❌ 400 Bad Request

Common Causes

• Malformed JSON body — missing comma, unclosed bracket, trailing comma
• Invalid query parameters — wrong type, out-of-range value, missing required param
• Wrong Content-Type header — sending JSON but declaring multipart/form-data
• Oversized request — body exceeds server's size limit
• Invalid URL — special characters not URL-encoded
• Missing required fields — API expects a field that wasn't included

Debugging 400 Errors

// Check your request in fetch
fetch('/api/users', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },  // ← required for JSON
  body: JSON.stringify({ name: 'Alice', email: 'alice@example.com' })
})
.then(async res => {
  if (!res.ok) {
    const error = await res.json();
    console.error('400 Error:', error);  // check the response body for details
  }
});

Steps to diagnose:

1. Check the response body — the server usually includes an error message
2. Validate your JSON with our JSON Formatter
3. Verify Content-Type matches your body format
4. Check API docs for required fields and their types

400 vs 422 vs 401