diff --git a/tests/tools/test_computer_use.py b/tests/tools/test_computer_use.py
index 9f4a6a5ca0513..1ea2f20a25877 100644
--- a/tests/tools/test_computer_use.py
+++ b/tests/tools/test_computer_use.py
@@ -1572,8 +1572,11 @@ class FakeProc:
             returncode = 0
             stderr = ""
             # Daemon returns a path, not inline base64.
-            stdout = ('{"element_count": 7, "tree_markdown": "- [0] AXButton",'
-                      ' "screenshot_file_path": "%s"}' % str(shot))
+            stdout = json.dumps({
+                "element_count": 7,
+                "tree_markdown": "- [0] AXButton",
+                "screenshot_file_path": str(shot),
+            })
 
         import subprocess as _sp
         orig_run = _sp.run
@@ -1687,6 +1690,44 @@ def cli_call(name, args, timeout):
         assert cap.app == "Finder"
 
 
+class TestCliFallbackUtf8Decoding:
+    """Bug A: `_call_tool_via_cli` must force UTF-8 decoding for cua-driver
+    stdout so non-cp932 window titles do not break Japanese Windows hosts."""
+
+    def test_call_tool_via_cli_uses_utf8_with_replace(self):
+        from typing import Any, cast
+        from tools.computer_use.cua_backend import _CuaDriverSession
+
+        session = cast(Any, _CuaDriverSession.__new__(_CuaDriverSession))
+        captured_kw = {}
+
+        class FakeProc:
+            returncode = 0
+            stderr = ""
+            stdout = ('{"element_count": 2, "tree_markdown": '
+                      '"- [0] AXWindow \\"⛂ Claude Code\\""}')
+
+        import subprocess as _sp
+        orig_run = _sp.run
+
+        def fake_run(cmd, **kw):
+            captured_kw.update(kw)
+            return FakeProc()
+
+        _sp.run = fake_run
+        try:
+            out = session._call_tool_via_cli(
+                "get_window_state", {"pid": 1, "window_id": 2}, 30.0
+            )
+        finally:
+            _sp.run = orig_run
+
+        assert captured_kw.get("encoding") == "utf-8"
+        assert captured_kw.get("errors") == "replace"
+        assert "2 elements" in out["data"]
+        assert out["isError"] is False
+
+
 class TestCaptureAppFilterNoMatch:
     """capture(app=X) must not silently fall back to the frontmost window
     when X matches nothing — on a non-English macOS, list_windows returns
diff --git a/tools/computer_use/cua_backend.py b/tools/computer_use/cua_backend.py
index 32dae8b7b8611..9eed89b8112ff 100644
--- a/tools/computer_use/cua_backend.py
+++ b/tools/computer_use/cua_backend.py
@@ -163,7 +163,9 @@ def _resolve_mcp_invocation(
         from tools.environments.local import _sanitize_subprocess_env
         proc = subprocess.run(
             [driver_cmd, "manifest"],
-            capture_output=True, text=True, timeout=timeout,
+            capture_output=True, text=True,
+            encoding="utf-8", errors="replace",
+            timeout=timeout,
             stdin=subprocess.DEVNULL,
             # cua-driver is a third-party binary — never hand it provider
             # API keys via inherited env (same policy as the MCP and CLI
@@ -254,7 +256,9 @@ def cua_driver_update_check(*, timeout: float = 8.0) -> Optional[Dict[str, Any]]
         from tools.environments.local import _sanitize_subprocess_env
         proc = subprocess.run(
             [_CUA_DRIVER_CMD, "check-update", "--json"],
-            capture_output=True, text=True, timeout=timeout,
+            capture_output=True, text=True,
+            encoding="utf-8", errors="replace",
+            timeout=timeout,
             # Some older drivers don't have the verb and fall through to a
             # stdin-reading mode rather than erroring — DEVNULL gives them EOF
             # so they exit fast instead of blocking until the timeout.
@@ -920,7 +924,9 @@ def _call_tool_via_cli(self, name: str, args: Dict[str, Any], timeout: float) ->
             for attempt in range(attempts):
                 try:
                     proc = _subprocess.run(
-                        cmd, capture_output=True, text=True, timeout=max(15.0, timeout),
+                        cmd, capture_output=True, text=True,
+                        encoding="utf-8", errors="replace",
+                        timeout=max(15.0, timeout),
                         env=_sanitize_subprocess_env(cua_driver_child_env()),
                     )
                 except Exception as e:  # pragma: no cover - subprocess spawn failure