[BUG] MCP tool calls hang when arguments contain Japanese (Unicode) characters

Environment

• Claude Desktop: 1.1.2156 (4e9bb2) 2026-02-05
• OS: macOS
• MCP Server: Custom stdio-based server calling external API

Description

MCP tool calls hang indefinitely when the tool arguments contain Japanese characters. The same call succeeds immediately when using English text only.

Steps to Reproduce

1. Create an MCP server with a tool that accepts string parameters
2. Call the tool with Japanese text in the arguments:

``json
{
"description": "センサーデータを記録するためのスキーマです。温度、湿度、気圧などの環境情報を保存します。",
"jsonSchema": {
"properties": {
"temperature": {
"type": "number",
"description": "温度（摂氏）"
},
"humidity": {
"type": "number",
"description": "相対湿度（%）"
}
}
}
}
``

1. The call hangs with a spinning indicator

Expected Behavior

Tool call should complete successfully regardless of language.

Actual Behavior

• Japanese text: Hangs indefinitely
• English text: Completes successfully

Workaround

Use English for all MCP tool arguments, then edit Japanese text via Web UI.

Notes

The issue appears to be related to Unicode escape sequences (\uXXXX) during JSON serialization of tool arguments.