What is custom-skill-builder?

A guide for creating or updating reusable Agent Skills in the open SKILL.md format. A skill is a focused set of instructions that an agent loads for a recurring project, team, domain, or tool task.

Use it to define skill triggers, write portable procedures, decide between extending an existing skill and creating a new one, and add references or scripts when they support repeatable work.

It helps ensure a skill activates for the right requests, stays focused, and contains information the agent would not reliably infer. It also helps avoid creating overlapping skills or adding unnecessary files.

custom-skill-builder — Skill for Codex by ashermahonin/agentic-skills

Install

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.

Any agent

npx skills add ashermahonin/agentic-skills --skill custom-skill-builder

Clone the repo

git clone --depth 1 https://github.com/ashermahonin/agentic-skills

Made for: Codex.

Wrote 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.

README.md

[![agentmods](https://agentmods.dev/badge/skills/ashermahonin/agentic-skills/custom-skill-builder/github.svg)](https://agentmods.dev/skills/ashermahonin/agentic-skills/custom-skill-builder)

Your own site

<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/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.

Your own site · 80×15

<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="Reviewed on agentmods" width="80" height="20"></a>

Per session 79 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 700 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.

Origin original No closer match found in the catalogue.

Token cost

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 5.5	$0.00032	$0.00280
Sonnet 5.5	$0.00016	$0.00140
Haiku 4.5	$0.00008	$0.00070

Measured 2mo ago against content hash 9d069d464e47, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

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 2mo 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.

agentic/skills/custom-skill-builder/SKILL.md · 62 lines

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 name and a description that 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 and assets/ only for files used in output.
Add client metadata such as agents/openai.yaml only 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.

Read the full file on GitHub · 62 lines

Files

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.

Changes

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.

2mo ago First seen · 62 lines · 79 tokens per session scan A 9d069d464e47

Subscribe to this mod's changes

custom-skill-builder is a skill published in the GitHub repository ashermahonin/agentic-skills (10 stars, last pushed 2mo ago), licensed MIT. It adds 79 tokens to every session and 700 once invoked, about $0.0003 per session on Opus 5.5. 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.

Building agents

custom-skill-builder next to 6 skills from other repositories
Mod	Type	Stars	Last push	Licence	Scan	Tokens	Works with
custom-skill-builderashermahonin/agentic-skills	Skill	10	2mo ago	MIT	A	79	Codex
imagegenopenai/codex	Skill	133k	yesterday	Apache-2.0	A	113	Codex
playwright-climicrosoft/playwright	Skill	99k	yesterday	Apache-2.0	A	19	Claude Code
local-ai-agentsmicrosoft/ai-agents-for-beginners	Skill	79k	4d ago	MIT	A	200	Claude Code, Codex
systematic-debuggingobra/superpowers	Skill	313k	3d ago	MIT	A	21	Claude Code
next-cache-components-adoptionvercel/next.js	Skill	143k	yesterday	MIT	A	95	Claude Code
next-partial-prefetching-adoptionvercel/next.js	Skill	143k	yesterday	MIT	A	103	Claude Code