[BUG] @ Mention Feature Not Working

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 typing @ in the Claude Code input field, the autocomplete menu does NOT
appear immediately. The dropdown only shows up after typing at least one
character following the @.

Current behavior: @ → nothing happens → @f → dropdown appears
Expected behavior: @ → dropdown appears immediately

This is a change from previous behavior where the dropdown would appear
instantly upon pressing @, allowing users to browse all available files
and MCP servers before typing.

What Should Happen?

When pressing @, a dropdown/autocomplete menu should appear IMMEDIATELY
(without needing to type additional characters), showing:

• Available files in the current project that can be mentioned
• Configured MCP servers that can be enabled/disabled

Previously, pressing @ alone would trigger the menu, allowing users to
browse options before narrowing down with typed characters.

Error Messages/Logs

No error messages displayed. The @ character is typed but the expected
  autocomplete/mention popup simply does not appear.

Steps to Reproduce

1. Open PowerShell on Windows 11
2. Navigate to any directory (e.g., cd C:\Users\user)
3. Run claude to start Claude Code CLI
4. In the input field, type ONLY the @ character (don't type anything after)
5. Expected: A dropdown menu should appear immediately with files and MCP options
6. Actual: Nothing happens - the dropdown does NOT appear

1. Now type one more character (e.g., @f)
2. Actual: NOW the dropdown appears and starts filtering

The issue is that @ alone no longer triggers the menu - you must type
@<character> for it to show.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.0.69 (Claude Code)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

This is likely a debounce/trigger threshold change in the autocomplete logic. Previously the trigger was "on @ keypress", now it seems to be "on @ + minimum 1 character". This could be:

• An intentional change that broke UX expectations
• A regression in input event handling
• A race condition where the menu checks for content length before rendering