diff --git a/selfdrive/test/fuzzy_generation.py b/selfdrive/test/fuzzy_generation.py
index 94eb0dfaa6..c9f2353713 100644
--- a/selfdrive/test/fuzzy_generation.py
+++ b/selfdrive/test/fuzzy_generation.py
@@ -1,11 +1,10 @@
 import capnp
 import hypothesis.strategies as st
 from typing import Any
+from types import ModuleType
 from collections.abc import Callable
 from functools import cache
 
-from cereal import log
-
 DrawType = Callable[[st.SearchStrategy], Any]
 
 
@@ -76,6 +75,6 @@ class FuzzyGenerator:
     return data
 
   @classmethod
-  def get_random_event_msg(cls, draw: DrawType, events: list[str], real_floats: bool = False) -> list[dict[str, Any]]:
+  def get_random_event_msg(cls, draw: DrawType, log_schema: ModuleType, events: list[str], real_floats: bool = False) -> list[dict[str, Any]]:
     fg = cls(draw, real_floats=real_floats)
-    return [draw(fg.generate_struct(log.Event.schema, e)) for e in sorted(events)]
+    return [draw(fg.generate_struct(log_schema.Event.schema, e)) for e in sorted(events)]
diff --git a/selfdrive/test/process_replay/test_fuzzy.py b/selfdrive/test/process_replay/test_fuzzy.py
index 723112163e..b6ee5a7b3f 100644
--- a/selfdrive/test/process_replay/test_fuzzy.py
+++ b/selfdrive/test/process_replay/test_fuzzy.py
@@ -25,7 +25,7 @@ class TestFuzzProcesses:
   @settings(phases=[Phase.generate, Phase.target], max_examples=MAX_EXAMPLES, deadline=1000,
             suppress_health_check=[HealthCheck.too_slow, HealthCheck.data_too_large])
   def test_fuzz_process(self, proc_name, cfg, data):
-    msgs = FuzzyGenerator.get_random_event_msg(data.draw, events=cfg.pubs, real_floats=True)
+    msgs = FuzzyGenerator.get_random_event_msg(data.draw, log_schema=log, events=cfg.pubs, real_floats=True)
     lr = [log.Event.new_message(**m).as_reader() for m in msgs]
     cfg.timeout = 5
     pr.replay_process(cfg, lr, fingerprint=TOYOTA.TOYOTA_COROLLA_TSS2, disable_progress=True)
diff --git a/selfdrive/test/process_replay/test_log_compat.py b/selfdrive/test/process_replay/test_log_compat.py
new file mode 100644
index 0000000000..fdc3b79653
--- /dev/null
+++ b/selfdrive/test/process_replay/test_log_compat.py
@@ -0,0 +1,48 @@
+import os
+import capnp
+import pytest
+import shutil
+import hypothesis.strategies as st
+from hypothesis import given, settings, HealthCheck
+
+from cereal import CEREAL_PATH
+from openpilot.selfdrive.test.fuzzy_generation import FuzzyGenerator
+from openpilot.tools.lib.logreader import LogReader
+from openpilot.common.run import run_cmd
+
+MAX_EXAMPLES = int(os.environ.get("MAX_EXAMPLES", "10"))
+TARGET_REMOTE = os.environ.get("TARGET_REMOTE", "origin")
+TARGET_BRANCH = os.environ.get("TARGET_BRANCH", "master")
+
+
+@pytest.fixture(scope="module")
+def parent_schema_file(tmp_path_factory):
+  tmp_dir = tmp_path_factory.mktemp("cereal")
+  # FIXME this is an ugly way to do this, but for some reason capnp.load ignores the `imports``, and only looks at dir where the file is
+  # how it supposed to work is: capnp.load(my_custom_log_capnp, imports=[CEREAL_PATH])
+  shutil.copytree(CEREAL_PATH, tmp_dir, dirs_exist_ok=True)
+
+  commit = run_cmd(["git", "merge-base", f"{TARGET_REMOTE}/{TARGET_BRANCH}", "HEAD"])
+  log_capnp_url = f"https://raw.githubusercontent.com/commaai/openpilot/{commit}/cereal/log.capnp"
+  tmp_log_capnp_path = tmp_dir / f"{commit}-log.capnp"
+  if not tmp_log_capnp_path.exists():
+    run_cmd(["curl", "-o", str(tmp_log_capnp_path), log_capnp_url])
+
+  return str(tmp_log_capnp_path)
+
+
+@given(st.data())
+@settings(max_examples=MAX_EXAMPLES, suppress_health_check=[HealthCheck.large_base_example])
+def test_log_backwards_compatibility(parent_schema_file, data):
+  # capnp global parser needs to be cleaned up to avoid schema/struct ID conflicts
+  capnp_parser = capnp.SchemaParser()
+  old_log = capnp_parser.load(os.path.abspath(parent_schema_file))
+
+  msgs_dicts = FuzzyGenerator.get_random_event_msg(data.draw, log_schema=old_log, events=old_log.Event.schema.union_fields, real_floats=True)
+  msgs = [old_log.Event.new_message(**m).as_reader() for m in msgs_dicts]
+  dat = b"".join(msg.as_builder().to_bytes() for msg in msgs)
+
+  lr = list(LogReader.from_bytes(dat))
+  assert len(lr) == len(msgs)
+  # calling which() on a removed union type will raise an exception
+  assert {m.which() for m in lr} == {msg.which() for msg in msgs}