Shared State

What is shared state?#

Agentic Copilots maintain a shared state that seamlessly connects your UI with the agent's execution. This shared state system allows you to:

Display the agent's current progress and intermediate results
Update the agent's state through UI interactions
React to state changes in real-time across your application

When should I use this?#

Use shared state when you want to facilitate collaboration between your agent and the user. Updates flow both ways — the agent's outputs are automatically reflected in the UI, and any inputs the user updates in the UI are automatically reflected in the agent's execution.

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Reading agent state#

Subscribe a component to the agent's state with useAgent. Any time the agent mutates its state — for example via a tool call — the hook fires and your UI re-renders with the new values.

page.tsx

  // Subscribe the component to agent state changes. Any time the agent  // mutates its state (e.g. via its `set_notes` tool) this hook fires,  // we re-render, and the sidebar panels reflect the new values.  const { agent } = useAgent({    agentId: "shared-state-read-write",    updates: [UseAgentUpdate.OnStateChanged],  });

The returned agent.state is just a plain object. Read it like any other piece of React state and render the parts you care about — agent-written notes, structured outputs, progress indicators, anything the agent has put there.

Writing agent state#

The same agent object exposes a setState setter. Calling it from a UI event handler pushes the new value into shared state, and the agent reads it back on its next turn — so the UI's writes visibly steer the model.

page.tsx

  // WRITE: every edit in the sidebar goes straight into agent state.  // On the agent's next turn, `_inject_preferences` reads this back out  // of state and prepends a preferences SystemMessage — so the UI's  // writes visibly steer the model.  const handlePreferencesChange = (next: Preferences) => {    agent.setState({      ...(agentState as object | undefined),      preferences: next,      notes: agentState?.notes ?? [],    } as RWAgentState);  };

This is what makes the channel two-way: the UI doesn't just observe the agent, it can hand the agent fresh inputs (preferences, selections, partial work) without going through the chat thread.

Rendering shared state in the UI#

Because agent.state is plain React data, the UI layer is whatever you'd normally build. The demo on this page wires the agent's outputs into a small card component and feeds user edits back through setState.

notes-card.tsx

// Read-side render: this card reflects the agent-authored `notes` slice// of shared state. The parent page passes `state.notes` in; we never// touch agent state ourselves — we just render it. The Clear button is// a small write-back, exposed as an `onClear` prop.export function NotesCard({ notes, onClear }: NotesCardProps) {  return (    <Card data-testid="notes-card" className="w-full">      <CardHeader>        <div className="flex items-start justify-between gap-3">          <div className="space-y-1.5">            <CardTitle>Agent Scratch pad</CardTitle>            <CardDescription>              The agent writes here via its{" "}              <code className="font-mono text-[11px] text-[#010507]">                set_notes              </code>{" "}              tool. The UI re-renders from shared state.            </CardDescription>          </div>          {notes.length > 0 && (            <Button              type="button"              onClick={onClear}              data-testid="notes-clear-button"              variant="destructive"              size="sm"              className="uppercase tracking-[0.14em] text-[10px]"            >              Clear            </Button>          )}        </div>      </CardHeader>      <CardContent>        {notes.length === 0 ? (          <div            data-testid="notes-empty"            className="text-sm text-[#838389] italic min-h-[160px] flex items-center justify-center text-center px-4 border border-dashed border-[#E9E9EF] rounded-xl bg-[#FAFAFC]"          >            the agent will make observations about you and note them here!          </div>        ) : (          <ul            data-testid="notes-list"            className="space-y-2 text-sm text-[#010507]"          >            {notes.map((note, i) => (              <li                key={i}                data-testid="note-item"                className="flex gap-2 rounded-lg border border-[#E9E9EF] bg-[#FAFAFC] px-3 py-2"              >                <span className="text-[#838389] font-mono text-xs leading-5 select-none">                  {String(i + 1).padStart(2, "0")}                </span>                <span className="flex-1">{note}</span>              </li>            ))}          </ul>        )}      </CardContent>    </Card>  );}

Streaming partial state updates#

By default, agent state only updates between node transitions, so a long-running tool call appears as one big burst at the end. State streaming forwards a specific tool argument straight into a state key as it's being generated, so the UI can watch the answer assemble token-by-token.

shared_state_streaming_agent.py

from __future__ import annotations

from ag_ui_adk import AGUIToolset
from ag_ui_adk.config import PredictStateMapping
from google.adk.agents import LlmAgent
from google.adk.tools import ToolContext

from agents.shared_chat import get_model, stop_on_terminal_text


def write_document(tool_context: ToolContext, document: str) -> dict:
    """Write a document into shared state.

    Whenever the user asks you to write or draft anything (essay, poem,
    email, summary, etc.), call this tool with the full content as a
    single string. The UI renders state["document"] live as you type.

    Argument name `document` mirrors langgraph-python's `write_document`
    signature so the shared D5 fixture (`tool_argument="document"`) and
    the LGP-aligned PredictStateMapping below stay in lock-step.
    """
    tool_context.state["document"] = document
    return {"status": "ok", "length": len(document)}


_INSTRUCTION = (
    "You are a collaborative writing assistant. Whenever the user asks "
    "you to write, draft, or revise any piece of text, ALWAYS call the "
    "`write_document` tool with the full content as a single string. "
    "Never paste the document into a chat message directly — the document "
    "belongs in shared state and the UI renders it live as you type."
)

shared_state_streaming_agent = LlmAgent(
    name="SharedStateStreamingAgent",
    model=get_model(),
    instruction=_INSTRUCTION,
    tools=[write_document, AGUIToolset()],
    after_model_callback=stop_on_terminal_text,
)


SHARED_STATE_STREAMING_PREDICT_STATE = [
    PredictStateMapping(
        state_key="document",
        tool="write_document",
        tool_argument="document",
        emit_confirm_tool=False,
        stream_tool_call=True,
    ),
]

See State streaming for the full walkthrough, including the corresponding useAgent subscription on the frontend.

Read-only context#

When the value is UI-owned and the agent should read it but never write it back — current user, selected record, scroll position — reach for useAgentContext instead of full shared state. It publishes values as a one-way UI → agent channel that auto-unregisters on unmount.

See Agent read-only context for the full pattern.