diff --git a/docs/compaction-refresh/probes/p3_d4_focused_run.py b/docs/compaction-refresh/probes/p3_d4_focused_run.py
index edf7280..b256888 100644
--- a/docs/compaction-refresh/probes/p3_d4_focused_run.py
+++ b/docs/compaction-refresh/probes/p3_d4_focused_run.py
@@ -510,7 +510,8 @@ def launch_verified_pi(
 ) -> PiRpc:
     command = [
         sys.executable,
-        "-I",
+        # -s preserves sys.path[0]=pinned/ for the launcher's sibling helper.
+        "-s",
         "-B",
         str(launcher),
         "--runtime",
@@ -726,6 +727,7 @@ def isolated_environment(
         "MOSAIC_PI_FORCE_SKILLS": "",
         "PI_SKIP_VERSION_CHECK": "1",
         "PYTHONDONTWRITEBYTECODE": "1",
+        "PYTHONNOUSERSITE": "1",
     }
     if "PI_CODING_AGENT" in os.environ:
         environment["PI_CODING_AGENT"] = os.environ["PI_CODING_AGENT"]