diff --git a/app/a/[id]/route.ts b/app/a/[id]/route.ts
index 697ae313..6fe742b3 100644
--- a/app/a/[id]/route.ts
+++ b/app/a/[id]/route.ts
@@ -12,7 +12,8 @@
 import { NextRequest, NextResponse } from "next/server";
 import { resolveClick } from "@/lib/ads/serve";
 import { serviceClient } from "@/lib/supabase/service";
-import { clientIpFromHeaders, lookupGeo } from "@/lib/tracker/geo";
+import { lookupGeo } from "@/lib/tracker/geo";
+import { adClickIp } from "@/lib/ads/client-ip";
 import { parseDevice } from "@/lib/tracker/device";
 import { isShortCode } from "@/lib/ads/shortcode";
 import { env } from "@/lib/env";
@@ -80,7 +81,7 @@ export async function GET(request: NextRequest, ctx: { params: Promise<{ id: str
     const imp = await findImpression(sb, id, byCode);
     if (!imp) return NextResponse.redirect(fallback, { status: 302 });
 
-    const ip = clientIpFromHeaders(request.headers);
+    const ip = adClickIp(request.headers);
     const geo = await lookupGeo(ip).catch(() => null);
     // Deliberately the STRICT classification here, unlike /api/ads/motd: a
     // terminal ad is served to curl, but it's clicked from a browser when the
diff --git a/app/api/ads/click/route.ts b/app/api/ads/click/route.ts
index 440c2c21..2b28496b 100644
--- a/app/api/ads/click/route.ts
+++ b/app/api/ads/click/route.ts
@@ -4,7 +4,8 @@
 
 import { NextRequest, NextResponse } from "next/server";
 import { resolveClick } from "@/lib/ads/serve";
-import { clientIpFromHeaders, lookupGeo } from "@/lib/tracker/geo";
+import { lookupGeo } from "@/lib/tracker/geo";
+import { adClickIp } from "@/lib/ads/client-ip";
 import { parseDevice } from "@/lib/tracker/device";
 import { env } from "@/lib/env";
 
@@ -21,7 +22,7 @@ export async function GET(request: NextRequest) {
     const creativeId = url.searchParams.get("cr");
     const visitorId = url.searchParams.get("v");
 
-    const ip = clientIpFromHeaders(request.headers);
+    const ip = adClickIp(request.headers);
     const geo = await lookupGeo(ip).catch(() => null);
     const device = parseDevice(request.headers.get("user-agent")).deviceType;
 
diff --git a/docs/ad-click-throttling.md b/docs/ad-click-throttling.md
new file mode 100644
index 00000000..8de3211d
--- /dev/null
+++ b/docs/ad-click-throttling.md
@@ -0,0 +1,35 @@
+# Ad click throttling
+
+A visitor or IP may produce one accepted ad click per five seconds across the
+network. The cooldown covers every campaign and publisher, including promotional
+and paper-auction clicks. A repeated click still redirects to the advertiser, but
+is recorded as invalid: no advertiser debit, publisher accrual or paper spend.
+
+An atomic Redis script claims both the salted IP and visitor buckets together.
+The keys expire after five seconds and rejected attempts do not extend that
+expiry. Using a shared Redis instance prevents simultaneous requests handled by
+different app instances from passing the same cooldown. Changing only a cookie
+or only an IP does not reset the other bucket. People sharing an IP also share
+this short cooldown.
+
+The existing six-hour campaign deduplication also covers legitimate free-tier
+delivery. Invalid bot traffic does not count as prior delivery. Missing identity
+and failed validation withhold billing. During a Redis outage, redirects continue
+and all cash and paper charges are withheld; an availability warning is logged
+at most once a minute per process. Configure `REDIS_URL` and `IP_HASH_SALT` on
+every app instance before deploying.
+
+On Railway, click accounting uses the edge-provided `X-Real-IP`, rather than
+letting a caller-supplied Cloudflare header replace it. See the
+[Railway request-header contract](https://docs.railway.com/networking/public-networking/specs-and-limits).
+
+Run the concurrency tests against a disposable Redis instance:
+
+```sh
+TEST_AD_REDIS_URL=redis://127.0.0.1:6379 npm test
+```
+
+These tests use unique temporary keys and cover concurrent connections, expiry,
+cookie/IP changes, rejection without extending the window, failed validation
+and every accounting tier. `TEST_AD_REDIS_URL` is deliberately separate from
+the application's `REDIS_URL`.
diff --git a/lib/ads/click-cooldown.ts b/lib/ads/click-cooldown.ts
new file mode 100644
index 00000000..3723b53c
--- /dev/null
+++ b/lib/ads/click-cooldown.ts
@@ -0,0 +1,68 @@
+import Redis from "ioredis";
+import { hashIp } from "@/lib/ipHash";
+
+export const CLICK_COOLDOWN_MS = 5_000;
+
+// One atomic operation across every app instance, campaign and publisher.
+// Both identity buckets must be clear; changing a visitor cookie cannot reset
+// an IP's cooldown. Rejected attempts do not keep extending the window.
+export const CLAIM_CLICK_LUA = `
+for _, key in ipairs(KEYS) do
+  if redis.call('EXISTS', key) == 1 then return 0 end
+end
+for _, key in ipairs(KEYS) do
+  redis.call('SET', key, '1', 'PX', ARGV[1])
+end
+return 1
+`;
+
+let redis: Redis | undefined;
+let lastWarning = 0;
+function unavailable(): ClickCooldown {
+  if (Date.now() - lastWarning >= 60_000) {
+    lastWarning = Date.now();
+    console.warn("[ads] Click cooldown unavailable; cash and paper charges withheld.");
+  }
+  return { allowed: false, reason: "cooldown_unavailable" };
+}
+function client(): Redis | null {
+  const url = process.env.REDIS_URL;
+  if (!url) return null;
+  if (!redis || redis.status === "end") {
+    redis = new Redis(url, {
+      lazyConnect: true,
+      maxRetriesPerRequest: 0,
+      connectTimeout: 1_000,
+      commandTimeout: 1_500,
+      retryStrategy: () => null,
+    });
+    // Errors are handled by claimClickCooldown. Never log connection URLs.
+    redis.on("error", () => {});
+  }
+  return redis;
+}
+
+export type ClickCooldown = { allowed: boolean; reason?: "click_cooldown" | "missing_identity" | "cooldown_unavailable" };
+
+export async function claimClickCooldown(input: {
+  visitorId?: string | null;
+  ip?: string | null;
+}): Promise<ClickCooldown> {
+  const visitor = input.visitorId?.trim();
+  const keys = [
+    ...(input.ip ? [`ad:click:ip:${hashIp(input.ip)}`] : []),
+    ...(visitor && /^[\w-]{1,128}$/.test(visitor)
+      ? [`ad:click:visitor:${hashIp(`ad-visitor:${visitor}`)}`] : []),
+  ];
+  if (!keys.length) return { allowed: false, reason: "missing_identity" };
+  try {
+    const connection = client();
+    if (!connection) return unavailable();
+    const accepted = await connection.eval(CLAIM_CLICK_LUA, keys.length, ...keys, CLICK_COOLDOWN_MS);
+    return accepted === 1 ? { allowed: true } : { allowed: false, reason: "click_cooldown" };
+  } catch {
+    // Redirects still work during an outage; cash, publisher accrual and paper
+    // charges require a positively confirmed admission.
+    return unavailable();
+  }
+}
diff --git a/lib/ads/client-ip.ts b/lib/ads/client-ip.ts
new file mode 100644
index 00000000..2ffc94f5
--- /dev/null
+++ b/lib/ads/client-ip.ts
@@ -0,0 +1,13 @@
+import { clientIpFromHeaders } from "@/lib/tracker/geo";
+
+/** Railway sets X-Real-IP at its edge. Other forwarded headers can be supplied
+ * by the caller and must not override that identity for click accounting.
+ * https://docs.railway.com/networking/public-networking/specs-and-limits
+ */
+export function adClickIp(headers: Headers): string | null {
+  if (!process.env.RAILWAY_ENVIRONMENT_ID) return clientIpFromHeaders(headers);
+  const trusted = new Headers();
+  const ip = headers.get("x-real-ip");
+  if (ip) trusted.set("x-real-ip", ip);
+  return clientIpFromHeaders(trusted);
+}
diff --git a/lib/ads/fraud.ts b/lib/ads/fraud.ts
index 4b93212c..08679631 100644
--- a/lib/ads/fraud.ts
+++ b/lib/ads/fraud.ts
@@ -71,6 +71,14 @@ export async function assessClickValidity(input: {
   ipHashes?: string[] | null;
   device?: string | null;
 }): Promise<ClickValidity> {
+  try {
+    return await checkClickValidity(input);
+  } catch {
+    return { valid: false, reason: "validation_unavailable" };
+  }
+}
+
+async function checkClickValidity(input: Parameters<typeof assessClickValidity>[0]): Promise<ClickValidity> {
   // 1. Bots never bill.
   if (isBotDevice(input.device)) return { valid: false, reason: "bot" };
 
@@ -79,11 +87,12 @@ export async function assessClickValidity(input: {
   // 2. Anti-forgery: if the click claims an impression, it must exist and match
   // the campaign/slot it says it clicked.
   if (input.impressionId) {
-    const { data: imp } = await sb
+    const { data: imp, error } = await sb
       .from("ad_impressions")
       .select("campaign_id, slot_id")
       .eq("id", input.impressionId)
       .maybeSingle();
+    if (error) return { valid: false, reason: "validation_unavailable" };
     if (!imp) return { valid: false, reason: "no_impression" };
     if (imp.campaign_id !== input.campaignId) return { valid: false, reason: "impression_mismatch" };
     if (input.slotId && imp.slot_id !== input.slotId) return { valid: false, reason: "impression_mismatch" };
@@ -94,14 +103,13 @@ export async function assessClickValidity(input: {
   const ipHashes = (input.ipHashes ?? [])
     .map((h) => safeId(h))
     .filter((h): h is string => h !== null);
-  if (!visitor && ipHashes.length === 0) return { valid: true }; // nothing to dedupe on
+  if (!visitor && ipHashes.length === 0) return { valid: false, reason: "missing_identity" };
 
   const since = new Date(Date.now() - CLICK_DEDUPE_WINDOW_MS).toISOString();
   const q = sb
     .from("ad_clicks")
     .select("id")
     .eq("campaign_id", input.campaignId)
-    .eq("valid", true)
     .gte("ts", since)
     .limit(1);
 
@@ -113,7 +121,8 @@ export async function assessClickValidity(input: {
     ...ipHashes.map((h) => `ip_hash.eq.${h}`),
   ];
 
-  const { data: dupe } = await q.or(terms.join(","));
+  const { data: dupe, error } = await q.or(`and(or(valid.eq.true,tier.eq.free),or(${terms.join(",")}))`);
+  if (error) return { valid: false, reason: "validation_unavailable" };
   if (dupe && dupe.length > 0) return { valid: false, reason: "duplicate" };
 
   return { valid: true };
diff --git a/lib/ads/serve.ts b/lib/ads/serve.ts
index 06a30a64..60ff9846 100644
--- a/lib/ads/serve.ts
+++ b/lib/ads/serve.ts
@@ -29,6 +29,7 @@ import { promoForCampaign } from "./promos";
 import { promoState, clickChargeCents } from "./trending";
 import { paperWeight, type PaperBudgetFields } from "./autobid";
 import { paperCharge } from "./bids";
+import { claimClickCooldown } from "./click-cooldown";
 
 // Server-side ad selection + metering. Runs under the service-role client so
 // the public serving endpoints can read cross-tenant campaigns/creatives and
@@ -626,7 +627,7 @@ export async function resolveClick(input: {
     // yesterday's too, or every check silently misses for the first hours after
     // the salt rotates.
     const ipHash = hashIpRotating(input.ctx?.ip ?? null);
-    const validity = await assessClickValidity({
+    let validity = await assessClickValidity({
       campaignId: campaign.id,
       slotId: input.slotId,
       impressionId: input.impressionId,
@@ -634,6 +635,10 @@ export async function resolveClick(input: {
       ipHashes: rotatingIpHashCandidates(input.ctx?.ip ?? null, CLICK_DEDUPE_WINDOW_MS),
       device: input.ctx?.device,
     });
+    if (validity.valid) {
+      const admission = await claimClickCooldown({ visitorId, ip: input.ctx?.ip });
+      if (!admission.allowed) validity = { valid: false, reason: admission.reason };
+    }
 
     // The 90-day promo, settled here rather than inside ad_charge_click.
     //
diff --git a/tests/contract/ads-click-accounting.test.ts b/tests/contract/ads-click-accounting.test.ts
new file mode 100644
index 00000000..7dd76d86
--- /dev/null
+++ b/tests/contract/ads-click-accounting.test.ts
@@ -0,0 +1,66 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const state = vi.hoisted(() => ({
+  admitted: true, promo: false, free: false, inserts: [] as Record<string, unknown>[],
+  rpc: vi.fn(), paper: vi.fn(),
+}));
+vi.mock("@/lib/supabase/service", () => ({ serviceClient: () => ({
+  rpc: state.rpc,
+  from(table: string) {
+    const q = {
+      select: () => q, eq: () => q,
+      insert(row: Record<string, unknown>) { state.inserts.push(row); return q; },
+      maybeSingle: async () => ({ data: table === "ad_campaigns"
+        ? { id: "campaign", destination_url: "https://example.com/offer", ref_slug: "ad-test", bid_credits: 4 }
+        : { id: "click" } }),
+    };
+    return q;
+  },
+}) }));
+vi.mock("@/lib/ads/fraud", async (original) => ({
+  ...await original<object>(), assessClickValidity: async () => ({ valid: true }),
+}));
+vi.mock("@/lib/ads/click-cooldown", () => ({ claimClickCooldown: async () => ({
+  allowed: state.admitted, reason: state.admitted ? undefined : "click_cooldown",
+}) }));
+vi.mock("@/lib/ads/promos", () => ({ promoForCampaign: async () => null }));
+vi.mock("@/lib/ads/trending", async (original) => ({
+  ...await original<object>(), promoState: () => ({ active: state.promo }),
+}));
+vi.mock("@/lib/ads/bids", () => ({ paperCharge: state.paper }));
+import { resolveClick } from "@/lib/ads/serve";
+
+beforeEach(() => {
+  state.admitted = true; state.promo = false; state.free = false;
+  state.inserts = []; state.rpc.mockReset(); state.paper.mockReset();
+  state.rpc.mockImplementation(async () => ({ data: [{ click_id: "click", valid: !state.free, charged_cents: state.free ? 0 : 20 }] }));
+});
+const click = () => resolveClick({ campaignId: "campaign", slotId: "slot", ctx: { ip: "8.8.8.8", visitorId: "visitor" } });
+
+describe("click admission before accounting", () => {
+  it.each(["paid", "promo", "free"])("withholds %s accounting on a rejected click while preserving the destination", async (tier) => {
+    state.admitted = false; state.promo = tier === "promo"; state.free = tier === "free";
+    expect(await click()).toBe("https://example.com/offer?ref=ad-test");
+    expect(state.rpc).not.toHaveBeenCalled();
+    expect(state.paper).not.toHaveBeenCalled();
+    expect(state.inserts).toHaveLength(1);
+    expect(state.inserts[0]).toMatchObject({ valid: false, tier: "paid", charged_cents: 0, publisher_earn_cents: 0 });
+  });
+  it("keeps admitted paid accounting intact", async () => {
+    await click();
+    expect(state.rpc).toHaveBeenCalledWith("ad_charge_click", expect.objectContaining({ p_campaign: "campaign", p_cpc_credits: 4 }));
+    expect(state.paper).not.toHaveBeenCalled();
+  });
+  it("keeps admitted free-tier paper accounting separate from cash", async () => {
+    state.free = true;
+    await click();
+    expect(state.paper).toHaveBeenCalledWith(expect.anything(), expect.objectContaining({ clickId: "click" }));
+  });
+  it("keeps admitted promo clicks unbilled", async () => {
+    state.promo = true;
+    await click();
+    expect(state.rpc).not.toHaveBeenCalled();
+    expect(state.inserts[0]).toMatchObject({ tier: "free", charged_cents: 0, publisher_earn_cents: 0 });
+    expect(state.paper).toHaveBeenCalledOnce();
+  });
+});
diff --git a/tests/contract/ads-click-cooldown-redis.test.ts b/tests/contract/ads-click-cooldown-redis.test.ts
new file mode 100644
index 00000000..da392dda
--- /dev/null
+++ b/tests/contract/ads-click-cooldown-redis.test.ts
@@ -0,0 +1,37 @@
+import Redis from "ioredis";
+import { randomUUID } from "node:crypto";
+import { afterAll, describe, expect, it } from "vitest";
+import { CLAIM_CLICK_LUA, CLICK_COOLDOWN_MS } from "@/lib/ads/click-cooldown";
+
+// Opt-in, disposable Redis only. Never points at the application's REDIS_URL.
+const testUrl = process.env.TEST_AD_REDIS_URL;
+describe.skipIf(!testUrl)("atomic click admission against Redis", () => {
+  const clients = testUrl ? Array.from({ length: 8 }, () => new Redis(testUrl)) : [];
+  const prefix = `test:ad-click:${randomUUID()}:`;
+  const keys: string[] = [];
+  const key = (name: string) => { const k = prefix + name; keys.push(k); return k; };
+  afterAll(async () => {
+    if (keys.length) await clients[0].del(...keys);
+    await Promise.all(clients.map((c) => c.quit()));
+  });
+  it("admits exactly one of 32 concurrent cross-campaign clicks across connections", async () => {
+    const ip = key("ip"), visitor = key("visitor");
+    const accepted = await Promise.all(Array.from({ length: 32 }, (_, i) =>
+      clients[i % clients.length].eval(CLAIM_CLICK_LUA, 2, ip, visitor, CLICK_COOLDOWN_MS)));
+    expect(accepted.filter((n) => n === 1)).toHaveLength(1);
+    expect(await clients[0].pttl(ip)).toBeGreaterThan(4_000);
+    expect(await clients[0].pttl(ip)).toBeLessThanOrEqual(5_000);
+    expect(await clients[1].eval(CLAIM_CLICK_LUA, 2, ip, key("rotated-visitor"), 5_000)).toBe(0);
+    expect(await clients[1].eval(CLAIM_CLICK_LUA, 2, key("rotated-ip"), visitor, 5_000)).toBe(0);
+    expect(await clients[1].eval(CLAIM_CLICK_LUA, 2, key("different-ip"), key("different-visitor"), 5_000)).toBe(1);
+  });
+  it("does not claim the other identity or extend expiry on rejection", async () => {
+    const ip = key("busy-ip"), visitor = key("new-visitor");
+    await clients[0].set(ip, "1", "PX", 100);
+    expect(await clients[1].eval(CLAIM_CLICK_LUA, 2, ip, visitor, 5_000)).toBe(0);
+    expect(await clients[0].exists(visitor)).toBe(0);
+    expect(await clients[0].pttl(ip)).toBeLessThanOrEqual(100);
+    await new Promise((r) => setTimeout(r, 120));
+    expect(await clients[1].eval(CLAIM_CLICK_LUA, 2, ip, visitor, 5_000)).toBe(1);
+  });
+});
diff --git a/tests/contract/ads-click-cooldown.test.ts b/tests/contract/ads-click-cooldown.test.ts
new file mode 100644
index 00000000..fdb775ab
--- /dev/null
+++ b/tests/contract/ads-click-cooldown.test.ts
@@ -0,0 +1,68 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+const mock = vi.hoisted(() => ({ eval: vi.fn() }));
+vi.mock("ioredis", () => ({ default: class {
+  status = "ready";
+  eval = mock.eval;
+  on() {}
+} }));
+import { claimClickCooldown, CLICK_COOLDOWN_MS } from "@/lib/ads/click-cooldown";
+import { adClickIp } from "@/lib/ads/client-ip";
+
+beforeEach(() => {
+  vi.stubEnv("REDIS_URL", "redis://localhost:6379");
+  mock.eval.mockReset().mockResolvedValue(1);
+});
+afterEach(() => vi.unstubAllEnvs());
+
+describe("network-wide click cooldown", () => {
+  it("claims both identities together for five seconds without storing raw identifiers", async () => {
+    expect(await claimClickCooldown({ ip: "8.8.8.8", visitorId: "visitor-123" })).toEqual({ allowed: true });
+    const [, count, ipKey, visitorKey, ttl] = mock.eval.mock.calls[0];
+    expect(count).toBe(2);
+    expect(ipKey).toMatch(/^ad:click:ip:[a-f0-9]{32}$/);
+    expect(visitorKey).toMatch(/^ad:click:visitor:[a-f0-9]{32}$/);
+    expect(ttl).toBe(5_000);
+    expect(CLICK_COOLDOWN_MS).toBe(ttl);
+    expect(JSON.stringify(mock.eval.mock.calls)).not.toContain("8.8.8.8");
+    expect(JSON.stringify(mock.eval.mock.calls)).not.toContain("visitor-123");
+  });
+  it("rejects an occupied identity instead of charging another campaign", async () => {
+    mock.eval.mockResolvedValue(0);
+    expect(await claimClickCooldown({ ip: "8.8.8.8" })).toEqual({ allowed: false, reason: "click_cooldown" });
+  });
+  it("withholds charges when Redis fails or is unconfigured", async () => {
+    mock.eval.mockRejectedValue(new Error("offline"));
+    expect((await claimClickCooldown({ ip: "8.8.8.8" })).reason).toBe("cooldown_unavailable");
+    vi.stubEnv("REDIS_URL", "");
+    expect((await claimClickCooldown({ ip: "8.8.8.8" })).reason).toBe("cooldown_unavailable");
+  });
+  it("rejects unidentified traffic and unsafe visitor IDs", async () => {
+    expect((await claimClickCooldown({})).reason).toBe("missing_identity");
+    expect((await claimClickCooldown({ visitorId: "forged),filter" })).reason).toBe("missing_identity");
+    expect(mock.eval).not.toHaveBeenCalled();
+  });
+  it("does not reset the IP bucket at UTC midnight", async () => {
+    vi.useFakeTimers();
+    try {
+      vi.setSystemTime(new Date("2026-09-13T23:59:59Z"));
+      await claimClickCooldown({ ip: "8.8.8.8" });
+      vi.setSystemTime(new Date("2026-09-14T00:00:01Z"));
+      await claimClickCooldown({ ip: "8.8.8.8" });
+      expect(mock.eval.mock.calls[0][2]).toBe(mock.eval.mock.calls[1][2]);
+    } finally { vi.useRealTimers(); }
+  });
+});
+
+describe("Railway click identity", () => {
+  it("cannot be overridden by caller-controlled forwarded headers", () => {
+    vi.stubEnv("RAILWAY_ENVIRONMENT_ID", "production");
+    expect(adClickIp(new Headers({
+      "x-real-ip": "8.8.8.8", "cf-connecting-ip": "1.1.1.1", "x-forwarded-for": "9.9.9.9",
+    }))).toBe("8.8.8.8");
+  });
+  it("does not fall back to a forged header when the trusted identity is missing", () => {
+    vi.stubEnv("RAILWAY_ENVIRONMENT_ID", "production");
+    expect(adClickIp(new Headers({ "cf-connecting-ip": "1.1.1.1" }))).toBeNull();
+  });
+});
diff --git a/tests/contract/ads-click-validity.test.ts b/tests/contract/ads-click-validity.test.ts
new file mode 100644
index 00000000..a1ca99b0
--- /dev/null
+++ b/tests/contract/ads-click-validity.test.ts
@@ -0,0 +1,46 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const state = vi.hoisted(() => ({ error: false, throws: false, duplicate: false, filter: "", impression: { campaign_id: "campaign", slot_id: "slot" } }));
+vi.mock("@/lib/supabase/service", () => ({ serviceClient: () => ({
+  from() {
+    if (state.throws) throw new Error("database unavailable");
+    const q = {
+      select: () => q, eq: () => q, gte: () => q, limit: () => q,
+      maybeSingle: async () => ({ data: state.impression, error: state.error ? {} : null }),
+      or: async (filter: string) => {
+        state.filter = filter;
+        return { data: state.duplicate ? [{ id: "previous" }] : [], error: state.error ? {} : null };
+      },
+    };
+    return q;
+  },
+}) }));
+import { assessClickValidity } from "@/lib/ads/fraud";
+beforeEach(() => {
+  state.error = false; state.throws = false; state.duplicate = false; state.filter = "";
+  state.impression = { campaign_id: "campaign", slot_id: "slot" };
+});
+const input = { campaignId: "campaign", slotId: "slot", visitorId: "visitor", ipHashes: ["abc123"] };
+describe("click validation", () => {
+  it("deduplicates both paid and free delivery while excluding invalid traffic", async () => {
+    state.duplicate = true;
+    expect(await assessClickValidity(input)).toEqual({ valid: false, reason: "duplicate" });
+    expect(state.filter).toBe("and(or(valid.eq.true,tier.eq.free),or(visitor_id.eq.visitor,ip_hash.eq.abc123))");
+  });
+  it("withholds billing if a database lookup fails or throws", async () => {
+    state.error = true;
+    expect((await assessClickValidity(input)).reason).toBe("validation_unavailable");
+    expect((await assessClickValidity({ ...input, impressionId: "impression" })).reason).toBe("validation_unavailable");
+    state.throws = true;
+    expect((await assessClickValidity(input)).reason).toBe("validation_unavailable");
+  });
+  it("rejects bots, mismatched impressions and missing identities", async () => {
+    expect((await assessClickValidity({ ...input, device: "bot" })).reason).toBe("bot");
+    state.impression.campaign_id = "forged";
+    expect((await assessClickValidity({ ...input, impressionId: "impression" })).reason).toBe("impression_mismatch");
+    expect((await assessClickValidity({ campaignId: "campaign", visitorId: "v),injected" })).reason).toBe("missing_identity");
+  });
+  it("admits a new recognized visitor after successful validation", async () => {
+    expect(await assessClickValidity(input)).toEqual({ valid: true });
+  });
+});