fix(a2a): resume a remote agent's task when the user answers its call

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

@@ -15,6 +15,7 @@
  */
 package com.google.adk.a2a.agent;
 
+import static com.google.common.base.Strings.emptyToNull;
 import static com.google.common.base.Strings.nullToEmpty;
 
 import com.fasterxml.jackson.core.JsonProcessingException;
@@ -30,6 +31,7 @@
 import com.google.adk.events.Event;
 import com.google.adk.utils.AgentEnums.AgentOrigin;
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.Iterables;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.genai.types.Content;
 import com.google.genai.types.CustomMetadata;
@@ -197,13 +199,17 @@ private Message.Builder newA2AMessage(
   }
 
   private Message prepareMessage(InvocationContext invocationContext) {
-    Event userCall = EventConverter.findUserFunctionCall(invocationContext.session().events());
-    if (userCall != null) {
+    List<Event> events = invocationContext.session().events();
+    Event callEvent = EventConverter.findUserFunctionCall(events);
+    if (callEvent != null) {
+      // The last event is the user's answer to the call.
+      Event answer = Iterables.getLast(events);
       ImmutableList<io.a2a.spec.Part<?>> parts =
-          EventConverter.contentToParts(userCall.content(), userCall.partial().orElse(false));
+          EventConverter.contentToParts(answer.content(), answer.partial().orElse(false));
+      // As in ADK Python, a call that came without ids, such as one in a plain message, sends none.
       return newA2AMessage(invocationContext, Message.Role.USER, parts)
-          .taskId(EventConverter.taskId(userCall))
-          .contextId(EventConverter.contextId(userCall))
+          .taskId(emptyToNull(EventConverter.taskId(callEvent)))
+          .contextId(emptyToNull(EventConverter.contextId(callEvent)))
           .build();
     }
     return newA2AMessage(

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

@@ -22,6 +22,7 @@
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Iterables;
 import com.google.genai.types.Content;
+import com.google.genai.types.CustomMetadata;
 import com.google.genai.types.FunctionResponse;
 import io.a2a.spec.Part;
 import java.util.Collection;
@@ -31,34 +32,40 @@
 
 /** Converter for ADK Events to A2A Messages. */
 public final class EventConverter {
+  /** Legacy task ID key, read only for events with neither {@code a2a:} ID key. */
   public static final String ADK_TASK_ID_KEY = "adk_task_id";
+
+  /** Legacy context ID key, read only for events with neither {@code a2a:} ID key. */
   public static final String ADK_CONTEXT_ID_KEY = "adk_context_id";
 
   private EventConverter() {}
 
   /**
    * Returns the task ID from the event.
    *
-   * <p>Task ID is stored in the event's custom metadata with the key {@link #ADK_TASK_ID_KEY}.
+   * <p>Task ID is stored in the event's custom metadata under {@code a2a:task_id}, where {@link
+   * ResponseConverter} and ADK Python write it. An event with neither {@code a2a:task_id} nor
+   * {@code a2a:context_id} is read from the legacy {@link #ADK_TASK_ID_KEY}.
    *
    * @param event The event to get the task ID from.
    * @return The task ID, or an empty string if not found.
    */
   public static String taskId(Event event) {
-    return metadataValue(event, ADK_TASK_ID_KEY);
+    return remoteId(event, AdkMetadataKey.TASK_ID.getType(), ADK_TASK_ID_KEY);
   }
 
   /**
    * 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}.
+   * <p>Context ID is stored in the event's custom metadata under {@code a2a:context_id}, where
+   * {@link ResponseConverter} and ADK Python write it. An event with neither {@code a2a:task_id}
+   * nor {@code a2a:context_id} is read from the legacy {@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);
+    return remoteId(event, AdkMetadataKey.CONTEXT_ID.getType(), ADK_CONTEXT_ID_KEY);
   }
 
   /**
@@ -196,14 +203,22 @@ private static Optional<Content> presentAsUserMessage(Event event) {
             .build());
   }
 
-  private static String metadataValue(Event event, String key) {
-    if (event.customMetadata().isEmpty()) {
-      return "";
-    }
-    return event.customMetadata().get().stream()
-        .filter(m -> m.key().map(k -> k.equals(key)).orElse(false))
-        .findFirst()
-        .flatMap(m -> m.stringValue())
+  /**
+   * Returns the ID under {@code key}, or under {@code legacyKey} if the event has neither new key.
+   */
+  private static String remoteId(Event event, String key, String legacyKey) {
+    boolean hasNewKeys =
+        findMetadata(event, AdkMetadataKey.TASK_ID.getType()).isPresent()
+            || findMetadata(event, AdkMetadataKey.CONTEXT_ID.getType()).isPresent();
+    return findMetadata(event, hasNewKeys ? key : legacyKey)
+        .flatMap(CustomMetadata::stringValue)
         .orElse("");
   }
+
+  private static Optional<CustomMetadata> findMetadata(Event event, String key) {
+    return event.customMetadata().stream()
+        .flatMap(List::stream)
+        .filter(m -> m.key().map(k -> k.equals(key)).orElse(false))
+        .findFirst();
+  }
 }

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

@@ -40,8 +40,10 @@
 import com.google.adk.sessions.Session;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.Iterables;
 import com.google.genai.types.Content;
 import com.google.genai.types.CustomMetadata;
+import com.google.genai.types.FunctionCall;
 import com.google.genai.types.FunctionResponse;
 import com.google.genai.types.Part;
 import io.a2a.client.Client;
@@ -652,6 +654,103 @@ public void runAsync_constructsRequestWithHistory() {
     assertThat(((TextPart) message.getParts().get(3)).getText()).isEqualTo("how are you?");
   }
 
+  @Test
+  public void runAsync_answeringRemoteCall_sendsAnswerInItsTask() {
+    // Recorded under the keys ResponseConverter writes.
+    Message message =
+        sendAnswerToRemoteCall(
+            ImmutableList.of(
+                CustomMetadata.builder().key("a2a:task_id").stringValue("task-1").build(),
+                CustomMetadata.builder().key("a2a:context_id").stringValue("context-1").build()));
+
+    assertThat(message.getTaskId()).isEqualTo("task-1");
+    assertThat(message.getContextId()).isEqualTo("context-1");
+    DataPart answer = (DataPart) Iterables.getOnlyElement(message.getParts());
+    assertThat(answer.getMetadata()).containsEntry("adk_type", "function_response");
+    assertThat(answer.getData()).containsEntry("id", "call-1");
+  }
+
+  @Test
+  public void runAsync_answeringRemoteCallWithoutIds_sendsNoIds() {
+    Message message = sendAnswerToRemoteCall(ImmutableList.of());
+
+    assertThat(message.getTaskId()).isNull();
+    assertThat(message.getContextId()).isNull();
+  }
+
+  /**
+   * Runs the agent on a session where the user answers a remote call recorded with {@code
+   * callMetadata}, and returns the message sent.
+   */
+  @SuppressWarnings("unchecked") // cast for Mockito
+  private Message sendAnswerToRemoteCall(ImmutableList<CustomMetadata> callMetadata) {
+    RemoteA2AAgent agent = createAgent();
+    Session session =
+        Session.builder("session-4")
+            .appName("demo")
+            .userId("user")
+            .events(
+                ImmutableList.of(
+                    Event.builder()
+                        .id("e1")
+                        .author("remote-agent")
+                        .content(
+                            Content.builder()
+                                .role("model")
+                                .parts(
+                                    ImmutableList.of(
+                                        Part.builder()
+                                            .functionCall(
+                                                FunctionCall.builder()
+                                                    .name("fn")
+                                                    .id("call-1")
+                                                    .build())
+                                            // As converted from the remote's function-call part.
+                                            .partMetadata(
+                                                ImmutableMap.of("adk_type", "function_call"))
+                                            .build()))
+                                .build())
+                        .customMetadata(callMetadata)
+                        .build(),
+                    Event.builder()
+                        .id("e2")
+                        .author("user")
+                        .content(
+                            Content.builder()
+                                .role("user")
+                                .parts(
+                                    ImmutableList.of(
+                                        Part.builder()
+                                            .functionResponse(
+                                                FunctionResponse.builder()
+                                                    .name("fn")
+                                                    .id("call-1")
+                                                    .response(ImmutableMap.of("result", "ok"))
+                                                    .build())
+                                            .build()))
+                                .build())
+                        .build()))
+            .build();
+    InvocationContext context =
+        InvocationContext.builder()
+            .sessionService(new InMemorySessionService())
+            .artifactService(new InMemoryArtifactService())
+            .pluginManager(new PluginManager())
+            .invocationId("invocation-4")
+            .agent(new TestAgent())
+            .session(session)
+            .runConfig(RunConfig.builder().build())
+            .build();
+    mockStreamResponse(consumer -> consumer.accept(createFinalEvent("ok"), agentCard));
+
+    var unused = agent.runAsync(context).toList().blockingGet();
+
+    ArgumentCaptor<Message> messageCaptor = ArgumentCaptor.forClass(Message.class);
+    verify(mockClient)
+        .sendMessage(messageCaptor.capture(), any(List.class), any(Consumer.class), any());
+    return messageCaptor.getValue();
+  }
+
   @Test
   @SuppressWarnings("unchecked") // cast for Mockito
   public void runAsync_constructsRequestWithFunctionResponse() {

a2a/src/test/java/com/google/adk/a2a/converters/EventConverterTest.java

@@ -53,6 +53,35 @@ public void testTaskId() {
     assertThat(EventConverter.taskId(e)).isEqualTo("task-123");
   }
 
+  @Test
+  public void taskIdAndContextId_readA2aKeysBeforeLegacyKeys() {
+    Event e =
+        Event.builder()
+            .customMetadata(
+                ImmutableList.of(
+                    metadata("a2a:task_id", "task-1"),
+                    metadata("a2a:context_id", "context-1"),
+                    metadata(EventConverter.ADK_TASK_ID_KEY, "legacy-task"),
+                    metadata(EventConverter.ADK_CONTEXT_ID_KEY, "legacy-context")))
+            .build();
+
+    assertThat(EventConverter.taskId(e)).isEqualTo("task-1");
+    assertThat(EventConverter.contextId(e)).isEqualTo("context-1");
+  }
+
+  @Test
+  public void taskId_eventWithA2aContextIdOnly_ignoresLegacyTaskId() {
+    Event e =
+        Event.builder()
+            .customMetadata(
+                ImmutableList.of(
+                    metadata("a2a:context_id", "context-1"),
+                    metadata(EventConverter.ADK_TASK_ID_KEY, "legacy-task")))
+            .build();
+
+    assertThat(EventConverter.taskId(e)).isEmpty();
+  }
+
   @Test
   public void testTaskId_empty() {
     Event e = Event.builder().build();
@@ -241,4 +270,8 @@ protected Flowable<Event> runLiveImpl(InvocationContext invocationContext) {
       return Flowable.empty();
     }
   }
+
+  private static CustomMetadata metadata(String key, String value) {
+    return CustomMetadata.builder().key(key).stringValue(value).build();
+  }
 }