I’m not imagining something that specifies policies at all. Capability based security systems shouldn’t need them. Instead, the main application can create capabilities programmatically - for example by opening a file for reading. The file handle is a capability. Libraries you call don’t get access to the OS, or anything outside their module scope that hasn’t been passed in as an explicit parameter. A 3rd party library can access the file handle you pass it as a function argument, but it can’t ask the OS to open any other files on your computer. Or open a network socket.

OS APIs aren’t in a scope that any of the libraries you use have access to.

There are a few things capabilities need to be able to do. They need to be able to be tightened. (So for example, if you have a file with RW flags, you can make a copy of the file handle that’s read only). And they need to be unforgeable (file handles can’t be created from an integer file descriptor). We’d also need a handful more “file handle” like objects - and this is where granularity matters. Like we need an equivalent for directories that gives you access to everything recursively in that directory and nothing outside of it.

If the library I call delegates some work to another library, it’s pretty simple. It just passes its file handle through to the other library as a function argument. There’s no policy to maintain or update. Nothing even changes from the point of view of the original caller.

The tricky part is stuff like debug output, environment variables (NODE_ENV=production) or stdout. Do libraries implicitly get access to stdout or not? Maybe they get stderr but not stdout, and they can open new temp files? You would also need to restrict JavaScript code from arbitrarily messing with the global scope or global libraries like String.

But honestly, any answer to these problems would be a massive improvement over what we have now. Being able to know your binary search library can’t make network requests, and that your database can’t interact with files outside its database directory would do so much for supply chain security it’s not funny. A good capability system should just change the APIs that your software can use a bit. Generally in good ways that make dependencies more explicit. Done right, there shouldn’t be any new syntax to learn or deno style flags to pass at the command line or anything like that.