diff --git a/.claude/skills/write-sglang-test/SKILL.md b/.claude/skills/write-sglang-test/SKILL.md
index 93bf5b786..51ab8919c 100644
--- a/.claude/skills/write-sglang-test/SKILL.md
+++ b/.claude/skills/write-sglang-test/SKILL.md
@@ -172,7 +172,7 @@ if __name__ == "__main__":
     unittest.main()
 ```
 
-Use `unittest.mock.patch` / `MagicMock` to mock dependencies and isolate the logic under test. If the module transitively imports GPU-only packages (e.g. `sgl_kernel`), they can be stubbed so the test runs on CPU CI. See `test/registered/unit/README.md` for details and examples.
+Use `unittest.mock.patch` / `MagicMock` to mock dependencies and isolate the logic under test. If the module transitively imports GPU-only packages (e.g. `sgl_kernel`), they can be stubbed so the test runs on CPU CI. Do not modify `sys.modules` at module level — use `patch.dict` (as a class decorator or with `start`/`stop`) to ensure cleanup and avoid cross-test pollution. See `test/registered/unit/README.md` for details and examples.
 
 **Quality bar** — test real logic (validation boundaries, state transitions, error paths, branching, etc.). Skip tests that just verify Python itself works (e.g., "does calling an abstract method raise `NotImplementedError`?", "does a dataclass store the field I assigned?"). Consolidate repetitive patterns into parameterized tests. No production code changes in test PRs.
 
diff --git a/test/registered/unit/README.md b/test/registered/unit/README.md
index f5023b431..914f28639 100644
--- a/test/registered/unit/README.md
+++ b/test/registered/unit/README.md
@@ -81,7 +81,11 @@ from sglang.srt.managers.scheduler import Scheduler
 register_cpu_ci(est_time=2, suite="stage-a-test-cpu")
 ```
 
-The same pattern can be applied to other GPU-only packages: try importing the real package, and if it fails, register a `sys.meta_path` finder that stubs it. See `maybe_stub_sgl_kernel()` in `python/sglang/test/test_utils.py` for the implementation.
+The same pattern (`sys.meta_path` finder) can be applied to other GPU-only packages.
+See `maybe_stub_sgl_kernel()` in `python/sglang/test/test_utils.py` for the
+implementation. Do not directly mutate `sys.modules` at module level — pytest
+imports all test files before running any, so such mutations pollute the entire
+process. If you must stub, use `patch.dict("sys.modules", ...)` with proper cleanup.
 
 ## Rules