Cadl is the former name of Microsoft’s open-source API design language, now called TypeSpec. With TypeSpec, teams describe an API in a reusable source definition; the compiler and emitters can then produce artifacts such as OpenAPI specifications and code. It is a design-time tool, not a replacement for implementing or running a service.
What is Cadl, and what is TypeSpec?
Cadl is the historical name for the language now branded TypeSpec. The project changelog records the rename in version 0.41.0 on March 3, 2023. Readers looking for current Microsoft documentation should search for TypeSpec, not assume Cadl is still the product name.
TypeSpec is an open-source language for designing APIs. Instead of treating each output format as the primary source, a team can describe an API in TypeSpec and generate artifacts from that definition. Microsoft describes it as “a powerful and flexible language for designing APIs.”
How does TypeSpec turn a definition into API artifacts?
A TypeSpec compiler processes the source definition, and emitters generate target artifacts. One important target is OpenAPI, which lets a TypeSpec-based workflow connect with established API documentation, testing, gateway, and client-generation tools.
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- Define the API: Write its structure and reusable definitions in TypeSpec.
- Compile and emit: Run the TypeSpec compiler with an emitter for the desired output.
- Use the generated artifacts: Review and integrate outputs such as an OpenAPI specification into the project’s existing workflow.
Generated output does not itself implement or operate the service. Teams still need a service implementation and should validate generated artifacts against their contract and project requirements.
What can TypeSpec generate, and how mature is code generation?
Microsoft’s overview lists client generation for .NET, JavaScript, Java, and Python, plus server-side stubs for .NET and JavaScript. Microsoft’s current status note says client and server code generation are in preview. Treat those targets as preview capabilities, and confirm their current support and fit before making them a production dependency.
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OpenAPI is a key interoperability path, but output availability does not mean every emitter or target has the same maturity. Check the specific emitter and generated artifact your project needs.
Can an existing OpenAPI specification be migrated to TypeSpec?
Yes. Microsoft’s overview describes an OpenAPI migration tool and conversion examples, so an existing specification can be a starting point rather than requiring a complete rewrite from scratch. A conversion should still be reviewed against the actual API contract, requirements, and conventions; do not assume the generated TypeSpec captures every project-specific decision without checking it.
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When is a TypeSpec-first workflow worth considering?
TypeSpec is most relevant when a team wants reusable, modular API definitions and a source that can feed multiple outputs. An OpenAPI-first workflow may remain suitable when the existing specification already serves the team well. The decision depends on the project, not on a documented universal productivity advantage.
- Reuse: Would shared definitions reduce duplication across APIs or outputs?
- Target fit: Are the needed emitters and languages supported at a maturity level the project can accept?
- Tool compatibility: Does generated OpenAPI fit existing documentation, testing, gateway, and client-generation processes?
- Migration and maintenance: Is the effort to convert and maintain TypeSpec justified by the team’s needs?
Microsoft’s materials describe the workflow capabilities, but do not establish independent comparative measurements of productivity or speed.
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How can you start learning TypeSpec?
Microsoft provides official documentation, getting-started guides and quickstarts, language references, videos, community resources, and an interactive TypeSpec Playground. A practical first exercise is to write a small definition, generate OpenAPI, and inspect the result. If starting from an existing API, use the documented OpenAPI migration path and review its conversion against the project’s contract.
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