GraphQL is a query language for APIs that allows clients to request exactly the data they need, nothing more and nothing less. Unlike REST where you typically hit multiple endpoints to gather related data, GraphQL uses a single endpoint. The client specifies the shape of the response, reducing over-fetching and under-fetching of data. This leads to more efficient network usage and faster development cycles.

A GraphQL schema defines the types, queries, mutations, and subscriptions available in an API. It serves as a contract between client and server. Introspection is a built-in feature that allows you to query the schema itself (using __schema and __type meta-fields). Click "Load Schema" to fetch the full type system, which powers autocomplete and documentation features in this playground.

Variables make your queries reusable and safe. Define them in the query with a $ prefix (e.g., query($id: ID!)), then provide their values in the Variables tab as a JSON object (e.g., {"id": "123"}). This playground sends variables alongside your query in the request body. Never interpolate user input directly into query strings to avoid injection risks.

CORS (Cross-Origin Resource Sharing) errors occur when the GraphQL server doesn't allow requests from browser-based tools like this playground. This is a server-side configuration issue. Solutions: (1) Use a public API that has CORS enabled (try our presets), (2) Configure your server to include Access-Control-Allow-Origin headers, (3) Use a browser extension that bypasses CORS during development, or (4) Run a local proxy server.

Click the "Headers" tab in the editor panel. Enter headers one per line in the format Header-Name: value. Common examples: Authorization: Bearer <token> for JWT/auth tokens, or X-API-Key: your-key for API key authentication. Headers are sent with every request to the endpoint.

Common errors include: Syntax errors (malformed query), Validation errors (query doesn't match schema, e.g. requesting non-existent fields), Resolver errors (server-side failures during data fetching), Authentication errors (missing/invalid credentials), and Rate limiting (too many requests). GraphQL returns errors in an errors array alongside partial data in the response.

Yes! This playground automatically saves your query history to your browser's local storage. Click the "History" button to view, load, or manage past queries. Your history includes the query text, variables, endpoint URL, and timestamp. No data is sent to any server — everything stays on your device. You can also download responses as JSON files.