feat: add formatting to the RemoteA2A agent so it filters out the previous agent responses and updates the context of the function calls and responses

a2a/src/main/java/com/google/adk/a2a/RemoteA2AAgent.java

@@ -171,6 +171,25 @@ public RemoteA2AAgent build() {
     }
   }
 
+  private Message.Builder newA2AMessage(Message.Role role, List<io.a2a.spec.Part<?>> parts) {
+    return new Message.Builder().messageId(UUID.randomUUID().toString()).role(role).parts(parts);
+  }
+
+  private Message prepareMessage(InvocationContext invocationContext) {
+    Event userCall = EventConverter.findUserFunctionCall(invocationContext.session().events());
+    if (userCall != null) {
+      ImmutableList<io.a2a.spec.Part<?>> parts =
+          EventConverter.contentToParts(userCall.content(), userCall.partial().orElse(false));
+      return newA2AMessage(Message.Role.USER, parts)
+          .taskId(EventConverter.taskId(userCall))
+          .contextId(EventConverter.contextId(userCall))
+          .build();
+    }
+    return newA2AMessage(
+            Message.Role.USER, EventConverter.messagePartsFromContext(invocationContext))
+        .build();
+  }
+
   @Override
   protected Flowable<Event> runAsyncImpl(InvocationContext invocationContext) {
     // Construct A2A Message from the last ADK event
@@ -181,14 +200,7 @@ protected Flowable<Event> runAsyncImpl(InvocationContext invocationContext) {
       return Flowable.empty();
     }
 
-    Optional<Message> a2aMessageOpt = EventConverter.convertEventsToA2AMessage(invocationContext);
-
-    if (a2aMessageOpt.isEmpty()) {
-      logger.warn("Failed to convert event to A2A message.");
-      return Flowable.empty();
-    }
-
-    Message originalMessage = a2aMessageOpt.get();
+    Message originalMessage = prepareMessage(invocationContext);
     String requestJson = serializeMessageToJson(originalMessage);
 
     return Flowable.create(

a2a/src/main/java/com/google/adk/a2a/converters/EventConverter.java

@@ -2,16 +2,19 @@
 
 import static com.google.common.collect.ImmutableList.toImmutableList;
 
+import com.google.adk.a2a.common.GenAiFieldMissingException;
 import com.google.adk.agents.InvocationContext;
+import com.google.adk.events.Event;
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.Iterables;
 import com.google.genai.types.Content;
-import io.a2a.spec.Message;
+import com.google.genai.types.FunctionResponse;
 import io.a2a.spec.Part;
 import java.util.Collection;
+import java.util.List;
 import java.util.Optional;
 import java.util.UUID;
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
+import org.jspecify.annotations.Nullable;
 
 /**
  * Converter for ADK Events to A2A Messages.
@@ -20,59 +23,176 @@
  * use in production code.
  */
 public final class EventConverter {
-  private static final Logger logger = LoggerFactory.getLogger(EventConverter.class);
+  public static final String ADK_TASK_ID_KEY = "adk_task_id";
+  public static final String ADK_CONTEXT_ID_KEY = "adk_context_id";
 
   private EventConverter() {}
 
+  private static String metadataValue(Event event, String key) {
+    if (event.customMetadata().isEmpty()) {
+      return "";
+    }
+    return event.customMetadata().get().stream()
+        .filter(m -> m.key().orElse("").equals(key))
+        .findFirst()
+        .map(m -> m.stringValue().orElse(""))
+        .orElse("");
+  }
+
   /**
-   * Converts an ADK InvocationContext to an A2A Message.
+   * Returns the task ID from the event.
    *
-   * <p>It combines all the events in the session, plus the user content, converted into A2A Parts,
-   * into a single A2A Message.
+   * <p>Task ID is stored in the event's custom metadata with the key {@link #ADK_TASK_ID_KEY}.
    *
-   * <p>If the context has no events, or no suitable content to build the message, an empty optional
-   * is returned.
-   *
-   * @param context The ADK InvocationContext to convert.
-   * @return The converted A2A Message.
+   * @param event The event to get the task ID from.
+   * @return The task ID, or an empty string if not found.
    */
-  public static Optional<Message> convertEventsToA2AMessage(InvocationContext context) {
-    if (context.session().events().isEmpty()) {
-      logger.warn("No events in session, cannot convert to A2A message.");
-      return Optional.empty();
-    }
-
-    ImmutableList.Builder<Part<?>> partsBuilder = ImmutableList.builder();
+  public static String taskId(Event event) {
+    return metadataValue(event, ADK_TASK_ID_KEY);
+  }
 
-    context
-        .session()
-        .events()
-        .forEach(
-            event ->
-                partsBuilder.addAll(
-                    contentToParts(event.content(), event.partial().orElse(false))));
-    partsBuilder.addAll(contentToParts(context.userContent(), false));
+  /**
+   * Returns the context ID from the event.
+   *
+   * <p>Context ID is stored in the event's custom metadata with the key {@link
+   * #ADK_CONTEXT_ID_KEY}.
+   *
+   * @param event The event to get the context ID from.
+   * @return The context ID, or an empty string if not found.
+   */
+  public static String contextId(Event event) {
+    return metadataValue(event, ADK_CONTEXT_ID_KEY);
+  }
 
-    ImmutableList<Part<?>> parts = partsBuilder.build();
+  /**
+   * Returns the last user function call event from the list of events.
+   *
+   * @param events The list of events to find the user function call event from.
+   * @return The user function call event, or null if not found.
+   */
+  public static @Nullable Event findUserFunctionCall(List<Event> events) {
+    Event candidate = Iterables.getLast(events);
+    if (!candidate.author().equals("user")) {
+      return null;
+    }
+    FunctionResponse functionResponse;
+    try {
+      functionResponse = findUserFunctionResponse(candidate);
+    } catch (GenAiFieldMissingException e) {
+      return null;
+    }
+    if (functionResponse == null || functionResponse.id().isEmpty()) {
+      return null;
+    }
+    for (int i = events.size() - 2; i >= 0; i--) {
+      Event event = events.get(i);
+      if (isUserFunctionCall(event, functionResponse.id().get())) {
+        return event;
+      }
+    }
+    return null;
+  }
 
-    if (parts.isEmpty()) {
-      logger.warn("No suitable content found to build A2A request message.");
-      return Optional.empty();
+  private static FunctionResponse findUserFunctionResponse(Event candidate) {
+    if (candidate.content().isEmpty() || candidate.content().get().parts().isEmpty()) {
+      throw new GenAiFieldMissingException("Event has no content or parts.");
     }
+    return candidate.content().get().parts().get().stream()
+        .filter(part -> part.functionResponse().isPresent())
+        .findFirst()
+        .orElseThrow(() -> new GenAiFieldMissingException("Event has no function response."))
+        .functionResponse()
+        .get();
+  }
 
-    return Optional.of(
-        new Message.Builder()
-            .messageId(UUID.randomUUID().toString())
-            .parts(parts)
-            .role(Message.Role.USER)
-            .build());
+  private static boolean isUserFunctionCall(Event event, String functionResponseId) {
+    if (event.content().isEmpty()) {
+      return false;
+    }
+    return event.content().get().parts().get().stream()
+        .anyMatch(
+            part ->
+                part.functionCall().isPresent()
+                    && part.functionCall().get().id().orElse("").equals(functionResponseId));
   }
 
+  /**
+   * Converts a GenAI Content object to a list of A2A Parts.
+   *
+   * @param content The GenAI Content object to convert.
+   * @param isPartial Whether the content is partial.
+   * @return A list of A2A Parts.
+   */
   public static ImmutableList<Part<?>> contentToParts(
       Optional<Content> content, boolean isPartial) {
     return content.flatMap(Content::parts).stream()
         .flatMap(Collection::stream)
         .map(part -> PartConverter.fromGenaiPart(part, isPartial))
         .collect(toImmutableList());
   }
+
+  /**
+   * Returns the parts from the context events that should be sent to the agent.
+   *
+   * <p>All session events from the previous remote agent response (or the beginning of the session
+   * in case of the first agent invocation) are included into the A2A message. Events from other
+   * agents are presented as user messages and rephased as if a user was telling what happened in
+   * the session up to the point.
+   *
+   * @param context The invocation context to get the parts from.
+   * @return A list of A2A Parts.
+   */
+  public static ImmutableList<Part<?>> messagePartsFromContext(InvocationContext context) {
+    if (context.session().events().isEmpty()) {
+      return ImmutableList.of();
+    }
+    List<Event> events = context.session().events();
+    int lastResponseIndex = -1;
+    String contextId = "";
+    for (int i = events.size() - 1; i >= 0; i--) {
+      Event event = events.get(i);
+      if (event.author().equals(context.agent().name())) {
+        lastResponseIndex = i;
+        contextId = contextId(event);
+        break;
+      }
+    }
+    ImmutableList.Builder<Part<?>> partsBuilder = ImmutableList.builder();
+    for (int i = lastResponseIndex + 1; i < events.size(); i++) {
+      Event event = events.get(i);
+      if (!event.author().equals("user") && !event.author().equals(context.agent().name())) {
+        event = presentAsUserMessage(event, contextId);
+      }
+      contentToParts(event.content(), event.partial().orElse(false)).forEach(partsBuilder::add);
+    }
+    return partsBuilder.build();
+  }
+
+  private static Event presentAsUserMessage(Event event, String contextId) {
+    Event.Builder userEvent =
+        new Event.Builder().id(UUID.randomUUID().toString()).invocationId(contextId).author("user");
+    ImmutableList<com.google.genai.types.Part> parts =
+        event.content().flatMap(Content::parts).stream()
+            .flatMap(Collection::stream)
+            // convert only non-thought parts to user message parts, skip thought parts as they are
+            // not meant to be shown to the user
+            .filter(part -> !part.thought().orElse(false))
+            .map(part -> PartConverter.remoteCallAsUserPart(event.author(), part))
+            .collect(toImmutableList());
+    if (parts.isEmpty()) {
+      return userEvent.build();
+    }
+    com.google.genai.types.Part forContext =
+        com.google.genai.types.Part.builder().text("For context:").build();
+    return userEvent
+        .content(
+            Content.builder()
+                .parts(
+                    ImmutableList.<com.google.genai.types.Part>builder()
+                        .add(forContext)
+                        .addAll(parts)
+                        .build())
+                .build())
+        .build();
+  }
 }

a2a/src/main/java/com/google/adk/a2a/converters/PartConverter.java

@@ -374,6 +374,50 @@ private static FilePart filePartToA2A(Part part, ImmutableMap.Builder<String, Ob
         metadata.buildOrThrow());
   }
 
+  /**
+   * Converts a remote call part to a user part.
+   *
+   * <p>Events are rephrased as if a user was telling what happened in the session up to the point.
+   * E.g.
+   *
+   * <pre>{@code
+   * For context:
+   * User said: Now help me with Z
+   * Agent A said: Agent B can help you with it!
+   * Agent B said: Agent C might know better.*
+   * }</pre>
+   *
+   * @param author The author of the part.
+   * @param part The part to convert.
+   * @return The converted part.
+   */
+  public static Part remoteCallAsUserPart(String author, Part part) {
+    if (part.text().isPresent()) {
+      String partText = String.format("[%s] said: %s", author, part.text().get());
+      return Part.builder().text(partText).build();
+    } else if (part.functionCall().isPresent()) {
+      FunctionCall functionCall = part.functionCall().get();
+      String partText =
+          String.format(
+              "[%s] called tool %s with parameters: %s",
+              author,
+              functionCall.name().orElse("<unknown>"),
+              functionCall.args().orElse(ImmutableMap.of()));
+      return Part.builder().text(partText).build();
+    } else if (part.functionResponse().isPresent()) {
+      FunctionResponse functionResponse = part.functionResponse().get();
+      String partText =
+          String.format(
+              "[%s] %s tool returned result: %s",
+              author,
+              functionResponse.name().orElse("<unknown>"),
+              functionResponse.response().orElse(ImmutableMap.of()));
+      return Part.builder().text(partText).build();
+    } else {
+      return part;
+    }
+  }
+
   @SuppressWarnings("unchecked") // safe conversion from objectMapper.readValue
   private static Map<String, Object> coerceToMap(Object value) {
     if (value == null) {

a2a/src/test/java/com/google/adk/a2a/RemoteA2AAgentTest.java

@@ -412,10 +412,11 @@ public void runAsync_constructsRequestWithHistory() {
         .sendMessage(messageCaptor.capture(), any(List.class), any(Consumer.class), any());
     Message message = messageCaptor.getValue();
     assertThat(message.getRole()).isEqualTo(Message.Role.USER);
-    assertThat(message.getParts()).hasSize(3);
+    assertThat(message.getParts()).hasSize(4);
     assertThat(((TextPart) message.getParts().get(0)).getText()).isEqualTo("hello");
-    assertThat(((TextPart) message.getParts().get(1)).getText()).isEqualTo("hi");
-    assertThat(((TextPart) message.getParts().get(2)).getText()).isEqualTo("how are you?");
+    assertThat(((TextPart) message.getParts().get(1)).getText()).isEqualTo("For context:");
+    assertThat(((TextPart) message.getParts().get(2)).getText()).isEqualTo("[model] said: hi");
+    assertThat(((TextPart) message.getParts().get(3)).getText()).isEqualTo("how are you?");
   }
 
   @Test