---name: graphql-schemadescription: >whenever you create or edit GraphQL SDL, such as a `schema.graphql` file or `typeDefs`, including when: license: MITcompatibility: Any GraphQL implementation (Apollo Server, graphql-js, Yoga, etc.)metadata: author: apollographqlversion: 1.0.2allowed-tools: Bash(npm:*) Bash(npx:*) Read Write Edit Glob Grep---

GraphQL Schema Design Guide

This guide covers best practices for designing GraphQL schemas that are intuitive, performant, and maintainable. Schema design is primarily a server-side concern that directly impacts API usability.

Schema Design Principles

1. Design for Client Needs

• Think about what queries clients will write
• Organize types around use cases, not database tables
• Expose capabilities, not implementation details

2. Be Explicit

• Use clear, descriptive names
• Make nullability intentional
• Document with descriptions

3. Design for Evolution

• Plan for backwards compatibility
• Use deprecation before removal
• Avoid breaking changes
• Give every new argument or input field a default value, or make it nullable. Existing clients don't send it, so a required field without a default makes their requests fail validation.

Quick Reference

Type Definition Syntax

"""
A user in the system.
"""
type User {
  id: ID!
  email: String!
  name: String
  posts(first: Int = 10, after: String): PostConnection!
  createdAt: DateTime!
}

Nullability Rules

Pattern	Meaning
String	Nullable - may be null
String!	Non-null - always has value
[String]	Nullable list, nullable items
[String!]	Nullable list, non-null items
[String]!	Non-null list, nullable items
[String!]!	Non-null list, non-null items

Best Practice: Use [Type!]! for lists - empty list over null, no null items.

Input vs Output Types

# Output type - what clients receive
type User {
  id: ID!
  email: String!
  createdAt: DateTime!
}

# Input type - what clients send
input CreateUserInput {
  email: String!
  name: String
}

# Mutation using input type
type Mutation {
  createUser(input: CreateUserInput!): User!
}

Interface Pattern

interface Node {
  id: ID!
}

type User implements Node {
  id: ID!
  email: String!
}

type Post implements Node {
  id: ID!
  title: String!
}

Union Pattern

union SearchResult = User | Post | Comment

type Query {
  search(query: String!): [SearchResult!]!
}

Reference Files

Detailed documentation for specific topics:

• Types - Type design patterns, interfaces, unions, and custom scalars
• Naming - Naming conventions for types, fields, and arguments
• Pagination - Connection pattern and cursor-based pagination
• Errors - Error modeling and result types
• Security - Security best practices for schema design

Key Rules

Type Design

• Define types based on domain concepts, not data storage
• Use interfaces for shared fields across types
• Use unions for mutually exclusive types
• Keep types focused (single responsibility)
• Avoid deep nesting - flatten when possible

Field Design

• Fields should be named from client's perspective
• Return the most specific type possible
• Make expensive fields explicit (consider arguments)
• Use arguments for filtering, sorting, pagination

Mutation Design

• Use single input argument pattern: mutation(input: InputType!)
• Return affected objects in mutation responses
• Model mutations around business operations, not CRUD
• Consider returning a union of success/error types

ID Strategy

• Use globally unique IDs when possible
• Implement Node interface for refetchability
• Base64-encode compound IDs if needed

Ground Rules

• ALWAYS add descriptions to types and fields
• ALWAYS use non-null (!) for fields that cannot be null
• ALWAYS use [Type!]! pattern for lists
• ALWAYS paginate lists that can grow without limit, such as Query.users or Post.comments: return a connection and give first a default page size
• NEVER expose database internals in schema
• NEVER break backwards compatibility without deprecation
• NEVER add a required argument or input field without a default value
• PREFER dedicated input types over many arguments
• PREFER enums over arbitrary strings for fixed values
• USE ID type for identifiers, not String or Int
• USE custom scalars for domain-specific values (DateTime, Email, URL)