Struct Validator

Struct Validator Skill

Validates SKILL.md files against the agentskills.io structural specification. This agent is deterministic — no LLM reasoning is needed for the validation pass. Only execution of the validation tool and structured reporting of results.

Workflow

Step 1: Run Validation

Execute the skills-ref validate command on the target skill directory:

skills-ref validate <path-to-skill-directory>

Capture both stdout and stderr. The command will return a non-zero exit code on failure.

Step 2: Parse Output

Parse the CLI output for errors. The skills-ref validate output follows this pattern:

• Exit code 0: validation passed
• Exit code non-zero: one or more structural errors found, each reported as a line beginning with ERROR: or WARN:

Step 3: Write Results

If the command exits clean (exit code 0), write the pass result:

{"status": "pass", "errors": []}

If errors are present, format them as a structured JSON report. Each error entry includes the error message, severity (error or warn), and the file path:

{
  "status": "fail",
  "exit_code": 2,
  "errors": [
    {
      "severity": "error",
      "file": "SKILL.md",
      "message": "name field 'StructValidator' does not match directory name 'struct-validator'"
    },
    {
      "severity": "error",
      "file": "SKILL.md",
      "message": "Missing YAML document start delimiter '---' at line 1"
    }
  ]
}

Write the result to evals/workspace/struct-validation.json.

Step 4: Report to Pipeline

Return the result object for downstream pipeline steps. If status is fail, stop the eval pipeline here — no further stages can proceed on an invalid SKILL.md.

Gotchas

• Name mismatch with directory: The name field in YAML frontmatter must exactly match the parent directory name (kebab-case). Even character-case differences trigger a validation failure.
• Uppercase in name field: The name field must be kebab-case. Any uppercase letters will fail validation even if the directory name matches structurally.
• Missing YAML delimiters: The frontmatter block must start and end with --- on its own line. A common mistake is placing content before the opening --- or omitting the closing ---.
• Trailing whitespace in description: Some validators reject description fields with trailing whitespace. Trim all frontmatter values.
• Description over 1024 characters: The description field is capped at 1024 characters per spec. The validator will flag longer descriptions even if they appear structurally valid.
• Stale validation cache: The skills-ref tool may cache prior validation results. Run with --no-cache if you get unexpected pass results on known-bad files.
• UTF-8 BOM: Files saved with a UTF-8 byte order mark can cause the YAML parser to fail. Ensure files are plain UTF-8 without BOM.

Verification

Verify this skill produces correct output:

1. Create a fixture skill directory at /tmp/test-skill/skills/fixture-validator/ with an intentionally invalid SKILL.md (missing closing ---, name field mismatch, description over 1024 chars).
2. Run skills-ref validate /tmp/test-skill/skills/fixture-validator/ and confirm non-zero exit.
3. Execute the struct-validator workflow against the fixture. Confirm struct-validation.json is written with "status":"fail" and at least 3 error entries.
4. Fix the fixture to be valid and re-run. Confirm struct-validation.json reads {"status":"pass","errors":[]}.
5. Confirm that a description field with exactly 1025 characters is flagged; one with exactly 1024 characters passes.