diff --git a/test/framework/src/main/java/org/elasticsearch/test/ESTestCase.java b/test/framework/src/main/java/org/elasticsearch/test/ESTestCase.java
index 7601eb7629d1c..730e0e6136c4c 100644
--- a/test/framework/src/main/java/org/elasticsearch/test/ESTestCase.java
+++ b/test/framework/src/main/java/org/elasticsearch/test/ESTestCase.java
@@ -1202,6 +1202,7 @@ public static String randomAlphaOfLength(int codeUnits) {
 
     /**
      * Generate a random string containing only alphanumeric characters.
+     * <b>The locale for the string is {@link Locale#ROOT}.</b>
      * @param length the length of the string to generate
      * @return the generated string
      */
diff --git a/x-pack/plugin/inference/qa/inference-service-tests/src/javaRestTest/java/org/elasticsearch/xpack/inference/InferenceCrudIT.java b/x-pack/plugin/inference/qa/inference-service-tests/src/javaRestTest/java/org/elasticsearch/xpack/inference/InferenceCrudIT.java
index f94b9dd2b867f..2e280f644add5 100644
--- a/x-pack/plugin/inference/qa/inference-service-tests/src/javaRestTest/java/org/elasticsearch/xpack/inference/InferenceCrudIT.java
+++ b/x-pack/plugin/inference/qa/inference-service-tests/src/javaRestTest/java/org/elasticsearch/xpack/inference/InferenceCrudIT.java
@@ -21,6 +21,7 @@
 import java.io.IOException;
 import java.util.Iterator;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Objects;
 import java.util.Set;
@@ -317,7 +318,7 @@ public void testSupportedStream() throws Exception {
             var events = streamInferOnMockService(modelId, TaskType.COMPLETION, input);
 
             var expectedResponses = Stream.concat(
-                input.stream().map(String::toUpperCase).map(str -> "{\"completion\":[{\"delta\":\"" + str + "\"}]}"),
+                input.stream().map(s -> s.toUpperCase(Locale.ROOT)).map(str -> "{\"completion\":[{\"delta\":\"" + str + "\"}]}"),
                 Stream.of("[DONE]")
             ).iterator();
             assertThat(events.size(), equalTo((input.size() + 1) * 2));
@@ -356,7 +357,9 @@ public void testUnifiedCompletionInference() throws Exception {
     }
 
     private static Iterator<String> expectedResultsIterator(List<String> input) {
-        return Stream.concat(input.stream().map(String::toUpperCase).map(InferenceCrudIT::expectedResult), Stream.of("[DONE]")).iterator();
+        // The Locale needs to be ROOT to match what the test service is going to respond with
+        return Stream.concat(input.stream().map(s -> s.toUpperCase(Locale.ROOT)).map(InferenceCrudIT::expectedResult), Stream.of("[DONE]"))
+            .iterator();
     }
 
     private static String expectedResult(String input) {
diff --git a/x-pack/plugin/inference/qa/test-service-plugin/src/main/java/org/elasticsearch/xpack/inference/mock/TestStreamingCompletionServiceExtension.java b/x-pack/plugin/inference/qa/test-service-plugin/src/main/java/org/elasticsearch/xpack/inference/mock/TestStreamingCompletionServiceExtension.java
index 8325017f8e390..a39be00d9e7fa 100644
--- a/x-pack/plugin/inference/qa/test-service-plugin/src/main/java/org/elasticsearch/xpack/inference/mock/TestStreamingCompletionServiceExtension.java
+++ b/x-pack/plugin/inference/qa/test-service-plugin/src/main/java/org/elasticsearch/xpack/inference/mock/TestStreamingCompletionServiceExtension.java
@@ -43,6 +43,7 @@
 import java.util.EnumSet;
 import java.util.HashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.Flow;
@@ -142,7 +143,7 @@ public void unifiedCompletionInfer(
         }
 
         private StreamingChatCompletionResults makeResults(List<String> input) {
-            var responseIter = input.stream().map(String::toUpperCase).iterator();
+            var responseIter = input.stream().map(s -> s.toUpperCase(Locale.ROOT)).iterator();
             return new StreamingChatCompletionResults(subscriber -> {
                 subscriber.onSubscribe(new Flow.Subscription() {
                     @Override
@@ -173,7 +174,7 @@ private ChunkedToXContent completionChunk(String delta) {
         }
 
         private StreamingUnifiedChatCompletionResults makeUnifiedResults(UnifiedCompletionRequest request) {
-            var responseIter = request.messages().stream().map(message -> message.content().toString().toUpperCase()).iterator();
+            var responseIter = request.messages().stream().map(message -> message.content().toString().toUpperCase(Locale.ROOT)).iterator();
             return new StreamingUnifiedChatCompletionResults(subscriber -> {
                 subscriber.onSubscribe(new Flow.Subscription() {
                     @Override