Hi again, I tried pretty hard to reproduce by firing off call from threads in parallel, but seemed okay. I haven't checked if xcall is thrad/process safe yet, but a quick glance in the code makes me think not. For this, and your issue, python-xcall now throws an AssertionError if it finds xcall already running. There is 20-30ms period after making one call where a second will still go through though. It should help a bit--and it might help you diagnose want wrong for you? The real solution is (from the outside of xcall at least) is a file lock--but those cause so much grief!