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> Discover all available pages from the documentation index: https://mastra.ai/llms.txt

# Signals

**Added in:** `@mastra/core@1.39.0`

Signals let you send input to an agent's [thread](https://mastra.ai/docs/memory/message-history) at any time, including while the agent is working. That input might be a user message, an update from a background job, or an external event. Each signal has a type that tells Mastra what kind of input it carries.

For an ongoing conversation, use signals to send input and a [thread stream](https://mastra.ai/docs/signals/thread-streams) to receive output. Subscribe to the stream first, then send input whenever it becomes available. By default, Mastra delivers it to the active run or starts a run when the thread is idle. You don't need to check whether the agent is running before sending.

For a one-off request, use [`agent.stream()`](https://mastra.ai/reference/streaming/agents/stream) to start a run and read its response directly. It can also use Memory, but it doesn't give you a subscription that follows later runs on the same thread.

> **📹 Watch:** Watch [Mastra signals overview](https://www.youtube.com/watch?v=7It2y89TVP4) to see how signals wake and steer long-running agents.

## Quickstart

Create an agent, subscribe to a thread, then send input to that thread. Subscribing first means the subscription is already open when the input starts a run.

### Use an Agent

This example stores thread history with [Memory](https://mastra.ai/docs/memory/overview), using a local libSQL database, so the same conversation is available across runs and process restarts. Memory accepts any supported storage adapter, and the thread is created on the first run.

```typescript
import { Agent } from '@mastra/core/agent'
import { Memory } from '@mastra/memory'
import { LibSQLStore } from '@mastra/libsql'

const agent = new Agent({
  id: 'support-agent',
  name: 'Support Agent',
  instructions: 'Help the user compare options.',
  model: 'openai/gpt-6.1-sol',
  memory: new Memory({
    storage: new LibSQLStore({
      id: 'signals-storage',
      url: 'file:./signals.db',
    }),
  }),
})

const thread = {
  resourceId: 'user_123',
  threadId: 'thread_456',
}

const subscription = await agent.subscribeToThread(thread)

const result = agent.sendMessage('Compare that with the previous option.', thread)
await result.accepted

for await (const chunk of subscription.stream) {
  console.log(chunk)
}
```

The loop prints output chunks as the agent responds. It stays subscribed after the run finishes, ready for output from later messages to the same thread. Call `subscription.unsubscribe()` when you no longer need output. [Sending signals](https://mastra.ai/docs/signals/sending-signals) explains how to send further messages and other kinds of input.

### Use the client SDK

The same order applies through [`client.getAgent()`](https://mastra.ai/reference/client-js/mastra-client): call `subscribeToThread()` first, then send a message to the same thread. The client returns the subscription stream and leaves run ownership on the server. [Thread streams](https://mastra.ai/docs/signals/thread-streams) shows the full client example.

## How signals reach a thread

Sending input to an active run doesn't always interrupt the model immediately. Mastra waits until the run can accept it. If the current answer needs to finish first, queue a follow-up instead. You can also store input for later without starting a run, or discard it based on the thread's state.

[Sending signals](https://mastra.ai/docs/signals/sending-signals) explains these delivery choices and how to check whether input was accepted. When your app runs on several processes, [configure a shared backend](https://mastra.ai/docs/signals/running-across-processes) so signals reach the process running the thread.

## Choose an API surface

| Surface                           | Use it for                                                         | Canonical reference                                                                                                                   |
| --------------------------------- | ------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------- |
| Direct `Agent` methods            | Server-side code that owns the agent instance                      | [Agent reference](https://mastra.ai/reference/agents/agent)                                                                           |
| HTTP or the JavaScript client SDK | Applications that call a running Mastra server                     | [Server agent routes](https://mastra.ai/reference/server/routes), [`client.getAgent()`](https://mastra.ai/reference/client-js/agents) |
| AgentController `Session`         | Long-running assistants and coding agents built on AgentController | [Agent Controller](https://mastra.ai/docs/harness/agent-controller)                                                                   |

Start with `Agent` methods when your code runs alongside the agent, or the client SDK when it calls a server. If you're already using [Agent Controller](https://mastra.ai/docs/harness/agent-controller), its `Session` manages the thread and subscription for you.

## Next steps

Use this table to find the right page for what you want to do next:

| Goal                                                                                                        | Start here                                                                          |
| ----------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------- |
| Stream a thread's output and lifecycle into your app                                                        | [Thread streams](https://mastra.ai/docs/signals/thread-streams)                     |
| Send a user message, typed signal, state signal, or notification                                            | [Sending signals](https://mastra.ai/docs/signals/sending-signals)                   |
| Understand when a signal interrupts a run or waits for the next step                                        | [When the agent sees your input](https://mastra.ai/docs/signals/sending-signals)    |
| Keep named, thread-scoped context across turns                                                              | [State signals](https://mastra.ai/docs/signals/state-signals)                       |
| Deliver external events through a durable inbox                                                             | [Notifications](https://mastra.ai/docs/signals/notifications)                       |
| Connect polling sources or webhooks to agents                                                               | [Signal providers](https://mastra.ai/docs/signals/signal-providers)                 |
| Coordinate threads across multiple processes                                                                | [Running across processes](https://mastra.ai/docs/signals/running-across-processes) |
| Look up notification records, priorities, and delivery policies                                             | [Notifications reference](https://mastra.ai/reference/signals/notifications)        |
| Observe queued messages with [`subscribeThreadEvents()`](https://mastra.ai/reference/signals/thread-events) | [Thread events reference](https://mastra.ai/reference/signals/thread-events)        |