diff --git a/server-common/src/main/java/io/a2a/server/requesthandlers/DefaultRequestHandler.java b/server-common/src/main/java/io/a2a/server/requesthandlers/DefaultRequestHandler.java
index fdc6ee11d..5e72d224d 100644
--- a/server-common/src/main/java/io/a2a/server/requesthandlers/DefaultRequestHandler.java
+++ b/server-common/src/main/java/io/a2a/server/requesthandlers/DefaultRequestHandler.java
@@ -1001,7 +1001,7 @@ private CompletableFuture<Void> cleanupProducer(@Nullable CompletableFuture<Void
         });
     }
 
-    private MessageSendSetup initMessageSend(MessageSendParams params, ServerCallContext context) {
+    private MessageSendSetup initMessageSend(MessageSendParams params, ServerCallContext context) throws A2AError {
         // Build RequestContext FIRST to get the real taskId (auto-generated if not provided)
         // This eliminates the need for temporary IDs - we use the same UUID throughout
         RequestContext requestContext = requestContextBuilder.get()
@@ -1026,6 +1026,17 @@ private MessageSendSetup initMessageSend(MessageSendParams params, ServerCallCon
 
         Task task = taskManager.getTask();
         if (task != null) {
+            // Reject messages to tasks that are in a terminal state (completed, canceled, rejected, failed).
+            // Per A2A spec section 3.1.1 (CORE-SEND-002): the SDK MUST return UnsupportedOperationError
+            // before forwarding the message to the AgentExecutor.
+            if (task.status().state().isFinal()) {
+                throw new UnsupportedOperationError(
+                        null,
+                        "Cannot send message to task " + task.id() +
+                        " - task is in a terminal state: " + task.status().state(),
+                        null);
+            }
+
             // Validate contextId matches the existing task's contextId
             String messageContextId = params.message().contextId();
             if (messageContextId != null && !messageContextId.equals(task.contextId())) {
diff --git a/server-common/src/test/java/io/a2a/server/requesthandlers/DefaultRequestHandlerTest.java b/server-common/src/test/java/io/a2a/server/requesthandlers/DefaultRequestHandlerTest.java
index 1e3217380..c03b81dfc 100644
--- a/server-common/src/test/java/io/a2a/server/requesthandlers/DefaultRequestHandlerTest.java
+++ b/server-common/src/test/java/io/a2a/server/requesthandlers/DefaultRequestHandlerTest.java
@@ -41,6 +41,7 @@
 import io.a2a.spec.TaskStatus;
 import io.a2a.spec.TaskStatusUpdateEvent;
 import io.a2a.spec.TextPart;
+import io.a2a.spec.UnsupportedOperationError;
 
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
@@ -516,15 +517,23 @@ void testAuthRequired_Resubscription() throws Exception {
      * Test: Reject SendMessage with mismatching contextId and taskId.
      * When a message references an existing task but provides a different contextId,
      * the request must be rejected with an InvalidParamsError.
+     * The task must not be in a terminal state, or the terminal-state guard fires first.
      */
     @Test
     void testRejectMismatchingContextId() throws Exception {
-        // Arrange: Create an initial task to get valid identifiers
-        CountDownLatch agentCompleted = new CountDownLatch(1);
+        // Arrange: Create an initial task – agent stays active (working) so the task is NOT terminal
+        CountDownLatch agentStarted = new CountDownLatch(1);
+        CountDownLatch agentRelease = new CountDownLatch(1);
 
         agentExecutorExecute = (context, emitter) -> {
+            emitter.startWork();
+            agentStarted.countDown();
+            try {
+                agentRelease.await(10, TimeUnit.SECONDS);
+            } catch (InterruptedException e) {
+                Thread.currentThread().interrupt();
+            }
             emitter.complete();
-            agentCompleted.countDown();
         };
 
         Message initialMessage = Message.builder()
@@ -534,6 +543,164 @@ void testRejectMismatchingContextId() throws Exception {
             .parts(new TextPart("initial message"))
             .build();
 
+        // returnImmediately=true so onMessageSend returns quickly (on first event)
+        MessageSendParams initialParams = MessageSendParams.builder()
+            .message(initialMessage)
+            .configuration(DEFAULT_CONFIG)
+            .build();
+
+        EventKind result = requestHandler.onMessageSend(initialParams, NULL_CONTEXT);
+        assertInstanceOf(Task.class, result);
+        Task task = (Task) result;
+
+        // Wait until agent has started (task is in WORKING state, not terminal)
+        assertTrue(agentStarted.await(5, TimeUnit.SECONDS));
+
+        try {
+            // Act & Assert: Send a follow-up message with matching taskId but wrong contextId
+            // The task is still WORKING, so the terminal guard does NOT fire.
+            // The contextId mismatch guard should fire instead.
+            Message mismatchedMessage = Message.builder()
+                .messageId("msg-2")
+                .role(Message.Role.ROLE_USER)
+                .taskId(task.id())
+                .contextId("wrong-context-does-not-exist")
+                .parts(new TextPart("follow-up message"))
+                .build();
+
+            MessageSendParams mismatchedParams = MessageSendParams.builder()
+                .message(mismatchedMessage)
+                .configuration(DEFAULT_CONFIG)
+                .build();
+
+            InvalidParamsError error = assertThrows(InvalidParamsError.class,
+                () -> requestHandler.onMessageSend(mismatchedParams, NULL_CONTEXT));
+            assertTrue(error.getMessage().contains(task.id()));
+        } finally {
+            // Release agent so it completes and doesn't leak
+            agentRelease.countDown();
+        }
+    }
+
+    /**
+     * Helper: creates a task, drives it to the given terminal state, then asserts that a
+     * follow-up SendMessage to that task throws UnsupportedOperationError (CORE-SEND-002).
+     */
+    private void assertSendMessageToTerminalStateThrows(TaskState terminalState) throws Exception {
+        CountDownLatch agentCompleted = new CountDownLatch(1);
+
+        agentExecutorExecute = (context, emitter) -> {
+            switch (terminalState) {
+                case TASK_STATE_COMPLETED -> emitter.complete();
+                case TASK_STATE_CANCELED  -> emitter.cancel();
+                case TASK_STATE_REJECTED  -> emitter.reject();
+                // Use fail() (no-arg) which emits TaskStatusUpdateEvent(FAILED) via the normal path,
+                // ensuring the task state is persisted to the store before we query it.
+                case TASK_STATE_FAILED    -> emitter.fail();
+                default -> throw new IllegalArgumentException("Unexpected state: " + terminalState);
+            }
+            agentCompleted.countDown();
+        };
+
+        Message initialMessage = Message.builder()
+            .messageId("msg-initial-" + terminalState)
+            .role(Message.Role.ROLE_USER)
+            .parts(new TextPart("create task"))
+            .build();
+
+        EventKind result = requestHandler.onMessageSend(
+            MessageSendParams.builder().message(initialMessage).configuration(DEFAULT_CONFIG).build(),
+            NULL_CONTEXT);
+        assertInstanceOf(Task.class, result);
+        Task task = (Task) result;
+        final String finalTaskId = task.id();
+
+        assertTrue(agentCompleted.await(5, TimeUnit.SECONDS), "Agent should complete");
+        Thread.sleep(200); // allow MainEventBusProcessor to persist the final state
+
+        Task storedTask = taskStore.get(finalTaskId);
+        assertNotNull(storedTask);
+        assertEquals(terminalState, storedTask.status().state(),
+            "Task should be in " + terminalState + " state");
+
+        // Reset: agent executor must NOT be called on the follow-up
+        agentExecutorExecute = (context, emitter) -> {
+            throw new AssertionError("AgentExecutor must NOT be invoked for a terminal task");
+        };
+
+        Message followUpMessage = Message.builder()
+            .messageId("msg-followup-" + terminalState)
+            .role(Message.Role.ROLE_USER)
+            .taskId(finalTaskId)
+            .parts(new TextPart("follow-up to terminal task"))
+            .build();
+
+        MessageSendParams followUpParams = MessageSendParams.builder()
+            .message(followUpMessage)
+            .configuration(DEFAULT_CONFIG)
+            .build();
+
+        UnsupportedOperationError error = assertThrows(UnsupportedOperationError.class,
+            () -> requestHandler.onMessageSend(followUpParams, NULL_CONTEXT),
+            "Expected UnsupportedOperationError when sending message to a " + terminalState + " task");
+
+        assertNotNull(error.getMessage(), "Error message should not be null");
+        assertTrue(error.getMessage().contains(finalTaskId),
+            "Error message should reference the task id");
+    }
+
+    /**
+     * CORE-SEND-002: SendMessage to a completed task must return UnsupportedOperationError.
+     */
+    @Test
+    void testSendMessage_ToCompletedTask_ThrowsUnsupportedOperationError() throws Exception {
+        assertSendMessageToTerminalStateThrows(TaskState.TASK_STATE_COMPLETED);
+    }
+
+    /**
+     * CORE-SEND-002: SendMessage to a canceled task must return UnsupportedOperationError.
+     */
+    @Test
+    void testSendMessage_ToCanceledTask_ThrowsUnsupportedOperationError() throws Exception {
+        assertSendMessageToTerminalStateThrows(TaskState.TASK_STATE_CANCELED);
+    }
+
+    /**
+     * CORE-SEND-002: SendMessage to a rejected task must return UnsupportedOperationError.
+     */
+    @Test
+    void testSendMessage_ToRejectedTask_ThrowsUnsupportedOperationError() throws Exception {
+        assertSendMessageToTerminalStateThrows(TaskState.TASK_STATE_REJECTED);
+    }
+
+    /**
+     * CORE-SEND-002: SendMessage to a failed task must return UnsupportedOperationError.
+     */
+    @Test
+    void testSendMessage_ToFailedTask_ThrowsUnsupportedOperationError() throws Exception {
+        assertSendMessageToTerminalStateThrows(TaskState.TASK_STATE_FAILED);
+    }
+
+    /**
+     * Test: SendStreamingMessage to a task in a terminal state must also return UnsupportedOperationError
+     * (CORE-SEND-002, streaming path).
+     */
+    @Test
+    void testSendMessageStream_ToCompletedTask_ThrowsUnsupportedOperationError() throws Exception {
+        // Arrange: Create and complete an initial task
+        CountDownLatch agentCompleted = new CountDownLatch(1);
+
+        agentExecutorExecute = (context, emitter) -> {
+            emitter.complete();
+            agentCompleted.countDown();
+        };
+
+        Message initialMessage = Message.builder()
+            .messageId("msg-initial-stream")
+            .role(Message.Role.ROLE_USER)
+            .parts(new TextPart("create task for stream test"))
+            .build();
+
         MessageSendParams initialParams = MessageSendParams.builder()
             .message(initialMessage)
             .configuration(DEFAULT_CONFIG)
@@ -542,24 +709,35 @@ void testRejectMismatchingContextId() throws Exception {
         EventKind result = requestHandler.onMessageSend(initialParams, NULL_CONTEXT);
         assertInstanceOf(Task.class, result);
         Task task = (Task) result;
-        assertTrue(agentCompleted.await(5, TimeUnit.SECONDS));
 
-        // Act & Assert: Send a follow-up message with matching taskId but wrong contextId
-        Message mismatchedMessage = Message.builder()
-            .messageId("msg-2")
+        assertTrue(agentCompleted.await(5, TimeUnit.SECONDS), "Agent should complete");
+        Thread.sleep(200); // allow MainEventBusProcessor to persist
+
+        // Verify task is in terminal state
+        Task storedTask = taskStore.get(task.id());
+        assertNotNull(storedTask);
+        assertEquals(TaskState.TASK_STATE_COMPLETED, storedTask.status().state());
+
+        // Reset: agent executor must NOT be called
+        agentExecutorExecute = (context, emitter) -> {
+            throw new AssertionError("AgentExecutor must NOT be invoked for a terminal task");
+        };
+
+        // Act & Assert: streaming follow-up to a terminal task must also be rejected
+        Message followUpMessage = Message.builder()
+            .messageId("msg-followup-stream")
             .role(Message.Role.ROLE_USER)
             .taskId(task.id())
-            .contextId("wrong-context-does-not-exist")
-            .parts(new TextPart("follow-up message"))
+            .parts(new TextPart("streaming follow-up to completed task"))
             .build();
 
-        MessageSendParams mismatchedParams = MessageSendParams.builder()
-            .message(mismatchedMessage)
+        MessageSendParams followUpParams = MessageSendParams.builder()
+            .message(followUpMessage)
             .configuration(DEFAULT_CONFIG)
             .build();
 
-        InvalidParamsError error = assertThrows(InvalidParamsError.class,
-            () -> requestHandler.onMessageSend(mismatchedParams, NULL_CONTEXT));
-        assertTrue(error.getMessage().contains(task.id()));
+        assertThrows(UnsupportedOperationError.class,
+            () -> requestHandler.onMessageSendStream(followUpParams, NULL_CONTEXT),
+            "Expected UnsupportedOperationError when streaming message to a completed task");
     }
 }