X discussion: Flask creator Armin Ronacher says an Opus 5.5 agent could re-implement all of CPython in about two weeks with no human input, sparking a debate over whether the value is in the test suite
Flask creator Armin Ronacher (@mitsuhiko) posted on November 1 that he is convinced an Opus 5.5 agent could now re-implement all of CPython in about two weeks against just the tests, with no human input, possibly with ABI compatibility, no GIL, a JIT, and better numbers on every measurable metric. The post had about 1,637 likes, 98K views and 162 replies. He later added that a rewrite does not strictly need an existing test suite, and that having the agent build its own tests is often better for porting. antirez agreed, while Mario Zechner and Meta engineer Kevin Newton pushed back. This is a personal prediction; no such rewrite has been done.
Key Takeaways
- Original post: @mitsuhiko, Nov 1, 11:52 UTC, about 1,637 likes, 269 bookmarks, 162 replies and 98K views at fetch time
- His core claim: given just the tests and no human input, an agent could rewrite CPython in about two weeks, "probably" with full compatibility including ABI, no GIL and a JIT, which he called "completely crazy"; it is a prediction, not an experiment
- Follow-up post (14:07 UTC, about 212 likes): an existing test suite is not required, and many pre-existing tests are bad for hill climbing; he says an agent he ran a few weeks ago re-implemented the Excel formula engine and built its own test suite from scratch
- antirez (creator of Redis) agreed that you can ask an agent to build a large test suite before starting a port; Mario Zechner disagreed, saying his problem with agents is trust and pre-existing test and conformance suites increase it
- Meta engineer Kevin Newton pushed back that very few Python JITs have succeeded because of complexity and compatibility, and that a good one without human input is unrealistic; others noted that under PEP 703 the free-threaded build is not ABI-compatible with the standard one










