diff --git a/lib/trinity/sim_server.ex b/lib/trinity/sim_server.ex
index 7a451f5..f50446d 100644
--- a/lib/trinity/sim_server.ex
+++ b/lib/trinity/sim_server.ex
@@ -31,6 +31,7 @@ defmodule Trinity.SimServer do
     end
   end
 
+  @spec receive_response(request_id, timeout) :: {:reply, term} | {:error, term} | :timeout
   def receive_response(request_id, timeout) do
     case get_sim() do
       nil -> :gen_server.receive_response(request_id, timeout)
@@ -38,6 +39,22 @@ defmodule Trinity.SimServer do
     end
   end
 
+  @spec cast(GenServer.server, term) :: :ok
+  def cast(server, request) do
+    case get_sim() do
+      nil -> GenServer.cast(server, request)
+      _sim -> sim_cast(server, request)
+    end
+  end
+
+  @spec reply(term, term) :: :ok
+  def reply(client, reply) do
+    case get_sim() do
+      nil -> GenServer.reply(client, reply)
+      _sim -> sim_reply(client, reply)
+    end
+  end
+
   defp sim_call(server, request, timeout) do
     req_id = send_request(server, request)
     case receive_response(req_id, timeout) do
@@ -66,11 +83,14 @@ defmodule Trinity.SimServer do
     result
   end
 
-  @spec cast(GenServer.server, term) :: :ok
-  def cast(server, request) do
-    case get_sim() do
-      nil -> GenServer.cast(server, request)
-      _sim -> :noop
-    end
+  defp sim_cast(server, request) do
+    SimProcess.send server, {:"$sim_cast", request}
+    :ok
+  end
+
+  defp sim_reply(client, reply) do
+    {_pid, [:alias | alias] = tag} = client
+    SimProcess.send alias, {tag, reply}
+    :ok
   end
 end