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API and Interface Design

A skill that guides Claude to design stable, hard-to-misuse REST/GraphQL endpoints, module boundaries, and type contracts.

Dev & CodingIntermediate★ 423⑂ 45AI score 8/10Last updated: Sep 13, 2026

What it does

Gives Claude a concrete rulebook for designing public interfaces.

  • Contract first: define typed input/output before implementing
  • Consistent error semantics: a single { error: { code, message, details } } shape plus an HTTP status mapping table
  • Validate at boundaries only: route handlers, forms, third-party responses (always untrusted), env loading — trust types internally
  • Prefer addition over modification: optional new fields instead of type changes or removals
  • Deep idempotency-key guidance: key derivation, atomic claim via unique constraint, payload-hash guard, in-flight duplicate strategies (409 / wait / 202), retention windows
  • Naming conventions table, rationalization rebuttals, red flags, and a final verification checklist

Who it's for

  • Backend engineers designing or refactoring REST/GraphQL APIs
  • Tech leads defining module boundaries and cross-team contracts
  • Anyone building payment/order flows where retries can double-charge
  • API reviewers who need a consistent standard

Examples

  1. "Design a REST API for task management" → resource paths, pagination envelope, PATCH partial updates, uniform error bodies
  2. "Add an Idempotency-Key to our charge endpoint" → stable key derivation, unique-constraint claim, 422 on same key with different payload
  3. "Is adding this field to our existing API safe?" → backward-compatibility rules checked through the lens of Hyrum's Law

· · · Install guide · · ·

Try it now, no install

Paste this into Claude to use the skill without installing anything.

Read the instructions in this file and follow them to help me:
https://raw.githubusercontent.com/aiskillstore/marketplace/HEAD/pending/addyosmani/api-and-interface-design/SKILL.md

What I want: (describe your task here)

If Claude can't open the link, open it yourself and paste the contents instead.

↓ If it works for you, download the ZIP below and install it. Then it runs on its own — no pasting each time.

Install in the Claude app (no terminal)
  1. Download the ZIP with the button below.
  2. In Claude, open Settings → Capabilities and turn on 'Code execution and file creation'. (one time)
  3. Go to Customize → Skills → + → 'Upload a skill' and upload the ZIP.
↓ Download ZIP
Install in Claude Code

Let Claude do it — paste this into Claude Code

Install the skill I found on Claude Skill Mart.
Copy the pending/addyosmani/api-and-interface-design folder from the GitHub repo aiskillstore/marketplace into my ~/.claude/skills/api-and-interface-design/.
When it's done, tell me in one line what this skill can do.

Install with a command instead

git clone https://github.com/aiskillstore/marketplace.git && mkdir -p ~/.claude/skills && cp -r marketplace/pending/addyosmani/api-and-interface-design ~/.claude/skills/

⚠ This is a third-party skill. Check the source repository before installing.

  1. Open a terminal.
  2. Clone the repo: git clone https://github.com/aiskillstore/marketplace.git
  3. Create the skills folder: mkdir -p ~/.claude/skills
  4. Copy the skill: cp -r marketplace/pending/addyosmani/api-and-interface-design ~/.claude/skills/
  5. Verify that ~/.claude/skills/api-and-interface-design/SKILL.md exists.
  6. Restart Claude Code and ask something like "design a REST API for ..." — the skill will be picked up automatically.
View source on GitHub ↗