## Summary
Type `chat.createStartSessionAction` against the chat agent so
`clientData` is typed end-to-end on the first turn. Closes the gap where
`useTriggerChatTransport`'s `startSession` callback already hands you a
typed `clientData` (via the transport generic) but the server-side
action couldn't accept it without untyped routing through the `metadata`
field.
## Design
`ChatStartSessionParams` gains a typed `clientData` field via the new
generic:
```ts
export type ChatStartSessionParams<TChat extends AnyTask = AnyTask> = {
chatId: string;
clientData?: InferChatClientData<TChat>;
triggerConfig?: Partial<SessionTriggerConfig>;
metadata?: Record<string, unknown>;
};
function createChatStartSessionAction<TChat extends AnyTask = AnyTask>(
taskId: string,
options?: CreateChatStartSessionActionOptions
): (params: ChatStartSessionParams<TChat>) => Promise<ChatStartSessionResult>
```
When provided, `clientData` is folded into the first run's
`triggerConfig.basePayload.metadata`, so `onPreload` / `onChatStart` see
the same shape per-turn `metadata` carries via the transport. The opaque
session-level `metadata` field stays exactly as before — it lands on the
Session row, not the run payload.
## Usage
```ts
// actions.ts
import { chat } from "@trigger.dev/sdk/ai";
import type { myChat } from "@/trigger/chat";
export const startChatSession = chat.createStartSessionAction<typeof myChat>("my-chat");
```
```tsx
// Chat.tsx
const transport = useTriggerChatTransport<typeof myChat>({
task: "my-chat",
accessToken: ({ chatId }) => mintChatAccessToken(chatId),
startSession: ({ chatId, clientData }) =>
startChatSession({ chatId, clientData }),
});
```
## Test plan
- [x] `pnpm run build --filter @trigger.dev/sdk` passes
- [ ] Verify a `chat.agent` with `clientDataSchema` reads the typed
clientData from `onPreload` payload metadata on the first turn
Eric Allam
9ff410bfa4