This is currently not possible because the filtering is done by the application calling CCT. The default CCT library implementation explicitly filters on the list of known Chrome app IDs and the app has to explicitly work around this filtering by trying the default browser first before falling back to the default CCT call.

It sounds like you're saying that this is currently not possible because it's currently not implemented that way? But of course! I'm saying they should change it so that the application asking for a custom tab can only request "please give me a custom tab".

I'm saying it's not possible at the OS level because the filtering is done in application code. CCT is not an OS service, it's a client library and a service IDL. Updating the library to remove the ability to filter by application ID is similarly not possible because binding to specific application IDs is not unique to CCT, it's simply how binding to services works on Android.

You lost me. Are you saying that the people who make Android OS can't reprogram Android OS? That is all this person is saying.

That's not what they're saying. The "chrome custom tabs" feature is a service interface exposed by an application, not the OS. There is no "open a custom tab" feature provided by Android. Therefore applications have to locate an installed browser that is known to support CCT before asking that application to start a Custom Tabs activity. The involvement of the OS in this interaction is to show the calling activity a list of applications that handle the ACTION_VIEW intent for URLs, and to bind the browser's CCT service to the interface for the client. The clients are the one choosing to only interact with Chrome at this point!