feat: add injectable time and UUID providers for timestamps and ids

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

@@ -48,12 +48,10 @@
 import io.reactivex.rxjava3.core.BackpressureStrategy;
 import io.reactivex.rxjava3.core.Flowable;
 import io.reactivex.rxjava3.core.FlowableEmitter;
-import java.time.Instant;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Objects;
 import java.util.Optional;
-import java.util.UUID;
 import java.util.function.BiConsumer;
 import org.jspecify.annotations.Nullable;
 import org.slf4j.Logger;
@@ -193,22 +191,25 @@ public boolean isStreaming() {
     return streaming;
   }
 
-  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.Builder newA2AMessage(
+      InvocationContext invocationContext, Message.Role role, List<io.a2a.spec.Part<?>> parts) {
+    return new Message.Builder().messageId(invocationContext.newUuid()).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)
+      return newA2AMessage(invocationContext, Message.Role.USER, parts)
           .taskId(EventConverter.taskId(userCall))
           .contextId(EventConverter.contextId(userCall))
           .build();
     }
     return newA2AMessage(
-            Message.Role.USER, EventConverter.messagePartsFromContext(invocationContext))
+            invocationContext,
+            Message.Role.USER,
+            EventConverter.messagePartsFromContext(invocationContext))
         .build();
   }
 
@@ -568,11 +569,11 @@ private Event createAggregatedEvent(Content content, @Nullable ClientEvent trigg
       }
 
       return Event.builder()
-          .id(UUID.randomUUID().toString())
+          .id(invocationContext.newUuid())
           .invocationId(invocationContext.invocationId())
           .author(agentName)
           .content(content)
-          .timestamp(Instant.now().toEpochMilli())
+          .timestamp(invocationContext.now().toEpochMilli())
           .customMetadata(aggMetadataBuilder.build())
           .build();
     }

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

@@ -27,7 +27,6 @@
 import java.util.Collection;
 import java.util.List;
 import java.util.Optional;
-import java.util.UUID;
 import org.jspecify.annotations.Nullable;
 
 /** Converter for ADK Events to A2A Messages. */
@@ -144,29 +143,36 @@ public static ImmutableList<Part<?>> messagePartsFromContext(InvocationContext c
     }
     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);
+      Optional<Content> content;
+      boolean partial;
       if (!event.author().equals("user") && !event.author().equals(context.agent().name())) {
-        event = presentAsUserMessage(event, contextId);
+        // Another agent's output is rephrased as a complete user message.
+        content = presentAsUserMessage(event);
+        partial = false;
+      } else {
+        content = event.content();
+        partial = event.partial().orElse(false);
       }
-      contentToParts(event.content(), event.partial().orElse(false)).forEach(partsBuilder::add);
+      contentToParts(content, partial).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");
+  /**
+   * Rephrases the content of an event authored by another agent as a user message, or returns empty
+   * when the event has no parts to present.
+   */
+  private static Optional<Content> presentAsUserMessage(Event event) {
     ImmutableList<com.google.genai.types.Part> parts =
         event.content().flatMap(Content::parts).stream()
             .flatMap(Collection::stream)
@@ -176,20 +182,18 @@ private static Event presentAsUserMessage(Event event, String contextId) {
             .map(part -> PartConverter.remoteCallAsUserPart(event.author(), part))
             .collect(toImmutableList());
     if (parts.isEmpty()) {
-      return userEvent.build();
+      return Optional.empty();
     }
     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();
+    return Optional.of(
+        Content.builder()
+            .parts(
+                ImmutableList.<com.google.genai.types.Part>builder()
+                    .add(forContext)
+                    .addAll(parts)
+                    .build())
+            .build());
   }
 
   private static String metadataValue(Event event, String key) {

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

@@ -44,12 +44,10 @@
 import io.a2a.spec.TaskArtifactUpdateEvent;
 import io.a2a.spec.TaskState;
 import io.a2a.spec.TaskStatusUpdateEvent;
-import java.time.Instant;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
 import java.util.Optional;
-import java.util.UUID;
 import org.jspecify.annotations.Nullable;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -402,12 +400,12 @@ private static void logDroppedMetadata(A2AMetadataKey key, Exception e) {
   private static Event emptyEvent(InvocationContext invocationContext) {
     Event.Builder builder =
         Event.builder()
-            .id(UUID.randomUUID().toString())
+            .id(invocationContext.newUuid())
             .invocationId(invocationContext.invocationId())
             .author(invocationContext.agent().name())
             .branch(invocationContext.branch().orElse(null))
             .content(Content.builder().role("user").parts(ImmutableList.of()).build())
-            .timestamp(Instant.now().toEpochMilli());
+            .timestamp(invocationContext.now().toEpochMilli());
     return builder.build();
   }
 
@@ -421,10 +419,10 @@ private static boolean hasParts(Event event) {
 
   private static Event.Builder remoteAgentEventBuilder(InvocationContext invocationContext) {
     return Event.builder()
-        .id(UUID.randomUUID().toString())
+        .id(invocationContext.newUuid())
         .invocationId(invocationContext.invocationId())
         .author(invocationContext.agent().name())
         .branch(invocationContext.branch().orElse(null))
-        .timestamp(Instant.now().toEpochMilli());
+        .timestamp(invocationContext.now().toEpochMilli());
   }
 }

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

@@ -67,6 +67,7 @@
 import io.reactivex.rxjava3.core.Maybe;
 import io.reactivex.rxjava3.plugins.RxJavaPlugins;
 import io.reactivex.rxjava3.subscribers.TestSubscriber;
+import java.time.Instant;
 import java.util.AbstractList;
 import java.util.ArrayList;
 import java.util.Collections;
@@ -76,6 +77,7 @@
 import java.util.Objects;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.BiConsumer;
 import java.util.function.Consumer;
@@ -220,6 +222,39 @@ public void runAsync_aggregatesPartialEvents() {
     assertResponseMetadata(finalEvent);
   }
 
+  @Test
+  @SuppressWarnings("unchecked") // cast for Mockito
+  public void runAsync_usesInvocationProvidersForMessageIdAndEvents() {
+    RemoteA2AAgent agent = createAgent();
+    AtomicInteger counter = new AtomicInteger();
+    InvocationContext providerContext =
+        invocationContext.toBuilder()
+            .instantSource(() -> Instant.ofEpochMilli(1234L))
+            .uuidProvider(() -> String.format("uuid-%04d", counter.getAndIncrement()))
+            .build();
+    mockStreamResponse(
+        consumer -> {
+          consumer.accept(createPartialEvent("Hello ", true, false), agentCard);
+          consumer.accept(createPartialEvent("World!", true, false), agentCard);
+          consumer.accept(createFinalEvent("Final artifact content"), agentCard);
+        });
+
+    List<Event> events = agent.runAsync(providerContext).toList().blockingGet();
+
+    ArgumentCaptor<Message> messageCaptor = ArgumentCaptor.forClass(Message.class);
+    verify(mockClient)
+        .sendMessage(messageCaptor.capture(), any(List.class), any(Consumer.class), any());
+    // The outbound A2A message id is the first id drawn from the invocation's provider: building
+    // the request parts draws none.
+    assertThat(messageCaptor.getValue().getMessageId()).isEqualTo("uuid-0000");
+    assertThat(events).hasSize(4);
+    assertThat(events.stream().map(Event::id).distinct()).hasSize(4);
+    for (Event event : events) {
+      assertThat(event.id()).startsWith("uuid-");
+      assertThat(event.timestamp()).isEqualTo(1234L);
+    }
+  }
+
   @Test
   public void runAsync_aggregatesInterleavedFunctionCalls() {
     RemoteA2AAgent agent = createAgent();

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

@@ -196,6 +196,7 @@ public void testMessagePartsFromContext() {
                         .build(),
                     Event.builder()
                         .author("other_agent")
+                        .partial(true)
                         .content(
                             Content.builder()
                                 .parts(ImmutableList.of(Part.builder().text("hey").build()))
@@ -208,12 +209,21 @@ public void testMessagePartsFromContext() {
             .session(session)
             .sessionService(new InMemorySessionService())
             .agent(agent)
+            // An explicit invocation id keeps build() from drawing one; rephrasing another agent's
+            // output must draw none.
+            .invocationId("invocation-id")
+            .uuidProvider(
+                () -> {
+                  throw new AssertionError("messagePartsFromContext must not draw ids");
+                })
             .build();
     ImmutableList<io.a2a.spec.Part<?>> parts = EventConverter.messagePartsFromContext(ctx);
 
     assertThat(parts).hasSize(2);
     assertThat(((TextPart) parts.get(0)).getText()).isEqualTo("For context:");
     assertThat(((TextPart) parts.get(1)).getText()).isEqualTo("[other_agent] said: hey");
+    // Another agent's output is rephrased as a complete message even when its event was partial.
+    assertThat(parts.get(1).getMetadata()).doesNotContainKey(A2AMetadataKey.PARTIAL.getType());
   }
 
   private static final class TestAgent extends BaseAgent {

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

@@ -49,6 +49,7 @@
 import io.a2a.spec.TaskStatusUpdateEvent;
 import io.a2a.spec.TextPart;
 import io.reactivex.rxjava3.core.Flowable;
+import java.time.Instant;
 import java.util.Arrays;
 import java.util.List;
 import java.util.Optional;
@@ -140,6 +141,41 @@ public void messageToEvent_convertsMessage() {
     assertThat(content.parts().get().get(0).text()).hasValue("test-message");
   }
 
+  @Test
+  public void messageToEvent_usesInvocationProvidersForIdAndTimestamp() {
+    InvocationContext providerContext =
+        invocationContext.toBuilder()
+            .instantSource(() -> Instant.ofEpochMilli(1234L))
+            .uuidProvider(() -> "fixed-uuid")
+            .build();
+    Message a2aMessage =
+        new Message.Builder()
+            .messageId("msg-1")
+            .role(Message.Role.USER)
+            .parts(ImmutableList.of(new TextPart("test-message")))
+            .build();
+
+    Event event = ResponseConverter.messageToEvent(a2aMessage, providerContext);
+
+    assertThat(event.id()).isEqualTo("fixed-uuid");
+    assertThat(event.timestamp()).isEqualTo(1234L);
+  }
+
+  @Test
+  public void taskToEvent_withNoMessage_emptyEventUsesInvocationProviders() {
+    InvocationContext providerContext =
+        invocationContext.toBuilder()
+            .instantSource(() -> Instant.ofEpochMilli(1234L))
+            .uuidProvider(() -> "fixed-uuid")
+            .build();
+    Task task = testTask().status(new TaskStatus(TaskState.WORKING, null, null)).build();
+
+    Event event = ResponseConverter.taskToEvent(task, providerContext);
+
+    assertThat(event.id()).isEqualTo("fixed-uuid");
+    assertThat(event.timestamp()).isEqualTo(1234L);
+  }
+
   @Test
   public void taskToEvent_withArtifacts_returnsEventFromLastArtifact() {
     io.a2a.spec.Part<?> a2aPart = new TextPart("Artifact content");

contrib/firestore-session-service/src/main/java/com/google/adk/sessions/FirestoreSessionService.java

@@ -17,6 +17,7 @@
 
 import com.google.adk.events.Event;
 import com.google.adk.events.EventActions;
+import com.google.adk.platform.UuidProvider;
 import com.google.adk.utils.ApiFutureUtils;
 import com.google.adk.utils.Constants;
 import com.google.api.core.ApiFuture;
@@ -38,6 +39,7 @@
 import io.reactivex.rxjava3.core.Maybe;
 import io.reactivex.rxjava3.core.Single;
 import java.time.Instant;
+import java.time.InstantSource;
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.HashSet;
@@ -64,6 +66,8 @@
 public class FirestoreSessionService implements BaseSessionService {
   private static final Logger logger = LoggerFactory.getLogger(FirestoreSessionService.class);
   private final Firestore firestore;
+  private final InstantSource instantSource;
+  private final UuidProvider uuidProvider;
   private static final String ROOT_COLLECTION_NAME = Constants.ROOT_COLLECTION_NAME;
   private static final String EVENTS_SUBCOLLECTION_NAME = Constants.EVENTS_SUBCOLLECTION_NAME;
   private static final String APP_STATE_COLLECTION = Constants.APP_STATE_COLLECTION;
@@ -81,7 +85,22 @@ public class FirestoreSessionService implements BaseSessionService {
 
   /** Constructor for FirestoreSessionService. */
   public FirestoreSessionService(Firestore firestore) {
-    this.firestore = firestore;
+    this(firestore, InstantSource.system(), UuidProvider.SYSTEM);
+  }
+
+  /**
+   * Creates a session service whose generated session IDs and {@code lastUpdateTime} values come
+   * from the given providers instead of {@link java.util.UUID#randomUUID()} and the wall clock.
+   *
+   * @param firestore The Firestore client.
+   * @param instantSource Supplies the {@code lastUpdateTime} of a newly created session.
+   * @param uuidProvider Supplies the session ID when {@code createSession} is not given one.
+   */
+  public FirestoreSessionService(
+      Firestore firestore, InstantSource instantSource, UuidProvider uuidProvider) {
+    this.firestore = Objects.requireNonNull(firestore, "firestore cannot be null");
+    this.instantSource = Objects.requireNonNull(instantSource, "instantSource cannot be null");
+    this.uuidProvider = Objects.requireNonNull(uuidProvider, "uuidProvider cannot be null");
   }
 
   /** Gets the sessions collection reference for a given userId. */
@@ -121,7 +140,7 @@ public Single<Session> createSession(
               Optional.ofNullable(sessionId)
                   .map(String::trim)
                   .filter(s -> !s.isEmpty())
-                  .orElseGet(() -> UUID.randomUUID().toString());
+                  .orElseGet(uuidProvider::newUuid);
 
           ConcurrentMap<String, Object> initialState =
               (state == null) ? new ConcurrentHashMap<>() : new ConcurrentHashMap<>(state);
@@ -131,7 +150,7 @@ public Single<Session> createSession(
               resolvedSessionId,
               initialState);
           List<Event> initialEvents = new ArrayList<>();
-          Instant now = Instant.now();
+          Instant now = instantSource.instant();
           Session newSession =
               Session.builder(resolvedSessionId)
                   .appName(appName)