Comment by simonw

Have you spent much time with Codex 5.1 or 5.2 in OpenAI Codex or a Claude Opus 4".5 in Claude code over the last ~6 weeks?

I think they represent a meaningful step change in what models can build. For me they are the moment we went from building relatively trivial things unassisted to building quite large and complex system that take multiple hours, often still triggered by a single prompt.

Some personal examples from the past few weeks.

- A spec-compliant HTML5 parsing library by Codex 5.2: https://simonwillison.net/2025/Dec/15/porting-justhtml/

- A CLI-based transcript export and publishing tool by Opus 4.5: https://simonwillison.net/2025/Dec/25/claude-code-transcript...

- A full JavaScript interpreter in dependency/free Python (!) https://github.com/simonw/micro-javascript - and here's that transcript published using the above-mentioned tool: https://static.simonwillison.net/static/2025/claude-code-mic...

- A WebAssembly runtime in Python which I haven't yet published

The above projects all took multiple prompts, but were still mostly built by prompting Claude Code for web on my iPhone in between Christmas family things.

I have a single-prompt one:

- A Datasette plugin that integrates Cloudflare's CAPTCHA system: https://github.com/simonw/datasette-turnstile - transcript: https://gistpreview.github.io/?2d9190335938762f170b0c0eb6060...

I'm not confident any of these projects would have worked with the coding agents and models we had had four months ago. There is no chance they would've worked with the January 2025 available models.