openapi-expert

An OpenAPI and Swagger guide for designing, checking, documenting, and generating code from API specifications. OpenAPI is a standard description of how software APIs work.

In plain words
What is it for?
Use it to design REST APIs, define security and versioning, validate specifications, and prepare them for documentation or code generation.
Why use it?
It helps prevent inconsistent API contracts, unclear documentation, broken consumers, and repeated schema definitions.

Skill for Claude CodeCodex

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 skills/librefang/librefang-registry/openapi-expert
Any agent
npx skills add librefang/librefang-registry --skill openapi-expert
Clone the repo
git clone --depth 1 https://github.com/librefang/librefang-registry

Made for: Claude Code, Codex.

Per session 20 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 816 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin 98% copy Near-identical to another mod 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 $0.00020 $0.00816
Opus 5 $0.00010 $0.00408
Sonnet 5 $0.00004 $0.00163
Haiku 4.5 $0.00002 $0.00082

Measured 2d ago against content hash bb9927a20634, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

openapi-expert 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.

Origin

This is a copy

98% identical to openapi-expert — 3 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.

skills/openapi-expert/SKILL.md · 43 lines

How it starts

The opening of the file, as written. The whole thing — 43 lines — stays where its author put it; the contents beside it link to each section on GitHub.

OpenAPI Expert

An API design architect with deep expertise in the OpenAPI Specification, RESTful API conventions, and the tooling ecosystem for validation, documentation, and code generation. This skill provides guidance for designing clear, consistent, and evolvable API contracts using OpenAPI 3.0 and 3.1, covering schema composition, security definitions, versioning strategies, and developer experience optimization.

Key Principles

  • Design the API specification before writing implementation code; the spec serves as the contract between frontend, backend, mobile, and third-party consumers
  • Use $ref extensively to define reusable schemas, parameters, and responses in the components section; duplication across paths leads to inconsistency and maintenance burden
  • Version your API explicitly through URL path prefixes (/v1/, /v2/) or custom headers; never break existing consumers by changing response shapes without a version boundary
  • Write meaningful descriptions for every path, parameter, schema property, and response; the spec doubles as your API documentation and should be understandable without reading source code
  • Validate the spec in CI using linting tools to catch breaking changes, missing descriptions, inconsistent naming, and schema errors before they reach production

Techniques

  • Structure the OpenAPI document with info (title, version, contact), servers (base URLs per environment), paths (endpoints), and components (schemas, securitySchemes, parameters, responses)
  • Compose schemas using allOf for inheritance (base object + extension), oneOf for polymorphism (exactly one match), and anyOf for flexible unions (at least one match)
  • Provide request and response examples at both the schema level and the media type level; tools like Swagger UI and Redoc render these prominently for developer reference
  • Define security schemes (Bearer JWT, API key, OAuth2 flows) in components/securitySchemes and apply them globally or per-operation with the security field
  • Distinguish path parameters (/users/{id}), query parameters (?page=2&limit=20), and header parameters for different use cases; path parameters identify resources, query parameters filter or paginate
  • Implement consistent pagination with limit/offset or cursor-based patterns, documenting the pagination metadata schema (total, next_cursor, has_more) in a reusable component
  • Generate server stubs and client SDKs using openapi-generator with language-specific templates; customize templates for your coding conventions

Read the full file on GitHub · 43 lines

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 · 43 lines · 20 tokens per session scan A bb9927a20634

Subscribe to this mod's changes

openapi-expert is a skill published in the GitHub repository librefang/librefang-registry (11 stars, last pushed 8d ago), licensed MIT. It adds 20 tokens to every session and 816 once invoked, about $0.0001 per session on Opus 5. A static security scan graded it A with 0 findings. It is 98% identical to openapi-expert, differing in 3 lines, and is treated as a copy.

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