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 ericrisco/rsc-harness --skill rubygit clone --depth 1 https://github.com/ericrisco/rsc-harnessWrote 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/ericrisco/rsc-harness/ruby)<a href="https://agentmods.dev/skills/ericrisco/rsc-harness/ruby"><img src="https://agentmods.dev/badge/skills/ericrisco/rsc-harness/ruby/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/ericrisco/rsc-harness/ruby"><img src="https://agentmods.dev/badge/skills/ericrisco/rsc-harness/ruby.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 | $0.00087 | $0.03371 |
| Opus 5.5 | $0.00035 | $0.01348 |
| Sonnet 5.5 | $0.00017 | $0.00674 |
| Haiku 4.5 | $0.00009 | $0.00337 |
Grade A, and why
ruby 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 — 296 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Idiomatic Ruby, the language
Write Ruby a senior Rubyist would sign off on: expression-oriented, block-driven,
leaning on Enumerable and Comparable instead of hand-rolled loops, with
frozen_string_literal hygiene and a real Gemfile/.gemspec when you package.
This is Ruby the language and its non-Rails ecosystem — blocks, modules, gems,
metaprogramming, Minitest/RSpec. Delegate: Rails apps, ActiveRecord models, controllers,
migrations, views, Hotwire — anything that loads Rails or uses ActiveSupport/ActiveRecord
idioms — to rails; the same task in another language to
python, go, rust,
elixir, php or
typescript; the CI pipeline that runs your Ruby tests to
github-actions — this skill writes the tests, not the workflow.
Targets Ruby 4.0 (released 2025-12-25) on 3.4 semantics: Prism is the default
parser, string literals warn on mutation without a magic comment, it is an implicit
block param, and Set is now a core class.
Mental model
Internalize these before writing a line:
- Everything is an expression and returns a value.
if,case, blocks, method bodies all yield a value — name and return it instead of threading a flag. - Everything is an object receiving messages.
5.times,nil.to_a,"x".freeze. There are no primitives to special-case. - Reach for
Enumerablebefore you write a loop. A manual index loop in Ruby is almost always amap/select/reduceyou have not spotted yet. - Prefer returning new values to mutating in place. Mutation is allowed but it is the exception you justify, not the default.
- Freeze your string literals. New files start with
# frozen_string_literal: true. - Dynamism is the trap, not the feature. Metaprogramming is a scalpel; if a plain
method, a
Struct, or aDatadoes the job, use that.
What ships with it
5 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.
- 3d ago First seen · 296 lines · 87 tokens per session scan A 97ffa368b6a6
ruby is a skill published in the GitHub repository ericrisco/rsc-harness (314 stars, last pushed today), licensed MIT. It adds 87 tokens to every session and 3,371 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 2026-10-24.
Other skills, from other repositories
ruby
Ruby programming language — syntax, core types, OOP, blocks/procs/lambdas, exceptions, I/O, concurrency, metaprogramming, gems, testing, style guide, C extensions, and tooling.
ruby
Ruby development guidelines covering idiomatic code style, Ruby 3.x features, testing with RSpec, and best practices for building maintainable Ruby applications.
rspec
RSpec testing best practices for Ruby and Rails applications, covering test organization, data management, and isolation patterns.
ruby-expert
Expert-level Ruby development with Rails, modern features, testing, and best practices. Use when the user mentions Ruby on Rails, RSpec, gem, Sinatra, or metaprogramming, or when the task involves Ruby 3+ Features, Object-Oriented, Functional Features, or Pattern Matching.
ruby
Ruby programming patterns and idioms.
Ruby Patterns
Use this skill when working in Ruby projects (Rails or otherwise) and you want clean object boundaries, predictable dependency management, and maintainable code organization.