Thank-you all for the discussion and the attempts to accommodate flit, but I'll bow out now. It's become clear that the way flit approaches packaging is fundamentally incompatible with the priorities other people have for the ecosystem. Namely, I see sdists as archival artifacts to be made approximately once per release, but the general trend is to make them a key part of the build pipeline. Making a guerilla tool with no concern for integration was fun. It became frustrating as people began to use it and expected it to play well with other tools, so I jumped on PEP 517 as a way to bring it into the fold. That didn't work out, and a tool that doesn't play well with pip can only be an attractive nuisance at best, even if it technically complies with the relevant specs. Flit is therefore deprecated, and I recommend anyone using it migrate back to setup.py packaging. Best wishes, Thomas