docs: multi-tab coordination, error stack truncation changelog

Author: ericallam

docs/ai-chat/changelog.mdx

@@ -4,6 +4,28 @@ sidebarTitle: "Changelog"
 description: "Pre-release updates for AI chat agents."
 ---
 
+<Update label="April 17, 2026" description="0.0.0-chat-prerelease-20260417152143" tags={["SDK"]}>
+
+## Multi-tab coordination
+
+Prevent duplicate messages when the same chat is open in multiple browser tabs. Enable with `multiTab: true` on the transport.
+
+```tsx
+const transport = useTriggerChatTransport({ task: "my-chat", multiTab: true, accessToken });
+const { messages, setMessages } = useChat({ id: chatId, transport });
+const { isReadOnly } = useMultiTabChat(transport, chatId, messages, setMessages);
+```
+
+Only one tab can send at a time. Other tabs enter read-only mode with real-time message updates via `BroadcastChannel`. When the active tab's turn completes, any tab can send next. Crashed tabs are detected via heartbeat timeout (10s).
+
+See [Multi-tab coordination](/ai-chat/frontend#multi-tab-coordination) and [`useMultiTabChat`](/ai-chat/reference#usemultitabchat).
+
+## Error stack truncation
+
+Large error stacks no longer OOM the worker process. Stacks are capped at 50 frames (top 5 + bottom 45), individual lines at 1024 chars, messages at 1000 chars. Applied in `parseError`, `sanitizeError`, and OTel span recording.
+
+</Update>
+
 <Update label="April 15, 2026" description="0.0.0-chat-prerelease-20260415164455" tags={["SDK"]}>
 
 ## Fix: `resume: true` hangs on completed turns

docs/ai-chat/frontend.mdx

@@ -406,6 +406,65 @@ for await (const chunk of stream) {
 }
 ```
 
+## Multi-tab coordination
+
+When the same chat is open in multiple browser tabs, `multiTab: true` prevents duplicate messages and syncs conversation state across tabs. Only one tab can send at a time. Other tabs enter read-only mode with real-time message updates.
+
+```tsx
+import { useTriggerChatTransport } from "@trigger.dev/sdk/chat/react";
+import { useMultiTabChat } from "@trigger.dev/sdk/chat/react";
+import { useChat } from "@ai-sdk/react";
+
+function Chat({ chatId }: { chatId: string }) {
+  const transport = useTriggerChatTransport({
+    task: "my-chat",
+    accessToken,
+    multiTab: true,
+  });
+
+  const { messages, setMessages, sendMessage } = useChat({
+    id: chatId,
+    transport,
+  });
+
+  const { isReadOnly } = useMultiTabChat(transport, chatId, messages, setMessages);
+
+  return (
+    <div>
+      {isReadOnly && (
+        <div className="bg-amber-50 text-amber-700 p-2 text-sm">
+          This chat is active in another tab. Messages are read-only.
+        </div>
+      )}
+      {/* message list */}
+      <input
+        disabled={isReadOnly}
+        placeholder={isReadOnly ? "Active in another tab" : "Type a message..."}
+      />
+    </div>
+  );
+}
+```
+
+### How it works
+
+1. When a tab sends a message, the transport "claims" the chatId via `BroadcastChannel`
+2. Other tabs detect the claim and enter read-only mode (`isReadOnly: true`)
+3. The active tab broadcasts its messages so read-only tabs see updates in real-time
+4. When the turn completes, the claim is released. Any tab can send next.
+5. Heartbeats detect crashed tabs (10s timeout clears stale claims)
+
+### What `useMultiTabChat` does
+
+- Returns `{ isReadOnly }` for disabling the input UI
+- Broadcasts `messages` from the active tab to other tabs
+- Calls `setMessages` on read-only tabs when messages arrive from the active tab
+- Tracks read-only state via the transport's `BroadcastChannel` coordinator
+
+<Note>
+  Multi-tab coordination is same-browser only (`BroadcastChannel` is a browser API). It gracefully degrades to a no-op in Node.js, SSR, or browsers without `BroadcastChannel` support. Cross-device coordination requires server-side involvement.
+</Note>
+
 ## Self-hosting
 
 If you're self-hosting Trigger.dev, pass the `baseURL` option:

docs/ai-chat/reference.mdx

@@ -534,6 +534,20 @@ const transport = useTriggerChatTransport({
 });
 ```
 
+### multiTab
+
+Enable multi-tab coordination. When `true`, only one browser tab can send messages to a given chatId at a time. Other tabs enter read-only mode with real-time message updates via `BroadcastChannel`.
+
+```ts
+const transport = useTriggerChatTransport({
+  task: "my-chat",
+  accessToken,
+  multiTab: true,
+});
+```
+
+No-op when `BroadcastChannel` is unavailable (SSR, Node.js). See [Multi-tab coordination](/ai-chat/frontend#multi-tab-coordination).
+
 ### triggerOptions
 
 Options forwarded to the Trigger.dev API when starting a new run. Only applies to the first message — subsequent messages reuse the same run.
@@ -629,6 +643,32 @@ const transport = useTriggerChatTransport<typeof myChat>({
 
 The transport is created once on first render and reused across re-renders. Pass a type parameter for compile-time validation of the task ID.
 
+## useMultiTabChat
+
+React hook for multi-tab message coordination. Import from `@trigger.dev/sdk/chat/react`.
+
+```tsx
+import { useMultiTabChat } from "@trigger.dev/sdk/chat/react";
+
+const { isReadOnly } = useMultiTabChat(transport, chatId, messages, setMessages);
+```
+
+| Parameter | Type | Description |
+|-----------|------|-------------|
+| `transport` | `TriggerChatTransport` | Transport instance with `multiTab: true` |
+| `chatId` | `string` | The chat session ID |
+| `messages` | `UIMessage[]` | Current messages from `useChat` |
+| `setMessages` | `(messages) => void` | Message setter from `useChat` |
+
+**Returns:** `{ isReadOnly: boolean }` — `true` when another tab is actively sending to this chatId.
+
+The hook handles:
+- Tracking read-only state from the transport's `BroadcastChannel` coordinator
+- Broadcasting messages when this tab is the active sender
+- Receiving messages from other tabs and updating via `setMessages`
+
+See [Multi-tab coordination](/ai-chat/frontend#multi-tab-coordination).
+
 ## Related
 
 - [Realtime Streams](/tasks/streams) — How streams work under the hood