Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx agentmods add skills/existential-birds/beagle/skill-buildernpx skills add existential-birds/beagle --skill skill-buildergit clone --depth 1 https://github.com/existential-birds/beagleWhat 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 | $0.00032 | $0.00490 |
| Opus 5 | $0.00016 | $0.00245 |
| Sonnet 5 | $0.00006 | $0.00098 |
| Haiku 4.5 | $0.00003 | $0.00049 |
Grade A, and why
skill-builder 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 3d 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.
How it starts
The opening of the file, as written. The whole thing — 47 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Skill Builder
Create, validate, and refine Agent Skills.
Quick Start
- Gather the capability, triggers, and required domain knowledge.
- Choose a simple single-file skill or a multi-file skill with references.
- Write
SKILL.mdwith concise, trigger-focused instructions. - Add reference files only for detail that would otherwise bloat
SKILL.md. - Validate YAML frontmatter, file layout, and naming.
- Test the skill with the natural language users are likely to say.
Workflow
- Start with requirements and scope control.
- Design the structure before writing content.
- Keep descriptions in third person and include trigger keywords.
- Use progressive disclosure for long examples, templates, and validation details.
Gates
Follow in order. Pass means a check you can satisfy with concrete artifacts (written bullets, paths, line counts, parsed YAML)—not an internal sense of “done.”
- Requirements — Pass: Capability, triggers, and any required domain knowledge (or explicit “none”) are written down or confirmed from the user.
- Structure — Pass: Single-file vs
SKILL.md+references/is chosen; heavy detail lives in references, not inlined inSKILL.md. - Draft — Pass: Frontmatter is valid YAML with
nameanddescription;SKILL.mdis ≤ 500 lines; every relative link fromSKILL.mdresolves to a path that exists under this skill directory; eachallowed-toolsentry (if present) is justified. - Trigger check — Pass: At least one natural-language user phrase plausibly matching the
descriptionis identified for a quick invocation test.
Validation
- Keep
SKILL.mdunder 500 lines. - Prefer one-level reference links.
- Avoid time-sensitive guidance.
- Confirm frontmatter is valid YAML.
- Check that any
allowed-toolsentries are necessary and correct.
Advanced Reference
For the full workflow, templates, examples, and validation checklist, see references/skill-builder-guide.md.
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.
- 3d ago First seen · 47 lines · 32 tokens per session scan A 03c3c47d21f0
skill-builder is a skill published in the GitHub repository existential-birds/beagle (79 stars, last pushed 23d ago), licensed Apache-2.0. It adds 32 tokens to every session and 490 once invoked, about $0.0002 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 2026-08-30.
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