Skill frontmatter is a YAML block at the top of a skill file that controls how the skill runs and when it triggers.

What it does

The frontmatter declares the skill's name, description, model preference, effort level, allowed tools, MCP servers, and other per-skill config. Claude reads these fields to decide when to auto-invoke the skill and how to execute it. The description field is especially important - it's what Claude uses to match user intent to the skill.

When to use it

Locking a skill to a specific model or effort level for predictable results.
Restricting tools so a skill can't do more than it should.
Tuning descriptions for reliable auto-invocation.
Declaring required MCP servers so the skill fails fast if they're missing.

Gotchas

Bad descriptions lead to bad auto-invocation. Write them like ads: clear triggers and scope.
Tool restrictions apply strictly. If a skill tries to use an unlisted tool, it fails.
Model settings in frontmatter can override the session's current model - intentional, but surprising.

Official docs: https://code.claude.com/docs/en/skills.md#frontmatter-reference