[BUG] WebSearch tool fails with "deepseek-reasoner does not support this tool_choice" for all Chinese and English queries

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

API Error: 400 deepseek-reasoner does not support this tool_choice

What Should Happen?

WebSearch工具应该正常返回中文和英文的搜索结果，包括网页标题、URL和摘要。

Error Messages/Logs

Steps to Reproduce

1. 在Claude Code会话中，使用WebSearch工具发起任意中文查询，例如：

• 股权回购 创始股东 责任上限 以股权为限 典型判决案例
• 民法典第466条 合同解释 案例 股权投资

1. 同样发起任意英文查询，例如：

• equity redemption founder liability cap China court case

1. 观察返回结果
2. 实际结果：所有查询均返回：

API Error: 400 deepseek-reasoner does not support this tool_choice

备注：该错误与具体查询内容无关，中文和英文、简单和复杂查询均受影响。经多次重试（超过10次）确认，非偶发问题。

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Claude Code v2.1.140

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_