The workflow keep looping with the tool calling without going to the end

[heodi510]

Hello everyone,

I encounter a problem that the workflow cannot route to END with the model is binded with a tool.

Here is a simple example to replicate my errors:

class State(TypedDict):
    # Messages have the type "list". The `add_messages` function
    # in the annotation defines how this state key should be updated
    # (in this case, it appends messages to the list, rather than overwriting them)
    messages: Annotated[list, add_messages]

@tool
def get_today_menu(input:str) -> str:
    """Get the menu for today."""
    return "Today menu is: \n1. Pizza\n2. Burger\n3. Salad\n4. Pasta\n5. Sushi"


def should_continue(state: State):
    messages = state["messages"]
    last_message = messages[-1]
    if last_message.tool_calls:
        return "tools"
    return END

tools = [get_today_menu]
tool_node = ToolNode(tools)
llm= ChatVertexAI(model="gemini-2.0-flash", temperature=0)
llm_with_tools=llm.bind_tools(tools, tool_choice="any")

def chatbot(state: State):
    return {"messages": [llm_with_tools.invoke(state["messages"])]}


graph_builder = StateGraph(State)
graph_builder.add_node("chatbot", chatbot)
graph_builder.add_node("tools", tool_node)
graph_builder.add_conditional_edges("chatbot",should_continue,["tools",END])

graph_builder.add_edge(START, "chatbot")
graph_builder.add_edge("tools", "chatbot")
graph = graph_builder.compile()

try:
    display(Image(graph.get_graph().draw_mermaid_png()))
except Exception:
    # This requires some extra dependencies and is optional
    pass

onfig = {
    "recursion_limit": 4,
    "configurable": {
        "thread_id": "1"
    }
}

for idx, event in enumerate(graph.stream(
    {   "messages": [HumanMessage(content="What is the menu for today?" ),]},
    config
    )):
    for value in event.values():
        
        print(f"Event {idx}")
        msg = value["messages"][-1]
        if msg.content:
            print("Assistant:", msg.content)
        elif hasattr(msg, "additional_kwargs") and "function_call" in msg.additional_kwargs:
            fn = msg.additional_kwargs["function_call"]
            print("Function Call:", fn["name"])
            print("Arguments:", fn["arguments"])
        else:
            print("Empty response or unexpected format.")

The error message are as following:

Event 0
Function Call: get_today_menu
Arguments: {"input": "today"}
Event 1
Assistant: Today menu is: 
1. Pizza
2. Burger
3. Salad
4. Pasta
5. Sushi
Event 2
Function Call: get_today_menu
Arguments: {"input": "today"}
Event 3
Assistant: Today menu is: 
1. Pizza
2. Burger
3. Salad
4. Pasta
5. Sushi
---------------------------------------------------------------------------
GraphRecursionError                       Traceback (most recent call last)
Cell In[5], [line 11](vscode-notebook-cell:?execution_count=5&line=11)
      [8](vscode-notebook-cell:?execution_count=5&line=8) with open('job_description.py', "r", encoding="utf-8") as f:
      [9](vscode-notebook-cell:?execution_count=5&line=9)     jd = f.read()
---> [11](vscode-notebook-cell:?execution_count=5&line=11) for idx, event in enumerate(graph.stream(
     [12](vscode-notebook-cell:?execution_count=5&line=12)     {   "messages": [HumanMessage(content="What is the menu for today?" ),]},
     [13](vscode-notebook-cell:?execution_count=5&line=13)     config
     [14](vscode-notebook-cell:?execution_count=5&line=14)     )):
     [15](vscode-notebook-cell:?execution_count=5&line=15)     for value in event.values():
     [17](vscode-notebook-cell:?execution_count=5&line=17)         print(f"Event {idx}")

File ~/Projects/cover-letter-tool/venv/lib/python3.12/site-packages/langgraph/pregel/__init__.py:2481, in Pregel.stream(self, input, config, stream_mode, output_keys, interrupt_before, interrupt_after, checkpoint_during, debug, subgraphs)
   [2472](https://file+.vscode-resource.vscode-cdn.net/Users/heodi/Projects/cover-letter-tool/~/Projects/cover-letter-tool/venv/lib/python3.12/site-packages/langgraph/pregel/__init__.py:2472) if loop.status == "out_of_steps":
   [2473](https://file+.vscode-resource.vscode-cdn.net/Users/heodi/Projects/cover-letter-tool/~/Projects/cover-letter-tool/venv/lib/python3.12/site-packages/langgraph/pregel/__init__.py:2473)     msg = create_error_message(
   [2474](https://file+.vscode-resource.vscode-cdn.net/Users/heodi/Projects/cover-letter-tool/~/Projects/cover-letter-tool/venv/lib/python3.12/site-packages/langgraph/pregel/__init__.py:2474)         message=(
   [2475](https://file+.vscode-resource.vscode-cdn.net/Users/heodi/Projects/cover-letter-tool/~/Projects/cover-letter-tool/venv/lib/python3.12/site-packages/langgraph/pregel/__init__.py:2475)             f"Recursion limit of {config['recursion_limit']} reached "
   (...)
   [2479](https://file+.vscode-resource.vscode-cdn.net/Users/heodi/Projects/cover-letter-tool/~/Projects/cover-letter-tool/venv/lib/python3.12/site-packages/langgraph/pregel/__init__.py:2479)         error_code=ErrorCode.GRAPH_RECURSION_LIMIT,
   [2480](https://file+.vscode-resource.vscode-cdn.net/Users/heodi/Projects/cover-letter-tool/~/Projects/cover-letter-tool/venv/lib/python3.12/site-packages/langgraph/pregel/__init__.py:2480)     )
-> [2481](https://file+.vscode-resource.vscode-cdn.net/Users/heodi/Projects/cover-letter-tool/~/Projects/cover-letter-tool/venv/lib/python3.12/site-packages/langgraph/pregel/__init__.py:2481)     raise GraphRecursionError(msg)
   [2482](https://file+.vscode-resource.vscode-cdn.net/Users/heodi/Projects/cover-letter-tool/~/Projects/cover-letter-tool/venv/lib/python3.12/site-packages/langgraph/pregel/__init__.py:2482) # set final channel values as run output
   [2483](https://file+.vscode-resource.vscode-cdn.net/Users/heodi/Projects/cover-letter-tool/~/Projects/cover-letter-tool/venv/lib/python3.12/site-packages/langgraph/pregel/__init__.py:2483) run_manager.on_chain_end(loop.output)

GraphRecursionError: Recursion limit of 4 reached without hitting a stop condition. You can increase the limit by setting the `recursion_limit` config key.
For troubleshooting, visit: https://python.langchain.com/docs/troubleshooting/errors/GRAPH_RECURSION_LIMIT

I have test other routing functions from different tutorials but the workflow still keeps looping. I am not sure if it is caused by wrong routing function or other settings. Thanks for your time to review my questions.
Example 1:

def route_tools(
    state: State,
):
    """
    Use in the conditional_edge to route to the ToolNode if the last message
    has tool calls. Otherwise, route to the end.
    """
    if isinstance(state, list):
        ai_message = state[-1]
    elif messages := state.get("messages", []):
        ai_message = messages[-1]
    else:
        raise ValueError(f"No messages found in input state to tool_edge: {state}")
    if hasattr(ai_message, "tool_calls") and len(ai_message.tool_calls) > 0:
        return "tools"
    return END
graph_builder.add_conditional_edges(
    "chatbot",
    route_tools,
    {"tools": "tools", END: END},
)