HTTP Error 400 = “Bad Request”
The server could not understand or process the request because the request is invalid.

Common causes

• Invalid URL or query parameters
• Missing required fields in API/body
• Malformed JSON/XML
• Invalid headers
• File upload too large or unsupported
• Corrupted cookies/cache
• Wrong request method (GET vs POST)
• Invalid authentication format

Fixes for users

• Refresh the page
• Clear browser cache/cookies
• Check the URL carefully
• Try another browser/device
• Re-login if authentication expired

Fixes for developers

1. Validate request body

Bad JSON example

json id=”tpfj8g”
{
“name”: “John”,
}

Correct JSON

json id=”kff6hz”
{
“name”: “John”
}

2. Set correct headers

http id=”vjlwmr”
Content-Type: application/json
Accept: application/json

3. Laravel validation example

php id=”jlwm8r”
$request->validate([
’email’ => ‘required|email’,
‘password’ => ‘required’
]);

4. Node.js Express example

js id=”6r7w2z”
app.use(express.json());

5. Check request method

php id=”9ucnjf”
Route::post(‘/login’, [AuthController::class, ‘login’]);

Calling this route with GET may return 400.

API debugging checklist

• Verify endpoint URL
• Verify request payload
• Check headers
• Check authentication token
• Inspect server logs
• Test with Postman or curl