diff --git a/core/src/main/java/com/google/adk/models/GeminiUtil.java b/core/src/main/java/com/google/adk/models/GeminiUtil.java
index 0ef0a50fd..540d0e06e 100644
--- a/core/src/main/java/com/google/adk/models/GeminiUtil.java
+++ b/core/src/main/java/com/google/adk/models/GeminiUtil.java
@@ -38,6 +38,8 @@ public final class GeminiUtil {
       "Continue output. DO NOT look at this line. ONLY look at the content before this line and"
           + " system instruction.";
 
+  private static final String AF_FUNCTION_CALL_ID_PREFIX = "adk-";
+
   private GeminiUtil() {}
 
   /**
@@ -170,12 +172,16 @@ public static LlmRequest removeClientFunctionCallId(LlmRequest llmRequest) {
   }
 
   private static Part removeClientFunctionCallIdFromPart(Part part) {
-    if (part.functionCall().isPresent() && part.functionCall().get().id().isPresent()) {
+    if (part.functionCall().isPresent()
+        && part.functionCall().get().id().isPresent()
+        && part.functionCall().get().id().get().startsWith(AF_FUNCTION_CALL_ID_PREFIX)) {
       return part.toBuilder()
           .functionCall(part.functionCall().get().toBuilder().clearId().build())
           .build();
     }
-    if (part.functionResponse().isPresent() && part.functionResponse().get().id().isPresent()) {
+    if (part.functionResponse().isPresent()
+        && part.functionResponse().get().id().isPresent()
+        && part.functionResponse().get().id().get().startsWith(AF_FUNCTION_CALL_ID_PREFIX)) {
       return part.toBuilder()
           .functionResponse(part.functionResponse().get().toBuilder().clearId().build())
           .build();
diff --git a/core/src/test/java/com/google/adk/models/GeminiUtilTest.java b/core/src/test/java/com/google/adk/models/GeminiUtilTest.java
index 31cbe76de..b0943aa50 100644
--- a/core/src/test/java/com/google/adk/models/GeminiUtilTest.java
+++ b/core/src/test/java/com/google/adk/models/GeminiUtilTest.java
@@ -460,7 +460,7 @@ public void removeClientFunctionCallId_stripsIds() {
             .functionCall(
                 FunctionCall.builder()
                     .name("foo")
-                    .id("id1")
+                    .id("adk-id1")
                     .args(ImmutableMap.of("key", "value"))
                     .build())
             .build();
@@ -469,7 +469,7 @@ public void removeClientFunctionCallId_stripsIds() {
             .functionResponse(
                 FunctionResponse.builder()
                     .name("bar")
-                    .id("id2")
+                    .id("adk-id2")
                     .response(ImmutableMap.of("key", "value"))
                     .build())
             .build();
@@ -490,6 +490,43 @@ public void removeClientFunctionCallId_stripsIds() {
     assertThat(resultPart2.functionResponse().get().name()).hasValue("bar");
   }
 
+  @Test
+  public void removeClientFunctionCallId_preservesNonClientIds() {
+    Part partWithFunctionCall =
+        Part.builder()
+            .functionCall(
+                FunctionCall.builder()
+                    .name("foo")
+                    .id("call_123")
+                    .args(ImmutableMap.of("key", "value"))
+                    .build())
+            .build();
+    Part partWithFunctionResponse =
+        Part.builder()
+            .functionResponse(
+                FunctionResponse.builder()
+                    .name("bar")
+                    .id("call_456")
+                    .response(ImmutableMap.of("key", "value"))
+                    .build())
+            .build();
+    LlmRequest request = toRequest(partWithFunctionCall, partWithFunctionResponse);
+
+    LlmRequest result = GeminiUtil.removeClientFunctionCallId(request);
+
+    assertThat(result.contents()).hasSize(1);
+    assertThat(result.contents().get(0).parts()).isPresent();
+    assertThat(result.contents().get(0).parts().get()).hasSize(2);
+    Part resultPart1 = result.contents().get(0).parts().get().get(0);
+    assertThat(resultPart1.functionCall()).isPresent();
+    assertThat(resultPart1.functionCall().get().id()).hasValue("call_123");
+    assertThat(resultPart1.functionCall().get().name()).hasValue("foo");
+    Part resultPart2 = result.contents().get(0).parts().get().get(1);
+    assertThat(resultPart2.functionResponse()).isPresent();
+    assertThat(resultPart2.functionResponse().get().id()).hasValue("call_456");
+    assertThat(resultPart2.functionResponse().get().name()).hasValue("bar");
+  }
+
   private static Content toContent(Part... parts) {
     return Content.builder().parts(ImmutableList.copyOf(parts)).build();
   }