Langgraph State Management

from langgraph.graph import StateGraph, START, END, MessagesState
from langchain_core.messages import AIMessage

class State(MessagesState):
    pass

def chat_node(state: State):
    return {"messages": [AIMessage(content="Hello!")]}

graph = StateGraph(State).add_node("chat", chat_node)
graph.add_edge(START, "chat").add_edge("chat", END)
app = graph.compile()

from langgraph.graph import MessagesState

class State(MessagesState):
    documents: list[str]
    query: str

import { StateGraph, StateSchema, MessagesValue, ReducedValue, START, END } from "@langchain/langgraph";
import { AIMessage } from "@langchain/core/messages";
import { z } from "zod/v4";

const State = new StateSchema({
  messages: MessagesValue,
  documents: z.array(z.string()).default(() => []),
  count: new ReducedValue(
    z.number().default(0),
    { reducer: (current, update) => current + update }
  ),
});

const graph = new StateGraph(State)
  .addNode("chat", (state) => ({ messages: [new AIMessage("Hello!")] }))
  .addEdge(START, "chat")
  .addEdge("chat", END)
  .compile();

from typing import Annotated
import operator
from langgraph.graph import MessagesState

class State(MessagesState):
    # Overwrite (no reducer)
    query: str

    # Sum integers
    count: Annotated[int, operator.add]

    # Custom reducer
    results: Annotated[list[str], lambda left, right: left + right]

const State = new StateSchema({
  query: z.string(),                    // Last-write-wins
  messages: MessagesValue,              // Built-in message reducer
  count: new ReducedValue(              // Custom reducer
    z.number().default(0),
    { reducer: (current, update) => current + update }
  ),
});

from langgraph.types import Overwrite

def reset_messages(state: State):
    return {"messages": Overwrite(["fresh start"])}

from langchain_core.messages import RemoveMessage
from langgraph.graph.message import REMOVE_ALL_MESSAGES

# Delete specific message
{"messages": [RemoveMessage(id="msg_123")]}

# Delete all messages
{"messages": [RemoveMessage(id=REMOVE_ALL_MESSAGES)]}

# Development
from langgraph.checkpoint.memory import InMemorySaver
graph = builder.compile(checkpointer=InMemorySaver())

# Production (PostgreSQL)
from langgraph.checkpoint.postgres import PostgresSaver

DB_URI = "postgresql://user:pass@host:5432/db"
with PostgresSaver.from_conn_string(DB_URI) as checkpointer:
    # checkpointer.setup()  # Run once for initial schema
    graph = builder.compile(checkpointer=checkpointer)

    result = graph.invoke(
        {"messages": [{"role": "user", "content": "Hi"}]},
        {"configurable": {"thread_id": "session-1"}}
    )

// Development
import { MemorySaver } from "@langchain/langgraph";
const graph = builder.compile({ checkpointer: new MemorySaver() });

// Production (PostgreSQL)
import { PostgresSaver } from "@langchain/langgraph-checkpoint-postgres";
const checkpointer = PostgresSaver.fromConnString(DB_URI);
// await checkpointer.setup();  // Run once
const graph = builder.compile({ checkpointer });

config = {"configurable": {"thread_id": "user-123-session-1"}}
result = graph.invoke({"messages": [...]}, config)

parent_graph = parent_builder.compile(checkpointer=checkpointer)
# Subgraphs inherit the checkpointer

subgraph = sub_builder.compile(checkpointer=True)

from typing import TypedDict, Annotated, Literal

class AgentState(TypedDict):
    messages: Annotated[list[BaseMessage], add_messages]
    next: Literal["agent1", "agent2", "FINISH"]
    context: dict

import { StateSchema, MessagesValue, ReducedValue, UntrackedValue } from "@langchain/langgraph";
import { z } from "zod/v4";

const AgentState = new StateSchema({
  messages: MessagesValue,
  currentStep: z.string(),
  retryCount: z.number().default(0),

  // Custom reducer
  allSteps: new ReducedValue(
    z.array(z.string()).default(() => []),
    { inputSchema: z.string(), reducer: (current, newStep) => [...current, newStep] }
  ),

  // Transient state (not checkpointed)
  tempCache: new UntrackedValue(z.record(z.string(), z.unknown())),
});

// Extract types for use outside the graph builder
type State = typeof AgentState.State;
type Update = typeof AgentState.Update;