[BUG]

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 [400]: Function call is missing a thought_signature in functionCall parts. This is required for tools to work correctly, and missing thought_signature may lead to degraded model performance. Additional data, function call default_api:PowerShell , position 3. Please refer to https://ai.google.dev/gemini-api/docs/thought-signatures for more details. [gemini/gemini-3-flash-preview (400), gemini/gemini-2.5-pro (404), gemini/gemini-3.1-flash-lite (400), gemini/gemini-3.1-flash-lite (400), gemini/gemini-2.5-flash-lite (404)... (+4)]

Environment

• CLI Tool: Claude Code (Windows CLI)
• OS: Windows 11 Pro (10.0.26200)
• Shell: PowerShell / Bash
• Model: auto/claude-sonnet (or any other model in use when the error triggers)

Steps to Reproduce

1. Run Claude Code on Windows.
2. Issue any command that triggers tool use (e.g., asking Claude to list files, read a file, or perform a search).
3. The platform throws a 400 error regarding the missing thought_signature.

What Should Happen?

fix it so there will be no issue with tools

Error Messages/Logs

API Error: 400 [400]: Function call is missing a thought_signature in functionCall parts. This is required for tools to work correctly, and missing thought_signature may lead to degraded model performance. Additional data, function call `default_api:PowerShell` , position 3. Please refer to https://ai.google.dev/gemini-api/docs/thought-signatures for more details. [gemini/gemini-3-flash-preview (400), gemini/gemini-2.5-pro (404), gemini/gemini-3.1-flash-lite (400), gemini/gemini-3.1-flash-lite (400), gemini/gemini-2.5-flash-lite (404)... (+4)]

Steps to Reproduce

1. Run Claude Code on Windows.
2. Issue any command that triggers tool use (e.g., asking Claude to list files, read a file, or perform a search).
3. The platform throws a 400 error regarding the missing thought_signature.

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

opus 5

Claude Code Version

v2.1.233

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

_No response_