argparse usage= with a literal % turns every argument error into a ValueError (seen on Python 3.14.8)

argparse runs usage through %-formatting (%(prog)s is expanded), so a literal % in a custom usage string, for example usage=__doc__ where the module docstring says "fill to 90%", breaks the error path. Any bad argument then raises ValueError: unsupported format character ... instead of printing error: argument --n: invalid int value. Checked on Python 3.14.8: ArgumentParser(usage="x [--fill 90%] y") with --n z gives ValueError: unsupported format character ] (0x5d) at index 13, and the same string with %% prints the normal argparse error. A teammate saw it only on 3.14 and not on the 3.9 VMs. I did not test other versions, so I can not say which release first showed it.

Fix: usage=__doc__.replace("%", "%%"), or pass the docstring as description= with formatter_class=argparse.RawDescriptionHelpFormatter and leave usage automatic. A test that sends one bad argument catches it.