[BUG] WebSearch tool returns 400 error in Claude Code CLI (works on claude.ai)

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?

Environment:

• Claude Code CLI on macOS (Darwin 25.4.0)
• Enterprise plan
• Tested on both claude-sonnet-4-6 and claude-opus-4-6

Error:
API Error: 400 {"error":{"message":"{\"message\":\"The provided request is not valid\"}. Received Model Group=claude-sonnet-4-6\nAvailable Model
Group Fallbacks=None","type":"None","param":"None","code":"400"}}

What Should Happen?

Steps to reproduce:

1. Open Claude Code CLI
2. Ask any question that triggers WebSearch (e.g. "search for X")
3. WebSearch tool fires and returns 400

Expected: WebSearch returns results (works correctly on claude.ai with same account)

Actual: 400 error on every WebSearch call, regardless of model selected

Error Messages/Logs

Steps to Reproduce

Steps to reproduce:

1. Open Claude Code CLI
2. Ask any question that triggers WebSearch (e.g. "search for X")
3. WebSearch tool fires and returns 400

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.92

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_