feat(sdk): type chat.createStartSessionAction against your chat agent (#3684)

## 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

Author: ericallam

.changeset/chat-start-session-action-typed-client-data.md

@@ -0,0 +1,22 @@
+---
+"@trigger.dev/sdk": patch
+---
+
+Type `chat.createStartSessionAction` against your chat agent so `clientData` is typed end-to-end on the first turn:
+
+```ts
+import { chat } from "@trigger.dev/sdk/ai";
+import type { myChat } from "@/trigger/chat";
+
+export const startChatSession = chat.createStartSessionAction<typeof myChat>("my-chat");
+
+// In the browser, threaded from the transport's typed startSession callback:
+const transport = useTriggerChatTransport<typeof myChat>({
+  task: "my-chat",
+  startSession: ({ chatId, clientData }) =>
+    startChatSession({ chatId, clientData }),
+  // ...
+});
+```
+
+`ChatStartSessionParams` gains a typed `clientData` field — folded into the first run's `payload.metadata` so `onPreload` / `onChatStart` see the same shape per-turn `metadata` carries via the transport. The opaque session-level `metadata` field is unchanged.

packages/trigger-sdk/src/v3/ai.ts

@@ -9040,17 +9040,29 @@ export type CreateChatStartSessionActionOptions = {
 /**
  * Params for the function returned by {@link createChatStartSessionAction}.
  */
-export type ChatStartSessionParams = {
+export type ChatStartSessionParams<TChat extends AnyTask = AnyTask> = {
   /** Conversation id (mapped to the Session's `externalId`). */
   chatId: string;
+  /**
+   * Typed client data — folded into the first run's `payload.metadata` so
+   * `onPreload`, `onChatStart`, etc. see the same `clientData` shape on the
+   * first turn as subsequent turns get via the transport's `clientData`
+   * option. Typed via the agent's `clientDataSchema` when the action is
+   * parameterised with `createStartSessionAction<typeof myChat>(...)`.
+   */
+  clientData?: InferChatClientData<TChat>;
   /**
    * Per-call trigger config. Shallow-merged over the action's default
    * `triggerConfig`. `basePayload` is the customer's wire payload (for
    * `chat.agent`: anything beyond `chatId`/`messages`/`trigger`/`metadata`,
    * which the runtime injects automatically).
    */
   triggerConfig?: Partial<SessionTriggerConfig>;
-  /** Pass-through metadata folded into the session row. */
+  /**
+   * Opaque session-level metadata stored on the Session row. Separate from
+   * the per-turn `clientData` above. Use this when you want to attach
+   * server-side metadata that doesn't go through the agent's `clientDataSchema`.
+   */
   metadata?: Record<string, unknown>;
 };
 
@@ -9078,33 +9090,37 @@ export type ChatStartSessionResult = {
  * Wrap in a Next.js server action (or any server-side handler) so the
  * customer's secret key never crosses to the browser.
  *
+ * Parameterise the action with `<typeof yourChatAgent>` to type the
+ * `clientData` field against your agent's `clientDataSchema`.
+ *
  * @example
  * ```ts
  * // actions.ts
  * "use server";
  * import { chat } from "@trigger.dev/sdk/ai";
+ * import type { myChat } from "@/trigger/chat";
  *
- * export const startChatSession = chat.createStartSessionAction("my-chat", {
- *   triggerConfig: { machine: "small-1x" },
- * });
+ * export const startChatSession = chat.createStartSessionAction<typeof myChat>(
+ *   "my-chat",
+ *   { triggerConfig: { machine: "small-1x" } }
+ * );
  * ```
  *
- * Then in the browser:
+ * Then in the browser, threading the typed `clientData` from the transport:
  * ```tsx
- * const transport = useTriggerChatTransport({
+ * const transport = useTriggerChatTransport<typeof myChat>({
  *   task: "my-chat",
- *   accessToken: async ({ chatId }) => {
- *     const { publicAccessToken } = await startChatSession({ chatId });
- *     return publicAccessToken;
- *   },
+ *   accessToken: ({ chatId }) => mintChatAccessToken(chatId),
+ *   startSession: ({ chatId, clientData }) =>
+ *     startChatSession({ chatId, clientData }),
  * });
  * ```
  */
-function createChatStartSessionAction(
+function createChatStartSessionAction<TChat extends AnyTask = AnyTask>(
   taskId: string,
   options?: CreateChatStartSessionActionOptions
-): (params: ChatStartSessionParams) => Promise<ChatStartSessionResult> {
-  return async (params: ChatStartSessionParams): Promise<ChatStartSessionResult> => {
+): (params: ChatStartSessionParams<TChat>) => Promise<ChatStartSessionResult> {
+  return async (params: ChatStartSessionParams<TChat>): Promise<ChatStartSessionResult> => {
     if (!params.chatId) {
       throw new Error(
         "chat.createStartSessionAction: params.chatId is required — used as the session externalId."
@@ -9117,21 +9133,25 @@ function createChatStartSessionAction(
     // `onPreload` fires, the runtime opens its `.in` subscription, the
     // first user message arrives moments later via `.in/append`.
     //
-    // `metadata` is the customer's transport-level `clientData`,
-    // threaded through so the agent's `clientDataSchema` validates on
-    // the very first turn (the typical schema requires `userId` etc.).
+    // `clientData` is folded into `basePayload.metadata` so the agent's
+    // `clientDataSchema` validates on the very first turn against the same
+    // shape per-turn `metadata` carries via the transport.
     // Auto-tag every chat.agent run with `chat:{chatId}` so the dashboard /
     // run-list filter by chat works without the customer having to wire it
     // up. Mirrors the browser-mediated `TriggerChatTransport.doStart` path.
     const userTags = params.triggerConfig?.tags ?? options?.triggerConfig?.tags ?? [];
     const tags = [`chat:${params.chatId}`, ...userTags].slice(0, 5);
 
+    const clientDataMetadata =
+      params.clientData !== undefined ? { metadata: params.clientData } : {};
+
     const triggerConfig: SessionTriggerConfig = {
       basePayload: {
         messages: [],
         trigger: "preload",
         ...(options?.triggerConfig?.basePayload ?? {}),
         ...(params.triggerConfig?.basePayload ?? {}),
+        ...clientDataMetadata,
         chatId: params.chatId,
       },
       ...(options?.triggerConfig?.machine || params.triggerConfig?.machine

packages/trigger-sdk/src/v3/createStartSessionAction.test.ts

@@ -0,0 +1,136 @@
+import { afterEach, describe, expect, expectTypeOf, it } from "vitest";
+import { z } from "zod";
+import type { CreateSessionRequestBody, CreatedSessionResponseBody } from "@trigger.dev/core/v3";
+
+import { chat } from "./ai.js";
+import {
+  __setSessionStartImplForTests,
+  __setSessionOpenImplForTests,
+  SessionHandle,
+} from "./sessions.js";
+import { apiClientManager } from "@trigger.dev/core/v3";
+
+// `auth.createPublicToken` is called by the action when no start token is
+// supplied. Provide a minimal API client config so the mint path doesn't
+// throw before we get to assert the captured request body.
+apiClientManager.setGlobalAPIClientConfiguration({
+  baseURL: "https://example.invalid",
+  accessToken: "tr_test_secret",
+});
+
+// Capture the request body the action would send to `sessions.start()`.
+let lastStartBody: CreateSessionRequestBody | undefined;
+
+function installStartFixture() {
+  __setSessionStartImplForTests(async (body): Promise<CreatedSessionResponseBody> => {
+    lastStartBody = body;
+    return {
+      id: "session_fixture",
+      externalId: body.externalId ?? null,
+      type: body.type,
+      taskIdentifier: body.taskIdentifier,
+      triggerConfig: body.triggerConfig,
+      currentRunId: "run_fixture",
+      tags: body.triggerConfig.tags ?? [],
+      metadata: body.metadata ?? null,
+      closedAt: null,
+      closedReason: null,
+      expiresAt: null,
+      createdAt: new Date(),
+      updatedAt: new Date(),
+      runId: "run_fixture",
+      publicAccessToken: "tr_pat_fixture",
+      isCached: false,
+    };
+  });
+  __setSessionOpenImplForTests(() => new SessionHandle("session_fixture"));
+}
+
+afterEach(() => {
+  __setSessionStartImplForTests(undefined);
+  __setSessionOpenImplForTests(undefined);
+  lastStartBody = undefined;
+});
+
+// Build a fake chat agent task shape that the generic can narrow against.
+// We only need the static type — the runtime never invokes this task because
+// `__setSessionStartImplForTests` intercepts the network call.
+const fakeChat = chat
+  .withClientData({
+    schema: z.object({
+      userId: z.string(),
+      plan: z.enum(["free", "pro"]),
+    }),
+  })
+  .agent({
+    id: "fake-chat",
+    run: async () => undefined as any,
+  });
+
+describe("chat.createStartSessionAction — runtime", () => {
+  it("folds typed clientData into basePayload.metadata so onChatStart sees it on the first turn", async () => {
+    installStartFixture();
+
+    const start = chat.createStartSessionAction<typeof fakeChat>("fake-chat");
+
+    const result = await start({
+      chatId: "chat-1",
+      clientData: { userId: "u-1", plan: "pro" },
+    });
+
+    expect(result.publicAccessToken).toBe("tr_pat_fixture");
+    expect(lastStartBody?.triggerConfig.basePayload).toMatchObject({
+      messages: [],
+      trigger: "preload",
+      metadata: { userId: "u-1", plan: "pro" },
+      chatId: "chat-1",
+    });
+  });
+
+  it("leaves basePayload.metadata unset when clientData is not provided", async () => {
+    installStartFixture();
+
+    const start = chat.createStartSessionAction("fake-chat");
+    await start({ chatId: "chat-2" });
+
+    expect(lastStartBody?.triggerConfig.basePayload).not.toHaveProperty("metadata");
+  });
+
+  it("keeps session-level metadata distinct from per-turn clientData", async () => {
+    installStartFixture();
+
+    const start = chat.createStartSessionAction<typeof fakeChat>("fake-chat");
+    await start({
+      chatId: "chat-3",
+      clientData: { userId: "u-3", plan: "free" },
+      metadata: { source: "marketing-site" },
+    });
+
+    // Per-turn shape (visible to onPreload / onChatStart):
+    expect(lastStartBody?.triggerConfig.basePayload).toMatchObject({
+      metadata: { userId: "u-3", plan: "free" },
+    });
+    // Session-row metadata (opaque, never typed via clientDataSchema):
+    expect(lastStartBody?.metadata).toEqual({ source: "marketing-site" });
+  });
+});
+
+describe("chat.createStartSessionAction — types", () => {
+  it("narrows clientData against the chat agent's clientDataSchema", () => {
+    const start = chat.createStartSessionAction<typeof fakeChat>("fake-chat");
+
+    // The clientData field is typed off the agent's schema.
+    expectTypeOf<Parameters<typeof start>[0]["clientData"]>().toEqualTypeOf<
+      { userId: string; plan: "free" | "pro" } | undefined
+    >();
+    // The agent's typed clientData is strictly narrower than `unknown`.
+    expectTypeOf<Parameters<typeof start>[0]["clientData"]>().not.toEqualTypeOf<unknown>();
+  });
+
+  it("defaults clientData to unknown when called without a generic", () => {
+    const start = chat.createStartSessionAction("fake-chat");
+    expectTypeOf(start).parameter(0).toHaveProperty("clientData");
+    // Untyped variant — clientData is `unknown`.
+    expectTypeOf<Parameters<typeof start>[0]["clientData"]>().toEqualTypeOf<unknown>();
+  });
+});