diff --git a/packages/vertexai/src/requests/response-helpers.ts b/packages/vertexai/src/requests/response-helpers.ts
index 3521dc1bc5f..0435f1711ce 100644
--- a/packages/vertexai/src/requests/response-helpers.ts
+++ b/packages/vertexai/src/requests/response-helpers.ts
@@ -36,6 +36,7 @@ export function createEnhancedContentResponse(
    * The Vertex AI backend omits default values.
    * This causes the `index` property to be omitted from the first candidate in the
    * response, since it has index 0, and 0 is a default value.
+   * See: https://github.com/firebase/firebase-js-sdk/issues/8566
    */
   if (response.candidates && !response.candidates[0].hasOwnProperty('index')) {
     response.candidates[0].index = 0;
diff --git a/packages/vertexai/src/requests/stream-reader.ts b/packages/vertexai/src/requests/stream-reader.ts
index da8eca71880..8162407d90b 100644
--- a/packages/vertexai/src/requests/stream-reader.ts
+++ b/packages/vertexai/src/requests/stream-reader.ts
@@ -151,7 +151,9 @@ export function aggregateResponses(
   for (const response of responses) {
     if (response.candidates) {
       for (const candidate of response.candidates) {
-        const i = candidate.index;
+        // Index will be undefined if it's the first index (0), so we should use 0 if it's undefined.
+        // See: https://github.com/firebase/firebase-js-sdk/issues/8566
+        const i = candidate.index || 0;
         if (!aggregatedResponse.candidates) {
           aggregatedResponse.candidates = [];
         }
diff --git a/scripts/update_vertexai_responses.sh b/scripts/update_vertexai_responses.sh
index 27c32a09ead..101eac90d9f 100755
--- a/scripts/update_vertexai_responses.sh
+++ b/scripts/update_vertexai_responses.sh
@@ -17,7 +17,7 @@
 # This script replaces mock response files for Vertex AI unit tests with a fresh
 # clone of the shared repository of Vertex AI test data.
 
-RESPONSES_VERSION='v3.*' # The major version of mock responses to use
+RESPONSES_VERSION='v5.*' # The major version of mock responses to use
 REPO_NAME="vertexai-sdk-test-data"
 REPO_LINK="https://github.com/FirebaseExtended/$REPO_NAME.git"