Why My API Works in Postman but Not in Browser

Definition: What is the Postman vs Browser API Issue?

The Postman vs Browser API Issue occurs when an API request works successfully in Postman (a desktop API testing tool) but fails when made from a web browser. This discrepancy happens because Postman and browsers handle HTTP requests differently, with browsers enforcing additional security restrictions that Postman bypasses.

The main difference is that browsers enforce CORS (Cross-Origin Resource Sharing) policies, security restrictions, and same-origin policies that prevent cross-origin requests unless the server explicitly allows them. Postman, being a desktop application, doesn't enforce these browser security restrictions, allowing requests that browsers would block.

This issue is common when developing web applications that consume APIs, testing APIs in browsers, or integrating third-party APIs. Understanding why APIs work in Postman but fail in browsers is essential for debugging API integration issues and ensuring APIs work correctly in web applications.

Key Point: APIs work in Postman but fail in browsers because browsers enforce CORS policies, security restrictions, and same-origin policies that Postman bypasses. The solution is to configure your API server to allow cross-origin requests with proper CORS headers.

What: Understanding the Differences

The differences between Postman and browsers that cause API issues:

CORS (Cross-Origin Resource Sharing)

Browsers enforce CORS policies that block cross-origin requests unless the server sends proper CORS headers (Access-Control-Allow-Origin, Access-Control-Allow-Methods). Postman doesn't enforce CORS, allowing any cross-origin request. This is the most common cause of browser API failures.

Example: Browser blocks request from localhost:3000 to api.example.com without CORS headers, while Postman allows it

Authentication Headers

Browsers restrict certain headers (Authorization, Custom-Headers) in cross-origin requests unless the server explicitly allows them via Access-Control-Allow-Headers. Postman allows any headers. Browsers also handle cookies and authentication differently than Postman.

Example: Browser blocks Authorization header in cross-origin request without CORS header allowing it

Preflight Requests

Browsers send OPTIONS preflight requests before certain cross-origin requests to check if the server allows them. If the server doesn't handle OPTIONS requests or return proper CORS headers, the preflight fails and the actual request is blocked. Postman doesn't send preflight requests.

Example: Browser sends OPTIONS request before POST, server must respond with CORS headers

Browser Security Restrictions

Browsers enforce same-origin policy, content security policies, and security restrictions that prevent certain requests. Postman, being a desktop app, doesn't have these restrictions. Browsers also block mixed content (HTTP from HTTPS) and certain request types.

Example: Browser blocks HTTP requests from HTTPS pages, Postman allows both

Important: Understanding CORS, authentication headers, preflight requests, and browser security restrictions is key to fixing API issues. The main difference is that browsers enforce security policies that Postman bypasses.

When: When This Issue Occurs

This issue occurs in these situations:

Cross-Origin API Requests

When making API requests from a web page to a different domain, protocol, or port, browsers enforce CORS policies. If the API server doesn't send proper CORS headers, the request fails in browsers but works in Postman.

Authentication in Web Apps

When using authentication headers (Bearer tokens, API keys) in browser requests, browsers may block them if the server doesn't allow them via CORS headers. Postman allows any authentication headers without restrictions.

Complex HTTP Requests

When making POST, PUT, DELETE requests with custom headers or JSON bodies, browsers send preflight OPTIONS requests. If the server doesn't handle preflight requests, the actual request fails in browsers but works in Postman.

Local Development

When developing locally with frontend on localhost:3000 and API on localhost:8000, browsers treat these as different origins and enforce CORS. Postman doesn't have this restriction, making local API testing easier in Postman than browsers.

Common Scenario: This issue is most common when making cross-origin API requests from web applications, using authentication in browsers, or developing locally with separate frontend and backend servers. CORS is the primary cause in most cases.

How To: Fix Browser API Issues

Follow these methods to fix API issues in browsers:

Method 1: Configure CORS Headers (Backend Fix)

Configure your API server to send proper CORS headers:

Express.js (Node.js)

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

// Enable CORS for all routes
app.use(cors({
  origin: 'https://yourdomain.com', // or ['http://localhost:3000'] for dev
  methods: ['GET', 'POST', 'PUT', 'DELETE'],
  allowedHeaders: ['Content-Type', 'Authorization'],
  credentials: true
}));

// Or manually set CORS headers
app.use((req, res, next) => {
  res.header('Access-Control-Allow-Origin', 'https://yourdomain.com');
  res.header('Access-Control-Allow-Methods', 'GET, POST, PUT, DELETE');
  res.header('Access-Control-Allow-Headers', 'Content-Type, Authorization');
  res.header('Access-Control-Allow-Credentials', 'true');
  
  // Handle preflight requests
  if (req.method === 'OPTIONS') {
    return res.sendStatus(200);
  }
  next();
});

Flask (Python)

from flask import Flask
from flask_cors import CORS

app = Flask(__name__)

# Enable CORS
CORS(app, resources={
    r"/api/*": {
        "origins": ["https://yourdomain.com"],
        "methods": ["GET", "POST", "PUT", "DELETE"],
        "allow_headers": ["Content-Type", "Authorization"],
        "supports_credentials": True
    }
})

# Or manually
@app.after_request
def after_request(response):
    response.headers.add('Access-Control-Allow-Origin', 'https://yourdomain.com')
    response.headers.add('Access-Control-Allow-Methods', 'GET, POST, PUT, DELETE')
    response.headers.add('Access-Control-Allow-Headers', 'Content-Type, Authorization')
    response.headers.add('Access-Control-Allow-Credentials', 'true')
    return response

Method 2: Use a Proxy (Frontend Fix)

Use a proxy to bypass CORS in development:

Next.js Proxy Configuration

// next.config.js
module.exports = {
  async rewrites() {
    return [
      {
        source: '/api/:path*',
        destination: 'https://api.example.com/:path*',
      },
    ];
  },
};

Create React App Proxy

// package.json
{
  "proxy": "https://api.example.com"
}

// Then use relative URLs in fetch
fetch('/api/users') // Proxied to https://api.example.com/api/users

Method 3: Handle Preflight Requests

Ensure your server handles OPTIONS preflight requests:

Express.js Preflight Handler

app.options('*', (req, res) => {
  res.header('Access-Control-Allow-Origin', req.headers.origin);
  res.header('Access-Control-Allow-Methods', 'GET, POST, PUT, DELETE, OPTIONS');
  res.header('Access-Control-Allow-Headers', 'Content-Type, Authorization');
  res.header('Access-Control-Max-Age', '86400'); // 24 hours
  res.sendStatus(200);
});

Method 4: Browser Fetch API with Proper Headers

Use fetch API correctly in browsers:

JavaScript Fetch with CORS

// Simple GET request
fetch('https://api.example.com/users', {
  method: 'GET',
  headers: {
    'Content-Type': 'application/json',
  },
  credentials: 'include' // Include cookies if needed
})
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.error('Error:', error));

// POST request with authentication
fetch('https://api.example.com/users', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
    'Authorization': 'Bearer ' + token
  },
  body: JSON.stringify({ name: 'John' }),
  credentials: 'include'
})
  .then(response => {
    if (!response.ok) {
      throw new Error('Network response was not ok');
    }
    return response.json();
  })
  .then(data => console.log(data))
  .catch(error => console.error('CORS or network error:', error));

Best Practice: Configure CORS headers on your backend server for production. Use proxies or CORS extensions only for development. Always handle preflight OPTIONS requests. Test API requests in browsers, not just Postman, to catch CORS issues early.

Why: Why This Happens

APIs work in Postman but fail in browsers for these reasons:

Browser Security Model

Browsers enforce same-origin policy and CORS to prevent malicious websites from making unauthorized requests to other domains. This security model protects users from cross-site request forgery (CSRF) attacks and data theft. Postman, being a desktop app, doesn't have these restrictions.

CORS Policy Enforcement

Browsers automatically enforce CORS policies by checking server responses for CORS headers. If headers are missing or incorrect, browsers block requests. Postman doesn't check CORS headers, allowing any cross-origin request. This is why APIs work in Postman but fail in browsers.

Preflight Request Requirement

Browsers send OPTIONS preflight requests before complex cross-origin requests to check server permissions. If servers don't handle preflight requests, browsers block the actual request. Postman doesn't send preflight requests, making it easier to test APIs.

Header Restrictions

Browsers restrict certain headers (Authorization, Custom-Headers) in cross-origin requests unless servers explicitly allow them via Access-Control-Allow-Headers. Postman allows any headers without restrictions. This causes authentication to work in Postman but fail in browsers.

Important: APIs work in Postman but fail in browsers because browsers enforce security policies (CORS, same-origin, preflight) that Postman bypasses. The solution is to configure your API server to allow cross-origin requests with proper CORS headers.

Frequently Asked Questions

Why does my API work in Postman but not in browser?

Common causes include CORS (Cross-Origin Resource Sharing) errors, missing authentication headers, preflight request failures, browser security restrictions, and different request handling. Postman bypasses browser security, while browsers enforce CORS policies and security restrictions that can block API requests.

How do I fix CORS error in browser?

Configure your API server to include CORS headers: Access-Control-Allow-Origin, Access-Control-Allow-Methods, Access-Control-Allow-Headers. For development, use a proxy or CORS browser extension. For production, configure your backend (Express.js, Flask, Django) to allow cross-origin requests from your domain.

Why does authentication work in Postman but not browser?

Postman allows custom headers and authentication methods that browsers may block. Browsers restrict certain headers (like Authorization) in cross-origin requests. Ensure your API server sends proper CORS headers allowing Authorization header, and use proper authentication methods (Bearer tokens, API keys) that browsers support.

What is a preflight request and why does it fail?

A preflight request is an OPTIONS request browsers send before certain cross-origin requests. It checks if the server allows the actual request. Preflight fails if the server doesn't respond with proper CORS headers or doesn't handle OPTIONS requests. Configure your server to handle OPTIONS requests and return appropriate CORS headers.

How do I test API in browser like Postman?

Use browser DevTools Network tab to inspect requests, use fetch() API in console, or create a simple HTML page with JavaScript. For CORS issues, use a proxy server, CORS browser extension, or configure your backend. For authentication, ensure headers are properly set and CORS allows them.