If I create an object in swift, that is defined in kotlin/common or kotlin/ios module, is it considered frozen by defauly?

no

I have client api class in common module

class ClientApi(
    private val versionName: String
) {

    private val client = HttpClient()
    suspend fun getFrom(path: String) = client
        .get<String>(urlString = path)
}

I have an extension method in ios module:

fun ClientApi.getFrom(path: String, block: (Result) -> Unit) {
GlobalScope.launch(Dispatchers.Default) {
        val result = getFrom(path)
        launch(MainLoopDispatcher) {
            block(screen)
        }
    }
}

I initialize client in swift and use an extension method. I’ve received an exception

Caused by: kotlin.native.concurrent.InvalidMutabilityException: mutation attempt of frozen io.ktor.client.request.HttpRequestPipeline@17e66c8

Can it be a result of using coroutines of 1.3.3-mt-native branch?

yes, you're passing

client

across thread boundaries, which feezes it and its parent object