diff --git a/CONTRIBUTING.rst b/CONTRIBUTING.rst
index 8b8fc0e791..21c09fada0 100644
--- a/CONTRIBUTING.rst
+++ b/CONTRIBUTING.rst
@@ -40,6 +40,16 @@ When modifying driver files, rebuilding Cython modules is often necessary.
 Without caching, each such rebuild may take over a minute. Caching usually brings it
 down to about 2-3 seconds.
 
+**Important:** After modifying any ``.py`` file under ``cassandra/`` that is
+Cython-compiled (such as ``query.py``, ``protocol.py``, ``cluster.py``, etc.),
+you **must** rebuild extensions before running tests::
+
+    python setup.py build_ext --inplace
+
+Without rebuilding, Python will load the stale compiled extension (``.so`` / ``.pyd``)
+instead of your modified ``.py`` source, and your changes will not actually be tested.
+The test suite will emit a warning if it detects this situation.
+
 Building the Docs
 =================
 
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000000..b6bf42677d
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,66 @@
+# Copyright ScyllaDB, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import importlib.machinery
+import os
+import warnings
+
+# Directory containing the Cython-compiled driver modules.
+_CASSANDRA_DIR = os.path.join(os.path.dirname(__file__), os.pardir, "cassandra")
+
+
+def pytest_configure(config):
+    """Warn when a compiled Cython extension is older than its .py source.
+
+    Python's import system prefers compiled extensions (.so / .pyd) over pure
+    Python (.py) files.  If a developer edits a .py file without rebuilding
+    the Cython extensions (``python setup.py build_ext --inplace``), the tests
+    will silently run the *old* compiled code, masking any regressions in the
+    Python source.
+
+    This hook detects such staleness at test-session startup so the developer
+    is alerted immediately.
+    """
+    stale = []
+    # Iterate over .py sources and, for each module, look for the first
+    # existing compiled extension in EXTENSION_SUFFIXES order.  This mirrors
+    # how Python's import machinery selects an extension module, and avoids
+    # globbing patterns like "*{suffix}" that can pick up ABI-tagged
+    # extensions built for other Python versions.
+    if os.path.isdir(_CASSANDRA_DIR):
+        for entry in os.listdir(_CASSANDRA_DIR):
+            if not entry.endswith(".py"):
+                continue
+            module_name, _ = os.path.splitext(entry)
+            py_path = os.path.join(_CASSANDRA_DIR, entry)
+            # For this module, find the first extension file Python would load.
+            for suffix in importlib.machinery.EXTENSION_SUFFIXES:
+                ext_path = os.path.join(_CASSANDRA_DIR, module_name + suffix)
+                if not os.path.exists(ext_path):
+                    continue
+                if os.path.getmtime(py_path) > os.path.getmtime(ext_path):
+                    stale.append((module_name, ext_path, py_path))
+                # Only consider the first matching suffix; this is the one
+                # the import system would actually use.
+                break
+
+    if stale:
+        names = ", ".join(m for m, _, _ in stale)
+        warnings.warn(
+            f"Stale Cython extension(s) detected: {names}. "
+            f"The .py source is newer than the compiled extension — tests "
+            f"will run the OLD compiled code, not your latest changes. "
+            f"Rebuild with:  python setup.py build_ext --inplace",
+            stacklevel=1,
+        )