Mock API Server
Design endpoints and simulate JSON responses locally in your browser.
Private browser simulator · no requests or configurations are uploaded
This is a local API simulator for designing and testing responses. Export the configuration to share it with your team or adapt it for JSON Server, Mockoon, or automated tests.
How to build a mock API
- Add an endpoint. Choose an existing one or select "Add endpoint."
- Configure the response. Set the method and path, then the status code, delay, headers and JSON body.
- Test it. Enter a method and path in the built-in request tester and select "Send request" to preview the matching response.
- Back it up. Export the configuration to share or save it, and Import to restore it later.
What happens to your configuration
Your endpoints are saved in your browser's local storage, on your device — they're not uploaded to OpenToolkit, and this tool does not create a real, network-accessible server or URL that other applications can call.
The request tester matches your rules and returns a simulated response inside the same browser tab. Because everything lives in local storage, clearing your browser data clears your configuration too — export it first if you want to keep it.
What this mock server supports
Common reasons to use a mock API
- Build a frontend against realistic responses before the real backend exists.
- Demo an app without depending on a live API or network connection.
- Reproduce a bug tied to a specific status code, error shape or slow response.
- Write client-side error handling against responses you fully control.
Frequently asked questions
Does this create a real API I can call from another app?
No — it's a browser-based simulator. Requests only work through the built-in tester in this tab, not from another app, device or domain.
Where is my configuration stored?
In your browser's local storage on this device. It is not uploaded to OpenToolkit.
Will I lose my endpoints if I clear my browser data?
Yes. Export your configuration to a JSON file first if you want to keep or move it.
Can I share a mock setup with a teammate?
Yes — export the configuration and have them import the same JSON file.
Can I simulate a slow or failing response?
Yes. Each endpoint has its own response delay and status code, including error codes like 404 or 500.
Does it check authentication headers on incoming requests?
You can return any custom response headers, but request-side auth isn't validated — this tool simulates responses, it doesn't enforce access control.
Last reviewed: August 2026