Can not stop sub graph when asyncio.CancelledError occurred

[mahonzhan]

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

async def call_subgraph(state, config):
    output = await subgraph.ainvoke(
        input=state,
        config=config,
    )
    messages = output["messages"]
    messages = messages[-1:]
    return {
        **output,
        "messages": messages,
    }

parent_graph.add_node(node="call_subgraph", action=call_subgraph)

Error Message and Stack Trace (if applicable)

Description

I use fastapi and graph.astream to provide event stream, when I cancel request, asyncio.CancelledError occurred, and the graph will stopped, but when I set a node to ainvoke subgraph, subgraph will not stop (continue to do ReAct loop) when asyncio.CancelledError occurred

System Info

System Information

OS: Linux
OS Version: #1 SMP PREEMPT_DYNAMIC Thu, 17 Jul 2025 21:05:29 +0000
Python Version: 3.12.11 (main, Jul 11 2025, 22:43:48) [Clang 20.1.4 ]

Package Information

langchain_core: 0.3.69
langchain: 0.3.26
langchain_community: 0.3.27
langsmith: 0.4.8
langchain_anthropic: 0.3.17
langchain_cohere: 0.4.4
langchain_deepseek: 0.1.3
langchain_experimental: 0.3.4
langchain_google_community: 2.0.7
langchain_google_genai: 2.1.8
langchain_mcp_adapters: 0.1.9
langchain_mistralai: 0.2.11
langchain_ollama: 0.3.5
langchain_openai: 0.3.28
langchain_tavily: 0.2.10
langchain_text_splitters: 0.3.8
langchain_xai: 0.2.4
langgraph_sdk: 0.1.74
langgraph_supervisor: 0.0.27
langgraph_swarm: 0.0.13

Optional packages not installed

langserve

Other Dependencies

aiohttp: 3.12.14
aiohttp<4,>=3.9.1: Installed. No version info available.
aiohttp<4.0.0,>=3.8.3: Installed. No version info available.
anthropic<1,>=0.57.0: Installed. No version info available.
async-timeout<5.0.0,>=4.0.0;: Installed. No version info available.
beautifulsoup4: 4.13.4
cohere: 5.16.1
dataclasses-json<0.7,>=0.5.7: Installed. No version info available.
db-dtypes: Installed. No version info available.
filetype: 1.2.0
gapic-google-longrunning: Installed. No version info available.
google-ai-generativelanguage: 0.6.18
google-api-core: 2.25.1
google-api-python-client: 2.176.0
google-auth: 2.40.3
google-auth-httplib2: 0.2.0
google-auth-oauthlib: Installed. No version info available.
google-cloud-aiplatform: Installed. No version info available.
google-cloud-bigquery: Installed. No version info available.
google-cloud-bigquery-storage: Installed. No version info available.
google-cloud-contentwarehouse: Installed. No version info available.
google-cloud-core: 2.4.3
google-cloud-discoveryengine: Installed. No version info available.
google-cloud-documentai: Installed. No version info available.
google-cloud-documentai-toolbox: Installed. No version info available.
google-cloud-speech: Installed. No version info available.
google-cloud-storage: Installed. No version info available.
google-cloud-texttospeech: Installed. No version info available.
google-cloud-translate: Installed. No version info available.
google-cloud-vision: Installed. No version info available.
googlemaps: Installed. No version info available.
grpcio: 1.73.1
httpx: 0.28.1
httpx-sse<1,>=0.3.1: Installed. No version info available.
httpx-sse<1.0.0,>=0.4.0: Installed. No version info available.
httpx<1,>=0.25.2: Installed. No version info available.
httpx>=0.25.2: Installed. No version info available.
jsonpatch<2.0,>=1.33: Installed. No version info available.
langchain-anthropic;: Installed. No version info available.
langchain-aws;: Installed. No version info available.
langchain-azure-ai;: Installed. No version info available.
langchain-cohere;: Installed. No version info available.
langchain-community;: Installed. No version info available.
langchain-core<0.4,>=0.3.36: Installed. No version info available.
langchain-core<0.4.0,>=0.3.40: Installed. No version info available.
langchain-core<1.0.0,>=0.3.47: Installed. No version info available.
langchain-core<1.0.0,>=0.3.51: Installed. No version info available.
langchain-core<1.0.0,>=0.3.62: Installed. No version info available.
langchain-core<1.0.0,>=0.3.66: Installed. No version info available.
langchain-core<1.0.0,>=0.3.68: Installed. No version info available.
langchain-core<1.0.0,>=0.3.69: Installed. No version info available.
langchain-core>=0.3.40: 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-openai;: Installed. No version info available.
langchain-openai<0.4,>=0.3.18: Installed. No version info available.
langchain-openai<1.0.0,>=0.3.9: Installed. No version info available.
langchain-perplexity;: Installed. No version info available.
langchain-text-splitters<1.0.0,>=0.3.8: Installed. No version info available.
langchain-together;: Installed. No version info available.
langchain-xai;: Installed. No version info available.
langchain<1.0.0,>=0.3.26: Installed. No version info available.
langgraph-prebuilt>=0.1.7: Installed. No version info available.
langgraph>=0.3.5: Installed. No version info available.
langgraph>=0.5: Installed. No version info available.
langsmith-pyo3: Installed. No version info available.
langsmith>=0.1.125: Installed. No version info available.
langsmith>=0.1.17: Installed. No version info available.
langsmith>=0.3.45: Installed. No version info available.
mcp>=1.9.2: Installed. No version info available.
numpy>=1.26.2;: Installed. No version info available.
numpy>=2.1.0;: Installed. No version info available.
ollama<1.0.0,>=0.5.1: Installed. No version info available.
openai-agents: Installed. No version info available.
openai<2.0.0,>=1.86.0: Installed. No version info available.
opentelemetry-api: Installed. No version info available.
opentelemetry-exporter-otlp-proto-http: Installed. No version info available.
opentelemetry-sdk: Installed. No version info available.
orjson: 3.11.0
orjson>=3.10.1: Installed. No version info available.
packaging: 25.0
packaging>=23.2: Installed. No version info available.
pandas: 2.3.1
pyarrow: Installed. No version info available.
pydantic: 2.11.7
pydantic-settings<3.0.0,>=2.4.0: Installed. No version info available.
pydantic<3,>=2: Installed. No version info available.
pydantic<3.0.0,>=2.7.4: Installed. No version info available.
pydantic>=2.7.4: Installed. No version info available.
pytest: Installed. No version info available.
PyYAML>=5.3: Installed. No version info available.
requests: 2.32.4
requests-toolbelt: 1.0.0
requests<3,>=2: Installed. No version info available.
rich: 14.0.0
SQLAlchemy<3,>=1.4: Installed. No version info available.
tenacity!=8.4.0,<10,>=8.1.0: Installed. No version info available.
tenacity!=8.4.0,<10.0.0,>=8.1.0: Installed. No version info available.
tiktoken<1,>=0.7: Installed. No version info available.
tokenizers<1,>=0.15.1: Installed. No version info available.
types-pyyaml: 6.0.12.20250516
typing-extensions>=4.14.0: Installed. No version info available.
typing-extensions>=4.7: Installed. No version info available.
zstandard: 0.23.0

[mahonzhan]

simple ways to reproduce in fastapi with langgraph:

@router.get("/test")
async def test(request: Request):
    async def generate():
        try:
            async for chunk in supervisor.astream(
                    {
                        "messages": [
                            {
                                "role": "user",
                                "content": "find US and New York state GDP in 2024. what % of US GDP was New York state?",
                            }
                        ]
                    },
                    subgraphs=True,
                    print_mode=["tasks"],
            ):
                yield chunk
        except asyncio.CancelledError:
            log.warning("cancel")

    return EventSourceResponse(generate())

when the research subagent call llm, cancel the request with CTRL+C, research subagent keep going with new llm calls. not work as expected.

[casparb]

Hey @mahonzhan. When you're documenting issues in langchain-ai, please include a minimal, fully self-contained example that directly demonstrates the issue that you're experiencing. I wrote a script but was unable to reproduce this behavior:

see code

import asyncio
from typing import TypedDict

import uuid
from langgraph.graph import StateGraph, START, END
from langgraph.checkpoint.memory import InMemorySaver

class State(TypedDict):
    i: int

async def work_node(state: State) -> State:
    print("child: started", flush=True)
    try:
        j = 0
        while True:
            await asyncio.sleep(0.1)
            print(f"child: tick {j}", flush=True)
            j += 1
    except asyncio.CancelledError:
        print("child: got CancelledError", flush=True)
        raise

subgraph_builder = StateGraph(State)
subgraph_builder.add_node("work", work_node)
subgraph_builder.set_entry_point("work")
subgraph_builder.add_edge("work", END)
subgraph = subgraph_builder.compile()

parent = StateGraph(State)
parent.add_node("child", subgraph)
parent.set_entry_point("child")
parent.add_edge("child", END)

graph = parent.compile(checkpointer=InMemorySaver())

config = {""
    "configurable": {
        "thread_id": str(uuid.uuid4())
    }
}

async def main():
    task = asyncio.create_task(graph.ainvoke({"i": 0}, config))

    await asyncio.sleep(0.5)

    print("parent: cancelling run now", flush=True)
    task.cancel()
    try:
        await task
    except asyncio.CancelledError:
        print("parent: run cancelled", flush=True)

    await asyncio.sleep(1.0)
    print("parent: done", flush=True)

if __name__ == "__main__":
    asyncio.run(main())

$ python ./repro_cancel_subgraph.py
child: started
child: tick 0
child: tick 1
child: tick 2
child: tick 3
parent: cancelling run now
child: got CancelledError
parent: run cancelled
parent: done

Closing this issue. If you're still having problems please comment with code example and I'll reopen.

[mahonzhan]

Hi, @casparb Thank you for your reply.

example code:

# main.py
import asyncio
from typing import TypedDict

import uuid

import uvicorn
from fastapi import FastAPI
from langgraph.graph import StateGraph, START, END
from langgraph.checkpoint.memory import InMemorySaver
from sse_starlette import EventSourceResponse

class State(TypedDict):
    i: int

async def work_node(state: State) -> State:
    print("child: started", flush=True)
    try:
        j = 0
        while True:
            await asyncio.sleep(0.1)
            print(f"child: tick {j}", flush=True)
            j += 1
    except asyncio.CancelledError:
        print("child: got CancelledError", flush=True)
        raise

subgraph_builder = StateGraph(State)
subgraph_builder.add_node("work", work_node)
subgraph_builder.set_entry_point("work")
subgraph_builder.add_edge("work", END)
subgraph = subgraph_builder.compile()

parent = StateGraph(State)
parent.add_node("child", subgraph)
parent.set_entry_point("child")
parent.add_edge("child", END)

graph = parent.compile(checkpointer=InMemorySaver())

app = FastAPI()

@app.get("/test")
async def test():
    async def generate():
        try:
            async for chunk in graph.astream(
                    input={
                        "i": 0
                    },
                    config={
                        "configurable": {
                            "thread_id": str(uuid.uuid4())
                        }
                    },
                    subgraphs=True,
                    print_mode=["tasks"],
                    durability="exit",
            ):
                yield chunk
        except asyncio.CancelledError:
            print("cancel")

    return EventSourceResponse(generate())


if __name__ == "__main__":
    uvicorn.run(app=app, port=8000)

# pyproject.toml
[project]
name = "langgraph-bug-report"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
requires-python = ">=3.12"
dependencies = [
    "fastapi>=0.117.1",
    "langgraph>=0.6.7",
    "sse-starlette>=3.0.2",
    "uvicorn>=0.36.0",
]

test command:

timeout 2 curl localhost:8000/test

result:

INFO:     127.0.0.1:37788 - "GET /test HTTP/1.1" 200 OK
[tasks] {'id': 'a8878420-a4b7-9e17-4d3e-eec03f0daff8', 'name': 'child', 'input': {'i': 0}, 'triggers': ('branch:to:child',)}
child: started
[tasks] [graph=('child:a8878420-a4b7-9e17-4d3e-eec03f0daff8',)] {'id': 'cbb21716-3145-7d8f-ba08-99bdc865d478', 'name': 'work', 'input': {'i': 0}, 'triggers': ('branch:to:work',)}
child: tick 0
child: tick 1
child: tick 2
child: tick 3
child: tick 4
child: tick 5
child: tick 6
child: tick 7
child: tick 8
child: tick 9
child: tick 10
child: tick 11
child: tick 12
child: tick 13
child: tick 14
child: tick 15
child: tick 16
child: tick 17
child: tick 18
cancel
child: tick 19
child: tick 20
child: tick 21
child: tick 22
child: tick 23
child: tick 24
child: tick 25
child: tick 26
child: tick 27
child: tick 28
child: tick 29
child: tick 30
child: tick 31
child: tick 32
child: tick 33
child: tick 34
child: tick 35
child: tick 36
child: tick 37
child: tick 38
child: tick 39

As mentioned earlier, child cannot stop.

[chengjinluo]

After some digging, I found that when the request disconnected, anyio (used by fastapi) will consistently cancel all new tasks awaited in the api handler coroutine, which also prevents the running of langgraph AsyncPregelLoop's finalizer aexit task