LangGraph Integration

If you already use LangGraph, Stroma can add contract validation to your existing graphs without rewriting them.

Install

uv add stroma[langgraph]

Decorating LangGraph nodes

Use @stroma_langgraph_node to attach contracts to your LangGraph node functions:

from pydantic import BaseModel
from stroma import (
    ContractRegistry,
    NodeContract,
    StromaRunner,
)
from stroma.adapters.langgraph import (
    LangGraphAdapter,
    stroma_langgraph_node,
)


class InputState(BaseModel):
    x: int


class OutputState(BaseModel):
    y: int


# Define contracts
registry = ContractRegistry()

contract = NodeContract(
    node_id="add_one",
    input_schema=InputState,
    output_schema=OutputState,
)
registry.register(contract)


@stroma_langgraph_node("add_one", contract)  # (1)!
async def add_one(state: InputState) -> dict:
    return {"y": state.x + 1}

1. Works like @stroma_node but is designed for LangGraph's state-passing pattern.

Wrapping a graph

The LangGraphAdapter discovers decorated nodes in your graph and wraps them with validation:

runner = StromaRunner.quick()
adapter = LangGraphAdapter(registry, runner)

# Assuming you have a compiled LangGraph graph
# graph = build_your_graph()
# wrapped = adapter.wrap(graph)  # (1)!

1. wrap() finds all nodes decorated with @stroma_langgraph_node, replaces them with validating wrappers, and returns the modified graph. Non-decorated nodes are left untouched.

How wrapping works

When the adapter wraps a node, it:

1. Extracts the state dict from LangGraph's state object
2. Validates it against the node's input schema
3. Calls the original node function
4. Validates the result against the node's output schema
5. Returns the validated output as a dict

If validation fails at either boundary, a ContractViolation is raised — just like with the standard runner.

State extraction

The adapter handles multiple state formats: plain dicts, Pydantic models, objects with .dict(), and objects with __dict__. This covers LangGraph's various state representations.

When to use the adapter vs. the runner

Use case	Recommendation
New pipeline from scratch	Use StromaRunner directly
Existing LangGraph graph	Use LangGraphAdapter to add validation
Gradual migration	Start with the adapter, migrate nodes to the runner over time

Tip

The adapter currently provides contract validation only. Full runner features (retries, checkpointing, cost tracking) require using StromaRunner directly.

Recap

• Install with uv add stroma[langgraph]
• Decorate nodes with @stroma_langgraph_node to attach contracts
• Use LangGraphAdapter.wrap(graph) to add validation to an existing graph
• The adapter validates inputs and outputs at every decorated node boundary

That's the end of the tutorial! For deeper dives, check the Concepts guide and the API Reference.