Mock Server

Scalar Mock Server Skill

Reference for implementing and debugging mock APIs with @scalar/mock-server. Use this when you need realistic API responses from an OpenAPI description document, custom request behavior, seeded data, or Docker-based mock environments.

Overview

• Package: @scalar/mock-server
• Runtime: Node.js (package engine: >=22)
• Main API: createMockServer(options)
• Docs:
  • Getting started: https://scalar.com/tools/mock-server/getting-started
  • Custom request handlers (x-handler): https://scalar.com/tools/mock-server/custom-request-handler
  • Data seeding (x-seed): https://scalar.com/tools/mock-server/data-seeding
  • Docker: https://scalar.com/tools/mock-server/docker

Quick Start

Fastest way to run a mock server from a local OpenAPI description:

npx @scalar/cli document mock openapi.json --watch

Programmatic setup:

import { serve } from '@hono/node-server'
import { createMockServer } from '@scalar/mock-server'

const app = await createMockServer({
  document: './openapi.yaml',
  onRequest({ context, operation }) {
    console.log(context.req.method, context.req.path, operation.operationId)
  },
})

serve({ fetch: app.fetch, port: 3000 })

createMockServer() Options

At least one of the following is required:

• document: OpenAPI description document as URL, file path, or object
• specification: deprecated alias for document

Optional:

• onRequest({ context, operation }): callback before each request is processed
• validateRequest: set to false to always return a mock response instead of rejecting contract violations with 422 (default: true)
• logger: pass false to silence the authentication instructions printed on startup, or a (line) => void sink to redirect them (default: logs to the console)

Built-in Behavior

When the server starts, it:

1. Processes and loads the OpenAPI description document.
2. Seeds schema data from x-seed extensions (idempotent: only when collection is empty).
3. Registers authentication routes for declared security schemes.
4. Registers operation routes for each path + method.
5. Exposes the source document at:
   • /openapi.json
   • /openapi.yaml

Custom Request Logic with x-handler

Use x-handler in an operation for dynamic behavior instead of static examples.

Helpers available in x-handler:

• store for in-memory persistence (list, get, create, update, delete, clear)
• faker for generated test data
• req for request data (body, params, query, headers)
• res for response examples by status code (res['200'], res['404'], ...)

Status behavior:

• store.get() / store.update() => 200 when found, 404 when not found
• store.create() => 201
• store.delete() => 204 when deleted, 404 when not found
• store.list() => 200
• Returning null/undefined triggers 404 (uses responses.404 example/schema when provided)

Seed Data with x-seed

Use x-seed on components.schemas.<SchemaName> to seed initial data at startup.

Helpers available in x-seed:

• seed.count(n, factory)
• seed(array)
• seed(factory) (single item shortcut)
• faker, store, and schema

Key rule: the schema key name is used as the collection name.

Docker Usage

Run the Docker image:

docker run -p 3000:3000 scalarapi/mock-server --url https://api.example.com/openapi.yaml

Document source priority (high to low):

1. --url <URL>
2. OPENAPI_DOCUMENT
3. OPENAPI_DOCUMENT_URL
4. /docs volume-mounted files

Useful routes:

• Mock endpoints: from your OpenAPI paths
• API reference UI: /scalar
• Description document: /openapi.json, /openapi.yaml

Troubleshooting Checklist

• Confirm the OpenAPI description document is valid and reachable.
• Confirm at least one document source is configured (document, --url, env var, or mounted file).
• If seeded data is missing, check x-seed exists on schema keys and the collection was empty on startup.
• If auth-protected routes return unauthorized responses, verify matching securitySchemes and request credentials.
• If custom logic fails, inspect x-handler runtime errors (mock server returns 500 with handler error details).
• For an unhandled 500 (one whose error is Internal Server Error), read its operation object — it names the method and OpenAPI path that failed, plus the operationId when the document declares one.