Fix: Preserve custom CookiePolicy in Session.prepare_request

src/requests/sessions.py

@@ -471,10 +471,20 @@ def prepare_request(self, request):
             cookies = cookiejar_from_dict(cookies)
 
         # Merge with session cookies
-        merged_cookies = merge_cookies(
-            merge_cookies(RequestsCookieJar(), self.cookies), cookies
-        )
-
+        # Preserve the cookie policy from the session's cookie jar
+        # Fixes issue where custom policies were lost during request preparation
+        if isinstance(self.cookies, RequestsCookieJar):
+            # Create a new jar with the same policy as the session's jar
+            jar = RequestsCookieJar()
+            jar.set_policy(self.cookies.get_policy())
+            merged_cookies = merge_cookies(
+                merge_cookies(jar, self.cookies), cookies
+            )
+        else:
+            # Fall back to original behavior for non-RequestsCookieJar instances
+            merged_cookies = merge_cookies(
+                merge_cookies(RequestsCookieJar(), self.cookies), cookies
+            )
         # Set environment's basic authentication if not explicitly set.
         auth = request.auth
         if self.trust_env and not auth and not self.auth:
@@ -496,7 +506,6 @@ def prepare_request(self, request):
             hooks=merge_hooks(request.hooks, self.hooks),
         )
         return p
-
     def request(
         self,
         method,

tests/test_requests.py

@@ -3038,3 +3038,4 @@ def test_json_decode_errors_are_serializable_deserializable():
     )
     deserialized_error = pickle.loads(pickle.dumps(json_decode_error))
     assert repr(json_decode_error) == repr(deserialized_error)
+