400 — Bad Request

The 400 Bad Request status code indicates that the server could not understand or process the client's request due to invalid syntax or a mistake in the request itself. This error is typically caused by issues in the request from the client, and it prevents the server from fulfilling the request.

Common Causes of 400 Bad Request

1. Malformed Syntax: If the client sends an improperly structured request (e.g., missing a necessary character or an incorrectly formatted URL), the server will return a 400 error because it cannot interpret the request.

2. Invalid Request Headers: Incorrect or unsupported headers in the request can trigger a 400 error. For instance, if the Content-Type header doesn’t match the type of data being sent (e.g., JSON data labeled as text/plain), the server might reject it.

3. Oversized Request: Some servers have limits on request size. If the request exceeds the allowed limit (e.g., sending too much data in a POST request), the server can return a 400 error.

4. URL Encoding Errors: When URLs are incorrectly encoded (e.g., spaces not replaced by %20), the server may reject them as invalid.

5. Invalid Query Parameters: If a request URL contains unexpected, unsupported, or invalid query parameters, the server might return a 400 error.

6. Corrupted Cookies: Sometimes, corrupted or expired cookies in the browser can also lead to a 400 error.

Example of a 400 Bad Request

Imagine a user wants to retrieve a specific product using a URL like:

https://example.com/api/products/1234

If the user accidentally types:

https://example.com/api//products//1234

The server might return a 400 Bad Request error because the URL format is incorrect.

How to Fix a 400 Bad Request Error

1. Check URL Syntax: Ensure that the URL is typed correctly, without extra slashes, missing characters, or incorrect encoding.

2. Clear Browser Cache and Cookies: Clearing corrupted cookies or cached data can often resolve 400 errors.

3. Validate Query Parameters: Ensure that any parameters in the request URL are correct and supported by the server.

4. Verify Request Headers: If manually constructing a request (e.g., in Postman or similar tools), confirm that headers are correct and match the expected content format.

5. Reduce Request Size: If possible, reduce the data size in the request, especially for POST or PUT requests that might exceed server limit