Fix Windows exec UTF-16 output decoding

nanobot/agent/tools/shell.py

@@ -65,6 +65,17 @@ def _reap_pid(pid: int) -> None:
         logger.debug("_reap_pid({}): {}", pid, exc)
 
 
+def _decode_process_output(data: bytes) -> str:
+    if not data:
+        return ""
+    if _IS_WINDOWS and b"\x00" in data[:200]:
+        try:
+            return data.decode("utf-16")
+        except UnicodeDecodeError:
+            pass
+    return data.decode("utf-8", errors="replace")
+
+
 # Policy note appended to recoverable workspace-boundary guard errors.
 _WORKSPACE_BOUNDARY_NOTE = (
     "\n\nNote: this is a hard policy boundary, not a transient failure. "
@@ -337,10 +348,10 @@ class ExecTool(Tool):
             output_parts = []
 
             if stdout:
-                output_parts.append(stdout.decode("utf-8", errors="replace"))
+                output_parts.append(_decode_process_output(stdout))
 
             if stderr:
-                stderr_text = stderr.decode("utf-8", errors="replace")
+                stderr_text = _decode_process_output(stderr)
                 if stderr_text.strip():
                     output_parts.append(f"STDERR:\n{stderr_text}")
 

tests/tools/test_exec_platform.py

@@ -465,6 +465,29 @@ class TestExecuteEndToEnd:
         assert "hello world" in result
         assert "Exit code: 0" in result
 
+    @pytest.mark.asyncio
+    async def test_windows_decodes_utf16_output(self):
+        """PowerShell output may arrive as UTF-16LE on Windows."""
+        mock_proc = AsyncMock()
+        mock_proc.communicate.return_value = (
+            "ok café\r\n".encode("utf-16-le"),
+            "warn λ\r\n".encode("utf-16-le"),
+        )
+        mock_proc.returncode = 0
+
+        with (
+            patch("nanobot.agent.tools.shell._IS_WINDOWS", True),
+            patch.object(ExecTool, "_spawn", return_value=mock_proc),
+            patch.object(ExecTool, "_guard_command", return_value=None),
+        ):
+            tool = ExecTool()
+            result = await tool.execute(command="echo ok")
+
+        assert "ok café" in result
+        assert "warn λ" in result
+        assert "\x00" not in result
+        assert "Exit code: 0" in result
+
     @pytest.mark.asyncio
     async def test_unix_full_path(self):
         """Full execute() flow on Unix: env, spawn, output formatting."""