diff --git a/conftest.py b/conftest.py
index acade85f..ab3c2886 100644
--- a/conftest.py
+++ b/conftest.py
@@ -1,5 +1,14 @@
+import os
+
 import pytest
 
+# The planner's numba kernels are compiled with fastmath for the host CPU, so an rsim game played
+# on CI's CPU can drift from the same game played locally, and a test that asserts how a game
+# unfolds passes on one machine and fails on the other. Compile them for a generic x86-64 CPU in
+# tests, so every machine runs the same code. Set before numba is imported; rsim worker
+# processes inherit it.
+os.environ.setdefault("NUMBA_CPU_NAME", "generic")
+
 
 def pytest_addoption(parser):
     parser.addoption(