Hello Patrick, I just had a look into the source code of HttpMethodBase. The HTTP client deals with 100-continue responses, but seemingly only when an "Expect:" header has been sent in the first place. So you might want to add that header.
You can also check for some intermediate proxies that add an "Expect:" header and do not filter the 100-continue responses. But this might be an issue for the HTTP client. RFC 2616 says in chapter 10.1: A client MUST be prepared to accept one or more 1xx status responses prior to a regular response, even if the client does not expect a 100 (Continue) status message. Unexpected 1xx status responses MAY be ignored by a user agent. regards, Roland
