ValidationNode

langgraph-prebuilt provides an implementation of a node that validates tool calls against a pydantic schema - ValidationNode:

from pydantic import BaseModel, field_validator
from langgraph.prebuilt import ValidationNode
from langchain_core.messages import AIMessage


class SelectNumber(BaseModel):
    a: int

    @field_validator("a")
    def a_must_be_meaningful(cls, v):
        if v != 37:
            raise ValueError("Only 37 is allowed")
        return v

validation_node = ValidationNode([SelectNumber])
validation_node.invoke({
    "messages": [AIMessage("", tool_calls=[{"name": "SelectNumber", "args": {"a": 42}, "id": "1"}])]
})