TypeSpec lets you describe an API and its data models in a structured source language, then compile that source into artifacts such as an OpenAPI specification. It defines the interface; your backend service still implements the API’s runtime behavior. For a REST project, the practical workflow is to initialize a TypeSpec project, define service metadata, models and HTTP operations, and compile the result.
How TypeSpec fits into an API workflow
Think of TypeSpec as the maintained description of an API, rather than the server itself. You write source that describes resources, operations, parameters, models and protocol details. The compiler and an emitter translate that description into an artifact such as OpenAPI for documentation, client tooling or other consumers. The application’s backend remains responsible for handling requests and implementing business logic.
For teams accustomed to writing OpenAPI directly, TypeSpec provides a higher-level authoring language. A TypeSpec model can become an OpenAPI schema, and a named model referenced in multiple places generally becomes a reusable component reference in the emitted document. The generated OpenAPI file is an output of the source model, not a substitute for maintaining that model.
Start a REST API project
The official setup guide documents a CLI-based path. Its template and package selections are current documented guidance rather than permanent requirements; check the TypeSpec documentation if the CLI offers different options in your installed version.
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Initialize a project by running
tsp init. Select the Generic REST API template, then select the@typespec/httpand@typespec/openapi3libraries when prompted. -
Review the starter files.
main.tspholds the API definitions,tspconfig.yamlconfigures compilation, andpackage.jsonrecords project metadata and dependencies. -
Compile the project from its root with
tsp compile .. The starter configuration emits an OpenAPI file undertsp-output/.Rank #2
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Inspect the output and revise the TypeSpec source when the interface needs to change. Treat the emitted file as a build artifact unless your project has deliberately chosen a different source-of-truth policy.
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The HTTP library supplies the HTTP decorators and models used to describe protocol behavior. The OpenAPI 3 library is needed to emit an OpenAPI specification, but is not required just to define the sample API in TypeSpec. The official setup guide also describes scaffolding and extensions for VS Code and Visual Studio.
Define the service, models and REST operations
A useful way to organize an API definition is in layers: identify the service, group related declarations in a namespace, define data models, and then describe operations and their HTTP bindings. The REST tutorial’s examples import @typespec/http and use its Http namespace for HTTP-related features.
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Describe the service and server
Use service metadata to give the API a title and other identifying information. The HTTP library’s @server decorator can associate a server URL with a namespace; multiple servers and parameterized server URLs are supported in the documented patterns. Keep environment-specific deployment details distinct from the routes that define the API’s operations.
Model data explicitly
Define request and response shapes as TypeSpec models. Their properties establish the data contract, while references to named models can be reused by operations and represented as reusable schemas in OpenAPI. This makes shared structures visible in the source instead of duplicating their definitions across endpoints.
Bind operations to HTTP
Operations describe what the API exposes; HTTP decorators describe how each operation is reached and how its inputs map to the protocol. The HTTP library includes decorators such as @get, @post, @put, @patch, @delete, @route, @path, @query and @header. Use the verb and route decorators to establish the endpoint, and parameter decorators where the contract requires a path, query or header value.
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Keep each operation’s parameters and return type aligned with its intended request and response. A compact definition should make it possible to answer, from the TypeSpec source, which route is exposed, which inputs it accepts, and which model or value it returns. The HTTP library reference and HTTP cheat sheet document the available patterns.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Keep API documentation with the definitions
TypeSpec supports documentation through doc comments and the @doc decorator. The language guide notes that comments are often preferred because they are less intrusive to the specification. Whichever form you choose, write descriptions in Markdown: TypeSpec tooling assumes that format.
Document declarations where readers encounter them. Explain an operation’s purpose, what a parameter means, and any model semantics that are not obvious from property names alone. This keeps useful context close to the definitions that generate the API description.
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Model API versions as deliberate changes
For an API that needs explicit version history, use the @typespec/versioning library. The documented workflow adds the dependency, declares supported versions with @versioned and an enum, then marks changes with versioning decorators. Tutorial examples include adding an operation in a later version and changing a field’s name and optionality.
The compiler can generate a separate OpenAPI specification for each declared version. This lets a team communicate the shape of each version from the same source model. Versioning support describes changes; it does not, by itself, establish that a change meets every client-compatibility rule or organizational policy. Review the generated contracts against the compatibility guarantees your service intends to make.
Convert an existing OpenAPI 3 definition cautiously
If you already have an OpenAPI 3 YAML or JSON file, the tsp-openapi3 CLI can convert it into TypeSpec files. The TypeSpec documentation describes the converter’s purpose as “a one time conversion to help you get started with TypeSpec” on its OpenAPI3 to TypeSpec page.
Review and take ownership of the generated TypeSpec source before making it your maintained definition. The documentation warns that generated output can change across future TypeSpec versions without that change being treated as a breaking change. Do not assume the conversion is a guaranteed lossless round trip or that regenerated files will remain identical over time.
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When to build a TypeSpec library or emitter
Most API teams can author services using the available language libraries and emitters. Custom extension work is a separate need: use tsp init --template library-ts to start a library project or tsp init --template emitter-ts to start an emitter project. The official authoring guide discusses package structure and TypeSpec dependencies; for libraries and compiler dependencies, it recommends peer dependencies. A monorepo can simplify development when maintaining multiple libraries.
These templates are for teams extending TypeSpec itself, not prerequisites for defining a REST API. See the library authoring guide for extension development details.
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