feat(trigger-sdk): add streamBaseURL to TriggerChatTransport (#3641)

`TriggerChatTransport` had a single `baseURL` option covering both the
`.in/append` POSTs and the long-lived `.out` SSE subscription. Customers
wanting to route the SSE through a proxy (e.g. a Cloudflare worker
capturing JA4 fingerprints for bot detection) had to send every append
through the proxy too, adding a hop to every user message.

New optional `streamBaseURL` overrides the SSE base URL only; appends
keep using `baseURL`. Falls back to `baseURL` when unset, so existing
transports are unchanged.

```ts
const transport = new TriggerChatTransport({
  task: "ai-chat",
  baseURL: "https://api.trigger.dev",
  streamBaseURL: "https://chat-proxy.example.com",
  accessToken,
  startSession,
});
```

Verified with a new test in `chat.test.ts` that asserts `.in/append`
routes through `baseURL` and `.out` SSE routes through `streamBaseURL`.
All existing tests still pass.

Author: ericallam

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

@@ -568,6 +568,40 @@ describe("TriggerChatTransport", () => {
       expect(subscribe!).toContain("/realtime/v1/sessions/chat-by-chatid/out");
     });
 
+    it("routes .out SSE through streamBaseURL while appends stay on baseURL", async () => {
+      const requests: string[] = [];
+      global.fetch = vi.fn().mockImplementation(async (url: string | URL) => {
+        const urlStr = typeof url === "string" ? url : url.toString();
+        requests.push(urlStr);
+        if (isSessionStreamAppendUrl(urlStr)) return defaultAppendResponse();
+        if (isSessionOutSubscribeUrl(urlStr)) return defaultSseResponse();
+        throw new Error(`Unexpected URL: ${urlStr}`);
+      });
+
+      const transport = new TriggerChatTransport({
+        task: "my-chat-task",
+        accessToken: () => "pat",
+        baseURL: "https://api.test.trigger.dev",
+        streamBaseURL: "https://chat-proxy.example.com",
+        sessions: { "chat-split": { publicAccessToken: "p" } },
+      });
+
+      const stream = await transport.sendMessages({
+        trigger: "submit-message",
+        chatId: "chat-split",
+        messageId: undefined,
+        messages: [createUserMessage("Hi")],
+        abortSignal: undefined,
+      });
+      await drainChunks(stream);
+
+      const append = requests.find(isSessionStreamAppendUrl);
+      const subscribe = requests.find(isSessionOutSubscribeUrl);
+      expect(append!.startsWith("https://api.test.trigger.dev/")).toBe(true);
+      expect(subscribe!.startsWith("https://chat-proxy.example.com/")).toBe(true);
+      expect(subscribe!).toContain("/realtime/v1/sessions/chat-split/out");
+    });
+
     it("for submit-message, only the latest message is delivered to .in", async () => {
       // Slim wire: each `.in/append` carries at most ONE new message in
       // `payload.message` (singular). Even if the caller hands sendMessages

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

@@ -225,6 +225,15 @@ export type TriggerChatTransportOptions<TClientData = unknown> = {
   /** Base URL for the Trigger.dev API. @default "https://api.trigger.dev" */
   baseURL?: string;
 
+  /**
+   * Base URL for the SSE stream subscription only (`GET .../sessions/{chatId}/out`).
+   * Falls back to `baseURL` when unset. Set this to route the long-lived
+   * stream through a custom proxy (e.g. a Cloudflare worker capturing JA4
+   * fingerprints for bot detection) while keeping append POSTs direct to
+   * `baseURL` to avoid an extra hop on every user message.
+   */
+  streamBaseURL?: string;
+
   /** Additional headers included in every API request. */
   headers?: Record<string, string>;
 
@@ -346,6 +355,7 @@ export class TriggerChatTransport implements ChatTransport<UIMessage> {
     | ((params: StartSessionParams<Record<string, unknown>>) => Promise<StartSessionResult>)
     | undefined;
   private readonly baseURL: string;
+  private readonly streamBaseURL: string;
   private readonly extraHeaders: Record<string, string>;
   private readonly streamTimeoutSeconds: number;
   private defaultMetadata: Record<string, unknown> | undefined;
@@ -367,6 +377,7 @@ export class TriggerChatTransport implements ChatTransport<UIMessage> {
       | ((params: StartSessionParams<Record<string, unknown>>) => Promise<StartSessionResult>)
       | undefined;
     this.baseURL = options.baseURL ?? DEFAULT_BASE_URL;
+    this.streamBaseURL = options.streamBaseURL ?? this.baseURL;
     this.extraHeaders = options.headers ?? {};
     this.streamTimeoutSeconds = options.streamTimeoutSeconds ?? DEFAULT_STREAM_TIMEOUT_SECONDS;
     this.defaultMetadata = options.clientData;
@@ -1021,7 +1032,7 @@ export class TriggerChatTransport implements ChatTransport<UIMessage> {
       );
     }
 
-    const streamUrl = `${this.baseURL}/realtime/v1/sessions/${encodeURIComponent(chatId)}/out`;
+    const streamUrl = `${this.streamBaseURL}/realtime/v1/sessions/${encodeURIComponent(chatId)}/out`;
 
     return new ReadableStream<UIMessageChunk>({
       start: async (controller) => {