Anthropic announced on May 18, 2026 that it was acquiring Stainless, a New York developer tools startup whose software turns an API specification into software development kits (SDKs), command-line tools and MCP servers. Anthropic did not disclose the terms. The Information had reported earlier that the two companies were in talks at a price above $300 million.
Stainless's team, led by founder and chief executive Alex Rattray, is joining Anthropic. The company told TechCrunch it would wind down all hosted Stainless products, including the SDK generator, and that the team would work on Claude Platform features and on connecting AI agents to APIs.
Existing customers keep ownership of the SDKs they have already generated and the right to change and extend them, Anthropic said. They lose access to the hosted service that kept those SDKs updated as their APIs changed.
What Stainless built
Rattray started Stainless in early 2022. He had previously been an engineer at Stripe, where he worked on API documentation and the system behind Stripe's client SDKs. Stainless applied the same idea for other companies: it reads a company's API specification and produces client libraries in languages including TypeScript, Python, Go, Java and Kotlin.
Stainless had generated every official Anthropic SDK since the early days of the Claude API. Its customers also included OpenAI, Google and Cloudflare, according to TechCrunch, and InfoWorld listed Google DeepMind, Perplexity and Groq. In its 2024 funding announcement, Stainless named Meta, Runway, Cerebras, LangChain, Metronome, Modern Treasury, Orb and Mux among its users.
“Agents are only as useful as what they can connect to.”
Why Anthropic is buying
Anthropic framed the deal around AI agents, which need reliable connections to outside software to be useful. Katelyn Lesse, Anthropic's head of platform engineering, said agents are only as useful as what they can connect to. Rattray said he started Stainless because SDKs deserve as much care as the APIs they wrap.
Anthropic created the Model Context Protocol, or MCP, an open standard for connecting AI models to tools and data. Stainless had added MCP server generation to its product, so the deal brings that tooling in-house.
Analysts quoted by InfoWorld said owning SDK generation gives Anthropic tighter control over how developers build on Claude. Lian Jye Su of Omdia said in-house control means closer integration with changes to Claude's API. InfoWorld placed the deal alongside Anthropic's earlier purchases of Bun and Vercept.
Steven Dickens, chief executive of HyperFRAME Research, told Techstrong.ai that buyers in deals of this kind get a team that is ready to work and intellectual property that has already been tested in production.
The founders and the funding
Stainless raised a $25 million Series A in December 2024 led by Jennifer Li at Andreessen Horowitz. Existing investors Sequoia and The General Partnership took part, along with new backers Felicis, Zapier and MongoDB Ventures, and individual investors including former Stripe chief operating officer Claire Hughes Johnson and Datadog president Amit Agarwal.
Stainless did not report its total funding or headcount in the sources reviewed, and Anthropic did not say how many people joined.
What happens to customers
Closing the hosted generator affects companies that relied on Stainless to keep their SDKs current, including rival AI labs. InfoWorld reported that some customers may need replacement tools or their own in-house systems to maintain SDKs as their APIs change. Let's Data Science noted that continuous automated updates would now require other tooling or self-hosting.
Commentators also pointed out that the deal turns a neutral supplier used across the AI industry into tooling owned by one lab. Forrester analyst Biswajeet Mahapatra warned in InfoWorld that quickly generated MCP servers can expose endpoints without proper authentication or monitoring, a risk for companies building agent connections at speed.
Anthropic did not publish a detailed plan for how Stainless technology will be used inside the Claude Platform, or say whether it will keep supporting SDKs it generates for other AI companies.
