I’m posting this for awareness. The WebSphere SAML SSO feature only works with an IdP-initiated SSO. (Refer to this article: Understanding the WebSphere Application Server SAML Trust Association Interceptor.)
The IdP must be aware of the SP Entity Id as well as the RelayState which is the URL to which the browser is forwarded upon successful assertion by the WebSphere SAML SSO. (Refer to this guide for further SAML SSO configuration: Configuring SAML Web Browser SSO in Liberty.)
I confirmed that the following configuration works for SimpleSAML IdP when the user goes to the Id’s URL…
For more information, see Examples of configuring SSO (IBM Knowledge Center).
[Admin: For convenience, here are the meanings of the acronyms: Security Assertion Markup Language (SAML), Single Sign-On (SSO), Identity Provider (IdP), Trust Association Interceptor (TAI), Service Provider (SP).]