Why does Postman work but my browser doesn't?

Postman is an API testing tool, not a web browser. It does not enforce CORS (Cross-Origin Resource Sharing) security policies, which are specifically designed to stop malicious scripts in web browsers.

I added CORS to my Express app but it still fails?

Make sure `app.use(cors())` is placed *before* your route definitions. If it's placed after, the routes will still block the requests.

Fix: API Works in Postman but Not in React

You've built your backend API. You tested it in Postman, and it returns a perfectly formatted JSON response with a 200 OK status.

Confident, you plug the same URL into your React application using `fetch` or `axios`. But instead of data, your console lights up red. You get a `Network Error`, or a `CORS policy` violation, or your request simply hangs.

This is one of the most frustrating but common issues developers face before deployment. In this guide, I will break down exactly *why* Postman succeeds where the browser fails, and how to debug and fix the root cause immediately.

TL;DR - Most Common Fixes

1Check if it's a CORS error in your browser console (Postman ignores CORS).
2Ensure you are not accidentally sending an OPTIONS request that your backend isn't configured to handle.
3Verify you are passing the correct `Content-Type: application/json` headers in your fetch/axios request.
4Check if your localhost URL matches exactly (e.g., `127.0.0.1` vs `localhost`).

Root Causes

Postman Bypasses CORS by Default

Browsers enforce the Same-Origin Policy for security. If your React app runs on `http://localhost:3000` and your server on `http://localhost:5000`, the browser blocks the request unless the server explicitly allows it via CORS headers. Postman is not a browser, so it completely ignores CORS.

Access to fetch at 'http://localhost:5000/api' from origin 'http://localhost:3000' has been blocked by CORS policy.

Missing or Incorrect Request Headers

Postman automatically attaches certain headers (like `Content-Type`) based on your body format. If you use `fetch` in React and forget to specify headers, your backend might reject the request or fail to parse the body.

fetch('/api/data', { method: 'POST', body: JSON.stringify(data) }) // Missing headers!

Preflight (OPTIONS) Request Failure

For complex requests (like POST with JSON), browsers first send an OPTIONS request to check if the server permits the actual request. If your backend doesn't handle OPTIONS routes properly, the preflight fails, and the main request is never sent.

Step-by-Step Fix Guide

Identify if it's a CORS Issue

Open your Chrome DevTools -> Console. Look for a red error message mentioning 'CORS policy'. If you see it, the issue is 100% on your backend configuration.

Always check the Network tab in DevTools, not just the Console.

Fix Backend CORS Configuration (Node/Express Example)

If you are using Node/Express, install the `cors` package and enable it correctly.

const cors = require('cors');
const app = express();

// Allow requests from your React app
app.use(cors({
  origin: 'http://localhost:3000',
  credentials: true
}));

Fix Frontend Request Headers

Ensure your React request explicitly tells the backend it is sending JSON data.

const response = await fetch('http://localhost:5000/api/endpoint', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
    'Accept': 'application/json'
  },
  body: JSON.stringify({ key: 'value' })
});

If using axios, `Content-Type` is usually set automatically, but verify it anyway.

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Related Errors

AxiosError: Network Error
Usually implies the backend server is not running, or CORS blocked the request so early that Axios couldn't read the response status.
415 Unsupported Media Type
You forgot the `Content-Type: application/json` header in your React request.

Prevention Strategy

Always set up a proper CORS configuration locally that mirrors your production setup.
Use proxying in React (e.g., adding `"proxy": "http://localhost:5000"` in `package.json` for CRA or using Next.js rewrites) to bypass CORS issues entirely during local development.
Standardize an API client utility (like an axios instance) in your frontend so headers are automatically applied to every request.

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API Works in Postman but Not in React?