Claude Skill Validator
Paste or open a SKILL.md and check it before you share the skill. Prettify parses the YAML frontmatter, reports a syntax error with its line, and shows the fields as an editable card. Skill Check then tests the file against the Agent Skills and Claude Code rules.
What Skill Check looks for
Skill Check reads the frontmatter and the body and reports problems in the Problems panel. It follows the Agent Skills specification for the name: 1 to 64 lowercase letters, numbers, and single hyphens, with no hyphen at the start or end. The specification also requires a description of 1,024 characters or fewer. Claude Code uses the first line of the body when the description is missing, but Claude reads the description to decide when to use the skill, so Skill Check reports a missing one.
It also warns when the description and when_to_use together pass the 1,536 characters Claude Code shows, when a key such as effort, context, or shell has a value Claude Code does not accept, and when the body is longer than the 500 lines the Claude Code docs recommend. Keys that Claude Code ignores are listed, so a misspelled key does not fail silently.
What is free and what is Pro
Free: the check on this page, which finds YAML errors in the frontmatter and lists its keys, plus the editor with its Preview, Raw view, and Properties card. In the editor, the Problems panel tells you how many skill issues it found.
Skill Check is a Pro feature with one free try per day. It shows each issue with its message and line. Pro is a one-time purchase that also adds Side by side editing, the Outline, Mermaid diagrams in the Preview, and export.
Fix issues where you find them
Click an issue in the Problems panel to jump to its line. Fix the name or description in the Properties card, or edit the frontmatter as YAML. Edit the instructions in the Preview or the Raw view. Prettier formats the file when you open it, so the result is clean before you commit it.
Frequently asked questions
What does the check on this page do?
It parses the frontmatter as YAML and reports an error with its line, lists the frontmatter keys, and warns about a code block with no closing fence. The skill rules run in the editor with Skill Check.
Is Skill Check free?
Skill Check is a Pro feature, and you get one free try per day. Without Pro, the editor still tells you how many skill issues it found. Pro is a one-time purchase.
Which rules does Skill Check use?
The Agent Skills specification and the Claude Code skills documentation: the name format and the description of up to 1,024 characters that the specification requires, the values Claude Code accepts for keys like effort and context, keys it ignores, and the 500-line body size that the Claude Code docs recommend.
Does it judge whether my instructions are good?
No. Skill Check tests the structure of the file against the published rules. It does not run the skill, and it does not grade the instructions you write in the body.
What is a SKILL.md file?
It is the file that defines a Claude skill: YAML frontmatter with a name and a description, then Markdown instructions. The SKILL.md format page lists every field and rule, with an example.
Is my skill file uploaded?
No. Parsing and checking run in your browser, and the file stays on your device.