Keystone's AI Engineer Bet: Founder Turns Down Acquisition, Open-Sources

Keystone is a San Francisco AI developer tools company founded in 2025, betting that 'AI can act as an on-call engineer': automatically triaging tickets, fixing bugs, and handling production incidents. Founder Pablo Hansen earned a master's degree in AI at age 19 and was the first employee at Onyx (YC W24); his company Keystone was selected for the YC S25 batch and had already landed AI-native customers such as Lovable before Demo Day.

At the S25 Demo Day in September 2025, YC investors interviewed by TechCrunch listed it as one of the companies they most wanted to invest in across the batch: the 20-year-old founder held a master's degree in AI, the product was already fixing production bugs for customers, and it had turned down a seven-figure acquisition offer. That month it completed a $500,000 seed round, with investors including M.G. Siegler, Harrison Metal, Partech, Tekton, and others.

The company did not continue along the 'closed-source bug-fixing tool' path; instead, it renamed to Proliferate and transformed the product into an MIT-licensed, self-hostable coding agent platform: agents have isolated sandboxes, access internal services through an action catalog, are triggered by any event, and teams can preview and take over in multi-user real time. The narrative was upgraded from 'AI that helps you fix bugs' to 'open-source infrastructure that helps you build company-level coding agents'.

As of September 2026, Proliferate is still operating and in early commercialization: the open-source repository has been published, and the team is validating engineering efficiency gains from background agents with a small number of customers. It demonstrates a path of upgrading the bet from a narrow product to an open-source platform—first proving single-point value with paying customers, then betting on infrastructure.