kotlinx-schema-generator-json

Runtime JSON Schema generation from Kotlin classes and functions.

Concrete implementations of schema generators using Kotlin reflection (JVM) and kotlinx-serialization (Multiplatform).

Platform Support:

Multiplatform: Transformers and serialization-based generators (Common)
JVM only: Reflection-based generators (require kotlin-reflect)

Generators

Reflection-Based (JVM Only)

kotlinx.schema.generator.json.ReflectionClassJsonSchemaGenerator - generates schemas from any KClass via reflection
kotlinx.schema.generator.json.ReflectionFunctionCallingSchemaGenerator - generates function calling schemas from KCallable

Serialization-Based (Multiplatform)

SerializationClassJsonSchemaGenerator - generates schemas from @Serializable classes (works on JVM, Native, JS, Wasm)

Examples

Reflection Generator (JVM)

// Class schema generation
val generator = ReflectionClassJsonSchemaGenerator.Default
val schema: JsonObject = generator.generateSchema(User::class)

// Function calling schema generation
val funcGenerator = ReflectionFunctionCallingSchemaGenerator.Default
val funcSchema = funcGenerator.generateSchema(::myFunction)

Serialization Generator (Multiplatform)

import kotlinx.serialization.Serializable
import kotlinx.schema.generator.json.serialization.SerializationClassJsonSchemaGenerator

@Serializable
data class User(val name: String, val email: String? = null)

val generator = SerializationClassJsonSchemaGenerator.Default
val schema = generator.generateSchema(User.serializer().descriptor)

Features

Reflection Generators (JVM only):

Analyze third-party classes without source modification
Extract default values from data class properties
Recognize foreign annotations (Jackson, Koog, LangChain4j)
OpenAI/Anthropic function calling format
Sealed class hierarchies with oneOf

Serialization Generators (Multiplatform):

Works on all kotlinx-serialization targets (JVM, Native, JS, Wasm)
Consistent with kotlinx-serialization behavior
@SerialSchemaIgnore to exclude sealed subtypes from generated schemas

Limitations

Reflection Generators:

Function parameter defaults cannot be extracted (data class property defaults work)
JVM only

Serialization Generators:

Requires @Serializable annotation
Cannot extract actual default values (only detects presence)
@Description is not visible in SerialDescriptor (it lacks @SerialInfo). Use @SerialDescription instead, which is recognized automatically without any extra configuration.