400 Bad Request

What This Error Means

The server cannot process the request due to a client error.

The request has malformed syntax or invalid request message framing.

Common Causes

• Invalid JSON/XML syntax
• Missing required parameters
• Malformed request headers
• Invalid query string

How to Fix It (For Visitors)

• Check the URL for typos
• Contact the site administrator

How to Fix It (For Developers/Admins)

• Validate request syntax
• Check API request format
• Review Content-Type header
• Validate JSON payload

Code Examples

Here's how to return a 400 status code in various programming languages:

Python (Flask)

from flask import Flask, jsonify

app = Flask(__name__)

@app.route('/endpoint')
def endpoint():
    return jsonify({"message": "Bad Request"}), 400

Node.js (Express)

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

app.get('/endpoint', (req, res) => {
    res.status(400).json({ message: 'Bad Request' });
});

PHP

<?php
http_response_code(400);
header('Content-Type: application/json');
echo json_encode(['message' => 'Bad Request']);
?>

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(400)
    json.NewEncoder(w).Encode(map[string]string{
        "message": "Bad Request",
    })
}

Java (Spring Boot)

@GetMapping("/endpoint")
public ResponseEntity<Map<String, String>> endpoint() {
    Map<String, String> response = new HashMap<>();
    response.put("message", "Bad Request");
    return ResponseEntity.status(400).body(response);
}

Ruby (Sinatra)

get '/endpoint' do
    status 400
    json message: 'Bad Request'
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 7231 Section 6.5.1.

View on IANA HTTP Status Code Registry →