304 Not Modified

What This Error 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

Code Examples

Here's how to return a 304 status code in various programming languages:

Python (Flask)

from flask import Flask, jsonify

app = Flask(__name__)

@app.route('/endpoint')
def endpoint():
    return jsonify({"message": "Not Modified"}), 304

Node.js (Express)

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

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

PHP

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

Go

package main

import (
    "encoding/json"
    "net/http"
)

func handler(w http.ResponseWriter, r *http.Request) {
    w.Header().Set("Content-Type", "application/json")
    w.WriteHeader(304)
    json.NewEncoder(w).Encode(map[string]string{
        "message": "Not Modified",
    })
}

Java (Spring Boot)

@GetMapping("/endpoint")
public ResponseEntity<Map<String, String>> endpoint() {
    Map<String, String> response = new HashMap<>();
    response.put("message", "Not Modified");
    return ResponseEntity.status(304).body(response);
}

Ruby (Sinatra)

get '/endpoint' do
    status 304
    json message: 'Not Modified'
end

Browser Compatibility

Browser	Support	Notes
Chrome	✓ Full Support	All versions
Firefox	✓ Full Support	All versions
Safari	✓ Full Support	All versions
Edge	✓ Full Support	All versions

Official Specification

This status code is defined in RFC 7232 Section 4.1.

View on IANA HTTP Status Code Registry →