diff --git a/src/requests/sessions.py b/src/requests/sessions.py
index 731550de88..c7d06f7103 100644
--- a/src/requests/sessions.py
+++ b/src/requests/sessions.py
@@ -471,9 +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