diff --git a/tests/tools/test_working_diff.py b/tests/tools/test_working_diff.py
index 94b2861ab2..6fec09030b 100644
--- a/tests/tools/test_working_diff.py
+++ b/tests/tools/test_working_diff.py
@@ -10,6 +10,7 @@ import subprocess
 
 import pytest
 
+import tools.working_diff as working_diff
 from tools.working_diff import collect_working_diff
 
 pytestmark = pytest.mark.skipif(
@@ -57,3 +58,66 @@ def test_unknown_mode_rejected(repo):
     result = collect_working_diff(str(repo), mode="bogus")
     assert result["success"] is False
     assert "bogus" in result["error"]
+
+
+def test_run_decodes_git_output_as_utf8(monkeypatch, repo):
+    """``_run`` must force UTF-8 decoding of git's output.
+
+    Without ``encoding="utf-8"`` (and a lossy ``errors=``), ``subprocess.run``
+    falls back to the platform locale encoding. On Windows that's typically
+    cp932: UTF-8 multibyte output (e.g. a Japanese filename or diff content)
+    then either raises ``UnicodeDecodeError`` or silently decodes as mojibake,
+    depending on the byte sequence. The raise violates the "Never raises on
+    git failure" contract in ``_run``'s docstring.
+    """
+    captured = {}
+    real_run = subprocess.run
+
+    def fake_run(*args, **kwargs):
+        captured.update(kwargs)
+        return real_run(*args, **kwargs)
+
+    monkeypatch.setattr(working_diff.subprocess, "run", fake_run)
+
+    working_diff._run(["status"], str(repo))
+
+    assert captured.get("encoding") == "utf-8"
+    assert captured.get("errors") == "replace"
+
+
+def test_non_ascii_untracked_file_does_not_raise(repo):
+    """A non-ASCII filename + content must decode cleanly, not raise.
+
+    Regression test for the UnicodeDecodeError observed on real Windows
+    machines when git output contains UTF-8 multibyte sequences (Japanese
+    filename/content here) and the platform locale is not UTF-8.
+    """
+    (repo / "日本語ファイル.py").write_text("x = 'こんにちは'\n", encoding="utf-8")
+
+    result = collect_working_diff(str(repo))
+
+    assert result["success"] is True
+    assert any("日本語ファイル.py" in f for f in result["untracked"])
+    assert "こんにちは" in result["diff"]
+
+
+def test_cp932_content_is_lossy_but_never_raises(repo):
+    """Non-UTF-8 blob content degrades to replacement characters, not a crash.
+
+    Git emits blob bytes uninterpreted, so a repo whose files are cp932-encoded
+    decodes lossily under the forced UTF-8 policy. That is the same trade-off
+    checkpoint_manager's ``_run_git`` already makes (utf-8 + errors="replace"
+    on every git call): a readable-but-lossy diff for legacy encodings, never
+    an exception. This test pins the trade-off so it stays a documented choice.
+    """
+    legacy = repo / "legacy.txt"
+    legacy.write_bytes("before=東京\n".encode("cp932"))
+    _git(repo, "add", ".")
+    _git(repo, "commit", "-q", "-m", "legacy")
+    legacy.write_bytes("after=日本語\n".encode("cp932"))
+
+    result = collect_working_diff(str(repo))
+
+    assert result["success"] is True
+    assert "legacy.txt" in result["diff"]
+    assert "\ufffd" in result["diff"]  # lossy by design, matching _run_git
diff --git a/tools/working_diff.py b/tools/working_diff.py
index cc0d0df101..a5df51d4f6 100644
--- a/tools/working_diff.py
+++ b/tools/working_diff.py
@@ -35,6 +35,7 @@ def _run(args: List[str], cwd: str, timeout: int = _GIT_TIMEOUT):
     proc = subprocess.run(
         ["git", "-c", "core.quotePath=false", *args],
         cwd=cwd, capture_output=True, text=True, timeout=timeout,
+        encoding="utf-8", errors="replace",
     )
     return proc.returncode, proc.stdout