Besides a focus on generative AI, what do AI startups like OpenAI, Anthropic and Together AI share in common? They use Stainless, a platform created by ex-Stripe staffer Alex Rattray, to generate SDKs for their APIs.

Rattray, who studied economics at the University of Pennsylvania, has been building things for as long as he can remember, from an underground newspaper in high school to a bike-share program in college. Rattray picked up programming on the side while at UPenn, which led to a job at Stripe as an engineer on the developer platform team.

At Stripe, Rattray helped to revamp API documentation and launch the system that powers Stripe’s API client SDK. It’s while working on those projects Rattray observed there wasn’t an easy way for companies, including Stripe, to build SDKs for their APIs at scale.

“Handwriting the SDKs couldn’t scale,” he told TechCrunch. “Today, every API designer has to settle a million and one ‘bikeshed’ questions all over again, and painstakingly enforce consistency around these decisions across their API.”

Now, you might be wondering, why would a company need an SDK if it offers an API? APIs are simply protocols, enabling software components to communicate with each other and transfer data. SDKs, on the other hand, offer a set of software-crafting tools that plug into APIs. Without an SDK to accompany an API, API users are forced to read API docs and build everything themselves, which isn’t the best experience.

Rattray’s solution is Stainless, which takes in an API spec and generates SDKs in a range of programming languages including Python, TypeScript, Kotlin, Go and Java. As APIs evolve and change, Stainless’ platform pushes those updates with options for versioning and publishing changelogs.

“API companies today have a team of several people building libraries in each new language to connect to their API,” Rattray said. “These libraries inevitably become inconsistent, fall out of date and require constant changes from specialist engineers. Stainless fixes that problem by generating them via code.”

Stainless isn’t the only API-to-SDK generator out there. There’s LibLab and Speakeasy, to name a couple, plus longstanding open source projects such as the OpenAPI Generator.

Stainless, however, delivers more “polish” than most others, Rattray said, thanks partly to its use of generative AI.

“Stainless uses generative AI to produce an initial ‘Stainless config’ for customers, which is then up to them to fine-tune to their API,” he explained. “This is particularly valuable for AI companies, whose huge user bases includes many novice developers trying to integrate with complex features like chat streaming and tools.”

Perhaps that’s what attracted customers like OpenAI, Anthropic and Together AI, along with Lithic, LangChain, Orb, Modern Treasury and Cloudflare. Stainless has “dozens” of paying clients in its beta, Rattray said, and some of the SDKs it’s generated, including OpenAI’s Python SDK, are getting millions of downloads per week.

“If your company wants to be a platform, your API is the bedrock of that,” he said. “Great SDKs for your API drive faster integration, broader feature adoption, quicker upgrades and trust in your engineering quality.”

Most customers are paying for Stainless’ enterprise tier, which comes with additional white-glove services and AI-specific functionality. Publishing a single SDK with Stainless is free. But companies have to fork over between $250 per month and $30,000 per year for multiple SDKs across multiple programming languages.

Rattray bootstrapped Stainless “with revenue from day one,” he said, adding that the company could be profitable as soon as this year; annual recurring revenue is hovering around $1 million. But Rattray opted instead to take on outside investment to build new product lines.

Stainless recently closed a $3.5 million seed round with participation from Sequoia and The General Partnership.

“Across the tech ecosystem, Stainless stands out as a beacon that elevates the developer experience, rivaling the high standard once set by Stripe,” said Anthony Kline, partner at The General Partnership. “As APIs continue to be the core building blocks of integrating services like LLMs into applications, Alex’s first-hand experience pioneering Stripe’s API codegen system uniquely positions him to craft Stainless into the quintessential platform for seamless, high-quality API interactions.”

Stainless has a 10-person team based in New York. Rattray expects headcount to grow to 15 or 20 by the end of the year.