cls.flask_port = cls.flask_server.server_port
+        cls.flask_server_running = True
 
-        cls.flask_thread = threading.Thread(target=run_flask, daemon=True)
+        cls.flask_thread = threading.Thread(target=cls.flask_server.serve_forever, daemon=True)
         cls.flask_thread.start()
 
         # Wait for server to be ready with health check
@@ -379,12 +381,15 @@ class TestImagePIL(unittest.TestCase):
             try:
                 with urllib.request.urlopen(f'http://127.0.0.1:{cls.flask_port}/health', timeout=1) as response:
                     if response.status == 200:
-                        break
+                        return
             except (urllib.error.URLError, ConnectionRefusedError, OSError):
                 pass
             time.sleep(wait_interval)
             elapsed += wait_interval
 
+        raise RuntimeError(
+            f"Test Flask server did not become ready on port {cls.flask_port} within {max_wait}s")
+
     def test_image_url_detection(self):
         """Test URL detection functionality."""
         img = Image()