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 skills add ashermahonin/agentic-skills --skill custom-skill-buildergit clone --depth 1 https://github.com/ashermahonin/agentic-skillsWrote 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/ashermahonin/agentic-skills/custom-skill-builder)<a href="https://agentmods.dev/skills/ashermahonin/agentic-skills/custom-skill-builder"><img src="https://agentmods.dev/badge/skills/ashermahonin/agentic-skills/custom-skill-builder.svg" alt="Measured on agentmods" 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 | $0.00079 | $0.00700 |
| Opus 5 | $0.00039 | $0.00350 |
| Sonnet 5 | $0.00016 | $0.00140 |
| Haiku 4.5 | $0.00008 | $0.00070 |
Grade A, and why
custom-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 8d 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 — 62 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Custom Skill Builder
Purpose
Create focused, portable skills that load at the right time and give an agent information it would not reliably infer on its own.
Skill brief
- Read
references/skill-authoring-contract.md. - Clarify the skill intent in one sentence: what recurring job should this skill make easier?
- Decide whether this is a new skill, an update, or a project-local skill.
- Check for overlap with existing skills. Extend an existing skill when the new behavior is a narrow variant; create a new skill when it has a distinct trigger, workflow, or ownership boundary.
- If the skill depends on current libraries, APIs, CLIs, MCP servers, or platform behavior, verify those docs with Context7 MCP before writing procedural guidance.
Authoring method
- Write a lowercase hyphenated
nameand adescriptionthat says what the skill does and when it should activate. - Keep the body focused on the recurring decision or procedure. Put conditional detail in directly linked references.
- Add
scripts/only for deterministic repeated work andassets/only for files used in output. - Add client metadata such as
agents/openai.yamlonly when it improves a supported client; the core skill must remain usable without it. - Register the skill in routing only when it participates in a multi-skill workflow.
- Validate against the Agent Skills specification and this repository's routing rules.
- Forward-test realistic prompts that should and should not activate the skill. Compare the result with the previous version or a no-skill baseline when the behavior is important.
Portability rules
- Preserve the user's chosen product, scope, and authorization boundary.
- Use current primary documentation for external technology behavior.
- Do not copy manuals into a skill. Keep only the decisions and procedures the agent needs.
- Do not require one fixed heading template. Structure the body around the actual task.
- Do not add a skill-level README, changelog, or installation guide unless it is part of the requested package.
What ships with it
2 files 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.
- 8d ago First seen · 62 lines · 79 tokens per session scan A 9d069d464e47
custom-skill-builder is a skill published in the GitHub repository ashermahonin/agentic-skills (10 stars, last pushed 14d ago), licensed MIT. It adds 79 tokens to every session and 700 once invoked, about $0.0004 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 2026-08-31.
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