HTTP 400 Bad Request

What It Means

HTTP 400 Bad Request indicates that the server cannot or will not process the request due to something perceived to be a client error. This could be malformed request syntax, invalid request message framing, deceptive request routing, or invalid input data.

Common Causes

• Malformed JSON in the request body
• Missing required request parameters or headers
• Invalid query string format
• Request body exceeds expected format
• Invalid URL encoding
• Corrupted or incomplete multipart form data
• Invalid Content-Type header for the request body

How to Fix

Debugging the request

# Use curl to inspect what you're sending
curl -v -X POST https://api.example.com/users \
  -H "Content-Type: application/json" \
  -d '{"name": "John", "email": "john@example.com"}'

# Validate JSON before sending
echo '{"name": "John"}' | python3 -m json.tool

Common fixes in JavaScript

// Problem: Sending data without Content-Type header
// Bad
fetch('/api/users', {
  method: 'POST',
  body: JSON.stringify({ name: 'John' })
});

// Good
fetch('/api/users', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify({ name: 'John' })
});

// Problem: Malformed JSON
// Bad - single quotes are not valid JSON
fetch('/api/users', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: "{'name': 'John'}"
});

// Good - use JSON.stringify
fetch('/api/users', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify({ name: 'John' })
});

Server-side validation (Express.js)

app.post('/api/users', (req, res) => {
  const { name, email } = req.body;

  if (!name || !email) {
    return res.status(400).json({
      error: 'Bad Request',
      message: 'Both name and email are required',
      fields: {
        name: !name ? 'Name is required' : null,
        email: !email ? 'Email is required' : null
      }
    });
  }

  // Process valid request
});

Python requests

import requests
import json

# Ensure proper JSON encoding
data = {"name": "John", "email": "john@example.com"}
response = requests.post(
    "https://api.example.com/users",
    json=data  # Automatically sets Content-Type and encodes JSON
)

if response.status_code == 400:
    print("Bad Request:", response.json())

Related Errors

• HTTP 401 - Unauthorized: The request lacks valid authentication credentials.
• HTTP 403 - Forbidden: The request is valid but the server refuses to act on it.
• HTTP 422 - Unprocessable Entity: The request is well-formed but semantically invalid.