[BUG] MCP slash commands unrecognized when followed by a prompt

Environment

• Platform (select one):
• [X] Anthropic API
• [ ] AWS Bedrock
• [ ] Google Vertex AI
• [ ] Other: <!-- specify -->
• Claude CLI version: 1.0.93
• Operating System: macOS 15.6.1
• Terminal: zsh

Bug Description

When I invoke any MCP slash command followed by a prompt, the command isn't recognized.

This works: /zen:chat but this doesn't: /zen:chat foo

The error message I get is Unknown slash command: /zen:chat.

This happens with all of my MCP connections, I have verified the problem does not exist on zen specifically.

Steps to Reproduce

1. Add MCP server of choice to claude
2. Invoke a slash command with a prompt after

Expected Behavior

The command should run, and the string following that slash command should be interpreted as input to that command.

Actual Behavior

I get the error Unknown slash command: /zen:chat