API Advice

Choosing an SDK vendor: Speakeasy vs Fern vs Stainless vs openapi-codegen

Nolan Sullivan

May 28, 2025 - 16 min read

---

Without an SDK for your API, your users must handle raw HTTP requests themselves, leading to integration delays, avoidable bugs, and higher support costs. Even with good documentation, not having an SDK can slow adoption, frustrate developers, and increase churn.

A well-designed SDK improves developer experience. It saves time, reduces integration errors, speeds up onboarding, and – importantly – can drive adoption and revenue.

Today, you no longer need to build SDKs manually. Several tools generate SDKs from an OpenAPI document, including Speakeasy , Fern , Stainless , and OpenAPI Typescript Codegen .

But with so many options available, how do you know which will save you the most time? Which generator will create SDKs that are easy to use, extend, and won’t break when your API grows? Which will let you focus on building features instead of fixing generated code?

In this article, we compare the main SDK generation options available today. We’ll explain their strengths, weaknesses, and differences to help you decide which best suits your needs.

What to look for in an SDK generator

It’s not enough to focus solely on the code produced when evaluating an SDK generator. You also need to consider factors impacting the SDK’s developer experience (for internal and external devs) and how well the tool fits your workflow and budget:

• OpenAPI support: Does the generator fully support the OpenAPI Specification, including handling schema validation and pagination?
• OAuth 2.0 support: Does the generator handle OAuth client credentials properly, including token fetching and refreshing?
• Webhooks support: Can the generator validate, parse, and process webhook events securely and reliably?
• Generated documentation and examples: Does the generated SDK come with ready-to-use documentation and copy-paste examples for everyday API operations, making it easier for developers to get started quickly?
• SDK quality and structure: Is the generated code readable, idiomatic, and easy to navigate? Does the generator produce ready-to-publish packages with minimal dependencies, or does it create bloated, messy code that your team would need to clean up?
• Streaming support: Does the generator produce SDKs that properly handle large uploads and downloads using streams?
• SDK automation and CI/CD integration: Can the generator integrate into your CI/CD pipeline and automatically regenerate SDKs as your API evolves?
• SDK customization options: Does the tool let you customize naming, structure, and the SDK’s behavior when necessary?
• Pricing: Is the pricing transparent, scalable, and startup-friendly?

Here’s a quick comparison of how each SDK generator performs across these criteria.

Not sure which tool to pick? Here’s how to choose based on your priorities:

• Choose Speakeasy if you want the most complete feature set with the least manual work and you’re OpenAPI-first.
• Choose Fern if you need maximum control over SDK structure and don’t mind a more complex setup.
• Choose OpenAPI Typescript Codegen if you need a fast, open-source solution for basic TypeScript SDKs.

Speakeasy

Speakeasy’s approach to SDK generation focuses on real-world API usage, offering advanced features and minimizing developer friction. Because Speakeasy uses OpenAPI documents to generate SDKs, it’s a compelling choice for API-first teams.

OpenAPI support

Speakeasy fully supports OpenAPI versions 3.0 and 3.1.

As Speakeasy relies on your OpenAPI documents to generate SDKs, the quality of those documents directly impacts the results. To help ensure that OpenAPI documents are clean, accurate, and complete, Speakeasy validates them before generation, catching issues like missing required fields in requests or server responses that don’t match the documented schema.

For example, when sending a payload, if your OpenAPI document marks a field as required, the SDK will enforce it at the client level:

If the email field is missing, the SDK throws a validation error without making a request.

Speakeasy also handles pagination patterns natively. Whether your API uses offset, cursor, or custom pagination logic, Speakeasy can generate helper functions that abstract pagination away from the user. For example:

No need to manually manage cursors, tokens, or page offsets.

SDK quality and structure

Speakeasy-generated SDKs are production-ready by default – idiomatic for each target language, aligned with community best practices, and free of bloated, deeply nested, or unreadable structures.

Each SDK is designed for immediate publishing to registries like npm, PyPI, and Maven Central, with sensible package structure, typed clients, and minimal external dependencies.

OAuth 2.0 support

Speakeasy supports OAuth 2.0 out of the box. If your OpenAPI document defines an OAuth 2.0 security scheme, Speakeasy automatically detects it and generates SDK code that manages tokens on behalf of your users, including requesting tokens, handling expirations, and refreshing them when needed.

Configuring OAuth in your OpenAPI document

Let’s take a look at how to define an OAuth 2.0 client credentials flow in an OpenAPI document:

Once this security scheme is defined in an OpenAPI document, Speakeasy-generated SDKs will prompt SDK users to provide a clientID and clientSecret when initializing the client:

Behind the scenes, the SDK will:

• Automatically fetch an access token from the token URL.
• Retry requests with a fresh token if the current one has expired.
• Add the Bearer <token> Authorization header for authenticated requests.

No additional code is needed to manage token lifetimes for this flow.

Customizing token retrieval

For more advanced scenarios, such as integrating with nonstandard token endpoints or using custom headers during token exchange, you can write your token retrieval logic and pass it to the SDK:

This allows complete control over how tokens are requested, validated, cached, and refreshed.

Webhooks support

Speakeasy offers automatic webhook event validation and parsing. For example, it can generate functions that automatically verify webhook signatures and parse the payload correctly according to the OpenAPI event schema:

This automatic verification and parsing ensures security and reduces the chances of mishandling or misparsing critical webhook notifications.

Streaming support

Speakeasy supports streaming for download and upload endpoints – essential when handling large files or long-lived server responses. Instead of buffering the whole file in memory, risking performance issues or crashes, SDKs expose the response as a stream.

To enable this behavior, define the endpoint in your OpenAPI document using the application/octet-stream content type with a binary format:

This tells the SDK generator to treat the response as a raw byte stream. In a TypeScript SDK, the download logic becomes clean and efficient:

Here, the sdk.streamable("UR123") call fetches a ReadableStream. The Node.js Writable.toWeb() method converts a standard file stream to a format compatible with the browser-style Streams API. Then pipeTo() streams the API’s ReadableStream directly into the writable file stream, avoiding full in-memory buffering.

Generated documentation and examples

Alongside SDKs, Speakeasy automatically generates documentation in README.md, RUNTIMES.md, FUNCTIONS.md, and USAGE.md files. The docs include concise usage examples (like creating a resource, retries, and authentication) based directly on your OpenAPI document.

Developers get the SDK package and a browsable documentation site located in the docs directory, ready to deploy.

SDK automation and CI/CD integration

Speakeasy supports fully automated SDK generation. It can integrate directly in your CI/CD pipelines (for example, in GitHub Actions or GitLab CI/CD) so that when you update your OpenAPI document, SDKs are rebuilt and published automatically, without manual intervention.

SDK customization options

Speakeasy provides flexible configuration for naming, error handling, retry strategies, pagination settings, and more, either through OpenAPI extensions or project-specific overrides.

You can fine-tune the SDK to match your API’s particular design patterns or your engineering team’s preferences.

Pricing

Speakeasy isn’t open source but offers a hosted SDK management platform with automated generation, publishing, and updates. The free tier includes one SDK in a single language with GitHub integration and access to the complete OpenAPI toolchain. Paid plans start at $250/month and unlock features like pagination support and multi-language output. Higher tiers offer webhook support, event streaming, test generation, and enterprise-grade service-level agreements (SLAs).

Fern

Fern is a modern SDK generator built around its domain-specific language, Fern Definition . While it supports OpenAPI, Fern works best when you adopt its format. Fern is positioned for teams that want to generate SDKs quickly without thinking about structure, naming, or layout. But choosing convenience over control comes with trade-offs: more generated files, tighter ecosystem lock-in, and reduced interoperability with standard tools.

OpenAPI support

Fern  supports OpenAPI 3.0 and 3.1. However, when relying solely on OpenAPI, you’ll likely hit limitations around pagination, OAuth, documentation, or advanced type handling.

The Fern OpenAPI parser covers most standard use cases. However, it is sensitive to inconsistent patterns and may miss edge cases like unconventional pagination and loosely defined endpoints. If standard pagination fields like next_cursor or offset are clearly described in the Fern Definition, the parser can recognize them and generate SDK methods accordingly.

Link pagination is not supported, and the customization  for pagination is limited, as you can only define the offset and results fields:

Speakeasy  allows you to define the type of pagination, whether the pagination inputs are in the request parameters or body, and also the pagination outputs. You can define fields on the outputs such as numPages, results, and nextCursor, so Speakeasy knows how to handle auto-pagination.

SDK quality and structure

Fern SDKs tend to be deeply nested and verbose. The generated TypeScript code includes multiple layers of abstraction and utility wrappers, making it hard to follow what’s being executed. Request logic is often hidden behind custom serializers, and the separation between core logic and API-specific behavior is unclear. Naming conventions are inconsistent, and common operations span too many files, making the SDKs harder to read, debug, or extend.

See the SDK structure section of our Speakeasy vs Fern post for an overview of the Fern SDK structure.

OAuth 2.0 support

Fern can generate SDKs that support bearer token authentication (OAuth 2.0 access tokens) . The client setup usually expects you to pass the token at initialization, and it automatically attaches it to API calls.

For example, a Fern TypeScript SDK configuration with OAuth 2.0 would look like this:

The Fern SDK supports authorization flows such as client-credentials and authorization-code, but doesn’t support more advanced or legacy flows like resource_owner_password. Also, you can’t implement custom auth flows with Fern because there’s no extensibility mechanism.

By contrast, Speakeasy provides full flexibility via SDK hooks , letting you inject custom authentication or refresh  logic, or even build support for entirely custom auth schemes when needed.

Webhooks support

Fern SDKs provide a constructEvent helper  to verify webhook signatures and parse incoming payloads into typed event objects. This improves security and reduces boilerplate for consumers handling webhooks.

That said, Fern leaves much of the responsibility for webhook handling to the developer. You must manually configure secrets, map the correct headers, and implement any error handling, replay protection, or logging logic.

Streaming support

Fern supports binary uploads using the bytes type in the Fern Definition, allowing you to model raw binary payloads (like MP3 or image files) as the request body. Fern expects type: file for responses, which allows the SDK to treat the returned data as a streamable file. However, this streaming behavior is only available when using Fern Definition. If you rely on OpenAPI, you must manually handle both uploads and downloads, without stream helpers.

Generated documentation and examples

If you use Fern Definition, the generated SDKs include well-structured inline documentation and developer-friendly examples of API operations. When working directly from OpenAPI without Fern-specific metadata, the generated SDK documentation is more basic and lacks detailed examples.

The SDKs still include method signatures and basic type hints, but they rely heavily on how much information is in your original OpenAPI document.

SDK automation and CI/CD integration

Fern provides tools that automate SDK generation  whenever your API specification changes. However, this automation is primarily designed for projects using Fern Definition. Pure OpenAPI projects may require additional configuration or custom scripts to achieve seamless CI/CD automation.

SDK customization options

If you adopt the Fern Definition, Fern is highly customizable. You can configure naming conventions, error handling structures, pagination strategies, custom SDK behavior, and even the generated documentation. However, this flexibility comes with vendor lock-in. In many cases, you’ll have to manage two sources of truth: your OpenAPI document and the Fern Definition.

Speakeasy, on the other hand, lets you use a single OpenAPI document as the source of truth. For more custom behavior, you can write SDK hooks, update the OpenAPI document directly, or use an Arazzo definition  for advanced use cases like defining workflows.

Fern also allows custom code augmentation , but only after the SDK has been generated.

Pricing

Fern’s starter plan  begins at $250/mo per SDK for up to 50 endpoints — the same price as Speakeasy’s Scale-Up plan, but with fewer features. You’ll need the $600/mo Pro plan to access OAuth, pagination, and webhook support. Even then, features like token refresh or signature validation are DIY. Enterprise support adds SLAs and migrations, but doesn’t close the feature gap.

Stainless

Stainless is an SDK generation platform built around a custom configuration format rather than native OpenAPI support. Stainless produces polished SDKs with a clean developer experience, but it requires additional setup and manual integration for many advanced features.

OpenAPI support

Stainless partially supports the OpenAPI Specification , so the best results require migrating to its custom Stainless format. Using plain OpenAPI documents can lead to missing features, such as pagination metadata, authentication flows, and webhook definitions. Stainless does not automatically infer pagination behavior from OpenAPI descriptions. Developers need to annotate their specs manually to enable proper pagination handling.

If your OpenAPI document is your stack’s single source of truth, you’ll run into limitations unless you migrate to the Stainless domain-specific language.

SDK quality and structure

Stainless prioritizes clean SDKs with minimal dependencies and idiomatic formatting. However, the SDK layout lacks structure: Models, operations, and helpers are often mixed without clear separation. Tracing what’s actually happening under the hood can take time.

Error handling is thin, so expect to wire up your retry logic, response decoding, and fallback behavior.

OAuth 2.0 support

Stainless-generated SDKs do not manage OAuth tokens for you. Instead, developers must fetch access tokens manually and inject them into the client’s headers.

There’s no built-in retry, refresh, or expiration handling — you’re on your own for all lifecycle logic.

Webhooks support

Stainless doesn’t support webhooks. If you define webhook events, you’ll need to implement your own verification (for example, HMAC or RSA) and deserialization logic. There’s no helper like constructEvent() or built-in type-safe handler — just raw payloads.

Streaming support

Streaming support is only available for Enterprise customers.

Generated documentation and examples

Stainless SDKs generated from OpenAPI documents have limited inline documentation. Migrating to the Stainless format improves documentation quality, but compared to other generators like Speakeasy or Fern, Stainless provides fewer ready-to-use examples.

SDK automation and CI/CD integration

Stainless supports automation for OpenAPI updates  through a GitHub Action or polling mechanism. The recommended setup uses a prebuilt GitHub Action that pushes OpenAPI document changes directly from your repository to Stainless. You can also configure the platform to poll a persistent URL (like a raw GitHub link) hourly for changes.

However, Stainless does not provide a full CI/CD pipeline for SDK generation and publication beyond uploading your OpenAPI document. You’re still responsible for managing SDK builds, versioning, and publishing to registries like npm or PyPI. Compared to platforms like Speakeasy that offer end-to-end automation – from OpenAPI document to SDK release – Stainless requires more manual setup and maintenance.

SDK customization options

Customization is only available when using the Stainless format. Using a plain OpenAPI document limits your ability to customize SDK behavior. Teams that want fine-grained control over SDK structure must migrate to Stainless’s configuration file format.

Pricing

Stainless offers a free plan with one local SDK and up to 50 endpoints . The Scale-Up plan starts at $250/month per SDK, with a limit of 50 endpoints per SDK. For larger teams, the Business plan is priced at $800/month per SDK and allows up to 200 endpoints per SDK. Enterprise pricing is custom and includes higher limits, SLAs, and premium support.

OpenAPI Typescript Codegen

OpenAPI Typescript Codegen is a lightweight, open-source CLI tool that generates TypeScript or JavaScript SDKs directly from OpenAPI documents. While the tool is minimalistic and simple to set up, it requires significant manual work to produce production-grade SDKs.

OpenAPI support

OpenAPI Typescript Codegen fully supports OpenAPI 2.0 and 3.0 but interprets OpenAPI documents very literally: If the document lacks examples, pagination metadata, or detailed schemas, the generated SDK will be incomplete or inaccurate. The tool provides no built-in understanding of pagination patterns (such as offset- or cursor-based approaches) or advanced specification features like polymorphism with oneOf and anyOf. The quality of the generated SDK depends heavily on the precision of the OpenAPI document provided.

SDK quality and structure

The SDK generated by OpenAPI Typescript Codegen is clean but extremely basic: A simple folder structure (/models, /services), low-level HTTP wrappers using Fetch or Axios, and minimal or no error handling for API failures. The SDK feels closer to an API client scaffold than a complete developer experience platform.

OAuth 2.0 support

OpenAPI Typescript Codegen does not offer built-in helpers for OAuth flows. The developer must manually handle authentication, and token management and refresh logic must be implemented outside the SDK.

Webhooks support

The generator does not natively support webhook validation, parsing, or handling. It focuses on REST API clients and does not extend to webhook server-side integrations.

Streaming support

Streaming is not natively supported. Large uploads and downloads must be manually handled by adjusting Fetch or Axios request configurations.

Generated documentation and examples

OpenAPI Typescript Codegen generates basic inline TypeScript typings and function signatures. It does not provide ready-to-use examples or rich developer documentation. Developers are expected to rely on TypeScript’s IntelliSense to navigate the SDK. There are no tutorials or built-in guides alongside the generated code.

SDK automation and CI/CD integration

Since it is a simple CLI tool, OpenAPI Typescript Codegen  easily integrates into any CI/CD pipeline.

A typical GitHub Action step might look like this:

Automation is straightforward but requires custom scripting for tasks like versioning, publishing, and changelog generation.

SDK customization options

OpenAPI Typescript Codegen has limited support for SDK customization. Developers can modify templates slightly, but advanced customization like changing naming conventions or response handling requires forking the tool or manually post-processing the output.

Pricing

OpenAPI Typescript Codegen is fully open source  and free to use. There is no official enterprise support channel or paid offering available.

Final thoughts

If you are building an API product, you probably already have an OpenAPI document available – or you should.

When choosing an SDK generator, prioritize tools that offer complete, native support for OpenAPI without requiring custom formats, workarounds, or extensive manual adjustments. Ultimately, the best SDK generator is the one that fits your team’s workflow, spec quality, and appetite for customization.

If you are looking for a tool that fully supports OpenAPI, helps you generate high-quality SDKs with minimal manual work, and makes it easier to ship faster, it’s worth checking out the Speakeasy tools.