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# Session

A `Session` is the isolated runtime for one resource and optional scope. It owns its event bus, thread binding, state, mode and model selections, run control, approvals, suspensions, follow-ups, and display state. The [`AgentController`](https://mastra.ai/reference/agent-controller/agent-controller-class) supplies shared agents, configuration, storage, workspaces, and services.

Create sessions through `controller.createSession()`. Direct construction and controller wiring methods aren't application APIs.

For a conceptual introduction, see the [AgentController overview](https://mastra.ai/docs/harness/agent-controller).

## Usage example

The following example uses the supported controller-to-session flow.

```typescript
await controller.init()

const session = await controller.createSession({ resourceId: 'project-42' })
const unsubscribe = session.subscribe(event => {
  if (event.type === 'display_state_changed') {
    render(event.displayState)
  }
})

await session.sendMessage({ content: 'Review the current project.' })
unsubscribe()
```

## Properties

The session is organized into sub-objects, each owning one domain of per-conversation state.

**identity** (`SessionIdentity`): Stable session, owner, and resource identity for the conversation. See identity methods below.

**thread** (`SessionThread`): Active thread binding and thread/message reads. See thread methods below.

**mode** (`SessionMode`): Active mode selection. See mode methods below.

**model** (`SessionModel`): Active model selection, including per-mode persistence. See model methods below.

**om** (`SessionOM`): Observer and reflector model settings for observational memory.

**permissions** (`SessionPermissions`): Tool and category permission policies represented in session state.

**subagents** (`SessionSubagents`): Global and per-agent-type subagent model selection.

**run** (`SessionRun`): Run and trace identity plus abort state for the in-flight run. See run methods below.

**stream** (`SessionStream`): The live subscription to the agent thread stream. See stream methods below.

**suspensions** (`SessionSuspensions`): Parked interactive tool calls awaiting a resume. See suspensions methods below.

**followUps** (`SessionFollowUps`): Queue of messages submitted while a run is in progress. See follow-up methods below.

**approval** (`SessionApproval`): The pending tool-approval gate. See approval methods below.

**displayState** (`SessionDisplayState`): The canonical AgentControllerDisplayState snapshot a UI renders from. See display-state methods below.

**state** (`AgentControllerRequestState<TState>`): The schema-validated, session-owned AgentController state. See state methods below.

**browser** (`MastraBrowser | undefined`): The browser automation instance for this session. Set at creation via createSession, or from the AgentController config default. Undefined when no browser is configured.

## Methods

### Identity and events

#### `getTags()`

Return a copy of the tags supplied when the session was created.

```typescript
const tags = session.getTags()
```

Returns: `Record<string, string>`

#### `subscribe(listener)`

Subscribe to this session's isolated event bus. The method returns an unsubscribe function.

```typescript
const unsubscribe = session.subscribe(event => {
  console.log(event.type)
})

unsubscribe()
```

Returns: `() => void`

### Messages and run control

#### `sendMessage({ content, files?, requestContext? })`

Send a user message. The session creates a thread first when no thread is active.

```typescript
await session.sendMessage({
  content: 'Summarize this file.',
  files: [{ data: fileContents, mediaType: 'text/plain', filename: 'notes.txt' }],
})
```

When `requestContext` carries `MASTRA_MESSAGE_AUTHOR_KEY`, the stored message names that sender under `content.providerMetadata.mastra.author`. See [Reserved keys](https://mastra.ai/docs/server/request-context).

#### `steer({ content, requestContext? })`

Queue steering content into an active run.

```typescript
await session.steer({ content: 'Focus on the failing tests.' })
```

#### `followUp({ content, requestContext? })`

Queue a follow-up while a run is active, or send it immediately while idle.

```typescript
await session.followUp({ content: 'Then propose a fix.' })
```

#### `getCurrentRunId()`

Return the active stream run identifier, the tracked run identifier, or `null` while idle.

```typescript
const runId = session.getCurrentRunId()
```

Returns: `string | null`

#### `abort()`

Abort the active run and clear pending suspension display state.

```typescript
session.abort()
```

### Workspace

#### `getWorkspace()`

Return the workspace resolved for this session. This preserves session-level overrides and workspaces selected from the session scope.

A workspace is optional. When neither the session nor the `AgentController` configures one, the session still runs without filesystem or sandbox access, and this returns `undefined`. Check the result before using it.

```typescript
const workspace = session.getWorkspace()
const skill = await workspace?.skills?.get('code-review')
```

Returns: `Workspace | undefined`

### Session grants

Session-scoped grants auto-approve tools without prompting. Grants are ephemeral: they reset when the session restarts and are never persisted.

#### `grantCategory(category)`

Grant a tool category for the current session. Tools in this category are auto-approved.

```typescript
session.grantCategory('edit')
```

#### `grantTool(toolName)`

Grant a specific tool for the current session.

```typescript
session.grantTool('mastra_workspace_execute_command')
```

#### `getGrants()`

Return the currently granted categories and tools.

```typescript
const grants = session.getGrants()
// { categories: string[], tools: string[] }
```

#### `hasCategoryGrant(category)`

Return whether a category has an in-memory session grant.

```typescript
const allowed = session.hasCategoryGrant('edit')
```

Returns: `boolean`

#### `hasToolGrant(toolName)`

Return whether a tool has an in-memory session grant.

```typescript
const allowed = session.hasToolGrant('write_file')
```

Returns: `boolean`

### Tool approvals

#### `resolveToolApproval(toolName)`

Return the effective policy after applying explicit tool rules, session grants, and category rules.

```typescript
const policy = session.resolveToolApproval('execute_command')
```

Returns: `PermissionPolicy`

#### `respondToToolApproval({ decision, toolCallId, requestContext?, declineContext? })`

Respond to a pending tool approval request, raised by a `tool_approval_required` event. `toolCallId` is required and identifies the parked call, so pass the `toolCallId` from the event. Pass `always_allow_category` to also grant the tool's whole category for the gate's thread.

```typescript
session.respondToToolApproval({ decision: 'approve', toolCallId: event.toolCallId })
session.respondToToolApproval({ decision: 'decline', toolCallId: event.toolCallId })
session.respondToToolApproval({ decision: 'always_allow_category', toolCallId: event.toolCallId })
```

#### `respondToToolSuspension({ resumeData, toolCallId?, requestContext? })`

Resume a suspended tool with application-provided data. Supply `toolCallId` when several tool calls are suspended.

```typescript
await session.respondToToolSuspension({
  toolCallId: event.toolCallId,
  resumeData: ['src'],
})
```

For `submit_plan`, pass `{ action: 'approved' }` or `{ action: 'rejected', feedback }`. Approval can switch to the mode configured by `transitionsTo` before the tool resumes.

### Token usage

#### `getTokenUsage()`

Return a copy of the running token-usage tally for the active thread.

```typescript
const usage = session.getTokenUsage()
// { promptTokens, completionTokens, totalTokens, ... }
```

## Identity

`session.identity` owns the stable identifiers for the conversation: the resource ID and session `id`, plus an `ownerId`. The `id` and `ownerId` are stable for the life of the session and don't change when the resource ID is switched. They mirror the `id` and `ownerId` fields on `SessionRecord` in storage.

### `session.identity.getId()`

Return the stable session identifier.

```typescript
const sessionId = session.identity.getId()
```

### `session.identity.getOwnerId()`

Return the stable owner identifier for the session.

```typescript
const ownerId = session.identity.getOwnerId()
```

### `session.identity.getResourceId()`

Return the current resource ID.

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

### `session.identity.getDefaultResourceId()`

Return the resource ID the session was created with.

```typescript
const defaultResourceId = session.identity.getDefaultResourceId()
```

To change the resource ID, use [`controller.setResourceId()`](https://mastra.ai/reference/agent-controller/agent-controller-class), which also clears the active thread. The session `id` and `ownerId` aren't affected by resource switches.

## Thread

`session.thread` owns the active thread binding and resource-scoped thread operations. Stored threads and messages can survive controller recreation when storage is configured. The live session and its event bus don't.

### `session.thread.create({ title?, id?, requestContext? })`

Create a thread, bind the session to it, and open its event stream. Pass `requestContext` when memory is a function that reads the signed-in user.

```typescript
const thread = await session.thread.create({
  id: 'thread-7',
  title: 'Investigate login failure',
})
```

Returns: `Promise<AgentControllerThread>`

### `session.thread.rename({ title })`

Rename the active stored thread.

```typescript
await session.thread.rename({ title: 'Fix login failure' })
```

### `session.thread.clone({ sourceThreadId?, title?, resourceId?, requestContext? })`

Clone an owned thread and its messages, then bind the session to the clone. Pass `requestContext` when memory is a function that reads the signed-in user.

```typescript
const clone = await session.thread.clone({
  sourceThreadId: 'thread-7',
  title: 'Alternative approach',
})
```

Returns: `Promise<AgentControllerThread>`

### `session.thread.switch({ threadId, emitEvent?, requestContext? })`

Switch to an owned stored thread and hydrate its mode, model, and observational memory settings. Pass `requestContext` when memory is a function that reads the signed-in user.

```typescript
await session.thread.switch({ threadId: 'thread-8' })
```

### `session.thread.delete({ threadId, requestContext? })`

Delete an owned thread. Deleting the active thread also clears the current binding. Pass `requestContext` when memory is a function that reads the signed-in user.

```typescript
await session.thread.delete({ threadId: 'thread-8' })
```

### `session.thread.getId()`

Return the active thread ID, or `null` when no thread is bound.

```typescript
const threadId = session.thread.getId()
```

### `session.thread.ensureId({ requestContext? })`

Return the active thread ID, creating and binding a new thread when none is bound. Concurrent calls share one thread. Use it with sessions created with `createInitialThread: false`.

```typescript
const threadId = await session.thread.ensureId()
```

### `session.thread.list(options?)`

List threads from storage. By default only threads for the current resource are returned, and transient forked subagent threads are hidden.

```typescript
const threads = await session.thread.list()
const allThreads = await session.thread.list({ allResources: true })
const everything = await session.thread.list({ includeForkedSubagents: true })
```

### `session.thread.getById({ threadId })`

Return a single thread by ID, or `null` if it doesn't exist.

```typescript
const thread = await session.thread.getById({ threadId: 'thread-abc123' })
```

### `session.thread.listActiveMessages(options?)`

Retrieve messages for the active thread. Returns an empty array when no thread is bound.

```typescript
const messages = await session.thread.listActiveMessages({ limit: 50 })
```

### `session.thread.listMessages({ threadId, limit? })`

Retrieve messages for a specific thread.

```typescript
const messages = await session.thread.listMessages({ threadId: 'thread-abc123' })
```

The message-reading methods `listActiveMessages`, `listMessages`, and `firstUserMessage` return `MastraDBMessage` objects, while `firstUserMessages` returns a `Map<string, MastraDBMessage>` keyed by thread ID. Each message has a `role`, an `id`, a `createdAt`, and a `content` object with `content.format` and a `content.parts` array. Read text, reasoning, tool calls, and attachments from `content.parts`. Signals such as system reminders and notifications are returned as separate messages with `role: 'signal'`.

### `session.thread.firstUserMessage({ threadId })`

Retrieve the first user message for a thread, or `null` if none.

```typescript
const firstMsg = await session.thread.firstUserMessage({
  threadId: 'thread-abc123',
})
```

### `session.thread.firstUserMessages({ threadIds })`

Retrieve the first user message for many threads at once, returned as a map.

```typescript
const firstByThread = await session.thread.firstUserMessages({
  threadIds: ['thread-a', 'thread-b'],
})
```

### `session.thread.getSetting({ key })`

Read a setting from the active thread metadata.

```typescript
const value = await session.thread.getSetting({ key: 'omThreshold' })
```

### `session.thread.setSetting({ key, value })`

Write a setting to the active thread metadata.

```typescript
await session.thread.setSetting({ key: 'omThreshold', value: 0.8 })
```

### `session.thread.deleteSetting({ key })`

Remove a setting from the active thread metadata.

```typescript
await session.thread.deleteSetting({ key: 'omThreshold' })
```

## Mode

`session.mode` owns the active mode selection.

### `session.mode.get()`

Return the active mode ID.

```typescript
const modeId = session.mode.get()
```

### `session.mode.resolve()`

Return the full `AgentControllerMode` object for the active mode, resolved against the controller's configured modes.

```typescript
const mode = session.mode.resolve()
```

### `session.mode.switch({ modeId })`

Switch to another mode and persist the mode on the active thread. The session emits `mode_changed`. Switching modes doesn't change the active model; switch the model separately when your host assigns different models to different modes.

```typescript
await session.mode.switch({ modeId: 'build' })
```

## Model

`session.model` owns the session's single active model selection.

### `session.model.get()`

Return the active model ID.

```typescript
const modelId = session.model.get()
```

### `session.model.displayName()`

Return the last segment of the active model ID as a short display name. Returns `'unknown'` when no model is selected.

```typescript
const name = session.model.displayName()
```

### `session.model.hasSelection()`

Check whether a model is currently selected.

```typescript
if (session.model.hasSelection()) {
  // Ready to send messages
}
```

### `session.model.set({ modelId })`

Set the active model in memory without persisting it, reporting model use, or emitting a `model_changed` event.

```typescript
session.model.set({ modelId: 'anthropic/claude-sonnet-4-6' })
```

### `session.model.switch(modelId, options?)`

Switch the session's active model. The model ID is persisted as the thread's `currentModelId` and restored with that thread. Reports the selection to the controller's `modelUseCountTracker` and emits a `model_changed` event.

Pass `{ thinkingLevel }` as the second argument to switch and persist both values in one operation. The level must be `'off'`, `'low'`, `'medium'`, `'high'`, `'xhigh'`, or `'max'`, and the controller's state schema must accept it. Both live values are updated before events are emitted. Omitting the option preserves the existing thinking-level override. Every `model_changed` event includes the current `{ modelId, thinkingLevel }`, even when the thinking level is unchanged. The level is `undefined` when the session has no override.

```typescript
await session.model.switch('anthropic/claude-sonnet-4-6', {
  thinkingLevel: 'high',
})
```

The client SDK exposes the same operation as `session.switchModel(modelId, { thinkingLevel })`.

## Observational Memory

The observational-memory model selection, grouped by role under `session.om.observer` and `session.om.reflector`. Both roles expose the same methods. Reads return the value from session state when set, falling back to the controller's `omConfig` defaults.

### `session.om.observer.modelId()` / `session.om.reflector.modelId()`

Return the role's model ID, or `undefined` when neither session state nor `omConfig` provides one.

```typescript
const observer = session.om.observer.modelId()
const reflector = session.om.reflector.modelId()
```

### `session.om.observer.threshold()` / `session.om.reflector.threshold()`

Return the role's threshold in tokens (observation threshold for the observer, reflection threshold for the reflector), or `undefined` when unset.

```typescript
const observationThreshold = session.om.observer.threshold()
const reflectionThreshold = session.om.reflector.threshold()
```

### `session.om.observer.switchModel({ modelId })` / `session.om.reflector.switchModel({ modelId })`

Switch the role's model. Persists the setting to thread metadata and emits an `om_model_changed` event.

```typescript
await session.om.observer.switchModel({
  modelId: 'anthropic/claude-haiku-4-5',
})
await session.om.reflector.switchModel({
  modelId: 'anthropic/claude-haiku-4-5',
})
```

### `session.om.observer.resolvedModel()` / `session.om.reflector.resolvedModel()`

Resolve the role's model ID to a model instance via the configured model gateways, or `undefined` when no model ID is set or no resolver is configured.

```typescript
const observerModel = session.om.observer.resolvedModel()
const reflectorModel = session.om.reflector.resolvedModel()
```

## Permissions

`session.permissions` owns the tool-approval policy represented in `session.state`: the per-category and per-tool rules consulted during approval resolution. These are distinct from the in-memory grants documented under [Session grants](#session-grants). Grants reset with the live session. Permission rules aren't durable unless the host restores the corresponding session state.

### `session.permissions.getRules()`

Return the current permission rules, or empty rules (`{ categories: {}, tools: {} }`) when none are set.

```typescript
const rules = session.permissions.getRules()
// { categories: { execute: 'ask' }, tools: { dangerous_tool: 'deny' } }
```

### `session.permissions.setForCategory({ category, policy })`

Set the approval policy (`'allow' | 'ask' | 'deny'`) for a tool category. Resolves once the change is persisted to session state.

```typescript
await session.permissions.setForCategory({ category: 'execute', policy: 'ask' })
```

### `session.permissions.setForTool({ toolName, policy })`

Set the approval policy for a specific tool. Per-tool policies take precedence over category policies. Resolves once persisted.

```typescript
await session.permissions.setForTool({ toolName: 'dangerous_tool', policy: 'deny' })
```

## Subagents

`session.subagents` owns subagent configuration. It currently exposes the subagent model selection under `session.subagents.model`.

### `session.subagents.model.get({ agentType? })`

Return the subagent model ID, preferring the per-`agentType` value when one is given, then the global subagent model, or `null` when neither is set.

```typescript
const modelId = session.subagents.model.get({ agentType: 'explore' })
```

### `session.subagents.model.set({ modelId, agentType? })`

Set the subagent model ID. Pass an `agentType` to set a per-type override, or omit it to set the global default. Persists to thread settings and emits a `subagent_model_changed` event.

```typescript
// Set the global subagent model
await session.subagents.model.set({ modelId: 'anthropic/claude-sonnet-4-6' })

// Set a per-type override
await session.subagents.model.set({
  modelId: 'anthropic/claude-haiku-4-5',
  agentType: 'explore',
})
```

## Run

`session.run` owns run and trace identity plus abort state for the in-flight run.

### `session.run.getRunId()` / `getTraceId()`

Return the stored run ID and trace ID for the current run, or `null` when idle.

```typescript
const runId = session.run.getRunId()
const traceId = session.run.getTraceId()
```

### `session.run.isRunning()`

Return whether a run is currently in progress.

```typescript
if (session.run.isRunning()) {
  // A run is active
}
```

## Stream

`session.stream` owns the live subscription to the agent thread stream and its dedup key.

### `session.stream.activeRunId()`

Return the run ID active on the live stream, or `null` when no stream is open.

```typescript
const runId = session.stream.activeRunId()
```

### `session.stream.isActive()`

Return whether the stream currently has an active run.

```typescript
if (session.stream.isActive()) {
  // The current thread's stream is producing output
}
```

## Suspensions

`session.suspensions` owns parked interactive tool calls (such as `ask_user` and `request_access`) awaiting a resume.

### `session.suspensions.hasPending()`

Return whether any tool is currently suspended.

```typescript
if (session.suspensions.hasPending()) {
  // At least one interactive tool is waiting for a response
}
```

### `session.suspensions.has({ toolCallId })`

Return whether a specific tool call is suspended.

```typescript
const waiting = session.suspensions.has({ toolCallId: event.toolCallId })
```

Resume a suspended tool with [`session.respondToToolSuspension()`](#tool-approvals).

## Follow-ups

`session.followUps` owns the FIFO queue of messages submitted while a run is in progress.

### `session.followUps.count()`

Return the number of queued follow-ups.

```typescript
const queued = session.followUps.count()
```

### `session.followUps.isEmpty()`

Return whether the follow-up queue is empty.

```typescript
if (!session.followUps.isEmpty()) {
  // Messages are waiting to be processed
}
```

## Approval

`session.approval` owns the pending tool-approval gate.

### `session.approval.isArmed()`

Return whether a tool is currently awaiting an approval decision.

```typescript
if (session.approval.isArmed()) {
  // Show the approval prompt
}
```

Respond with [`session.respondToToolApproval()`](#tool-approvals).

## Display state

`session.displayState` owns the canonical `AgentControllerDisplayState` snapshot a UI renders from, and the reducer that keeps it in sync with every session event.

### `session.displayState.get()`

Return the current `AgentControllerDisplayState` snapshot for UI rendering.

```typescript
const displayState = session.displayState.get()
```

### `session.displayState.restoreTasks(tasks)`

Restore the task portion of the snapshot after a UI replays persisted task tool history. This is a pure update of the snapshot and doesn't emit an event, so re-render explicitly after calling it.

```typescript
session.displayState.restoreTasks(replayedTasks)
```

After every event, the session emits `display_state_changed` with the latest snapshot. Subscribe with [`session.subscribe()`](#identity-and-events) or read the current value from `session.displayState.get()`.

## State

`session.state` owns the schema-validated AgentController state for the conversation. It holds the current snapshot and validates updates against the `stateSchema` passed to the AgentController. Updates are serialized, and every change emits a `state_changed` event.

### `session.state.get()`

Return a readonly copy of the current state snapshot.

```typescript
const state = session.state.get()
```

### `session.state.set(updates)`

Merge a partial update into the state. The queue applies concurrent calls in order and validates them against the schema before emitting `state_changed` with the changed keys.

```typescript
await session.state.set({ yolo: true })
```

### `session.state.update(updater)`

Run an updater against the current snapshot and apply its result atomically within the write queue. Use this for read-modify-write changes that must see the latest state. The updater returns mergeable `updates` and optional `events`, plus the `result` value that `update()` resolves to.

```typescript
const added = await session.state.update(current => ({
  updates: { count: (current.count ?? 0) + 1 },
  result: (current.count ?? 0) + 1,
}))
```

## Persistence boundaries

A `Session` is a live runtime object. Its event bus, arbitrary `session.state`, permission rules, permission grants, pending approvals, suspensions, follow-ups, run state, and stream state don't automatically survive controller or process recreation. The host must restore any of this state when recreating a session.

Configured storage persists threads and messages along with token usage, while thread settings restore mode and model choices. Restored settings can include observational memory and subagent model selections with per-agent-type overrides. A chat channel can map back to stored threads. However, `AgentControllerChannels` keeps channel-to-session and auto-approval state in memory.

## Related

- [AgentController class](https://mastra.ai/reference/agent-controller/agent-controller-class)
- [AgentController overview](https://mastra.ai/docs/harness/agent-controller)
- [Threads and state](https://mastra.ai/docs/harness/agent-controller)
- [Tool approvals](https://mastra.ai/docs/harness/agent-controller)