Anthropic Buys Stainless, the Toolmaker Behind Its Rivals' SDKs

Anthropic announced on 18 May that it has acquired Stainless, the developer-tools startup whose software generates SDKs for OpenAI, Google, Cloudflare and a long list of other AI labs. Founded in 2022 by former Stripe engineer Alex Rattray, Stainless turns an API specification into client libraries across TypeScript, Python, Go, Java and Kotlin. The Information reported last week the deal was likely to clear $300 million; Anthropic did not disclose terms.

Anthropic has used Stainless to generate every official Claude SDK since its API's earliest days, so the integration is less a pivot than a formal absorption. Katelyn Lesse, who heads platform engineering at Anthropic, framed the rationale plainly in the company's announcement: "Agents are only as useful as what they can connect to." Rattray, for his part, described Anthropic as one of the first customers willing to bet that "SDKs deserve as much care as the APIs they wrap."

What changes for other AI labs

Stainless will wind down its hosted products, including the SDK generator that competing labs have come to depend on. Customers retain ownership of the SDKs they have already shipped and full rights to modify them, but new generation work will move in-house at Anthropic. For OpenAI, Google and the rest of the roster, that means rebuilding the tooling, switching providers, or doing without. Either way, a piece of shared infrastructure that quietly powered the whole industry is now sitting under one roof.

The agent connectivity bet

The acquisition fits Anthropic's broader push around the Model Context Protocol, the open standard for letting AI agents reach into external tools and services. Stainless already produces MCP server scaffolding alongside its SDK generators, which is exactly the layer Anthropic wants to control as Claude moves further into autonomous, tool-using territory. Commentary on Hacker News noted that whoever owns the connective tissue between APIs and agents has unusual leverage over the rest of the stack.

For developers, the practical effect is small in the short run. Existing SDKs keep working and MCP servers keep responding. The longer-term question is whether Anthropic uses its new asset to advantage its own platform, or keeps the generator broadly neutral the way Stripe famously kept its tools available to its competitors. The valuation Anthropic now commands, recently reported as approaching $900 billion, gives it room to make either call. The next twelve months should make clear which one.