fix(sdk): preserve x-trigger-branch on override POSTs and handle Request input in SSE wrapper

When chat.createStartSessionAction was given a baseURL or fetch
override, the inline POSTs for /api/v1/sessions and
/api/v1/auth/jwt/claims forgot to forward apiClientManager.branchName as
the x-trigger-branch header. Callers running against a preview branch
(via TRIGGER_PREVIEW_BRANCH or VERCEL_GIT_COMMIT_REF) would route to
the wrong environment in override mode. Extracted the header builder
to a shared helper.

The SSE fetch wrappers in TriggerChatTransport.subscribeToSessionStream
and AgentChat.subscribeToSessionStream cast to typeof fetch but coerced
non-string input via .toString(), which yields "[object Request]" on
Request objects and discards the Request's intrinsic method/headers/
signal. Replaced the coercion with explicit branches for string, URL,
and Request — Request inputs now extract url + init properly with any
caller-provided init taking precedence (matches fetch(Request, init)
semantics).

Author: ericallam

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

@@ -8648,6 +8648,22 @@ function resolveChatStartBaseURL(
   return raw.replace(/\/$/, "");
 }
 
+function overrideRequestHeaders(accessToken: string): Record<string, string> {
+  const headers: Record<string, string> = {
+    "Content-Type": "application/json",
+    Authorization: `Bearer ${accessToken}`,
+    "x-trigger-source": "sdk",
+  };
+  // Forward the preview-branch hint so override-mode requests land on the
+  // same env the standard ApiClient path would have routed to. Mirrors
+  // ApiClient.#getHeaders. Read from TRIGGER_PREVIEW_BRANCH /
+  // VERCEL_GIT_COMMIT_REF via apiClientManager.branchName.
+  if (apiClientManager.branchName) {
+    headers["x-trigger-branch"] = apiClientManager.branchName;
+  }
+  return headers;
+}
+
 async function callSessionsCreateWithOverride(args: {
   chatId: string;
   body: { type: "chat.agent"; externalId: string; taskIdentifier: string; triggerConfig: SessionTriggerConfig; metadata?: Record<string, unknown> };
@@ -8664,11 +8680,7 @@ async function callSessionsCreateWithOverride(args: {
   const url = `${resolveChatStartBaseURL("sessions", args.chatId, args.baseURLOption)}/api/v1/sessions`;
   const init: RequestInit = {
     method: "POST",
-    headers: {
-      "Content-Type": "application/json",
-      Authorization: `Bearer ${accessToken}`,
-      "x-trigger-source": "sdk",
-    },
+    headers: overrideRequestHeaders(accessToken),
     body: JSON.stringify(args.body),
   };
   const response = args.fetchOverride
@@ -8698,11 +8710,7 @@ async function mintPublicTokenWithOverride(args: {
   const url = `${resolveChatStartBaseURL("auth", args.chatId, args.baseURLOption)}/api/v1/auth/jwt/claims`;
   const init: RequestInit = {
     method: "POST",
-    headers: {
-      "Content-Type": "application/json",
-      Authorization: `Bearer ${accessToken}`,
-      "x-trigger-source": "sdk",
-    },
+    headers: overrideRequestHeaders(accessToken),
   };
   const response = args.fetchOverride
     ? await args.fetchOverride(url, init, ctx)

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

@@ -683,13 +683,28 @@ export class AgentChat<TAgent = unknown> {
     const onTurnComplete = this.onTurnComplete;
     const chatId = this.chatId;
     const sseCtx: AgentChatEndpointContext = { endpoint: "out", chatId };
-    const sseFetchClient: typeof fetch | undefined = this.fetchOverride
-      ? ((input, init) =>
-          this.fetchOverride!(
-            typeof input === "string" ? input : (input as URL | Request).toString(),
-            init ?? {},
+    const fetchOverride = this.fetchOverride;
+    const sseFetchClient: typeof fetch | undefined = fetchOverride
+      ? ((input, init) => {
+          if (typeof input === "string") {
+            return fetchOverride(input, init ?? {}, sseCtx);
+          }
+          if (input instanceof URL) {
+            return fetchOverride(input.toString(), init ?? {}, sseCtx);
+          }
+          // Request — preserve its url + intrinsic init, let any provided
+          // init override on top (matches fetch(Request, init) semantics).
+          return fetchOverride(
+            input.url,
+            {
+              method: input.method,
+              headers: input.headers,
+              signal: input.signal,
+              ...(init ?? {}),
+            },
             sseCtx
-          )) as typeof fetch
+          );
+        }) as typeof fetch
       : undefined;
 
     const internalAbort = new AbortController();

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

@@ -1191,13 +1191,29 @@ export class TriggerChatTransport implements ChatTransport<UIMessage> {
             : () => {};
 
         const sseCtx: ChatTransportEndpointContext = { endpoint: "out", chatId };
-        const sseFetchClient: typeof fetch | undefined = this.fetchOverride
-          ? ((input, init) =>
-              this.fetchOverride!(
-                typeof input === "string" ? input : (input as URL | Request).toString(),
-                init ?? {},
+        const fetchOverride = this.fetchOverride;
+        const sseFetchClient: typeof fetch | undefined = fetchOverride
+          ? ((input, init) => {
+              if (typeof input === "string") {
+                return fetchOverride(input, init ?? {}, sseCtx);
+              }
+              if (input instanceof URL) {
+                return fetchOverride(input.toString(), init ?? {}, sseCtx);
+              }
+              // Request — preserve its url + intrinsic init, let any
+              // provided init override on top (matches fetch(Request, init)
+              // semantics).
+              return fetchOverride(
+                input.url,
+                {
+                  method: input.method,
+                  headers: input.headers,
+                  signal: input.signal,
+                  ...(init ?? {}),
+                },
                 sseCtx
-              )) as typeof fetch
+              );
+            }) as typeof fetch
           : undefined;
         const connectSseOnce = async (token: string) => {
           const subscription = new SSEStreamSubscription(streamUrl, {