Mocking API Responses

When your backend isn’t ready—or you want to simulate specific responses—Inspectr lets you mock APIs based on an OpenAPI specification and override responses in real time.

Inspectr leverages Stoplight Prism behind the scenes, giving you access to both static and dynamic mocking powered by Faker and JSON Schema Faker.

Want a fast demo? Jump to the Mocking Quick Start →.

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Use Cases

• Frontend development before backend is available
• Testing edge cases (timeouts, errors, unexpected payloads)
• Demos and offline API development
• Simulating invalid or large payloads for robustness testing

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Providing OpenAPI for mocking

You can provide the OpenAPI documents as a local file or as a public URL by using the --mock-backend parameter.

Local File:

inspectr --listen=:8080 --mock-backend=./openapi.yaml

Public URL:

inspectr --listen=:8080 --mock-backend=https://inspectr.dev/demo/hello.openapi.yaml

Quick Launch OpenAPI mock

You can also launch an OpenAPI Mock using Inspectr, by just using the URL https://inspectr.dev/launch with a reference to any remote OpenAPI document:

https://inspectr.dev/launch?openapi=https://inspectr.dev/demo/hello.openapi.yaml

Replace the openapi parameter with the URL of your own OpenAPI file. Inspectr will start immediately with mocking enabled.

You can easily mock the typical OpenAPI example APIs using Inspectr, by clicking the launch link:

• Museum API by Redocly

  Click here to Launch Museum API or visit https://inspectr.dev/launch?openapi=https://inspectr.dev/demo/museum.openapi.yaml

• Train Travel API by Bump.sh

  Click here Train Travel API or visit https://inspectr.dev/launch?openapi=https://inspectr.dev/demo/train-travel-api-openapi-source.yaml

• Galaxy API by Scalar

  Click here Galaxy API or visit https://inspectr.dev/launch?openapi=https://inspectr.dev/demo/galaxy.openapi.yaml

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Static Mocking

Static mocking uses predefined examples in your OpenAPI spec to return consistent, predictable responses.

Step 1: Prepare Your OpenAPI File

paths:
  /items:
    get:
      responses:
        '200':
          description: Success
          content:
            application/json:
              examples:
                default:
                  value:
                    items: ["A", "B", "C"]

Save it as openapi.yaml.

Step 2: Start Inspectr in Static Mock Mode

inspectr --listen=:8080 --mock-backend=./openapi.yaml

• Returns mock responses based on OpenAPI examples
• No backend service needed
• Captures requests in the Inspectr UI at http://localhost:4004

Step 3: Use Response Overrides

Use headers to simulate conditions or return different examples:

Header	Description
inspectr-response-status	Override status code (e.g. 404, 500)
inspectr-response-delay	Add artificial delay in milliseconds
inspectr-response-example	Choose a specific OpenAPI example by name
inspectr-response-content-type	Override Content-Type header

Example:

curl http://localhost:8080/items \
  -H "inspectr-response-status: 404"

See the guide Response Overrides → for more examples

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Dynamic Mocking

Dynamic mocking generates data using Faker and JSON Schema Faker based on your OpenAPI schema and x-faker extensions.

Step 1: Extend Your OpenAPI with x-faker

components:
  schemas:
    Pet:
      type: object
      properties:
        id:
          type: integer
          format: int64
        name:
          type: string
          x-faker: name.firstName
        photoUrls:
          type: array
          items:
            type: string
            x-faker: image.imageUrl

If x-faker is missing or invalid, Prism falls back to JSON Schema Faker.

Step 2: Start Inspectr in Dynamic Mode

inspectr --mock-backend=./openapi.yaml --mock-dynamic=true

Example Response (Generated):

{
  "id": 12608726,
  "name": "Addison",
  "photoUrls": [
    "http://lorempixel.com/640/480",
    "http://lorempixel.com/640/480"
  ]
}

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Comparison: Static vs. Dynamic Mocking

Mode	Behavior
Static	Returns predefined examples from OpenAPI spec
Dynamic	Generates data using Faker/JSF from schema and x-faker annotations

Use static mode for stable mocks based on the provided examples. Use dynamic mode to simulate variability and more dynamic data.

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Summary

Mock mode in Inspectr gives you full flexibility when developing against APIs. Whether you need consistent fixtures or random data, mocking accelerates frontend and QA workflows.