OpenApiWeaver is an incremental Roslyn source generator that generates strongly typed C# HTTP clients from OpenAPI 3.x documents, including OpenAPI 3.2, at build time. It does not rely on runtime code generation or reflection, and emits plain C# during compilation.

Quick Start

1. Install the package

<ItemGroup>
  <PackageReference Include="OpenApiWeaver" Version="x.y.z" PrivateAssets="all" />
</ItemGroup>

2. Add your OpenAPI document

<ItemGroup>
  <OpenApiWeaverDocument Include="openapi\petstore.yaml"
                         ClientName="PetstoreClient"
                         Namespace="Contoso.Generated" />
</ItemGroup>

Declare the document as an OpenApiWeaverDocument item rather than AdditionalFiles. The package's MSBuild targets project these items into compiler inputs automatically.

3. Use the generated client

var client = new PetstoreClient(accessToken: "your-token");

// Operations are grouped by OpenAPI tag
var pet = await client.Pets.GetAsync(petId: 1);

You can also pass an existing HttpClient to the generated root client. When you do, the generated client reuses that instance, preserves an existing BaseAddress, and applies security credentials per request according to document-level or operation-level security requirements instead of mutating DefaultRequestHeaders.

The package includes the source generator and all required analyzer assemblies, so no additional dependencies are required.

Features

• Incremental source generation — uses the Roslyn incremental generator pipeline to keep rebuilds efficient
• OpenAPI 3.0-3.2 support — reads .json, .yaml, and .yml documents, including OpenAPI 3.2 features such as response summaries and nullable type arrays
• Tag-based sub-clients — groups operations by OpenAPI tags and exposes them as properties on the root client
• Typed request / response models — generates classes, enums, nested inline types, dictionaries, and composition-aware schema mappings from components/schemas, including discriminator-based polymorphic models
• Multiple request body formats — supports application/json, application/x-www-form-urlencoded, and multipart/form-data
• Security scheme support — supports OAuth2 and bearer tokens, together with API keys in headers, query strings, and cookies, while honoring operation-level security requirements
• Runtime error handling — throws OpenApiException, with typed OpenApiException<TError> when error schemas are available
• OpenAPI-driven XML docs — generates IntelliSense comments from document, tag, operation, response, and schema metadata, with HTML removed automatically
• Build-time diagnostics — reports errors and warnings as standard compiler diagnostics

Comparison

How OpenApiWeaver compares to other popular OpenAPI-to-C# client generators:

	OpenApiWeaver	Kiota	NSwag	Refitter
Mechanism	Roslyn incremental source generator	CLI (kiota generate)	CLI / MSBuild	Refit interface generator (source generator / CLI)
Code generation	Automatic on build, incremental	Manual re-run / kiota update	Manual or MSBuild	Automatic (source generator)
Generated files in repo	Not needed (emitted in-memory)	Committed to the repo	Committed to the repo	Not needed (source generator)
Runtime dependencies	None (plain HttpClient)	Microsoft.Kiota.* packages	JSON library (e.g. Newtonsoft.Json)	Refit runtime
Runtime reflection	None	None	None	None (Refit source generator)
Distribution	Single NuGet (PrivateAssets="all")	dotnet tool + runtime packages	dotnet tool / NuGet	NuGet + Refit
Languages	C#	C#, Java, Go, TypeScript, Python, …	C#, TypeScript	C#

Choose OpenApiWeaver when you want clients to regenerate automatically on every build with no committed code, no runtime dependencies, and a single NuGet package — without a separate CLI step or generated files to maintain. For multi-language generation or a large, mature ecosystem, Kiota and NSwag remain strong choices.

Requirements

• .NET SDK 10.0 or later

Documentation

For detailed guides, configuration options, and schema type mapping, see the documentation site.

License

This project is licensed under the MIT License.