pwasm 0.2a0

Simon Willison released pwasm 0.2a0, an alpha-tagged pure Python WebAssembly engine, after Claude Opus 5.5 produced 42 commits that brought it to near-complete WASM specification coverage. The PyPI wheel now bundles working WASM builds of MicroPython, QuickJS and Micro QuickJS, which Willison said he would not trust for production use. Willison originally built pwasm in January 2026 as a vibe-coded project and had not touched it since.

Release: pwasm 0.2a0 https://github.com/simonw/pwasm/releases/tag/0.2a0 pwasm is one of my folly projects - an entirely vibe-coded pure Python WebAssembly engine that I built in January during my first bout of AI mania https://simonwillison.net/2026/Sep/27/2026-in-llms-so-far/ simon-willison-2026-in-llms.012.webp . I hadn't touched it since January, so I decided to let Claude Opus 5.5 loose on it and see if it could make any significant improvements: Evaluate current state of pwasm - then consider what it would take to get the MicroPython and micro JavaScript experiments from the research repo https://github.com/simonw/research working under it - and what it would take to speed it up 42 commits later https://github.com/simonw/pwasm/pull/12 with minimal follow-up prompting it now handles almost all of the WASM specification, and the wheel from PyPI bundles working WASM builds of MicroPython https://micropython.org/ , QuickJS https://github.com/quickjs-ng/quickjs and Micro QuickJS https://github.com/bellard/mquickjs . I wouldn't trust this thing at all - hence the alpha version tag - but it's interesting seeing how today's models can improve on the work of models from 10 months ago. Tags: python https://simonwillison.net/tags/python , webassembly https://simonwillison.net/tags/webassembly , vibe-coding https://simonwillison.net/tags/vibe-coding