API specifications are the underpinning source of truth and governance layer that makes the API world work. Getting systems to understand how your API functions — and ensuring that the API actually does function in that way — are two critical steps in the development process.

In this realm, two approaches are commonly discussed: OpenAPI and TypeSpec. Today, we’re going to dive into what these solutions are, how they work, and why you should care. Finally, we’ll answer the critical question on everyone’s mind: is TypeSpec a production-ready replacement for OpenAPI? Read on to find out!

What is OpenAPI?

OpenAPI is a specification format that offers a standardized method to describe RESTful APIs in a machine-readable, structured format. Developers can detail API endpoints, authentication and authorization flows, as well as the methods, parameters, and expected request and response schemas. The specification is typically written in JSON or YAML, allowing for a comprehensive specification that is still human-readable and easy to follow.

OpenAPI emerged in 2011 as an open-source initiative initially called Swagger, and as Swagger gained popularity in the developer community, it started to attract bids for acquisition. In 2015, after an acquisition of Swagger by SmartBear, the specification was donated to the Linux Foundation. At this point, it was renamed to the OpenAPI Specification, or OAS, and was released to the broader development community.

This open sourcing of OpenAPI was a huge step in its evolution, unlocking community-driven governance and development. Since then, it has become a foundational tool in API development, with broad support across tools like Spectral, Postman, and Swagger UI.

OpenAPI Benefits

OpenAPI offers a lot of quality-of-life benefits and features for developers, which have made it a strong product offering up to this point. Firstly, OpenAPI has some world-class comprehensive documentation. This level of documentation and community support has meant that new adopters have a straightforward path to building their specification and resolving issues.

There is also the fact that OpenAPI is broadly adopted, which has led to significant maturity. For people who run into issues of lacking extensions or solutions, it’s likely that there is a tool or integration that will solve that problem. Particular CI/CD pipelines call for particular solutions, and having OpenAPI be as mature and widely adopted as it is means that there are integrations for almost every tool you can imagine.

Finally, OpenAPI is built for explicit control and validation out of the box. Support for JSON and YAML specifications makes for highly granular definitions, giving developers precise control over API behavior, validation rules, and request-response details. This unlocks some pretty spectacular validation and linting solutions as a tack-on benefit, allowing you to further govern your API definitions against best practices and development processes.

OpenAPI Drawbacks

As with any standard of course, not everything is perfect. OpenAPI is widely adopted and relatively mature, but it really focuses on a specific subset of APIs and processes, which can introduce some blind spots and drawbacks.

One concern is around how verbose OpenAPI is. With great control comes great complexity, and when large, complex APIs expand their frontiers, API bloat can be a serious issue in OpenAPI. This can make the specification difficult to manage over time, requiring more intentional specification management.

As such, OpenAPI itself can be complex. It has a specific way of documenting things, requiring a learning curve. While this curve is offset somewhat by the large and supportive community, it requires an understanding of schema construction and deep contextual development, often creating an “OpenAPI wizard” on a team in isolation.

Finally, there’s the reality that OpenAPI is of the RESTful era. While REST APIs are almost ubiquitous at this point, there are other standards — outside of old standards like SOAP, there are also WebSockets and GraphQL services, and those can be hard to document and govern using OpenAPI.

What is TypeSpec?

TypeSpec is a language for defining APIs and data models released by Microsoft in 2024. Initially developed as Cadl, it was designed as a language with a syntax similar to TypeScript, designed for describing APIs. The idea was to create a simple solution that offered a familiar methodology and structure to allow developers to automatically create specifications from their APIs.

In many ways, TypeSpec feels like an answer to some of the common complaints with OpenAPI. But notably, it’s not built simply as a replacement for OpenAPI. In fact, TypeSpec can generate an OpenAPI Specification file using a feature called emitters, allowing you to develop and define your API and then build a specification based on the in-language definition tooling.

TypeSpec Benefits

TypeSpec offers some specific benefits that have made it quite popular in the development community.

First and foremost, it’s a definition language, and as such, necessitates accuracy in the construction phase. This construction-oriented view also opens up your code to a wide variety of specification formats. While you can emit an OpenAPI file quite easily, you can also output your specification in JSON Schema, Protobufs, and more, allowing you more flexibility in your ultimate output. In many cases, you can even do more than one: you can output both an OpenAPI specification file and a JSON Schema file, tying your structures to a single contract and reducing bugs that arise from translation.

That single source of truth also makes it uniquely suited for large-scale API governance. Whereas OpenAPI can get wonky with larger and more complex APIs, TypeSpec requires definition in the build structure, meaning that the code documents its own specification. This helps encourage consistency and modularity, aligning your API governance with your design methodology and approach.

Finally, the sheer extensibility that comes with decorators and emitters means you can tailor your outputs and functionality to a wide range of use cases. This can be as simple as tailoring your output or as complex as encapsulating your standards into shared libraries, and is a huge benefit to communal development.

TypeSpec Drawbacks

While TypeSpec offers some great features, there are some drawbacks that developers should consider. First, TypeSpec has a much smaller ecosystem compared to OpenAPI. It’s a newer tool with a smaller official integration footprint, and as such, support for some areas — like emitters — might not be as ubiquitous as you’d think. For instance, the gRPC emitters are still in novel development, and as such, are not widely adopted.

There’s also just the reality that this is a domain-specific language (DSL). It’s a clean syntax, but it’s developer-centric. This can make it difficult to approach compared to a simple YAML tool, and while it can certainly output to OpenAPI for that purpose, that’s another step in the process, which might feel hard to justify to non-engineering stakeholders.

TypeSpec also heavily favors a model-driven design, which can be overkill for simple APIs or projects with minimal schema complexity. This can result in TypeSpec feeling heavier than is necessary for some implementations, whereas OpenAPI, with all of its drawbacks with hand-written specifications, can feel a bit simpler and more direct.

Finally, it should be noted that this is a very Microsoft-centric tool. Most of the documentation for large-scale implementation comes from Microsoft itself via Azure, and as such, third-party and community support in terms of examples or experiences is much more limited compared to OpenAPI.

Does TypeSpec Replace OpenAPI?

All of this begs one question: is TypeSpec a replacement for OpenAPI? The simple answer is no, but it augments it. You can think of TypeSpec as a complementary, high-level abstraction that can make OpenAPI better from the developer perspective.

TypeSpec’s main value proposition is simple — instead of building the specification after construction, why not construct using the definition itself? In this reality, TypeSpec becomes a design-time tool that allows you to use a simple DSL to define your service in practice. Once you’ve built the service, you can then emit an OpenAPI specification file — or any number of other files — as an output artifact.

Asking if TypeSpec replaces OpenAPI is a bit like asking if CAD, a computer-aided design system, replaces blueprints. A blueprint may still be output from CAD, but you use CAD to model your structure as you plan. In the same way, OpenAPI is a potential output of TypeSpec, and is not something that TypeSpec can outright replace.

So, when does it make sense to use each? To answer this, we should look at this not as a “TypeSpec vs. OpenAPI” question, but as a question of when to favor each tool as the source of truth and definition.

TypeSpec And OpenAPI: Final Thoughts

TypeSpec offers a lot of great features, but it should be seen as an addendum to OpenAPI, not a replacement. Both can live comfortably with one another, and choosing whether to use both or one or the other is going to come down to the specific value you need from each tool. Leveraging TypeSpec, you can support a wide range of outputs, but if you only need an OpenAPI spec and you need other non-technical eyes on the prize, then OpenAPI might be simpler and easier to integrate.