actually bubbles up original error w.o pickling TypeErrors wrapping it

src/core/runner.py

@@ -1,22 +1,28 @@
 import pickle
 import time
-from django.test.runner import DiscoverRunner, ParallelTestSuite, RemoteTestRunner
+from django.test.runner import DiscoverRunner, ParallelTestSuite, RemoteTestResult, RemoteTestRunner
 
 
-class _Py313SafeRemoteTestRunner(RemoteTestRunner):
+class _Py313SafeRemoteTestResult(RemoteTestResult):
     """Prevents Python 3.13 'cannot pickle traceback' crash in parallel workers.
 
     In Python 3.13, traceback objects are not picklable. Django's parallel
-    runner tries to pickle test errors to pass them back to the main process.
-    When a Selenium test crashes the browser, the resulting exception traceback
-    fails to pickle, aborting the entire parallel run with a confusing TypeError.
+    runner serialises test errors via multiprocessing (pickle) to send them
+    from worker subprocesses back to the main process. When a Selenium test
+    crashes the browser, the resulting exception traceback fails to pickle,
+    aborting the entire parallel run with a confusing TypeError.
 
     Fix: strip the traceback (replace with None) before it reaches pickle.
     Error type and message are preserved; only the traceback is lost in transit.
+
+    Note: the fix must be on RemoteTestResult (not RemoteTestRunner) because
+    addError/addFailure — where check_picklable is called — live on the result
+    object, not the runner. RemoteTestRunner.run() instantiates resultclass()
+    and passes it to the test suite; overriding it on the runner has no effect.
     """
 
     @staticmethod
-    def _strip_tb_if_needed(err):
+    def _safe_err(err):
         exc_type, exc_value, exc_tb = err
         try:
             pickle.dumps((exc_type, exc_value, exc_tb))
@@ -25,10 +31,14 @@ class _Py313SafeRemoteTestRunner(RemoteTestRunner):
         return err
 
     def addError(self, test, err):
-        super().addError(test, self._strip_tb_if_needed(err))
+        super().addError(test, self._safe_err(err))
 
     def addFailure(self, test, err):
-        super().addFailure(test, self._strip_tb_if_needed(err))
+        super().addFailure(test, self._safe_err(err))
+
+
+class _Py313SafeRemoteTestRunner(RemoteTestRunner):
+    resultclass = _Py313SafeRemoteTestResult
 
 
 class _SafeParallelTestSuite(ParallelTestSuite):