diff --git a/src/a2a/server/tasks/task_manager.py b/src/a2a/server/tasks/task_manager.py
index e5d899c1e..c9dfc879f 100644
--- a/src/a2a/server/tasks/task_manager.py
+++ b/src/a2a/server/tasks/task_manager.py
@@ -12,7 +12,7 @@
     TaskStatus,
     TaskStatusUpdateEvent,
 )
-from a2a.utils.errors import InvalidParamsError
+from a2a.utils.errors import InvalidAgentResponseError, InvalidParamsError
 from a2a.utils.telemetry import trace_function
 
 
@@ -74,12 +74,16 @@ def append_artifact_to_task(task: Task, event: TaskArtifactUpdateEvent) -> None:
             dict(new_artifact_data.metadata.items())
         )
     else:
-        # We received a chunk to append, but we don't have an existing artifact.
-        # we will ignore this chunk
-        logger.warning(
-            'Received append=True for nonexistent artifact index %s in task %s. Ignoring chunk.',
-            artifact_id,
-            task.id,
+        # A2A spec (Section 4.2.2 TaskArtifactUpdateEvent,
+        # https://a2a-protocol.org/v1.0.0/specification/#422-taskartifactupdateevent):
+        #   "append — If true, the content of this artifact should be
+        #    appended to a previously sent artifact."
+        # append=True with no prior artifact of this ID is an agent
+        # protocol violation (#1038).
+        raise InvalidAgentResponseError(
+            f'append=True for nonexistent artifact_id={artifact_id!r} '
+            f'in task {task.id!r}. The artifact must be created (append=False) '
+            f'before appending parts to it.'
         )
 
 
diff --git a/tests/server/tasks/test_task_manager.py b/tests/server/tasks/test_task_manager.py
index eba8d2f14..7daa3ab3d 100644
--- a/tests/server/tasks/test_task_manager.py
+++ b/tests/server/tasks/test_task_manager.py
@@ -18,7 +18,7 @@
     TaskStatus,
     TaskStatusUpdateEvent,
 )
-from a2a.utils.errors import InvalidParamsError
+from a2a.utils.errors import InvalidAgentResponseError, InvalidParamsError
 
 
 class SampleUser(User):
@@ -429,6 +429,7 @@ def test_append_artifact_to_task():
     assert len(task.artifacts[1].parts) == 1
 
     # Test appending part to a task that does not have a matching artifact
+    # should raise InvalidAgentResponseError instead of silently dropping (#1038)
     non_existing_artifact_with_parts = Artifact(
         artifact_id='artifact-456', parts=[Part(text='Part 1')]
     )
@@ -438,7 +439,8 @@ def test_append_artifact_to_task():
         task_id='123',
         context_id='123',
     )
-    append_artifact_to_task(task, append_event_5)
-    assert len(task.artifacts) == 2
-    assert len(task.artifacts[0].parts) == 2
-    assert len(task.artifacts[1].parts) == 1
+    with pytest.raises(
+        InvalidAgentResponseError,
+        match='append=True for nonexistent artifact_id',
+    ):
+        append_artifact_to_task(task, append_event_5)