HTTP 417 Expectation Failed

The server cannot meet the expectation in the request's Expect header — in practice, Expect: 100-continue.

Defined in RFC 9110 §15.5.18 — 417 Expectation Failed · MDN reference

Where you meet HTTP 417 in production

Proxies and older servers that do not implement 100-continue, answering large POSTs from curl, .NET's HttpClient and similar clients that send the header automatically.

Why you would test it

Relevant if your client sends Expect: 100-continue before large uploads (curl does by default above a size threshold). Test it falls back to sending the body without the expectation.

What your client should do about a 417

Retry without the Expect header and send the body straight away. Most clients can be configured to stop sending it — curl with -H "Expect:", for example.

417 versus the codes it gets confused with

• 417 vs 100 — 100 Continue is the success case of the same handshake: the server says "go ahead, send the body".
• 417 vs 400 Bad Request — Some servers answer an unsupported Expect with a plain 400 instead.

Endpoints that return 417

1 endpoint in this playground answers with 417. 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 417

What is Expect: 100-continue for?

Letting a client ask "will you accept this?" before sending a large body, so a request that will be rejected does not waste the upload.

How do I stop curl sending it?

Add -H "Expect:" with an empty value.

Other client error codes

• 400 Bad Request
• 401 Unauthorized
• 402 Payment Required
• 403 Forbidden
• 404 Not Found
• 405 Method Not Allowed
• 406 Not Acceptable
• 407 Proxy Authentication Required

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

Last updated 2 October 2026