diff --git a/tests/unit/test_block_step_sync.py b/tests/unit/test_block_step_sync.py
index 51b76ca..3590fde 100644
--- a/tests/unit/test_block_step_sync.py
+++ b/tests/unit/test_block_step_sync.py
@@ -1,6 +1,7 @@
 """Host contract of PeriodicBlockStepper.step(n): one stream wait per call (mocked driver)."""
 import ast
 from contextlib import nullcontext
+import importlib.util
 from pathlib import Path
 import types
 from unittest.mock import Mock
@@ -8,14 +9,15 @@ import unittest
 
 import numpy as np
 
-import CelerisLab.lbm.block_stepper as b
 from CelerisLab.config import LBMConfig
 from CelerisLab.lbm.blocks import BlockLayout2D
 
 
 def host_module():
     """block_stepper with pycuda/compiler replaced by mocks; no GPU context."""
-    path = Path(b.__file__)
+    # Locate the source without importing it: importing block_stepper loads pycuda.driver,
+    # which CPU-only import-boundary tests (test_build_contracts) must never see.
+    path = Path(importlib.util.find_spec("CelerisLab.lbm.block_stepper").origin)
     tree = ast.parse(path.read_text())
     tree.body = [node for node in tree.body if not (
         isinstance(node, ast.Import) and any(a.name == "pycuda.driver" for a in node.names)