304 Not Modified

What HTTP 304 Not Modified Means

The resource has not been modified since the last request.

Use cached version of the resource.

Common Causes

• If-Modified-Since conditional request
• ETag validation
• Resource unchanged

How to Fix It (For Visitors)

• Browser uses cached version

How to Fix It (For Developers/Admins)

• Implement ETag and Last-Modified headers
• Support conditional requests

Returning a 304 Not Modified (Code Examples)

If you build APIs or web apps, here is how to send an HTTP 304 response and how to test for it:

Node.js (Express)

app.get('/resource', (req, res) => {
  res.status(304).json({ error: 'Not Modified' });
});

Python (Flask)

@app.route('/resource')
def resource():
    return jsonify(error='Not Modified'), 304

PHP

<?php
http_response_code(304);
header('Content-Type: application/json');
echo json_encode(['error' => 'Not Modified']);

Check the status with curl

curl -I https://example.com/resource
# Look for: HTTP/1.1 304 Not Modified

Frequently Asked Questions

What does HTTP 304 Not Modified mean?

Use cached version of the resource. In short: The resource has not been modified since the last request.

How should I handle an HTTP 304 Not Modified response?

Implement ETag and Last-Modified headers. Support conditional requests.

Official Specification

The 304 Not Modified status code is defined in RFC 7232 Section 4.1.

View the IANA HTTP Status Code Registry →