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<@U4CP1J0QP> I stumbled on a file where you have done some work  <https://github.com/ktorio/ktor/blob/master/ktor-client/ktor-client-mock/jvm/test/io/ktor/client/tests/mock/MockEngineExtendedTests.kt> :slightly_smiling_face:

Do you happen to know if the MockEngine is thread safe? I'm seeing some flaky test where I fire of 10 concurrent requests towards the a mocked httpClient

```mockEngineConfig.addHandler { request -&gt;
    respondOk()
}
ktorMockEngine = MockEngine(mockEngineConfig)
val httpClient = HttpClient(ktorMockEngine)

// fire off 10 concurrent requests, ackording to the logs all of them are sent but I end up with
Assertions.assertThat(ktorMockEngine.requestHistory.size).isEqualTo(10)
Expecting:
 &lt;8&gt;
to be equal to:
 &lt;10&gt;```


actually I every now and then see this in the logs:
```10:00:09.259 [DefaultDispatcher-worker-8 @coroutine#8] ERROR c.m.o.s.core.sqs.SqsMessagePipeline - Unable to process message with id: '3'.
java.lang.IllegalStateException: Unhandled <http://localhost:8081/query>
	at io.ktor.client.engine.mock.MockEngine.execute(MockEngine.kt:44)```
Do I need to wait for the MockEngine to get initialized? Is there anyway I can check if that is ready?