APIs are excellent to interact with data. But at some point you need to consume those same APIs from inside real application code, and that means typed client libraries in the languages your team actually uses. SDK generators take the OpenAPI spec you already reference and turn it into idiomatic, versioned client libraries across multiple languages. This guide explains what SDKs are, why generators matter, and which ones are worth your time.
Table of Contents
An SDK (Software Development Kit) is a package or library that describes the foundations of how to interact with a public or private REST API. It abstracts away the endpoints, authentication requirements, and boilerplate code, providing ready-to-use methods so developers can get exactly what they need.
⚡
Faster time-to-first-call
Developers get working code in minutes instead of reading docs for hours
🌍
Multi-language reach
Publish SDKs in the languages your users actually work with
🔒
Auth handled for you
Token management, retries, and error handling baked in
📦
Zero boilerplate
No more writing the same fetch wrapper five times in five languages
Without an SDK, your users write raw HTTP calls, manage auth headers manually, parse responses themselves, and repeat this across every language they work in. An SDK takes all of that off their plate and lets them focus on what they are actually building.
Writing SDKs by hand sounds reasonable until you have three languages to support, a fast-moving API, and a small team. Then it becomes a serious problem. Here is where things break down in practice.
Development costs
As a startup or solo developer, it is fundamentally difficult to hire individuals who can consistently craft polished SDKs across multiple language targets, especially when API requirements keep changing. You end up rewriting the same logic multiple times in different languages, with no guarantee that quality stays consistent. For a startup, this process can be time-consuming and expensive, and that is assuming you already have the knowledge or the team to create and maintain SDKs in the first place.
Maintenance never stops
SDKs are expected to evolve in parallel with the REST API. But in practice, APIs change fast and requirements shift constantly. The result is SDKs shipped with broken docs, failing test cases, and an abundance of todo! comments. Developers can only move so fast, and the gap between API and SDK grows quietly until it becomes a real problem.
Hiring costs
The cost of hiring and onboarding developers is significant and time-consuming, with no guarantee they will understand your user base or business well enough to produce high-quality SDKs over the long run. Hiring a specialist for every language your users request is slow and expensive. An SDK generator solves this problem entirely.
Dependency hell
If you are a JavaScript developer, you are already well-acquainted with managing an unholy spread of HTTP clients: fetch, node-fetch, and the entire axios universe alongside zod, platform-specific builds for Browser, Node.js, and Cloudflare Workers, vulnerability reports, new releases, patches, and missing types. A good SDK generator handles this for you from a single spec.
As a Bruno user you need a way to generate SDK from your API collections and that is where SDK generators come in.
Bruno workflow SDK generator workflow ───────────────────── ───────────────────────────────── Explore endpoints Take the OpenAPI spec you reference Test request/response Generate typed client libraries Verify auth flows Publish to npm, PyPI, Maven, etc. Debug in real time Consume the API from app code
If you are building or regularly consuming an API, you will eventually need typed client libraries rather than hand-writing fetch calls yourself. These generators take the OpenAPI or Swagger spec you already reference in Bruno and turn it into idiomatic, versioned SDKs for multiple languages. When the API changes, you regenerate. The client code stays in sync without manual effort.
The short version: Bruno helps you understand an API. An SDK generator helps your team and your users ship code that consumes it reliably.
Not all generators are built the same. Here are the criteria worth evaluating before picking one.
| Criteria | Why It Matters |
|---|---|
| CLI support | Lets you build automations, run generation in CI/CD, and work faster from the terminal |
| Embedded documentation | Markdown docs that render on GitHub mean users can read the docs right next to the code |
| Tests and assertions | A tested SDK lets you catch breakage before it reaches customers, no extra setup needed |
| Minimal setup | Too many prompts, pop-ups, and config files drive developers away before they get started |
| Single installation | Requiring extra runtimes (Java, Python, etc.) on top of the generator itself adds friction |
| Sensible defaults | A good generator works out of the box: upload your spec, fill in a few fields, and generate |
The following five generators cover the main options available today, from free open-source tools to enterprise platforms. Each entry covers what it does well, where it falls short, and who it is best suited for.
OpenAPI Generator is a free, open-source, template-based SDK generator. It supports over 20 programming languages, which makes it a strong fit for wide adoption. It ships a Node.js CLI powered by Java under the hood. The TypeScript output is functional but fairly verbose, with utilities spread across multiple files and some unused code imports. Getting to your first HTTP call requires a small amount of boilerplate setup. On the positive side, documentation is output as Markdown, which looks good on GitHub.
Pros
Cons
Best for: Teams with a solid OpenAPI spec who need SDKs in as many languages as possible, prefer open-source, and are comfortable doing some initial setup.
FumaStudio is an opinionated, Bruno-compatible SDK generator. It uses a custom TypeScript-like Domain Specific Language (DSL) to document API contracts, enabling type safety, readability, and collaboration. It is relatively new but excels at integrating natively with Bruno. Rather than reading a deeply nested OpenAPI blob, you work with a Zod-like syntax that lives right alongside your .bru files in plain sight.
By treating your Bruno collections as the single source of truth, your SDK generation and API documentation all flow from the same place. No context switching, no separate spec drifting out of sync every time an endpoint changes. The generated SDK has zero runtime dependencies and produces excellent types.
Why it stands out for Bruno users
.env.* and /environments/*.{bru,yaml} turn into typed client parameters automaticallyPros
Cons
Best for: Bruno users who want the tightest possible integration between their collections and their SDK, with zero-dependency TypeScript output and built-in documentation.
Fern is an SDK generator and documentation platform that treats your API definition as a single source of truth. From one spec it generates idiomatic client libraries and a synchronized documentation website, keeping reference docs and SDKs aligned without manual maintenance.
Fern produces idiomatic code in nine languages: TypeScript, Python, Go, Java, C#, PHP, Ruby, Swift, and Rust. Beyond generation, it manages the full package life cycle, compiling each SDK and publishing directly to registries like npm, PyPI, Maven, and NuGet.
Pros
Cons
Best for: Teams that need polished developer tooling at scale. Fern goes beyond generation and handles the entire SDK life cycle from creation through publishing and documentation.
Speakeasy helps teams build great developer experiences for their APIs. It provides tools, workflows, and infrastructure to generate and manage high-quality SDKs, Terraform providers, and API documentation directly from an OpenAPI spec. The platform covers generation, publishing, CI/CD integration, and changelog automation end to end.
Pros
Cons
Best for: Organizations that have already committed to OpenAPI and want everything else automated: SDK generation, release pipeline, CI/CD, changelogs, and governance tooling all in one platform.
Stainless was a compelling option before being acquired by Anthropic. New sign-ups are no longer being accepted. It is similar to Speakeasy in scope, but with a stronger focus on AI-native infrastructure. Beyond SDK generation, the team was building tools to help developers create AI-powered applications and infrastructure, including support for Model Context Protocol (MCP) servers.
Stainless is not currently available for new users. It is included here because it represents where the SDK generation space is heading: treating generation as one piece of a broader AI-native developer platform rather than the end goal.
All five solutions are strong in certain areas and fall short in others. Here is a breakdown of the top pick for each specific need.
Developer Experience
Winner: FumaStudio
FumaStudio treats your Bruno collections as the single source of truth. SDK generation and documentation flow from the same place, with no context switching and no separate spec that drifts out of sync. The DSL is Zod-like in syntax, lives right beside your .bru files, and produces zero-dependency TypeScript with clear, readable code.
SDK Life-Cycle Management
Winner: Fern
If you need polished developer tooling at scale, Fern goes beyond generation. It reimagines the entire SDK life cycle: generation, testing, publishing, and documentation. The documentation platform is one of the best in the market, and multi-protocol support makes it suitable for modern APIs rather than just REST.
OpenAPI Ecosystem and Compatibility
Winner: OpenAPI Generator
If your API already has a solid OpenAPI spec and you need SDKs in as many languages as possible, this is still one of the safest choices. It is free, open source, offline-first, and nothing comes close to its language coverage and community ecosystem. It may not generate the most polished SDKs out of the box, but it is extraordinarily flexible through templates.
Enterprise Automation
Winner: Speakeasy
Speakeasy is built for organizations that have already settled on OpenAPI and want everything else automated. SDK generation is only part of the offering; the platform also handles release pipelines, CI/CD integration, changelog generation, and AI governance tooling. If your team ships SDKs across multiple languages every week, the time savings can easily justify the cost.
| Feature | OpenAPI Gen | FumaStudio | Fern | Speakeasy |
|---|---|---|---|---|
| Free tier | Free | Free | Paid | Trial only |
| Bruno-native | No | Yes | No | No |
| Languages | 20+ | TypeScript | 9 | Multiple |
| Offline support | Yes | Soon | No | No |
| Auto-publishes packages | No | No | Yes | Yes |
| Built-in tests | No | Yes | Yes | Yes |
| API docs site | No | Yes | Yes | Yes |
| Multi-protocol | Limited | HTTP only | Yes | Yes |
| MCP support | No | No | Yes | Yes |
Building and maintaining SDKs by hand is a tax that compounds over time. Every API change creates a ripple of updates across multiple languages, every new language target demands specialized knowledge, and every gap between your API and your SDK erodes developer trust. The handwritten approach does not scale, not for a solo developer, and definitely not for a growing team.
If you are a Bruno user, pick based on what you actually need:
The OpenAPI spec you already have open in Bruno is all most of these tools need to get started. Pick one, run it against your spec, and see how much boilerplate disappears.