diff --git a/docs/compaction-refresh/probes/p3_d4_focused_run.py b/docs/compaction-refresh/probes/p3_d4_focused_run.py
index 6f8643d..4e465c3 100644
--- a/docs/compaction-refresh/probes/p3_d4_focused_run.py
+++ b/docs/compaction-refresh/probes/p3_d4_focused_run.py
@@ -24,6 +24,12 @@ from pathlib import Path
 from typing import Any, Callable
 
 HERE = Path(__file__).resolve().parent
+# WI-2 is deliberately kept in its reviewed worktree until the release package
+# contains the gated launcher.  This harness pins the source it will execute;
+# falling back to the released `mosaic` binary is forbidden.
+GATED_WI_ROOT = HERE.parents[3].parent / "stack-827-probe3"
+GATED_WI_HEAD = "abd2791f59b3f06f46dd08e55298ced72f6aa7c2"
+GATED_LAUNCHER = GATED_WI_ROOT / "packages/mosaic/framework/tools/lease-broker/launch-runtime.py"
 
 
 class PiRpc:
@@ -220,6 +226,30 @@ export default function register(pi: ExtensionAPI): void {
     )
 
 
+def gated_launcher_precondition(root: Path, socket_path: Path, environment: dict[str, str]) -> None:
+    """Refuse to launch unless the exact WI launcher binds this run's fixture."""
+
+    if environment.get("MOSAIC_LEASE_BROKER_SOCKET") != str(socket_path):
+        raise RuntimeError("D4 precondition: lease broker socket is not this fixture")
+    if socket_path.parent != root or root.parent != Path(tempfile.gettempdir()):
+        raise RuntimeError("D4 precondition: fixture socket is outside this run's temporary root")
+    try:
+        head = subprocess.check_output(
+            ["git", "-C", str(GATED_WI_ROOT), "rev-parse", "HEAD"], text=True
+        ).strip()
+        launcher_source = GATED_LAUNCHER.read_text(encoding="utf-8")
+    except (OSError, subprocess.CalledProcessError) as error:
+        raise RuntimeError("D4 precondition: gated WI launcher is unavailable") from error
+    if head != GATED_WI_HEAD:
+        raise RuntimeError(f"D4 precondition: gated WI head mismatch: {head}")
+
+    register = launcher_source.find('"action": "register_anchor"')
+    execute = launcher_source.find("execute(command[0], command, environment)")
+    fixture_socket = launcher_source.find('source_environment["MOSAIC_LEASE_BROKER_SOCKET"]')
+    if register < 0 or execute < 0 or register >= execute or fixture_socket < 0:
+        raise RuntimeError("D4 precondition: launcher is not register-before-exec fixture-bound")
+
+
 def assert_d4(records: list[dict[str, Any]]) -> dict[str, object]:
     lifecycle = [record for record in records if record.get("event") == "runtime_generation_bump"]
     promotion = next(record for record in records if record.get("event") == "probe_lease_promoted")
@@ -293,20 +323,6 @@ def run_once(index: int) -> Path:
     extension = root / "d4_extension.ts"
     write_extension(extension)
 
-    broker = subprocess.Popen(
-        [
-            sys.executable,
-            str(HERE / "p3_generation_broker.py"),
-            "--socket",
-            str(socket_path),
-            "--log",
-            str(generation_log),
-        ],
-        text=True,
-        stdout=subprocess.PIPE,
-        stderr=subprocess.STDOUT,
-    )
-    wait_path(socket_path)
     environment = os.environ.copy()
     # Do not let the gated launcher inherit or fall back to its live/default
     # broker.  Both its registration handshake and D4 lifecycle use this one
@@ -321,10 +337,30 @@ def run_once(index: int) -> Path:
             "PI_SKIP_VERSION_CHECK": "1",
         }
     )
+    # Must run before the fixture broker or Pi process is launched.  It proves
+    # the launcher registers before exec and can only read this fixture socket.
+    gated_launcher_precondition(root, socket_path, environment)
+    broker = subprocess.Popen(
+        [
+            sys.executable,
+            str(HERE / "p3_generation_broker.py"),
+            "--socket",
+            str(socket_path),
+            "--log",
+            str(generation_log),
+        ],
+        text=True,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.STDOUT,
+    )
+    wait_path(socket_path)
     pi = PiRpc(
         [
-            "mosaic",
-            "yolo",
+            sys.executable,
+            str(GATED_LAUNCHER),
+            "--runtime",
+            "pi",
+            "--",
             "pi",
             "--mode",
             "rpc",