[BUG] claude code prefilling assistant messages when using opus 4.6

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?

When using the latest Claude Code version (v2.1.56) and switching to Opus (1M context), I found that Claude Code performs prefilling of assistant messages (last-assistant-turn prefills) on some requests, resulting in HTTP 400 errors.

However, according to the official documentation, Opus 4.6 no longer supports this behavior (https://platform.claude.com/docs/en/about-claude/models/whats-new-claude-4-6#prefill-removal).

What Should Happen?

When using the latest Claude Code version (v2.1.56) and switching to Opus 4.6, Claude Code should not performs prefilling of assistant messages.

Error Messages/Logs

Steps to Reproduce

Switch to Opus 4.6 and have it explore any code repository

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.56 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

_No response_