diff --git a/src/server/services/__tests__/anomaly-detection-job.test.ts b/src/server/services/__tests__/anomaly-detection-job.test.ts
index f242f1e3..7fe23605 100644
--- a/src/server/services/__tests__/anomaly-detection-job.test.ts
+++ b/src/server/services/__tests__/anomaly-detection-job.test.ts
@@ -13,12 +13,20 @@ vi.mock("@/server/services/anomaly-detector", () => ({
   },
 }));
 
+// SC-3: control leadership so the tick guard can be exercised both ways.
+// Defaults to leader so the existing tick-driven tests keep doing work.
+vi.mock("@/server/services/leader-election", () => ({
+  isLeader: vi.fn(() => true),
+}));
+
 import { prisma } from "@/lib/prisma";
 import { AnomalyDetectionService } from "@/server/services/anomaly-detection-job";
 import { evaluateAllPipelines } from "@/server/services/anomaly-detector";
+import { isLeader } from "@/server/services/leader-election";
 
 const prismaMock = prisma as unknown as DeepMockProxy<PrismaClient>;
 const mockEvaluateAll = evaluateAllPipelines as ReturnType<typeof vi.fn>;
+const mockIsLeader = isLeader as ReturnType<typeof vi.fn>;
 
 // ─── Tests ──────────────────────────────────────────────────────────────────
 
@@ -28,6 +36,7 @@ describe("AnomalyDetectionService", () => {
   beforeEach(() => {
     mockReset(prismaMock);
     vi.clearAllMocks();
+    mockIsLeader.mockReturnValue(true);
     vi.useFakeTimers();
     // The tick now iterates orgs from prisma.organization.findMany. Default
     // mock returns a single default org so existing single-org test cases
@@ -155,4 +164,36 @@ describe("AnomalyDetectionService", () => {
     // Release the hung evaluate so the test exits cleanly.
     releaseFirst([]);
   });
+
+  // ── SC-3: leadership guard (de-SPOF schedulers) ──────────────────────────
+
+  describe("leadership guard", () => {
+    it("tick is a no-op when the instance is no longer leader", async () => {
+      mockIsLeader.mockReturnValue(false);
+      prismaMock.organization.findMany.mockResolvedValue([
+        { id: "org-a" } as never,
+      ]);
+
+      service.init();
+      await vi.advanceTimersByTimeAsync(60_000);
+
+      // A demoted instance must do no work: no org scan, no pipeline
+      // evaluation — otherwise it duplicates the new leader's anomaly runs.
+      expect(prismaMock.organization.findMany).not.toHaveBeenCalled();
+      expect(mockEvaluateAll).not.toHaveBeenCalled();
+    });
+
+    it("tick proceeds normally while the instance is leader", async () => {
+      mockIsLeader.mockReturnValue(true);
+      prismaMock.organization.findMany.mockResolvedValue([
+        { id: "org-a" } as never,
+      ]);
+
+      service.init();
+      await vi.advanceTimersByTimeAsync(60_000);
+
+      expect(prismaMock.organization.findMany).toHaveBeenCalledTimes(1);
+      expect(mockEvaluateAll).toHaveBeenCalledWith({ organizationId: "org-a" });
+    });
+  });
 });
diff --git a/src/server/services/__tests__/fleet-alert-service.test.ts b/src/server/services/__tests__/fleet-alert-service.test.ts
index 214d41d0..389e11e9 100644
--- a/src/server/services/__tests__/fleet-alert-service.test.ts
+++ b/src/server/services/__tests__/fleet-alert-service.test.ts
@@ -33,6 +33,12 @@ vi.mock("@/server/services/cost-alert", () => ({
   evaluateCostAlerts: vi.fn().mockResolvedValue(undefined),
 }));
 
+// SC-3: control leadership so the tick guard can be exercised both ways.
+// Defaults to leader so the existing tick-driven tests keep doing work.
+vi.mock("@/server/services/leader-election", () => ({
+  isLeader: vi.fn(() => true),
+}));
+
 import { prisma } from "@/lib/prisma";
 import { FleetAlertService } from "@/server/services/fleet-alert-service";
 import {
@@ -44,8 +50,10 @@ import {
 } from "@/server/services/fleet-metrics";
 import { deliverToChannels } from "@/server/services/channels";
 import { getVersionDrift } from "@/server/services/drift-metrics";
+import { isLeader } from "@/server/services/leader-election";
 
 const prismaMock = prisma as unknown as DeepMockProxy<PrismaClient>;
+const mockIsLeader = isLeader as ReturnType<typeof vi.fn>;
 
 const mockGetFleetErrorRate = getFleetErrorRate as ReturnType<typeof vi.fn>;
 const mockGetFleetEventVolume = getFleetEventVolume as ReturnType<typeof vi.fn>;
@@ -130,6 +138,8 @@ describe("FleetAlertService", () => {
     mockGetFleetThroughputDrop.mockReset();
     mockGetNodeLoadImbalance.mockReset();
     (deliverToChannels as ReturnType<typeof vi.fn>).mockReset().mockResolvedValue([]);
+    mockIsLeader.mockReset();
+    mockIsLeader.mockReturnValue(true);
 
     service = new FleetAlertService();
   });
@@ -672,4 +682,43 @@ describe("FleetAlertService", () => {
       expect(prismaMock.alertRule.findMany).not.toHaveBeenCalled();
     });
   });
+
+  // ── SC-3: leadership guard (de-SPOF schedulers) ──────────────────────────
+
+  describe("leadership guard", () => {
+    it("tick is a no-op when the instance is no longer leader", async () => {
+      mockIsLeader.mockReturnValue(false);
+      prismaMock.organization.findMany.mockResolvedValue([
+        { id: "org-a" } as never,
+      ]);
+      prismaMock.alertRule.findMany.mockResolvedValue([]);
+
+      service.start();
+      await vi.advanceTimersByTimeAsync(30_000);
+
+      // A demoted instance must do no work: no org scan, no rule evaluation,
+      // no channel delivery — otherwise it double-fires with the new leader.
+      expect(prismaMock.organization.findMany).not.toHaveBeenCalled();
+      expect(prismaMock.alertRule.findMany).not.toHaveBeenCalled();
+      expect(deliverToChannels).not.toHaveBeenCalled();
+    });
+
+    it("tick proceeds normally while the instance is leader", async () => {
+      mockIsLeader.mockReturnValue(true);
+      prismaMock.organization.findMany.mockResolvedValue([
+        { id: "org-a" } as never,
+      ]);
+      prismaMock.alertRule.findMany.mockResolvedValue([]);
+
+      service.start();
+      await vi.advanceTimersByTimeAsync(30_000);
+
+      expect(prismaMock.organization.findMany).toHaveBeenCalledTimes(1);
+      expect(prismaMock.alertRule.findMany).toHaveBeenCalledWith(
+        expect.objectContaining({
+          where: expect.objectContaining({ organizationId: "org-a" }),
+        }),
+      );
+    });
+  });
 });
diff --git a/src/server/services/__tests__/git-sync-retry.test.ts b/src/server/services/__tests__/git-sync-retry.test.ts
index 69e2be9a..3d248314 100644
--- a/src/server/services/__tests__/git-sync-retry.test.ts
+++ b/src/server/services/__tests__/git-sync-retry.test.ts
@@ -23,16 +23,42 @@ vi.mock("@/server/services/event-alerts", () => ({
 vi.mock("@/server/services/sse-broadcast", () => ({
   broadcastSSE: vi.fn(),
 }));
+vi.mock("@/server/services/leader-election", () => ({
+  isLeader: vi.fn(() => true),
+}));
+
 
 import { prisma } from "@/lib/prisma";
 import { gitSyncCommitPipeline } from "@/server/services/git-sync";
 import { fireEventAlert } from "@/server/services/event-alerts";
 import { GitSyncRetryService, getNextRetryAt, createGitSyncJob } from "../git-sync-retry";
+import { isLeader } from "@/server/services/leader-election";
 
 const prismaMock = prisma as unknown as DeepMockProxy<PrismaClient>;
 const commitMock = vi.mocked(gitSyncCommitPipeline);
 const fireAlertMock = vi.mocked(fireEventAlert);
 
+// Keep leadership true by default so the per-tick guard never suppresses the
+// existing tick tests; individual tests opt into the not-leader path.
+beforeEach(() => {
+  vi.mocked(isLeader).mockReturnValue(true);
+});
+
+describe("GitSyncRetryService leadership guard", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    vi.mocked(isLeader).mockReturnValue(true);
+  });
+
+  it("skips the tick (no work) when this instance is not the leader", async () => {
+    vi.mocked(isLeader).mockReturnValue(false);
+    const service = new GitSyncRetryService();
+    await (service as unknown as { tick(): Promise<void> }).tick();
+    expect(prismaMock.organization.findMany).not.toHaveBeenCalled();
+    expect(commitMock).not.toHaveBeenCalled();
+  });
+});
+
 describe("getNextRetryAt", () => {
   it("returns 30s delay for attempt 0", () => {
     const result = getNextRetryAt(0);
diff --git a/src/server/services/anomaly-detection-job.ts b/src/server/services/anomaly-detection-job.ts
index c45a3c12..fe83595c 100644
--- a/src/server/services/anomaly-detection-job.ts
+++ b/src/server/services/anomaly-detection-job.ts
@@ -4,7 +4,8 @@ import {
   evaluateAllPipelines,
   ANOMALY_CONFIG,
 } from "@/server/services/anomaly-detector";
-import { infoLog, errorLog } from "@/lib/logger";
+import { debugLog, infoLog, errorLog } from "@/lib/logger";
+import { isLeader } from "@/server/services/leader-election";
 
 // ─── Constants ──────────────────────────────────────────────────────────────
 
@@ -59,6 +60,14 @@ export class AnomalyDetectionService {
    * tenant's analysis (or failure) does not stall another's.
    */
   private async tick(): Promise<void> {
+    // SC-3: re-check leadership each tick. A demoted leader's setInterval keeps
+    // firing for up to one TTL (~15s) after Redis renewals fail; without this
+    // guard the old + new leader both run anomaly analysis, duplicating events.
+    // Guard only — the timer stays so it resumes if leadership is re-acquired.
+    if (!isLeader()) {
+      debugLog("anomaly-detection", "Skipping tick — instance is no longer leader");
+      return;
+    }
     if (this.tickInFlight) {
       infoLog(
         "anomaly-detection",
diff --git a/src/server/services/backup-scheduler.ts b/src/server/services/backup-scheduler.ts
index f2886c29..a103e369 100644
--- a/src/server/services/backup-scheduler.ts
+++ b/src/server/services/backup-scheduler.ts
@@ -5,6 +5,7 @@ import { getOrgSettings } from "@/lib/org-settings";
 import { debugLog, infoLog, errorLog } from "@/lib/logger";
 import { createBackup, runRetentionCleanup, runOrphanCleanup } from "./backup";
 import { fireEventAlert } from "./event-alerts";
+import { isLeader } from "@/server/services/leader-election";
 
 /**
  * Per-organization backup scheduler.
@@ -98,6 +99,17 @@ function scheduleJobForOrg(
   }
 
   const task = cron.schedule(cronExpression, async () => {
+    // SC-3: a demoted leader's cron tasks keep firing for up to one TTL (~15s)
+    // after Redis renewals fail. Re-check leadership at the top of the per-org
+    // callback so a demoted instance skips the backup instead of racing the new
+    // leader (duplicate backups). Guard only — the cron task is left registered.
+    if (!isLeader()) {
+      debugLog(
+        "backup-scheduler",
+        `Skipping scheduled backup for org=${organizationId} — instance is no longer leader`,
+      );
+      return;
+    }
     infoLog(
       "backup-scheduler",
       `Starting scheduled backup for org=${organizationId}`,
diff --git a/src/server/services/cost-optimizer-scheduler.ts b/src/server/services/cost-optimizer-scheduler.ts
index cf07da58..ef6c0bff 100644
--- a/src/server/services/cost-optimizer-scheduler.ts
+++ b/src/server/services/cost-optimizer-scheduler.ts
@@ -8,6 +8,7 @@ import {
   cleanupExpiredRecommendations,
 } from "@/server/services/cost-recommendations";
 import { generateAiRecommendations } from "@/server/services/cost-optimizer-ai";
+import { isLeader } from "@/server/services/leader-election";
 
 /**
  * Cost-optimizer scheduler — single global cron tick fans out across orgs.
@@ -86,6 +87,15 @@ function scheduleJob(
   }
 
   const task = cron.schedule(cronExpression, async () => {
+    // SC-3: both the daily and continuous passes route through this callback.
+    // A demoted leader's cron tasks keep firing for up to one TTL (~15s) after
+    // Redis renewals fail, so re-check leadership here — a demoted instance
+    // skips the run instead of racing the new leader (duplicate analysis runs).
+    // Guard only — the cron task is left registered.
+    if (!isLeader()) {
+      debugLog("cost-optimizer", `Skipping ${label} — instance is no longer leader`);
+      return;
+    }
     infoLog("cost-optimizer", `Starting ${label}...`);
     try {
       await run();
diff --git a/src/server/services/fleet-alert-service.ts b/src/server/services/fleet-alert-service.ts
index 02160116..3c0e9507 100644
--- a/src/server/services/fleet-alert-service.ts
+++ b/src/server/services/fleet-alert-service.ts
@@ -17,7 +17,8 @@ import type { LoadImbalanceResult, ThroughputDropDetail } from "@/server/service
 import { getVersionDrift } from "@/server/services/drift-metrics";
 import { checkCertificateExpiry } from "@/server/services/cert-expiry-checker";
 import { evaluateCostAlerts } from "@/server/services/cost-alert";
-import { infoLog, errorLog } from "@/lib/logger";
+import { debugLog, infoLog, errorLog } from "@/lib/logger";
+import { isLeader } from "@/server/services/leader-election";
 
 // Re-export the constant for downstream use (e.g. T03 validation)
 export { FLEET_METRICS } from "@/server/services/alert-evaluator";
@@ -83,6 +84,15 @@ export class FleetAlertService {
    * their own queries.
    */
   private async tick(): Promise<void> {
+    // SC-3: a demoted leader's timers keep firing for up to one TTL (~15s)
+    // after Redis renewals fail. Without this guard the old + new leader both
+    // evaluate every rule, double-firing alerts. Re-check leadership each tick
+    // so a demoted instance becomes a no-op (we keep the timer, not tear it
+    // down, so it resumes cleanly if it re-acquires leadership).
+    if (!isLeader()) {
+      debugLog("fleet-alert", "Skipping tick — instance is no longer leader");
+      return;
+    }
     if (this.tickInFlight) {
       infoLog(
         "fleet-alert",
diff --git a/src/server/services/git-sync-retry.ts b/src/server/services/git-sync-retry.ts
index 194f6ee6..4b79bc31 100644
--- a/src/server/services/git-sync-retry.ts
+++ b/src/server/services/git-sync-retry.ts
@@ -5,6 +5,7 @@ import { gitSyncCommitPipeline, gitSyncDeletePipeline } from "@/server/services/
 import { fireEventAlert } from "@/server/services/event-alerts";
 import { loadOrgDataKeyCiphertext } from "@/server/services/crypto-v3-callsite";
 import { broadcastSSE } from "@/server/services/sse-broadcast";
+import { isLeader } from "@/server/services/leader-election";
 
 // --- Constants ---
 
@@ -61,6 +62,10 @@ export class GitSyncRetryService {
    * Single tick: iterate orgs and process retries per org.
    */
   private async tick(): Promise<void> {
+    if (!isLeader()) {
+      debugLog("git-sync-retry", "Skipping tick — instance is no longer leader");
+      return;
+    }
     if (this.tickInFlight) {
       infoLog(
         "git-sync-retry",
diff --git a/src/server/services/lake/lake-alerts.ts b/src/server/services/lake/lake-alerts.ts
index 72d52b75..ba6fdff2 100644
--- a/src/server/services/lake/lake-alerts.ts
+++ b/src/server/services/lake/lake-alerts.ts
@@ -1,7 +1,8 @@
 import { adminPrisma, prisma } from "@/lib/prisma";
 import { withOrgTx } from "@/lib/with-org-tx";
 import { runWithOrgContext } from "@/lib/org-context";
-import { infoLog, errorLog } from "@/lib/logger";
+import { debugLog, infoLog, errorLog } from "@/lib/logger";
+import { isLeader } from "@/server/services/leader-election";
 import type { LakeAlertRule } from "@/generated/prisma";
 import { isLakeEnabled } from "./clickhouse";
 import { aggregateValue, type LakeAggFunction } from "./lake-query";
@@ -279,6 +280,14 @@ let timer: NodeJS.Timeout | null = null;
 let tickInFlight = false;
 
 async function tick(): Promise<void> {
+  // SC-3: re-check leadership each tick. A demoted leader's setInterval keeps
+  // firing for up to one TTL (~15s) after Redis renewals fail; without this
+  // guard the old + new leader both evaluate lake alert rules, double-firing.
+  // Guard only — the timer stays so it resumes if leadership is re-acquired.
+  if (!isLeader()) {
+    debugLog("lake-alert", "Skipping tick — instance is no longer leader");
+    return;
+  }
   if (tickInFlight) return; // setInterval does not skip overlapping callbacks
   tickInFlight = true;
   try {
diff --git a/src/server/services/metrics-rollup.ts b/src/server/services/metrics-rollup.ts
index c42688cd..e894cff5 100644
--- a/src/server/services/metrics-rollup.ts
+++ b/src/server/services/metrics-rollup.ts
@@ -1,6 +1,7 @@
 import { adminPrisma } from "@/lib/prisma";
 import { withOrgTx } from "@/lib/with-org-tx";
-import { infoLog, errorLog } from "@/lib/logger";
+import { infoLog, errorLog, debugLog } from "@/lib/logger";
+import { isLeader } from "@/server/services/leader-election";
 
 /**
  * Long-retention metric rollups (B5).
@@ -602,6 +603,10 @@ const ROLLUP_INTERVAL_MS = HOUR_MS;
 let timer: ReturnType<typeof setInterval> | null = null;
 
 async function runRollupTick(): Promise<void> {
+  if (!isLeader()) {
+    debugLog("metrics-rollup", "Skipping tick — instance is no longer leader");
+    return;
+  }
   for (const granularity of ["HOUR", "DAY"] as const) {
     try {
       const result = await rollupMetrics({ granularity });
diff --git a/src/server/services/retry-service.ts b/src/server/services/retry-service.ts
index d560029f..f9883727 100644
--- a/src/server/services/retry-service.ts
+++ b/src/server/services/retry-service.ts
@@ -7,7 +7,8 @@ import { getDriver } from "@/server/services/channels";
 import { decryptChannelConfig } from "@/server/services/channel-secrets";
 import type { ChannelPayload } from "@/server/services/channels/types";
 import { deliverOutboundWebhook, isPermanentFailure } from "@/server/services/outbound-webhook";
-import { infoLog, errorLog } from "@/lib/logger";
+import { debugLog, infoLog, errorLog } from "@/lib/logger";
+import { isLeader } from "@/server/services/leader-election";
 
 // ─── Constants ──────────────────────────────────────────────────────────────
 
@@ -51,6 +52,15 @@ export class RetryService {
    * tracking wrappers, which handle backoff scheduling automatically.
    */
   async processRetries(): Promise<void> {
+    // SC-3: a demoted leader's setInterval keeps firing for up to one TTL
+    // (~15s) after Redis renewals fail. Re-check leadership so a demoted
+    // instance doesn't re-deliver alerts/webhooks the new leader is already
+    // retrying (duplicate deliveries). Covers processOutboundRetries too (it
+    // runs below). Guard only — the timer stays so it resumes on re-acquisition.
+    if (!isLeader()) {
+      debugLog("retry-service", "Skipping poll — instance is no longer leader");
+      return;
+    }
     let dueRetries;
     try {
       dueRetries = await prisma.deliveryAttempt.findMany({
diff --git a/src/server/services/telemetry-scheduler.ts b/src/server/services/telemetry-scheduler.ts
index cd257157..275ba727 100644
--- a/src/server/services/telemetry-scheduler.ts
+++ b/src/server/services/telemetry-scheduler.ts
@@ -1,5 +1,7 @@
 import cron, { type ScheduledTask } from "node-cron";
 import { sendTelemetryHeartbeat } from "./telemetry-sender";
+import { debugLog } from "@/lib/logger";
+import { isLeader } from "@/server/services/leader-election";
 
 // Daily at 03:42 UTC. Jitter across instances happens naturally because each
 // VF install runs at a different wall clock when the cron crosses the boundary.
@@ -10,6 +12,13 @@ let task: ScheduledTask | null = null;
 export function initTelemetryScheduler(): void {
   if (task) return;
   task = cron.schedule(DAILY_CRON, async () => {
+    // SC-3: a demoted leader's cron keeps firing for up to one TTL (~15s) after
+    // Redis renewals fail. Re-check leadership so only the current leader sends
+    // the telemetry heartbeat. Guard only — the cron task is left registered.
+    if (!isLeader()) {
+      debugLog("telemetry", "Skipping heartbeat — instance is no longer leader");
+      return;
+    }
     try {
       await sendTelemetryHeartbeat();
     } catch (err) {