What they did
The poster, crc32 on Hacker News (sjmelia on GitHub), set LLMs the job of reconstructing the C and assembler source for the MS-DOS release of Sid Meier's Civilization. The target was strict: the output had to build with the period toolchain (Microsoft C 5.1, MASM 5.10, LINK 5.01, run in DOSBox-X) and produce the original CIV.EXE exactly. That fixed target let the models check their own work against pinned hashes and iterate. The poster pointed the models at OpenCiv1, an earlier decompilation project, and used the result to write a technical book, an annotated story of the source, and a second repo that builds the game for modern systems. Code and docs were edited and verified by hand after the models wrote them.
How it went
The rebuilt source produces a byte-for-byte match with version 474.05 of the executable, apart from a four-byte startup correction the poster believes was a patch applied after linking. The graphics driver, sound driver and MISC.EXE are checked against hashes too. It took a couple of months of on-and-off use of roughly 15 pounds a month subscriptions from OpenAI, Anthropic and Google, without hitting their limits. The poster has since started the same approach on a newer, less documented game, with no result yet.
Worth knowing
Matching the binary does not mean this is the original source: names, comments and some types are guesses, though the compiler's layout depends on identifier names and length limits, which narrows them. The poster credits OpenCiv1 and notes it was likely in the models' training data, so a less studied program may go much worse. Legality was raised in the comments and left open. The poster did not say which tools or harness ran the models.