ChatGPT reverse-engineers .7z without tools

This is less a "magic unzipper" story than a frontier model getting cornered into pure constraint-solving. The most useful prompt pattern is probably just a blunt spec that bans external tools and pushes the model to work with standard-library primitives.

• –Front-load the limits: no pip, no apt, no internet, only the standard library.
• –The impressive part is compositional reasoning, not 7z trivia: file signatures, varints, metadata layout, then lzma and struct to synthesize a parser.
• –Commenters name Qwen3.5 Coder, Mistral Small 4, and Claude-style systems as the closest bets, but frontier thinking models still have the highest ceiling.
• –Treat the story as anecdotal, not benchmark-grade; some of the "manual parse" effect may come from tool logs, hallucinated availability, or orchestration quirks.