Choosing the right SDK generation platform is a strategic decision for engineering teams. Tools like Fern and Speakeasy promise faster integrations and cleaner SDKs, but they differ significantly in language coverage, deployment flexibility, and documentation capabilities. Whether you're building a public-facing developer platform or supporting mobile and frontend teams alongside your backend, understanding these differences is key to selecting a tool that scales with your stack. This SDK generator comparison breaks down each platform's strengths and trade-offs to help you choose the right fit for your API strategy.
TLDR:
- Fern generates SDKs in 9 languages (including Swift, Ruby, Rust) vs Speakeasy's 6 languages.
- Fern produces synchronized docs and SDKs from one spec vs Speakeasy's SDK-only approach.
- Fern offers both local CI/CD pipeline execution and cloud generation vs Speakeasy's cloud-only approach.
- Fern creates zero-dependency TypeScript SDKs optimized for frontend performance.
- Fern generates idiomatic, hierarchical code that mirrors API structure for better navigation.
What is Speakeasy?
Speakeasy is an SDK generator that parses OpenAPI specifications to build client libraries. It supports engineering teams that need type-safe SDKs across multiple languages, including TypeScript, Python, Go, Java, and C#. The tool uses code generation to reduce the maintenance burden of manually writing API clients for different environments.
Speakeasy integrates into CI/CD pipelines via GitHub Actions, triggering SDK updates whenever your API definition changes. This treats SDK creation as a standard step in the release process rather than a manual chore, keeping generated code in sync with your API.
What is Fern?
Fern is an SDK generator and documentation platform that treats your API spec as a single source of truth. Beyond generating SDKs, Fern produces synchronized documentation websites, so reference docs and client libraries stay aligned without manual upkeep.
Fern supports a broader set of languages than Speakeasy, producing idiomatic code in TypeScript, Python, Go, Java, C#, Ruby, Swift, PHP, and Rust. It also manages the full package lifecycle, compiling code and publishing directly to registries like npm, PyPI, and Maven.
The core compiler is open source and can run locally within your CI pipeline, giving security-conscious teams full control over where generation happens.
SDK generation and language support
Speakeasy generates SDKs for six languages: TypeScript, Python, Go, Java, C#, and PHP. However, it doesn't support Swift or Ruby, which limits teams that need to reach a wider range of developer stacks. Speakeasy also relies exclusively on OpenAPI, with no native support for AsyncAPI, OpenRPC, or gRPC.
Fern generates SDKs for nine languages: TypeScript, Python, Go, Java, C#, Swift, Ruby, PHP, and Rust. Beyond broader language coverage, Fern produces idiomatic code—Python SDKs use Pydantic models, and Go libraries use functional options. Fern can also ingest multiple specification formats, including OpenAPI, AsyncAPI, OpenRPC, gRPC, and its own Fern Definition, so teams aren't locked into a single way of describing their APIs.
Documentation integration
Speakeasy doesn't build its own documentation engine. In late 2024, it launched Speakeasy Docs through a partnership with Scalar, and teams can also integrate with tools like ReadMe or Mintlify. However, documentation remains a separate workflow—code samples sync via OpenAPI overlays rather than generating from the same source as the SDKs.
Fern generates both SDKs and documentation from a single API definition. API reference documentation generates alongside SDKs, ensuring the interactive API Explorer and language-specific snippets stay aligned with the underlying library logic. Any spec update triggers CI to regenerate SDKs and deploy updated docs in a single operation. This architecture supports custom domains, RBAC, and a true single source of truth—reducing tool sprawl and improving developer experience.
SDK structure and code quality
Speakeasy generates working code with type safety but often defaults to a flattened structure. This works for simple interfaces but complicates usability as an API expands. Large APIs become difficult to navigate when resources lack logical grouping.
Fern organizes generated code hierarchically to match your API's resource structure, allowing developers to traverse endpoints using IDE autocomplete. The output follows native conventions: TypeScript SDKs offer browser compatibility without dependencies, while Python clients use native validation patterns. The result is SDKs that feel native to each language ecosystem.
Frontend and mobile development
Speakeasy's output is largely configured for server-side usage. Its TypeScript libraries often carry dependencies that add overhead to browser bundles, and without Swift support, iOS engineers must write manual networking code—introducing risk of API drift.
Fern supports full-stack development with zero-dependency TypeScript SDKs compatible with React, Vue, and Next.js, plus native Swift generation for iOS. This lets teams distribute type-safe libraries to backend, browser, and mobile consumers from a single workflow.
Deployment flexibility and security
Both Fern and Speakeasy default to cloud-based generation, where teams upload OpenAPI specifications to receive generated code. For most teams, this works well. But organizations with strict data governance—those complying with HIPAA or FedRAMP, for example—often can't share sensitive API definitions with external vendors.
Fern offers an alternative: you can run the open source generators directly within your own CI/CD pipeline, meaning API definitions never leave your systems. This flexibility extends to documentation, which you can export as Docker containers for hosting behind firewalls or within a VPC. Speakeasy doesn't offer a self-hosted option, making Fern the better fit for regulated industries like finance and healthcare.
Why Fern provides a better developer experience
Speakeasy is a solid choice for teams operating strictly within its six supported backend languages and already maintaining separate documentation pipelines. However, it lacks native support for mobile clients, frontend-optimized libraries, and a self-hosted deployment option.
Fern covers more ground: nine languages including Swift and Ruby, zero-dependency TypeScript for frontend performance, and documentation generated alongside SDKs to prevent drift. For security-conscious enterprises, Fern's open source generators can run entirely within your own infrastructure. The result is a unified workflow where every developer receives accurate, idiomatic libraries from a single source of truth.
Final thoughts on the Fern and Speakeasy comparison
Your choice of SDK generator shapes how developers interact with your API. Fern produces idiomatic, hierarchically structured code that feels native to each language ecosystem—easier to read, navigate, and maintain as your API grows. By treating documentation as a code artifact, Fern ensures your reference docs and SDKs stay in sync automatically. If you need mobile support, frontend-optimized SDKs, or self-hosted generation, Fern covers those requirements.
FAQ
What's the main difference between Fern and Speakeasy's approach to SDK generation?
Fern generates both SDKs and documentation from a single API definition, keeping them synchronized automatically. Speakeasy doesn't build its own documentation engine—it offers a docs solution through a partnership with Scalar, and teams can integrate with tools like ReadMe or Mintlify. However, documentation remains a separate workflow from SDK generation, which can lead to drift between your docs and client libraries.
Why does code structure matter in generated SDKs?
Proper code structure directly impacts developer experience, readability, and long-term maintainability. Well-structured, idiomatic SDKs mirror the API’s resource hierarchy and language conventions, making them easier to navigate with IDE tools, quicker to learn, and safer to extend as the API grows. Poorly structured or flattened SDKs may be simpler initially, but they become harder to understand, error-prone, and costly to maintain at scale. Fern generates hierarchical structures that follow these principles.
Can I use Fern if my organization has strict data security requirements?
Yes. Fern's open source generators run entirely within your own CI/CD pipeline, meaning your API definitions never leave your infrastructure. You can also export documentation as Docker containers for self-hosting behind firewalls or within a VPC, making it suitable for HIPAA, FedRAMP, and other regulated environments.
When should I consider switching from Speakeasy to Fern?
If you need Swift SDKs for mobile apps, zero-dependency TypeScript for frontend performance, or Ruby/Rust support, Fern covers those languages while Speakeasy doesn't. You should also consider switching if maintaining separate documentation tools leads to drift or if your security requirements prevent using cloud-based generation services.
How does Fern handle SDK structure for large APIs?
Fern generates hierarchical SDK structures that mirror your API's resource organization, allowing developers to navigate large APIs using IDE autocomplete. This contrasts with flattened structures that become difficult to navigate as APIs grow, helping developers discover and use endpoints intuitively.
