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# State signals

A state signal keeps the model informed about something that changes over time, such as the user's browser page or a background task's progress. It has a stable name on a thread, such as `browser`, so updates describe the same piece of context across turns. Those updates provide context rather than chat messages.

Your application code, background job, or integration supplies the updates. This code is called the **producer**. It sends either a full description of the current state, called a **snapshot**, or a description of what changed, called a **delta**. Mastra stores the update and includes its text in the model's context. Your producer is responsible for comparing and combining values.

## How state signals work

The named slot identified by a state `id` is also called a **state lane**. Start it with a snapshot. After that, send deltas while the model still has the snapshot it needs to understand them. Send a fresh snapshot if the old one has left the model's context, or if you're unsure a delta is sufficient.

Mastra tracks the current `cacheKey` and mode for each state ID. If both match an incoming update, [`sendStateSignal()`](https://mastra.ai/reference/agents/agent) skips it and returns `{ skipped: true, reason: 'unchanged' }`. Change the cache key when the state changes.

State signals are durable, so they survive later turns and, with shared storage, process restarts. Because every accepted update becomes context, batch or summarize frequent changes before sending.

For user input, use [`sendMessage()`](https://mastra.ai/reference/agents/agent). If context only matters for one model request, send a [transient reactive signal](https://mastra.ai/docs/signals/sending-signals) instead. External events that need a durable inbox record and delivery policy belong in [notifications](https://mastra.ai/docs/signals/notifications).

## Send a state signal

Before calling `sendStateSignal()`, configure [Memory](https://mastra.ai/docs/memory/overview) on the agent and ensure the target thread exists. Unlike a user message, this call can't create a missing thread.

When the producer detects a change, send these fields:

- `id`: the name of the state, such as `browser`.
- `mode`: `snapshot` for the full state or `delta` for changes.
- `cacheKey`: a version identifier used to skip duplicate updates.
- `contents`: the text the model reads.
- `value`: optional structured data for your application and processors. It isn't automatically shown to the model.

The [Agent reference](https://mastra.ai/reference/agents/agent) lists all fields and return values.

```typescript
await agent.sendStateSignal(
  {
    id: 'browser',
    mode: 'snapshot',
    cacheKey: 'browser:https://example.com:3-tabs',
    contents: 'Browser is open. Active tab URL: https://example.com. 3 open tabs.',
    value: {
      activeUrl: 'https://example.com',
      tabCount: 3,
      open: true,
    },
  },
  {
    resourceId: 'user_123',
    threadId: 'thread_456',
  },
)
```

## Compute state in a processor

Use [`computeStateSignal()`](https://mastra.ai/reference/processors/processor-interface) when a processor owns a state lane. Mastra calls it once per model input step after [`processInputStep()`](https://mastra.ai/reference/processors/processor-interface) and before the model request is finalized. The processor reads `activeStateSignals`, the last snapshot, and `deltasSinceSnapshot`, and returns `undefined` when nothing changed. The [Processors guide](https://mastra.ai/docs/agents/processors) covers its place in the execution order.

```typescript
import type { ComputeStateSignalArgs, Processor } from '@mastra/core/processors'

export const browserStateProcessor: Processor = {
  id: 'browser-state',
  stateId: 'browser',
  computeStateSignal(args: ComputeStateSignalArgs) {
    const browser = readCurrentBrowserState()
    const previous = readMostRecentBrowserState(args.activeStateSignals)
    const changed = previous ? diffBrowserState(previous, browser) : browser
    const shouldRefreshSnapshot = Boolean(args.lastSnapshot && !args.contextWindow.hasSnapshot)

    if (previous && Object.keys(changed).length === 0 && !shouldRefreshSnapshot) {
      return
    }

    const isDelta = Boolean(previous && !shouldRefreshSnapshot)

    return {
      mode: isDelta ? 'delta' : 'snapshot',
      cacheKey: stableBrowserStateCacheKey(browser),
      contents: isDelta ? describeBrowserDelta(changed) : describeBrowserSnapshot(browser),
      value: browser,
      ...(isDelta ? { delta: changed } : {}),
    }
  },
}
```

The browser-reading, comparison, cache-key, and description helpers in this example are application code you supply. Mastra provides `lastSnapshot` and `deltasSinceSnapshot`, loading them from message history if needed. Your processor uses them to decide what changed and what text to send.

## Built-in uses

Mastra uses state signals for long-lived context, and the same patterns apply to your application:

- The [browser](https://mastra.ai/docs/browser) context processor keeps the active page and tabs under `browser`.
- [Goals](https://mastra.ai/docs/harness/goals) keep the current objective under the `goal` id.
- [Task tracking](https://mastra.ai/docs/agents/tools) stores task state through [`TaskStateProcessor`](https://mastra.ai/reference/signals/task-signal-provider).
- [Working memory](https://mastra.ai/docs/memory/working-memory) can move out of the system prompt and into a `working-memory` state signal with `useStateSignals: true`.

## Persistence and truncation

State signals can't be transient. Mastra rebuilds tracking metadata from persisted history, so a delivery-only signal would break deduplication. With shared storage, persisted state and its tracking survive a restart; without it, a new process starts without the lane.

`contextWindow.hasSnapshot` tells the processor whether the active window already contains a snapshot for the lane. If it's `false`, return a fresh `snapshot` so the model sees current state after older state messages are trimmed.