diff --git a/nanobot/agent/tools/_windows_job.py b/nanobot/agent/tools/_windows_job.py
index 83a7111f6..19ca985d3 100644
--- a/nanobot/agent/tools/_windows_job.py
+++ b/nanobot/agent/tools/_windows_job.py
@@ -181,18 +181,17 @@ class WindowsJob:
             self.close()
             raise error
 
-        assigned = False
+        if not _kernel32.AssignProcessToJobObject(self._handle, process):
+            error = _win_error("AssignProcessToJobObject")
+            _kernel32.TerminateProcess(process, 1)
+            _close_handle(process)
+            self.close()
+            raise error
+
         try:
-            if not _kernel32.AssignProcessToJobObject(self._handle, process):
-                raise _win_error("AssignProcessToJobObject")
-            assigned = True
             _resume_primary_thread(pid)
         except Exception:
-            if assigned:
-                self.terminate()
-            else:
-                _kernel32.TerminateProcess(process, 1)
-                self.close()
+            self.terminate()
             raise
         finally:
             _close_handle(process)
diff --git a/nanobot/agent/tools/shell.py b/nanobot/agent/tools/shell.py
index 8583968d1..9dfda7db9 100644
--- a/nanobot/agent/tools/shell.py
+++ b/nanobot/agent/tools/shell.py
@@ -12,7 +12,7 @@ import sys
 from contextlib import suppress
 from dataclasses import dataclass
 from pathlib import Path, PureWindowsPath
-from typing import Any, Protocol, cast, runtime_checkable
+from typing import Any, Protocol, cast
 
 from loguru import logger
 from pydantic import Field
@@ -45,14 +45,11 @@ _IS_WINDOWS = sys.platform == "win32"
 _PROCESS_TREE_OWNER_ATTR = "_nanobot_process_tree_owner"
 
 
-@runtime_checkable
 class _ProcessTreeOwner(Protocol):
     creation_flags: int
 
     def assign_and_resume(self, pid: int) -> None: ...
 
-    def close(self) -> None: ...
-
     def release(self) -> None: ...
 
     def terminate(self) -> None: ...
@@ -554,6 +551,7 @@ class ExecTool(Tool):
         """Launch *command* in a platform-appropriate shell."""
         if _IS_WINDOWS:
             windows_job = None
+            process = None
             creation_flags = 0
             if process_tree and sys.platform == "win32":
                 windows_job = ExecTool._create_windows_job()
@@ -601,6 +599,8 @@ class ExecTool(Tool):
             except BaseException:
                 if windows_job is not None:
                     windows_job.terminate()
+                if process is not None:
+                    await ExecTool._kill_process(process)
                 raise
         shell_program = shell_program or shutil.which("bash") or "/bin/bash"
         args: list[str] = [shell_program]
@@ -757,11 +757,8 @@ class ExecTool(Tool):
     def _process_tree_owner(
         process: asyncio.subprocess.Process,
     ) -> _ProcessTreeOwner | None:
-        attributes = getattr(process, "__dict__", None)
-        if not isinstance(attributes, dict):
-            return None
-        owner = cast(dict[str, object], attributes).get(_PROCESS_TREE_OWNER_ATTR)
-        return owner if isinstance(owner, _ProcessTreeOwner) else None
+        # _spawn is the only writer for this private ownership marker.
+        return cast(_ProcessTreeOwner | None, vars(process).get(_PROCESS_TREE_OWNER_ATTR))
 
     @staticmethod
     def _create_windows_job() -> _ProcessTreeOwner:
diff --git a/tests/tools/test_exec_platform.py b/tests/tools/test_exec_platform.py
index 49ddf7e75..890c8b29b 100644
--- a/tests/tools/test_exec_platform.py
+++ b/tests/tools/test_exec_platform.py
@@ -8,7 +8,7 @@ platform-specific binaries (all subprocess calls are mocked).
 import asyncio
 import shutil
 import sys
-from unittest.mock import AsyncMock, patch
+from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
 
@@ -27,9 +27,6 @@ class _FakeWindowsJob:
     def assign_and_resume(self, pid: int) -> None:
         pass
 
-    def close(self) -> None:
-        pass
-
     def release(self) -> None:
         pass
 
@@ -149,6 +146,32 @@ class TestSpawnUnix:
 
 class TestSpawnWindows:
 
+    @pytest.mark.asyncio
+    async def test_job_assignment_failure_kills_suspended_process(self):
+        env = {"PATH": ""}
+        process = AsyncMock()
+        process.pid = 123
+        process.returncode = None
+        process.kill = MagicMock()
+        process.wait.return_value = -9
+        job = MagicMock(spec=_FakeWindowsJob)
+        job.creation_flags = 0x4
+        job.assign_and_resume.side_effect = OSError("OpenProcess failed")
+
+        with (
+            patch("nanobot.agent.tools.shell._IS_WINDOWS", True),
+            patch("nanobot.agent.tools.shell.sys.platform", "win32"),
+            patch("asyncio.create_subprocess_exec", new_callable=AsyncMock) as mock_exec,
+            patch.object(ExecTool, "_create_windows_job", return_value=job),
+            pytest.raises(OSError, match="OpenProcess failed"),
+        ):
+            mock_exec.return_value = process
+            await ExecTool._spawn("echo hi", r"C:\work", env, process_tree=True)
+
+        job.terminate.assert_called_once_with()
+        process.kill.assert_called_once_with()
+        process.wait.assert_awaited_once_with()
+
     @pytest.mark.asyncio
     async def test_single_line_uses_powershell(self):
         """Single-line commands on Windows now route through PowerShell."""