Just tried out the "SwiftToolsSupport-auto" and you have to make sure in all sub dependencies it is also added as "SwiftToolsSupport-auto" otherwise it will not work in sandboxed applications. I tested this by adding one of my projects to xcode 12 project as a swift package dependency. This is because I want to use this on iOS too.

Finaly figured out why this is. The public headers should not be in a folder named the same as the framework you are building. See https://stackoverflow.com/questions/27776497/include-of-non-modular-header-inside-framework-module/67869384#67869384 The reason behind this is not clear to me but renaming the folder fixed the issue. And then in the umbrella header make sure to only use imports <> and not the "" kind. https://github.com/dooZdev/mixed-swift-objc-framework

It should work now as of xcode 14 beta3

Did you manage to find a solution?