Anthropic acquired developer-tools startup Stainless on May 18, 2026, the companies confirmed in separate announcements that day. The deal brings the SDK and MCP server tooling provider in-house for Anthropic's Claude Platform and immediately shut Stainless's hosted products to new customers. Stainless had supplied auto-generated client libraries and tooling for a roster of companies including OpenAI and Cloudflare, making the acquisition a significant event in the developer-infrastructure market for agentic AI.
The acquisition removes a shared SDK-generation service from the market, forcing several major API providers to find alternative ways to maintain their client libraries. For Anthropic, the move adds a proven team that has built every official Anthropic SDK since the start of its API and deepens the lab's investment in the infrastructure that connects models to external data—a focus area as AI labs race to build agentic systems.
Stainless generated SDKs in languages including TypeScript, Python, Go, Java, and Kotlin, as well as command-line interfaces and tooling for the Model Context Protocol (MCP), from API specifications. Its customers included OpenAI, whose Python SDK repository explicitly states it is generated with Stainless, and Cloudflare, whose Go library also carries that attribution. The company also listed Google DeepMind among its customers, although evidence of that usage is less publicly documented.
As of May 18, all hosted Stainless products stopped accepting new signups and projects. Customers who had already generated SDKs retain ownership and can modify them, but the automated generator is no longer available as a service. Stainless said it will wind down the hosted products entirely, though it did not provide a timeline for the transition or say how long existing services will remain operational.
The company was founded in 2022 by Alex Rattray, a former Stripe engineer who worked on SDK tooling. It raised a $25 million Series A in December 2024, led by the venture firm a16z with participation from Sequoia and The General Partnership. Secondary reports from The Information and Axios pegged total funding at about $35 million, but only the Series A has been confirmed by primary sources.
Anthropic said in its announcement that "Stainless has powered the generation of every official Anthropic SDK since the earliest days of the Anthropic API" and that the team will join work on the Claude Platform, strengthening the ability of Claude to connect to data and tools. The statement frames the acquisition as a natural step in building the agentic layer that links models to external systems, a theme highlighted by Anthropic's earlier creation and donation of the MCP to the Linux Foundation's Agentic AI Foundation.
The acquisition leaves a gap in the developer-tooling market. Alternatives exist—Speakeasy, Fern, APIMatic, LibLab, and the open-source OpenAPI Generator—but migrating from a service that was deeply woven into release pipelines takes time. Neither OpenAI nor Cloudflare has commented on how they will handle the loss of Stainless's hosted services. The reported deal price remains unconfirmed: The Information reported on May 12 that Anthropic was in advanced talks to buy Stainless for at least $300 million, but neither company disclosed terms.
The final deal structure and the number of employees joining Anthropic were not disclosed. It is also unclear whether Stainless's MCP server generation tools will survive in any open-source form after the wind-down, or how long existing services will remain accessible for current customers.
The acquisition reinforces Anthropic's position in the agent-connectivity stack, even as it removes a shared tool from the broader ecosystem. Anthropic created the MCP protocol and has co-founded the Agentic AI Foundation with Block and OpenAI, with backing from Google, Microsoft, AWS, Cloudflare, and Bloomberg. For companies betting on agentic workflows, the concentration of a widely used SDK and MCP generation tool inside a major AI lab is a development that will shape infrastructure choices in the months ahead.