ValueError
langgraph.errors.ValueError
Stack trace
Traceback (most recent call last):
File "app.py", line 42, in run_graph
next_node = graph.get_next_node(current_node, condition)
File "/usr/local/lib/python3.10/site-packages/langgraph/graph.py", line 88, in get_next_node
raise ValueError(f"Conditional edge returned invalid node: {next_node}")
ValueError: Conditional edge returned invalid node: 'node_xyz' Why it happens
LangGraph expects conditional edges to return a valid node identifier that exists in the graph's node set. If the condition logic returns a node key that is missing or misspelled, the graph cannot proceed and raises this error. This often happens due to typos in condition functions or incomplete graph definitions.
Detection
Add validation checks after conditional edge evaluation to assert the returned node exists in the graph nodes dictionary before proceeding, and log the returned node for debugging.
Causes & fixes
Conditional function returns a node key that is not defined in the graph nodes.
Ensure all node keys returned by conditional edges are present in the graph's node dictionary and spelled correctly.
Typo or mismatch in node identifiers between graph definition and conditional logic.
Standardize node naming conventions and verify conditional edge functions return exact node keys as defined.
Conditional edge function returns None or an unexpected type instead of a valid node key string.
Add explicit return statements in conditional functions to always return a valid node key string; add type checks if needed.
Code: broken vs fixed
from langgraph import Graph
graph = Graph(nodes={'start': {}, 'end': {}}, edges={
'start': lambda cond: 'node_xyz' if cond else 'end' # invalid node key 'node_xyz'
})
next_node = graph.get_next_node('start', True) # This line raises ValueError
print(next_node)import os
from langgraph import Graph
# Use environment variable for config if needed
graph = Graph(nodes={'start': {}, 'end': {}}, edges={
'start': lambda cond: 'end' if cond else 'end' # fixed to valid node key 'end'
})
next_node = graph.get_next_node('start', True) # No error now
print(next_node) # Outputs: endWorkaround
Wrap calls to get_next_node in try/except ValueError, log the invalid node returned, and fallback to a default safe node or halt execution gracefully.
Prevention
Implement strict validation of all conditional edge return values during graph construction and unit tests to ensure all nodes referenced exist in the graph definition.