api-and-interface-design

name	api-and-interface-design
description	Guides stable API and interface design. Use when designing APIs, module boundaries, or any public interface. Use when creating REST or GraphQL endpoints, defining type contracts between modules, or establishing boundaries between frontend and backend.
zh_description	用于API、interface、设计，支持任务规划、执行、评审和验证。
version	1.0.0
author	addyosmani
source	github:addyosmani/agent-skills
source_url	https://github.com/addyosmani/agent-skills/blob/main/skills/api-and-interface-design/SKILL.md
license	MIT
tags	["agent", "ai", "api-and-interface-design", "engineering", "workflow"]
created_at	2026-07-27
updated_at	2026-07-27
quality	5
complexity	advanced
upstream_slug	api-and-interface-design

API and Interface Design

Overview

Design stable, well-documented interfaces that are hard to misuse. Good interfaces make the right thing easy and the wrong thing hard. This applies to REST APIs, GraphQL schemas, module boundaries, component props, and any surface where one piece of code talks to another.

When to Use

• Designing new API endpoints
• Defining module boundaries or contracts between teams
• Creating component prop interfaces
• Establishing database schema that informs API shape
• Changing existing public interfaces

Core Principles

Hyrum's Law

With a sufficient number of users of an API, all observable behaviors of your system will be depended on by somebody, regardless of what you promise in the contract.

This means: every public behavior — including undocumented quirks, error message text, timing, and ordering — becomes a de facto contract once users depend on it. Design implications:

• Be intentional about what you expose. Every observable behavior is a potential commitment.
• Don't leak implementation details. If users can observe it, they will depend on it.
• Plan for deprecation at design time. See deprecation-and-migration for how to safely remove things users depend on.
• Tests are not enough. Even with perfect contract tests, Hyrum's Law means "safe" changes can break real users who depend on undocumented behavior.

The One-Version Rule

Avoid forcing consumers to choose between multiple versions of the same dependency or API. Diamond dependency problems arise when different consumers need different versions of the same thing. Design for a world where only one version exists at a time — extend rather than fork.

1. Contract First

Define the interface before implementing it. The contract is the spec — implementation follows.

// Define the contract first
interface TaskAPI {
  // Creates a task and returns the created task with server-generated fields
  createTask(input: CreateTaskInput): Promise<Task>;

  // Returns paginated tasks matching filters
  (: ): <<>>;

  
  (: ): <>;

  
  (: , : ): <>;

  
  (: ): <>;
}