diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/controllers/KeysController.java b/service/src/main/java/org/whispersystems/textsecuregcm/controllers/KeysController.java
index efbf6d0b4..8549cdd70 100644
--- a/service/src/main/java/org/whispersystems/textsecuregcm/controllers/KeysController.java
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/controllers/KeysController.java
@@ -74,6 +74,8 @@ import org.whispersystems.textsecuregcm.storage.Device;
 import org.whispersystems.textsecuregcm.storage.KeysManager;
 import org.whispersystems.textsecuregcm.util.HeaderUtils;
 import org.whispersystems.textsecuregcm.util.Util;
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
 
 @SuppressWarnings("OptionalUsedAsFieldOrParameterType")
 @Path("/v2/keys")
@@ -388,43 +390,36 @@ public class KeysController {
     }
 
     final List<Device> devices = parseDeviceId(deviceId, target);
-    final List<PreKeyResponseItem> responseItems = new ArrayList<>(devices.size());
 
-    final List<CompletableFuture<Void>> tasks = devices.stream().map(device -> {
-          final CompletableFuture<Optional<ECPreKey>> unsignedEcPreKeyFuture =
-              keysManager.takeEC(targetIdentifier.uuid(), device.getId());
+    final List<PreKeyResponseItem> responseItems = Flux.fromIterable(devices)
+        .flatMap(device -> Mono.zip(
+            Mono.just(device),
+            Mono.fromFuture(keysManager.takeEC(targetIdentifier.uuid(), device.getId())),
+            Mono.fromFuture(keysManager.getEcSignedPreKey(targetIdentifier.uuid(), device.getId())),
+            Mono.fromFuture(keysManager.takePQ(targetIdentifier.uuid(), device.getId()))))
+        .flatMap(deviceAndPreKeys -> {
+          final Device device = deviceAndPreKeys.getT1();
+          final KEMSignedPreKey pqPreKey = deviceAndPreKeys.getT4().orElse(null);
+          final ECPreKey unsignedEcPreKey = deviceAndPreKeys.getT2().orElse(null);
+          final ECSignedPreKey signedEcPreKey = deviceAndPreKeys.getT3().orElse(null);
 
-          final CompletableFuture<Optional<ECSignedPreKey>> signedEcPreKeyFuture =
-              keysManager.getEcSignedPreKey(targetIdentifier.uuid(), device.getId());
+          Metrics.counter(GET_KEYS_COUNTER_NAME, Tags.of(
+                  UserAgentTagUtil.getPlatformTag(userAgent),
+                  Tag.of(IDENTITY_TYPE_TAG_NAME, targetIdentifier.identityType().name()),
+                  Tag.of("oneTimeEcKeyAvailable", String.valueOf(unsignedEcPreKey != null)),
+                  Tag.of("pqKeyAvailable", String.valueOf(pqPreKey != null))))
+              .increment();
 
-          final CompletableFuture<Optional<KEMSignedPreKey>> pqPreKeyFuture =
-              keysManager.takePQ(targetIdentifier.uuid(), device.getId());
-
-          return CompletableFuture.allOf(unsignedEcPreKeyFuture, signedEcPreKeyFuture, pqPreKeyFuture)
-              .thenAccept(ignored -> {
-                final KEMSignedPreKey pqPreKey = pqPreKeyFuture.join().orElse(null);
-                final ECPreKey unsignedEcPreKey = unsignedEcPreKeyFuture.join().orElse(null);
-                final ECSignedPreKey signedEcPreKey = signedEcPreKeyFuture.join().orElse(null);
-
-                Metrics.counter(GET_KEYS_COUNTER_NAME, Tags.of(
-                        UserAgentTagUtil.getPlatformTag(userAgent),
-                        Tag.of(IDENTITY_TYPE_TAG_NAME, targetIdentifier.identityType().name()),
-                        Tag.of("oneTimeEcKeyAvailable", String.valueOf(unsignedEcPreKey != null)),
-                        Tag.of("pqKeyAvailable", String.valueOf(pqPreKey != null))))
-                    .increment();
-
-                if (signedEcPreKey != null || unsignedEcPreKey != null || pqPreKey != null) {
-                  final int registrationId = device.getRegistrationId(targetIdentifier.identityType());
-
-                  responseItems.add(
-                      new PreKeyResponseItem(device.getId(), registrationId, signedEcPreKey, unsignedEcPreKey,
-                          pqPreKey));
-                }
-              });
+          if (signedEcPreKey != null || unsignedEcPreKey != null || pqPreKey != null) {
+            final int registrationId = device.getRegistrationId(targetIdentifier.identityType());
+            return Mono.just(new PreKeyResponseItem(
+                device.getId(), registrationId, signedEcPreKey, unsignedEcPreKey, pqPreKey));
+          } else {
+            return Mono.empty();
+          }
         })
-        .toList();
-
-    CompletableFuture.allOf(tasks.toArray(new CompletableFuture[0])).join();
+        .collectList()
+        .block();
 
     final IdentityKey identityKey = target.getIdentityKey(targetIdentifier.identityType());