diff --git a/app/api/presence/visitors/route.ts b/app/api/presence/visitors/route.ts
index c0b2969..019a10a 100644
--- a/app/api/presence/visitors/route.ts
+++ b/app/api/presence/visitors/route.ts
@@ -7,6 +7,7 @@ import { updateVisitorPresence } from "@/lib/presence";
 
 const inputSchema = z.strictObject({
   tabId: z.string().uuid(),
+  browserId: z.string().uuid(),
   guideId: z.string().uuid().nullable(),
   active: z.boolean().default(true),
 });
diff --git a/hooks/use-presence.ts b/hooks/use-presence.ts
index 465d41c..50d83c2 100644
--- a/hooks/use-presence.ts
+++ b/hooks/use-presence.ts
@@ -58,7 +58,8 @@ function useHeartbeat<T>({
         method: "POST",
         headers: { "Content-Type": "application/json" },
         body: JSON.stringify({
-          tabId: identifyBy === "browser" ? visitorId() : tabId(),
+          tabId: tabId(),
+          ...(identifyBy === "browser" ? { browserId: visitorId() } : {}),
           guideId,
           ...(section !== undefined ? { section } : {}),
           active,
@@ -82,7 +83,7 @@ function useHeartbeat<T>({
       if (document.visibilityState === "visible") void send();
     };
     const leave = () => {
-      if (identifyBy === "tab") void send(false);
+      void send(false);
     };
     window.addEventListener("focus", refresh);
     window.addEventListener("pagehide", leave);
diff --git a/lib/presence.ts b/lib/presence.ts
index 126ea9b..4405491 100644
--- a/lib/presence.ts
+++ b/lib/presence.ts
@@ -36,16 +36,19 @@ async function purge(
 
 export async function updateVisitorPresence({
   tabId,
+  browserId,
   guideId,
   active,
 }: {
   tabId: string;
+  browserId: string;
   guideId: string | null;
   active: boolean;
 }) {
   const redis = await getRedisClient();
   const activeKey = `${prefix()}:visitors`;
   const scopeKey = `${prefix()}:visitor-scopes`;
+  const browserKey = `${prefix()}:visitor-browsers`;
   const now = Date.now();
   const serialized = await redis.eval(`
     local now = tonumber(ARGV[1])
@@ -53,31 +56,52 @@ export async function updateVisitorPresence({
     for _, id in ipairs(expired) do
       redis.call('ZREM', KEYS[1], id)
       redis.call('HDEL', KEYS[2], id)
+      redis.call('HDEL', KEYS[3], id)
     end
     if ARGV[5] == '1' then
       if redis.call('ZCARD', KEYS[1]) >= 10000 and not redis.call('ZSCORE', KEYS[1], ARGV[2]) then
         return redis.error_reply('presence capacity reached')
       end
+      local previousScope = redis.call('HGET', KEYS[2], ARGV[2])
+      local previousBrowser = redis.call('HGET', KEYS[3], ARGV[2])
       redis.call('ZADD', KEYS[1], now + tonumber(ARGV[4]), ARGV[2])
       redis.call('HSET', KEYS[2], ARGV[2], ARGV[3])
+      redis.call('HSET', KEYS[3], ARGV[2], ARGV[6])
+      if previousScope ~= ARGV[3] or previousBrowser ~= ARGV[6] then redis.call('DEL', KEYS[4]) end
     else
       redis.call('ZREM', KEYS[1], ARGV[2])
       redis.call('HDEL', KEYS[2], ARGV[2])
+      redis.call('HDEL', KEYS[3], ARGV[2])
+      redis.call('DEL', KEYS[4])
     end
     redis.call('PEXPIRE', KEYS[1], tonumber(ARGV[4]) * 2)
     redis.call('PEXPIRE', KEYS[2], tonumber(ARGV[4]) * 2)
-    local snapshot = redis.call('GET', KEYS[3])
+    redis.call('PEXPIRE', KEYS[3], tonumber(ARGV[4]) * 2)
+    if #expired > 0 then redis.call('DEL', KEYS[4]) end
+    local snapshot = redis.call('GET', KEYS[4])
     if snapshot then return snapshot end
     local ids = redis.call('ZRANGEBYSCORE', KEYS[1], now + 1, '+inf')
     local counts = {}
+    local browsers = {}
+    local guideBrowsers = {}
     for _, id in ipairs(ids) do
+      local browser = redis.call('HGET', KEYS[3], id) or id
       local scope = redis.call('HGET', KEYS[2], id)
-      if scope and scope ~= '' then counts[scope] = (counts[scope] or 0) + 1 end
+      browsers[browser] = true
+      if scope and scope ~= '' then
+        local key = scope .. ':' .. browser
+        if not guideBrowsers[key] then
+          counts[scope] = (counts[scope] or 0) + 1
+          guideBrowsers[key] = true
+        end
+      end
     end
-    snapshot = cjson.encode({global = #ids, byGuide = counts})
-    redis.call('SET', KEYS[3], snapshot, 'PX', 5000)
+    local global = 0
+    for _ in pairs(browsers) do global = global + 1 end
+    snapshot = cjson.encode({global = global, byGuide = counts})
+    redis.call('SET', KEYS[4], snapshot, 'PX', 5000)
     return snapshot
-  `, 3, activeKey, scopeKey, `${prefix()}:visitor-counts`, now, tabId, guideId ?? "", PRESENCE_TTL_MS, active ? "1" : "0") as string;
+  `, 4, activeKey, scopeKey, browserKey, `${prefix()}:visitor-counts`, now, tabId, guideId ?? "", PRESENCE_TTL_MS, active ? "1" : "0", browserId) as string;
   const snapshot = JSON.parse(serialized) as { global: number; byGuide: Record<string, number> };
   return { ...snapshot, guide: guideId ? snapshot.byGuide[guideId] ?? 0 : null };
 }
diff --git a/tests/security-redis.integration.test.ts b/tests/security-redis.integration.test.ts
index 15dceb9..6843144 100644
--- a/tests/security-redis.integration.test.ts
+++ b/tests/security-redis.integration.test.ts
@@ -54,14 +54,33 @@ describeWithRedis("shared Redis security paths", () => {
   });
 
   it("tracks visitor counts without a separate Redis round trip per visitor", async () => {
-    const first = await updateVisitorPresence({ tabId: `${runId}:1`, guideId: "guide-a", active: true });
+    const first = await updateVisitorPresence({ tabId: `${runId}:1`, browserId: `${runId}:browser-1`, guideId: "guide-a", active: true });
     expect(first.global).toBe(1);
     expect(first.guide).toBe(1);
-    await updateVisitorPresence({ tabId: `${runId}:2`, guideId: "guide-a", active: true });
+    const second = await updateVisitorPresence({ tabId: `${runId}:2`, browserId: `${runId}:browser-2`, guideId: "guide-a", active: true });
+    expect(second.global).toBe(2);
+    expect(second.guide).toBe(2);
     const count = await publisher.zcard(`${process.env.REDIS_PRESENCE_PREFIX}:visitors`);
     expect(count).toBe(2);
   });
 
+  it("counts a browser once across tabs without losing either guide", async () => {
+    const browserId = `${runId}:browser-3`;
+    const firstTab = `${runId}:3`;
+    const secondTab = `${runId}:4`;
+    const firstGuide = await updateVisitorPresence({ tabId: firstTab, browserId, guideId: "guide-c", active: true });
+    const secondGuide = await updateVisitorPresence({ tabId: secondTab, browserId, guideId: "guide-d", active: true });
+    expect(secondGuide.global).toBe(firstGuide.global);
+    expect(secondGuide.guide).toBe(1);
+    expect(secondGuide.byGuide["guide-c"]).toBe(1);
+    const sameGuide = await updateVisitorPresence({ tabId: secondTab, browserId, guideId: "guide-c", active: true });
+    expect(sameGuide.guide).toBe(1);
+    expect(sameGuide.byGuide["guide-d"]).toBeUndefined();
+    const afterLeave = await updateVisitorPresence({ tabId: secondTab, browserId, guideId: "guide-d", active: false });
+    expect(afterLeave.byGuide["guide-c"]).toBe(1);
+    expect(afterLeave.byGuide["guide-d"]).toBeUndefined();
+  });
+
   it("fans one subscribed topic out to two SSE clients", async () => {
     const topic = `page:${runId}`;
     const first = new AbortController();