[BUG] MCP tool calls hang when arguments contain Japanese (Unicode) characters
Status Fixed / completed
Maintainer reply None cached
Workaround ✓ Mentioned in thread ↓
Activity 4 comments · opened Feb 7, 2026 · closed Feb 7, 2026
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
- Create an MCP server with a tool that accepts string parameters
- 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（%）"
}
}
}
}
- 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.
4 Comments
Found 3 possible duplicate issues:
This issue will be automatically closed as a duplicate in 3 days.
🤖 Generated with Claude Code
This issue is distinct from the suggested duplicates:
This issue is language-specific:
The problem appears to be in how Claude Desktop handles Unicode escape sequences (
\uXXXX) during JSON serialization of MCP tool arguments, not in the object/string serialization itself.Workaround: Use English for MCP tool arguments, then edit Japanese text via Web UI.
Closing this issue as the root cause is unclear. The problem may not be specific to Japanese/Unicode characters. We'll investigate further on our side.
This issue has been automatically locked since it was closed and has not had any activity for 7 days. If you're experiencing a similar issue, please file a new issue and reference this one if it's relevant.