ruby

A set of conventions for writing Ruby code, including common language patterns, error handling, design choices, and RSpec tests. Ruby is a programming language often used for web applications and scripts.

In plain words
What is it for?
Use it when writing or reviewing Ruby classes, modules, methods, error handling, enumerable operations, and RSpec tests.
Why use it?
It keeps Ruby code consistent and easier to read while reducing unnecessary inheritance, deeply nested logic, and weak test structure.

Cursor rule

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.

agentmods
npx agentmods add rules/nedcodes-ok/cursorrules-collection/ruby
Clone the repo
git clone --depth 1 https://github.com/nedcodes-ok/cursorrules-collection
Per session 267 This file is loaded in full into every session.
When invoked 267 The same file — it is already loaded in full.
Security scan A 0 findings. Scan, not verified.
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.

ModelPer sessionOnce invoked
Fable 5 $0.00267 $0.00267
Opus 5 $0.00133 $0.00133
Sonnet 5 $0.00053 $0.00053
Haiku 4.5 $0.00027 $0.00027

Measured 2d ago against content hash cfc8f210e015, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

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 2d 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.

rules-mdc/languages/ruby.mdc · 39 lines

What it actually says

Ruby Cursor Rules

You are an expert Ruby developer. Follow these rules:

Code Style

  • Follow Ruby Style Guide (rubocop defaults)
  • 2-space indentation. snake_case methods, PascalCase classes
  • Prefer single-quoted strings without interpolation
  • Use trailing conditionals: return if invalid?

Design

  • Favor duck typing and composition over inheritance
  • Use modules for shared behavior (mixins)
  • Keep methods under 15 lines, classes under 150
  • Use Struct or Data (Ruby 3.2+) for value objects

Idioms

  • Use &:method_name for simple blocks: users.map(&:name)
  • Use guard clauses to reduce nesting
  • Use Enumerable methods over manual loops
  • Use frozen_string_literal: true in all files

Error Handling

  • Custom errors inherit StandardError, not Exception
  • Rescue at method level, not individual lines
  • Provide meaningful messages with context

Testing

  • Use RSpec with descriptive context/it blocks
  • Use let for lazy setup, subject for the tested thing
  • Use factories (FactoryBot) over fixtures
  • Use shared_examples for common interfaces
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.

  1. 2d ago First seen · 39 lines · 267 tokens per session scan A cfc8f210e015

Subscribe to this mod's changes

ruby is a cursor rule published in the GitHub repository nedcodes-ok/cursorrules-collection (37 stars, last pushed 6mo ago), licensed MIT. It adds 267 tokens to every session, about $0.0013 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.