react-agent-js/react_agent/utils/state.ts

import { BaseMessage } from "@langchain/core/messages";
import { Annotation } from "@langchain/langgraph";
import { messagesStateReducer } from "@langchain/langgraph";

// This is the primary state of your agent, where you can store any information
export const State = Annotation.Root({
  /**
   * Messages track the primary execution state of the agent.

    Typically accumulates a pattern of:

    1. HumanMessage - user input
    2. AIMessage with .tool_calls - agent picking tool(s) to use to collect
         information
    3. ToolMessage(s) - the responses (or errors) from the executed tools
    
        (... repeat steps 2 and 3 as needed ...)
    4. AIMessage without .tool_calls - agent responding in unstructured
        format to the user.

    5. HumanMessage - user responds with the next conversational turn.

        (... repeat steps 2-5 as needed ... )
    
    Merges two lists of messages, updating existing messages by ID.

    By default, this ensures the state is "append-only", unless the
    new message has the same ID as an existing message.

    Returns:
        A new list of messages with the messages from \`right\` merged into \`left\`.
        If a message in \`right\` has the same ID as a message in \`left\`, the
        message from \`right\` will replace the message from \`left\`.`
   */
  messages: Annotation<BaseMessage[]>({
    reducer: messagesStateReducer,
    default: () => [],
  }),
  /**
   * Set to 'true' if the step is recursion_limit - 1 (meaning it's the last step before the graph will raise an error)
   *
   * This is a 'managed' variable (meaning it is managed by the state machine rather than your code).
   */
  is_last_step: Annotation<boolean>({
    reducer: (existing: boolean, newValue: boolean) => newValue ?? existing,
    default: () => false,
  }),
  // Feel free to add additional attributes to your state as needed.
  // Common examples include retrieved documents, extracted entities, API connections, etc.
});

export type StateT = typeof State.State;