Run graph nodes in parallel

[jsxzhub]

Checked other resources

• This is a bug, not a usage question. For questions, please use the LangChain Forum (https://forum.langchain.com/).
• I added a clear and detailed title that summarizes the issue.
• I read what a minimal reproducible example is (https://stackoverflow.com/help/minimal-reproducible-example).
• I included a self-contained, minimal example that demonstrates the issue INCLUDING all the relevant imports. The code run AS IS to reproduce the issue.

Example Code

import operator
from typing import Annotated, Any
from typing_extensions import TypedDict
from langgraph.graph import StateGraph, START, END
class State(TypedDict):
# The operator.add reducer fn makes this append-only
aggregate: Annotated[list, operator.add]
def a(state: State):
print(f'Adding "A" to {state["aggregate"]}')
return {"aggregate": ["A"]}
def b(state: State):
print(f'Adding "B" to {state["aggregate"]}')
return {"aggregate": ["B"]}
def b_2(state: State):
print(f'Adding "B_2" to {state["aggregate"]}')
return {"aggregate": ["B_2"]}
def c(state: State):
print(f'Adding "C" to {state["aggregate"]}')
return {"aggregate": ["C"]}
def c_2(state: State):
print(f'Adding "C_2" to {state["aggregate"]}')
return {"aggregate": ["C_2"]}
def d(state: State):
print(f'Adding "D" to {state["aggregate"]}')
return {"aggregate": ["D"]}
builder = StateGraph(State)
builder.add_node(a)
builder.add_node(b)
builder.add_node(b_2)
builder.add_node(c)
builder.add_node(d)
builder.add_edge(START, "a")
builder.add_edge("a", "b")
builder.add_edge("a", "c")
builder.add_edge("b", "b_2")
builder.add_edge("b_2", "d")
builder.add_edge("c", "d")
builder.add_edge("d", END)
graph = builder.compile()
from IPython.display import Image, display
display(Image(graph.get_graph().draw_mermaid_png()))
print(graph.invoke({"aggregate": []}))

Error Message and Stack Trace (if applicable)

Description

result：
Adding "A" to []

Adding "B" to ['A']

Adding "C" to ['A']

Adding "B_2" to ['A', 'B', 'C']

Adding "D" to ['A', 'B', 'C']

Adding "D" to ['A', 'B', 'C', 'B_2', 'D']

{'aggregate': ['A', 'B', 'C', 'B_2', 'D', 'D']}

Why did "D" execute twice?

System Info

System Information

OS: Linux
OS Version: #1 SMP PREEMPT_DYNAMIC Debian 6.1.148-1 (2025-08-26)
Python Version: 3.13.9 | packaged by Anaconda, Inc. | (main, Oct 21 2025, 19:16:10) [GCC 11.2.0]

Package Information

langchain_core: 1.0.0
langchain: 1.0.2
langsmith: 0.4.37
langchain_openai: 1.0.1
langgraph_api: 0.4.46
langgraph_cli: 0.4.4
langgraph_runtime_inmem: 0.14.1
langgraph_sdk: 0.2.9

Optional packages not installed

langserve

Other Dependencies

blockbuster: 1.5.25
claude-agent-sdk: Installed. No version info available.
click: 8.3.0
cloudpickle: 3.1.1
cryptography: 44.0.3
grpcio: 1.76.0
grpcio-tools: 1.76.0
httpx: 0.28.1
jsonpatch: 1.33
jsonschema-rs: 0.29.1
langchain-anthropic: Installed. No version info available.
langchain-aws: Installed. No version info available.
langchain-community: Installed. No version info available.
langchain-deepseek: Installed. No version info available.
langchain-fireworks: Installed. No version info available.
langchain-google-genai: Installed. No version info available.
langchain-google-vertexai: Installed. No version info available.
langchain-groq: Installed. No version info available.
langchain-huggingface: Installed. No version info available.
langchain-mistralai: Installed. No version info available.
langchain-ollama: Installed. No version info available.
langchain-perplexity: Installed. No version info available.
langchain-together: Installed. No version info available.
langchain-xai: Installed. No version info available.
langgraph: 1.0.1
langgraph-checkpoint: 3.0.0
langsmith-pyo3: Installed. No version info available.
openai: 2.6.0
openai-agents: Installed. No version info available.
opentelemetry-api: 1.38.0
opentelemetry-exporter-otlp-proto-http: 1.38.0
opentelemetry-sdk: 1.38.0
orjson: 3.11.3
packaging: 25.0
protobuf: 6.33.0
pydantic: 2.12.3
pyjwt: 2.10.1
pytest: Installed. No version info available.
python-dotenv: 1.1.1
pyyaml: 6.0.3
requests: 2.32.5
requests-toolbelt: 1.0.0
rich: Installed. No version info available.
sse-starlette: 2.1.3
starlette: 0.48.0
structlog: 25.4.0
tenacity: 9.1.2
tiktoken: 0.12.0
truststore: 0.10.4
typing-extensions: 4.15.0
uvicorn: 0.38.0
vcrpy: Installed. No version info available.
watchfiles: 1.1.1
zstandard: 0.25.0