diff --git a/src/billing/meter.test.ts b/src/billing/meter.test.ts
index 0e6c1a6..d7c4df5 100644
--- a/src/billing/meter.test.ts
+++ b/src/billing/meter.test.ts
@@ -8,7 +8,7 @@ const TMP = fs.mkdtempSync(path.join(os.tmpdir(), "lisa-billing-"));
 process.env.LISA_HOME = TMP;
 
 const { homeScope, homeForUid } = await import("../paths.js");
-const { recordUsage, readUsage, summarizeUsage } = await import("./meter.js");
+const { recordUsage, readUsage, summarizeUsage, claimAnomalyAlert } = await import("./meter.js");
 const { costMicroUSD, priceForModel, modelTier, formatMicroUSD, MARGIN } = await import("./prices.js");
 
 const U = (i: number, o: number, cr = 0, cw = 0) => ({
@@ -94,3 +94,88 @@ describe("meter ledger", () => {
     assert.equal(rows.length, 1);
   });
 });
+
+describe("anomaly alert claim (cross-instance dedup)", () => {
+  // The claim is what stops MAX_INSTANCES>1 from paging the operator once per
+  // instance for the same $10 day. Its contract is asymmetric on purpose:
+  // only a precondition failure is treated as "someone else has it".
+  const DAY = "2026-08-21";
+  const uid = "u-anomaly-1";
+
+  const withFirestore = async <T>(fn: () => Promise<T>): Promise<T> => {
+    const prev = {
+      on: process.env.LISA_FIRESTORE,
+      tok: process.env.LISA_FIRESTORE_TOKEN,
+      proj: process.env.LISA_FIRESTORE_PROJECT,
+    };
+    process.env.LISA_FIRESTORE = "1";
+    process.env.LISA_FIRESTORE_TOKEN = "test-token";
+    process.env.LISA_FIRESTORE_PROJECT = "test-project";
+    try {
+      return await fn();
+    } finally {
+      if (prev.on === undefined) delete process.env.LISA_FIRESTORE;
+      else process.env.LISA_FIRESTORE = prev.on;
+      if (prev.tok === undefined) delete process.env.LISA_FIRESTORE_TOKEN;
+      else process.env.LISA_FIRESTORE_TOKEN = prev.tok;
+      if (prev.proj === undefined) delete process.env.LISA_FIRESTORE_PROJECT;
+      else process.env.LISA_FIRESTORE_PROJECT = prev.proj;
+    }
+  };
+
+  const reply = (status: number): typeof fetch =>
+    (async () =>
+      new Response(status === 200 ? "{}" : "denied", { status })) as unknown as typeof fetch;
+
+  test("Firestore off → always claims (Mac edition keeps the in-process Set)", async () => {
+    let called = false;
+    const spy = (async () => {
+      called = true;
+      return new Response("{}", { status: 200 });
+    }) as unknown as typeof fetch;
+    assert.equal(await claimAnomalyAlert(DAY, spy), true);
+    assert.equal(called, false, "must not touch the network when Firestore is off");
+  });
+
+  test("Firestore on but no uid scope → claims without a write", async () => {
+    await withFirestore(async () => {
+      let called = false;
+      const spy = (async () => {
+        called = true;
+        return new Response("{}", { status: 200 });
+      }) as unknown as typeof fetch;
+      assert.equal(await claimAnomalyAlert(DAY, spy), true);
+      assert.equal(called, false);
+    });
+  });
+
+  test("create succeeds → this instance owns the alert", async () => {
+    await withFirestore(async () => {
+      await homeScope.run(homeForUid(uid), async () => {
+        assert.equal(await claimAnomalyAlert(DAY, reply(200)), true);
+      });
+    });
+  });
+
+  for (const status of [409, 412]) {
+    test(`precondition failure ${status} → another instance already alerted`, async () => {
+      await withFirestore(async () => {
+        await homeScope.run(homeForUid(uid), async () => {
+          assert.equal(await claimAnomalyAlert(DAY, reply(status)), false);
+        });
+      });
+    });
+  }
+
+  test("Firestore unavailable → still alerts (a missed burn beats a duplicate)", async () => {
+    await withFirestore(async () => {
+      await homeScope.run(homeForUid(uid), async () => {
+        assert.equal(await claimAnomalyAlert(DAY, reply(503)), true);
+        const boom = (async () => {
+          throw new Error("network down");
+        }) as unknown as typeof fetch;
+        assert.equal(await claimAnomalyAlert(DAY, boom), true);
+      });
+    });
+  });
+});
diff --git a/src/billing/meter.ts b/src/billing/meter.ts
index 6efaffd..2202c3a 100644
--- a/src/billing/meter.ts
+++ b/src/billing/meter.ts
@@ -13,7 +13,9 @@
  * recording NEVER throws (metering must not take chat down).
  */
 import path from "node:path";
-import { lisaHome } from "../paths.js";
+import { lisaHome, scopedUid } from "../paths.js";
+import { logError, redactId } from "../log.js";
+import { firestoreEnabled, setDoc, FirestoreError } from "../cloud/firestore.js";
 import { appendLine, readTextOrEmpty, atomicWrite } from "../fs-utils.js";
 import { withFileLock } from "../soul/lock.js";
 import type { ProviderUsage } from "../providers/types.js";
@@ -83,7 +85,7 @@ export async function recordUsage(
     await appendLine(usageFile(), JSON.stringify(rec));
     void trimIfNeeded();
   } catch (err) {
-    console.error(`[billing] usage audit append failed (turn still priced + debited): ${(err as Error).message}`);
+    logError(`[billing] usage audit append failed (turn still priced + debited): ${(err as Error).message}`);
   }
   // Global daily cap accounting + anomaly alert (B7). Best-effort, and run
   // whether or not the audit line landed.
@@ -92,8 +94,13 @@ export async function recordUsage(
   return rec;
 }
 
-// One alert per home per process-day: a single account burning > $10 face in a
-// day is worth an operator's eyes (PLAN §6.5).
+// One alert per home per DAY: a single account burning > $10 face in a day is
+// worth an operator's eyes (PLAN §6.5).
+//
+// The Set below is only the in-PROCESS fast path. On the cloud it is not
+// sufficient on its own — each instance keeps its own copy and a cold start
+// wipes it, so with MAX_INSTANCES>1 the same $10 day would page the operator
+// once per instance. claimAnomalyAlert() is the cross-instance arbiter.
 const ALERT_THRESHOLD_MICRO = 10_000_000;
 const alerted = new Set<string>();
 
@@ -103,20 +110,63 @@ export function setAnomalySink(sink: ((text: string) => void) | null): void {
   anomalySink = sink;
 }
 
+/**
+ * Claim today's anomaly alert for the ACTIVE tenant. True means this process
+ * won the claim and must alert; false means another instance already did.
+ *
+ * The claim is a create-only write, so a second writer fails its precondition
+ * — that failure IS the dedup signal. Any OTHER failure (network, permission,
+ * Firestore down) returns true on purpose: a duplicate alert is a nuisance, a
+ * missed one is an unnoticed $10+/day burn. Fail loud, never silent.
+ *
+ * Returns true unconditionally with Firestore off or outside a per-uid scope
+ * (Mac edition, shared-token demo) — there the in-process Set already decides,
+ * exactly as before.
+ *
+ * Exported for tests; `fetchFn` follows the firestore.ts injection convention.
+ */
+export async function claimAnomalyAlert(
+  day: string,
+  fetchFn: typeof fetch = fetch,
+): Promise<boolean> {
+  if (!firestoreEnabled()) return true;
+  const uid = scopedUid();
+  if (!uid) return true;
+  try {
+    await setDoc(
+      `lisa-anomaly-alerts/${uid}_${day}`,
+      { uid, day, at: new Date().toISOString() },
+      { exists: false },
+      fetchFn,
+    );
+    return true;
+  } catch (err) {
+    if (err instanceof FirestoreError && (err.status === 409 || err.status === 412)) return false;
+    return true;
+  }
+}
+
 async function alertIfAnomalous(now: Date): Promise<void> {
   try {
-    const key = `${lisaHome()}:${now.toISOString().slice(0, 10)}`;
+    const day = now.toISOString().slice(0, 10);
+    const key = `${lisaHome()}:${day}`;
     if (alerted.has(key)) return;
     const today = await summarizeUsage(new Date(now).setUTCHours(0, 0, 0, 0));
-    if (today.microUSD > ALERT_THRESHOLD_MICRO) {
-      alerted.add(key);
-      const text = `${lisaHome()} spent ${(today.microUSD / 1e6).toFixed(2)} USD face today (${today.turns} turns)`;
-      console.error(`[billing] ⚠ anomaly: ${text}`);
-      try {
-        anomalySink?.(text);
-      } catch {
-        /* alerting must never break metering */
-      }
+    if (today.microUSD <= ALERT_THRESHOLD_MICRO) return;
+    // Mark before the (network-bound) claim so concurrent turns in THIS
+    // process don't all pile into it.
+    alerted.add(key);
+    if (!(await claimAnomalyAlert(day))) return; // another instance owns today
+    // The cloud home path embeds the uid, so name the tenant by a redacted id
+    // there; the Mac edition's local path is meaningful and carries no uid.
+    const uid = scopedUid();
+    const who = uid ? `uid ${redactId(uid)}` : lisaHome();
+    const text = `${who} spent ${(today.microUSD / 1e6).toFixed(2)} USD face today (${today.turns} turns)`;
+    logError(`[billing] ⚠ anomaly: ${text}`);
+    try {
+      anomalySink?.(text);
+    } catch {
+      /* alerting must never break metering */
     }
   } catch {
     /* observability only */