diff --git a/scripts/run-test-wave.py b/scripts/run-test-wave.py
index 44a16c23c..b3998f541 100755
--- a/scripts/run-test-wave.py
+++ b/scripts/run-test-wave.py
@@ -27,6 +27,14 @@
 SKIPPED = re.compile(r"(?:^|, )(?P<skipped>[0-9]+) skipped")
 SLOW_SUITES = frozenset(("incremental", "store_arch", "daemon_runtime"))
 POLL_SECONDS = 0.05
+# Floor for how long an external Windows helper (taskkill.exe, powershell.exe)
+# may take to answer. This is deliberately NOT --kill-grace: that flag budgets
+# how long a doomed process may take to die, while this budgets process spawn
+# plus CIM startup on a loaded runner, which routinely exceeds a second. Wiring
+# the two together made a small kill grace flake the whole wave -- taskkill
+# timing out is reported as "could not prove cleanup", and the descendant probe
+# that runs afterwards fails closed on its own timeout.
+WINDOWS_HELPER_TIMEOUT_SECONDS = 30
 
 
 @dataclass
@@ -123,6 +131,10 @@ def start_suite(
     )
 
 
+def windows_helper_timeout(kill_grace: int) -> int:
+    return max(kill_grace, WINDOWS_HELPER_TIMEOUT_SECONDS)
+
+
 def windows_descendants(pid: int, timeout: int) -> bool:
     """True if any live process still claims `pid` as its parent.
 
@@ -167,7 +179,7 @@ def terminate_process_tree(active: ActiveSuite, kill_grace: int) -> None:
             # how a deliberately-hanging fixture suite reddened a release run.
             # taskkill /T cannot walk a tree from a dead PID, so prove cleanup
             # the only way still available -- nothing is parented to it.
-            if windows_descendants(process.pid, kill_grace):
+            if windows_descendants(process.pid, windows_helper_timeout(kill_grace)):
                 raise RuntimeError(
                     f"suite {active.name!r} leader exited leaving live descendants"
                 )
@@ -185,7 +197,7 @@ def terminate_process_tree(active: ActiveSuite, kill_grace: int) -> None:
                 stdin=subprocess.DEVNULL,
                 stdout=subprocess.DEVNULL,
                 stderr=subprocess.DEVNULL,
-                timeout=kill_grace,
+                timeout=windows_helper_timeout(kill_grace),
             )
         except (OSError, subprocess.TimeoutExpired):
             completed = None
diff --git a/tests/test_parallel_harness_contract.sh b/tests/test_parallel_harness_contract.sh
index a658dc56a..511fb25a0 100755
--- a/tests/test_parallel_harness_contract.sh
+++ b/tests/test_parallel_harness_contract.sh
@@ -256,6 +256,7 @@ python3 - "$scheduler" "$fixture" "$(command -v python3)" <<'PY'
 from __future__ import annotations
 
 import ctypes
+import importlib.util
 import os
 import pathlib
 import signal
@@ -275,6 +276,29 @@ release = barrier / "timeout_exit_race.release"
 descendant_path = fixture / "descendant.pid"
 
 
+def scheduler_wait_budget() -> int:
+    """Seconds to allow the scheduler to finish refusing.
+
+    On POSIX the refusal is signal-driven and lands well inside --kill-grace.
+    On Windows it costs external helper spawns (taskkill.exe, and powershell.exe
+    for the descendant probe), which the scheduler deliberately budgets with its
+    own floor rather than --kill-grace. Read that floor from the scheduler
+    instead of restating it: hard-coding a budget here silently turns a slow
+    runner into a harness failure the moment the two numbers drift apart. The
+    refusal path can spend the floor twice -- once proving descendants, once in
+    the cleanup re-entry -- so allow both plus interpreter startup.
+    """
+    if os.name != "nt":
+        return 8
+    spec = importlib.util.spec_from_file_location("cbm_run_test_wave", scheduler)
+    module = importlib.util.module_from_spec(spec)
+    # Register before exec: @dataclass resolves annotations through
+    # sys.modules[cls.__module__], which is None for an unregistered module.
+    sys.modules[spec.name] = module
+    spec.loader.exec_module(module)
+    return module.WINDOWS_HELPER_TIMEOUT_SECONDS * 2 + 10
+
+
 def process_state(pid: int) -> str:
     if os.name == "nt":
         handle = ctypes.windll.kernel32.OpenProcess(0x101000, False, pid)
@@ -370,7 +394,7 @@ try:
             raise SystemExit("FAIL: scheduler did not observe the forced leader exit")
         time.sleep(0.02)
     release.write_text("release\n", encoding="utf-8")
-    stdout, stderr = process.communicate(timeout=8)
+    stdout, stderr = process.communicate(timeout=scheduler_wait_budget())
 
     if os.name == "nt":
         # Assert the PROPERTY, not the wording. This used to require the phrase