[BUG]

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) 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?

Claude Code never sends the MCP server/discover negotiation probe — modern-only (2026-07-28) MCP servers are unreachable

### What Should Happen?

Modern-only MCP servers should connect correctly. Verified as working on Linux, but not on macOS.

### Error Messages/Logs

```shell

```

### Steps to Reproduce

[cc-20260831-225403-7b4f45.zip](https://github.com/user-attachments/files/31666454/cc-20260831-225403-7b4f45.zip)

### Claude Model

Opus

### Is this a regression?

I don't know

### Last Working Version

_No response_

### Claude Code Version

2.1.252

### Platform

Other

### Operating System

macOS

### Terminal/Shell

Other

### Additional Information

macOS 15.7.9

No source file or test is named. Start by inspecting the attached session archive and comparing MCP connection behavior on macOS 15.7.9 with the reported working Linux behavior in Claude Code 2.1.252; done means modern-only MCP servers connect successfully on macOS.

Written by the indexing model from the issue text.