Keep Files After Reload

You have a sandbox chat. The agent writes files. The provider sandbox then goes away. The next run starts empty.

Portable snapshots save those files after each successful terminal run. A later run restores the latest checkpoint into a new private sandbox. By the end of this page, chat() writes and restores those checkpoints.

Create one persistence value. Pass that exact value to withPersistence and to the snapshots object. Put withPersistence before withSandbox.

This page is enough for automatic save and restore. When you need a named version, a fork, a download, or a file policy, add Save a Named Version, Branch From a Version, Send a Frozen File, or Pick Which Files to Keep.

Create new persistence#

Use memorySandboxSnapshots for local development. It creates persistence, a checkpoint store, and the snapshot methods as one object. It does not load @tanstack/ai-persistence at runtime.

import { chat } from '@tanstack/ai'
import { grokBuildText } from '@tanstack/ai-grok-build'
import { withPersistence } from '@tanstack/ai-persistence'
import {
  defineSandbox,
  defineWorkspace,
  InMemorySandboxInstanceStore,
  memorySandboxSnapshots,
  withSandbox,
} from '@tanstack/ai-sandbox'
import { dockerSandbox } from '@tanstack/ai-sandbox-docker'

const instances = new InMemorySandboxInstanceStore()
const userId = 'user-123'

const sandbox = defineSandbox({
  id: 'app-builder',
  provider: dockerSandbox({ image: 'node:22' }),
  workspace: defineWorkspace({ source: { type: 'none' } }),
  lifecycle: { reuse: 'thread' },
})

const snapshots = await memorySandboxSnapshots({
  sandbox,
  instances,
})

const result = chat({
  threadId: 'app-thread',
  context: { userId },
  adapter: grokBuildText('composer-2.5'),
  messages: [{ role: 'user', content: 'Create a landing page.' }],
  middleware: [
    withPersistence(snapshots.persistence),
    withSandbox(sandbox, {
      instances,
      snapshots,
    }),
  ],
})

void result

You can bind sandbox, instances, tenant, and locks at create time. A later snapshots.save call can override those values. See Save a Named Version.

Keep instances in the same server module as this middleware. A named save must use this same instance store.

Pass the session userId in context for every run. Pass that same user id as tenant.userId on snapshots.save.

Reuse existing persistence#

If withPersistence already uses a persistence object, pass that same object to createSandboxSnapshots. Do not create a second message store.

The persistence object must include these stores:

• messages

• artifacts (with listForThread)

• blobs

  You also need a checkpoint store. That store is not a persistence store.

  If you already keep generated files, you already have artifacts and blobs. Use those same stores.

  If you only have messages, add artifacts and blobs to that same adapter. See Which stores do you need? and Build a generation adapter.

import { chat } from '@tanstack/ai'
import { grokBuildText } from '@tanstack/ai-grok-build'
import { withPersistence } from '@tanstack/ai-persistence'
import {
  createSandboxSnapshots,
  InMemorySandboxCheckpointStore,
  withSandbox,
} from '@tanstack/ai-sandbox'
import { instances, persistence, sandbox } from './sandbox-server'

const snapshots = createSandboxSnapshots({
  persistence,
  checkpoints: new InMemorySandboxCheckpointStore(),
  sandbox,
  instances,
})

const result = chat({
  threadId: 'app-thread',
  adapter: grokBuildText('composer-2.5'),
  messages: [{ role: 'user', content: 'Create a landing page.' }],
  middleware: [
    withPersistence(snapshots.persistence),
    withSandbox(sandbox, {
      instances,
      snapshots,
    }),
  ],
})

void result

Use durable stores in production#

memorySandboxSnapshots is for local development and tests. Production needs durable message, artifact, and blob stores, plus a durable checkpoint store.

The React chat example exports sqliteSandboxSnapshots() for a Node 22.5+ server. That function is an example adapter. It is not a package export.

Use one SQLite transaction for every checkpoint write, head update, and blob reference count update. Use one transaction for a fork. The fork transaction must also copy the source conversation. It must reject a destination thread that already has persisted state.

Writer leases#

Each thread has one checkpoint writer lease.

• A second run for the same thread gets a writer conflict while the lease is active.

• The middleware renews the lease while the run is active.

• Pause and detach paths release the lease. They do not publish a partial checkpoint.

• If the writer loses the lease, the middleware does not publish the checkpoint. A later successful run can create a new checkpoint.

What happens on the next run#

A later run restores the latest checkpoint only into a new private sandbox. A live resumed sandbox keeps its current files. See Portable Sandbox Snapshots for that restore rule.

Instance durability finds a provider sandbox across server processes. When that sandbox is gone, portable snapshots rebuild the workspace. See Instance Durability.

The default policy excludes .git, node_modules, and .env* paths. To keep only some files, or one file, see Pick Which Files to Keep. Read What a Snapshot Stores for secrets and restore safety.