diff --git a/docs/compaction-refresh/probes/p3_d4_focused_run.py b/docs/compaction-refresh/probes/p3_d4_focused_run.py
index 00f2f96..edf7280 100644
--- a/docs/compaction-refresh/probes/p3_d4_focused_run.py
+++ b/docs/compaction-refresh/probes/p3_d4_focused_run.py
@@ -495,6 +495,12 @@ def gated_launcher_precondition(
     return closure
 
 
+def reject_pinned_bytecode(pinned_directory: Path) -> None:
+    cache_directory = pinned_directory / "__pycache__"
+    if cache_directory.exists() or any(pinned_directory.rglob("*.pyc")):
+        raise RuntimeError("D4 precondition: pinned bytecode cache is forbidden")
+
+
 def launch_verified_pi(
     launcher: Path,
     workspace: Path,
@@ -504,6 +510,8 @@ def launch_verified_pi(
 ) -> PiRpc:
     command = [
         sys.executable,
+        "-I",
+        "-B",
         str(launcher),
         "--runtime",
         "pi",
@@ -523,6 +531,7 @@ def launch_verified_pi(
         "--extension",
         str(extension),
     ]
+    reject_pinned_bytecode(launcher.parent)
     # This is deliberately the statement immediately before Popen (inside
     # PiRpc): the fixture-pinned launcher bytes are re-hashed then executed.
     if hashlib.sha256(launcher.read_bytes()).hexdigest() != GATED_LAUNCHER_SHA256:
@@ -531,10 +540,16 @@ def launch_verified_pi(
 
 
 def launch_verified_broker(
-    broker_path: Path, generation_path: Path, socket_path: Path, log_path: Path
+    broker_path: Path,
+    generation_path: Path,
+    socket_path: Path,
+    log_path: Path,
+    environment: dict[str, str],
 ) -> subprocess.Popen[str]:
     command = [
         sys.executable,
+        "-I",
+        "-B",
         str(broker_path),
         "--socket",
         str(socket_path),
@@ -543,6 +558,7 @@ def launch_verified_broker(
         "--generation-module",
         str(generation_path),
     ]
+    reject_pinned_bytecode(broker_path.parent)
     if hashlib.sha256(broker_path.read_bytes()).hexdigest() != GATED_BROKER_SHA256:
         raise RuntimeError("D4 precondition: pinned broker hash mismatch")
     # The final helper re-hash is immediately adjacent to the broker Popen.
@@ -550,6 +566,7 @@ def launch_verified_broker(
         raise RuntimeError("D4 precondition: pinned helper hash mismatch")
     return subprocess.Popen(
         command,
+        env=environment,
         text=True,
         stdout=subprocess.PIPE,
         stderr=subprocess.STDOUT,
@@ -708,6 +725,7 @@ def isolated_environment(
         "MOSAIC_HOME": str(fixture_mosaic_home),
         "MOSAIC_PI_FORCE_SKILLS": "",
         "PI_SKIP_VERSION_CHECK": "1",
+        "PYTHONDONTWRITEBYTECODE": "1",
     }
     if "PI_CODING_AGENT" in os.environ:
         environment["PI_CODING_AGENT"] = os.environ["PI_CODING_AGENT"]
@@ -744,7 +762,7 @@ def run_once(index: int) -> Path:
         # the launcher registers before exec and can only read this fixture socket.
         closure = gated_launcher_precondition(root, socket_path, environment)
         broker = launch_verified_broker(
-            closure.broker, closure.generation, socket_path, generation_log
+            closure.broker, closure.generation, socket_path, generation_log, environment
         )
         wait_path(socket_path)
         pi = launch_verified_pi(closure.launcher, workspace, sessions, extension, environment)