fix(core): align Gemini placeholder user turns with ADK Python

core/src/main/java/com/google/adk/models/GeminiUtil.java

@@ -35,9 +35,20 @@
 /** Request / Response utilities for {@link Gemini}. */
 public final class GeminiUtil {
 
+  /**
+   * Text of the user turn appended when a request has no contents, so that the model acts on the
+   * system instruction. Same wording as ADK Python and ADK TypeScript.
+   */
+  public static final String HANDLE_SYSTEM_INSTRUCTION_MESSAGE =
+      "Handle the requests as specified in the System Instruction.";
+
+  /**
+   * Text of the user turn appended when the last content is not from the user, so that the model
+   * keeps producing output. Same wording as ADK Python, ADK TypeScript and ADK Go.
+   */
   public static final String CONTINUE_OUTPUT_MESSAGE =
-      "Continue output. DO NOT look at this line. ONLY look at the content before this line and"
-          + " system instruction.";
+      "Continue processing previous requests as instructed. Exit or provide a summary if no more"
+          + " outputs are needed.";
 
   private GeminiUtil() {}
 
@@ -193,9 +204,10 @@ private static Part removeClientFunctionCallIdFromPart(Part part) {
    * Ensures that the content is conducive to prompting a model response by ensuring the last
    * content part is from the user.
    *
-   * <p>If the list is empty or the last message is not from the user, a new "user" content part
-   * with a {@link #CONTINUE_OUTPUT_MESSAGE} is appended to the list. This is necessary to prompt
-   * the model to generate a response.
+   * <p>If the list is empty, a new "user" content part with {@link
+   * #HANDLE_SYSTEM_INSTRUCTION_MESSAGE} is appended. If the last message is not from the user, a
+   * new "user" content part with {@link #CONTINUE_OUTPUT_MESSAGE} is appended. This is necessary to
+   * prompt the model to generate a response.
    *
    * @param contents The original list of {@link Content}.
    * @return A list of {@link Content} where the last element is guaranteed to be from the "user".
@@ -204,11 +216,10 @@ static List<Content> ensureModelResponse(List<Content> contents) {
     // Last content must be from the user, otherwise the model won't respond.
     if (contents.isEmpty()
         || !Ascii.equalsIgnoreCase(Iterables.getLast(contents).role().orElse(""), Role.USER)) {
+      String text =
+          contents.isEmpty() ? HANDLE_SYSTEM_INSTRUCTION_MESSAGE : CONTINUE_OUTPUT_MESSAGE;
       Content userContent =
-          Content.builder()
-              .parts(ImmutableList.of(Part.fromText(CONTINUE_OUTPUT_MESSAGE)))
-              .role(Role.USER)
-              .build();
+          Content.builder().parts(ImmutableList.of(Part.fromText(text))).role(Role.USER).build();
       return Stream.concat(contents.stream(), Stream.of(userContent)).collect(toImmutableList());
     }
     return contents;

core/src/test/java/com/google/adk/models/GeminiUtilTest.java

@@ -37,8 +37,16 @@
 @RunWith(JUnit4.class)
 public final class GeminiUtilTest {
 
+  // Same wording as the user turns that ADK Python appends in
+  // BaseLlm._maybe_append_user_content.
+  private static final Content SYSTEM_INSTRUCTION_CONTENT =
+      Content.fromParts(
+          Part.fromText("Handle the requests as specified in the System Instruction."));
   private static final Content CONTINUE_CONTENT =
-      Content.fromParts(Part.fromText(GeminiUtil.CONTINUE_OUTPUT_MESSAGE));
+      Content.fromParts(
+          Part.fromText(
+              "Continue processing previous requests as instructed. Exit or provide a summary if"
+                  + " no more outputs are needed."));
 
   @Test
   public void getPart0FromLlmResponse_noContent_returnsEmpty() {
@@ -334,12 +342,12 @@ public void sanitizeRequestForGeminiApi_multipleContents_sanitizesAll() {
   }
 
   @Test
-  public void ensureModelResponse_emptyList_appendsContinueMessage() {
+  public void ensureModelResponse_emptyList_appendsSystemInstructionMessage() {
     ImmutableList<Content> contents = ImmutableList.of();
 
     List<Content> result = GeminiUtil.ensureModelResponse(contents);
 
-    assertThat(result).containsExactly(CONTINUE_CONTENT);
+    assertThat(result).containsExactly(SYSTEM_INSTRUCTION_CONTENT);
   }
 
   @Test
@@ -405,12 +413,13 @@ public void ensureModelResponse_lastContentIsNotUser_appendsContinueMessage() {
   }
 
   @Test
-  public void prepareGenenerateContentRequest_emptyRequest_returnsRequestWithContinueContent() {
+  public void
+      prepareGenenerateContentRequest_emptyRequest_returnsRequestWithSystemInstructionContent() {
     LlmRequest request = LlmRequest.builder().build();
 
     LlmRequest result = GeminiUtil.prepareGenenerateContentRequest(request, true);
 
-    assertThat(result.contents()).containsExactly(CONTINUE_CONTENT);
+    assertThat(result.contents()).containsExactly(SYSTEM_INSTRUCTION_CONTENT);
     assertThat(result.config()).isEmpty();
   }