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fix(langchain): neutralize write_todos tool message #37698

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fix(langchain): neutralize write_todos tool message #37698

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db7460c
fix(langchain): neutralize `write_todos` tool message
nick-hollon-lc May 26, 2026
dc13974
fix(langchain): collapse `_write_todos` `ToolMessage` to one line
nick-hollon-lc May 26, 2026
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6 changes: 2 additions & 4 deletions libs/langchain_v1/langchain/agents/middleware/todo.py
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Original file line number Diff line number Diff line change
Expand Up @@ -136,7 +136,7 @@ def write_todos(
return Command(
update={
"todos": todos,
"messages": [ToolMessage(f"Updated todo list to {todos}", tool_call_id=tool_call_id)],
"messages": [ToolMessage("Todo list updated.", tool_call_id=tool_call_id)],
}
)

Expand All @@ -149,9 +149,7 @@ def _write_todos(
return Command(
update={
"todos": todos,
"messages": [
ToolMessage(f"Updated todo list to {todos}", tool_call_id=runtime.tool_call_id)
],
"messages": [ToolMessage("Todo list updated.", tool_call_id=runtime.tool_call_id)],
}
)

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55 changes: 22 additions & 33 deletions libs/langchain_v1/tests/unit_tests/agents/middleware/implementations/test_todo.py
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Original file line number Diff line number Diff line change
Expand Up @@ -296,43 +296,32 @@ def mock_handler(req: ModelRequest) -> ModelResponse:


@pytest.mark.parametrize(
("todos", "expected_message"),
"todos",
[
([], "Updated todo list to []"),
(
[{"content": "Task 1", "status": "pending"}],
"Updated todo list to [{'content': 'Task 1', 'status': 'pending'}]",
),
(
[
{"content": "Task 1", "status": "pending"},
{"content": "Task 2", "status": "in_progress"},
],
(
"Updated todo list to ["
"{'content': 'Task 1', 'status': 'pending'}, "
"{'content': 'Task 2', 'status': 'in_progress'}]"
),
),
(
[
{"content": "Task 1", "status": "pending"},
{"content": "Task 2", "status": "in_progress"},
{"content": "Task 3", "status": "completed"},
],
(
"Updated todo list to ["
"{'content': 'Task 1', 'status': 'pending'}, "
"{'content': 'Task 2', 'status': 'in_progress'}, "
"{'content': 'Task 3', 'status': 'completed'}]"
),
),
[],
[{"content": "Task 1", "status": "pending"}],
[
{"content": "Task 1", "status": "pending"},
{"content": "Task 2", "status": "in_progress"},
],
[
{"content": "Task 1", "status": "pending"},
{"content": "Task 2", "status": "in_progress"},
{"content": "Task 3", "status": "completed"},
],
],
)
def test_todo_middleware_write_todos_tool_execution(
todos: list[dict[str, Any]], expected_message: str
todos: list[dict[str, Any]],
) -> None:
"""Test that the write_todos tool executes correctly."""
"""Test that the write_todos tool executes correctly.

The tool message is a neutral "Todo list updated." ack; the actual
state lives in the `todos` field of the Command update. Dumping the
full list inline (including `status: completed` entries) was priming
some models to interpret the tool result as "everything done, wrap
up briefly" instead of continuing to deliver the substantive answer.
"""
tool_call = {
"args": {"todos": todos},
"name": "write_todos",
Expand All @@ -341,7 +330,7 @@ def test_todo_middleware_write_todos_tool_execution(
}
result = write_todos.invoke(tool_call)
assert result.update["todos"] == todos
assert result.update["messages"][0].content == expected_message
assert result.update["messages"][0].content == "Todo list updated."


@pytest.mark.parametrize(
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