diff --git a/src/apm_cli/config.py b/src/apm_cli/config.py
index 6e758526d..7ce73396c 100644
--- a/src/apm_cli/config.py
+++ b/src/apm_cli/config.py
@@ -16,7 +16,7 @@ def ensure_config_exists():
         os.makedirs(CONFIG_DIR)
 
     if not os.path.exists(CONFIG_FILE):
-        with open(CONFIG_FILE, "w") as f:
+        with open(CONFIG_FILE, "w", encoding="utf-8") as f:
             json.dump({"default_client": "vscode"}, f)
 
 
@@ -32,7 +32,7 @@ def get_config():
     if _config_cache is not None:
         return _config_cache
     ensure_config_exists()
-    with open(CONFIG_FILE) as f:
+    with open(CONFIG_FILE, encoding="utf-8") as f:
         _config_cache = json.load(f)
     return _config_cache
 
@@ -53,7 +53,7 @@ def update_config(updates):
     config = get_config()
     config.update(updates)
 
-    with open(CONFIG_FILE, "w") as f:
+    with open(CONFIG_FILE, "w", encoding="utf-8") as f:
         json.dump(config, f, indent=2)
     _invalidate_config_cache()
 
@@ -135,7 +135,7 @@ def unset_temp_dir() -> None:
     config = get_config()
     if "temp_dir" in config:
         del config["temp_dir"]
-        with open(CONFIG_FILE, "w") as f:
+        with open(CONFIG_FILE, "w", encoding="utf-8") as f:
             json.dump(config, f, indent=2)
     _invalidate_config_cache()
 
@@ -185,7 +185,7 @@ def unset_copilot_cowork_skills_dir() -> None:
     config = get_config()
     if "copilot_cowork_skills_dir" in config:
         del config["copilot_cowork_skills_dir"]
-        with open(CONFIG_FILE, "w") as f:
+        with open(CONFIG_FILE, "w", encoding="utf-8") as f:
             json.dump(config, f, indent=2)
     _invalidate_config_cache()
 
diff --git a/src/apm_cli/marketplace/client.py b/src/apm_cli/marketplace/client.py
index 1d32a2c14..f33b850e4 100644
--- a/src/apm_cli/marketplace/client.py
+++ b/src/apm_cli/marketplace/client.py
@@ -88,13 +88,13 @@ def _read_cache(name: str) -> dict | None:
     if not os.path.exists(data_path) or not os.path.exists(meta_path):
         return None
     try:
-        with open(meta_path) as f:
+        with open(meta_path, encoding="utf-8") as f:
             meta = json.load(f)
         fetched_at = meta.get("fetched_at", 0)
         ttl = meta.get("ttl_seconds", _CACHE_TTL_SECONDS)
         if time.time() - fetched_at > ttl:
             return None  # Expired
-        with open(data_path) as f:
+        with open(data_path, encoding="utf-8") as f:
             return json.load(f)
     except (json.JSONDecodeError, OSError, KeyError) as exc:
         logger.debug("Cache read failed for '%s': %s", name, exc)
@@ -107,7 +107,7 @@ def _read_stale_cache(name: str) -> dict | None:
     if not os.path.exists(data_path):
         return None
     try:
-        with open(data_path) as f:
+        with open(data_path, encoding="utf-8") as f:
             return json.load(f)
     except (json.JSONDecodeError, OSError):
         return None
@@ -118,9 +118,9 @@ def _write_cache(name: str, data: dict) -> None:
     data_path = _cache_data_path(name)
     meta_path = _cache_meta_path(name)
     try:
-        with open(data_path, "w") as f:
+        with open(data_path, "w", encoding="utf-8") as f:
             json.dump(data, f, indent=2)
-        with open(meta_path, "w") as f:
+        with open(meta_path, "w", encoding="utf-8") as f:
             json.dump(
                 {"fetched_at": time.time(), "ttl_seconds": _CACHE_TTL_SECONDS},
                 f,
diff --git a/src/apm_cli/marketplace/registry.py b/src/apm_cli/marketplace/registry.py
index 9faa4c0ec..4248349ef 100644
--- a/src/apm_cli/marketplace/registry.py
+++ b/src/apm_cli/marketplace/registry.py
@@ -32,7 +32,7 @@ def _ensure_file() -> str:
     ensure_config_exists()
     path = _marketplaces_path()
     if not os.path.exists(path):
-        with open(path, "w") as f:
+        with open(path, "w", encoding="utf-8") as f:
             json.dump({"marketplaces": []}, f, indent=2)
     return path
 
@@ -51,7 +51,7 @@ def _load() -> list[MarketplaceSource]:
             return list(_registry_cache)
         path = _ensure_file()
         try:
-            with open(path) as f:
+            with open(path, encoding="utf-8") as f:
                 data = json.load(f)
         except (json.JSONDecodeError, OSError) as exc:
             logger.warning("Failed to read %s: %s", path, exc)
@@ -72,7 +72,7 @@ def _save(sources: list[MarketplaceSource]) -> None:
     path = _ensure_file()
     data = {"marketplaces": [s.to_dict() for s in sources]}
     tmp = path + ".tmp"
-    with open(tmp, "w") as f:
+    with open(tmp, "w", encoding="utf-8") as f:
         json.dump(data, f, indent=2)
     os.replace(tmp, path)
     with _registry_lock:
diff --git a/src/apm_cli/models/plugin.py b/src/apm_cli/models/plugin.py
index 85c6c65c9..5db8fdcab 100644
--- a/src/apm_cli/models/plugin.py
+++ b/src/apm_cli/models/plugin.py
@@ -113,7 +113,7 @@ def from_path(cls, plugin_path: Path) -> "Plugin":
                 f"Plugin metadata not found in any expected location: {plugin_path}"
             )
 
-        with open(metadata_file) as f:
+        with open(metadata_file, encoding="utf-8") as f:
             metadata_dict = json.load(f)
 
         metadata = PluginMetadata.from_dict(metadata_dict)
diff --git a/src/apm_cli/runtime/copilot_runtime.py b/src/apm_cli/runtime/copilot_runtime.py
index 184ff1c84..b344c2831 100644
--- a/src/apm_cli/runtime/copilot_runtime.py
+++ b/src/apm_cli/runtime/copilot_runtime.py
@@ -207,7 +207,7 @@ def get_mcp_servers(self) -> dict[str, Any]:
             return {}
 
         try:
-            with open(mcp_config_path) as f:
+            with open(mcp_config_path, encoding="utf-8") as f:
                 config = json.load(f)
                 return config.get("servers", {})
         except Exception as e:
diff --git a/tests/unit/marketplace/test_marketplace_client.py b/tests/unit/marketplace/test_marketplace_client.py
index 92fd9583f..93718cdfa 100644
--- a/tests/unit/marketplace/test_marketplace_client.py
+++ b/tests/unit/marketplace/test_marketplace_client.py
@@ -429,3 +429,36 @@ def test_different_hosts_different_keys(self):
         s1 = MarketplaceSource(name="mkt", owner="o", repo="r", host="a.com")
         s2 = MarketplaceSource(name="mkt", owner="o", repo="r", host="b.com")
         assert client_mod._cache_key(s1) != client_mod._cache_key(s2)
+
+
+class TestCacheUtf8RoundTrip:
+    """Cache I/O preserves non-ASCII content (Windows cp1252/cp950 guard)."""
+
+    def test_write_and_read_non_ascii(self, tmp_path):
+        data = {
+            "name": "Marketplace -- cafe",
+            "description": "\u4e2d\u6587 description",
+            "plugins": [{"name": "skill-\u958b\u59cb", "author": "cafe"}],
+        }
+        client_mod._write_cache("utf8-mkt", data)
+
+        cached = client_mod._read_cache("utf8-mkt")
+        assert cached is not None
+        assert cached["name"] == "Marketplace -- cafe"
+        assert cached["description"] == "\u4e2d\u6587 description"
+        assert cached["plugins"][0]["name"] == "skill-\u958b\u59cb"
+
+    def test_stale_cache_read_non_ascii(self, tmp_path):
+        import os as _os
+
+        data = {"plugins": [{"name": "\u4e2d\u6587-skill"}]}
+        client_mod._write_cache("stale-mkt", data)
+
+        # Drop the meta file so _read_cache treats the entry as missing and
+        # _read_stale_cache is the only path that returns content.
+        _os.remove(client_mod._cache_meta_path("stale-mkt"))
+        assert client_mod._read_cache("stale-mkt") is None
+
+        stale = client_mod._read_stale_cache("stale-mkt")
+        assert stale is not None
+        assert stale["plugins"][0]["name"] == "\u4e2d\u6587-skill"
diff --git a/tests/unit/marketplace/test_marketplace_registry.py b/tests/unit/marketplace/test_marketplace_registry.py
index 1310933ce..b4204fecc 100644
--- a/tests/unit/marketplace/test_marketplace_registry.py
+++ b/tests/unit/marketplace/test_marketplace_registry.py
@@ -92,3 +92,45 @@ def test_corrupted_file_returns_empty(self, tmp_path):
 
         registry_mod._invalidate_cache()
         assert registry_mod.get_registered_marketplaces() == []
+
+
+class TestRegistryUtf8RoundTrip:
+    """Registry persistence preserves non-ASCII content (Windows cp1252/cp950 guard)."""
+
+    def test_add_and_read_non_ascii_marketplace(self):
+        # Note: name/owner/repo are typically ASCII per the marketplace spec,
+        # but the registry file itself must still be UTF-8 to handle any
+        # non-ASCII content that may flow through future fields. We use a
+        # description-bearing source by writing a custom entry directly.
+        src = MarketplaceSource(name="cafe-mkt", owner="cafe-org", repo="plugins-\u958b\u59cb")
+        registry_mod.add_marketplace(src)
+
+        # Force re-load from disk by clearing the cache.
+        registry_mod._invalidate_cache()
+        fetched = registry_mod.get_marketplace_by_name("cafe-mkt")
+        assert fetched is not None
+        assert fetched.repo == "plugins-\u958b\u59cb"
+
+    def test_registry_file_is_readable_with_utf8_external_writes(self, tmp_path):
+        """A registry file written externally with raw UTF-8 (ensure_ascii=False)
+        must still load cleanly. This is the regression case for cp1252/cp950
+        Windows locales where the default open() would fail to decode."""
+        import json as _json
+
+        path = registry_mod._marketplaces_path()
+        # Ensure parent dir exists.
+        registry_mod._ensure_file()
+        payload = {
+            "marketplaces": [
+                {"name": "cafe-mkt", "owner": "o", "repo": "repo-\u4e2d\u6587"},
+            ]
+        }
+        # Write raw UTF-8 (no \uXXXX escaping) to mimic what a non-Python
+        # tool or a future writer with ensure_ascii=False would produce.
+        with open(path, "w", encoding="utf-8") as f:
+            _json.dump(payload, f, ensure_ascii=False)
+
+        registry_mod._invalidate_cache()
+        fetched = registry_mod.get_marketplace_by_name("cafe-mkt")
+        assert fetched is not None
+        assert fetched.repo == "repo-\u4e2d\u6587"
diff --git a/tests/unit/test_config.py b/tests/unit/test_config.py
new file mode 100644
index 000000000..82db51f65
--- /dev/null
+++ b/tests/unit/test_config.py
@@ -0,0 +1,53 @@
+"""Tests for apm_cli.config module-level config file I/O.
+
+These tests exercise the round-trip of non-ASCII content through the global
+config file to guard against the cp1252/cp950 UnicodeDecodeError class of
+bugs on Windows when ``open()`` is called without an explicit encoding.
+"""
+
+import json
+
+import pytest
+
+from apm_cli import config as config_mod
+
+
+@pytest.fixture
+def isolated_config(tmp_path, monkeypatch):
+    """Point CONFIG_DIR / CONFIG_FILE to a temp directory and clear cache."""
+    config_dir = tmp_path / ".apm"
+    config_file = config_dir / "config.json"
+    monkeypatch.setattr(config_mod, "CONFIG_DIR", str(config_dir))
+    monkeypatch.setattr(config_mod, "CONFIG_FILE", str(config_file))
+    monkeypatch.setattr(config_mod, "_config_cache", None)
+    return config_file
+
+
+class TestConfigUtf8RoundTrip:
+    """Round-trip non-ASCII content through the config file."""
+
+    def test_update_config_preserves_non_ascii(self, isolated_config):
+        non_ascii_value = "/Users/cafe/projets/\u958b\u59cb"
+        config_mod.update_config({"copilot_cowork_skills_dir": non_ascii_value})
+
+        # Force re-read from disk by invalidating the cache.
+        config_mod._invalidate_config_cache()
+        loaded = config_mod.get_config()
+
+        assert loaded["copilot_cowork_skills_dir"] == non_ascii_value
+
+    def test_config_file_is_utf8_on_disk(self, isolated_config):
+        non_ascii_value = "# \u958b\u59cb -- cafe"
+        config_mod.update_config({"note": non_ascii_value})
+
+        # Read raw bytes and decode as UTF-8 to assert the on-disk encoding.
+        raw = isolated_config.read_bytes()
+        decoded = json.loads(raw.decode("utf-8"))
+        assert decoded["note"] == non_ascii_value
+
+    def test_ensure_config_exists_uses_utf8(self, isolated_config, monkeypatch):
+        # Force ensure_config_exists() to create the file.
+        config_mod.ensure_config_exists()
+        assert isolated_config.exists()
+        # File must be readable as UTF-8 JSON.
+        json.loads(isolated_config.read_bytes().decode("utf-8"))
diff --git a/tests/unit/test_copilot_runtime.py b/tests/unit/test_copilot_runtime.py
index cd9455cad..172786d68 100644
--- a/tests/unit/test_copilot_runtime.py
+++ b/tests/unit/test_copilot_runtime.py
@@ -154,3 +154,30 @@ def test_str_representation(self):
             str_repr = str(runtime)
             assert "CopilotRuntime" in str_repr
             assert "test-model" in str_repr
+
+
+class TestMcpConfigUtf8RoundTrip:
+    """Reading MCP config preserves non-ASCII content (Windows cp1252/cp950 guard)."""
+
+    def test_get_mcp_servers_reads_non_ascii(self, tmp_path):
+        import json as _json
+
+        mcp_path = tmp_path / "mcp-config.json"
+        servers = {
+            "servers": {
+                "demo-cafe": {
+                    "command": "node",
+                    "args": ["server.js"],
+                    "description": "\u4e2d\u6587 description -- cafe",
+                }
+            }
+        }
+        mcp_path.write_bytes(_json.dumps(servers).encode("utf-8"))
+
+        with patch.object(CopilotRuntime, "is_available", return_value=True):
+            runtime = CopilotRuntime()
+            with patch.object(CopilotRuntime, "get_mcp_config_path", return_value=mcp_path):
+                got = runtime.get_mcp_servers()
+
+        assert "demo-cafe" in got
+        assert got["demo-cafe"]["description"] == "\u4e2d\u6587 description -- cafe"
diff --git a/tests/unit/test_plugin.py b/tests/unit/test_plugin.py
new file mode 100644
index 000000000..a597e160a
--- /dev/null
+++ b/tests/unit/test_plugin.py
@@ -0,0 +1,31 @@
+"""Tests for apm_cli.models.plugin module I/O.
+
+Round-trips non-ASCII content through Plugin.from_path to guard against
+cp1252/cp950 UnicodeDecodeError on Windows when reading plugin.json.
+"""
+
+import json
+
+from apm_cli.models.plugin import Plugin
+
+
+class TestPluginUtf8RoundTrip:
+    """Round-trip non-ASCII content through plugin.json reads."""
+
+    def test_from_path_reads_non_ascii_metadata(self, tmp_path):
+        metadata = {
+            "id": "demo-plugin",
+            "name": "Demo plugin -- cafe",
+            "version": "1.0.0",
+            "description": "Plugin de demo with \u4e2d\u6587 description",
+            "author": "Cafe Author",
+        }
+        plugin_json = tmp_path / "plugin.json"
+        plugin_json.write_bytes(json.dumps(metadata).encode("utf-8"))
+
+        plugin = Plugin.from_path(tmp_path)
+
+        assert plugin.metadata.id == "demo-plugin"
+        assert plugin.metadata.name == "Demo plugin -- cafe"
+        assert plugin.metadata.description == "Plugin de demo with \u4e2d\u6587 description"
+        assert plugin.metadata.author == "Cafe Author"