Hello experts, I’m currently building an agent app using LangGraph in Python, and facing an issue with node transitions.
In a certain situation, although there’s an edge from the current state to the next state, it exits the graph execution.
I turned the debug and verbose logging with set_debug(True) and set_verbose(True) but there’s no clue so far. Also there’s no exceptions that I could catch.
Is there any case or condition LangGraph does not execute a node or stop the graph execution? Also, is there any better way to debug this sort of issues?
there are several conditions under which a LangGraph run will stop even if you believe a next edge exists. The most common are:
END selected by a conditional edge: If your conditional edge returns END, the graph terminates.
No next tasks scheduled: If the current step produces no destinations or writes that trigger another node, the internal loop ends with status “done”.
Writes to unknown state keys: If a node returns updates for keys that aren’t declared in the graph’s state (channels), those writes are ignored and may result in no triggers.
Explicit finish edge or node: A node connected to END (or set via set_finish_point) will terminate execution when reached.
Interrupts/breakpoints: Dynamic interrupt() or static interrupt_before/after intentionally pause execution; if you’re using plain invoke without resuming, this can look like an exit.
Recursion limit: If you loop without reaching a stop condition until the recursion limit, LangGraph raises a recursion error.
Better debugging than set_debug/set_verbose:
Stream debug events: Use graph.stream(..., stream_mode=["debug","updates","values"]) to see per-step tasks, triggers, results, and state.
Visualize your compiled graph: graph.get_graph().draw_mermaid_png() to confirm edges are in the compiled graph (and not added after compile).
Step with static breakpoints: Compile/run with interrupt_before/interrupt_after to pause before/after specific nodes.
Time-travel and replay: Inspect checkpoints and replay/fork runs to see how the routing decision was made.
Minimal examples
# Stream with rich debug output
for event in graph.stream(
user_input,
config={"configurable": {"thread_id": "t1"}},
stream_mode=["debug", "updates", "values"],
):
print(event)
# Visualize the compiled graph (e.g., in a notebook)
from IPython.display import Image, display
display(Image(graph.get_graph().draw_mermaid_png()))
# Step through with breakpoints (compile time)
graph = builder.compile(
checkpointer=checkpointer,
interrupt_before=["node_a"],
interrupt_after=["node_b", "node_c"],
)
# If using dynamic interrupts inside nodes
from langgraph.types import interrupt, Command
def my_node(state):
val = interrupt({"ask": "Need human approval?"})
return {"approved": val}
# Resume later with a Command(...)
graph.invoke(Command(resume=True), config={"configurable": {"thread_id": "t1"}})
What to check in your graph
Verify the conditional edge’s return values exactly match mapping keys or node names. Consider typing the return with Literal[...] to catch mismatches early.
Confirm the node updates only declared state keys (channels). Writes to unknown keys are ignored and won’t trigger downstream nodes.
Ensure you didn’t accidentally set an edge to END or call set_finish_point on the current node.
If you use interrupt(), make sure a checkpointer is set and that you resume with Command(resume=...).
Use streaming debug to confirm that after your node runs, a task for the next node was actually scheduled.
Thank you for your detailed comment, I can’t share much details except the graph structure, the issue occurs typically the edge from C to B which is a conditional edge. I checked the method is returning a correct value and this issue does not always happen. I’m using my own checkpointer so I’ll look into further following your advice.
Visualize your compiled graph: graph.get_graph().draw_mermaid_png() to confirm edges are in the compiled graph (and not added after compile)
After I compiled the graph and output it, it doesn’t look like the one above. All nodes present, but edges are missing, however it seems it’s working for most of the cases. Does the draw_mermaid_png supposed to output the complete graph structure when I call it after compiling the graph?
Especially nodes and graph definitions