Don't emit AG-UI text message content events with empty text part deltas (#2779)

Author: celeritatem

pydantic_ai_slim/pydantic_ai/ag_ui.py

@@ -488,10 +488,11 @@ async def _handle_model_request_event(  # noqa: C901
     elif isinstance(agent_event, PartDeltaEvent):
         delta = agent_event.delta
         if isinstance(delta, TextPartDelta):
-            yield TextMessageContentEvent(
-                message_id=stream_ctx.message_id,
-                delta=delta.content_delta,
-            )
+            if delta.content_delta:  # pragma: no branch
+                yield TextMessageContentEvent(
+                    message_id=stream_ctx.message_id,
+                    delta=delta.content_delta,
+                )
         elif isinstance(delta, ToolCallPartDelta):  # pragma: no branch
             assert delta.tool_call_id, '`ToolCallPartDelta.tool_call_id` must be set'
             yield ToolCallArgsEvent(

tests/test_ag_ui.py

@@ -864,6 +864,7 @@ async def stream_function(
     ) -> AsyncIterator[DeltaThinkingCalls | str]:
         yield {0: DeltaThinkingPart(content='')}
         yield "Let's do some thinking"
+        yield ''
         yield {1: DeltaThinkingPart(content='Thinking ')}
         yield {1: DeltaThinkingPart(content='about the weather')}
         yield {2: DeltaThinkingPart(content='')}