diff --git a/clients/c/Makefile b/clients/c/Makefile
index 2c77cce..bfad189 100644
--- a/clients/c/Makefile
+++ b/clients/c/Makefile
@@ -5,9 +5,6 @@ LIBS ?= -lcurl -ljansson -luuid -lpthread
 
 all: libcosmopolite.so
 
-test: test.o cosmopolite.o
-	$(CC) $(LDFLAGS) -o test test.o cosmopolite.o $(LIBS)
-
 libcosmopolite.so: cosmopolite.o
 	$(CC) -shared $(LDFLAGS) -o libcosmopolite.so cosmopolite.o $(LIBS)
 
@@ -23,5 +20,8 @@ install: libcosmopolite.so cosmopolite.h
 clean:
 	rm -f test libcosmopolite.so *.o
 
-valgrind:
+test: test.o cosmopolite.o
+	$(CC) $(LDFLAGS) -o test test.o cosmopolite.o $(LIBS)
+
+runtest: test
 	valgrind --leak-check=full --show-reachable=yes --num-callers=20 --suppressions=suppressions ./test
diff --git a/clients/c/cosmopolite.c b/clients/c/cosmopolite.c
index 7b80853..6ff7fc5 100644
--- a/clients/c/cosmopolite.c
+++ b/clients/c/cosmopolite.c
@@ -180,6 +180,11 @@ static void cosmo_handle_message(cosmo *instance, json_t *event) {
 }
 
 static void cosmo_handle_event(cosmo *instance, json_t *event) {
+  json_t *event_id = json_object_get(event, "event_id");
+  if (event_id) {
+    json_array_append(instance->ack, event_id);
+  }
+
   const char *event_type = json_string_value(json_object_get(event, "event_type"));
   if (strcmp(event_type, "message") == 0) {
     cosmo_handle_message(instance, event);
@@ -194,13 +199,19 @@ static json_t *cosmo_command(const char *name, const json_t *arguments) {
 }
 
 // Takes ownership of commands.
-static json_t *cosmo_send_rpc(cosmo *instance, json_t *commands) {
-  // Always poll.
-  json_array_append_new(commands, cosmo_command("poll", json_array()));
+// Takes ownership of ack.
+static json_t *cosmo_send_rpc(cosmo *instance, json_t *commands, json_t *ack) {
+  json_t *int_commands = json_array();
 
-  char *request = cosmo_build_rpc(instance, commands);
+  // Always poll.
+  json_t *arguments = json_pack("{so}", "ack", ack);
+  json_array_append_new(int_commands, cosmo_command("poll", arguments));
+  json_array_extend(int_commands, commands);
+
+  char *request = cosmo_build_rpc(instance, int_commands);
 
   char *response = cosmo_send_http(instance, request);
+  json_decref(int_commands);
   if (!response) {
     return commands;
   }
@@ -243,7 +254,8 @@ static json_t *cosmo_send_rpc(cosmo *instance, json_t *commands) {
   json_t *to_retry = json_array();
   json_t *command;
   json_array_foreach(commands, index, command) {
-    json_t *command_response = json_array_get(command_responses, index);
+    // +1 for the poll offset from int_commands.
+    json_t *command_response = json_array_get(command_responses, index + 1);
     json_t *result = json_object_get(command_response, "result");
     if (!result) {
       fprintf(stderr, "response lacks \"result\" key\n");
@@ -251,24 +263,6 @@ static json_t *cosmo_send_rpc(cosmo *instance, json_t *commands) {
       continue;
     }
     if (!strcmp(json_string_value(result), "retry")) {
-      // One giant hack to change queued subscribe messages to ask for less data.
-      // Remove this once we get channels working; it's not worth the yuck.
-      const char *name = json_string_value(json_object_get(command, "command"));
-      if (!strcmp(name, "subscribe")) {
-        json_t *arguments = json_object_get(command, "arguments");
-        json_t *subject = json_object_get(arguments, "subject");
-        json_t *last_message = cosmo_get_last_message(instance, subject);
-        if (last_message) {
-          json_t *last_id = json_object_get(arguments, "last_id");
-          json_int_t last_id_val = last_id ? json_integer_value(last_id) : 0;
-          json_int_t local_last_id = json_integer_value(json_object_get(last_message, "id"));
-          if (local_last_id > last_id_val) {
-            json_object_del(arguments, "messages");
-            json_object_set_new(arguments, "last_id", json_integer(local_last_id));
-          }
-          json_decref(last_message);
-        }
-      }
       json_array_append(to_retry, command);
       continue;
     }
@@ -287,12 +281,14 @@ static void *cosmo_thread_main(void *arg) {
   while (!instance->shutdown) {
     json_t *commands = instance->command_queue;
     instance->command_queue = json_array();
+    json_t *ack = instance->ack;
+    instance->ack = json_array();
 
     instance->next_delay_ms = CYCLE_MS;
     instance->next_delay_ms += rand_r(&instance->seedp) % (instance->next_delay_ms / CYCLE_STAGGER_FACTOR);
 
     assert(!pthread_mutex_unlock(&instance->lock));
-    json_t *to_retry = cosmo_send_rpc(instance, commands);
+    json_t *to_retry = cosmo_send_rpc(instance, commands, ack);
     assert(!pthread_mutex_lock(&instance->lock));
 
     json_array_extend(instance->command_queue, to_retry);
@@ -456,6 +452,8 @@ cosmo *cosmo_create(const char *base_url, const char *client_id, const cosmo_cal
   instance->profile = NULL;
   instance->command_queue = json_array();
   assert(instance->command_queue);
+  instance->ack = json_array();
+  assert(instance->ack);
   instance->subscriptions = json_array();
   assert(instance->subscriptions);
   instance->next_delay_ms = 0;
@@ -475,6 +473,7 @@ void cosmo_shutdown(cosmo *instance) {
   assert(!pthread_mutex_destroy(&instance->lock));
   assert(!pthread_cond_destroy(&instance->cond));
   json_decref(instance->command_queue);
+  json_decref(instance->ack);
   json_decref(instance->subscriptions);
   free(instance->profile);
   curl_easy_cleanup(instance->curl);
diff --git a/clients/c/cosmopolite.h b/clients/c/cosmopolite.h
index b1b9071..77c02f5 100644
--- a/clients/c/cosmopolite.h
+++ b/clients/c/cosmopolite.h
@@ -24,6 +24,7 @@ typedef struct {
   bool shutdown;
   char *profile;
   json_t *command_queue;
+  json_t *ack;
   json_t *subscriptions;
   uint64_t next_delay_ms;
   unsigned int seedp;
diff --git a/clients/c/test.c b/clients/c/test.c
index 53f81e8..2c14d72 100644
--- a/clients/c/test.c
+++ b/clients/c/test.c
@@ -87,6 +87,7 @@ bool test_message_round_trip() {
 
   json_decref(subject);
   json_decref(message_out);
+
   cosmo_shutdown(client);
   return true;
 }