[BUG] Claude Code Web silently errors on bad slash command input

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?

In Claude Code Web, if you send a slash + space + query , it silently errors.
For example:

"""
/ hi how are you
"""

returns

Hook SessionStart:startup

and then stops

Nothing is shown in the CLI view as well. Typing the same thing in the CLI shows:
"""
Commands are in the form /command [args]
"""

<img width="791" height="98" alt="Image" src="https://github.com/user-attachments/assets/9d34eb32-dd23-453e-a6ab-878c7edacf52" />

What Should Happen?

It should show the same error message we get in the CLI:
"""
Commands are in the form /command [args]
"""

<img width="398" height="41" alt="Image" src="https://github.com/user-attachments/assets/f0b4f55a-7f70-44f5-b3ee-16d5795a9e83" />

Error Messages/Logs

Steps to Reproduce

• Go to Claude Code Web
• Create a new session
• Type " / hi "

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.0.72

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Other

Additional Information

I'm using the Claude Code Web feature on Claude Code Desktop on Mac