Anthropic revealed on Monday that it has acquired Stainless, a startup created by former Stripe engineer Alex Rattray. The company’s software is widely used by competing AI labs, including OpenAI and Google. The financial terms of the deal were not disclosed by Anthropic. However, according to a report from The Information last week, Anthropic had been in negotiations to purchase Stainless—backed by Sequoia Capital and Andreessen Horowitz—for over $300 million. This acquisition effectively removes a key infrastructure provider from the reach of Anthropic’s rivals.

An Anthropic spokesperson confirmed that current Stainless customers will retain ownership of the software development kits (SDKs) they have already generated, along with full rights to modify and extend them as they see fit. Founded in 2022 and based in New York, Stainless gained prominence in the emerging AI sector by automating the creation and upkeep of SDKs—the libraries that developers rely on to interact with APIs. Rattray developed software capable of transforming API specifications into production-ready SDKs across multiple programming languages, including Python, TypeScript, Kotlin, Go, and Java. The platform became popular because it automatically updates SDKs whenever APIs change, eliminating the tedious manual maintenance process.

This technology is especially valuable for companies like Anthropic, OpenAI, Google, Replicate, Runway, and Cloudflare, which are building AI agents that connect to external software to complete tasks on behalf of users. Stainless’s SDK tools offer a straightforward way to create and sustain those connections—but moving forward, the tools will be exclusive to Anthropic, leaving its competitors without access. According to Anthropic, Stainless software has been behind the generation of every official Anthropic SDK since the earliest days of its API.

“I started Stainless because SDKs deserve as much care as the APIs they wrap,” Rattray said in a press release issued Monday. “Anthropic was one of the first teams to bet on this with us. We have been watching what developers have built on Claude over the last few years, which made bringing our teams together an easy decision. The team gets to keep doing the work we love, on the platform where it matters most.”