Hello world from undertow, enriched with HttpContinueAcceptingHandler and HTTP/2 support: public class HelloWorldServer { public static void main( final String [] args) { Undertow server = Undertow.builder() .addHttpListener(8080, "localhost" ) .setServerOption(UndertowOptions.ENABLE_HTTP2, true ) .setHandler( new HttpContinueAcceptingHandler( new HttpHandler() { @Override public void handleRequest( final HttpServerExchange exchange) throws Exception { exchange.getResponseHeaders().put(Headers.CONTENT_TYPE, "text/plain" ); exchange.getResponseSender().send( "Hello World" ); } })).build(); server.start(); } } Sending something with curl: $ curl --http2 -X PUT localhost:8080/ -d foo -X PUT -H 'Expect: 100- continue ' --http2-prior-knowledge -v * Trying 127.0.0.1:8080... * TCP_NODELAY set * Connected to localhost (127.0.0.1) port 8080 (#0) * Using HTTP2, server supports multi-use * Connection state changed (HTTP/2 confirmed) * Copying HTTP/2 data in stream buffer to connection buffer after upgrade: len=0 * Using Stream ID: 1 (easy handle 0x5642a7ea1e30) > PUT / HTTP/2 > Host: localhost:8080 > user-agent: curl/7.68.0 > accept: */* > expect: 100- continue > content-length: 3 > content-type: application/x-www-form-urlencoded > * Connection state changed (MAX_CONCURRENT_STREAMS == 4294967295)! * http2 error: Invalid HTTP header field was received: frame type: 1, stream: 1, name: [content-length], value: [0] * HTTP/2 stream 0 was not closed cleanly: PROTOCOL_ERROR (err 1) * stopped the pause stream! * Connection #0 to host localhost left intact curl: (92) HTTP/2 stream 0 was not closed cleanly: PROTOCOL_ERROR (err 1) I am trying to reproduce an issue we initially had with haproxy: haproxy doing TLS reencrypt, having negotiated HTTP/1.1 with client and HTTP2 with backend client sends a curl -X PUT that automatically adds a Expect traffic breaks in the middle From the RFC I read as example: HTTP/1.1 100 Continue HEADERS Extension-Field: bar ==> - END_STREAM + END_HEADERS :status = 100 extension-field = bar I am not sure the content-length should be sent, but also it looks like the 100-continue replied by Undertow contains the "END_STREAM" flag.