Anthropic is in advanced talks to acquire Stainless, a four-year-old developer tools company that uses AI to automatically convert complex APIs into SDKs (software development kits) for developers and AI agents. The deal is valued at a minimum of $300 million.
The list of Stainless’s clients is particularly telling: OpenAI, Google, and Anthropic itself all rely on its tools to streamline model access. If the acquisition goes through, Anthropic would gain direct control over a critical access point for competitors’ models. OpenAI previously maintained its own SDK, but according to a senior engineer, the maintenance was costly and resource-intensive, leading the company to switch to Stainless. Alternatives like Speakeasy, LibLab, and the open-source OpenAPI Generator exist, but Stainless holds a dominant market position.
Stainless, headquartered in New York with about 80 employees, has raised $35 million from investors including a16z, Sequoia, and Felicis. Its last valuation in December 2024 was $150 million, meaning the acquisition price represents roughly double that. Anthropic, valued at $350 billion in February, may use a mix of stock to pay for the deal. Negotiations are not yet finalized, and terms could still change.
Beyond serving the three major model makers, Stainless also provides SDK tools for non-model companies like Cloudflare and Cerebras, and built companion software for Anthropic’s Model Context Protocol (MCP) launched in November 2024. This acquisition marks Anthropic’s fourth in six months: following last December’s purchase of JavaScript runtime Bun, it has also acquired computer operation company Vercept and AI biotech firm Coefficient Bio.