diff --git a/scripts/boot-public-runtime.ps1 b/scripts/boot-public-runtime.ps1
index ff9b33c..8704cd8 100644
--- a/scripts/boot-public-runtime.ps1
+++ b/scripts/boot-public-runtime.ps1
@@ -240,8 +240,15 @@ function Test-VoiceSidecarStack {
     "--tts-language", "KR",
     "--timeout-seconds", "5"
   )
-  & $Python @probeArgs 1>$null 2>$null
-  return $LASTEXITCODE -eq 0
+  $previousErrorActionPreference = $ErrorActionPreference
+  try {
+    $ErrorActionPreference = "Continue"
+    & $Python @probeArgs 1>$null 2>$null
+    $probeExit = $LASTEXITCODE
+  } finally {
+    $ErrorActionPreference = $previousErrorActionPreference
+  }
+  return $probeExit -eq 0
 }
 
 Write-BootLog "================ boot start ================"
diff --git a/scripts/start-public-runtime.ps1 b/scripts/start-public-runtime.ps1
index 3f5c735..e6c3efe 100644
--- a/scripts/start-public-runtime.ps1
+++ b/scripts/start-public-runtime.ps1
@@ -139,8 +139,15 @@ function Test-VoiceSidecarReady {
     "--tts-language", $MeloTtsLanguage,
     "--timeout-seconds", "5"
   )
-  & $Python @probeArgs 1>$null 2>$null
-  return $LASTEXITCODE -eq 0
+  $previousErrorActionPreference = $ErrorActionPreference
+  try {
+    $ErrorActionPreference = "Continue"
+    & $Python @probeArgs 1>$null 2>$null
+    $probeExit = $LASTEXITCODE
+  } finally {
+    $ErrorActionPreference = $previousErrorActionPreference
+  }
+  return $probeExit -eq 0
 }
 
 function Wait-VoiceSidecarReady {
diff --git a/scripts/test_public_runtime_watchdog_provenance.py b/scripts/test_public_runtime_watchdog_provenance.py
index 86dee61..4807e16 100644
--- a/scripts/test_public_runtime_watchdog_provenance.py
+++ b/scripts/test_public_runtime_watchdog_provenance.py
@@ -24,6 +24,7 @@ INSTALLER_SOURCE = INSTALLER.read_text(encoding="utf-8")
 BOOT_SOURCE = BOOT.read_text(encoding="utf-8")
 BOOT_REGISTER_SOURCE = BOOT_REGISTER.read_text(encoding="utf-8")
 HIDDEN_TRIGGER_SOURCE = HIDDEN_TRIGGER.read_text(encoding="utf-8")
+START_SOURCE = START.read_text(encoding="utf-8")
 RUNBOOK_SOURCE = RUNBOOK.read_text(encoding="utf-8")
 LOCAL_DEVELOPMENT_SOURCE = LOCAL_DEVELOPMENT.read_text(encoding="utf-8")
 
@@ -264,6 +265,79 @@ if ($result.Detail -ne 'exact readiness probe failed') {
                 msg=f"stdout={completed.stdout}\nstderr={completed.stderr}",
             )
 
+    def test_boolean_sidecar_probes_survive_native_stderr_under_powershell_51(self) -> None:
+        powershell = shutil.which("powershell.exe")
+        python = shutil.which("python.exe")
+        if powershell is None or python is None:
+            self.skipTest("Windows PowerShell 5.1 and Python are required")
+
+        contracts = (
+            (
+                START_SOURCE[
+                    START_SOURCE.index("function Test-VoiceSidecarReady") :
+                    START_SOURCE.index("function Wait-VoiceSidecarReady")
+                ].strip(),
+                "$result = Test-VoiceSidecarReady -Component 'stt'",
+                "start",
+            ),
+            (
+                BOOT_SOURCE[
+                    BOOT_SOURCE.index("function Test-VoiceSidecarStack") :
+                    BOOT_SOURCE.index('Write-BootLog "================ boot start')
+                ].strip(),
+                "$result = Test-VoiceSidecarStack",
+                "boot",
+            ),
+        )
+        python_escaped = python.replace("'", "''")
+        probe_escaped = str(VOICE_PROBE).replace("'", "''")
+
+        with tempfile.TemporaryDirectory() as temporary_directory:
+            for contract, invocation, role in contracts:
+                harness = Path(temporary_directory) / f"{role}-probe-contract.ps1"
+                harness.write_text(
+                    "$ErrorActionPreference = 'Stop'\n"
+                    + f"$Python = '{python_escaped}'\n"
+                    + f"$VoiceSidecarProbe = '{probe_escaped}'\n"
+                    + f"$voiceSidecarProbe = '{probe_escaped}'\n"
+                    + "$WhisperPort = 1\n"
+                    + "$MeloTtsPort = 2\n"
+                    + "$WhisperModel = 'small'\n"
+                    + "$WhisperLanguage = 'ko'\n"
+                    + "$WhisperDevice = 'cpu'\n"
+                    + "$MeloTtsModel = 'melotts-korean'\n"
+                    + "$MeloTtsLanguage = 'KR'\n"
+                    + contract
+                    + "\n"
+                    + invocation
+                    + "\nif ($result) { throw 'unreachable sidecars were accepted' }\n",
+                    encoding="utf-8-sig",
+                )
+                completed = subprocess.run(
+                    [
+                        powershell,
+                        "-NoLogo",
+                        "-NoProfile",
+                        "-NonInteractive",
+                        "-ExecutionPolicy",
+                        "Bypass",
+                        "-File",
+                        str(harness),
+                    ],
+                    capture_output=True,
+                    text=True,
+                    encoding="utf-8",
+                    errors="replace",
+                    timeout=30,
+                    check=False,
+                )
+                with self.subTest(role=role):
+                    self.assertEqual(
+                        completed.returncode,
+                        0,
+                        msg=f"stdout={completed.stdout}\nstderr={completed.stderr}",
+                    )
+
     def test_hidden_trigger_never_executes_a_workspace_script_directly(self) -> None:
         for expected in (
             "VignettePublicRuntimeWatchdog",