State Streaming

What is this?#

By default, agent state only updates between backend checkpoints, so a long-running tool call (writing a full document, drafting an email) appears to the UI as one big burst at the end. For agent-native apps, that feels broken: users expect to watch the output materialise.

State streaming forwards the value of a specific tool argument straight into an agent state key as the argument is being generated. The UI, subscribed via useAgent, re-renders every token.

When should I use this?#

Use state streaming whenever a tool's output is long-form text or a growing structured value and you want the user to see it assemble in real time. Common shapes:

• A collaborative writing agent that emits a document
• A research agent that accumulates a list of findings
• A planning agent that builds up a step-by-step plan

Without streaming, the user stares at a spinner. With streaming, they see the answer grow token-by-token.

The backend: one streaming state mapping#

The backend pattern is always the same: map one streaming tool argument to one shared-state key. Middleware-backed frameworks usually expose this as a declarative mapping — for example, LangGraph Python's StateStreamingMiddleware with StateItem(...) entries, or copilotkitCustomizeConfig with an emitIntermediateState mapping for LangGraph TypeScript graphs. Direct SDK adapters do the same work in their streaming loop by parsing partial tool arguments and emitting STATE_SNAPSHOT whenever the mapped value changes. When the LLM streams that argument, CopilotKit writes every partial value into shared state before the tool even finishes executing.

A few things to note:

• The state key must exist in your agent state (document in this demo).
• The tool and argument names must match the exact LLM-facing tool call you want to forward (write_document.document here).
• When the tool call completes, its final return value is written to the same key, so the streamed partial eventually becomes the authoritative final value.

The frontend: useAgent + OnStateChanged#

The UI side is identical to any other shared-state subscription: useAgent with OnStateChanged gives you a reactive agent.state. Add OnRunStatusChanged if you want a "LIVE" / "done" indicator.

From there, agent.state.document is just a string that grows on every token, and agent.isRunning tells you whether to show a streaming indicator.

Related#

• Shared State (overview) — the bidirectional read + write pattern this extends.
• Agent read-only context — for the inverse, UI → agent one-way channel.