diff --git a/src/a2a/server/tasks/task_manager.py b/src/a2a/server/tasks/task_manager.py
index e5d899c1e..43c79bd5a 100644
--- a/src/a2a/server/tasks/task_manager.py
+++ b/src/a2a/server/tasks/task_manager.py
@@ -74,12 +74,14 @@ 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,
+        # We received a chunk to append, but there is no existing artifact
+        # with this id.  Silently dropping the chunk would hide a real bug
+        # in the caller (e.g. generating a fresh artifact_id on every
+        # add_artifact call instead of pinning one), so we raise.
+        raise ValueError(
+            f'append=True but no artifact with id {artifact_id!r} exists on '
+            f'task {task.id!r}. Create the artifact first (append=False) '
+            f'before appending to it.'
         )
 
 
diff --git a/tests/server/tasks/test_task_manager.py b/tests/server/tasks/test_task_manager.py
index eba8d2f14..d3412dc96 100644
--- a/tests/server/tasks/test_task_manager.py
+++ b/tests/server/tasks/test_task_manager.py
@@ -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
+    # raises ValueError instead of silently dropping the chunk (#1038)
     non_existing_artifact_with_parts = Artifact(
         artifact_id='artifact-456', parts=[Part(text='Part 1')]
     )
@@ -438,7 +439,9 @@ def test_append_artifact_to_task():
         task_id='123',
         context_id='123',
     )
-    append_artifact_to_task(task, append_event_5)
+    with pytest.raises(ValueError, match='append=True but no artifact with id'):
+        append_artifact_to_task(task, append_event_5)
+    # Artifacts unchanged — the bad chunk was not silently added
     assert len(task.artifacts) == 2
     assert len(task.artifacts[0].parts) == 2
     assert len(task.artifacts[1].parts) == 1