Content negotiation and serialization in Ktor Client

The ContentNegotiation plugin serves two primary purposes:

Negotiating media types between the client and server. For this, it uses the Accept and Content-Type headers.
Serializing/deserializing the content in a specific format when sending requests and receiving responses. Ktor supports the following formats out-of-the-box: JSON, XML, CBOR, and ProtoBuf.

Add dependencies

ContentNegotiation

To use ContentNegotiation, you need to include the ktor-client-content-negotiation artifact in the build script:

implementation("io.ktor:ktor-client-content-negotiation:$ktor_version")

implementation "io.ktor:ktor-client-content-negotiation:$ktor_version"

        
                    <dependency>
                        <groupId>io.ktor</groupId>
                        <artifactId>ktor-client-content-negotiation-jvm</artifactId>
                        <version>${ktor_version}</version>
                    </dependency>
                    
        

You can learn more about artifacts required by the Ktor client from Adding client dependencies.

Note that serializers for specific formats require additional artifacts. For example, kotlinx.serialization requires the ktor-serialization-kotlinx-json dependency for JSON. Depending on the included artifacts, Ktor chooses a default serializer automatically. If required, you can specify the serializer explicitly and configure it.

Serialization

Before using kotlinx.serialization converters, you need to add the Kotlin serialization plugin as described in the Setup section.

JSON

To serialize/deserialize JSON data, you can choose one of the following libraries: kotlinx.serialization, Gson, or Jackson.

Add the ktor-serialization-kotlinx-json artifact in the build script:

implementation("io.ktor:ktor-serialization-kotlinx-json:$ktor_version")

implementation "io.ktor:ktor-serialization-kotlinx-json:$ktor_version"

        
                    <dependency>
                        <groupId>io.ktor</groupId>
                        <artifactId>ktor-serialization-kotlinx-json-jvm</artifactId>
                        <version>${ktor_version}</version>
                    </dependency>
                    
        

Add the ktor-serialization-gson artifact in the build script:

implementation("io.ktor:ktor-serialization-gson:$ktor_version")

implementation "io.ktor:ktor-serialization-gson:$ktor_version"

        
                    <dependency>
                        <groupId>io.ktor</groupId>
                        <artifactId>ktor-serialization-gson-jvm</artifactId>
                        <version>${ktor_version}</version>
                    </dependency>
                    
        

Add the ktor-serialization-jackson artifact in the build script:

implementation("io.ktor:ktor-serialization-jackson:$ktor_version")

implementation "io.ktor:ktor-serialization-jackson:$ktor_version"

        
                    <dependency>
                        <groupId>io.ktor</groupId>
                        <artifactId>ktor-serialization-jackson-jvm</artifactId>
                        <version>${ktor_version}</version>
                    </dependency>
                    
        

XML

To serialize/deserialize XML, add the ktor-serialization-kotlinx-xml in the build script:

implementation("io.ktor:ktor-serialization-kotlinx-xml:$ktor_version")

implementation "io.ktor:ktor-serialization-kotlinx-xml:$ktor_version"

        
                    <dependency>
                        <groupId>io.ktor</groupId>
                        <artifactId>ktor-serialization-kotlinx-xml-jvm</artifactId>
                        <version>${ktor_version}</version>
                    </dependency>
                    
        

CBOR

To serialize/deserialize CBOR, add the ktor-serialization-kotlinx-cbor in the build script:

implementation("io.ktor:ktor-serialization-kotlinx-cbor:$ktor_version")

implementation "io.ktor:ktor-serialization-kotlinx-cbor:$ktor_version"

        
                    <dependency>
                        <groupId>io.ktor</groupId>
                        <artifactId>ktor-serialization-kotlinx-cbor-jvm</artifactId>
                        <version>${ktor_version}</version>
                    </dependency>
                    
        

ProtoBuf

To serialize/deserialize ProtoBuf, add the ktor-serialization-kotlinx-protobuf in the build script:

implementation("io.ktor:ktor-serialization-kotlinx-protobuf:$ktor_version")

implementation "io.ktor:ktor-serialization-kotlinx-protobuf:$ktor_version"

        
                    <dependency>
                        <groupId>io.ktor</groupId>
                        <artifactId>ktor-serialization-kotlinx-protobuf-jvm</artifactId>
                        <version>${ktor_version}</version>
                    </dependency>
                    
        

Install ContentNegotiation

To install ContentNegotiation, pass it to the install function inside a client configuration block:

val client = HttpClient(CIO) {
    install(ContentNegotiation)
}

Now you can configure the required JSON serializer.

Configure a serializer

JSON serializer

To register the JSON serializer in your application, call the json method:

import io.ktor.client.plugins.contentnegotiation.*
import io.ktor.serialization.kotlinx.json.*

val client = HttpClient(CIO) {
    install(ContentNegotiation) {
        json()
    }
}

In the json constructor, you can access the JsonBuilder API, for example:

        val client = HttpClient(CIO) {
            install(ContentNegotiation) {
                json(Json {
                    prettyPrint = true
                    isLenient = true
                })
            }
        }

You can find the full example here: client-json-kotlinx.

To register the Gson serializer in your application, call the gson method:

import io.ktor.client.plugins.contentnegotiation.*
import io.ktor.serialization.gson.*

val client = HttpClient(CIO) {
    install(ContentNegotiation) {
        gson()
    }
}

The gson method also allows you to adjust serialization settings provided by GsonBuilder.

To register the Jackson serializer in your application, call the jackson method:

import io.ktor.client.plugins.contentnegotiation.*
import io.ktor.serialization.jackson.*

val client = HttpClient(CIO) {
    install(ContentNegotiation) {
        jackson()
    }
}

The jackson method also allows you to adjust serialization settings provided by ObjectMapper.

XML serializer

To register the XML serializer in your application, call the xml method:

import io.ktor.client.plugins.contentnegotiation.*
import io.ktor.serialization.kotlinx.xml.*

val client = HttpClient(CIO) {
    install(ContentNegotiation) {
        xml()
    }
}

The xml method also allows you to access XML serialization settings, for example:

import io.ktor.client.plugins.contentnegotiation.*
import io.ktor.serialization.kotlinx.xml.*
import nl.adaptivity.xmlutil.*
import nl.adaptivity.xmlutil.serialization.*

val client = HttpClient(CIO) {
    install(ContentNegotiation) {
        xml(format = XML {
            xmlDeclMode = XmlDeclMode.Charset
        })
    }
}

CBOR serializer

To register the CBOR serializer in your application, call the cbor method:

import io.ktor.client.plugins.contentnegotiation.*
import io.ktor.serialization.kotlinx.cbor.*

val client = HttpClient(CIO) {
    install(ContentNegotiation) {
        cbor()
    }
}

The cbor method also allows you to access CBOR serialization settings provided by CborBuilder, for example:

import io.ktor.client.plugins.contentnegotiation.*
import io.ktor.serialization.kotlinx.cbor.*
import kotlinx.serialization.cbor.*

val client = HttpClient(CIO) {
    install(ContentNegotiation) {
        cbor(Cbor {
            ignoreUnknownKeys = true
        })
    }
}

ProtoBuf serializer

To register the ProtoBuf serializer in your application, call the protobuf method:

import io.ktor.client.plugins.contentnegotiation.*
import io.ktor.serialization.kotlinx.protobuf.*

val client = HttpClient(CIO) {
    install(ContentNegotiation) {
        protobuf()
    }
}

The protobuf method also allows you to access ProtoBuf serialization settings provided by ProtoBufBuilder, for example:

import io.ktor.client.plugins.contentnegotiation.*
import io.ktor.serialization.kotlinx.protobuf.*
import kotlinx.serialization.protobuf.*

val client = HttpClient(CIO) {
    install(ContentNegotiation) {
        protobuf(ProtoBuf {
            encodeDefaults = true
        })
    }
}

Receive and send data

Create a data class

To receive and send data, you need to have a data class, for example:

data class Customer(val id: Int, val firstName: String, val lastName: String)

If you use kotlinx.serialization, make sure that this class has the @Serializable annotation:

@Serializable
data class Customer(val id: Int, val firstName: String, val lastName: String)

Serializing/deserializing of the following types is supported by the kotlinx.serialization library:

Builtin classes
Deserializing of Sequences
Serializing of Flows

Send data

To send a class instance within a request body as JSON, assign this instance using setBody function and set the content type to application/json by calling contentType:

        val response: HttpResponse = client.post("http://localhost:8080/customer") {
            contentType(ContentType.Application.Json)
            setBody(Customer(3, "Jet", "Brains"))
        }

To send data as XML or CBOR, set contentType to ContentType.Application.Xml or ContentType.Application.Cbor, respectively.

Receive data

When a server sends a response with the application/json, application/xml, or application/cbor content, you can deserialize it by specifying a data class as a parameter of a function used to receive a response payload (body in the example below):

val customer: Customer = client.get("http://localhost:8080/customer/3").body()

You can find the full example here: client-json-kotlinx.

20 October 2025