diff --git a/web/workspace/src/lib/workspace/multiplexer.browser.spec.ts b/web/workspace/src/lib/workspace/multiplexer.browser.spec.ts
index 23eaa7a5d..77ab8f56d 100644
--- a/web/workspace/src/lib/workspace/multiplexer.browser.spec.ts
+++ b/web/workspace/src/lib/workspace/multiplexer.browser.spec.ts
@@ -32,6 +32,13 @@ class FakeWebSocket {
   }
 
   close(code?: number, reason?: string): void {
+    if (
+      code !== undefined &&
+      code !== 1000 &&
+      (code < 3000 || code > 4999)
+    ) {
+      throw new DOMException('Invalid WebSocket close code', 'InvalidAccessError');
+    }
     this.closes.push([code, reason]);
     if (this.readyState === FakeWebSocket.CLOSED) return;
     this.readyState = FakeWebSocket.CLOSED;
@@ -91,7 +98,7 @@ test('invalid inbound frame closes before dispatch and reconnects for a fresh sn
   );
 
   expect(onFrame).not.toHaveBeenCalled();
-  expect(first.closes).toEqual([[1002, 'Invalid workspace protocol frame']]);
+  expect(first.closes).toEqual([[4002, 'Invalid workspace protocol frame']]);
   expect(onStatus).toHaveBeenLastCalledWith('closed', 'Workspace protocol frame rejected');
   expect(JSON.stringify(onStatus.mock.calls)).not.toContain('untrusted detail');
 
diff --git a/web/workspace/src/lib/workspace/multiplexer.ts b/web/workspace/src/lib/workspace/multiplexer.ts
index a1ca52865..453aaf9d5 100644
--- a/web/workspace/src/lib/workspace/multiplexer.ts
+++ b/web/workspace/src/lib/workspace/multiplexer.ts
@@ -30,6 +30,7 @@ export type WorkspaceMultiplexerSubscription = {
 };
 
 const multiplexers = new Map<string, WorkspaceMultiplexer>();
+const INVALID_PROTOCOL_CLOSE_CODE = 4002;
 let nextMultiplexerSequence = 0;
 
 function nextMultiplexerId(kind: 'client' | 'request'): string {
@@ -205,7 +206,7 @@ export class WorkspaceMultiplexer {
 
   #rejectInboundFrame(): void {
     this.#disconnectMessage = 'Workspace protocol frame rejected';
-    this.#socket?.close(1002, 'Invalid workspace protocol frame');
+    this.#socket?.close(INVALID_PROTOCOL_CLOSE_CODE, 'Invalid workspace protocol frame');
   }
 
   #sendWorkerMethod(clientId: string, method: Method): void {