5 HTTP Edge Cases That Break APIs (And How to Fix Them)

Most API developers nail the happy path. But HTTP is a protocol with decades of baggage, and some edge cases can silently break your API in production. Here are 5 HTTP edge cases every API developer should know about — with code examples and fixes. 1. Duplicate HTTP Headers The HTTP spec (RFC 9110) allows multiple headers with the same name. Most frameworks merge them — but not always the way you expect. // Express.js example // If a client sends: X-Custom: foo AND X-Custom: bar app . get ( ' /test ' , ( req , res ) => { console . log ( req . headers [ ' x-custom ' ]); // Output: 'foo, bar' (comma-joined string) }); The trap: If you're parsing a header value and expecting a single string, duplicate headers will silently corrupt your logic. Fix: Always validate header values. If you expect a single value, take the first one explicitly: const value = req . headers [ ' x-custom ' ]?. split ( ' , ' )[ 0 ]?. trim (); 2. Empty Body on 204 No Content... or Is It? HTTP 204 means "no content" —