No useful traceback when using pydantic schema and failing validation

[eyurtsev]

Privileged issue

• I am a LangChain maintainer, or was asked directly by a LangChain maintainer to create an issue here.

Issue Content

The code below fails on entry to ok_node, but the stack trace has no information about which node failed.

Given that the same schema is often shared between all nodes in the graph (at least by default) -- it makes it impossible to determine where the run time error is occurring.

from langgraph.graph import StateGraph, START, END
from typing_extensions import TypedDict

from pydantic import BaseModel

# The overall state of the graph (this is the public state shared across nodes)
class OverallState(BaseModel):
    a: str

def bad_node(state: OverallState):
    return {
        "a": 123 # Invalid
    }

def ok_node(state: OverallState):
    return {
        "a": "goodbye"
    }


# Build the state graph
builder = StateGraph(OverallState)
builder.add_node(bad_node)
builder.add_node(ok_node)
builder.add_edge(START, "bad_node")
builder.add_edge("bad_node", "ok_node")
builder.add_edge("ok_node", END)
graph = builder.compile()

# Test the graph with a valid input
graph.invoke({ "a": "hello"})

Part of error trace

354 return tasks

File ~/.pyenv/versions/3.11.4/envs/core/lib/python3.11/site-packages/langgraph/pregel/algo.py:495, in prepare_single_task(task_path, task_id_checksum, checkpoint, processes, channels, managed, config, step, for_execution, store, checkpointer, manager)
485 if triggers := sorted(
486 chan
487 for chan in proc.triggers
(...)
492 > seen.get(chan, null_version)
493 ):
494 try:
--> 495 val = next(
496 _proc_input(
497 step, proc, managed, channels, for_execution=for_execution
498 )
499 )
500 except StopIteration:
501 return

File ~/.pyenv/versions/3.11.4/envs/core/lib/python3.11/site-packages/langgraph/pregel/algo.py:627, in _proc_input(step, proc, managed, channels, for_execution)
625 # If the process has a mapper, apply it to the value
626 if for_execution and proc.mapper is not None:
--> 627 val = proc.mapper(val)
629 yield val

File ~/.pyenv/versions/3.11.4/envs/core/lib/python3.11/site-packages/langgraph/graph/state.py:704, in _coerce_state(schema, input)
703 def _coerce_state(schema: Type[Any], input: dict[str, Any]) -> dict[str, Any]:
--> 704 return schema(**input)

File ~/.pyenv/versions/3.11.4/envs/core/lib/python3.11/site-packages/pydantic/main.py:212, in BaseModel.init(self, **data)
210 # __tracebackhide__ tells pytest and some other tools to omit this function from tracebacks
211 tracebackhide = True
--> 212 validated_self = self.pydantic_validator.validate_python(data, self_instance=self)
213 if self is not validated_self:
214 warnings.warn(
215 'A custom validator is returning a value other than self.\n'
216 "Returning anything other than self from a top level model validator isn't supported when validating via __init__.\n"
217 'See the model_validator docs (https://docs.pydantic.dev/latest/concepts/validators/#model-validators) for more details.',
218 category=None,
219 )

ValidationError: 1 validation error for OverallState
a
Input should be a valid string [type=string_type, input_value=123, input_type=int]
For further information visit https://errors.pydantic.dev/2.9/v/string_type