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Migrate from Stainless to Fern.

Fern is built for the long-term API developer and agent experience. Our team can migrate you within a week.

Book a demoRead the full comparison

30-minute call. Migration plan tailored to your API.

payment-service
payment-gateway.ts
payment-gateway.tsPaymentGatewaycreatePayment

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// src/payment-gateway.ts
export interface PaymentInput {
amount: number;
sourceId: string;
}

export class PaymentGateway {
async createPayment(input: PaymentInput) {
// TODO: implement Square payment
return null;
}
}
Problems0
Output
Debug Console
Terminal
Ports
Spell Checker1
zsh+
Cursor Tab
Ln 8, Col 28Spaces: 4UTF-8LFTypeScript

Built for the API developer and agent experience.

Stainless is a dashboard. Fern is a developer-first platform with CLI workflows, multi-protocol coverage, and production-ready docs.

9+ languages from one spec

TypeScript, Python, Go, Java, C#, PHP, Ruby, Swift, and Rust. Each follows that language's conventions and passes its standard linters. Stainless ships 8.

REST, WebSockets, and gRPC

Multi-protocol SDKs generated from the same OpenAPI source. Stainless is REST only — Fern ships strongly-typed WebSocket and gRPC clients alongside.

CLI in your CI, not a dashboard

Generation lives in a fern/ directory of YAML. CI runs fern generate on every spec change and opens a PR with updated SDKs. Reviewable, version-controlled, no lock-in.

API Explorer, versioning, MCP server

Fern Docs generates from the same spec as your SDKs, so reference and code never drift. Includes Ask Fern, llms.txt, and a built-in MCP server for agents.

On-premise and SOC 2 Type II

Self-hosted deployment for regulated industries. SOC 2 Type II compliant, 99.9% uptime SLA. Stainless is cloud-only.

We used an alternative SDK generator, OpenAPI Generator, for years, and customers constantly complained. Since adopting Fern, customers have migrated rapidly to our new SDKs and have been blown away by the quality.

Gil FeigCTO, Merge

Seamless cutover. Our team does the work.

You don't run the migration alone. A Fern engineer pairs with your team from spec import through production cutover.

  1. 01

    Spec import

    Bring your existing OpenAPI spec. Our team mirrors your current SDK surface so consumers don't break.

  2. 02

    Side-by-side validation

    We run your existing Stainless-generated tests against Fern output. Same payloads, same behaviors.

  3. 03

    Cut over in a week

    Publish from Fern's CI to npm, PyPI, Maven, and the rest. Old versions stay live until your customers migrate.

Common questions from teams evaluating Fern.

Want the full feature-by-feature comparison? Read the Stainless to Fern blog post.

How long does a typical migration take?
Most teams cut over in 5 to 10 business days. Single-spec APIs are faster; multi-spec or auth-heavy APIs may take 2 to 3 weeks. Our team embeds with yours through the cutover.
Will my existing SDK consumers break?
No. We mirror your current SDK surface (class names, method signatures, error types) so your published versions stay backward-compatible. New versions ship from Fern's CI thereafter.
Do you support our language?
Fern generates idiomatic SDKs in TypeScript, Python, Go, Java, C#, PHP, Ruby, Swift, and Rust. Each follows that language's conventions and passes its standard linters.
Can we self-host?
Yes. Fern offers on-premise and self-hosted deployment for regulated industries. SOC 2 Type II compliant; SLAs available on enterprise plans.

Get a tailored migration plan in 30 minutes.

A Fern engineer walks you through your current spec, your SDK surface, and the exact cutover plan. No sales pitch, no slide deck.

Book a demo