[BUG] Disabling MCP tools stopped working (they are available, even though they show as "disabled")

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?

MCP tool disabling no longer works.

What Should Happen?

Disabled MCP tools should not be available and should not have tools descriptions in context

Error Messages/Logs

Steps to Reproduce

Run Claude code with MCP servers installed, disable them if they are enabled, or enable them if they are disabled and then re-disable them, and see that they are still present in context and that Claude can still use them.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.1.0

Claude Code Version

2.1.5, edit: 2.1.7 still has the problem

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

Workaround is to exit and resume the conversation with desired mcp state. (disabling before exiting and then resuming)