The Mapping[L, H] trait defines a bi-directional mapping between values of type L and values of type H.

Low-level values of type L can be decoded to a higher-level value of type H. The decoding can fail; this is represented by a result of type DecodeResult.Failure. Failures might occur due to format errors, wrong arity, exceptions, or validation errors. Validators can be added through the validate method.

High-level values of type H can be encoded as a low-level value of type L.

Mappings can be chained using one of the map functions.


A Codec[L, H, CF] is a Mapping[L, H], with additional meta-data: a schema and the format of the low-level value (more on that below).

There are built-in codecs for most common types such as String, Int etc. Codecs are usually defined as implicit values and resolved implicitly when they are referenced. However, they can also be provided explicitly as needed.

For example, a query[Int]("quantity") specifies an input parameter which corresponds to the quantity query parameter and will be mapped as an Int. There’s an implicit Codec[List[String], Int, TextPlain] value that is referenced by the query method (which is defined in the sttp.tapir package).

In this example, the low-level value is a List[String], as a given query parameter can be absent, have a single or many values. The high-level value is an Int. The codec will verify that there’s a single query parameter with the given name, and parse it as an int. If any of this fails, a failure will be reported.

In a server setting, if the value cannot be parsed as an int, a decoding failure is reported, and the endpoint won’t match the request, or a 400 Bad Request response is returned (depending on configuration).

As each codec is also a Mapping, codecs can be used to map endpoint inputs and outputs. In such cases, the additional meta-data (schema & format) isn’t taken into account.

Optional and multiple parameters

Some inputs/outputs allow optional, or multiple parameters:

  • path segments are always required
  • query and header values can be optional or multiple (repeated query parameters/headers)
  • bodies can be optional, but not multiple

In general, optional parameters are represented as Option values, and multiple parameters as List values. For example, header[Option[String]]("X-Auth-Token") describes an optional header. An input described as query[List[String]]("color") allows multiple occurrences of the color query parameter, with all values gathered into a list.


A codec contains a schema, which describes the high-level type. The schema is used when generating documentation and enforcing validation rules.

Schema consists of:

  • the schema type, which is one of the values defined in SchemaType, such as SString, SBinary, SArray or SProduct/SCoproduct (for ADTs). This is the shape of the encoded value - as it is sent over the network
  • meta-data: value optionality, description and low-level format name
  • validation rules

For primitive types, the schema values are built-in, and defined in the Schema companion object.

The schema is left unchanged when mapping a codec, or an input/output, as the underlying representation of the value doesn’t change. However, schemas can be changed for individual inputs/outputs using the .schema(Schema) method.

When codecs are derived for complex types, e.g. for json mapping, schemas are looked up through implicit Schema[T] values. See custom types for more details.

Codec format

Codecs contain an additional type parameter, which specifies the codec format. Each format corresponds to a media type, which describes the low-level format of the raw value (to which the codec encodes). Some built-in formats include text/plain, application/json and multipart/form-data. Custom formats can be added by creating an implementation of the sttp.tapir.CodecFormat trait.

Thanks to codecs being parametrised by codec formats, it is possible to have a Codec[String, MyCaseClass, TextPlain] which specifies how to serialize a case class to plain text, and a different Codec[String, MyCaseClass, Json], which specifies how to serialize a case class to json. Both can be implicitly available without implicit resolution conflicts.

Different codec formats can be used in different contexts. When defining a path, query or header parameter, only a codec with the TextPlain media type can be used. However, for bodies, any media types is allowed. For example, the input/output described by jsonBody[T] requires a json codec.