> ## Documentation Index
> Fetch the complete documentation index at: https://trigger.dev/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Upgrade Guide: prerelease → Sessions-as-run-manager

> Migrating chat.agent code from the prerelease API to the Sessions-as-run-manager release.

<Warning>
  The AI Agents and Prompts surface ships as part of the **v4.5 release candidate**. Install with `@trigger.dev/sdk@rc` (or pin `4.5.0-rc.0` or later) to use these features — they aren't yet on the latest stable, and APIs may still change before the 4.5.0 GA. See [supported AI SDK versions](/ai-chat/reference#compatibility) and the [AI chat changelog](/ai-chat/changelog) for details.
</Warning>

This guide is for customers who tried `chat.agent` during the prerelease period.
The public surface of `chat.agent({...})`, `useTriggerChatTransport`,
`AgentChat`, `chat.defer`, and `chat.history` is largely
unchanged — but the transport's auth callbacks and the server-side helpers
that feed them were reshaped, so most prerelease apps need a small wiring
update.

## TL;DR

<CodeGroup>
  ```ts before.ts theme={"theme":"css-variables"}
  // Single accessToken callback, dispatches on purpose
  accessToken: async ({ chatId, purpose }) => {
    if (purpose === "trigger") {
      return chat.createAccessToken<typeof myChat>("my-chat");
    }
    // purpose === "preload" — same call, same trigger token
    return chat.createAccessToken<typeof myChat>("my-chat");
  };
  ```

  ```ts after.ts theme={"theme":"css-variables"}
  // Two callbacks: pure refresh + server action that creates the session
  accessToken: ({ chatId }) => mintChatAccessToken(chatId),
  startSession: ({ chatId, clientData }) =>
        startChatSession({ chatId, clientData }),
  ```
</CodeGroup>

What changed:

* `accessToken` is now a **pure session-PAT mint** — called only on 401/403
  to refresh. It must return a token scoped to the session, not a
  `trigger:tasks` JWT.
* `startSession` is a **new callback** that wraps a server action calling
  `chat.createStartSessionAction(taskId)`. The transport invokes it on
  `transport.preload(chatId)` and lazily on the first `sendMessage` for
  any chatId without a cached PAT.
* `ChatSession` persistable state drops `runId` — store only
  `{publicAccessToken, lastEventId?}`.
* Per-call options on `transport.preload(chatId, ...)` are gone. Trigger
  config (machine, idleTimeoutInSeconds, tags, queue, maxAttempts) lives
  server-side in `chat.createStartSessionAction(taskId, options)`.

<Note>
  The architectural shift is that `chat.agent` no longer rolls its own
  per-run streams. It runs on top of a durable **Session** row that owns
  its current run, persists across run lifecycles, and orchestrates
  upgrades server-side. The customer-facing surface is similar; the wire
  path beneath it changed completely.
</Note>

## Step 1: Replace your access-token server action with two server actions

The old pattern was a single helper that minted a trigger token:

```ts app/actions.ts (before) theme={"theme":"css-variables"}
"use server";

import { chat } from "@trigger.dev/sdk/ai";
import type { myChat } from "@/trigger/chat";

export const getChatToken = () =>
  chat.createAccessToken<typeof myChat>("my-chat");
```

Replace with two helpers — one for session creation, one for PAT refresh:

```ts app/actions.ts (after) theme={"theme":"css-variables"}
"use server";

import { auth } from "@trigger.dev/sdk";
import { chat } from "@trigger.dev/sdk/ai";

// Server-side wrapper for session creation. Idempotent on (env, chatId).
// The customer's server is the only entry point that creates Session rows;
// the browser never holds a `trigger:tasks` JWT.
export const startChatSession = chat.createStartSessionAction("my-chat");

// Pure session-PAT mint for the transport's 401/403 retry path.
export async function mintChatAccessToken(chatId: string) {
  return auth.createPublicToken({
    scopes: {
      read: { sessions: chatId },
      write: { sessions: chatId },
    },
    expirationTime: "1h",
  });
}
```

`chat.createStartSessionAction(taskId)` returns a server action that:

1. Creates the Session row for `chatId` (idempotent on the
   `(env, externalId)` unique pair).
2. Triggers the agent task's first run with
   `basePayload: {messages: [], trigger: "preload"}` defaults plus any
   overrides you pass.
3. Returns `{sessionId, runId, publicAccessToken}` to the browser.

## Step 2: Update the transport wiring

The transport now takes two callbacks instead of one:

```tsx app/components/chat.tsx (after) theme={"theme":"css-variables"}
"use client";

import { useChat } from "@ai-sdk/react";
import { useTriggerChatTransport } from "@trigger.dev/sdk/chat/react";
import type { myChat } from "@/trigger/chat";
import { mintChatAccessToken, startChatSession } from "@/app/actions";

export function Chat() {
  const transport = useTriggerChatTransport<typeof myChat>({
    task: "my-chat",
    accessToken: ({ chatId }) => mintChatAccessToken(chatId),
    startSession: ({ chatId, clientData }) =>
      startChatSession({ chatId, clientData }),
  });

  const { messages, sendMessage, status } = useChat({ transport });
  // ...
}
```

The transport calls them in two distinct flows:

| Trigger                                                          | Callback fired              |
| ---------------------------------------------------------------- | --------------------------- |
| `transport.preload(chatId)`                                      | `startSession`              |
| First `sendMessage` for a chatId with no cached PAT              | `startSession` (auto)       |
| Any 401/403 from `.in/append`, `.out` SSE, or `end-and-continue` | `accessToken`               |
| Page hydrates with `sessions: { [chatId]: ... }`                 | Neither (uses hydrated PAT) |

`startSession` is deduped via an in-flight promise — concurrent
`preload` + `sendMessage` calls converge to one server action invocation.

## Step 3: Drop transport-level trigger config

The prerelease transport accepted `triggerConfig`, `triggerOptions`, and
per-call options on `preload`. All of that moved server-side:

```ts before theme={"theme":"css-variables"}
const transport = useTriggerChatTransport({
  task: "my-chat",
  accessToken: getChatToken,
  triggerConfig: { basePayload: { /* ... */ } },
  triggerOptions: { tags: [...], machine: "small-1x", maxAttempts: 3 },
});

transport.preload(chatId, { idleTimeoutInSeconds: 60, metadata: { ... } });
```

```ts after theme={"theme":"css-variables"}
// Trigger config now lives in chat.createStartSessionAction
export const startChatSession = chat.createStartSessionAction("my-chat", {
  triggerConfig: {
    machine: "small-1x",
    maxAttempts: 3,
    tags: ["my-tag"],
    idleTimeoutInSeconds: 60,
  },
});

// Browser side
const transport = useTriggerChatTransport<typeof myChat>({
  task: "my-chat",
  accessToken: ({ chatId }) => mintChatAccessToken(chatId),
  startSession: ({ chatId, clientData }) =>
      startChatSession({ chatId, clientData }),
});

transport.preload(chatId);  // no second arg
```

For metadata that varies per chat, use `clientData` on the transport (see
the next step) — it's typed and threaded through `startSession` automatically.

## Step 4: Use `clientData` for typed payload metadata

If your agent uses `withClientData({schema})`, the transport's `clientData`
option is now the canonical place to set it. The same value:

* Is passed to your `startSession` callback as `params.clientData`, where
  you forward it into `chat.createStartSessionAction`'s
  `triggerConfig.basePayload.metadata`. The agent's first run sees it in
  `payload.metadata` (visible to `onPreload` / `onChatStart`).
* Merges into per-turn `metadata` on every `.in/append` chunk
  (visible to `onTurnStart` / inside `run` via `turn.clientData`).

```tsx theme={"theme":"css-variables"}
const transport = useTriggerChatTransport<typeof myChat>({
  task: "my-chat",
  accessToken: ({ chatId }) => mintChatAccessToken(chatId),
  startSession: ({ chatId, clientData }) =>
      startChatSession({ chatId, clientData }),
  clientData: {
    userId: currentUser.id,
    plan: currentUser.plan,
  },
});
```

The `clientData` value is live-updated when the option changes (the hook
calls `setClientData` under the hood), so dynamic values work without
reconstructing the transport.

<Tip>
  Server-side authorization can still override or augment the
  browser-claimed `clientData` inside `startSession` — never trust the
  browser's identity claim. A typical pattern: the server action looks up
  the user from the request session, then merges the trusted server fields
  on top of `params.clientData`.
</Tip>

## Step 5: Update your `ChatSession` persistence

If you persist session state across page loads, drop the `runId` field:

```ts before theme={"theme":"css-variables"}
type ChatSession = {
  runId: string;
  publicAccessToken: string;
  lastEventId?: string;
};
```

```ts after theme={"theme":"css-variables"}
type ChatSession = {
  publicAccessToken: string;
  lastEventId?: string;
};
```

If your DB has a `runId` column, you can drop it (the transport doesn't
read it) or keep it for telemetry. The current run ID lives on the
Session row server-side now.

Hydration on page reload is unchanged:

```tsx theme={"theme":"css-variables"}
const transport = useTriggerChatTransport<typeof myChat>({
  // ...
  sessions: persistedSession
    ? { [chatId]: persistedSession }
    : {},
});
```

## `chat.requestUpgrade()`: same call, faster handoff

Calling `chat.requestUpgrade()` inside `onTurnStart` /
`onValidateMessages` still ends the current run so the next message starts
on the latest version. What changed is the mechanism:

* **Before:** the agent emitted a `trigger:upgrade-required` chunk on
  `.out`; the transport consumed it browser-side and triggered a new run.
* **After:** the agent calls `endAndContinueSession` server-to-server;
  the webapp triggers a new run and atomically swaps `Session.currentRunId`
  via optimistic locking. The browser's existing SSE subscription keeps
  receiving chunks across the swap — no transport-side bookkeeping.

The new run is recorded in a `SessionRun` audit row with
`reason: "upgrade"` for dashboard provenance.

## Hitting raw URLs

If your code talks to the realtime API directly instead of going through
the SDK, the URL shapes changed:

| Before                                                            | After                                                                                |
| ----------------------------------------------------------------- | ------------------------------------------------------------------------------------ |
| `GET /realtime/v1/streams/{runId}/chat`                           | `GET /realtime/v1/sessions/{chatId}/out`                                             |
| `POST /realtime/v1/streams/{runId}/{target}/chat-messages/append` | `POST /realtime/v1/sessions/{chatId}/in/append` (body: `{kind: "message", payload}`) |
| `POST /realtime/v1/streams/{runId}/{target}/chat-stop/append`     | `POST /realtime/v1/sessions/{chatId}/in/append` (body: `{kind: "stop"}`)             |

The session-scoped PAT
(`read:sessions:{chatId} + write:sessions:{chatId}`) authorizes both the
externalId form (`/sessions/my-chat-id/...`) and the friendlyId form
(`/sessions/session_abc.../...`). The transport always uses the
externalId form; the friendlyId form is available for dashboard tooling
and direct API consumers.

## What didn't change

* `chat.agent({...})` definition — `id`, `idleTimeoutInSeconds`,
  `clientDataSchema`, `actionSchema`, `hydrateMessages`, `onPreload`,
  `onChatStart`, `onValidateMessages`, `onTurnStart`, `onTurnComplete`,
  `onChatSuspend`, `run`. All callbacks have the same signature and
  fire at the same lifecycle points.
* `onAction` is still defined the same way, but its semantics changed
  in the [May 6 prerelease](/ai-chat/changelog) — actions are no longer
  turns, and `onAction` returning a `StreamTextResult` produces a model
  response.
* `chat.customAgent({...})` and the `chat.createSession(payload, ...)`
  helper for building a session loop manually inside a custom agent.
* `chat.defer` (deferred work) and `chat.history` (imperative history
  mutations from inside `onAction`).
* `AgentChat` (server-side chat client) — `agent`, `id`, `clientData`,
  `session`, `onTriggered`, `onTurnComplete`, `sendMessage`, `text()`.
* `useTriggerChatTransport` React semantics (created once, kept in a
  ref, callbacks updated under the hood).
* Multi-tab coordination (`multiTab: true`),
  [pending messages / steering](/ai-chat/pending-messages),
  [background injection](/ai-chat/background-injection),
  [compaction](/ai-chat/compaction).
* Per-turn `metadata` flowing through
  `sendMessage({ text }, { metadata })` to `turn.metadata` server-side.

## Verifying the migration

After updating, the smoke check is the same as before: send a message,
confirm the assistant streams a response, reload mid-stream, confirm
resume.

A few new things worth verifying once you've cut over:

* **Eager preload.** Click the button (or call `transport.preload(id)`
  programmatically) — your `startSession` callback should fire and a
  Session row + first run should be created before you send a message.
* **Idle-timeout continuation.** Wait past the agent's
  `idleTimeoutInSeconds` so the run exits, then send another message —
  the transport's `.in/append` should boot a new run on the same
  Session, with a `SessionRun` row of `reason: "continuation"`.
* **PAT refresh.** Force a stale PAT in your DB (corrupt the signature)
  and reload — the first request should 401, your `accessToken`
  callback should fire, and the retry should succeed.

If any of those misfire, check that:

* Your `accessToken` callback returns a token minted via
  `auth.createPublicToken({ scopes: { read: { sessions: chatId }, write: { sessions: chatId } } })`, **not**
  `chat.createAccessToken` or `auth.createTriggerPublicToken`. The
  transport rejects trigger tokens now.
* Your `startSession` callback returns
  `{publicAccessToken: string}` — the result of
  `chat.createStartSessionAction(taskId)({chatId, ...})` already has
  this shape.
* You haven't left a stale `getStartToken` option on the transport;
  it's not part of `TriggerChatTransportOptions` anymore.

## v4.5 wire format change

A second migration lands on top of the Sessions release. v4.5 removes the full-history wire payload — clients now ship at most one new `UIMessage` per `.in/append`, and the agent rebuilds prior history from a durable JSON snapshot in object storage plus a replay of the `session.out` tail.

If you use the built-in `TriggerChatTransport` / `AgentChat` and don't reach into the wire shape directly, **most apps need no changes** — the change is below the customer-facing surface. Customers who built custom transports, hit `/realtime/v1/sessions/{id}/in/append` directly, or rely on specific behaviors of `hydrateMessages` / `onChatStart` should read this section.

### Why the change

Long chats with heavy tool results were hitting the realtime API's 512 KiB body cap on `/in/append` once the accumulated `UIMessage[]` history (which the wire shipped in full on every send) crossed the limit. The 413 surfaced as a CORS error in browsers and stalled chats around turn 10–30 with tool use.

The wire is now **delta-only**: each `.in/append` carries at most one new `UIMessage`. The agent rebuilds prior history at run boot. The 512 KiB ceiling stops being pressure — typical payloads are a few KB regardless of chat length.

### Object-store configuration

Snapshot read/write uses Trigger.dev's existing object-store infrastructure — the same presigned-URL routes used for large payloads. Set the standard `OBJECT_STORE_*` env vars on your webapp deployment if you haven't already; MinIO and S3-compatible stores work via `OBJECT_STORE_DEFAULT_PROTOCOL`.

| Env var                          | Purpose                            |
| -------------------------------- | ---------------------------------- |
| `OBJECT_STORE_BASE_URL`          | Endpoint URL (S3, MinIO, R2, etc.) |
| `OBJECT_STORE_ACCESS_KEY_ID`     | Access key                         |
| `OBJECT_STORE_SECRET_ACCESS_KEY` | Secret key                         |
| `OBJECT_STORE_DEFAULT_PROTOCOL`  | `s3` (default), `minio`, etc.      |

Snapshots are written under `packets/{projectRef}/{envSlug}/sessions/{sessionId}/snapshot.json`. Each snapshot is small (typically tens of KB) and overwritten every turn — no append-only growth.

<Warning>
  **No object store + no `hydrateMessages` = conversations don't survive run boundaries.** With neither piece of state, a continuation boots empty and the agent can't reconstruct prior turns. Either configure an object store or register `hydrateMessages`. The runtime logs a warning at agent registration time when both are missing.
</Warning>

### Custom transports

If you've built your own transport (Slack bot, CLI, native app) against the [Client Protocol](/ai-chat/client-protocol), the `ChatTaskWirePayload` shape changed:

```ts before theme={"theme":"css-variables"}
type ChatTaskWirePayload = {
  messages: UIMessage[];        // full history
  chatId: string;
  trigger: "submit-message" | "regenerate-message" | "preload" | "close" | "action";
  // ...
};
```

```ts after theme={"theme":"css-variables"}
type ChatTaskWirePayload = {
  message?: UIMessage;          // singular, optional
  headStartMessages?: UIMessage[];  // chat.headStart only, "handover-prepare"
  chatId: string;
  trigger:
    | "submit-message"
    | "regenerate-message"
    | "preload"
    | "close"
    | "action"
    | "handover-prepare";
  // ...
};
```

What to send per trigger:

| Trigger                              | What to put in the payload                                                         |
| ------------------------------------ | ---------------------------------------------------------------------------------- |
| `submit-message`                     | The new user message (or a tool-approval-responded assistant message) in `message` |
| `regenerate-message`                 | No `message` — the agent trims its own tail                                        |
| `preload` / `close` / `action`       | No `message`                                                                       |
| `handover-prepare` (head-start only) | Full prior history in `headStartMessages` (route handler — not on `/in/append`)    |

The full wire breakdown is in the rewritten [Client Protocol](/ai-chat/client-protocol).

### `hydrateMessages` consumers

The hook signature is unchanged. Two behavior tightenings worth knowing:

1. **`incomingMessages` is now consistently 0-or-1-length.** Previously some triggers (`regenerate-message`, continuation) shipped full history; now all triggers ship at most one. If you assumed `incomingMessages` could contain multiple messages and acted on them as a batch, the loop now runs zero or one times. Patterns like the one below work the same — they just iterate fewer messages:

```ts theme={"theme":"css-variables"}
hydrateMessages: async ({ incomingMessages }) => {
  for (const msg of incomingMessages) {  // 0-or-1 iterations
    for (const r of chat.history.extractNewToolResults(msg)) {
      await auditLog.record({ id: r.toolCallId, output: r.output });
    }
  }
  return await db.getMessages(chatId);
}
```

2. **Registering `hydrateMessages` short-circuits snapshot+replay.** The runtime trusts your hook to be the source of truth, so it doesn't read or write the JSON snapshot or replay `session.out`. Zero object-store traffic. Trade-off: you own persistence end-to-end.

### `onChatStart` is now once-per-chat

`onChatStart` no longer fires on continuation runs (post-`endRun`, post-waitpoint-timeout, post-`chat.requestUpgrade`, post-cancel, post-crash) or on OOM-retry attempts. It fires **exactly once per chat**, on the very first user message of the chat's lifetime. The `continuation` and `previousRunId` fields on `ChatStartEvent` are now `@deprecated` (always `false` / `undefined` when the hook fires).

This makes once-per-chat setup code (create the Chat DB row, mint chat-scoped resources) safe to write without continuation gates. Drop any `if (continuation) return;` checks from `onChatStart`:

```ts before theme={"theme":"css-variables"}
onChatStart: async ({ continuation, chatId, clientData }) => {
  if (continuation) return;           // ❌ no longer needed — fires only on first message ever
  await db.chat.create({ /* ... */ });
}
```

```ts after theme={"theme":"css-variables"}
onChatStart: async ({ chatId, clientData }) => {
  await db.chat.create({ /* ... */ });  // ✅ guaranteed first-message-of-chat
}
```

If you need per-turn setup that **does** run on continuations, move it to [`onTurnStart`](/ai-chat/lifecycle-hooks#onturnstart) — that hook still fires on every turn, including the first turn of a continuation run.

### Move `chat.local` init from `onChatStart` to `onBoot`

Because `onChatStart` no longer fires on continuation runs, **`chat.local`** state initialized there will be missing when a continuation run starts — `run()` then crashes with `"chat.local can only be modified after initialization"`. The fix is to move per-process initialization to the new [`onBoot`](/ai-chat/lifecycle-hooks#onboot) hook, which fires once per worker boot (initial, preloaded, AND continuation):

```ts before theme={"theme":"css-variables"}
const userContext = chat.local<{ name: string; plan: string }>({ id: "userContext" });

onChatStart: async ({ clientData }) => {
  const user = await db.user.findUnique({ where: { id: clientData.userId } });
  userContext.init({ name: user.name, plan: user.plan }); // ❌ never runs on continuation
}
```

```ts after theme={"theme":"css-variables"}
const userContext = chat.local<{ name: string; plan: string }>({ id: "userContext" });

onBoot: async ({ clientData }) => {
  const user = await db.user.findUnique({ where: { id: clientData.userId } });
  userContext.init({ name: user.name, plan: user.plan }); // ✅ runs on every fresh worker
}
```

Anything else that's per-process (DB connection pools, sandbox handles, in-memory caches) belongs in `onBoot` for the same reason. Branch on `continuation` inside `onBoot` if you need to re-load state from your DB on takeover.

### Client-side `setMessages` doesn't round-trip

The new wire makes one thing explicit that was implicit before: **mutating `useChat()`'s messages on the client doesn't change the agent's history.** Full-history mutations were silently overwritten by the wire's accumulator before this release; now they aren't even shipped.

For history compaction, summarization, or branch-swap, mutate the agent's accumulator inside `onTurnStart` using [`chat.setMessages()`](/ai-chat/backend) or [`chat.history.set()`](/ai-chat/backend#chat-history). The client's `useChat` will reconcile against the next `session.out` payload.

### Verifying the v4.5 migration

After updating, the smoke check is the same as for v4.4:

* Send a message, confirm the assistant streams a response.
* Reload mid-stream, confirm resume.
* Send 30+ turns with tool calls — `.in/append` body sizes stay under \~5 KB the entire time. (Pre-change baseline: payloads grew past 512 KB around turn 10-30.)
* Idle out a run, send another message — the new run reads the snapshot, replays the tail, and continues seamlessly.

If continuations boot empty:

* Confirm `OBJECT_STORE_*` env vars are set on the webapp.
* Confirm the bucket key `packets/{projectRef}/{envSlug}/sessions/{sessionId}/snapshot.json` exists after a successful turn.
* Or — register `hydrateMessages` and let your DB be the source of truth.

## Reference

* [TriggerChatTransport options](/ai-chat/reference#triggerchattransport-options)
* [`chat.createStartSessionAction`](/ai-chat/reference)
* [Backend setup](/ai-chat/backend)
* [Frontend setup](/ai-chat/frontend)
* [Client Protocol](/ai-chat/client-protocol) — wire format reference
* [Persistence and replay](/ai-chat/patterns/persistence-and-replay) — snapshot model end-to-end