[Bug] Skill frontmatter "model" field causes 404 API error

Description

The model field in skill YAML frontmatter does not work as documented. Any value (alias or full model name) causes a 404 API error.

According to the official documentation, the model field should specify which model to use when the skill is active.

Steps to Reproduce

1. Create a skill with model field in frontmatter:

---
name: test-skill
description: Test skill for model field
model: sonnet
---

Test instruction content.

1. Invoke the skill with /test-skill

Expected Behavior

The skill should execute using the specified model (Sonnet).

Actual Behavior

API returns 404 error:

API Error: 404 {"type":"error","error":{"type":"not_found_error","message":"model: sonnet"}}

Additional Testing

| Value | Result |
|-------|--------|
| model: sonnet | 404 error |
| model: haiku | 404 error |
| model: claude-sonnet-4-5-20250514 | 404 error |
| (no model field) | Works |

All model values cause the same error, confirming this is a bug with the model field implementation itself.

The error message suggests the model field value is being passed incorrectly to the API (prefixed with "model: ").

Environment

• Claude Code version: 2.1.72
• OS: macOS (Darwin 25.3.0)
• Available models: Opus 4.5, Sonnet 4.5, Haiku 4.5

Workaround

Remove the model field from skill frontmatter. The skill will inherit the model from the parent session.

References

• Documentation: https://code.claude.com/docs/en/skills (Frontmatter reference table lists model as supported)
• Documentation: https://code.claude.com/docs/en/model-config (Model aliases and full names)