@@ -715,7 +715,8 @@ private Task doCancelTask(CancelTaskParams params, ServerCallContext context) th
 
     @Override
     @SuppressWarnings("NullAway")
-    public EventKind onMessageSend(MessageSendParams params, ServerCallContext context) throws A2AError {
+    public EventKind onMessageSend(MessageSendParams rawParams, ServerCallContext context) throws A2AError {
+        MessageSendParams params = normalizeBlankIds(rawParams);
         LOGGER.debug("onMessageSend - task: {}; context {}", params.message().taskId(), params.message().contextId());
         String msgTaskId = params.message().taskId();
         if (msgTaskId != null) {
@@ -927,7 +928,8 @@ public EventKind onMessageSend(MessageSendParams params, ServerCallContext conte
     @Override
     @SuppressWarnings("NullAway")
     public Flow.Publisher<StreamingEventKind> onMessageSendStream(
-            MessageSendParams params, ServerCallContext context) throws A2AError {
+            MessageSendParams rawParams, ServerCallContext context) throws A2AError {
+        MessageSendParams params = normalizeBlankIds(rawParams);
         LOGGER.debug("onMessageSendStream START - task: {}; context: {}; runningAgents: {}",
                 params.message().taskId(), params.message().contextId(), runningAgents.size());
         String msgTaskId = params.message().taskId();
@@ -1352,6 +1354,38 @@ private CompletableFuture<Void> cleanupProducer(@Nullable CompletableFuture<Void
         });
     }
 
+    /**
+     * Treats a blank {@code taskId}/{@code contextId} on the message the same as absent.
+     * <p>
+     * Proto3 scalar fields have no wire-level presence, so a client that never set these
+     * fields and a client that explicitly set them to {@code ""} are indistinguishable once
+     * the message crosses a transport that always serializes them (see {@code emptyToNull} in
+     * {@code A2ACommonFieldMapper} for the equivalent normalization on the gRPC mapping path).
+     * Without this, a blank {@code taskId} is mistaken for a reference to an existing task
+     * with id {@code ""} instead of a request to start a new task.
+     */
+    private static MessageSendParams normalizeBlankIds(MessageSendParams params) {
+        Message message = params.message();
+        String taskId = message.taskId();
+        String contextId = message.contextId();
+        boolean blankTaskId = taskId != null && taskId.isEmpty();
+        boolean blankContextId = contextId != null && contextId.isEmpty();
+        if (!blankTaskId && !blankContextId) {
+            return params;
+        }
+
+        Message normalizedMessage = Message.builder(message)
+                .taskId(blankTaskId ? null : taskId)
+                .contextId(blankContextId ? null : contextId)
+                .build();
+        return MessageSendParams.builder()
+                .message(normalizedMessage)
+                .configuration(params.configuration())
+                .metadata(params.metadata())
+                .tenant(params.tenant())
+                .build();
+    }
+
     @SuppressWarnings("NullAway") // shouldAddPushInfo guarantees pushConfigStore != null
     private MessageSendSetup initMessageSend(MessageSendParams params, ServerCallContext context) throws A2AError {
         Task task = authorizeTaskAccess(params, context);
@@ -1406,7 +1440,7 @@ private AgentExecutor resolveAgentExecutor(@Nullable String tenant) {
 
     @Override
     public void authorizeTaskAccess(@Nullable String requestedTaskId, ServerCallContext context, TaskOperation operation) throws A2AError {
-        if (requestedTaskId == null) {
+        if (requestedTaskId == null || requestedTaskId.isEmpty()) {
             return;
         }
         enforceRead(context, requestedTaskId, operation);
 
@@ -845,6 +845,115 @@ void testSendMessageStream_WithNonExistentTaskId_ThrowsTaskNotFoundError() {
             "Expected TaskNotFoundError when onMessageSendStream references a non-existent taskId");
     }
 
+    /**
+     * A blank (empty-string) taskId/contextId must be treated the same as absent (null).
+     * <p>
+     * Proto3 scalar fields have no wire-level presence, so a client that never set
+     * taskId/contextId and one that serialized them as "" are indistinguishable once the
+     * message crosses a JsonFormat-based transport (see {@code JSONRPCUtils}'s use of
+     * {@code alwaysPrintFieldsWithNoPresence()}). Before this fix, a blank taskId was
+     * mistaken for a reference to an existing task with id "", causing a spurious
+     * TaskNotFoundError instead of starting a new task.
+     */
+    @Test
+    void testSendMessage_WithBlankTaskId_TreatedAsNewTask() throws Exception {
+        CountDownLatch agentCompleted = new CountDownLatch(1);
+        agentExecutorExecute = (context, emitter) -> {
+            emitter.complete();
+            agentCompleted.countDown();
+        };
+
+        Message message = Message.builder()
+            .messageId("msg-blank-task-id")
+            .role(Message.Role.ROLE_USER)
+            .taskId("")
+            .contextId("")
+            .parts(new TextPart("hello"))
+            .build();
+
+        MessageSendParams params = MessageSendParams.builder()
+            .message(message)
+            .configuration(DEFAULT_CONFIG)
+            .build();
+
+        EventKind result = requestHandler.onMessageSend(params, NULL_CONTEXT);
+
+        assertInstanceOf(Task.class, result, "A blank taskId should start a new task, not fail lookup");
+        Task task = (Task) result;
+        assertNotNull(task.id());
+        assertFalse(task.id().isEmpty(), "A newly generated taskId must not be blank");
+        assertNotNull(task.contextId());
+        assertFalse(task.contextId().isEmpty(), "A newly generated contextId must not be blank");
+
+        assertTrue(agentCompleted.await(5, TimeUnit.SECONDS), "Agent should have been invoked for a new task");
+        assertNull(taskStore.get(""), "No task should ever be stored under the empty-string id");
+    }
+
+    /**
+     * Streaming counterpart of {@link #testSendMessage_WithBlankTaskId_TreatedAsNewTask()}.
+     */
+    @Test
+    void testSendMessageStream_WithBlankTaskId_TreatedAsNewTask() throws Exception {
+        CountDownLatch agentCompleted = new CountDownLatch(1);
+        agentExecutorExecute = (context, emitter) -> {
+            emitter.complete();
+            agentCompleted.countDown();
+        };
+
+        Message message = Message.builder()
+            .messageId("msg-stream-blank-task-id")
+            .role(Message.Role.ROLE_USER)
+            .taskId("")
+            .contextId("")
+            .parts(new TextPart("hello"))
+            .build();
+
+        MessageSendParams params = MessageSendParams.builder()
+            .message(message)
+            .configuration(DEFAULT_CONFIG)
+            .build();
+
+        CountDownLatch streamDone = new CountDownLatch(1);
+        AtomicReference<Throwable> errorRef = new AtomicReference<>();
+        AtomicReference<String> taskIdRef = new AtomicReference<>();
+
+        Flow.Publisher<StreamingEventKind> publisher =
+                requestHandler.onMessageSendStream(params, contextWithVersion("1.0"));
+        publisher.subscribe(new Flow.Subscriber<>() {
+            @Override
+            public void onSubscribe(Flow.Subscription s) {
+                s.request(Long.MAX_VALUE);
+            }
+
+            @Override
+            public void onNext(StreamingEventKind item) {
+                if (item instanceof Task t) {
+                    taskIdRef.set(t.id());
+                } else if (item instanceof TaskStatusUpdateEvent e) {
+                    taskIdRef.set(e.taskId());
+                }
+            }
+
+            @Override
+            public void onError(Throwable t) {
+                errorRef.set(t);
+                streamDone.countDown();
+            }
+
+            @Override
+            public void onComplete() {
+                streamDone.countDown();
+            }
+        });
+
+        assertTrue(streamDone.await(5, TimeUnit.SECONDS), "Stream should complete");
+        assertNull(errorRef.get(), "A blank taskId should start a new task, not fail lookup: " + errorRef.get());
+        assertTrue(agentCompleted.await(5, TimeUnit.SECONDS), "Agent should have been invoked for a new task");
+        assertNotNull(taskIdRef.get());
+        assertFalse(taskIdRef.get().isEmpty(), "A newly generated taskId must not be blank");
+        assertNull(taskStore.get(""), "No task should ever be stored under the empty-string id");
+    }
+
     /**
      * Verification for Codex adversarial review finding:
      * When a follow-up message includes taskId but omits contextId,
 
@@ -225,21 +225,21 @@ public Builder messageId(String messageId) {
         /**
          * Sets the conversation context identifier.
          *
-         * @param contextId the context identifier (optional)
+         * @param contextId the context identifier, or {@code null} to clear it (optional)
          * @return this builder for method chaining
          */
-        public Builder contextId(String contextId) {
+        public Builder contextId(@Nullable String contextId) {
             this.contextId = contextId;
             return this;
         }
 
         /**
          * Sets the task identifier this message is associated with.
          *
-         * @param taskId the task identifier (optional)
+         * @param taskId the task identifier, or {@code null} to clear it (optional)
          * @return this builder for method chaining
          */
-        public Builder taskId(String taskId) {
+        public Builder taskId(@Nullable String taskId) {
             this.taskId = taskId;
             return this;
         }