Simon Willison's Weblog: charlie-marsh

Thoughts on OpenAI acquiring Astral and uv/ruff/ty

2026-03-19T16:45:15+00:00

The big news this morning: Astral to join OpenAI (on the Astral blog) and OpenAI to acquire Astral (the OpenAI announcement). Astral are the company behind uv, ruff, and ty - three increasingly load-bearing open source projects in the Python ecosystem. I have thoughts!

The official line from OpenAI and Astral

The Astral team will become part of the Codex team at OpenAI.

Charlie Marsh has this to say:

Open source is at the heart of that impact and the heart of that story; it sits at the center of everything we do. In line with our philosophy and OpenAI's own announcement, OpenAI will continue supporting our open source tools after the deal closes. We'll keep building in the open, alongside our community -- and for the broader Python ecosystem -- just as we have from the start. [...]

After joining the Codex team, we'll continue building our open source tools, explore ways they can work more seamlessly with Codex, and expand our reach to think more broadly about the future of software development.

OpenAI's message has a slightly different focus (highlights mine):

As part of our developer-first philosophy, after closing OpenAI plans to support Astral’s open source products. By bringing Astral’s tooling and engineering expertise to OpenAI, we will accelerate our work on Codex and expand what AI can do across the software development lifecycle.

This is a slightly confusing message. The Codex CLI is a Rust application, and Astral have some of the best Rust engineers in the industry - BurntSushi alone (Rust regex, ripgrep, jiff) may be worth the price of acquisition!

So is this about the talent or about the product? I expect both, but I know from past experience that a product+talent acquisition can turn into a talent-only acquisition later on.

uv is the big one

Of Astral's projects the most impactful is uv. If you're not familiar with it, uv is by far the most convincing solution to Python's environment management problems, best illustrated by this classic XKCD:

Switch from python to uv run and most of these problems go away. I've been using it extensively for the past couple of years and it's become an essential part of my workflow.

I'm not alone in this. According to PyPI Stats uv was downloaded more than 126 million times last month! Since its release in February 2024 - just two years ago - it's become one of the most popular tools for running Python code.

Ruff and ty

Astral's two other big projects are ruff - a Python linter and formatter - and ty - a fast Python type checker.

These are popular tools that provide a great developer experience but they aren't load-bearing in the same way that uv is.

They do however resonate well with coding agent tools like Codex - giving an agent access to fast linting and type checking tools can help improve the quality of the code they generate.

I'm not convinced that integrating them into the coding agent itself as opposed to telling it when to run them will make a meaningful difference, but I may just not be imaginative enough here.

What of pyx?

Ever since uv started to gain traction the Python community has been worrying about the strategic risk of a single VC-backed company owning a key piece of Python infrastructure. I wrote about one of those conversations in detail back in September 2024.

The conversation back then focused on what Astral's business plan could be, which started to take form in August 2025 when they announced pyx, their private PyPI-style package registry for organizations.

I'm less convinced that pyx makes sense within OpenAI, and it's notably absent from both the Astral and OpenAI announcement posts.

Competitive dynamics

An interesting aspect of this deal is how it might impact the competition between Anthropic and OpenAI.

Both companies spent most of 2025 focused on improving the coding ability of their models, resulting in the November 2025 inflection point when coding agents went from often-useful to almost-indispensable tools for software development.

The competition between Anthropic's Claude Code and OpenAI's Codex is fierce. Those $200/month subscriptions add up to billions of dollars a year in revenue, for companies that very much need that money.

Anthropic acquired the Bun JavaScript runtime in December 2025, an acquisition that looks somewhat similar in shape to Astral.

Bun was already a core component of Claude Code and that acquisition looked to mainly be about ensuring that a crucial dependency stayed actively maintained. Claude Code's performance has increased significantly since then thanks to the efforts of Bun's Jarred Sumner.

One bad version of this deal would be if OpenAI start using their ownership of uv as leverage in their competition with Anthropic.

Astral's quiet series A and B

One detail that caught my eye from Astral's announcement, in the section thanking the team, investors, and community:

Second, to our investors, especially Casey Aylward from Accel, who led our Seed and Series A, and Jennifer Li from Andreessen Horowitz, who led our Series B. As a first-time, technical, solo founder, you showed far more belief in me than I ever showed in myself, and I will never forget that.

As far as I can tell neither the Series A nor the Series B were previously announced - I've only been able to find coverage of the original seed round from April 2023.

Those investors presumably now get to exchange their stake in Astral for a piece of OpenAI. I wonder how much influence they had on Astral's decision to sell.

Forking as a credible exit?

Armin Ronacher built Rye, which was later taken over by Astral and effectively merged with uv. In August 2024 he wrote about the risk involved in a VC-backed company owning a key piece of open source infrastructure and said the following (highlight mine):

However having seen the code and what uv is doing, even in the worst possible future this is a very forkable and maintainable thing. I believe that even in case Astral shuts down or were to do something incredibly dodgy licensing wise, the community would be better off than before uv existed.

Astral's own Douglas Creager emphasized this angle on Hacker News today:

All I can say is that right now, we're committed to maintaining our open-source tools with the same level of effort, care, and attention to detail as before. That does not change with this acquisition. No one can guarantee how motives, incentives, and decisions might change years down the line. But that's why we bake optionality into it with the tools being permissively licensed. That makes the worst-case scenarios have the shape of "fork and move on", and not "software disappears forever".

I like and trust the Astral team and I'm optimistic that their projects will be well-maintained in their new home.

OpenAI don't yet have much of a track record with respect to acquiring and maintaining open source projects. They've been on a bit of an acquisition spree over the past three months though, snapping up Promptfoo and OpenClaw (sort-of, they hired creator Peter Steinberger and are spinning OpenClaw off to a foundation), plus closed source LaTeX platform Crixet (now Prism).

If things do go south for uv and the other Astral projects we'll get to see how credible the forking exit strategy turns out to be.

Tags: python, ai, rust, openai, ruff, uv, astral, charlie-marsh, coding-agents, codex-cli, ty

pyx: a Python-native package registry, now in Beta

2025-08-13T18:36:51+00:00

pyx: a Python-native package registry, now in Beta

Since its first release, the single biggest question around the uv Python environment management tool has been around Astral's business model: Astral are a VC-backed company and at some point they need to start making real revenue.

Back in September Astral founder Charlie Marsh said the following:

I don't want to charge people money to use our tools, and I don't want to create an incentive structure whereby our open source offerings are competing with any commercial offerings (which is what you see with a lost of hosted-open-source-SaaS business models).

What I want to do is build software that vertically integrates with our open source tools, and sell that software to companies that are already using Ruff, uv, etc. Alternatives to things that companies already pay for today.

An example of what this might look like (we may not do this, but it's helpful to have a concrete example of the strategy) would be something like an enterprise-focused private package registry. [...]

It looks like those plans have become concrete now! From today's announcement:

TL;DR: pyx is a Python-native package registry --- and the first piece of the Astral platform, our next-generation infrastructure for the Python ecosystem.

We think of pyx as an optimized backend for uv: it's a package registry, but it also solves problems that go beyond the scope of a traditional "package registry", making your Python experience faster, more secure, and even GPU-aware, both for private packages and public sources (like PyPI and the PyTorch index).

pyx is live with our early partners, including Ramp, Intercom, and fal [...]

This looks like a sensible direction to me, and one that stays true to Charlie's promises to carefully design the incentive structure to avoid corrupting the core open source project that the Python community is coming to depend on.

Via @charliermarsh

Tags: open-source, packaging, python, uv, astral, charlie-marsh

Quoting Charlie Marsh

2025-01-29T18:53:56+00:00

We’re building a new static type checker for Python, from scratch, in Rust. From a technical perspective, it’s probably our most ambitious project yet. We’re about 800 PRs deep!

Like Ruff and uv, there will be a significant focus on performance. The entire system is designed to be highly incremental so that it can eventually power a language server (e.g., only re-analyze affected files on code change). [...]

We haven't publicized it to-date, but all of this work has been happening in the open, in the Ruff repository.

— Charlie Marsh

Tags: python, rust, ruff, uv, astral, charlie-marsh

uv python install --reinstall 3.13

2025-01-07T20:43:00+00:00

uv python install --reinstall 3.13

I couldn't figure out how to upgrade the version of Python 3.13 I had previous installed using uv - I had Python 3.13.0.rc2. Thanks to Charlie Marsh I learned the command for upgrading to the latest uv-supported release:

uv python install --reinstall 3.13

I can confirm it worked using:

uv run --python 3.13 python -c 'import sys; print(sys.version)'

Caveat from Zanie Blue on my PR to document this:

There are some caveats we'd need to document here, like this will break existing tool installations (and other virtual environments) that depend on the version. You'd be better off doing uv python install 3.13.X to add the new patch version in addition to the existing one.

Tags: python, uv, charlie-marsh

TIL: Using uv to develop Python command-line applications

2024-10-24T05:56:21+00:00

TIL: Using uv to develop Python command-line applications

I've been increasingly using uv to try out new software (via uvx) and experiment with new ideas, but I hadn't quite figured out the right way to use it for developing my own projects.

It turns out I was missing a few things - in particular the fact that there's no need to use uv pip at all when working with a local development environment, you can get by entirely on uv run (and maybe uv sync --extra test to install test dependencies) with no direct invocations of uv pip at all.

I bounced a few questions off Charlie Marsh and filled in the missing gaps - this TIL shows my new uv-powered process for hacking on Python CLI apps built using Click and my simonw/click-app cookecutter template.

Tags: cli, packaging, pip, python, til, cookiecutter, uv, astral, charlie-marsh

uv under discussion on Mastodon

2024-09-08T16:23:31+00:00

uv under discussion on Mastodon

Jacob Kaplan-Moss kicked off this fascinating conversation about uv on Mastodon recently. It's worth reading the whole thing, which includes input from a whole range of influential Python community members such as Jeff Triplett, Glyph Lefkowitz, Russell Keith-Magee, Seth Michael Larson, Hynek Schlawack, James Bennett and others. (Mastodon is a pretty great place for keeping up with the Python community these days.)

The key theme of the conversation is that, while uv represents a huge set of potential improvements to the Python ecosystem, it comes with additional risks due its attachment to a VC-backed company - and its reliance on Rust rather than Python.

Here are a few comments that stood out to me.

Russell:

As enthusiastic as I am about the direction uv is going, I haven't adopted them anywhere - because I want very much to understand Astral’s intended business model before I hook my wagon to their tools. It's definitely not clear to me how they're going to stay liquid once the VC money runs out. They could get me onboard in a hot second if they published a "This is what we're planning to charge for" blog post.

Hynek:

As much as I hate VC, [...] FOSS projects flame out all the time too. If Frost loses interest, there’s no PDM anymore. Same for Ofek and Hatch(ling).

I fully expect Astral to flame out and us having to fork/take over—it’s the circle of FOSS. To me uv looks like a genius sting to trick VCs into paying to fix packaging. We’ll be better off either way.

Glyph:

Even in the best case, Rust is more expensive and difficult to maintain, not to mention "non-native" to the average customer here. [...] And the difficulty with VC money here is that it can burn out all the other projects in the ecosystem simultaneously, creating a risk of monoculture, where previously, I think we can say that "monoculture" was the least of Python's packaging concerns.

Hynek on Rust:

I don’t think y’all quite grok what uv makes so special due to your seniority. The speed is really cool, but the reason Rust is elemental is that it’s one compiled blob that can be used to bootstrap and maintain a Python development. A blob that will never break because someone upgraded Homebrew, ran pip install or any other creative way people found to fuck up their installations. Python has shown to be a terrible tech to maintain Python.

Christopher Neugebauer:

Just dropping in here to say that corporate capture of the Python ecosystem is the #1 keeps-me-up-at-night subject in my community work, so I watch Astral with interest, even if I'm not yet too worried.

I'm reminded of this note from Armin Ronacher, who created Rye and later donated it to uv maintainers Astral:

However having seen the code and what uv is doing, even in the worst possible future this is a very forkable and maintainable thing. I believe that even in case Astral shuts down or were to do something incredibly dodgy licensing wise, the community would be better off than before uv existed.

I'm currently inclined to agree with Armin and Hynek: while the risk of corporate capture for a crucial aspect of the Python packaging and onboarding ecosystem is a legitimate concern, the amount of progress that has been made here in a relatively short time combined with the open license and quality of the underlying code keeps me optimistic that uv will be a net positive for Python overall.

Update: uv creator Charlie Marsh joined the conversation:

I don't want to charge people money to use our tools, and I don't want to create an incentive structure whereby our open source offerings are competing with any commercial offerings (which is what you see with a lost of hosted-open-source-SaaS business models).

What I want to do is build software that vertically integrates with our open source tools, and sell that software to companies that are already using Ruff, uv, etc. Alternatives to things that companies already pay for today.

An example of what this might look like (we may not do this, but it's helpful to have a concrete example of the strategy) would be something like an enterprise-focused private package registry. A lot of big companies use uv. We spend time talking to them. They all spend money on private package registries, and have issues with them. We could build a private registry that integrates well with uv, and sell it to those companies. [...]

But the core of what I want to do is this: build great tools, hopefully people like them, hopefully they grow, hopefully companies adopt them; then sell software to those companies that represents the natural next thing they need when building with Python. Hopefully we can build something better than the alternatives by playing well with our OSS, and hopefully we are the natural choice if they're already using our OSS.

Tags: armin-ronacher, jacob-kaplan-moss, open-source, packaging, python, russell-keith-magee, rust, hynek-schlawack, mastodon, glyph, uv, astral, charlie-marsh, seth-michael-larson

Docker images using uv's python

2024-09-06T23:54:29+00:00

Docker images using uv's python

Michael Kennedy interviewed uv/Ruff lead Charlie Marsh on his Talk Python podcast, and was inspired to try uv with Talk Python's own infrastructure, a single 8 CPU server running 17 Docker containers (status page here).

The key line they're now using is this:

RUN uv venv --python 3.12.5 /venv

Which downloads the uv selected standalone Python binary for Python 3.12.5 and creates a virtual environment for it at /venv all in one go.

Via @mkennedy

Tags: python, docker, uv, charlie-marsh

uv: Python packaging in Rust

2024-02-15T19:57:13+00:00

uv: Python packaging in Rust

"uv is an extremely fast Python package installer and resolver, written in Rust, and designed as a drop-in replacement for pip and pip-tools workflows."

From Charlie Marsh and Astral, the team behind Ruff, who describe it as a milestone in their pursuit of a "Cargo for Python".

Also in this announcement: Astral are taking over stewardship of Armin Ronacher's Rye packaging tool, another Rust project.

uv is reported to be 8-10x faster than regular pip, increasing to 80-115x faster with a warm global module cache thanks to copy-on-write and hard links on supported filesystems - which saves on disk space too.

It also has a --resolution=lowest option for installing the lowest available version of dependencies - extremely useful for testing, I've been wanting this for my own projects for a while.

Also included: uv venv - a fast tool for creating new virtual environments with no dependency on Python itself.

Via @charliermarsh

Tags: armin-ronacher, pip, python, rust, rye, ruff, uv, astral, charlie-marsh

Quoting Charlie Marsh

2024-02-04T19:41:16+00:00

Sometimes, performance just doesn't matter. If I make some codepath in Ruff 10x faster, but no one ever hits it, I'm sure it could get some likes on Twitter, but the impact on users would be meaningless.

And yet, it's good to care about performance everywhere, even when it doesn't matter. Caring about performance is cultural and contagious. Small wins add up. Small losses add up even more.

— Charlie Marsh

Tags: performance, ruff, charlie-marsh