[BUG] SSH: Handshake fails with Teleport proxy — no matching host key format (SSH certificates not supported)

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?

Summary

Claude Code Desktop's built-in SSH connection fails when connecting through a Teleport proxy with the error:

Handshake failed: no matching host key format

Root Cause

Teleport's SSH proxy only presents SSH certificate host keys (ecdsa-sha2-nistp256-cert-v01@openssh.com). The embedded SSH library in Claude Code Desktop does not support the -cert-v01@openssh.com certificate host key format, causing the handshake to fail.

The system's OpenSSH client handles this fine — ssh <host> from a terminal works without issues.

SSH Config

Host myhost
  HostName <hostname>
  User <user>
  ProxyCommand "tsh" proxy ssh --cluster=<cluster>.teleport.sh --proxy=<cluster>.teleport.sh:443 %r@%h:%p

Verbose SSH Output (relevant lines)

debug2: peer server KEXINIT proposal
debug2: host key algorithms: ecdsa-sha2-nistp256-cert-v01@openssh.com
debug1: kex: host key algorithm: ecdsa-sha2-nistp256-cert-v01@openssh.com

The server offers only the certificate variant. The embedded SSH library has no matching algorithm.

What Should Happen?

Claude Code Desktop should support SSH certificate host key formats (*-cert-v01@openssh.com), or optionally delegate to the system SSH binary for connection handling.

Error Messages/Logs

Steps to Reproduce

1. Configure an SSH host that connects through a Teleport proxy using tsh proxy ssh as the ProxyCommand
2. In Claude Code Desktop, add an SSH connection pointing to that host
3. Attempt to connect — handshake fails immediately

Claude Model

None

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

1.2581.0 (Desktop App)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Other

Additional Information

_No response_