fix(copilot_acp): read/write ACP files as UTF-8

agent/copilot_acp_client.py

@@ -703,7 +703,11 @@ def _handle_server_message(
                 if block_error:
                     raise PermissionError(block_error)
                 try:
-                    content = path.read_text()
+                    # Force UTF-8: read_text() with no encoding falls back to
+                    # the system locale (cp1252/GBK on Windows) and raises
+                    # UnicodeDecodeError on any non-ASCII source/doc the peer
+                    # asked to read. The codebase treats all text files as UTF-8.
+                    content = path.read_text(encoding="utf-8")
                 except FileNotFoundError:
                     content = ""
                 line = params.get("line")
@@ -732,7 +736,13 @@ def _handle_server_message(
                         f"Write denied: '{path}' is a protected system/credential file."
                     )
                 path.parent.mkdir(parents=True, exist_ok=True)
-                path.write_text(str(params.get("content") or ""))
+                # Force UTF-8: write_text() with no encoding falls back to the
+                # system locale (cp1252/GBK on Windows) and raises
+                # UnicodeEncodeError on any non-ASCII content the peer asked to
+                # write (unicode in source, docs, emoji, CJK).
+                path.write_text(
+                    str(params.get("content") or ""), encoding="utf-8"
+                )
                 response = {
                     "jsonrpc": "2.0",
                     "id": message_id,

tests/agent/test_copilot_acp_client.py

@@ -226,6 +226,53 @@ def test_write_text_file_reuses_write_denylist(self) -> None:
         self.assertIn("error", response)
         self.assertFalse(target.exists())
 
+    def test_write_text_file_writes_utf8(self) -> None:
+        """Non-ASCII content must persist as UTF-8, not the system locale
+        (cp1252/GBK on Windows would raise UnicodeEncodeError)."""
+        with tempfile.TemporaryDirectory() as tmpdir:
+            root = Path(tmpdir)
+            target = root / "note.md"
+            payload = "# Заголовок ✅\nステータス: 完了\n"
+
+            response = self._dispatch(
+                {
+                    "jsonrpc": "2.0",
+                    "id": 6,
+                    "method": "fs/write_text_file",
+                    "params": {"path": str(target), "content": payload},
+                },
+                cwd=str(root),
+            )
+
+            self.assertNotIn("error", response)
+            # Decode as UTF-8 and normalise platform newline translation
+            # (text-mode write turns \n into \r\n on Windows) — the point is
+            # that the non-ASCII bytes survived, not the line ending.
+            written = target.read_bytes().decode("utf-8").replace("\r\n", "\n")
+            self.assertEqual(written, payload)
+
+    def test_read_text_file_reads_utf8(self) -> None:
+        """A UTF-8 file with non-ASCII content must decode regardless of the
+        host locale."""
+        with tempfile.TemporaryDirectory() as tmpdir:
+            root = Path(tmpdir)
+            src = root / "src.txt"
+            payload = "переменная = '完了' # ✅\n"
+            src.write_bytes(payload.encode("utf-8"))
+
+            response = self._dispatch(
+                {
+                    "jsonrpc": "2.0",
+                    "id": 7,
+                    "method": "fs/read_text_file",
+                    "params": {"path": str(src)},
+                },
+                cwd=str(root),
+            )
+
+            content = ((response.get("result") or {}).get("content") or "")
+            self.assertEqual(content, payload)
+
     def test_write_text_file_respects_safe_root(self) -> None:
         with tempfile.TemporaryDirectory() as tmpdir:
             root = Path(tmpdir)