Can a browser get a 505?

Practically never; browsers negotiate HTTP versions properly. It comes from custom clients and tools.

No. The same request with the same version will fail the same way.

HTTP 505 HTTP Version Not Supported

The server does not support the major HTTP version used in the request.

Defined in RFC 9110 §15.6.6 — 505 HTTP Version Not Supported · MDN reference

Where you meet HTTP 505 in production

Malformed request lines from hand-rolled clients and scripts, very old servers receiving newer protocol versions, and misconfigured proxies mangling the version string.

Why you would test it

Hard to provoke with a normal client, which is why it is worth simulating: a client that assumes every 5xx is transient will retry a 505 forever.

What your client should do about a 505

Do not retry unchanged — the same version fails the same way. Switch the client to a version the server supports, which is usually HTTP/1.1.

505 versus the codes it gets confused with

505 vs 426 Upgrade Required — 426 asks you to upgrade to another protocol. 505 refuses the version you used.
505 vs 400 Bad Request — A malformed request line is often answered with 400 instead.

Endpoints that return 505

1 endpoint in this playground answers with 505. Every one is free, needs no signup, and can be called from the browser or with curl.

GET https://funapi.dev/api/faults/v1/status/{code} — Returned on request (Fault Injection API) Open & run this endpoint

Questions about HTTP 505

Can a browser get a 505?: Practically never; browsers negotiate HTTP versions properly. It comes from custom clients and tools.
Is a 505 transient?: No. The same request with the same version will fail the same way.

Other server error codes

All HTTP status codes · All 39 mock REST APIs · Getting started guide

Last updated 2 October 2026