fix(sdk): subagents: update prompt and make fetching of last message more robust (#3406)

libs/deepagents/deepagents/middleware/subagents.py

@@ -12,7 +12,7 @@ from langchain.agents.middleware.types import AgentMiddleware, ContextT, ModelRe
 from langchain.agents.structured_output import ResponseFormat
 from langchain.tools import BaseTool, ToolRuntime
 from langchain_core.language_models import BaseChatModel
-from langchain_core.messages import HumanMessage, ToolMessage
+from langchain_core.messages import AIMessage, HumanMessage, ToolMessage
 from langchain_core.runnables import Runnable, RunnableConfig
 from langchain_core.tools import StructuredTool
 from langgraph.types import Command
@@ -168,7 +168,10 @@ class CompiledSubAgent(TypedDict):
     """
 
 
-DEFAULT_SUBAGENT_PROMPT = "In order to complete the objective that the user asks of you, you have access to a number of standard tools."
+DEFAULT_SUBAGENT_PROMPT = """In order to complete the objective that the user asks of you, you have access to a number of standard tools.
+
+The calling agent only sees your final assistant message, not your intermediate work, tool results, or status tracking. Ensure your final
+response contains the complete answer."""
 
 # State keys that are excluded when passing state to subagents and when returning
 # updates from subagents.
@@ -430,8 +433,17 @@ def _build_task_tool(  # noqa: C901, PLR0915
             else:
                 content = json.dumps(structured)
         else:
-            # Strip trailing whitespace to prevent API errors with Anthropic
-            content = result["messages"][-1].text.rstrip() if result["messages"][-1].text else ""
+            # Walk back to the last AIMessage with non-empty text. Anthropic
+            # occasionally emits a trailing empty `end_turn` AIMessage after a
+            # successful final tool call, which would otherwise be forwarded
+            # as an empty ToolMessage.
+            content = ""
+            for msg in reversed(result["messages"]):
+                if isinstance(msg, AIMessage):
+                    text = msg.text.rstrip() if msg.text else ""
+                    if text:
+                        content = text
+                        break
 
         return Command(
             update={

libs/deepagents/tests/unit_tests/test_subagents.py

@@ -1367,6 +1367,66 @@ class TestSubAgents:
         task_tool_message = tool_messages[0]
         assert task_tool_message.content == "Plain text result without structured response"
 
+    def test_fallback_skips_trailing_empty_ai_message(self) -> None:
+        """Skip a trailing empty AIMessage and use the last AIMessage with text.
+
+        Anthropic/Bedrock occasionally emits an empty `end_turn` AIMessage after
+        a successful final tool call. The middleware should walk back to the
+        prior AIMessage carrying the real answer instead of forwarding an empty
+        ToolMessage.
+        """
+        mock_subagent = RunnableLambda(
+            lambda _: {
+                "messages": [
+                    AIMessage(content="The real answer from the subagent."),
+                    AIMessage(content=""),
+                ],
+            }
+        )
+
+        parent_chat_model = GenericFakeChatModel(
+            messages=iter(
+                [
+                    AIMessage(
+                        content="",
+                        tool_calls=[
+                            {
+                                "name": "task",
+                                "args": {
+                                    "description": "Do work",
+                                    "subagent_type": "worker",
+                                },
+                                "id": "call_trailing_empty",
+                                "type": "tool_call",
+                            }
+                        ],
+                    ),
+                    AIMessage(content="Done"),
+                ]
+            )
+        )
+
+        agent = create_deep_agent(
+            model=parent_chat_model,
+            checkpointer=InMemorySaver(),
+            subagents=[
+                CompiledSubAgent(
+                    name="worker",
+                    description="A worker agent",
+                    runnable=mock_subagent,
+                ),
+            ],
+        )
+
+        result = agent.invoke(
+            {"messages": [HumanMessage(content="Test")]},
+            config={"configurable": {"thread_id": f"test-trailing-empty-{uuid.uuid4().hex}"}},
+        )
+
+        tool_messages = [msg for msg in result["messages"] if msg.type == "tool"]
+        assert len(tool_messages) == 1
+        assert tool_messages[0].content == "The real answer from the subagent."
+
     def test_subagent_streaming_emits_messages_and_updates_from_subgraph(self) -> None:
         """Test end-to-end subagent streaming with `subgraphs=True`.