Core concept: A SKILL.md file is markdown with YAML frontmatter on top. The frontmatter is the part that is always loaded, so it has to carry the skill's identity and its trigger. The Agent Skills spec fixes exactly two required fields.

Why A and B are correct:

The Agent Skills documentation states it plainly: "Required fields: name and description".
name is the identifier. It is capped at 64 characters and may hold only lowercase letters, numbers and hyphens.
description "must include both what the Skill does and when Claude should use it", because Claude matches your request against it to decide whether to trigger the skill.
These two are the only metadata loaded at startup: "until a Skill is triggered, only its name and description occupy context."

Why the others are wrong:

C: allowed-tools is optional. It pre-approves tools for the turn that invokes the skill, and the frontmatter reference marks it "Required: No".
D: model is optional as well. It overrides the model while the skill is active and is also marked "Required: No".

Remember: Two fields are mandatory. name says what it is, description says when to use it. Everything else is a tuning knob.

Question 155