src/agent/graph.ts

import { StateGraph, START, END, Annotation } from "@langchain/langgraph";
import { BaseMessage } from "@langchain/core/messages";

// 1. Define Agent State Annotation (March 2026 Fleet Standard)
export const AgentState = Annotation.Root({
  messages: Annotation<BaseMessage[]>({
    reducer: (state, update) => state.concat(update),
    default: () => [],
  }),
  context: Annotation<Record<string, any>>({
    reducer: (state, update) => ({ ...state, ...update }),
    default: () => ({}),
  }),
});

// 2. Define Nodes (Placeholders for A2A Logic)
async function reasoningNode(state: typeof AgentState.State) {
  // Bind MCP Tools to LLM here
  return { context: { reasoned: true } };
}

async function toolNode(state: typeof AgentState.State) {
  // Wrap MCP Tools via @langchain/langgraph ToolNode
  return { messages: [] };
}

// 3. Construct A2A Graph Workflow
export function createAgentGraph() {
  const workflow = new StateGraph(AgentState)
    .addNode("reasoning", reasoningNode)
    .addNode("tools", toolNode)
    .addEdge(START, "reasoning")
    .addEdge("reasoning", "tools")
    .addEdge("tools", END);

  return workflow.compile(); // Optional: pass { checkpointer: new SupabaseSaver() } here
}