The Shibboleth SP does not have an application API per se, but the SessionInitiator mechanism does support a simple redirect protocol capable of triggering, and influencing, the creation of authentication requests.

This protocol supports a small set of query string parameters that correspond to identically named attributes that can be supplied either directly on a <SessionInitiator> element or as content settings on a per-resource basis.

When a query string parameter is used, it overrides any other source of the same setting/property.

Not all SessionInitiator handlers support all the possible parameters. In fact, most are specific to the SAML2 handler. Unsupported parameters are ignored.