What they did
The poster, u/rnatau, wanted to play Pro Pinball: The Web again, a 1995 PC game that never ran smoothly for them on a Windows 98 machine or in DOSBox. They used Codex with Sol 6.1 on high reasoning to reverse engineer it, working in small steps: physics first, then animations, then a first playable demo, with a lot of their own playtesting in between. Codex disassembled the original, turned parts of it into unit tests run against an emulator, and ran long gameplay simulations that it compared frame by frame with the same sequences in the original game. After the faithful version, the two built an enhanced edition that fixes some old quirks and decouples rendering from the 60 Hz loop with 17 physics microsteps per frame.
How it went
The poster says the port reached a fully faithful state and is released as open source with Windows and Linux builds; it needs the original data files from archive.org. Codex suggested redrawing the dot matrix display instead of scaling it, which kept it sharp in full screen. It found the cause of the old Windows 98 stutter (window timer jitter) and a once-per-second stutter after gamepad support was added, though only after the poster pushed back. It needed to be told how to compute only the microsteps up to the current time to raise the frame rate. The enhanced edition's display menu flow is still rough.
Worth knowing
The method that made it work was verification against the original: emulator-backed unit tests and frame-by-frame comparison of simulated play. The parts the poster did not specify themselves are the parts that still feel loose. Players still need to supply the original game's data files.