Anthropic has completed the acquisition of the developer tools startup Stainless, bringing a widely used software platform in-house.
Stainless, founded in 2022 by former Stripe engineer Alex Rattray, provides tools that automate the creation and maintenance of software development kits (SDKs) across multiple programming languages. The company’s software has been used by major AI labs including OpenAI, Google, and Cloudflare.
The financial terms of the acquisition have not been officially disclosed. Reports indicate the deal may exceed $300 million, more than doubling Stainless’s previous valuation of $150 million from December 2025. Stainless was backed by venture firms Sequoia Capital and Andreessen Horowitz.
Following the acquisition, Anthropic will wind down all hosted Stainless products, including its SDK generator. The company confirmed that existing Stainless customers retain ownership of the SDKs they generated, with full rights to modify or extend them.
Stainless specializes in converting API specifications into production-ready SDKs in languages such as Python, TypeScript, Kotlin, Go, and Java. The platform automatically updates SDKs as APIs evolve, removing the need for manual maintenance. Its tools have been particularly useful to AI companies developing agents that connect to external software to complete user tasks.
“SDKs deserve as much care as the APIs they wrap,” said Alex Rattray. “Anthropic was one of the first teams to bet on this with us. We have been watching what developers have built on Claude, which made bringing our teams together an easy decision,” he further added.
The software has powered the generation of every official Anthropic SDK since the company launched its API. Hundreds of companies previously relied on Stainless to generate SDKs, CLIs, and Model Context Protocol (MCP) servers.
By acquiring Stainless, Anthropic gains exclusive control over a key infrastructure supplier previously accessible to its competitors. The company aims to integrate the Stainless team to focus on connecting its AI platform, Claude, with external tools and data sources.
Katelyn Lesse, Head of Platform Engineering at Anthropic, said, “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 tools were previously used by OpenAI, Google, and other leading AI labs. With the acquisition, future SDK generation will be limited to Anthropic, giving the company an internal advantage in maintaining and scaling agent connectivity.
This acquisition marks Anthropic’s fourth purchase in six months, following the acquisitions of the JavaScript runtime Bun, the computer operations firm Vercept, and AI biotech company Coefficient Bio. The integration of Stainless is expected to enhance Claude’s tool integration and simplify development workflows for Anthropic.
The startup’s technology will continue to support existing SDKs, while all new developments will now occur exclusively within Anthropic. Rattray emphasized that the team will continue its work on the platform ‘where it matters most,’ maintaining focus on API connectivity and SDK reliability.
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