Creating Mock APIs from HAR Files

Building frontend applications often means waiting for backend APIs that don't exist yet. Or testing against third-party APIs with rate limits. Or debugging issues that only happen in production.

What if you could record real API traffic and instantly spin up a mock server that behaves exactly the same way? That's what ProxyKit's HAR-to-Mock feature does.

Why Mock APIs from HAR Files?

HAR (HTTP Archive) files capture real API traffic, including:

• Request URLs, methods, and headers
• Request bodies (POST/PUT/PATCH data)
• Response status codes
• Response headers and bodies

This makes HAR files the perfect source for mock servers because the responses are real data from real APIs. No guessing, no manual mocking, no keeping mock data in sync with production.

Use Cases

HAR-based mock servers solve real problems:

• Frontend development without backend: Start building the UI before the API is ready. Record traffic from a prototype or similar API.
• Offline development: Work on planes, trains, or anywhere without internet. Your mock server runs locally.
• Testing against rate-limited APIs: Stripe, Twilio, and other APIs have rate limits. Mock them for unlimited local testing.
• Reproducing production bugs: Capture the exact API responses that caused a bug, then replay them locally for debugging.
• Demo environments: Create consistent demo data that won't change during customer presentations.

How It Works

ProxyKit's mock server generator works in three steps:

1. Capture a HAR file

   Use browser DevTools, Charles Proxy, or any HTTP capture tool to record API traffic as a HAR file.

2. Upload to ProxyKit

   Visit the HAR Viewer and upload your HAR file. ProxyKit parses all requests and responses.

3. Generate your mock server

   Click "Generate Mock Server" and get a unique URL. Every endpoint from your HAR file is now available as a working API.

Step-by-Step: Creating Your First Mock Server

1. Capture the HAR File

Open Chrome DevTools (F12 or Cmd+Option+I), go to the Network tab, and perform the API operations you want to mock. Then:

1. Right-click anywhere in the request list
2. Select "Save all as HAR with content"
3. Save the file

2. Upload and Review

Go to ProxyKit's HAR Viewer and drag your HAR file onto the page. You'll see all captured endpoints organized by domain.

Review the endpoints to make sure they include the APIs you want to mock. Use the filters to narrow down by HTTP method or status code.

3. Generate the Mock Server

Click the "Generate Mock Server" button. ProxyKit will:

• Deduplicate endpoints (same method + path = one mock route)
• Create a unique URL for your mock server
• Return the mock server URL and a list of available routes

4. Use Your Mock Server

Replace your API base URL with the mock server URL. For example, if your real API was:

https://api.example.com/v1/users

And your mock server is:

https://proxykit-7xjg.polsia.app/mock/a1b2c3d4

Then call:

https://proxykit-7xjg.polsia.app/mock/a1b2c3d4/v1/users

The mock server responds with the exact same data from your HAR file.

What Gets Mocked

Each mock endpoint preserves:

Attribute	Preserved?
HTTP Method (GET, POST, etc.)	Yes
URL Path	Yes
Response Status Code	Yes
Response Headers	Yes (most)
Response Body	Yes
Query Parameters	Stored, not matched

Mock Server Features

CORS Enabled

All mock endpoints include CORS headers, so you can call them from any origin. No proxy configuration needed.

Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: GET, POST, PUT, DELETE, PATCH, OPTIONS
Access-Control-Allow-Headers: *

Automatic Deduplication

If your HAR file contains multiple requests to the same endpoint (same method + path), ProxyKit uses the first one. This keeps your mock server clean.

7-Day Expiration

Mock servers expire after 7 days to keep things clean. Need a longer-lived mock? Export to OpenAPI and use a dedicated mock service.

Viewing Available Routes

After generating a mock server, you can view all available routes by visiting:

https://proxykit-7xjg.polsia.app/mock/{your-slug}

This returns a JSON object listing every route, its HTTP method, and status code.

When to Use Mock Servers vs OpenAPI

ProxyKit can export to both mock servers and OpenAPI specs. Here's when to use each:

Use Case	Mock Server	OpenAPI
Quick frontend testing	Best	-
Demos and presentations	Best	-
API documentation	-	Best
Code generation	-	Best
CI/CD integration	Good	Best

Limitations

HAR-based mock servers work great for static responses. They don't support:

• Dynamic responses: Every request to the same endpoint returns the same response
• Request validation: The mock server doesn't validate request bodies
• Stateful operations: POST doesn't actually create data, DELETE doesn't remove it

For dynamic mocking with request validation and stateful behavior, export to OpenAPI and use a tool like Prism or Mockoon.

Conclusion

HAR-to-Mock is the fastest way to create realistic mock APIs. No code, no configuration, just real responses from real traffic.

Next time you're waiting for a backend API, blocked by rate limits, or debugging a production issue, capture a HAR file and spin up a mock server in seconds.