If your company ships an API, customers eventually ask for an SDK — a ready-made Python or TypeScript library instead of raw HTTP calls. Hand-writing and maintaining that library in six languages, keeping it in sync every time the API changes, is a real ongoing job. A newer generation of tools skips that: point them at your OpenAPI spec and they generate the SDKs, the documentation site, a CLI, and — increasingly — an MCP server so AI agents can call your API directly, all kept in sync automatically as the spec changes. Fern and Stainless are the two names that keep coming up in this space, and they list each other as direct competitors. Here's what actually differs.
Fern
Fern generates polished API documentation and client SDKs in multiple languages directly from an OpenAPI spec, and it goes further than most competitors on protocol coverage — REST, WebSockets, GraphQL, AsyncAPI and gRPC, not just REST. It's genuinely open source (Apache-2.0) with a real self-hosting option if you don't want to depend on a vendor for something as core as your public docs.
Pricing: Hobby is free (250 AI credits). Team is $150/month (1,000 AI credits, 5 team members, version switching). Enterprise is custom, with dedicated support, SSO and translated content.
Strengths: open-source with self-hosting, the widest protocol coverage of the two (gRPC and AsyncAPI support is unusual at this price point), and pricing that's public and predictable from the free tier up.
Limits: output quality depends entirely on how clean your source OpenAPI spec already is — a messy spec produces a messy SDK regardless of the generator — and the $150/month Team tier costs more than some documentation-only competitors like ReadMe or Mintlify that don't also generate SDKs.
Stainless
Stainless generates hand-crafted-feeling SDKs, documentation, a CLI, and MCP servers automatically from an OpenAPI spec — and its strongest credential is that it's the same generator behind OpenAI's, Anthropic's and Cloudflare's own official client libraries. It covers more languages than Fern out of the box, including Go, Java, Kotlin, Ruby, C#, PHP and even Terraform providers, and it names Fern directly as one of its competitors.
Pricing: no public pricing — quote-based, contact sales required.
Strengths: real production validation (three of the industry's most-copied SDKs are built on it), the broadest language coverage of the two, and it generates an MCP server alongside the SDK — useful now that AI agents calling your API directly is a real integration path, not a future one. Worth noting: the Stainless team joined Anthropic in 2026, which likely means continued investment specifically in agent-facing tooling like MCP server generation.
Limits: no public pricing means you can't compare cost without a sales call, its roadmap is now tied to Anthropic's priorities post-acquisition rather than running fully independently, and whether its SDK output is meaningfully better than Fern's for your specific API is something you'd need to test directly rather than take on reputation alone.
Side-by-side
| Fern | Stainless | |
|---|---|---|
| Pricing | Free tier; Team $150/mo; Enterprise custom | Custom quote only |
| Open source / self-host | Yes (Apache-2.0) | No |
| Protocols covered | REST, WebSockets, GraphQL, AsyncAPI, gRPC | Primarily REST-first (OpenAPI spec) |
| Languages | Multiple (fewer than Stainless) | TypeScript, Python, Go, Java, Kotlin, Ruby, C#, PHP, Terraform |
| MCP server generation | Not a stated focus | Yes |
| Notable users | API-first companies, dev platform teams | OpenAI, Anthropic, Cloudflare (official SDKs) |
Verdict
Pick Fern if: you want a transparent, public price tag you can budget against from day one, you need protocol coverage beyond plain REST (GraphQL, gRPC, AsyncAPI), or self-hosting the generator matters to you.
Pick Stainless if: you want the generator with the strongest real-world proof — it's what OpenAI, Anthropic and Cloudflare ship to their own developers — you need the widest language list including Go, Java or Terraform, or generating an MCP server alongside your SDK is part of the plan.
Both start from the same input — a clean OpenAPI spec — so the fastest way to actually decide is to run your own spec through each free tier or trial and compare the generated SDK output directly, rather than picking on reputation alone.