Spring ShallowEtagHeaderFilter is failing when used with async controllers and undertow. The reason is that it assumes that the servlet container uses ServletResponse form AsyncContext for subsequent async processing. I do not know the spec, but it works with other servlet containers so I assume that it's a bug in Undertow. It's hard to describe, please run https://github.com/lukas-krecan-lt/async-demo/blob/undertow/src/test/java/com/example/asyncdemo/AsyncControllerIntegrationTest.java in https://github.com/lukas-krecan-lt/async-demo/tree/undertow (undertow branch) and see for yourself.