[BUG] Custom agent cannot be invoked with @-mention in VSCode extension

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?

Custom agents can be invoked with @-mention syntax (see https://code.claude.com/docs/en/sub-agents#invoke-subagents-explicitly)
This works in the Claude Code CLI but if you try the same thing in VSCode its basically ignored

What Should Happen?

Behaviour should be the same as in the Claude Code CLI:

• When typing @ and the name of the custom agent and then starting to type its name, it should appear in the typeahead list e.g. @my-agent (agent)
• when submitting the message, the agent should be run

Error Messages/Logs

Steps to Reproduce

Create a new agent, e.g. name it "my-agent"

1. In the VSCode extension input box, type "@my-ag" and observe that the list does not show the agent entry
2. Manually write @my-agent (agent) into the input box and submit and observe the agent not beeing executed

Claude Model

Not sure / Multiple models

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.89

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

VS Code integrated terminal

Additional Information

VSCode extension version 2.1.89