A Codec[L, H, CF] is a bi-directional mapping between low-level values of type L and high-level values of type H. Low level values are formatted as CF. A codec also contains the schema of the high-level value, which is used for validation and documentation.

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. A query input requires a codec, where the low-level value is a List[String] (representing potentially 0, one, or multiple parameters with the given name in the URL). Hence, an implicit Codec[List[String], Int, TextPlain] value will be looked up when using the query method (which is defined in the sttp.tapir package).

In this example, the codec will verify that there’s a single query parameter with the given name, and parse it as an integer. 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).

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, example, default value 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.

Schemas are typically referenced indirectly through codecs, and are specified when the codec is created.

As part of deriving a codec, to support a custom or complex type (e.g. for json mapping), schemas can be looked up implicitly and derived as well. 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 type is allowed. For example, the input/output described by jsonBody[T] requires a json codec.


Read on about custom types.