diff --git a/graphify/cache.py b/graphify/cache.py
index 7f73db069..02127d9ac 100644
--- a/graphify/cache.py
+++ b/graphify/cache.py
@@ -55,8 +55,8 @@ def load_cached(path: Path, root: Path = Path(".")) -> dict | None:
     if not entry.exists():
         return None
     try:
-        return json.loads(entry.read_text())
-    except (json.JSONDecodeError, OSError):
+        return json.loads(entry.read_text(encoding="utf-8"))
+    except (json.JSONDecodeError, OSError, UnicodeDecodeError):
         return None
 
 
@@ -70,7 +70,10 @@ def save_cached(path: Path, result: dict, root: Path = Path(".")) -> None:
     entry = cache_dir(root) / f"{h}.json"
     tmp = entry.with_suffix(".tmp")
     try:
-        tmp.write_text(json.dumps(result))
+        # ensure_ascii=False keeps non-ASCII identifiers/labels readable in
+        # the cache file; encoding="utf-8" prevents UnicodeEncodeError on
+        # Windows where the default cp1252 codec cannot encode CJK/emoji.
+        tmp.write_text(json.dumps(result, ensure_ascii=False), encoding="utf-8")
         os.replace(tmp, entry)
     except Exception:
         tmp.unlink(missing_ok=True)
diff --git a/graphify/detect.py b/graphify/detect.py
index e9dc701f0..e684a6617 100644
--- a/graphify/detect.py
+++ b/graphify/detect.py
@@ -427,7 +427,7 @@ def detect(root: Path, *, follow_symlinks: bool = False) -> dict:
 def load_manifest(manifest_path: str = _MANIFEST_PATH) -> dict[str, float]:
     """Load the file modification time manifest from a previous run."""
     try:
-        return json.loads(Path(manifest_path).read_text())
+        return json.loads(Path(manifest_path).read_text(encoding="utf-8"))
     except Exception:
         return {}
 
@@ -442,7 +442,13 @@ def save_manifest(files: dict[str, list[str]], manifest_path: str = _MANIFEST_PA
             except OSError:
                 pass  # file deleted between detect() and manifest write - skip it
     Path(manifest_path).parent.mkdir(parents=True, exist_ok=True)
-    Path(manifest_path).write_text(json.dumps(manifest, indent=2))
+    # ensure_ascii=False so file paths with non-ASCII characters (CJK, accented
+    # Latin) survive a roundtrip; encoding="utf-8" prevents UnicodeEncodeError
+    # on Windows where the default cp1252 codec cannot encode those paths.
+    Path(manifest_path).write_text(
+        json.dumps(manifest, indent=2, ensure_ascii=False),
+        encoding="utf-8",
+    )
 
 
 def detect_incremental(root: Path, manifest_path: str = _MANIFEST_PATH) -> dict:
diff --git a/tests/test_encoding.py b/tests/test_encoding.py
new file mode 100644
index 000000000..ad37b202c
--- /dev/null
+++ b/tests/test_encoding.py
@@ -0,0 +1,116 @@
+"""Verify cache and manifest I/O survive non-ASCII content cross-platform.
+
+On Windows the default text codec is cp1252, which raises UnicodeEncodeError
+when writing CJK / emoji / accented Latin content unless an explicit
+encoding="utf-8" is passed. This test pins the behavior so any future
+regression is caught immediately.
+"""
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+import pytest
+
+from graphify.cache import load_cached, save_cached
+from graphify.detect import load_manifest, save_manifest
+
+
+# Sample content that crashes default-codec write_text on Windows
+NON_ASCII_PAYLOADS = [
+    "ascii_only",
+    "中文测试",                      # CJK
+    "日本語コメント",                 # Japanese
+    "한글 테스트",                    # Korean
+    "café résumé naïve",              # Accented Latin
+    "emoji 🚀 mixed 中 with 한",     # Mixed scripts + emoji
+]
+
+
+def _make_source_file(tmp_path: Path, name: str = "module.py") -> Path:
+    """Create a temporary source file we can hash and cache."""
+    source = tmp_path / name
+    source.write_text("x = 1\n", encoding="utf-8")
+    return source
+
+
+@pytest.mark.parametrize("payload", NON_ASCII_PAYLOADS)
+def test_cache_roundtrip_preserves_non_ascii_labels(tmp_path, payload):
+    """save_cached / load_cached must roundtrip non-ASCII node labels."""
+    source = _make_source_file(tmp_path)
+    result = {
+        "nodes": [{"id": "n1", "label": payload, "source_file": str(source)}],
+        "edges": [],
+    }
+    save_cached(source, result, root=tmp_path)
+    loaded = load_cached(source, root=tmp_path)
+
+    assert loaded is not None
+    assert loaded["nodes"][0]["label"] == payload
+
+
+def test_cache_roundtrip_preserves_non_ascii_in_source_file_path(tmp_path):
+    """Cache must handle source_file paths containing non-ASCII characters."""
+    # Create a directory and file with CJK characters in the name
+    cjk_dir = tmp_path / "中文目录"
+    cjk_dir.mkdir()
+    source = cjk_dir / "模块.py"
+    source.write_text("x = 1\n", encoding="utf-8")
+
+    result = {
+        "nodes": [{"id": "n1", "label": "MyClass", "source_file": str(source)}],
+        "edges": [],
+    }
+    save_cached(source, result, root=tmp_path)
+    loaded = load_cached(source, root=tmp_path)
+
+    assert loaded is not None
+    assert loaded["nodes"][0]["source_file"] == str(source)
+
+
+def test_cache_file_is_valid_utf8_on_disk(tmp_path):
+    """The cache file written to disk must be readable as UTF-8 by other tools."""
+    source = _make_source_file(tmp_path)
+    payload = "中文 emoji 🚀"
+    result = {"nodes": [{"id": "n1", "label": payload}], "edges": []}
+    save_cached(source, result, root=tmp_path)
+
+    # Find the cache entry on disk and verify it's valid UTF-8 JSON
+    cache_files = list((tmp_path / "graphify-out" / "cache").glob("*.json"))
+    assert len(cache_files) == 1
+    raw = cache_files[0].read_bytes()
+    # Round-trip via UTF-8 decode + JSON parse must succeed
+    parsed = json.loads(raw.decode("utf-8"))
+    assert parsed["nodes"][0]["label"] == payload
+
+
+@pytest.mark.parametrize("path_name", [
+    "ascii_module.py",
+    "中文模块.py",
+    "日本語.py",
+    "café.py",
+])
+def test_manifest_roundtrip_preserves_non_ascii_paths(tmp_path, path_name):
+    """save_manifest / load_manifest must roundtrip non-ASCII file paths."""
+    source = tmp_path / path_name
+    source.write_text("x = 1\n", encoding="utf-8")
+
+    manifest_path = str(tmp_path / "manifest.json")
+    save_manifest({"code": [str(source)]}, manifest_path=manifest_path)
+
+    loaded = load_manifest(manifest_path=manifest_path)
+    assert str(source) in loaded
+    assert loaded[str(source)] == source.stat().st_mtime
+
+
+def test_manifest_file_is_valid_utf8_on_disk(tmp_path):
+    """The manifest file on disk must be readable as UTF-8."""
+    source = tmp_path / "中文.py"
+    source.write_text("x = 1\n", encoding="utf-8")
+
+    manifest_path = str(tmp_path / "manifest.json")
+    save_manifest({"code": [str(source)]}, manifest_path=manifest_path)
+
+    raw = Path(manifest_path).read_bytes()
+    parsed = json.loads(raw.decode("utf-8"))
+    assert str(source) in parsed