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/ne11nn/cantos-plugin/skill-buildernpx skills add ne11nn/cantos-plugin --skill skill-buildergit clone --depth 1 https://github.com/ne11nn/cantos-pluginWrote 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/ne11nn/cantos-plugin/skill-builder)<a href="https://agentmods.dev/skills/ne11nn/cantos-plugin/skill-builder"><img src="https://agentmods.dev/badge/skills/ne11nn/cantos-plugin/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 | $0.00028 | $0.03035 |
| Opus 5 | $0.00014 | $0.01517 |
| Sonnet 5 | $0.00006 | $0.00607 |
| Haiku 4.5 | $0.00003 | $0.00303 |
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.
This is a copy
100% identical to skill-builder — 0 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.
How it starts
The opening of the file, as written. The whole thing — 286 lines — stays where its author put it; the contents beside it link to each section on GitHub.
What This Skill Does
Guides the creation and optimization of Claude Code skills using official best practices. Use this whenever:
- Building a new skill from scratch
- Optimizing or auditing an existing skill
- Deciding on advanced features (subagent execution, hooks, dynamic context, etc.)
- Troubleshooting a skill that isn't working correctly
For the complete technical reference on all frontmatter fields, advanced patterns, and troubleshooting, see reference.md.
Quick Start: What Is a Skill?
A skill is a reusable set of instructions that tells Claude Code how to handle a specific task. Skills live in .claude/skills/[skill-name]/SKILL.md inside your project. When you type /skill-name or describe what you need in natural language, Claude loads the skill's instructions and follows them.
Think of skills as SOPs for Claude. Instead of re-explaining a workflow every conversation, you write it once and invoke it forever.
How they work under the hood:
- Your project's
CLAUDE.mdinstructions are always loaded, every conversation - Skill descriptions (from frontmatter) are always loaded so Claude knows what's available
- The full skill content only loads when the skill is actually invoked
- Once loaded, Claude follows the skill's instructions while still respecting your CLAUDE.md rules
Mode 1: Build a New Skill
When building a new skill, run the Discovery Interview first. Do NOT start writing files until discovery is complete.
Discovery Interview
Ask questions using AskUserQuestion, one round at a time. Each round covers one topic. Move to the next round only after the user answers. Keep going until you're 95% confident you understand the skill well enough to build it without further clarification.
Round 1: Goal & Name
Why this matters: A clear goal prevents scope creep. The name becomes the /slash-command, so it needs to be memorable and specific.
- What does this skill do? What problem does it solve or what workflow does it automate?
- What should we call it? (Suggest a name based on their answer -- lowercase, hyphens, max 64 chars)
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 · 286 lines · 28 tokens per session scan A 0dd3dcaf8d9e
skill-builder is a skill published in the GitHub repository ne11nn/cantos-plugin (1 stars, last pushed 3d ago), licensed MIT. It adds 28 tokens to every session and 3,035 once invoked, about $0.0001 per session on Opus 5. A static security scan graded it A with 0 findings. It is 100% identical to skill-builder, differing in 0 lines, and is treated as a copy.
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