LangGraph conditional-router resume reproduction

This experiment independently verifies the behavior reported in LangGraph issue #8834: when a node writes state successfully but its conditional-edge router raises, resuming with invoke(None, same_config) returns normally without retrying the router or executing the downstream node.

Environment

• Python 3.10.2
• langgraph==1.2.11
• langgraph-checkpoint-sqlite==3.1.1
• InMemorySaver and SqliteSaver
• No model, API key, network request, or custom saver is used by the reproduction itself.

Run

From this directory:

./run.sh

From the XBSTACK monorepo checkout:

bash experiments/langgraph-conditional-router-resume-repro/run.sh

A successful run prints both REPRO_CONFIRMED and WORKAROUND_CONFIRMED. The runner writes structured evidence into logs/.

Reproduced behavior

The control case raises once inside an ordinary node. Resume retries that node, evaluates the router, calls sink, and returns {"value": 2}.

The failure case raises once inside the conditional router after the node state update. Resume returns {"value": 1} with route=1, sink=0, no pending task, and no second router call. The same result is reproduced with both InMemorySaver and SqliteSaver.

Verified application-level workaround

fixed/workaround.py moves the fallible routing work into a normal router_node. The conditional edge becomes a pure selector that only reads route_decision from state.

In the tested failure case, the checkpoint now exposes pending_before=["router_node"]. Resuming retries that node, evaluates the pure selector, executes sink, and reaches value=2. Both tested savers pass this workaround fixture.

This is an application-level containment pattern, not an upstream LangGraph patch.

Evidence boundary

This repository confirms the published-package behavior on LangGraph 1.2.11 for synchronous StateGraph.invoke with the two savers above. It does not establish behavior for async execution, other persistence backends, subgraphs, LangGraph Platform execution, or a future fixed release.

As checked on 2026-09-11, upstream issue #8834 remained open and this repository had not verified a released framework fix.

References

• Upstream issue: langchain-ai/langgraph#8834
• XBSTACK English write-up: https://www.xbstack.com/en/ai/langgraph-conditional-router-resume-skips-downstream/?utm_source=github&utm_medium=referral&utm_campaign=langgraph_conditional_router_resume&utm_content=repository_readme&ref=github
• XBSTACK Chinese write-up: https://www.xbstack.com/ai/langgraph-conditional-router-resume-skips-downstream/?utm_source=github&utm_medium=referral&utm_campaign=langgraph_conditional_router_resume&utm_content=repository_readme_zh&ref=github