mitsuhiko: an Opus 5.5 agent could reimplement all of CPython in two weeks against just the tests
thomasahle · x · 2026-11-01
Well-known developer mitsuhiko (Armin Ronacher) claims that at this point an Opus 5.5 agent could re-implement all of CPython within about two weeks with no human input, judged only by the test suite — likely achieving full compatibility including ABI, no GIL, a JIT, and beating the original on all measurable metrics.
Respondent thomasahle is working on related territory: lean-surfaces, a project embedding Python (plus C++, Rust, SystemVerilog) deeply in Lean 4 to prove real programs correct, with differentially-tested semantics. He notes progress was fast early on but has slowed as support climbed above 99%.
Related event: mitsuhiko: Coding Agents Can Rewrite CPython in Two Weeks Without Humans(3 posts)→
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