Anthropic Buys Stainless for ~$300M, Pulling SDK Plumbing Out From Under OpenAI and Google

Anthropic announced on May 18 that it has acquired Stainless, the developer-tools startup that has quietly powered the generation of official SDKs across nearly every frontier AI lab. Terms were not disclosed by Anthropic, but The Information pegged the deal at more than $300 million, making it one of the largest tooling acquisitions in the AI infrastructure stack to date.

Founded in 2022 by former Stripe engineer Alex Rattray, Stainless automates the creation and maintenance of software development kits — the language libraries developers use to call any modern API. The platform generates production-ready clients in Python, TypeScript, Kotlin, Go, and Java from an OpenAPI spec, plus the MCP server tooling that has become essential plumbing for agentic applications. Stainless's customer list reads like a who's who of model providers: OpenAI, Google, Cloudflare, Replicate, and Runway all relied on it to ship official API libraries.

"I started Stainless because SDKs deserve as much care as the APIs they wrap," Rattray said in a statement, noting that Anthropic was "one of the first teams to bet on this." Stainless has powered the generation of every official Anthropic SDK since the earliest days of the API, and the company will now wind down all hosted Stainless products, including its SDK generator. Existing customers retain full ownership of any SDKs they have generated to date and the rights to modify and extend them, but they will need to migrate to a self-hosted alternative or rebuild the tooling internally.

The strategic implication is unsubtle: Anthropic just removed a shared infrastructure supplier that several of its largest rivals were depending on. OpenAI, Google, and others now face a forced choice between rebuilding the SDK pipeline in-house or migrating to a competitor — work that consumes engineering cycles those companies would rather spend on models and agents. For Anthropic, absorbing the team also brings expertise in MCP server tooling directly into the org responsible for Model Context Protocol itself, tightening the loop between protocol design and the libraries developers actually ship against.

The deal also slots cleanly into a pattern. Over the past two months Anthropic has hired former OpenAI co-founder Andrej Karpathy to push its "Software 3.0" agenda, opened the Claude Security public beta, and locked up enterprise deals with PwC and the Gates Foundation. Pulling in the company that made shipping API libraries effortless is the toolchain piece of a broader bet that the winners of the agentic era will be the ones who own the rails developers use to build on top of them.