Every rule the validator checks.

Frontmatter structure

1. ErrorSKM-F01: No YAML frontmatter block found

   The file must open with a line containing only `---`, followed by YAML, followed by a closing `---`.

2. ErrorSKM-F02: Frontmatter block is not closed

   An opening `---` was found with no matching closing `---` line after it.

3. ErrorSKM-F03: Frontmatter does not parse as YAML

   The block between the `---` delimiters is not valid YAML.

4. ErrorSKM-F04: Frontmatter parses, but isn't a mapping

   The frontmatter must be a YAML mapping (`key: value` pairs) at the top level, not a list or a scalar.

5. WarningSKM-F05: Duplicate top-level key in frontmatter

   The same key appears twice at the top level — YAML keeps only the last value, silently.

6. WarningSKM-F06: Non-standard whitespace or characters in frontmatter

   Tabs, a byte-order mark, or smart-quote characters were found in the frontmatter block.

The name field

1. ErrorSKM-N01: `name` is missing

   `name` is one of the two required fields.

2. ErrorSKM-N02: `name` is longer than 64 characters

   The specification caps `name` at 64 characters.

3. ErrorSKM-N03: `name` contains disallowed characters

   `name` may contain only lowercase letters, digits, and hyphens.

4. ErrorSKM-N04: `name` has a leading, trailing, or consecutive hyphen

   The specification forbids leading, trailing, and consecutive hyphens in `name`.

5. ErrorSKM-N05: `name` does not match the parent directory

   The spec requires `name` to match the folder the SKILL.md lives in — a mismatch is a silent failure.

The description field

1. ErrorSKM-D01: `description` is missing

   `description` is the other required field, and the sole basis an agent uses to decide whether to load the skill.

2. ErrorSKM-D02: `description` is longer than 1024 characters

   The specification caps `description` at 1024 characters.

3. ErrorSKM-D03: `description` is empty

   An empty description can't do the job of triggering the skill in any runtime.

The body

1. WarningSKM-B01: Body is longer than 500 lines

   The specification recommends keeping SKILL.md under 500 lines and moving detail into `references/`.

2. WarningSKM-B02: Body is around or over 5,000 tokens

   The specification recommends keeping the activated body under roughly 5,000 tokens.

The rest of the field set

1. WarningSKM-K01: Unknown top-level frontmatter field

   This field isn't in the documented portable field set (name, description, license, compatibility, metadata, allowed-tools) or a known runtime extension.

2. WarningSKM-K02: `metadata` is not a map of strings to strings

   The specification defines `metadata` as a map of string keys to string values.

3. WarningSKM-K03: `compatibility` is longer than 500 characters

   The specification caps `compatibility` at 500 characters.

Portability across runtimes

1. PortabilitySKM-P01: `paths` is a Cursor-only field

   `paths` scopes a skill to files matching glob patterns — it works only in Cursor, and it doesn't just get ignored elsewhere.

2. PortabilitySKM-P02: `allowed-tools` is experimental

   The specification marks `allowed-tools` experimental, with support that 'may vary between agent implementations.' Don't treat it as a security control.

3. PortabilitySKM-P03: `context: fork` / `context: background` is a Claude Code-only field

   Runs the skill as a forked subagent — a Claude Code mechanism with no equivalent elsewhere.

4. PortabilitySKM-P04: `metadata.openclaw.*` / `command-dispatch` / `user-invocable` are OpenClaw-only fields

   Dependency gating, tool-dispatch, and invocation controls specific to OpenClaw.

5. PortabilitySKM-P05: `metadata.hermes.*` is a Hermes Agent-only field

   Conditional activation against available toolsets — a Hermes Agent mechanism.

6. PortabilitySKM-P06: `disable-model-invocation` isn't recognised everywhere the same way

   Turns off automatic selection in Cursor, OpenClaw, and Claude Code. Codex uses a different mechanism entirely (`policy.allow_implicit_invocation` in `agents/openai.yaml`); Hermes documents nothing.

7. PortabilitySKM-P07: `description` is longer than the tightest runtime budget

   1,024 characters is the spec ceiling, but OpenClaw's own authoring guidance asks for under 160 and practical Claude Code reports suggest under 200 — a description written for the tightest constraint is safe everywhere.

Filename and directory

1. ErrorSKM-X01: Filename is not exactly `SKILL.md`

   `SKILL.md` is case-sensitive on the filesystems that enforce it — `skill.md` is a documented cause of a skill silently never appearing.