Generating OpenAPI documentation

To expose documentation, endpoints first need to be interpreted into an OpenAPI yaml or json. Then, the generated description of our API can be exposed using a UI such as Swagger or Redoc.

These two operations can be done in a single step, using the SwaggerInterpreter or RedocInterpreter. Or, if needed, these steps can be done separately, giving you complete control over the process.

Generating and exposing documentation in a single step

Using Swagger

To generate OpenAPI documentation and expose it using the Swagger UI in a single step, first add the dependency:

"com.softwaremill.sttp.tapir" %% "tapir-swagger-ui-bundle" % "1.1.2"

Then, you can interpret a list of endpoints using SwaggerInterpreter. The result will be a list of file-serving server endpoints, which use the yaml corresponding to the endpoints passed originally. These swagger endpoints, together with the endpoints for which the documentation is generated, will need in turn to be interpreted using your server interpreter. For example:

import sttp.tapir._
import sttp.tapir.swagger.bundle.SwaggerInterpreter
import sttp.tapir.server.akkahttp.AkkaHttpServerInterpreter

import scala.concurrent.Future

val myEndpoints: List[AnyEndpoint] = ???

// first interpret as swagger ui endpoints, backend by the appropriate yaml
val swaggerEndpoints = SwaggerInterpreter().fromEndpoints[Future](myEndpoints, "My App", "1.0")

// add to your akka routes
val swaggerRoute = AkkaHttpServerInterpreter().toRoute(swaggerEndpoints)

By default, the documentation will be available under the /docs path. The path, as well as other options can be changed when creating the SwaggerInterpreter and invoking fromEndpoints. If the Swagger UI endpoints are deployed within a context, and you don’t want Swagger to use relative paths, you’ll need to set the useRelativePaths options to false, and specify the contextPath one.

Moreover, model generation can be configured - see below for more details on OpenAPIDocsOptions and the method parameters of fromEndpoints. Finally, the generated model can be customised. See the scaladocs for SwaggerInterpreter.

The swagger server endpoints can be secured using ServerLogic.prependSecurity, see server logic for details.

Using Redoc

Similarly as above, you’ll need the following dependency:

"com.softwaremill.sttp.tapir" %% "tapir-redoc-bundle" % "1.1.2"

And the server endpoints can be generated using the sttp.tapir.redoc.bundle.RedocInterpreter class.

Generating OpenAPI documentation separately

To generate the docs in the OpenAPI yaml format, add the following dependencies:

"com.softwaremill.sttp.tapir" %% "tapir-openapi-docs" % "1.1.2"
"com.softwaremill.sttp.apispec" %% "openapi-circe-yaml" % "..." // see

The case-class based model of the openapi data structures is present in the sttp-apispec project.

An endpoint can be converted to an instance of the model by importing the object:

import sttp.apispec.openapi.OpenAPI
import sttp.tapir._

val booksListing =[String]("bookId"))

val docs: OpenAPI = OpenAPIDocsInterpreter().toOpenAPI(booksListing, "My Bookshop", "1.0")

Such a model can then be refined, by adding details which are not auto-generated. Working with a deeply nested case class structure such as the OpenAPI one can be made easier by using a lens library, e.g. Quicklens.

The documentation is generated in a large part basing on schemas. Schemas can be automatically derived and customised.

Quite often, you’ll need to define the servers, through which the API can be reached. To do this, you can modify the returned OpenAPI case class either directly or by using a helper method:

import sttp.apispec.openapi.Server

val docsWithServers: OpenAPI = OpenAPIDocsInterpreter().toOpenAPI(booksListing, "My Bookshop", "1.0")
  .servers(List(Server("").description("Production server")))

Multiple endpoints can be converted to an OpenAPI instance by calling the method on a list of endpoints:

OpenAPIDocsInterpreter().toOpenAPI(List(addBook, booksListing, booksListingByGenre), "My Bookshop", "1.0")

The openapi case classes can then be serialised to YAML using Circe:

import sttp.apispec.openapi.circe.yaml._


Or to JSON:

import io.circe.Printer
import io.circe.syntax._
import sttp.apispec.openapi.circe._


Exposing generated OpenAPI documentation

Exposing the OpenAPI can be done using Swagger UI or Redoc. You can either both interpret endpoints to OpenAPI’s yaml and expose them in a single step (see above), or you can do that separately.

The modules tapir-swagger-ui and tapir-redoc contain server endpoint definitions, which given the documentation in yaml format, will expose it using the given context path. To use, add as a dependency either tapir-swagger-ui:

"com.softwaremill.sttp.tapir" %% "tapir-swagger-ui" % "1.1.2"

or tapir-redoc:

"com.softwaremill.sttp.tapir" %% "tapir-redoc" % "1.1.2"

Then, you’ll need to pass the server endpoints to your server interpreter. For example, using akka-http:

import sttp.apispec.openapi.circe.yaml._
import sttp.tapir._
import sttp.tapir.server.akkahttp.AkkaHttpServerInterpreter
import sttp.tapir.swagger.SwaggerUI

import scala.concurrent.Future

val myEndpoints: Seq[AnyEndpoint] = ???
val docsAsYaml: String = OpenAPIDocsInterpreter().toOpenAPI(myEndpoints, "My App", "1.0").toYaml

// add to your akka routes
val swaggerUIRoute = AkkaHttpServerInterpreter().toRoute(SwaggerUI[Future](docsAsYaml))


Options can be customised by providing an instance of OpenAPIDocsOptions to the interpreter:

  • operationIdGenerator: each endpoint corresponds to an operation in the OpenAPI format and should have a unique operation id. By default, the name of endpoint is used as the operation id, and if this is not available, the operation id is auto-generated by concatenating (using camel-case) the request method and path.

  • defaultDecodeFailureOutput: if an endpoint does not define a Bad Request response in errorOut, tapir will try to guess if decoding of inputs may fail, and add a 400 response if necessary. You can override this option to customize the mapping of endpoint’s inputs to a default error response. If you’d like to disable this feature, just provide a function that always returns None:

    OpenAPIDocsOptions.default.copy(defaultDecodeFailureOutput = _ => None)

Inlined and referenced schemas

All named schemas (that is, schemas which have the property defined) will be referenced at point of use, and their definitions will be part of the components section. If you’d like a schema to be inlined, instead of referenced, modify the schema removing the name.

Authentication inputs and security requirements

Multiple non-optional authentication inputs indicate that all the given authentication values should be provided, that is they will form a single security requirement, with multiple schemes, e.g.:

import sttp.model.headers.WWWAuthenticateChallenge
import sttp.tapir._

val multiAuthEndpoint =
    .securityIn(auth.apiKey(header[String]("token"), WWWAuthenticateChallenge("ApiKey").realm("realm")))
    .securityIn(auth.apiKey(header[String]("signature"), WWWAuthenticateChallenge("ApiKey").realm("realm")))

A single optional authentication method can be described by mapping to optional types, e.g. bearer[Option[String]]. Hence, two security requirements will be created: an empty one, and one corresponding to the given authentication input.

If there are multiple optional authentication methods, they will be treated as alternatives, and separate alternative security requirements will be created for them. However, this will not include the empty requirement, making authentication mandatory. If authentication should be optional, an empty security requirement will be added if an emptyAuth input is added (which doesn’t map to any values in the request, but only serves as a marker).


Note that even though multiple optional authentication methods might be rendered as alternatives in the documentation, when running the server, you’ll need to additionally check that at least one authentication input is provided. This can be done in the security logic, server logic, or by mapping the inputs using .mapDecode, as in the below example:

import sttp.model.headers.WWWAuthenticateChallenge
import sttp.tapir._

val alternativeAuthEndpoint = endpoint.securityIn(
  // auth.apiKey(...).and(auth.apiKey(..)) will map the request headers to a tuple (Option[String], Option[String])
  auth.apiKey(header[Option[String]]("token-old"), WWWAuthenticateChallenge("ApiKey").realm("realm"))
    .and(auth.apiKey(header[Option[String]]("token-new"), WWWAuthenticateChallenge("ApiKey").realm("realm")))
    // mapping this tuple to an Either[String, String], and reporting a decode error if both values are missing
    .mapDecode {
      case (Some(oldToken), _) => DecodeResult.Value(Left(oldToken))
      case (_, Some(newToken)) => DecodeResult.Value(Right(newToken))
      case (None, None)        => DecodeResult.Missing
    } {
      case Left(oldToken)  => (Some(oldToken), None)
      case Right(newToken) => (None, Some(newToken))
val alternativeOptionalAuthEndpoint = alternativeAuthEndpoint.securityIn(emptyAuth)

Finally, optional authentication inputs can be grouped into security requirements using Group names are arbitrary (and aren’t rendered in the documentation), but they need to be the same for a single group. Groups should only be used on optional authentication inputs. All such inputs in a single group will become a single security requirement when rendered in OpenAPI. As before, the fact that values for all inputs in a group are provided, needs to be checked on the server-side, either through decoding or server logic.

OpenAPI Specification Extensions

It’s possible to extend specification with extensions.

Specification extensions can be added by first importing an extension method, and then calling the docsExtension method which manipulates the appropriate attribute on the schema, endpoint or endpoint input/output:

import sttp.apispec.openapi._
import sttp.apispec.openapi.circe._
import sttp.apispec.openapi.circe.yaml._
import sttp.tapir._
import sttp.tapir.json.circe._


case class FruitAmount(fruit: String, amount: Int)

case class MyExtension(string: String, int: Int)

implicit val fruitAmountSchemaWithMyExtension: Schema[FruitAmount] =
  Schema.derived[FruitAmount].docsExtension("hello", MyExtension("world", 42))

val sampleEndpoint =
    .in("path-hello" / path[String]("world").docsExtension("x-path", 22))
    .in(query[String]("hi").docsExtension("x-query", 33))
    .in(jsonBody[FruitAmount].docsExtension("x-request", MyExtension("a", 1)))
    .out(jsonBody[FruitAmount].docsExtension("x-response", List("array-0", "array-1")).docsExtension("x-response", "foo"))
    .errorOut(stringBody.docsExtension("x-error", "error-extension"))
    .docsExtension("x-endpoint-level-string", "world")
    .docsExtension("x-endpoint-level-int", 11)
    .docsExtension("x-endpoint-obj", MyExtension("42.42", 42))

val rootExtensions = List(
  DocsExtension.of("x-root-bool", true),
  DocsExtension.of("x-root-list", List(1, 2, 4))

val openAPIYaml = OpenAPIDocsInterpreter().toOpenAPI(sampleEndpoint, Info("title", "1.0"), rootExtensions).toYaml

However, to add extensions to other unusual places (like, License or Server, etc.) you should modify the OpenAPI object manually or using a tool such as quicklens.

Hiding inputs/outputs

It’s possible to hide an input/output from the OpenAPI description using following syntax:

import sttp.tapir._

val acceptHeader: EndpointInput[String] = header[String]("Accept").schema(_.hidden(true))

Using SwaggerUI with sbt-assembly

The tapir-swagger-ui and tapir-swagger-ui-bundle modules rely on a file in the META-INF directory tree, to determine the version of the Swagger UI. You need to take additional measures if you package your application with sbt-assembly because the default merge strategy of the assembly task discards most artifacts in that directory. To avoid a NullPointerException, you need to include the following file explicitly:

assemblyMergeStrategy in assembly := {
  case PathList("META-INF", "maven", "org.webjars", "swagger-ui", "") =>
  case x =>
    val oldStrategy = (assemblyMergeStrategy in assembly).value