# Threads and state

Threads and state are how a Harness conversation survives beyond a single exchange. A **thread** is the persistent record of a conversation — its messages and metadata, saved to storage so a user can close the app and resume the same conversation later, or switch between several conversations. **State** is structured data attached to the conversation — values like model preferences, feature flags, or progress — that agents and your UI read and write as the conversation runs.

The two work together: the thread is the message history, and state is the shared scratchpad alongside it. Both persist across mode switches, model changes, and restarts, so nothing is lost when a user switches from plan mode to build mode or reopens the app the next day.

Thread _lifecycle_ transitions — create, switch, clone, delete — live on the Harness because they coordinate the shared thread lock and emit events. The active thread binding and thread/message _reads_ live on the [`Session`](https://mastra.ai/docs/harness/session) as `harness.session.thread`.

## Threads

A thread holds one conversation's full message history. The Harness binds the Session to one active thread at a time; messages you send and the agent's replies are appended to that thread and saved to storage. Threads let users resume a past conversation, keep several conversations side by side, or branch one into alternatives.

### Creating and selecting threads

On startup, call `selectOrCreateThread()` to resume the most recent thread or create a new one:

```typescript
await harness.init()
const thread = await harness.selectOrCreateThread()
```

Create a thread explicitly with a title:

```typescript
const thread = await harness.createThread({ title: 'New conversation' })
```

### Switching threads

Switch to an existing thread. The harness aborts any in-progress generation, acquires a lock on the new thread, and emits a `thread_changed` event:

```typescript
await harness.switchThread({ threadId: 'thread-abc123' })
```

### Listing threads

List threads for the current resource. Forked subagent threads are hidden by default:

```typescript
const threads = await harness.session.thread.list()

// Include all resources
const allThreads = await harness.session.thread.list({ allResources: true })
```

### Cloning threads

Clone a thread to create a branch of the conversation. The harness copies all messages and switches to the clone:

```typescript
const cloned = await harness.cloneThread({ title: 'Alternative approach' })
```

### Thread locking

Pass a `threadLock` to the Harness constructor to prevent concurrent access from multiple processes. The lock is acquired before any thread operation and released on switch or delete:

```typescript
const harness = new Harness({
  id: 'my-agent',
  threadLock: {
    acquire: async threadId => {
      /* acquire lock or throw */
    },
    release: async threadId => {
      /* release lock */
    },
  },
})
```

## State

Where a thread stores the conversation's messages, state stores structured values that describe the conversation but aren't messages — model preferences, feature flags, UI settings, or progress markers. Agents can read and update state during a run, and your UI can react to changes, so state is how the agent and the interface stay in sync on shared facts. You define its shape with a schema, and every update is validated against that schema before it's applied.

### Defining a state schema

Pass a `stateSchema` (Standard JSON Schema) to validate state and extract defaults:

```typescript
import { Agent } from '@mastra/core/agent'
import { Harness } from '@mastra/core/harness'
import { z } from 'zod'

const agent = new Agent({
  name: 'assistant',
  instructions: 'Help the user manage a stateful session.',
  model: 'openai/gpt-5.5',
})

const harness = new Harness({
  id: 'stateful-agent',
  agent,
  modes: [{ id: 'default', name: 'Default', metadata: { default: true } }],
  stateSchema: z.object({
    currentModelId: z.string().optional(),
    theme: z.enum(['light', 'dark']).default('dark'),
  }),
})
```

### Reading and writing state

State is owned by the [`Session`](https://mastra.ai/docs/harness/session) as `harness.session.state`:

```typescript
// Read the current state snapshot
const state = harness.session.state.get()

// Update state — validates against schema and emits state_changed
await harness.session.state.set({ theme: 'light' })
```

State changes emit a `state_changed` event with the new state and the set of changed keys.

## Resource IDs

Threads are scoped to a resource ID, which groups a conversation's threads by project, user, or workspace. Set it on the Harness constructor; it defaults to the harness `id` when omitted:

```typescript
const harness = new Harness({
  id: 'my-agent',
  resourceId: 'project-xyz',
})
```

The resource ID is part of a conversation's identity, so you read it from the Session:

```typescript
const resourceId = harness.session.identity.getResourceId()
```

## Related

- [Harness overview](https://mastra.ai/docs/harness/overview)
- [Session](https://mastra.ai/docs/harness/session)
- [Modes](https://mastra.ai/docs/harness/modes)
- [API reference](https://mastra.ai/reference/harness/harness-class)