Simon Willison’s Weblog

Marimo is Joining CoreWeave (via) I don't usually cover startup acquisitions here, but this one feels relevant to several of my interests.

Marimo (previously) provide an open source (Apache 2 licensed) notebook tool for Python, with first-class support for an additional WebAssembly build plus an optional hosted service. It's effectively a reimagining of Jupyter notebooks as a reactive system, where cells automatically update based on changes to other cells - similar to how Observable JavaScript notebooks work.

The first public Marimo release was in January 2024 and the tool has "been in development since 2022" (source).

CoreWeave are a big player in the AI data center space. They started out as an Ethereum mining company in 2017, then pivoted to cloud computing infrastructure for AI companies after the 2018 cryptocurrency crash. They IPOd in March 2025 and today they operate more than 30 data centers worldwide and have announced a number of eye-wateringly sized deals with companies such as Cohere and OpenAI. I found their Wikipedia page very helpful.

They've also been on an acquisition spree this year, including:

• Weights & Biases in March 2025 (deal closed in May), the AI training observability platform.
• OpenPipe in September 2025 - a reinforcement learning platform, authors of the Agent Reinforcement Trainer Apache 2 licensed open source RL framework.
• Monolith AI in October 2025, a UK-based AI model SaaS platform focused on AI for engineering and industrial manufacturing.
• And now Marimo.

Marimo's own announcement emphasizes continued investment in that tool:

Marimo is joining CoreWeave. We’re continuing to build the open-source marimo notebook, while also leveling up molab with serious compute. Our long-term mission remains the same: to build the world’s best open-source programming environment for working with data.

marimo is, and always will be, free, open-source, and permissively licensed.

Give CoreWeave's buying spree only really started this year it's impossible to say how well these acquisitions are likely to play out - they haven't yet established a track record.