Getting it into your agent
It runs from inside its repository, so the clone comes first — what it calls does not travel with the file alone.
git clone --depth 1 https://github.com/davidtoby/agent-skillsnpx agentmods add skills/davidtoby/agent-skills/agent-skills-frontmatter-compatibilityWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/davidtoby/agent-skills/agent-skills-frontmatter-compatibility)<a href="https://agentmods.dev/skills/davidtoby/agent-skills/agent-skills-frontmatter-compatibility"><img src="https://agentmods.dev/badge/skills/davidtoby/agent-skills/agent-skills-frontmatter-compatibility/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/davidtoby/agent-skills/agent-skills-frontmatter-compatibility"><img src="https://agentmods.dev/badge/skills/davidtoby/agent-skills/agent-skills-frontmatter-compatibility.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00067 | $0.00332 |
| Opus 5 | $0.00034 | $0.00166 |
| Sonnet 5 | $0.00013 | $0.00066 |
| Haiku 4.5 | $0.00007 | $0.00033 |
Grade A, and why
agent-skills-frontmatter-compatibility scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 9d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
What it actually says
Agent Skills Frontmatter Compatibility
Use the smallest metadata-only fix that keeps the skill meaning intact.
Quick checklist
- Inspect
SKILL.mdfrontmatter before changing the body. - Remove unsupported frontmatter fields; in this repo,
version:is a common offender. - Rewrite
description:as a plain single-line YAML string. - Avoid YAML block scalars for descriptions (
>and|). - Avoid brittle placeholder-heavy description text such as raw angle-bracket tokens when the packager/parser is failing.
- Re-run a full rebuild after the metadata fix because repo validation checks the whole repo, not only the changed skill.
Preferred rebuild step
Run:
python3 scripts/rebuild_all_packages.py
If you were iterating on one skill first, still finish with a full rebuild once the frontmatter fix is in place.
Practical rule
Prefer changing metadata formatting over editing substantive instructions. If the skill body is reasonable, leave it alone.
Known symptom pattern
If rebuild_all_packages.py fails immediately on a newly promoted local-only skill, suspect frontmatter compatibility before deeper refactors.
Reference
Read references/examples.md for concrete fixes that already worked in Toby's repo.
What ships with it
1 file beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 9d ago First seen · 40 lines · 67 tokens per session scan A 9540502b0e34
agent-skills-frontmatter-compatibility is a skill published in the GitHub repository davidtoby/agent-skills (10 stars, last pushed 1mo ago), licensed MIT. It adds 67 tokens to every session and 332 once invoked, about $0.0003 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-03.
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