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io.ktor.features.ContentNegotiation.Configuration (extensions in package io.ktor.jackson)
io.ktor.jackson.JacksonConverter
    install(ContentNegotiation) {
       register(ContentType.Application.Json, JacksonConverter())
    }

    to be able to modify the objectMapper (eg. using specific modules and/or serializers and/or
    configuration options, you could use the following (as seen in the ktor-samples):

    install(ContentNegotiation) {
        jackson {
            configure(SerializationFeature.INDENT_OUTPUT, true)
            registerModule(JavaTimeModule())