My summary comment on these things is that Bob is right and the standard could be improved by saying that the en passant target square should be specified if and only if there is at least one valid en passant capture. (There could be two.)
Yes, it means that a FEN using program has to know about legality testing. That's a reasonable requirement, and it's already necessary for valid SAN generation.
Programmer bug hunt challenge
Moderators: hgm, Rebel, chrisw
Re: Programmer bug hunt challenge
It appears that some are asking for Fen 2. Fen 1 served us well but could care less of chess logic. It was not a program of chess but rather a tool to place pieces on a board. 100's of programs work well with Fen 1. Can Fen 2 simplify things for chess programmers, YES. Is it really needed? 100's of programs say NO.