diff --git a/nanobot/process_runtime.py b/nanobot/process_runtime.py
index 366963dac..b05514b12 100644
--- a/nanobot/process_runtime.py
+++ b/nanobot/process_runtime.py
@@ -351,7 +351,10 @@ class ManagedProcessRuntime(Generic[_StartOptionsT]):
         return process_is_running(pid, platform_name=self.platform_name)
 
     def _process_identity(self, pid: int) -> str | int | None:
-        if self.platform_name == "Windows":
+        # Process inspection must follow the host API even when tests inject a
+        # target platform.  On Windows, falling through to POSIX calls is not
+        # merely unsupported: ``os.kill(pid, 0)`` broadcasts CTRL_C_EVENT.
+        if _platform_name() == "Windows" or self.platform_name == "Windows":
             return _windows_process_identity(pid)
         try:
             process_group = os.getpgid(pid)
@@ -441,7 +444,8 @@ def process_is_running(pid: int, *, platform_name: str | None = None) -> bool:
     """Probe a PID without delivering a control event on Windows."""
     if pid <= 0:
         return False
-    if (platform_name or _platform_name()) == "Windows":
+    host_platform = _platform_name()
+    if host_platform == "Windows" or platform_name == "Windows":
         # On Windows ``os.kill(pid, 0)`` sends CTRL_C_EVENT (whose value is 0)
         # instead of performing the harmless POSIX existence probe.
         return _windows_process_identity(pid) is not None
diff --git a/tests/gateway/test_runtime.py b/tests/gateway/test_runtime.py
index cb24e5e5d..d1b6e642d 100644
--- a/tests/gateway/test_runtime.py
+++ b/tests/gateway/test_runtime.py
@@ -440,6 +440,37 @@ def test_windows_process_probe_never_sends_ctrl_c(monkeypatch):
     assert process_is_running(54321, platform_name="Windows") is False
 
 
+def test_windows_host_probe_stays_safe_when_target_platform_is_posix(monkeypatch):
+    monkeypatch.setattr("nanobot.process_runtime._platform_name", lambda: "Windows")
+    monkeypatch.setattr(
+        "nanobot.process_runtime._windows_process_identity",
+        lambda pid: "created-at" if pid == 12345 else None,
+    )
+    monkeypatch.setattr(
+        "nanobot.process_runtime.os.kill",
+        lambda *_args: pytest.fail("Windows process probes must not call os.kill(pid, 0)"),
+    )
+
+    assert process_is_running(12345, platform_name="Linux") is True
+    assert process_is_running(54321, platform_name="Darwin") is False
+
+
+def test_windows_host_identity_stays_safe_when_target_platform_is_posix(tmp_path, monkeypatch):
+    runtime = GatewayRuntime(paths=_paths(tmp_path), platform_name="Linux")
+    monkeypatch.setattr("nanobot.process_runtime._platform_name", lambda: "Windows")
+    monkeypatch.setattr(
+        "nanobot.process_runtime._windows_process_identity",
+        lambda pid: "created-at" if pid == 12345 else None,
+    )
+    monkeypatch.setattr(
+        "nanobot.process_runtime.os.getpgid",
+        lambda *_args: pytest.fail("Windows process identities must not use POSIX APIs"),
+        raising=False,
+    )
+
+    assert runtime.process_identity(12345) == "created-at"
+
+
 def test_status_clears_stale_state(tmp_path, monkeypatch):
     runtime = GatewayRuntime(paths=_paths(tmp_path), platform_name="Linux")
     runtime.paths.run_dir.mkdir(parents=True)