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title Generate OpenAPI documents
author captainsafia
description Learn how to generate and customize OpenAPI documents in an ASP.NET Core app
ms.author safia
monikerRange >= aspnetcore-6.0
ms.custom mvc
ms.date 3/18/2025
uid fundamentals/openapi/aspnetcore-openapi

Generate OpenAPI documents

:::moniker range=">= aspnetcore-10.0"

The Microsoft.AspNetCore.OpenApi package provides built-in support for OpenAPI document generation in ASP.NET Core. The package provides the following features:

  • Support for generating OpenAPI version 3.1 documents.
  • Support for JSON Schema draft 2020-12.
  • Support for generating OpenAPI documents at run time and accessing them via an endpoint on the app.
  • Support for "transformer" APIs that allow modifying the generated document.
  • Support for generating multiple OpenAPI documents from a single app.
  • Takes advantage of JSON schema support provided by System.Text.Json.
  • Is compatible with native AoT.

The default OpenAPI version for generated documents is3.1. The version can be changed by explicitly setting the OpenApiVersion property of the OpenApiOptions in the configureOptions delegate parameter of AddOpenApi:

builder.Services.AddOpenApi(options =>
{
    // Specify the OpenAPI version to use.
    options.OpenApiVersion = Microsoft.OpenApi.OpenApiSpecVersion.OpenApi3_0;
});

When generating the OpenAPI document at build time, the OpenAPI version can be selected by setting the --openapi-version in the OpenApiGenerateDocumentsOptions MSBuild item.

    <!-- Configure build-time OpenAPI generation to produce an OpenAPI 3.1 document. -->
    <OpenApiGenerateDocumentsOptions>--openapi-version OpenApi3_1</OpenApiGenerateDocumentsOptions>

Package installation

Install the Microsoft.AspNetCore.OpenApi package:

Visual Studio

Run the following command from the Package Manager Console:

Install-Package Microsoft.AspNetCore.OpenApi

.NET CLI

Run the following command:

dotnet add package Microsoft.AspNetCore.OpenApi

Configure OpenAPI document generation

The following code:

  • Adds OpenAPI services using the xref:Microsoft.Extensions.DependencyInjection.OpenApiServiceCollectionExtensions.AddOpenApi%2A extension method on the app builder's service collection.
  • Maps an endpoint for viewing the OpenAPI document in JSON format with the xref:Microsoft.AspNetCore.Builder.OpenApiEndpointRouteBuilderExtensions.MapOpenApi%2A extension method on the app.

[!code-csharp]

Launch the app and navigate to https://localhost:<port>/openapi/v1.json to view the generated OpenAPI document.

Options to Customize OpenAPI document generation

The following sections demonstrate how to customize OpenAPI document generation.

Generate OpenAPI document in YAML format

The OpenAPI document can be generated in either JSON or YAML format. By default, the OpenAPI document is generated in JSON format. To generate the OpenAPI document in YAML format, specify the endpoint in the MapOpenApi call with a ".yaml" or ".yml" suffix, as shown in the following example:

app.MapOpenApi("/openapi/{documentName}.yaml");

Generating penAPI documents in YAML format at build time is currently not supported, but planned in a future preview.

Customize the OpenAPI document name

Each OpenAPI document in an app has a unique name. The default document name that is registered is v1.

builder.Services.AddOpenApi(); // Document name is v1

The document name can be modified by passing the name as a parameter to the xref:Microsoft.Extensions.DependencyInjection.OpenApiServiceCollectionExtensions.AddOpenApi%2A call.

builder.Services.AddOpenApi("internal"); // Document name is internal

The document name surfaces in several places in the OpenAPI implementation.

When fetching the generated OpenAPI document, the document name is provided as the documentName parameter argument in the request. The following requests resolve the v1 and internal documents.

GET http://localhost:5000/openapi/v1.json
GET http://localhost:5000/openapi/internal.json

Customize the OpenAPI version of a generated document

By default, OpenAPI document generation creates a document that is compliant with OpenAPI version 3.1. The following code demonstrates how to modify the default version of the OpenAPI document:

builder.Services.AddOpenApi(options =>
{
    options.OpenApiVersion = OpenApiSpecVersion.OpenApi3_0;
});

Customize the OpenAPI endpoint route

By default, the OpenAPI endpoint registered via a call to xref:Microsoft.AspNetCore.Builder.OpenApiEndpointRouteBuilderExtensions.MapOpenApi%2A exposes the document at the /openapi/{documentName}.json endpoint. The following code demonstrates how to customize the route at which the OpenAPI document is registered:

app.MapOpenApi("/openapi/{documentName}/openapi.json");

It's possible, but not recommended, to remove the documentName route parameter from the endpoint route. When the documentName route parameter is removed from the endpoint route, the framework attempts to resolve the document name from the query parameter. Not providing the documentName in either the route or query can result in unexpected behavior.

Customize the OpenAPI endpoint

Because the OpenAPI document is served via a route handler endpoint, any customization that is available to standard minimal endpoints is available to the OpenAPI endpoint.

Limit OpenAPI document access to authorized users

The OpenAPI endpoint doesn't enable any authorization checks by default. However, authorization checks can be applied to the OpenAPI document. In the following code, access to the OpenAPI document is limited to those with the tester role:

[!code-csharp]

Cache generated OpenAPI document

The OpenAPI document is regenerated every time a request to the OpenAPI endpoint is sent. Regeneration enables transformers to incorporate dynamic app state into their operation. For example, regenerating a request with details of the HTTP context. When applicable, the OpenAPI document can be cached to avoid executing the document generation pipeline on each HTTP request.

[!code-csharp]

Generate multiple OpenAPI documents

In some scenarios, it's helpful to generate multiple OpenAPI documents with different content from a single ASP.NET Core API app. These scenarios include:

  • Generating OpenAPI documentation for different audiences, such as public and internal APIs.
  • Generating OpenAPI documentation for different versions of an API.
  • Generating OpenAPI documentation for different parts of an app, such as a frontend and backend API.

To generate multiple OpenAPI documents, call the xref:Microsoft.Extensions.DependencyInjection.OpenApiServiceCollectionExtensions.AddOpenApi%2A extension method once for each document, specifying a different document name in the first parameter each time.

builder.Services.AddOpenApi("v1");
builder.Services.AddOpenApi("v2");

Each invocation of xref:Microsoft.Extensions.DependencyInjection.OpenApiServiceCollectionExtensions.AddOpenApi%2A can specify its own set of options, so that you can choose to use the same or different customizations for each OpenAPI document.

The framework uses the xref:Microsoft.AspNetCore.OpenApi.OpenApiOptions.ShouldInclude delegate method of xref:Microsoft.AspNetCore.OpenApi.OpenApiOptions to determine which endpoints to include in each document.

For each document, the xref:Microsoft.AspNetCore.OpenApi.OpenApiOptions.ShouldInclude delegate method is called for each endpoint in the app, passing the xref:Microsoft.AspNetCore.Mvc.ApiExplorer.ApiDescription object for the endpoint. The method returns a boolean value indicating whether the endpoint should be included in the document. The xref:Microsoft.AspNetCore.Mvc.ApiExplorer.ApiDescription object:

  • contains information about the endpoint, such as the HTTP method, route, and response types
  • Metadata attached to the endpoint via attributes or extension methods.

The default implementation of this delegate uses the xref:Microsoft.AspNetCore.Mvc.ApiExplorer.ApiDescription.GroupName field of xref:Microsoft.AspNetCore.Mvc.ApiExplorer.ApiDescription. The delegate is set on an endpoint using either the xref:Microsoft.AspNetCore.Builder.RoutingEndpointConventionBuilderExtensions.WithGroupName%2A extension method or the xref:Microsoft.AspNetCore.Routing.EndpointGroupNameAttribute attribute. WithGroupName or the EndpointGroupName attribute determines which endpoints to include in the document. Any endpoint that has not been assigned a group name is included all OpenAPI documents.

    // Include endpoints without a group name or with a group name that matches the document name
    ShouldInclude = (description) => description.GroupName == null || description.GroupName == DocumentName;

You can customize the xref:Microsoft.AspNetCore.OpenApi.OpenApiOptions.ShouldInclude delegate method to include or exclude endpoints based on any criteria you choose.

Generate OpenAPI documents at build-time

In typical web apps, OpenAPI documents are generated at run-time and served via an HTTP request to the app server.

In some scenarios, it's helpful to generate the OpenAPI document during the app's build step. These scenarios include:

  • Generating OpenAPI documentation that is committed into source control.
  • Generating OpenAPI documentation that is used for spec-based integration testing.
  • Generating OpenAPI documentation that is served statically from the web server.

To add support for generating OpenAPI documents at build time, install the Microsoft.Extensions.ApiDescription.Server package:

Visual Studio

Run the following command from the Package Manager Console:

Install-Package Microsoft.Extensions.ApiDescription.Server

.NET CLI

Run the following command in the directory that contains the project file:

dotnet add package Microsoft.Extensions.ApiDescription.Server

Upon installation, this package:

  • Automatically generates the Open API document(s) associated with the app during build.
  • Populates the Open API documents in the app's output directory.

If multiple documents are registered, and document name is not v1, it's post-fixed with the document name. E.g., {ProjectName}_{DocumentName}.json.

Visual Studio Code

dotnet build
type obj\{ProjectName}.json

.NET Core CLI

dotnet build
cat obj/{ProjectName}.json

Customizing build-time document generation

Modifying the output directory of the generated Open API file

By default, the generated OpenAPI document will be emitted to the app's output directory. To modify the location of the emitted file, set the target path in the OpenApiDocumentsDirectory property.

<PropertyGroup>
  <OpenApiDocumentsDirectory>.</OpenApiDocumentsDirectory>
</PropertyGroup>

The value of OpenApiDocumentsDirectory is resolved relative to the project file. Using the . value above will emit the OpenAPI document in the same directory as the project file.

Modifying the output file name

By default, the generated OpenAPI document will have the same name as the app's project file. To modify the name of the emitted file, set the --file-name argument in the OpenApiGenerateDocumentsOptions property.

<PropertyGroup>
  <OpenApiGenerateDocumentsOptions>--file-name my-open-api</OpenApiGenerateDocumentsOptions>
</PropertyGroup>

Selecting the OpenAPI document to generate

Some apps may be configured to emit multiple OpenAPI documents. Multiple OpenAPI documents may be generated for different versions of an API or to distinguish between public and internal APIs. By default, the build-time document generator emits files for all documents that are configured in an app. To only emit for a single document name, set the --document-name argument in the OpenApiGenerateDocumentsOptions property.

<PropertyGroup>
  <OpenApiGenerateDocumentsOptions>--document-name v2</OpenApiGenerateDocumentsOptions>
</PropertyGroup>

Customizing run-time behavior during build-time document generation

Build-time OpenAPI document generation functions by launching the apps entrypoint with a mock server implementation. A mock server is required to produce accurate OpenAPI documents because all information in the OpenAPI document can't be statically analyzed. Because the apps entrypoint is invoked, any logic in the apps startup is invoked. This includes code that injects services into the DI container or reads from configuration. In some scenarios, it's necessary to restrict the code paths that will run when the apps entry point is being invoked from build-time document generation. These scenarios include:

  • Not reading from certain configuration strings.
  • Not registering database-related services.

In order to restrict these code paths from being invoked by the build-time generation pipeline, they can be conditioned behind a check of the entry assembly:

:::code language="csharp" source="~/fundamentals/openapi/samples/9.x/AspireApp1/AspireApp1.Web/Program.cs" highlight="5-8":::

AddServiceDefaults adds common .NET Aspire services such as service discovery, resilience, health checks, and OpenTelemetry.

Trimming and Native AOT

OpenAPI in ASP.NET Core supports trimming and native AOT. The following steps create and publish an OpenAPI app with trimming and native AOT:

Create a new ASP.NET Core Web API (Native AOT) project.

dotnet new webapiaot

Publish the app.

dotnet publish

:::moniker-end

[!INCLUDE]

[!INCLUDE]