Anthropic made its biggest acquisition to date on May 18, snapping up Stainless, a New York-based developer tools startup founded by former Stripe engineer Alex Rattray, in a deal valued at more than 00 million. The move hands Anthropic direct control over the software that generates and maintains the official software development kits for not just its own Claude API, but also those of rivals OpenAI, Google, and Cloudflare -- and it is winding down the hosted product so that only Anthropic benefits going forward.

A Quiet Backbone of the AI Industry

Stainless, founded in 2022 and backed by Sequoia Capital and Andreessen Horowitz, built technology that takes an API specification and automatically generates production-ready SDKs across Python, TypeScript, Go, Java, and Kotlin. Those SDKs are the code libraries developers use every day to integrate with AI platforms. For a 10-person team based in New York, Stainless punched far above its weight: one customer case study showed Cloudflare shipping over 1,000 endpoints across three language SDKs using the platform.

The startup had powered every official Anthropic SDK since the earliest days of the Claude API. But it was not exclusive to Anthropic. OpenAI, Google, Replicate, and Runway were also customers, making Stainless a rare piece of shared infrastructure in an industry defined by fierce competition.

"Stainless has shaped how developers experience the Claude API since the start, and it has been great to work with them on that," said Katelyn Lesse, Head of Platform Engineering at Anthropic. "Agents are only as useful as what they can connect to. We are excited to bring the Stainless team into Anthropic to advance Claude's ability to connect to data and tools."

Cutting Off the Competition

The strategic implications are hard to overstate. By acquiring Stainless and shutting down its hosted products, Anthropic is removing a tool that its direct competitors relied on to reach their own developers. New signups, projects, and SDK generation through the Stainless platform are no longer available. Existing customers retain full ownership of SDKs they have already generated and can modify or extend them freely, but they will need to find alternative tooling for future SDK maintenance.

Alex Rattray, Stainless founder and CEO, framed the acquisition as a natural evolution. "I started Stainless because SDKs deserve as much care as the APIs they wrap. Anthropic was one of the first teams to bet on this with us," Rattray said. "We have been watching what developers have built on Claude over the last few years, which made bringing our teams together an easy decision."

The reported 00 million price tag, first surfaced by The Information before the deal closed, would make this one of the largest acquisitions of a developer tools company in the AI era, particularly notable given Stainless had only a 10-person team. Anthropic did not officially confirm the price.

Why This Matters

This acquisition sits at the intersection of two trends reshaping the AI industry. First, the race to control developer experience has become as important as the race to build better models. SDK quality directly affects how quickly developers can adopt an API, test new features, and ship integrations. By owning the tooling that generates those SDKs, Anthropic gains a structural advantage in developer relations that no amount of documentation or marketing can replicate.

Second, the deal underscores how the AI industry is shifting from models to agents. Anthropic created the Model Context Protocol to standardize how AI agents connect to external tools and data sources. Stainless was already building MCP server tooling alongside its SDK generator. Folding that capability in-house gives Anthropic end-to-end control over the connective tissue between Claude and the outside world, from the protocol layer to the code libraries developers use to build on it.

The competitive dynamics are also striking. OpenAI and Google must now find or build replacement SDK tooling, adding friction at a moment when developer adoption is a key battleground. It is a reminder that in the platform wars of AI, controlling infrastructure can be as powerful as controlling intelligence.

What to Watch Next

The immediate question is timing: Anthropic has not announced a precise shutdown date for hosted Stainless services, and affected customers -- including some of Anthropic's biggest rivals -- will be watching closely. Beyond that, the acquisition signals that Anthropic is willing to make aggressive moves to lock in developer ecosystem advantages. Watch for whether OpenAI or Google respond with their own developer tooling acquisitions, and whether Anthropic integrates Stainless technology into a broader push to make Claude the default platform for AI agent development.