Invalid model IDs generated for skills/agents without explicit model specification

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 running custom skills or agents that don't have an explicit model: specified in their
frontmatter, Claude Code generates invalid model IDs that result in 404 errors.

What Should Happen?

claude should run my /daily skill

Error Messages/Logs

API Error: 404 {"type":"error","error":{"type":"not_found_error","message":"model 'claude-sonnet-4-20250514' not                                   
     found"},"request_id":"req_84b4947a1431da288c07665b"}

Steps to Reproduce

1. Create a custom skill in .claude/skills/{name}/SKILL.md without a model: field
2. Run the skill (e.g., /daily)
3. Observe 404 error with malformed model ID

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.12

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_