diff --git a/src/a2a/server/tasks/task_manager.py b/src/a2a/server/tasks/task_manager.py
index c9dfc879f..54b4563ff 100644
--- a/src/a2a/server/tasks/task_manager.py
+++ b/src/a2a/server/tasks/task_manager.py
@@ -19,6 +19,37 @@
 logger = logging.getLogger(__name__)
 
 
+TERMINAL_TASK_STATES = {
+    TaskState.TASK_STATE_COMPLETED,
+    TaskState.TASK_STATE_CANCELED,
+    TaskState.TASK_STATE_FAILED,
+    TaskState.TASK_STATE_REJECTED,
+}
+
+
+def validate_state_transition(
+    current_state: TaskState, new_state: TaskState
+) -> None:
+    """Validates a task state transition before it is persisted.
+
+    Terminal states are final: a task in COMPLETED/CANCELED/FAILED/REJECTED
+    must never move to a different state (including back to SUBMITTED).
+    Re-persisting the same terminal state is tolerated for idempotency.
+
+    Raises:
+        InvalidAgentResponseError: If the transition moves a task away from
+            a terminal state.
+    """
+    if current_state in TERMINAL_TASK_STATES and new_state != current_state:
+        raise InvalidAgentResponseError(
+            message=(
+                f'Illegal state transition from terminal state '
+                f'{TaskState.Name(current_state)} to '
+                f'{TaskState.Name(new_state)}'
+            )
+        )
+
+
 @trace_function()
 def append_artifact_to_task(task: Task, event: TaskArtifactUpdateEvent) -> None:
     """Helper method for updating a Task object with new artifact data from an event.
@@ -204,6 +235,7 @@ async def save_task_event(
             logger.debug(
                 'Updating task %s status to: %s', task.id, event.status.state
             )
+            validate_state_transition(task.status.state, event.status.state)
             if task.status.HasField('message'):
                 task.history.append(task.status.message)
             if event.metadata:
diff --git a/tests/server/tasks/test_task_manager.py b/tests/server/tasks/test_task_manager.py
index 7daa3ab3d..dff1b0f0c 100644
--- a/tests/server/tasks/test_task_manager.py
+++ b/tests/server/tasks/test_task_manager.py
@@ -444,3 +444,76 @@ def test_append_artifact_to_task():
         match='append=True for nonexistent artifact_id',
     ):
         append_artifact_to_task(task, append_event_5)
+
+
+@pytest.mark.parametrize(
+    ('current', 'new'),
+    [
+        (TaskState.TASK_STATE_COMPLETED, TaskState.TASK_STATE_SUBMITTED),
+        (TaskState.TASK_STATE_COMPLETED, TaskState.TASK_STATE_WORKING),
+        (TaskState.TASK_STATE_CANCELED, TaskState.TASK_STATE_SUBMITTED),
+        (TaskState.TASK_STATE_FAILED, TaskState.TASK_STATE_WORKING),
+        (TaskState.TASK_STATE_REJECTED, TaskState.TASK_STATE_COMPLETED),
+    ],
+)
+def test_validate_state_transition_blocks_terminal_escape(current, new):
+    """A terminal state must never transition to a different state."""
+    from a2a.server.tasks.task_manager import validate_state_transition
+
+    with pytest.raises(InvalidAgentResponseError):
+        validate_state_transition(current, new)
+
+
+@pytest.mark.parametrize(
+    ('current', 'new'),
+    [
+        (TaskState.TASK_STATE_COMPLETED, TaskState.TASK_STATE_COMPLETED),
+        (TaskState.TASK_STATE_SUBMITTED, TaskState.TASK_STATE_WORKING),
+        (TaskState.TASK_STATE_SUBMITTED, TaskState.TASK_STATE_COMPLETED),
+        (TaskState.TASK_STATE_WORKING, TaskState.TASK_STATE_COMPLETED),
+        (TaskState.TASK_STATE_WORKING, TaskState.TASK_STATE_INPUT_REQUIRED),
+        (TaskState.TASK_STATE_INPUT_REQUIRED, TaskState.TASK_STATE_WORKING),
+    ],
+)
+def test_validate_state_transition_allows_legal_transitions(current, new):
+    """Legal forward transitions (and idempotent repeats) are accepted."""
+    from a2a.server.tasks.task_manager import validate_state_transition
+
+    validate_state_transition(current, new)
+
+
+@pytest.mark.asyncio
+async def test_save_task_event_blocks_terminal_overwrite():
+    """save_task_event must reject a terminal -> non-terminal status update.
+
+    Regression test for BUG-43: the status was blindly overwritten, so a
+    misbehaving agent could move a completed task back to SUBMITTED.
+    """
+    from a2a.server.tasks import InMemoryTaskStore
+    from a2a.server.tasks.task_manager import TaskManager
+
+    task_store = InMemoryTaskStore()
+    task_manager = TaskManager(
+        task_id=MINIMAL_TASK_ID,
+        context_id=MINIMAL_CONTEXT_ID,
+        task_store=task_store,
+        initial_message=None,
+        context=TEST_CONTEXT,
+    )
+
+    task = create_minimal_task()
+    task.status.state = TaskState.TASK_STATE_COMPLETED
+    await task_store.save(task, TEST_CONTEXT)
+
+    illegal_event = TaskStatusUpdateEvent(
+        task_id=MINIMAL_TASK_ID,
+        context_id=MINIMAL_CONTEXT_ID,
+        status=TaskStatus(state=TaskState.TASK_STATE_SUBMITTED),
+    )
+
+    with pytest.raises(InvalidAgentResponseError):
+        await task_manager.save_task_event(illegal_event)
+
+    # The task state must be unchanged.
+    stored = await task_store.get(MINIMAL_TASK_ID, TEST_CONTEXT)
+    assert stored.status.state == TaskState.TASK_STATE_COMPLETED