diff --git a/extensions/whatsapp/src/inbound/monitor.ts b/extensions/whatsapp/src/inbound/monitor.ts
index 4f2d5541b6a..5337c5d6a43 100644
--- a/extensions/whatsapp/src/inbound/monitor.ts
+++ b/extensions/whatsapp/src/inbound/monitor.ts
@@ -413,7 +413,13 @@ export async function monitorWebInbox(options: {
 
       // If this is history/offline catch-up, mark read above but skip auto-reply.
       if (upsert.type === "append") {
-        continue;
+        const APPEND_RECENT_GRACE_MS = 60_000;
+        const msgTsRaw = msg.messageTimestamp;
+        const msgTsNum = msgTsRaw != null ? Number(msgTsRaw) : NaN;
+        const msgTsMs = Number.isFinite(msgTsNum) ? msgTsNum * 1000 : 0;
+        if (msgTsMs < connectedAtMs - APPEND_RECENT_GRACE_MS) {
+          continue;
+        }
       }
 
       const enriched = await enrichInboundMessage(msg);
diff --git a/extensions/whatsapp/src/monitor-inbox.append-upsert.test.ts b/extensions/whatsapp/src/monitor-inbox.append-upsert.test.ts
new file mode 100644
index 00000000000..e5746455432
--- /dev/null
+++ b/extensions/whatsapp/src/monitor-inbox.append-upsert.test.ts
@@ -0,0 +1,149 @@
+import "./monitor-inbox.test-harness.js";
+import { describe, expect, it, vi } from "vitest";
+import { monitorWebInbox } from "./inbound.js";
+import {
+  DEFAULT_ACCOUNT_ID,
+  getAuthDir,
+  getSock,
+  installWebMonitorInboxUnitTestHooks,
+} from "./monitor-inbox.test-harness.js";
+
+describe("append upsert handling (#20952)", () => {
+  installWebMonitorInboxUnitTestHooks();
+  type InboxOnMessage = NonNullable<Parameters<typeof monitorWebInbox>[0]["onMessage"]>;
+
+  async function tick() {
+    await new Promise((resolve) => setImmediate(resolve));
+  }
+
+  async function startInboxMonitor(onMessage: InboxOnMessage) {
+    const listener = await monitorWebInbox({
+      verbose: false,
+      onMessage,
+      accountId: DEFAULT_ACCOUNT_ID,
+      authDir: getAuthDir(),
+    });
+    return { listener, sock: getSock() };
+  }
+
+  it("processes recent append messages (within 60s of connect)", async () => {
+    const onMessage = vi.fn(async () => {});
+    const { listener, sock } = await startInboxMonitor(onMessage);
+
+    // Timestamp ~5 seconds ago — recent, should be processed.
+    const recentTs = Math.floor(Date.now() / 1000) - 5;
+    sock.ev.emit("messages.upsert", {
+      type: "append",
+      messages: [
+        {
+          key: { id: "recent-1", fromMe: false, remoteJid: "120363@g.us" },
+          message: { conversation: "hello from group" },
+          messageTimestamp: recentTs,
+          pushName: "Tester",
+        },
+      ],
+    });
+    await tick();
+
+    expect(onMessage).toHaveBeenCalledTimes(1);
+
+    await listener.close();
+  });
+
+  it("skips stale append messages (older than 60s before connect)", async () => {
+    const onMessage = vi.fn(async () => {});
+    const { listener, sock } = await startInboxMonitor(onMessage);
+
+    // Timestamp 5 minutes ago — stale history sync, should be skipped.
+    const staleTs = Math.floor(Date.now() / 1000) - 300;
+    sock.ev.emit("messages.upsert", {
+      type: "append",
+      messages: [
+        {
+          key: { id: "stale-1", fromMe: false, remoteJid: "120363@g.us" },
+          message: { conversation: "old history sync" },
+          messageTimestamp: staleTs,
+          pushName: "OldTester",
+        },
+      ],
+    });
+    await tick();
+
+    expect(onMessage).not.toHaveBeenCalled();
+
+    await listener.close();
+  });
+
+  it("skips append messages with NaN/non-finite timestamps", async () => {
+    const onMessage = vi.fn(async () => {});
+    const { listener, sock } = await startInboxMonitor(onMessage);
+
+    // NaN timestamp should be treated as 0 (stale) and skipped.
+    sock.ev.emit("messages.upsert", {
+      type: "append",
+      messages: [
+        {
+          key: { id: "nan-1", fromMe: false, remoteJid: "120363@g.us" },
+          message: { conversation: "bad timestamp" },
+          messageTimestamp: NaN,
+          pushName: "BadTs",
+        },
+      ],
+    });
+    await tick();
+
+    expect(onMessage).not.toHaveBeenCalled();
+
+    await listener.close();
+  });
+
+  it("handles Long-like protobuf timestamps correctly", async () => {
+    const onMessage = vi.fn(async () => {});
+    const { listener, sock } = await startInboxMonitor(onMessage);
+
+    // Baileys can deliver messageTimestamp as a Long object (from protobufjs).
+    // Number(longObj) calls valueOf() and returns the numeric value.
+    const recentTs = Math.floor(Date.now() / 1000) - 5;
+    const longLike = { low: recentTs, high: 0, unsigned: true, valueOf: () => recentTs };
+    sock.ev.emit("messages.upsert", {
+      type: "append",
+      messages: [
+        {
+          key: { id: "long-1", fromMe: false, remoteJid: "120363@g.us" },
+          message: { conversation: "long timestamp" },
+          messageTimestamp: longLike,
+          pushName: "LongTs",
+        },
+      ],
+    });
+    await tick();
+
+    expect(onMessage).toHaveBeenCalledTimes(1);
+
+    await listener.close();
+  });
+
+  it("always processes notify messages regardless of timestamp", async () => {
+    const onMessage = vi.fn(async () => {});
+    const { listener, sock } = await startInboxMonitor(onMessage);
+
+    // Very old timestamp but type=notify — should always be processed.
+    const oldTs = Math.floor(Date.now() / 1000) - 86400;
+    sock.ev.emit("messages.upsert", {
+      type: "notify",
+      messages: [
+        {
+          key: { id: "notify-1", fromMe: false, remoteJid: "999@s.whatsapp.net" },
+          message: { conversation: "normal message" },
+          messageTimestamp: oldTs,
+          pushName: "User",
+        },
+      ],
+    });
+    await tick();
+
+    expect(onMessage).toHaveBeenCalledTimes(1);
+
+    await listener.close();
+  });
+});