Anthropic announced on Monday that it has acquired Stainless, the developer tools startup whose SDK generation software is used by OpenAI, Google, Replicate, Runway, and Cloudflare. The company immediately shut down all of Stainless’s hosted products, cutting competitors off from the tooling.
Terms were not disclosed, but The Information reported last week that the deal was valued at more than $300 million. Stainless is backed by Sequoia Capital and Andreessen Horowitz.
What Stainless Does
Founded in 2022 by former Stripe engineer Alex Rattray, Stainless automates the creation and maintenance of SDKs: the software libraries developers use to interact with APIs. The platform takes an API specification and generates production-ready SDKs across Python, TypeScript, Go, Java, Kotlin, and other languages, then automatically updates them as APIs change.
That capability matters far more in an agentic world than it did when Stainless launched. AI agents need to call external APIs to do anything useful, according to Anthropic’s announcement, and the SDK layer is the connective tissue between an agent and every service it touches. Stainless also builds MCP server tooling, which ties directly into Anthropic’s Model Context Protocol strategy for agent connectivity.
According to the Stainless blog post announcing the deal, the company estimates that roughly a quarter of the world’s professional software developers have used a Stainless-generated SDK or visited a documentation site built with its tools.
Competitors Lose a Shared Supplier
The immediate shutdown is the aggressive part. As TechCrunch reported, Anthropic told the publication that Stainless customers will retain ownership of SDKs they have already generated, with full rights to modify and extend them. But new signups, new projects, and new SDK generation are no longer available as of Monday.
OpenAI, Google, and Cloudflare all used Stainless to generate and maintain their developer SDKs. Those companies now need to either build equivalent tooling in-house, find an alternative vendor, or manually maintain their SDK libraries going forward.
Katelyn Lesse, Anthropic’s head of platform engineering, told TechCrunch: “Agents are only as useful as what they can connect to. We’re excited to bring the Stainless team into Anthropic to advance Claude’s ability to connect to data and tools.”
The Infrastructure Play
Stainless has powered every official Anthropic SDK since the earliest days of the Claude API, according to Anthropic. This is not a new relationship becoming an acquisition. It is a long-standing supplier being absorbed to lock down a strategic layer.
The acquisition fits a pattern. Anthropic created MCP to standardize how agents connect to external tools and data. Stainless builds the SDKs and MCP servers that make those connections work in practice. Owning both the protocol and the tooling that implements it gives Anthropic end-to-end control of the agent integration stack.
“I started Stainless because SDKs deserve as much care as the APIs they wrap,” Rattray said in Anthropic’s announcement. “Anthropic was one of the first teams to bet on this with us.”
For companies building on Claude, the acquisition means tighter integration and better SDK tooling over time. For everyone else building AI agents, it means one fewer shared infrastructure provider and a clear signal: the companies winning the agent race are buying the plumbing, not just training bigger models.