SubgraphStateSchemaMismatchError
langgraph.errors.SubgraphStateSchemaMismatchError
Stack trace
langgraph.errors.SubgraphStateSchemaMismatchError: Subgraph state does not match the expected schema. at langgraph.subgraph.Subgraph.load_state (subgraph.py:123) at langgraph.graph.Graph.load_subgraph_state (graph.py:87) at main.py:45
Why it happens
LangGraph enforces strict schema validation on subgraph states to ensure data integrity. This error occurs when the stored or incoming subgraph state structure differs from the schema defined in the subgraph's type or model, often due to version mismatches or manual state modifications.
Detection
Monitor subgraph state loading operations and catch SubgraphStateSchemaMismatchError exceptions; log the raw state data and schema version to identify discrepancies early.
Causes & fixes
The subgraph state schema was updated but existing saved states were not migrated.
Perform a state migration to update all saved subgraph states to the new schema version before loading.
Manual edits or corruption of the subgraph state JSON caused schema fields to be missing or malformed.
Validate and sanitize the subgraph state data before saving, and restore from backups if corruption is detected.
Different versions of LangGraph or subgraph definitions are used between saving and loading state.
Ensure consistent LangGraph and subgraph package versions across environments and deploy schema version checks.
Code: broken vs fixed
from langgraph import Graph
graph = Graph()
subgraph_state = graph.load_subgraph_state('user_profiles') # Raises SubgraphStateSchemaMismatchError
print(subgraph_state)import os
from langgraph import Graph
os.environ['LANGGRAPH_API_KEY'] = os.getenv('LANGGRAPH_API_KEY') # Use env var for auth
graph = Graph()
# Run migration before loading state
graph.migrate_subgraph_state('user_profiles') # Fixes schema mismatch
subgraph_state = graph.load_subgraph_state('user_profiles')
print(subgraph_state) # Now loads without errorWorkaround
Catch SubgraphStateSchemaMismatchError and parse the raw state JSON manually to fix or patch missing fields before retrying the load.
Prevention
Implement automated schema versioning and migration pipelines for subgraph states, and enforce consistent LangGraph versions across all environments.