SibylFS is a formal specification in Lem of the POSIX file system API and its real-world variations as found in Linux, OS X, and FreeBSD. The specification is executable so that the more than 20,000 test cases do not require inclusion of expected behavior -- the expectation envelope can be automatically extracted and checked against a real file system! SibylFS has found numerous bugs in many different file system, VFS, and libc configurations.