plan-and-execute example => TypeError: can only concatenate list (not "tuple") to list

[nick-youngblut]

Checked other resources

• I added a very descriptive title to this issue.
• I searched the LangGraph/LangChain documentation with the integrated search.
• I used the GitHub search to find a similar question and didn't find it.
• I am sure that this is a bug in LangGraph/LangChain rather than my code.
• I am sure this is better as an issue rather than a GitHub discussion, since this is a LangGraph bug and not a design question.

Example Code

Simply running the [Plan-and-Execute](https://langchain-ai.github.io/langgraph/tutorials/plan-and-execute/plan-and-execute) code

Error Message and Stack Trace (if applicable)

{
	"name": "TypeError",
	"message": "can only concatenate list (not \"tuple\") to list",
	"stack": "---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[47], line 3
      1 config = {\"recursion_limit\": 50}
      2 inputs = {\"input\": \"what is the hometown of the 2024 Australia open winner?\"}
----> 3 async for event in app.astream(inputs, config=config):
      4     for k, v in event.items():
      5         if k != \"__end__\":

File /usr/local/lib/python3.11/site-packages/langgraph/pregel/__init__.py:1446, in Pregel.astream(self, input, config, stream_mode, output_keys, input_keys, interrupt_before, interrupt_after, debug)
   1441     print_step_writes(
   1442         step, pending_writes, self.stream_channels_list
   1443     )
   1445 # apply writes to channels
-> 1446 _apply_writes(
   1447     checkpoint,
   1448     channels,
   1449     pending_writes,
   1450     (
   1451         self.checkpointer.get_next_version
   1452         if self.checkpointer
   1453         else _increment
   1454     ),
   1455 )
   1457 # yield current values
   1458 if \"values\" in stream_modes:

File /usr/local/lib/python3.11/site-packages/langgraph/pregel/__init__.py:1758, in _apply_writes(checkpoint, channels, pending_writes, get_next_version)
   1756 if chan in channels:
   1757     try:
-> 1758         updated = channels[chan].update(vals)
   1759     except InvalidUpdateError as e:
   1760         raise InvalidUpdateError(
   1761             f\"Invalid update for channel {chan} with values {vals}\"
   1762         ) from e

File /usr/local/lib/python3.11/site-packages/langgraph/channels/binop.py:96, in BinaryOperatorAggregate.update(self, values)
     94     values = values[1:]
     95 for value in values:
---> 96     self.value = self.operator(self.value, value)
     97 return True

TypeError: can only concatenate list (not \"tuple\") to list"
}

Description

I'm running the code in a Jupyter notebook in a VS Code devcontainer on my macbook.

System Info

System Information

OS: Linux
OS Version: #1 SMP PREEMPT Wed Dec 6 17:08:31 UTC 2023
Python Version: 3.11.9 (main, Jul 2 2024, 21:48:22) [GCC 10.2.1 20210110]

Package Information

langchain_core: 0.2.12
langchain: 0.2.7
langchain_community: 0.2.7
langsmith: 0.1.84
langchain_google_vertexai: 1.0.6
langchain_groq: 0.1.6
langchain_openai: 0.1.14
langchain_text_splitters: 0.2.2
langchain_weaviate: 0.0.2
langchainhub: 0.1.20
langgraph: 0.1.6

Packages not installed (Not Necessarily a Problem)

The following packages were not found:

langserve

[tisage]

The tutorial omitted a pair of square brackets [] in the past_steps line.
Fixed version:

    return {
        # "past_steps": (task, agent_response["messages"][-1].content),
        "past_steps": [(task, agent_response["messages"][-1].content)],
    }