Api Design Principles

When to use this skill

Use Api Design Principles when an AI agent needs a reusable SKILL.md workflow for this job: Master REST and GraphQL API design principles to build intuitive, scalable, and maintainable APIs that delight developers. Use when designing new APIs, reviewing API specifications, or establishing API design standards.

When not to use it

Skip Api Design Principles when the task is outside the coding category, or when a more specific skill in this directory already covers the same workflow with clearer triggers.

How to install

1. Personal install: create ~/.claude/skills/api-design-principles/SKILL.md (and any bundled scripts) so Claude Code, Claude Desktop, and compatible agents can load it in every project.
2. Project install: commit the same folder at .claude/skills/api-design-principles/ so teammates get the skill with the repo.
3. Restart the agent session after copying files so it re-scans the skills directory, then ask for the task in words that match the skill description.

Full install guide for Claude, Cursor, and Codex

What this skill does

# API Design Principles

Master REST and GraphQL API design principles to build intuitive, scalable, and maintainable APIs that delight developers and stand the test of time.

## When to Use This Skill

- Designing new REST or GraphQL APIs - Refactoring existing APIs for better usability - Establishing API design standards for your team - Reviewing API specifications before implementation - Migrating between API paradigms (REST to GraphQL, etc.) - Creating developer-friendly API documentation - Optimizing APIs for specific use cases (mobile, third-party integrations)

## Core Concepts

### 1. RESTful Design Principles

**Resource-Oriented Architecture**

- Resources are nouns (users, orders, products), not verbs - Use HTTP methods for actions (GET, POST, PUT, PATCH, DELETE) - URLs represent resource hierarchies - Consistent naming conventions

**HTTP Methods Semantics:**

- `GET`: Retrieve resources (idempotent, safe) - `POST`: Create new resources - `PUT`: Replace entire resource (idempotent) - `PATCH`: Partial resource updates - `DELETE`: Remove resources (idempotent)

### 2. GraphQL Design Principles

**Schema-First Development**

- Types define your domain model - Queries for reading data - Mutations for modifying data - Subscriptions for real-time updates

**Query Structure:**

- Clients request exactly what they need - Single endpoint, multiple operations - Strongly typed schema - Introspection built-in

### 3. API Versioning Strategies

**URL Versioning:**

``` /api/v1/users /api/v2/users ```

**Header Versioning:**

``` Accept: application/vnd.api+json; version=1 ```

**Query Parameter Versioning:**

``` /api/users?version=1 ```

## Detailed patterns and worked examples

Detailed pattern documentation lives in `references/details.md`. Read that file when the navigation tier above is insufficient.

## Best Practices

### REST APIs

1. **Consistent Naming**: Use plural nouns for collections (`/users`, not `/user`) 2. **Stateless**: Each request contains all necessary information 3. **Use HTTP Status Codes Correctly**: 2xx success, 4xx client errors, 5xx server errors 4. **Version Your API**: Plan for breaking changes from day one 5. **Pagination**: Always paginate large collections 6. **Rate Limiting**: Protect your API with rate limits 7. **Documentation**: Use OpenAPI/Swagger for interactive docs

### GraphQL APIs

1. **Schema First**: Design schema before writing resolvers 2. **Avoid N+1**: Use DataLoaders for efficient data fetching 3. **Input Validation**: Validate at schema and resolver levels 4. **Error Handling**: Return structured errors in mutation payloads 5. **Pagination**: Use cursor-based pagination (Relay spec) 6. **Deprecation**: Use `@deprecated` directive for gradual migration 7. **Monitoring**: Track query complexity and execution time

## Common Pitfalls

- **Over-fetching/Under-fetching (REST)**: Fixed in GraphQL but requires DataLoaders - **Breaking Changes**: Version APIs or use deprecation strategies - **Inconsistent Error Formats**: Standardize error responses - **Missing Rate Limits**: APIs without limits are vulnerable to abuse - **Poor Documentation**: Undocumented APIs frustrate developers - **Ignoring HTTP Semantics**: POST for idempotent operations breaks expectations - **Tight Coupling**: API structure shouldn't mirror database schema

Intended uses

• Designing new REST or GraphQL APIs
• Refactoring existing APIs for better usability
• Establishing API design standards for your team
• Reviewing API specifications before implementation
• Migrating between API paradigms (REST to GraphQL, etc.)
• Creating developer-friendly API documentation
• Optimizing APIs for specific use cases (mobile, third-party integrations)