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

What Visitors Should Do

• Browser uses cached version

Notes for Developers

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

How Conditional Requests Produce a 304

A 304 is the server's answer to a conditional GET or HEAD. The browser stores a validator from the first response and sends it back next time:

• ETag in the response, returned as If-None-Match in the next request
• Last-Modified in the response, returned as If-Modified-Since

If the resource has not changed, the server answers 304 Not Modified with no body, and the browser uses its cached copy. You can test it yourself:

curl -I https://example.com/style.css           # note the ETag value
curl -I -H 'If-None-Match: "abc123"' https://example.com/style.css

If a page keeps showing outdated content, the server is probably returning 304 because its ETag or Last-Modified value did not change after a deploy. Version your asset URLs (for example main.css?v=2) or purge the cache.

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 9110 Section 15.4.5.

View the IANA HTTP Status Code Registry →