fix: address PR 3173 review feedback (closure + unhandled rejection)

- playground transport: route startSession through a ref so sidebar
  edits (tags, machine, maxAttempts, maxDuration, version, region) made
  before the first send aren't silently ignored. Mirrors the existing
  clientDataJsonRef pattern.
- chat.handover: swallow the rethrown error after the recovery path
  has already run so processes started with --unhandled-rejections=throw
  don't crash. User-facing behavior unchanged.

Author: ericallam

apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.playground.$agentParam/route.tsx

@@ -260,6 +260,15 @@ function PlaygroundChat() {
   // Resource route prefix — all realtime traffic goes through session-authed routes
   const playgroundBaseURL = `${apiOrigin}/resources/orgs/${organization.slug}/projects/${project.slug}/env/${environment.slug}/playground`;
 
+  // The transport is constructed once (guarded ref below); reading
+  // `startSession` directly there would freeze its closure to the
+  // first render's sidebar values, so subsequent edits to tags /
+  // machine / maxAttempts / maxDuration / version / region would be
+  // silently ignored on the first send. Mirror the `clientDataJsonRef`
+  // pattern so the transport always calls the latest `startSession`.
+  const startSessionRef = useRef(startSession);
+  startSessionRef.current = startSession;
+
   // Create TriggerChatTransport directly (not via useTriggerChatTransport hook
   // to avoid React version mismatch between SDK and webapp)
   const transportRef = useRef<TriggerChatTransport | null>(null);
@@ -274,8 +283,8 @@ function PlaygroundChat() {
       // session-PAT-authed request. Wiring the same call to both
       // keeps the Preload button working without a separate refresh
       // route.
-      startSession: async () => ({ publicAccessToken: await startSession() }),
-      accessToken: () => startSession(),
+      startSession: async () => ({ publicAccessToken: await startSessionRef.current() }),
+      accessToken: () => startSessionRef.current(),
       baseURL: playgroundBaseURL,
       clientData: JSON.parse(clientDataJson || "{}") as Record<string, unknown>,
       ...(activeConversation?.publicAccessToken

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

@@ -587,7 +587,16 @@ async function openHandoverSession(opts: {
         generateMessageId: () => turnMessageId,
       })
     );
-    void handoverWhenDone(result).finally(() => clearTimeout(idleTimer));
+    // `handoverWhenDone` re-throws on dispatch failure for visibility,
+    // but the recovery (resolveDecision + handoverSkip) has already run
+    // by then and `stitchHandoverStream` closes the response cleanly via
+    // `decisionPromise`. The user-facing path is fine; we only suppress
+    // the unhandled-rejection so processes started with
+    // `--unhandled-rejections=throw` don't crash on what is effectively
+    // a logged failure with no further action to take.
+    void handoverWhenDone(result)
+      .finally(() => clearTimeout(idleTimer))
+      .catch(() => {});
 
     const stitched = stitchHandoverStream(teed);