# Schemas

A GraphQL schema will be derived automatically at compile-time (no reflection) from the types present in your resolver. The table below shows how common Scala types are converted to GraphQL types.

Scala Type GraphQL Type
Boolean Boolean
Int Int
Float Float
Double Float
String String
java.util.UUID ID
Unit Unit (custom scalar)
Long Long (custom scalar)
BigInt BigInt (custom scalar)
BigDecimal BigDecimal (custom scalar)
java.time.Instant Instant (custom scalar)
java.time.LocalDate LocalDate (custom scalar)
java.time.LocalTime LocalTime (custom scalar)
java.time.LocalDateTime LocalDateTime (custom scalar)
java.time.OffsetDateTime OffsetDateTime (custom scalar)
java.time.ZonedDateTime ZonedDateTime (custom scalar)
Case Class Object
Sealed Trait Enum or Union
Option[A] Nullable A
List[A] List of A
Set[A] List of A
Seq[A] List of A
Vector[A] List of A
A => B A and B
(A, B) Object with 2 fields _1 and _2
Either[A, B] Object with 2 nullable fields left and right
Map[A, B] List of Object with 2 fields key and value
ZIO[R, Nothing, A] A
ZIO[R, Throwable, A] Nullable A
Future[A] Nullable A
ZStream[R, Throwable, A] A (subscription) or List of A (query, mutation)
Json (from Circe (opens new window)) Json (custom scalar, need import caliban.interop.circe.json._)
Json (from play-json (opens new window)) Json (custom scalar, need import caliban.interop.play.json._)

See the Custom Types section to find out how to support your own types.

If you want Caliban to support other standard types, feel free to file an issue (opens new window) or even a PR.

# Enums, unions, interfaces

A sealed trait will be converted to a different GraphQL type depending on its content:

  • a sealed trait with only case objects will be converted to an ENUM
  • a sealed trait with only case classes will be converted to a UNION

GraphQL does not support empty objects, so in case a sealed trait mixes case classes and case objects, a union type will be created and the case objects will have a "fake" field named _ which is not queryable.

sealed trait Origin
object Origin {
  case object EARTH extends Origin
  case object MARS  extends Origin
  case object BELT  extends Origin

The snippet above will produce the following GraphQL type:

enum Origin {

Here's an example of union:

sealed trait Role
object Role {
  case class Captain(shipName: String) extends Role
  case class Engineer(specialty: String) extends Role
  case object Mechanic extends Role

The snippet above will produce the following GraphQL type:

union Role = Captain | Engineer | Mechanic

type Captain {
  shipName: String!

type Engineer {
  specialty: String!

type Mechanic {
  _: Boolean!

If your type needs to be shared between multiple unions you can use the @GQLValueType annotation to have caliban proxy to another type beyond the sealed trait.

case class Pilot(callSign: String)

sealed trait Role
object Role {
  case class Captain(shipName: String) extends Role
  case class Engineer(specialty: String) extends Role
  case class Proxy(pilot: Pilot)

This will produce the following GraphQL Types:

union Role = Captain | Engineer | Pilot

type Captain {
  shipName: String!

type Engineer {
  specialty: String!

type Pilot {
  callSign: String!

If you prefer an Interface instead of a Union type, add the @GQLInterface annotation to your sealed trait. An interface will be created with all the fields that are common to the case classes extending the sealed trait, as long as they return the same type.

If you prefer to have a Union type instead of an Enum, even when the sealed trait contains only objects, add the @GQLUnion annotation.

# Case classes and sealed traits

The transformation between Scala types and GraphQL types is handled by a typeclass named Schema. As mentioned earlier, Caliban provides instances of Schema for all basic Scala types, but inevitably you will need to support your own types, in particular case classes and sealed traits.

Caliban is able to generate instances of Schema for case classes and sealed traits. You have two choices for doing that: auto derivation and semi-auto derivation.

# Auto derivation

Auto derivation is achieved easily by adding the following import:

import caliban.schema.Schema.auto._

Using this import, Caliban will generate Schema instances for all the case classes and sealed traits that are found inside your resolver.

Auto derivation limitations

Auto derivation is the easiest way to get started, but it has some drawbacks:

  • If a type is referenced in several places inside your resolver, a Schema will be generated for each occurrence, which can lead to longer compilation times and a high amount of generated code (a sign of this is that the compiler will suggest increasing -Xmax-inlines in Scala 3).
  • When a Schema is missing for a nested type inside your resolver, it can sometimes be difficult to find out which type is missing when using auto derivation, because the error message will mention the root type and not the nested one.
  • The macro that generates the Schema instances sometimes gets confused when there are a lot of nested or recursive types, and can mistakenly generate a Schema for types that already have a Schema in scope. For this reason, semi-auto derivation is recommended for non-trivial schemas.

# Semi-auto derivation

Semi-auto derivation is achieved as follows for each type that needs a Schema instance (MyClass in the example):

In Scala 3, derivation doesn't support value classes and opaque types. You can use Schema.genDebug to print the generated code in the console.

# Combining auto and semi-auto derivation

For some types such as enums, it might be desirable to use auto derivation to reduce boilerplate schema definitions:

# Deriving fields from case class methods (Scala 3 only)

In certain cases, your type might contain fields whose value depends on other fields. For example, you might have a Person type with a fullName field that is derived from the firstName and lastName fields. In this case, you can use the @GQLField annotation to indicate that the field should be derived from the method with the same name.

import caliban.schema.Schema
import caliban.schema.Annotations.GQLField

case class Person(
  firstName: String,
  lastName: String
) derives Schema.SemiAuto {
  @GQLField def fullName: String = s"$firstName $lastName"

This case class will generate the following GraphQL type:

type Person {
  firstName: String!
  lastName: String!
  fullName: String!

The methods annotated with @GQLField can return any type for which a Schema is defined for, including effects such as ZIO and ZQuery. In addition, you can use any other annotation that is supported for case class arguments, such as @GQLName, @GQLDescription and @GQLDeprecated.


Derivation of fields via the @GQLField annotation can be convenient in certain cases, but has the following limitations:

  • The method cannot take arguments. If you need to derive a field that requires arguments, you can return a function instead.
  • The method must be public (i.e. not private or protected).
  • It currently only works with methods (i.e., def). If you need to cache the output of the method, you can create a private lazy val and return it from the method.
  • It is not compatible with ahead-of-time compilation (e.g., generating a GraalVM native-image executable).

# Arguments

To declare a field that take arguments, create a dedicated case class representing the arguments and make the field a function from this class to the result type.

case class Character(name: String, origin: Origin)
case class FilterArgs(origin: Option[Origin])
case class Queries(characters: FilterArgs => List[Character])

The snippet above will produce the following GraphQL type:

type Queries {
  characters(origin: Origin): [Character!]!

Caliban provides auto-derivation for common types such as Int, String, List, Option, etc. but you can also support your own types by providing an implicit instance of caliban.schema.ArgBuilder that defines how incoming arguments from that types should be extracted. You also need a Schema for those types.

Derivation of ArgBuilder for case classes works similarly to Schema derivation. You can use auto derivation by adding the following import, or via the derives keyword (Scala 3 only):

Or you can use semi-auto derivation as follows:


There is no ArgBuilder for tuples. If you have multiple arguments, use a case class containing all of them instead of a tuple.

# Custom types

Caliban provides auto-derivation for common types such as Int, String, List, Option, etc. but you can also support your own types by providing an implicit instance of caliban.schema.Schema. Note that you don't have to do this if your types are just case classes composed of common types.

An easy way to do this is to reuse existing instances and use contramap to map from your type to the original type. Here's an example of creating an instance for refined (opens new window)'s NonEmptyString reusing existing instance for String (if you use refined, you might want to look at caliban-refined (opens new window)):

import caliban.schema._
implicit val nonEmptyStringSchema: Schema[Any, NonEmptyString] = Schema.stringSchema.contramap(_.value)

You can also use the scalarSchema helper to create your own scalar types, providing a name, an optional description, and a function from your type to a ResponseValue:

import caliban.schema._
import caliban.ResponseValue.ObjectValue

implicit val unitSchema: Schema[Any, Unit] = Schema.scalarSchema("Unit", None, None, None, _ => ObjectValue(Nil))

If you are using a custom type as part of the input you also have to provide an implicit instance of caliban.schema.ArgBuilder. For example here's how to do that for java.time.LocalDate:

import java.time.LocalDate
import scala.util.Try

import caliban.Value
import caliban.CalibanError.ExecutionError
import caliban.schema.ArgBuilder

implicit val localDateArgBuilder: ArgBuilder[LocalDate] = {
  case Value.StringValue(value) =>
      .fold(ex => Left(ExecutionError(s"Can't parse $value into a LocalDate", innerThrowable = Some(ex))), Right(_))
  case other => Left(ExecutionError(s"Can't build a LocalDate from input $other"))

Value classes (case class SomeWrapper(self: SomeType) extends AnyVal) will be unwrapped by default in Scala 2 (this is not supported by Scala 3 derivation).

# Effects

Fields can return ZIO effects. This allows you to leverage all the features provided by ZIO: timeouts, retries, access to ZIO environment, memoizing, etc. An effect will be run every time a query requiring the corresponding field is executed.

import zio._

type CharacterName = String
case class Character(name: CharacterName)
case class Queries(characters: Task[List[Character]],
                   character: CharacterName => RIO[Console, Character])

If you don't use ZIO environment (R = Any), there is nothing special to do to get it working.

If you require a ZIO environment and use Scala 2, you can't use Schema.gen or the import we saw previously because they expect R to be Any. Instead, you need to make a new object that extends caliban.schema.GenericSchema[R] for your custom R. Then you can use gen or auto from that object to generate your schema.

import caliban._
import caliban.schema._

type MyEnv = Console 

object customSchema extends GenericSchema[MyEnv]
import customSchema.auto._

// if you use semi-auto generation, use this instead:
// implicit val characterSchema: Schema[MyEnv, Character] = customSchema.gen
// implicit val queriesSchema: Schema[MyEnv, Queries] = customSchema.gen

val queries = Queries(ZIO.attempt(???), _ => ZIO.succeed(???))
val api = graphQL(RootResolver(queries))

If you require a ZIO environment and use Scala 3, things are simpler since you don't need GenericSchema. Just make sure to use Schema.gen with the proper R type parameter. To make sure Caliban uses the proper environment, you need to specify it explicitly to graphQL(...), unless you already have Schema instances for your root operations in scope.

val queries = Queries(ZIO.attempt(???), _ => ZIO.succeed(???))
val api = graphQL[MyEnv, Queries, Unit, Unit](RootResolver(queries))
// or
// implicit val queriesSchema: Schema[MyEnv, Queries] = Schema.gen
// val api = graphQL(RootResolver(queries)) // it will infer MyEnv thanks to the instance above

# Annotations

Caliban supports a few annotations to enrich data types:

  • @GQLName("name") allows you to specify a different name for a data type or a field.
  • @GQLInputName("name") allows you to specify a different name for a data type used as an input (by default, the suffix Input is appended to the type name).
  • @GQLDescription("description") lets you provide a description for a data type or field. This description will be visible when your schema is introspected.
  • @GQLDeprecated("reason") allows deprecating a field or an enum value.
  • @GQLExcluded allows you to hide a field from the generated schema.
  • @GQLInterface to force a sealed trait generating an interface instead of a union.
  • @GQLDirective(directive: Directive) to add a directive to a field or type.
  • @GQLValueType(isScalar) forces a type to behave as a value type for derivation. Meaning that caliban will ignore the outer type and take the first case class parameter as the real type. If isScalar is true, it will generate a scalar named after the case class (default: false).
  • @GQLDefault("defaultValue") allows you to specify a default value for an input field using GraphQL syntax. The default value will be visible in your schema's SDL and during introspection.

# Java 8 Time types

Caliban provides implicit Schema types for the standard java.time types, by default these will use the ISO standard strings for serialization and deserialization. However, you can customize this behavior by using explicit constructor available under the ArgBuilder companion object. For instance, you can specify an instantEpoch to handle instants which are encoded using a Long from the standard java epoch time (January 1st 1970 00:00:00). For some time formats you can also specify a specific DateTimeFormatter to handle your particular date time needs.

# Code generation

Caliban can automatically generate Scala code from a GraphQL schema.

In order to use this feature, add the caliban-codegen-sbt sbt plugin to your project/plugins.sbt file:

addSbtPlugin("com.github.ghostdogpr" % "caliban-codegen-sbt" % "2.6.0")

And enable it in your build.sbt file:


Then call the calibanGenSchema sbt command.

calibanGenSchema schemaPath outputPath [--scalafmtPath path] [--headers name:value,name2:value2] [--packageName name] [--effect fqdn.Effect] [--scalarMappings gqlType:f.q.d.n.Type,gqlType2:f.q.d.n.Type2] [--imports a.b.c._,c.d.E] [--abstractEffectType true|false]

calibanGenSchema project/schema.graphql src/main/MyAPI.scala

This command will create a Scala file in outputPath containing all the types defined in the provided GraphQL schema defined at schemaPath. Instead of a file, you can provide a URL and the schema will be obtained using introspection.

The generated code will be formatted with Scalafmt using the configuration defined by --scalafmtPath option (default: .scalafmt.conf). If you provide a URL for schemaPath, you can provide request headers with --headers option.

The package of the generated code is derived from the folder of outputPath. This can be overridden by providing an alternative package with the --packageName option.

By default, each Query and Mutation will be wrapped into a zio.UIO effect. This can be overridden by providing an alternative effect with the --effect option.

You can also indicate that the effect type is abstract via --abstractEffectType true, in which case Query will be replaced by Query[F[_]] and so on (note F will be used unless --effect <effect> is explicitly given in which case <effect> would be used in place of F).

By default the suffix Input is appended to the type name of input types in the derived schema. Use the --preserveInputNames flag to disable this.

If you use scala3, you can enable --addDerives flag to automatically add derives clauses to the generated code. It will add type class instance derivation that create schema.

In case you use derives and ZIO Environment other than Any, you need to pass type info or your schema generation will fail. Use the --envForDerives flag to pass in the type alias for your ZIO Environment.

If you want to force a mapping between a GraphQL type and a Scala class (such as scalars), you can use the --scalarMappings option. Also you can add additional imports by providing --imports option.

Since Caliban 1.3.0, you can generate schemas using an sbt sourceGenerator, which means your schemas will be generated every time you compile (or when you import your build into Metals (opens new window)). This can be configured with the same settings as the client generators, but you have to specify .genType(Codegen.GenType.Schema) in the calibanSettings entry for a given file.

# Building Schemas by hand

Sometimes for whatever reason schema generation fails. This can happen if your schema has co-recursive types and Magnolia is unable to generate a schema for them. In cases like these you may need to instead create your own schema by hand.

Consider the case where you have three types which create cyclical dependencies on one another

import zio.UIO

case class Group(id: String, users: UIO[List[User]], parent: UIO[Option[Group]], organization: UIO[Organization])
case class Organization(id: String, groups: UIO[List[Group]])
case class User(id: String, group: UIO[Group])

These three types all depend on one another and if you attempt to generate a schema from them you will either end up with compiler errors or you will end up with a nasty runtime error from a NullPointerException. To help the compiler out we can hand generate the types for these case classes instead.

import caliban.schema.Schema
import caliban.schema.Schema.{obj, field}

implicit lazy val groupSchema: Schema[Any, Group] = obj("Group", Some("A group of users"))(
  implicit ft =>
implicit lazy val orgSchema: Schema[Any, Organization] = obj("Organization", Some("An organization of groups"))(
  implicit ft =>

implicit lazy val userSchema: Schema[Any, User] = obj("User", Some("A user of the service"))(
  implicit ft =>