SDKs

Choosing an SDK generator: Speakeasy vs Stainless vs Fern vs APIMatic vs OpenAPI Generator

Nolan Sullivan

February 11, 2026 - 9 min read

SDKs

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An SDK generator automatically produces language-specific client libraries from an API specification like OpenAPI, giving API consumers idiomatic, ready-to-use code instead of hand-written HTTP calls. As API ecosystems grow more complex, choosing the right generator has become a critical infrastructure decision.

There are now several commercial and open-source options for generating SDKs from an API specification. Choosing the right one affects developer experience, maintenance burden, and time-to-market.

This post compares five prominent SDK generators: OpenAPI Generator , APIMatic , Fern  (recently acquired by Postman), Stainless , and Speakeasy . For deeper technical dives, we have dedicated 1:1 comparison posts linked throughout.

What matters in an SDK generator

Before diving into individual tools, here are the criteria that matter most when evaluating SDK generators:

• Language support: How many languages does the generator target, and how idiomatic is the output in each?
• OpenAPI fidelity: Does the generator treat OpenAPI as the source of truth, or does it rely on a custom DSL or intermediate layer?
• Type safety: Does the SDK validate data at runtime, or does it rely solely on compile-time type checking?
• Dependencies and bundle size: How lean is the generated SDK? Fewer dependencies mean a smaller attack surface and easier auditing.
• Enterprise features: Built-in support for OAuth 2.0, pagination, retries, webhooks, server-sent events (SSE), and React Hooks.
• CI/CD and deployment flexibility: Can the generator run anywhere, including air-gapped environments and custom pipelines?
• Code customization: Support for lifecycle hooks, custom code injection, and patching for regeneration-safe modifications.

OpenAPI Generator

OpenAPI Generator  is the most widely used open-source SDK generator. It supports 50+ language targets and has a large community of contributors. For many teams, it’s the first tool they try when generating SDKs from an OpenAPI specification.

Strengths

• Massive language breadth: Over 50 generators covering languages, server stubs, and documentation formats. No other tool comes close in sheer target count.
• Free and open source: No licensing costs. The full source is available to fork and customize.
• Large community: Thousands of contributors and a decade of accumulated templates and configurations.

Limitations

• Spotty API feature coverage: Features like OAuth 2.0, pagination, webhooks, and discriminated unions are either missing or inconsistently implemented across language targets.
• Non-idiomatic output: The project’s Java origins show through in many generators. Python, TypeScript, and Go output often feels Java-like rather than native to those ecosystems.
• Maintenance burden: As of early 2025, the repository has 4,500+ open issues , growing roughly 10% per year. Many language templates are community-driven and inconsistently maintained.
• No runtime type safety: Generated SDKs don’t validate response data at runtime. Type annotations exist but aren’t enforced.
• Requires Docker for CI/CD: The Java-based CLI requires a JVM or Docker container, complicating lightweight CI/CD setups.
• Significant engineering investment: Enterprise teams that use OpenAPI Generator in production typically dedicate 3+ FTEs to maintaining forks, fixing templates, and patching gaps.

Best for

Teams that need a free starting point, have niche language requirements covered by community templates, and are prepared to invest engineering time in customization and maintenance.

For an in-depth technical comparison, see our Speakeasy vs OpenAPI Generator post, as well as language-specific comparisons for Go, Python, and Java.

APIMatic

APIMatic  is the longest-running commercial SDK generator, operating since 2014. It offers SDK generation alongside a developer portal and API documentation product.

Strengths

• Mature product: Over a decade of development and enterprise customer experience.
• Developer portal: APIMatic includes a built-in developer portal product, useful for teams that want an integrated documentation and SDK experience.
• Low entry price: Plans start at $15/month, making it the most affordable commercial option for small teams.
• Documentation and test generation: Generates SDK documentation and test suites alongside the SDKs themselves.

Limitations

• Limited language support: Supports seven languages. No Terraform provider generation, Unity, or Kotlin support.
• No runtime type safety: Like OpenAPI Generator, APIMatic SDKs unsafely cast response data without runtime validation.
• Heavy dependency tree: Generated TypeScript SDKs include 41+ dependencies with known supply chain risks and unmaintained packages.
• Limited customization: Supports naming customization but not structural customization of the generated SDK.
• CLI limitations: The APIMatic CLI depends on Node.js and several packages, making it less portable than a standalone binary. In testing, the CLI was also less reliable than the web interface.
• No free tier: Only a limited free trial, compared to the free plans offered by Speakeasy, Stainless, and Fern.

Best for

Teams that value a mature, all-in-one platform with built-in developer portals and are working with standard API patterns that don’t require deep SDK customization.

For an in-depth technical comparison, see our Speakeasy vs APIMatic post.

Fern

Fern  was founded in 2022 and was acquired by Postman in January 2026 . It generates SDKs in seven languages and also produces API reference documentation. Fern is built on a custom DSL with optional OpenAPI support.

Strengths

• Postman backing: The Postman acquisition brings resources, distribution, and a large existing user base.
• Documentation generation: Fern has a strong documentation product that generates clean API reference pages.
• Server stubs: Fern is one of the few generators that supports server-side stub generation in addition to SDKs.
• Webhook support: Fern-generated SDKs include webhook validation and payload parsing.

Limitations

• DSL-based, not OpenAPI-native: Fern is built on its own DSL. OpenAPI support is optional and treated as an import format, not the source of truth. This means your OpenAPI spec and your Fern definition can drift apart.
• Node.js-only TypeScript: Fern-generated TypeScript SDKs are designed for Node.js and can’t be bundled for the browser out of the box.
• More dependencies: Fern SDKs include a custom validation library and multiple third-party packages, increasing the attack surface and making supply chain auditing harder.
• Larger bundle size: The additional dependencies result in bundle sizes roughly 3x larger than Speakeasy for the same API.
• Limited SDK customization: Supports naming customization but not structural customization of the SDK.
• Missing enterprise features: No streaming uploads, caching, or React Hooks support. OAuth 2.0 is only available on paid plans.
• No free plan: Fern’s starter plan is paid ($250/month per SDK), while competitors offer free tiers.

Best for

Teams already in the Postman ecosystem, those that need combined SDK and documentation generation, or teams that prefer a DSL-first approach to API definition.

For an in-depth technical comparison, see our Speakeasy vs Fern post.

Stainless

Stainless  was founded in 2021 and has strong adoption from AI companies. The company raised a $25M Series A in December 2024.

Strengths

• Strong AI company adoption: Stainless generates the official SDKs for OpenAI, Anthropic, and Cloudflare, giving it credibility in high-profile use cases.
• Broad feature set: Supports retries, pagination, streaming uploads, webhooks, custom dependency injection, and union types.
• Custom code support: Stainless supports injecting arbitrary custom code into generated SDKs.

Limitations

• Custom DSL, not OpenAPI-native: Stainless uses a custom configuration layer (the Stainless config ) as an intermediary between your API spec and the generated SDKs. This adds complexity and means OpenAPI isn’t the single source of truth.
• No runtime type safety: Stainless SDKs unsafely cast response data to expected types rather than validating at runtime with a library like Zod. This provides a false sense of security that can lead to unexpected runtime errors as APIs evolve.
• Requires cloud connectivity: SDK generation requires network access to the Stainless cloud service, ruling out air-gapped deployments.
• Tightly coupled CI/CD: Generation orchestration is coupled to the Stainless platform, limiting flexibility for teams with custom pipeline requirements.
• Heavier dependency tree: The Cloudflare SDK (generated by Stainless) includes 25+ dependencies compared to Speakeasy’s single Zod dependency.

Best for

Teams building AI-focused APIs that want to follow the same SDK generation approach as OpenAI and Anthropic, and don’t require air-gapped deployment or deep CI/CD customization.

For an in-depth technical comparison, see our Speakeasy vs Stainless post.

Speakeasy

Speakeasy  was founded in 2022 and raised a $15M Series A in October 2024. The platform generates SDKs in ten languages and also supports contract testing. Notable customers include Vercel, Clerk, and Kong.

Strengths

• OpenAPI-native: Speakeasy treats your OpenAPI specification as the single source of truth. No DSL, no intermediate config language. This makes Speakeasy interoperable with the rest of the API tooling ecosystem.
• Runtime type safety: Speakeasy SDKs use Zod  to validate data at both compile time and runtime, catching type errors before HTTP requests are made. As we wrote in type safe vs type faith, genuine safety requires runtime enforcement, not just type annotations.
• Minimal dependencies: Generated TypeScript SDKs have a single dependency (Zod), compared to 25+ for Stainless and 41+ for APIMatic. This reduces attack surface, simplifies auditing, and results in smaller bundles.
• Runs anywhere: Speakeasy is a standalone CLI binary that runs on-prem, in air-gapped environments, or in any CI/CD pipeline. Teams can fully eject from the Speakeasy platform and run generation within their own infrastructure.
• Rapid support: Dedicated Slack channels with median response times under three minutes.

Limitations

• No server stubs: Speakeasy focuses on client-side SDK generation and doesn’t generate server-side stubs.
• No first party api docs: Speakeasy doesn’t include a built-in api docs product. We partner with Mintlify and Scalar to integrate your SDKs into your favorite docs provider.

Best for

Teams that want high-quality, idiomatic SDKs with runtime type safety, minimal dependencies, and deep enterprise features, while keeping OpenAPI as their single source of truth. Particularly strong for teams that need flexible CI/CD integration, air-gapped deployment support, or broad language coverage.

Summary comparison

Platform

Language support

SDK features

Pricing

Overall recommendation

Every tool on this list has legitimate use cases:

• OpenAPI Generator is the right starting point for budget-constrained teams, niche language requirements, or situations where engineers are willing to invest in maintaining a fork. It’s free, battle-tested, and has unmatched language breadth, but expect significant ongoing maintenance costs.
• APIMatic is a solid choice for teams that need an integrated developer portal alongside SDK generation, especially at the lower end of the pricing spectrum. It has the deepest history among commercial options.
• Fern is worth evaluating if you’re already in the Postman ecosystem or want combined SDK and documentation generation from a DSL-first workflow. The Postman acquisition brings stability and distribution.
• Stainless has earned its reputation through high-profile AI customers. If you’re building an AI-focused API and want to follow the same approach as OpenAI and Anthropic, it’s a natural choice.
• Speakeasy is our recommendation for teams that want the best combination of SDK quality, enterprise features, and operational flexibility. Runtime type safety with Zod, minimal dependencies, OpenAPI as the single source of truth, a standalone CLI that runs anywhere, and the broadest commercial language support make it the strongest all-around option. We’re biased, but we’ve shown our work: the details are in the 1:1 comparisons linked above.

For deeper technical comparisons, see our dedicated posts:

• Speakeasy vs OpenAPI Generator
• Speakeasy vs APIMatic
• Speakeasy vs Fern
• Speakeasy vs Stainless

To get started with Speakeasy, see the guides on generating SDKs, code samples, and mock server generation.