Anthropic just made a move that says a lot about where the AI industry is heading. The company has acquired Stainless, a startup that built the developer tools used by none other than its rival, OpenAI.

The goal, according to Anthropic, is to "improve developer experience and the connections between agents and external systems." In plain English: making it easier for developers to hook up AI agents to real-world data and tools.

Stainless was founded in 2022 by Alex Rattray in New York. The company specialized in building high-quality software development kits (SDKs) and developer interfaces for APIs. Think of it as the plumbing that lets developers actually use an API without pulling their hair out.

"I started Stainless because SDKs deserve as much care as the APIs they wrap," Rattray said. "Anthropic was one of the first teams to bet on this with us. 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 team gets to keep doing the work we love, on the platform where it matters most."

Katelyn Lesse, head of Platform Engineering at Anthropic, added: "Stainless has shaped how developers experience the Claude API since the start, and it's been great to work with them on that. 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."

Stainless will wind down its existing products, including its SDK generator, and halt all new products. Its client list was a who's who of AI: Google DeepMind, Meta, and OpenAI. OpenAI originally built its own internal SDK but switched to Stainless after maintaining it became too complex and resource-intensive. Stainless also worked on tools tied to Anthropic's Model Context Protocol (MCP), a framework launched in November 2024.

The deal is worth about $300 million, according to reports from before the transaction closed.

This acquisition comes as Anthropic is reportedly in talks for a new funding round that would value the company at $800 billion — more than double its $350 billion valuation from February. The company's annualized revenue hit $30 billion in April, surpassing OpenAI's roughly $24 billion run rate. That's a jump from $9 billion at the end of 2025, with over 1,000 companies now spending more than $1 million annually on Anthropic products.

Anthropic also recently launched Claude Mythos Preview, a model designed to identify software vulnerabilities, but restricted access due to cybersecurity concerns. That move has only strengthened its positioning ahead of a potential IPO later this year.