Tapir supports validation for primitive types. Validation of composite values, whole data structures, business rules enforcement etc. should be done as part of the server logic of the endpoint, using the dedicated error output (the E in Endpoint[I, E, O, S]) to report errors.

Single type validation

Validation rules are part of the Schema for a given type, and can be added either directly to the schema, or via the Codec or EndpointInput/EndpointOutput. For example, when defining a codec for a type, we have the .validate() method:

import sttp.tapir._
import sttp.tapir.CodecFormat.TextPlain

case class MyId(id: String)

implicit val myIdCodec: Codec[String, MyId, TextPlain] = Codec.string

Validators can also be added to individual inputs/outputs, in addition to whatever the codec provides:

import sttp.tapir._

val e =

For optional/iterable inputs/outputs, to validate the contained value(s), use:

import sttp.tapir._

query[List[Int]]("item").validateIterable(Validator.min(0)) // validates each repeated parameter

Validation rules added using the built-in validators are translated to OpenAPI documentation.

Validation rules and automatic codec derivation

As validators are parts of schemas, they are looked up as part of the implicit Schema[T] values. When customising schemas, use the .validate method on the schema to add a validator.

Decode failures

Codecs support reporting decoding failures, by returning a DecodeResult from the Codec.decode method. However, this is meant for input/output values which are in an incorrect low-level format, when parsing a “raw value” fails. In other words, decoding failures should be reported for format failures, not business validation errors.

To customise error messages that are returned upon validation/decode failures by the server, see error handling.

Enum validators

Validators for enumerations can be created using:

  • Validator.derivedEnumeration, which takes a type parameter. This should be an abstract, sealed base type, and using a macro determines the possible values

  • Validator.enumeration, which takes the list of possible values

To properly represent possible values in documentation, the enum validator additionally needs an encode method, which converts the enum value to a raw type (typically a string). This can be specified by:

  • explicitly providing it using the overloaded enumeration method with an encode parameter

  • by using one of the .encode methods on the Validator.Enumeration instance

  • when the values possible values are of a basic type (numbers, strings), the encode function is inferred if not present

  • by adding the validator directly to a codec using .validate (the encode function is then taken from the codec)

To simplify creation of schemas and codec, with a derived enum validator, Schema.derivedEnumeration and Codec.derivedEnumeration helper methods are available. For example:

import sttp.tapir._
import sttp.tapir.Codec.PlainCodec

sealed trait Color
case object Blue extends Color
case object Red extends Color

implicit def plainCodecForColor: PlainCodec[Color] = {
  Codec.derivedEnumeration[String, Color](
    (_: String) match {
      case "red"  => Some(Red)
      case "blue" => Some(Blue)
      case _      => None

If the enum is nested within an object and its values aren’t of a “basic” type (numbers, strings), regardless of whether the codec for that object is defined by hand or derived, we need to specify the encode function by hand:

// providing the enum values by hand
implicit def colorSchema: Schema[Color] = Schema.string.validate(
  Validator.enumeration(List(Blue, Red), (c: Color) => Some(c.toString.toLowerCase)))

// or deriving the enum values and using the helper function
implicit def colorSchema2: Schema[Color] = Schema.derivedEnumeration[Color](encode = Some(_.toString.toLowerCase))


Read on about content types.