API Response Simulator

Mock API responses, status codes, and headers for front-end testing. Simulate successful calls, errors, and timeouts locally.

Simulation Configuration

Define how your "mock API" should behave when called.

Response Preview

Status: 200 OK
Latency: --ms
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What is the API Response Simulator?

        
The API Response Simulator is a client-side mock development tool designed to simulate HTTP endpoints, response headers, and JSON payloads. By configuring status codes and latency values, frontend engineers can test how their web applications handle server responses without setting up a backend server.

        
This simulator evaluates your JSON body and generates response payloads locally in your browser's private memory. Since no headers, status codes, or payload objects are sent to external databases, your application's API architecture and JSON structures remain private.


        
How to Use the API Response Simulator

        
    
          
  1. Set the Endpoint Path: Type your mock URL path (e.g., /api/v1/users) into the endpoint input box.
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  2. Choose the Status Code: Select your target HTTP status code (such as 200 OK, 401 Unauthorized, 404 Not Found, or 500 Internal Error) from the dropdown list.
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  3. Configure Simulation Delay: Drag the delay slider to set a mock latency (from 0 to 5000 milliseconds) to test loading states and network timeouts.
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  4. Assign Custom Headers: Enter header key-value pairs (like Content-Type: application/json) and click "Add Header" to include them in the simulation response.
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  5. Write the JSON Body: Paste or edit your JSON response body inside the payload editor textarea.
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  6. Simulate Response: Click "Simulate Response." The right panel will display the simulated response code, headers, and the formatted JSON body after the selected delay.
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Key Features

        
    
          
  • Realistic Latency Emulation: Simulate network delays up to 5 seconds to test UI spinner animations and skeleton screen loaders.
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  • Broad Status Code Selections: Test edge cases by simulating 2xx success codes, 4xx client errors, and 5xx backend server faults.
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  • Custom Header Additions: Append custom metadata keys (such as token authorization headers or pagination counts) to simulated responses.
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  • Local Syntax Verification: Validates JSON strings instantly to prevent formatting errors on simulated outputs.
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  • 100% Client-Side Sandbox: Compiles mock payloads in browser RAM without network tracking.
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Common Use Cases

        
    
          
  • Frontend Autonomy: Build and test user interface components before backend developers complete the actual API endpoints.
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  • Error State Validation: Verify that your application displays clear warning alerts when API endpoints return 401, 403, or 500 server errors.
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  • Offline App Demos: Run mock demos of your client-side application during presentations without relying on active staging environments or internet connections.
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  • UI Loading State Checks: Emulate slow network connections to fine-tune loading spinners and prevent layout shifting.
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Frequently Asked Questions

        

          

            
            

              
No. The simulator operates entirely in your browser. It intercepts status selections and payload inputs locally to compile response data in your tab session, making it completely offline-capable.

            

          

          

            
            

              
Yes. All mock responses, headers, and JSON structures are processed locally in your browser memory. No data is sent to external databases or stored on 99tool.in servers.

            

          

          

            
            

              
The parser requires standard JSON formatting. Ensure that your keys and string values are enclosed in double quotes (not single quotes) and that there are no trailing commas at the end of lists or objects.

            

          

          

            
            

              
This simulator is designed to mock the response payload and metadata returned to your frontend code, helping you verify how your app displays data. It operates independently of the request method used.

            

          

          

            
            

              
Yes. The tool saves your active configurations to your browser's local storage automatically. When you return to the page, your endpoints and JSON configurations will reload.

            

          

        

      


      

        
🔒 Privacy & Data Note: API mock configurations, headers, and JSON validations are processed locally in your browser. No data is sent to our servers.