Skill · AI & Development

API Contract Designer

API Contract Designer is a Claude skill for AI & Development. Design consistent, production-ready REST and GraphQL contracts with OpenAPI specs. It costs $9.99 on Agensi. It runs on any paid Claude plan. Part of the StrategistKit catalog of 190+ focused Claude skills.

Category: AI & Development
Deliverable: 1 .skill bundle
Outputs: —
Last updated: 13 Jun 2026

$9.99 One-time · lifetime updates

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Overview

What API Contract Designer does.

API Contract Designer works by asking four scoping questions first — your domain, stack, desired output type, and who will consume the result — then proceeds through a fixed sequence of design decisions: resource modeling, naming conventions, error envelopes, pagination strategy, versioning, auth patterns, idempotency, webhooks, and an OpenAPI skeleton. Every cross-cutting decision is locked down once so no individual endpoint diverges. The skill adapts its output to your requested format: structured audit report, copy-paste OpenAPI YAML, or a prioritized checklist.

A typical input: a developer building a multi-tenant SaaS platform tells the skill they need a REST API for an order management domain, targeting third-party integrators, using Node.js, with cursor-based pagination and OAuth 2.0. They want a copy-paste OpenAPI skeleton plus an idempotency audit.

The skill would return a resource map (Order, LineItem, Fulfillment with lifecycle states), a locked error envelope schema, cursor pagination parameters, an Idempotency-Key header contract with retry semantics for POST /orders, and an OAS 3.1 skeleton with path stubs, component schemas, and security scheme definitions — structured so a developer can open it directly in Swagger Editor or Stoplight.

Who it's for

Backend and full-stack developers, API platform teams, and technical leads who need a consistent, reviewable API contract before implementation begins — particularly those handing off specs to external integrators, mobile clients, or other internal teams where breaking changes are costly.

How it works

Three steps. About two minutes.

Install

Add the .skill file to your Claude app. About 10 seconds.

Run it on your work

Invoke the skill and paste in your material.

Apply the output

Review, keep what works, and use it.

In depth

Why a Claude skill beats a prompt template.

A copy-paste prompt runs one static pass and stops. A skill is a bundled program — instructions, examples, and a workflow Claude runs as a unit: it asks for the right input, applies the same pattern every time, and returns the structured outputs above.

FAQ

Common questions.

What output formats does this skill actually produce?

It produces four types depending on what you ask for: structured audit reports with findings and recommendations, copy-paste OpenAPI 3.x YAML skeletons, direct answers with bullet-point summaries, or action checklists. You specify the format in your initial input or let the skill default based on your request type.

Does it support GraphQL as well as REST, or only REST?

Both are supported. You tell the skill which architectural style you are working in during the scoping step, and it adjusts its resource modeling, schema design, and output conventions accordingly. For GraphQL it focuses on type definitions, query/mutation naming, and resolver contracts rather than HTTP verbs and paths.

What do I need to provide to get useful output?

At minimum: a description of your domain (what the API is for), who will consume it, and what you want back (audit, spec, plan). Stack details and constraints improve the specificity of naming conventions, auth recommendations, and the OAS skeleton. If you skip any of these, the skill proceeds with documented defaults.

Can I use this to review an existing API rather than design a new one?

Yes. Tell the skill you want an audit report and paste in your existing endpoint list, schema fragments, or OpenAPI file. It will evaluate consistency, flag non-idempotent operations, identify missing error handling, and produce a prioritized set of findings with recommendations.

Does the skill generate a complete, deployable OpenAPI file or just a skeleton?

It generates a production-ready skeleton: all paths stubbed, component schemas defined, security schemes declared, and standard response shapes filled in. Request and response bodies are modeled from your domain inputs, but business-logic-specific field validation and example values require your review before the spec is final.