Anthropic acquires Stainless for $300 million

Anthropic said on May 18 that it had acquired Stainless, the developer-tools startup that has generated Anthropic’s official software development kits since the early days of its API. Stainless said its team would join Anthropic’s Claude Platform group, and Anthropic said it planned to bring Stainless’s SDK and Model Context Protocol server tooling deeper into its developer stack. The deal matters because Stainless was not just an Anthropic supplier. TechCrunch reported that Stainless software was also used by OpenAI, Google and Cloudflare, while The New Stack reported the transaction at $300 million and said Anthropic’s purchase would shut down a shared SDK generator used across rival platforms. Anthropic and Stainless did not disclose financial terms in their own announcements. (anthropic.com) ### What exactly did Anthropic buy? Anthropic described Stainless as “a leader in SDKs and MCP server tooling,” and said the company had powered every official Anthropic SDK from the start. In practice, that means code libraries, command-line tools and related infrastructure that help outside developers connect applications to Claude and keep those integrations updated as APIs change. (techcrunch.com) Stainless founder Alex Rattray said in a company blog post that the startup was founded in early 2022 to “reinvent the API layer of the internet” and improve developer experience. He said existing customers should visit Stainless’s transition page for next steps. ### Why does SDK generation matter here? Anthropic said the “frontier of AI is shifting from models that answer to agents that act,” and tied the Stainless acquisition to that shift. (anthropic.com) The company said agents are only as capable as the systems they can reach, making SDK generation and MCP server tooling part of the plumbing that lets models connect to outside software and data. (stainless.com) InfoWorld, citing Anthropic product lead Cat Wu Su, reported that Stainless had supported Anthropic’s libraries, command-line tools and connectors, and that tighter in-house control would help Anthropic integrate tooling with changes to the Claude API. Anthropic’s statement itself framed the same point more narrowly, saying the acquisition would extend Claude’s reach into external systems. (anthropic.com) ### Who else was using Stainless? TechCrunch reported on May 18 that Stainless was widely used by rival AI labs, including OpenAI and Google. The New Stack went further, saying the hosted SDK generator had been shared by OpenAI, Google and Cloudflare. That distinction matters because Anthropic’s own announcement focused on Anthropic’s use of Stainless, while outside reporting described a broader customer base. (infoworld.com) Anthropic did not name rival customers in its statement, and neither OpenAI nor Google appeared in the company’s announcement. ### Did Anthropic say it is shutting Stainless down for rivals? (techcrunch.com) Stainless said only that customers should consult its transition page, and Anthropic’s announcement did not spell out service terms for rival labs or other outside users. The New Stack reported that the acquisition would remove a shared SDK generator from OpenAI and Google, and Forbes separately reported that the hosted product was being shut down for everyone except Anthropic, with existing customers keeping what had already been generated. (anthropic.com) Those details came from outside reporting, not from the companies’ official posts. ### Where does this fit in Anthropic’s broader push? Anthropic’s newsroom shows the Stainless deal as part of a run of product and platform announcements in May, including Claude for Small Business on May 13 and a KPMG alliance on May 19. The company has also made other acquisitions tied to developer and agent infrastructure, including Bun in late 2025 and Vercept in March 2026. (thenewstack.io) May 18 is the key date for this deal: that is when Anthropic and Stainless published their announcements. For developers affected by the change, the immediate next step is the Stainless transition page, while Anthropic said the Stainless team will now work inside the Claude Platform organization. (anthropic.com 1) (anthropic.com 2)