diff --git a/headroom/install/runtime.py b/headroom/install/runtime.py
index d6fd560a5..57fe8d00f 100644
--- a/headroom/install/runtime.py
+++ b/headroom/install/runtime.py
@@ -173,6 +173,26 @@ def _read_pid(profile: str) -> int | None:
         return None
 
 
+def _pid_alive(pid: int) -> bool:
+    """Return True if ``pid`` names a live process."""
+
+    if pid <= 0:
+        return False
+    try:
+        import psutil
+
+        return bool(psutil.pid_exists(pid))
+    except Exception:
+        pass
+    try:
+        os.kill(pid, 0)
+    except PermissionError:
+        return True
+    except (ProcessLookupError, OSError, SystemError):
+        return False
+    return True
+
+
 def _clear_pid(profile: str) -> None:
     path = pid_path(profile)
     if path.exists():
diff --git a/tests/test_install/test_runtime.py b/tests/test_install/test_runtime.py
index c9f144fc4..5705dd48f 100644
--- a/tests/test_install/test_runtime.py
+++ b/tests/test_install/test_runtime.py
@@ -14,6 +14,7 @@
     _clear_pid,
     _deployment_env,
     _mount_source,
+    _pid_alive,
     _read_pid,
     _runtime_env,
     _write_pid,
@@ -253,6 +254,34 @@ def test_write_read_and_clear_pid(monkeypatch, tmp_path: Path) -> None:
     assert _read_pid("default") is None
 
 
+def test_pid_alive_uses_psutil_without_signal(monkeypatch) -> None:
+    fake_psutil = types.ModuleType("psutil")
+    fake_psutil.pid_exists = lambda pid: True if pid > 0 else False  # type: ignore[attr-defined]
+    monkeypatch.setitem(sys.modules, "psutil", fake_psutil)
+    monkeypatch.setattr(
+        "headroom.install.runtime.os.kill",
+        lambda pid, sig: (_ for _ in ()).throw(AssertionError("os.kill should not run")),
+    )
+
+    assert _pid_alive(123) is True
+
+
+def test_pid_alive_falls_back_and_handles_systemerror(monkeypatch) -> None:
+    fake_psutil = types.ModuleType("psutil")
+
+    def _raise_pid_exists(pid: int) -> bool:
+        raise RuntimeError("boom")
+
+    fake_psutil.pid_exists = _raise_pid_exists  # type: ignore[attr-defined]
+    monkeypatch.setitem(sys.modules, "psutil", fake_psutil)
+    monkeypatch.setattr(
+        "headroom.install.runtime.os.kill",
+        lambda pid, sig: (_ for _ in ()).throw(SystemError("WinError 87")),
+    )
+
+    assert _pid_alive(123) is False
+
+
 def test_runtime_start_lock_is_nonblocking(monkeypatch, tmp_path: Path) -> None:
     monkeypatch.setattr(Path, "home", lambda: tmp_path)
 
@@ -272,7 +301,12 @@ def test_runtime_start_lock_blocks_another_process(monkeypatch, tmp_path: Path)
         "with acquire_runtime_start_lock('default') as acquired:\n"
         "    print(acquired)\n"
     )
-    env = {**os.environ, "HOME": str(tmp_path), "PYTHONPATH": str(Path.cwd())}
+    env = {
+        **os.environ,
+        "HOME": str(tmp_path),
+        "USERPROFILE": str(tmp_path),
+        "PYTHONPATH": str(Path.cwd()),
+    }
 
     with acquire_runtime_start_lock("default") as acquired:
         assert acquired is True
@@ -519,6 +553,8 @@ def __init__(self, stdout: str = "") -> None:
     assert runtime_status(python_manifest) == "stopped"
 
     _write_pid("default", 125)
+    monkeypatch.setattr("headroom.install.runtime.pid_alive", lambda pid: True)
+    assert runtime_status(python_manifest) == "running"
     monkeypatch.setattr("headroom.install.runtime.pid_alive", lambda pid: False)
     assert runtime_status(python_manifest) == "stopped"