Tool Overview

The GraphQL Mock Server generates mock responses for GraphQL queries based on a schema that you provide. You paste or edit a GraphQL schema and a query, and the tool returns realistic JSON data that matches the types, fields, and relationships in the schema. The response looks similar to what a real GraphQL API would send, but it is generated instantly in your browser.

This solves a common problem in frontend and API development. Frontend teams often want to build and test user interfaces before the backend is finished. Backend teams want a safe way to try queries without running a live database. Building a full mock backend from scratch is slow and requires extra code. This tool gives you mock data in seconds, without any server setup.

The tool is designed for developers and technical users who are familiar with GraphQL schemas and queries. It is useful for frontend engineers, API designers, QA testers, and anyone who needs sample data for demos or tests. You do not need to write any mock resolvers yourself. The tool reads the schema, validates the query, and fills in reasonable values for each field.

Background & Concept Explanation

GraphQL is a query language for APIs. Clients send queries that describe exactly which fields they need, and servers respond with JSON data that matches the query shape. A GraphQL schema written in SDL (Schema Definition Language) defines the types, fields, and relationships that the API supports. The schema tells both humans and tools what data is available.

During development, teams often need mock data that has the same structure as the final API. Mock data helps with UI design, state management, testing, and documentation. Without a mock server, developers might hand-write JSON objects, which is time-consuming and easy to get wrong. They might guess field shapes that later differ from the real schema, causing rework.

A GraphQL mock server solves this by using the schema as the single source of truth. Instead of guessing shapes, it reads the types and fields from the schema. When you run a query, the mock server checks that the query matches the schema, then generates values that fit each field’s type. This gives you consistent, schema-driven mock responses without writing resolvers or database code.

Key Features

• Schema editor with SDL support. The tool includes a dedicated editor for your GraphQL schema. It uses a plain text area with monospaced font and shows the language as GraphQL. You can paste an existing schema, edit the default sample schema, or replace it with your own SDL. The editor enforces a maximum schema length to protect performance.
• Query editor for GraphQL operations. A second editor lets you write or paste GraphQL queries. It accepts queries that reference types defined in your schema. The editor also enforces a maximum query length to prevent very large inputs from causing delays.
• Automatic mock response generation. Whenever you change the schema or the query, the tool runs the mock generation logic after a short delay. It parses the schema, parses the query, validates that the query matches the schema, and then executes the query against a generated resolver that returns mock values. The result is shown as pretty-printed JSON in a response panel.
• Schema and query validation. Before generating data, the tool validates your schema and query. If the schema is empty, missing, or too long, it returns a clear error. If the query is empty, missing, or too long, it also returns an error. It uses GraphQL validation to ensure that fields and types in the query exist in the schema. Validation messages appear in the UI so you can fix issues quickly.
• Type-aware mock value generation. The mock engine generates values that respect GraphQL scalar types. Int fields get random integers, Float fields get decimal numbers, Boolean fields alternate between true and false, ID fields get id-style strings, and String fields get readable text. For string fields, the tool uses field names as hints: names become person-like names, email fields become email-like strings, title fields become sample titles, and url fields become URL-like strings.
• Support for objects and lists. When a field is an object type, the mock server recursively generates an object and fills in each sub-field. When a field is a list type, it generates arrays containing multiple mock items. This creates realistic nested structures that mirror real GraphQL responses.
• Error handling and feedback. If schema parsing fails, query parsing fails, or validation errors occur, the tool returns a response with isValid set to false and a list of error messages. These errors show up in the response panel and next to the relevant editor. This feedback helps you correct syntax errors and mismatched fields.
• Sample schemas and queries. The tool ships with built-in schema and query samples for common domains like e‑commerce and blogging. Each sample set includes a schema and a matching query. Buttons below the schema editor let you load these samples with a single click, which is useful for learning or quick demos.
• AI-powered schema generation. An integrated modal lets you describe your API in natural language. You click a Generate Schema button, type a description like “an e‑commerce API with products, categories, and orders,” and the tool calls an AI backend to generate a GraphQL schema. The returned SDL is inserted into the schema editor, so you can start mocking responses right away.
• AI-powered query suggestion. A button in the query editor can request a suggested query from the AI backend based on your current schema. If a schema is present, the tool sends it to the AI, which responds with a GraphQL query string. The query editor is then updated with this suggested query, giving you a starting point for testing.
• Real-time processing state indicators. When the mock engine or AI operations are running, the tool shows small loading spinners and “Processing” messages. This tells you that the system is working, especially for larger schemas or slower network calls.
• Copy to clipboard actions. Each editor panel includes a copy button that copies the schema, query, or response text to your clipboard. You can paste this code into your own project, documentation, or testing tools without manually selecting text.
• Response viewer in JSON format. The response panel uses the same editor component configured for JSON. It shows mock data with indentation and line breaks so you can read it easily. Errors related to execution or validation can appear below the response area to explain why a response is empty.
• Input limits and safety guards. To keep the tool responsive, it imposes limits on schema size and query size. If you exceed these limits, the tool shows a clear error message and does not attempt to parse or execute the input. This prevents performance problems in the browser.

Common Use Cases

A frontend team wants to build a user interface for a new GraphQL API before the backend is ready. They define a schema for users, posts, and comments in the schema editor and write queries for the UI screens. The GraphQL Mock Server generates mock JSON responses, allowing them to design components, manage state, and test data flows without a live backend.

An API designer wants to explore schema ideas and share them with the team. They describe the domain in natural language and use AI schema generation to create an initial schema. They refine the schema in the editor, use AI query suggestions to generate sample queries, and then show the resulting mock responses to stakeholders. This speeds up discussion and feedback.

A QA engineer needs stable test data for automated tests. They use the tool to define a schema that matches the production API and write queries that mirror real usage. The mock server generates predictable structures with realistic values, which they export and embed in test cases, reducing dependence on unstable staging data.

A developer is learning GraphQL and wants to understand how schemas, queries, and responses connect. They load the built-in e‑commerce or blog schemas, run the default queries, and study the resulting JSON. This helps them see how type definitions map to query shapes and returned data.

How to Use This Tool (Step-by-Step)

1. Open the GraphQL Mock Server. You will see three main panels: Schema, Query, and Response. Above them, there is an action bar with a Generate Schema button and status indicators.
2. Start with a schema. In the Schema panel, either keep the default schema, paste your own, or click one of the sample buttons (such as “E‑commerce” or “Blog”) to load a ready-made schema and matching query. Make sure your schema uses valid GraphQL SDL.
3. Review or write a query. In the Query panel, review the default query that matches your schema. You can edit it to request different fields or add arguments. If you have a schema but no query, click the AI query suggestion button to let the tool propose a query.
4. Wait for automatic mock generation. The tool debounces changes, so after you pause typing for a moment, it parses the schema and query, validates them together, and executes the query against a mock resolver. You do not need to press a run button; the response updates automatically.
5. Inspect the response. In the Response panel, review the JSON data returned. It will follow the shape of your query, with values that match the types and field names in your schema. Nested objects and lists will be populated with example items. If there are validation or execution errors, they will appear below the response.
6. Fix any errors shown. If the schema is invalid or the query references fields that do not exist, error messages will appear. Go back to the Schema or Query panels, correct the issues, and let the tool re-run the mock generation.
7. Use AI to generate a new schema if needed. If you do not have a schema yet, click the Generate Schema button in the action bar. In the modal that opens, describe your API in simple language. Click Generate, and the tool will replace the schema editor content with AI-generated SDL based on your description.
8. Use AI to suggest a query. Once you have a schema, click the AI button in the Query panel. The tool sends your schema to the AI service, which returns a query that matches the types and fields defined. You can edit this query further if needed.
9. Copy schema, query, or response. When you are happy with the mock setup, use the copy buttons in each panel to copy the current schema, query, or mock response. Paste them into your application code, documentation, or testing scripts.
10. Iterate as your API evolves. As your real backend schema changes, update the schema editor, adjust queries, and view updated mock responses. This keeps your front-end and test data in sync with your API design.

Calculations & Logic

The GraphQL Mock Server follows a defined sequence when generating mock responses. First, it validates that the schema string and query string are non-empty, within configured maximum lengths, and are of type string. If these checks fail, it returns a result marked as invalid with appropriate error messages.

Next, it builds a GraphQL schema object from the SDL string using the GraphQL library’s schema builder. It parses the query string into a document object. It then calls the GraphQL validator with the schema and document. If validation errors exist, it collects their messages and returns a result marked as invalid, including those messages.

When validation passes, the tool creates a custom field resolver. For each field in the query, this resolver inspects the GraphQL return type, including whether it is a list, non-null, or scalar. It uses helper functions to strip wrappers and get the underlying named type.

For scalar types like Int, Float, Boolean, ID, and String, the resolver delegates to a mock value generator. For Int, it returns random integers within a reasonable range. For Float, it returns random decimal numbers rounded to two decimal places. For Boolean, it randomly returns true or false. For ID, it builds simple id-like strings using random numbers. For String, it inspects the field name and returns context-aware values such as email-like, name-like, title-like, or URL-like strings if the field name suggests those meanings. Otherwise, it returns generic mock strings.

For object types, the resolver recursively creates an object. It fetches the object type’s fields from the schema, then for each field, it calls the mock value generator again. This recursion continues until all nested objects are filled with mock values. For list types, it generates arrays containing multiple mock values by calling the generator for the list’s element type.

The tool then calls the GraphQL execute function with the built schema, parsed document, and custom resolver as the fieldResolver. It passes an empty root value, because the resolver handles all fields. The execution result contains data and possibly errors. The tool wraps this in an object with isValid, data, and errors properties before sending it back to the UI.

AI schema generation and query suggestion follow a simple request-response pattern. For schema generation, the tool sends the user’s prompt string to a backend service. The backend returns a GraphQL SDL string, which replaces the schema editor content. For query suggestion, the tool sends the current schema SDL to the backend, which returns a GraphQL query string that replaces the query editor content.

Tips, Limitations & Best Practices

Start with a clear schema. The quality of mock responses depends on the clarity and accuracy of your schema. Define types, fields, and relationships that match your intended API. If you are not sure where to start, use the built-in samples or the AI schema generator as a base, then refine the SDL manually.

Use realistic field names. Because the mock engine uses field names to choose certain string values, names like email, name, title, and url produce more realistic mock data. This can make your mock responses look closer to real production data, which is useful for UI demos.

Keep schemas and queries within size limits. Very large schemas or queries can slow down parsing and execution in the browser. The tool enforces maximum lengths and returns friendly error messages if limits are exceeded. If you hit these limits, consider reducing test scope or splitting schemas.

Remember that mock data is not persistent. Each time you change the schema or query, the tool regenerates mock data. Do not expect the same ID values or random numbers on each run. Use mock responses for structure and layout, not as a permanent data store.

Check validation errors carefully. When the tool reports schema or query validation errors, read the messages closely, as they come from the GraphQL validator and often point directly to the problematic field or type. Fixing these issues early prevents confusion later in development.

Use AI features as helpers, not as final truth. AI-generated schemas and queries are starting points. Review them, adjust field names and types to match your real domain, and ensure they follow your team’s conventions. Do not rely on AI to infer all business logic or constraints.

Integrate mock responses into your workflow. Once you are happy with a schema and query, copy the response JSON and use it in storybooks, unit tests, or local fixtures. This reduces reliance on flaky staging environments and speeds up frontend development.

Finally, remember that a mock server is a development asset, not a replacement for real backend testing. Use the GraphQL Mock Server to move quickly in early stages, then connect your clients to real APIs as soon as they are ready to validate performance, authorization, and real data behavior.