[BUG] model alias not working in slash command frontmatter

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 model alias is specified in the YAML frontmatter of a slash command in a plugin, an API error is received.

API Error (haiku): 400 The provided model identifier is invalid.

If you specify the full model ID, the command succeeds successfully.
If you have the same command installed via a local slash command in the project it resolves correctly.

What Should Happen?

There should be a consistent support for the model alias in the slash commands installed as a plugin.

Error Messages/Logs

Steps to Reproduce

1. Create a test slash command locally and one in a git based plugin.
2. Use the below content in the slash commands.

---
description: A test slash command for outputting the model ID.
model: haiku
---

Output the current model that you are utilizing including the full ID. Nothing more.

1. Launch claude and execute both slash commands.
2. Plugin based slash command fails to parse the alias while the local slash command successfully converts it to the proper model ID.

Claude Model

Not sure / Multiple models

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.0.61

Platform

AWS Bedrock

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

<img width="951" height="191" alt="Image" src="https://github.com/user-attachments/assets/6c08cf49-4aa5-4e07-841a-a1eb9535296b" />