GraphQL Code Generator Core Knowledge

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: graphql-codegen for comprehensive documentation.

Installation

npm install -D @graphql-codegen/cli @graphql-codegen/typescript \
  @graphql-codegen/typescript-operations @graphql-codegen/client-preset

Basic Configuration

// codegen.ts
import type { CodegenConfig } from '@graphql-codegen/cli';

const config: CodegenConfig = {
  schema: 'http://localhost:4000/graphql',
  documents: ['src/**/*.tsx', 'src/**/*.ts'],
  ignoreNoDocuments: true,
  generates: {
    './src/gql/': {
      preset: 'client',
      config: {
        documentMode: 'string',
      },
    },
  },
};

export default config;

Run Generation

npx graphql-codegen
npx graphql-codegen --watch  # Watch mode

Client Preset (Recommended)

The client preset generates everything needed for type-safe GraphQL.

// codegen.ts
const config: CodegenConfig = {
  schema: 'http://localhost:4000/graphql',
  documents: ['src/**/*.tsx'],
  generates: {
    './src/gql/': {
      preset: 'client',
      plugins: [],
      presetConfig: {
        gqlTagName: 'gql',
        fragmentMasking: { unmaskFunctionName: 'getFragmentData' },
      },
    },
  },
};

Usage

import { gql } from '../gql';
import { useQuery } from '@apollo/client';

const GET_USER = gql(`
  query GetUser($id: ID!) {
    user(id: $id) {
      id
      name
      email
    }
  }
`);

function UserProfile({ id }: { id: string }) {
  const { data, loading } = useQuery(GET_USER, {
    variables: { id },
  });

  if (loading) return <Spinner />;
  // data.user is fully typed
  return <div>{data?.user?.name}</div>;
}

---

TanStack Query Integration

npm install -D @graphql-codegen/typescript-react-query

// codegen.ts
const config: CodegenConfig = {
  schema: 'http://localhost:4000/graphql',
  documents: ['src/**/*.graphql'],
  generates: {
    './src/gql/index.ts': {
      plugins: [
        'typescript',
        'typescript-operations',
        'typescript-react-query',
      ],
      config: {
        fetcher: {
          func: './fetcher#fetcher',
          isReactHook: false,
        },
        reactQueryVersion: 5,
        addInfiniteQuery: true,
      },
    },
  },
};

Fetcher

// src/gql/fetcher.ts
export const fetcher = <TData, TVariables>(
  query: string,
  variables?: TVariables
): (() => Promise<TData>) => {
  return async () => {
    const response = await fetch('http://localhost:4000/graphql', {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        Authorization: `Bearer ${getToken()}`,
      },
      body: JSON.stringify({ query, variables }),
    });

    const json = await response.json();
    if (json.errors) {
      throw new Error(json.errors[0].message);
    }
    return json.data;
  };
};

Generated Hooks Usage

import { useGetUserQuery, useCreateUserMutation } from './gql';

function UserProfile({ id }: { id: string }) {
  const { data, isLoading } = useGetUserQuery({ id });
  const createMutation = useCreateUserMutation();

  const handleCreate = () => {
    createMutation.mutate({
      input: { name: 'John', email: 'john@example.com' },
    });
  };

  if (isLoading) return <Spinner />;
  return <div>{data?.user?.name}</div>;
}

---

Fragment Colocation

// components/UserAvatar.tsx
import { gql, FragmentType, getFragmentData } from '../gql';

export const USER_AVATAR_FRAGMENT = gql(`
  fragment UserAvatar on User {
    id
    name
    avatarUrl
  }
`);

interface Props {
  user: FragmentType<typeof USER_AVATAR_FRAGMENT>;
}

export function UserAvatar({ user }: Props) {
  const data = getFragmentData(USER_AVATAR_FRAGMENT, user);
  return <img src={data.avatarUrl} alt={data.name} />;
}

// Usage in parent query
const GET_USER = gql(`
  query GetUser($id: ID!) {
    user(id: $id) {
      id
      ...UserAvatar
    }
  }
`);

---

Production Readiness

Schema Polling

// codegen.ts
const config: CodegenConfig = {
  schema: [
    {
      'http://localhost:4000/graphql': {
        headers: {
          Authorization: `Bearer ${process.env.GRAPHQL_TOKEN}`,
        },
      },
    },
  ],
  // ...
};

Multiple Schemas

const config: CodegenConfig = {
  generates: {
    './src/gql/user-api/': {
      schema: 'http://user-api:4000/graphql',
      documents: ['src/features/user/**/*.tsx'],
      preset: 'client',
    },
    './src/gql/product-api/': {
      schema: 'http://product-api:4001/graphql',
      documents: ['src/features/product/**/*.tsx'],
      preset: 'client',
    },
  },
};

CI/CD Integration

# .github/workflows/codegen.yml
name: GraphQL Codegen

on:
  push:
    paths:
      - 'src/**/*.graphql'
      - 'src/**/*.tsx'

jobs:
  generate:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - run: npm ci
      - run: npx graphql-codegen
      - name: Check for changes
        run: |
          if [[ -n $(git status --porcelain src/gql) ]]; then
            echo "Generated files changed"
            exit 1
          fi

Package Scripts

{
  "scripts": {
    "codegen": "graphql-codegen",
    "codegen:watch": "graphql-codegen --watch"
  }
}

Checklist

• Schema source configured (URL or local)
• Documents path matches source files
• Client preset for optimal output
• Fetcher configured with auth
• Fragment colocation pattern
• Watch mode for development
• CI validation of generated code
• TypeScript strict mode compatible

When NOT to Use This Skill

• REST API type generation (use openapi-codegen skill)
• tRPC type-safe APIs (use trpc skill)
• GraphQL schema design (use graphql skill)
• Non-TypeScript projects
• Simple GraphQL queries without type generation needs

Anti-Patterns

Anti-Pattern	Why It's Bad	Solution
Committing generated files to git	Merge conflicts, outdated types	Add to .gitignore, generate in CI
Not using client preset	Verbose configuration	Use client preset for modern setup
Ignoring schema changes	Type mismatches at runtime	Run codegen in watch mode during dev
Missing operationId or query names	Poor generated hook names	Name all queries/mutations
Not using fragments	Code duplication	Use fragments for reusable fields
Generating types only	Missing runtime validation	Combine with schema validation
Hardcoding schema URL	Environment coupling	Use env variables for schema source
Not versioning generator packages	Inconsistent output across team	Pin generator versions

Quick Troubleshooting

Issue	Possible Cause	Solution
Generation fails	Invalid schema or documents	Validate schema, check GraphQL syntax
Type errors after generation	Schema/code mismatch	Regenerate types, check schema changes
Missing types	Documents path not matching files	Check documents glob pattern
Duplicate operation names	Same query name in multiple files	Use unique operation names
Fragment not found	Fragment not in documents	Include fragment file in documents
Hook not generated	Not using React Query plugin	Add typescript-react-query plugin
"Cannot find module './gql'"	Generation didn't run	Run npm run codegen
Slow generation	Too many documents	Optimize glob patterns, use ignoreNoDocuments
Type inference not working	Wrong import path	Import from generated gql folder

Reference Documentation

• Client Preset
• React Query Plugin
• Typed Document Node