diff --git a/src/common/lib/client/restannotations.ts b/src/common/lib/client/restannotations.ts
index b6f0f99804..f7d3cd021c 100644
--- a/src/common/lib/client/restannotations.ts
+++ b/src/common/lib/client/restannotations.ts
@@ -78,6 +78,8 @@ class RestAnnotations {
       headers = Defaults.defaultPostHeaders(client.options, { format }),
       params = {};
 
+    Utils.mixin(headers, client.options.headers);
+
     const requestBody = Utils.encodeBody([wireAnnotation], client._MsgPack, format);
 
     await Resource.post(
diff --git a/test/browser/modular.test.js b/test/browser/modular.test.js
index e44f9a74e5..b949db16be 100644
--- a/test/browser/modular.test.js
+++ b/test/browser/modular.test.js
@@ -889,7 +889,10 @@ function registerAblyModularTests(Helper) {
         it('doesn’t break when it receives an ANNOTATION ProtocolMessage', async function () {
           const helper = this.test.helper;
           const rxClient = new BaseRealtime(
-            helper.ablyClientOptions({ plugins: { WebSocketTransport, FetchRequest } }),
+            helper.ablyClientOptions({
+              clientId: Helper.randomString(10),
+              plugins: { WebSocketTransport, FetchRequest },
+            }),
           );
           const channelName = 'mutable:annotation-1';
 
@@ -902,6 +905,7 @@ function registerAblyModularTests(Helper) {
 
             const txClient = new BaseRealtime(
               this.test.helper.ablyClientOptions({
+                clientId: Helper.randomString(10),
                 plugins: {
                   WebSocketTransport,
                   FetchRequest,
@@ -935,6 +939,7 @@ function registerAblyModularTests(Helper) {
           const channelName = 'mutable:annotation-2';
           const rxClient = new BaseRealtime(
             helper.ablyClientOptions({
+              clientId: Helper.randomString(10),
               plugins: {
                 WebSocketTransport,
                 FetchRequest,
@@ -949,6 +954,7 @@ function registerAblyModularTests(Helper) {
           await helper.monitorConnectionThenCloseAndFinishAsync(async () => {
             const txRealtime = new BaseRealtime(
               this.test.helper.ablyClientOptions({
+                clientId: Helper.randomString(10),
                 plugins: {
                   WebSocketTransport,
                   FetchRequest,
@@ -958,6 +964,7 @@ function registerAblyModularTests(Helper) {
             );
             const txRest = new BaseRest(
               this.test.helper.ablyClientOptions({
+                clientId: Helper.randomString(10),
                 plugins: {
                   FetchRequest,
                   Annotations,
diff --git a/test/realtime/annotations.test.js b/test/realtime/annotations.test.js
index 3c36bc1d02..2136afce5d 100644
--- a/test/realtime/annotations.test.js
+++ b/test/realtime/annotations.test.js
@@ -14,13 +14,13 @@ define(['shared_helper', 'chai'], function (Helper, chai) {
           done(err);
           return;
         }
-        rest = helper.AblyRest();
+        rest = helper.AblyRest({ clientId: Helper.randomString(10) });
         done();
       });
     });
 
     beforeEach(async () => {
-      realtime = helper.AblyRealtime();
+      realtime = helper.AblyRealtime({ clientId: Helper.randomString(10) });
     });
 
     afterEach(async () => {