diff --git a/shared/utils/intervalWorker.ts b/shared/utils/intervalWorker.ts
new file mode 100644
index 0000000..408a372
--- /dev/null
+++ b/shared/utils/intervalWorker.ts
@@ -0,0 +1,36 @@
+// A plain Worker's timers are not subject to the background-tab throttling
+// that setInterval/requestAnimationFrame get on the main thread (Chrome clamps
+// background-tab setInterval down to ~1/min after a few minutes; rAF stops
+// entirely) — used by anything in WebSim that must keep ticking while its tab
+// is backgrounded (the intended usage: WebSim in one tab, /live-atc in
+// another). docs/plans/2026-07-16-codex-fixes-live-atc-loop-websim.md WP2 Fix 1.
+export function createIntervalWorker(intervalMs: number, onTick: () => void): { stop: () => void } | null {
+  if (typeof Worker === 'undefined') return null
+
+  const source = `
+    let timer = null
+    self.onmessage = (e) => {
+      if (e.data === 'start') {
+        timer = setInterval(() => self.postMessage('tick'), ${intervalMs})
+      } else if (e.data === 'stop') {
+        if (timer !== null) clearInterval(timer)
+        self.close()
+      }
+    }
+  `
+  const blob = new Blob([source], { type: 'application/javascript' })
+  const url = URL.createObjectURL(blob)
+  const worker = new Worker(url)
+  worker.onmessage = (e: MessageEvent) => {
+    if (e.data === 'tick') onTick()
+  }
+  worker.postMessage('start')
+
+  return {
+    stop: () => {
+      worker.postMessage('stop')
+      worker.terminate()
+      URL.revokeObjectURL(url)
+    },
+  }
+}