"can receive only one value per step" error when running retry in graph parallel processing

[saeu5407]

Hello.
I'm using a langraph in parallel.

When one of the parallel nodes runs incorrectly, I gave it a conditional edge to retry it, and the error "can receive only one value per step" occurs.

There is no problem if you clear the conditional edge running retry.

When I printed it out, in the process of continuing to run retries on parallelized nodes, other nodes seem to be running the next node without waiting for it, but I want to know a solution.

Attached is the sample and graph structure that simply changed my code.

Thank you.

CODE

from typing import TypedDict, Annotated
from langgraph.graph import StateGraph, START, END

class State(TypedDict):
    count_a : int
    count_b : int
    count_ab : int
    count_b_true : bool
    count_ab_check : int


def pass_node(state):
    print("pass_node")
    print(state)
    return state


def pass_node_b(state):
    print("pass_node_b")
    count = state.get('count_a', None)
    count += 1
    state['count_a'] = count
    print(state)
    return {'count_a': count}


def pass_node_c(state):
    print("pass_node_c")
    count = state.get('count_b', None)
    count_ab_check = state.get('count_ab_check', None)
    if count is None:
        count = 0
    if count_ab_check is None:
        count_ab_check = 0
    count += 1
    count_ab_check += 1
    state['count_b'] = count
    print(state)
    return {'count_b': count, "count_ab_check": count_ab_check, "count_b_true": False}


def pass_node_c_condition(state):
    print("pass_node_c_condition")
    if not state['count_b_true']:
        if state['count_ab_check'] >= 3:
            print("next: but not true")
            return "next"
        else:
            print("retry")
            return "retry"
    else:
        print("retry")
        return "next"


def pass_node_d(state):
    print("pass_node_d")
    count = state.get('count_ab', None)
    if count is None:
        count = 0
    count += state['count_a'] + state['count_b']
    state['count_ab'] = count
    print(state)
    return state


builder = StateGraph(State)
builder.add_node("a", pass_node)
builder.add_edge(START, "a")
builder.add_node("b", pass_node_b)
builder.add_node("c", pass_node_c)
builder.add_node("d", pass_node)
builder.add_edge("a", "b")
builder.add_edge("a", "c")
builder.add_edge("b", "d")
builder.add_conditional_edges("c", pass_node_c_condition,
                              {
                                  "retry": "c",
                                  "next": "d"
                              })
builder.add_edge("d", END)
graph = builder.compile()


graph.invoke({'count_a':1})

PRINT & ERROR

pass_node
{'count_a': 1, 'count_b': None, 'count_ab': None, 'count_b_true': None, 'count_ab_check': None}
pass_node_c
{'count_a': 1, 'count_b': 1, 'count_ab': None, 'count_b_true': None, 'count_ab_check': None}
pass_node_c_condition
retry
pass_node_b
{'count_a': 2, 'count_b': None, 'count_ab': None, 'count_b_true': None, 'count_ab_check': None}
pass_node_c
{'count_a': 2, 'count_b': 2, 'count_ab': None, 'count_b_true': False, 'count_ab_check': 1}
pass_node_c_condition
retry
pass_node
{'count_a': 2, 'count_b': 1, 'count_ab': None, 'count_b_true': False, 'count_ab_check': 1}

langgraph.errors.InvalidUpdateError: At key 'count_b': Can receive only one value per step. Use an Annotated key to handle multiple values.

GRAPH

[vbarda]

Closing since a duplicate of the issue here #2336