"""Optional LangGraph integration.

The runtime works without LangGraph installed. This module exposes a factory for
projects that install the optional dependency.
"""

from __future__ import annotations


def build_langgraph():
    try:
        from langgraph.graph import END, START, StateGraph
    except ImportError as exc:  # pragma: no cover - depends on optional package
        raise RuntimeError(
            "langgraph is not installed. Install the optional dependency with "
            "`pip install -e .[langgraph]`."
        ) from exc

    graph = StateGraph(dict)
    graph.add_node("start", lambda state: state)
    graph.add_edge(START, "start")
    graph.add_edge("start", END)
    return graph.compile()