HTTP 400 Bad Request

What it means

Bad Request describes how the server processed the request and what the client should do next.

Why it happens

Common triggers include protocol conditions, routing decisions, and application behavior.

Common causes

• Malformed request format
• Authentication or authorization mismatch
• Missing resource or route

How to fix it

• Validate request URL, method, and headers
• Fix authentication/authorization flow
• Correct client payload and retry

Example request/response

GET /example HTTP/1.1
Host: errorlookup.com

HTTP/1.1 400 Bad Request

Browser/dev/server context

Use browser network tools, server logs, and APM traces together when diagnosing HTTP 400.

Related status codes

FAQ

Is HTTP 400 officially assigned?

Yes. This code is assigned in the IANA registry.