diff --git a/src/seqta/utils/calendarSync/eventFingerprint.ts b/src/seqta/utils/calendarSync/eventFingerprint.ts
new file mode 100644
index 00000000..bbede560
--- /dev/null
+++ b/src/seqta/utils/calendarSync/eventFingerprint.ts
@@ -0,0 +1,42 @@
+/** Stable content fingerprint for skip-unchanged calendar sync. */
+
+export type FingerprintableEvent = {
+  summary: string;
+  location?: string;
+  description?: string;
+  startDateTime: string;
+  endDateTime: string;
+  timeZone: string;
+};
+
+export function eventFingerprint(event: FingerprintableEvent): string {
+  return [
+    event.summary.trim(),
+    (event.location ?? "").trim(),
+    (event.description ?? "").trim(),
+    event.startDateTime.trim(),
+    event.endDateTime.trim(),
+    event.timeZone.trim(),
+  ].join("\n");
+}
+
+/** Parse `Key: {seqtaKey}` from Outlook event body text. */
+export function parseOutlookSeqtaKey(bodyContent: string | undefined | null): string | undefined {
+  if (!bodyContent) return undefined;
+  const match = bodyContent.match(/^Key:\s*(.+)$/m);
+  const key = match?.[1]?.trim();
+  return key && key.length > 0 ? key : undefined;
+}
+
+export function outlookDescriptionWithKey(
+  description: string | undefined,
+  seqtaKey: string,
+): string {
+  const base = (description ?? "Synced by BetterSEQTA+").trim();
+  const withoutKey = base
+    .split("\n")
+    .filter((line) => !/^Key:\s*/.test(line))
+    .join("\n")
+    .trim();
+  return `${withoutKey}\nKey: ${seqtaKey}`;
+}
diff --git a/src/seqta/utils/calendarSync/eventMap.ts b/src/seqta/utils/calendarSync/eventMap.ts
index b83415ae..617703b8 100644
--- a/src/seqta/utils/calendarSync/eventMap.ts
+++ b/src/seqta/utils/calendarSync/eventMap.ts
@@ -3,6 +3,8 @@ import browser from "webextension-polyfill";
 export interface EventMapEntry {
   id: string;
   date: string;
+  /** Last-synced content fingerprint; used to skip unchanged events. */
+  fingerprint?: string;
 }
 
 export type EventMapRecord = Record<string, string | EventMapEntry>;
@@ -17,11 +19,21 @@ export function normalizeEventMapEntry(
   if (value == null) return undefined;
   if (typeof value === "string") return { id: value, date: "" };
   if (typeof value.id === "string" && value.id.length > 0) {
-    return { id: value.id, date: value.date ?? "" };
+    return {
+      id: value.id,
+      date: value.date ?? "",
+      fingerprint: value.fingerprint,
+    };
   }
   return undefined;
 }
 
+export function getStoredFingerprint(
+  value: string | EventMapEntry | undefined,
+): string | undefined {
+  return normalizeEventMapEntry(value)?.fingerprint;
+}
+
 export function getStoredEventId(
   value: string | EventMapEntry | undefined,
 ): string | undefined {
diff --git a/src/seqta/utils/calendarSync/lessonSyncShared.test.ts b/src/seqta/utils/calendarSync/lessonSyncShared.test.ts
index 49dc997a..7b86f0c7 100644
--- a/src/seqta/utils/calendarSync/lessonSyncShared.test.ts
+++ b/src/seqta/utils/calendarSync/lessonSyncShared.test.ts
@@ -4,8 +4,21 @@ jest.mock("@/utils/verboseLog", () => ({
   verboseLog: jest.fn(),
 }));
 
-import { reportSyncProgress } from "./lessonSyncShared";
+import {
+  eventFingerprint,
+  outlookDescriptionWithKey,
+  parseOutlookSeqtaKey,
+} from "./eventFingerprint";
+import {
+  buildLessonSyncResult,
+  entriesToPrune,
+  formatLessonSyncResultMessage,
+  mergeRemoteEventsIntoMap,
+  reportSyncProgress,
+  upsertLessonEvents,
+} from "./lessonSyncShared";
 import type { GoogleCalendarSyncProgress } from "@/seqta/utils/googleCalendar/types";
+import { syncWindowRange } from "@/seqta/utils/googleCalendar/syncDateRange";
 
 describe("reportSyncProgress", () => {
   it("calls onProgress when provided", () => {
@@ -24,3 +37,189 @@ describe("reportSyncProgress", () => {
     expect(onProgress).toHaveBeenCalledWith(progress);
   });
 });
+
+describe("eventFingerprint", () => {
+  it("is stable for equivalent event content", () => {
+    const base = {
+      summary: "Math",
+      location: "MA1",
+      description: "Synced by BetterSEQTA+",
+      startDateTime: "2026-07-13T09:00:00",
+      endDateTime: "2026-07-13T10:00:00",
+      timeZone: "Australia/Adelaide",
+    };
+    expect(eventFingerprint(base)).toBe(eventFingerprint({ ...base }));
+    expect(eventFingerprint(base)).not.toBe(
+      eventFingerprint({ ...base, summary: "English" }),
+    );
+  });
+});
+
+describe("outlook Key helpers", () => {
+  it("embeds and parses Key lines", () => {
+    const withKey = outlookDescriptionWithKey("Synced by BetterSEQTA+\nTeacher: A", "origin:cal:1");
+    expect(withKey).toContain("Key: origin:cal:1");
+    expect(parseOutlookSeqtaKey(withKey)).toBe("origin:cal:1");
+    expect(outlookDescriptionWithKey(withKey, "origin:cal:2")).toContain("Key: origin:cal:2");
+    expect(outlookDescriptionWithKey(withKey, "origin:cal:2").match(/^Key:/gm)).toHaveLength(1);
+  });
+});
+
+describe("entriesToPrune", () => {
+  const origin = "https://school.seqta.com.au";
+  const weeksAhead = 12;
+  const syncDate = syncWindowRange(weeksAhead).from;
+  const mapKey = `${origin}::${origin}:cal:1`;
+  const cancelledKey = `${origin}::${origin}:cal:cancelled`;
+
+  it("returns nothing for incremental mode", () => {
+    expect(
+      entriesToPrune(
+        { [mapKey]: { id: "evt-1", date: syncDate } },
+        origin,
+        "incremental",
+        weeksAhead,
+        new Set([mapKey]),
+      ),
+    ).toEqual([]);
+  });
+
+  it("prunes cancelled lessons still inside the sync window", () => {
+    const pruned = entriesToPrune(
+      {
+        [mapKey]: { id: "evt-1", date: syncDate },
+        [cancelledKey]: { id: "evt-cancelled", date: syncDate },
+      },
+      origin,
+      "full",
+      weeksAhead,
+      new Set([mapKey]),
+    );
+    expect(pruned).toEqual([[cancelledKey, "evt-cancelled"]]);
+  });
+
+  it("prunes events whose date is outside the sync window", () => {
+    const staleKey = `${origin}::${origin}:cal:old`;
+    const pruned = entriesToPrune(
+      { [staleKey]: { id: "evt-old", date: "2020-01-06" } },
+      origin,
+      "full",
+      weeksAhead,
+      new Set(),
+    );
+    expect(pruned).toEqual([[staleKey, "evt-old"]]);
+  });
+});
+
+describe("mergeRemoteEventsIntoMap", () => {
+  const origin = "https://school.seqta.com.au";
+  const mapKey = (o: string, seqtaKey: string) => `${o}::${seqtaKey}`;
+
+  it("recovers remote IDs for the current origin only", () => {
+    const eventMap: Record<string, { id: string; date: string; fingerprint?: string }> = {};
+    mergeRemoteEventsIntoMap(
+      eventMap,
+      origin,
+      [
+        {
+          seqtaKey: `${origin}:cal:1`,
+          id: "remote-1",
+          date: "2026-07-13",
+          fingerprint: "fp-1",
+        },
+        {
+          seqtaKey: "https://other.seqta.com.au:cal:2",
+          id: "remote-other",
+          date: "2026-07-13",
+          fingerprint: "fp-other",
+        },
+      ],
+      mapKey,
+    );
+
+    expect(eventMap[`${origin}::${origin}:cal:1`]).toMatchObject({
+      id: "remote-1",
+      fingerprint: "fp-1",
+    });
+    expect(eventMap[`${origin}::https://other.seqta.com.au:cal:2`]).toBeUndefined();
+  });
+
+  it("keeps an existing local fingerprint when reconciling", () => {
+    const key = `${origin}::${origin}:cal:1`;
+    const eventMap = {
+      [key]: { id: "old-id", date: "2026-07-13", fingerprint: "local-fp" },
+    };
+    mergeRemoteEventsIntoMap(
+      eventMap,
+      origin,
+      [
+        {
+          seqtaKey: `${origin}:cal:1`,
+          id: "remote-1",
+          date: "2026-07-13",
+          fingerprint: "remote-fp",
+        },
+      ],
+      mapKey,
+    );
+    expect(eventMap[key]).toMatchObject({ id: "remote-1", fingerprint: "local-fp" });
+  });
+});
+
+describe("formatLessonSyncResultMessage", () => {
+  it("includes unchanged counts", () => {
+    expect(
+      formatLessonSyncResultMessage(
+        { success: true, created: 1, updated: 2, deleted: 0, skipped: 3 },
+        "Google Calendar",
+      ),
+    ).toBe("Google Calendar updated (1 new, 2 updated, 3 unchanged).");
+  });
+
+  it("reports up to date when nothing changed", () => {
+    expect(
+      formatLessonSyncResultMessage(
+        buildLessonSyncResult(0, 0, 0, 0, 0, Date.now()),
+        "Outlook Calendar",
+      ),
+    ).toBe("Outlook Calendar is up to date.");
+  });
+});
+
+describe("upsertLessonEvents skip unchanged", () => {
+  it("skips API writes when fingerprint matches", async () => {
+    const origin = "https://school.seqta.com.au";
+    const event = {
+      seqtaKey: `${origin}:cal:1`,
+      summary: "Math",
+      description: "Synced by BetterSEQTA+",
+      startDateTime: "2026-07-13T09:00:00",
+      endDateTime: "2026-07-13T10:00:00",
+      timeZone: "UTC",
+    };
+    const fp = eventFingerprint(event);
+    const key = `${origin}::${event.seqtaKey}`;
+    const eventMap = {
+      [key]: { id: "existing", date: "2026-07-13", fingerprint: fp },
+    };
+    const upsert = jest.fn(async () => "existing");
+
+    const result = await upsertLessonEvents({
+      events: [event],
+      eventMap,
+      origin,
+      staleEntryCount: 0,
+      totalSteps: 1,
+      lastSyncAt: Date.now(),
+      initialFailed: 0,
+      getAccessToken: async () => "token",
+      mapKey: (o, k) => `${o}::${k}`,
+      upsert,
+      writeState: async () => undefined,
+      logLabel: "Test",
+    });
+
+    expect(upsert).not.toHaveBeenCalled();
+    expect(result).toMatchObject({ created: 0, updated: 0, skipped: 1, failed: 0 });
+  });
+});
diff --git a/src/seqta/utils/calendarSync/lessonSyncShared.ts b/src/seqta/utils/calendarSync/lessonSyncShared.ts
index 0dcf2c01..d010a371 100644
--- a/src/seqta/utils/calendarSync/lessonSyncShared.ts
+++ b/src/seqta/utils/calendarSync/lessonSyncShared.ts
@@ -1,13 +1,17 @@
 import { verboseLog } from "@/utils/verboseLog";
 import {
   getStoredEventId,
+  getStoredFingerprint,
   lessonDateFromSeqtaKey,
   normalizeEventMapEntry,
   type EventMapRecord,
 } from "@/seqta/utils/calendarSync/eventMap";
+import { eventFingerprint } from "@/seqta/utils/calendarSync/eventFingerprint";
+import type { RemoteSyncedEvent } from "@/seqta/utils/calendarSync/remoteEvents";
 import {
   isDateInRange,
   syncWindowRange,
+  type SyncDateRange,
 } from "@/seqta/utils/googleCalendar/syncDateRange";
 import type {
   GoogleCalendarDeleteResult,
@@ -20,7 +24,12 @@ export const EVENT_MAP_PERSIST_EVERY = 10;
 
 export type MappedLessonEvent = {
   seqtaKey: string;
+  summary: string;
+  location?: string;
+  description?: string;
   startDateTime: string;
+  endDateTime: string;
+  timeZone: string;
 };
 
 export function reportSyncProgress(
@@ -65,15 +74,49 @@ export function entriesToPrune(
     if (!mapKey.startsWith(prefix)) continue;
     const entry = normalizeEventMapEntry(raw);
     if (!entry) continue;
-    const stale = entry.date
-      ? !isDateInRange(entry.date, window)
-      : !currentMapKeys.has(mapKey);
-    if (stale) entries.push([mapKey, entry.id]);
+    const missingFromTimetable = !currentMapKeys.has(mapKey);
+    const outsideWindow = entry.date ? !isDateInRange(entry.date, window) : false;
+    if (missingFromTimetable || outsideWindow) {
+      entries.push([mapKey, entry.id]);
+    }
   }
 
   return entries;
 }
 
+/** Merge remote BS+ events into the local eventMap (origin-scoped keys). */
+export function mergeRemoteEventsIntoMap(
+  eventMap: EventMapRecord,
+  origin: string,
+  remoteEvents: RemoteSyncedEvent[],
+  mapKey: (origin: string, seqtaKey: string) => string,
+): void {
+  for (const remote of remoteEvents) {
+    if (!remote.seqtaKey.startsWith(origin)) continue;
+    const key = mapKey(origin, remote.seqtaKey);
+    const existing = normalizeEventMapEntry(eventMap[key]);
+    eventMap[key] = {
+      id: remote.id,
+      date: remote.date || existing?.date || lessonDateFromSeqtaKey(remote.seqtaKey) || "",
+      // Prefer last-written local fingerprint so skip stays stable across API format quirks.
+      fingerprint: existing?.fingerprint ?? remote.fingerprint,
+    };
+  }
+}
+
+export function reconcileRangeForMode(
+  mode: "full" | "incremental",
+  weeksAhead: number,
+  ranges: {
+    syncWindowRange: (weeks: number) => SyncDateRange;
+    trailingWeekRange: (weeks: number) => SyncDateRange;
+  },
+): SyncDateRange {
+  return mode === "incremental"
+    ? ranges.trailingWeekRange(weeksAhead)
+    : ranges.syncWindowRange(weeksAhead);
+}
+
 export function notConfiguredSyncResult(error: string): GoogleCalendarSyncResult {
   return { success: false, configured: false, error };
 }
@@ -98,10 +141,12 @@ export function formatLessonSyncResultMessage(
   const created = result.created ?? 0;
   const updated = result.updated ?? 0;
   const deleted = result.deleted ?? 0;
+  const skipped = result.skipped ?? 0;
   const parts: string[] = [];
   if (created > 0) parts.push(`${created} new`);
   if (updated > 0) parts.push(`${updated} updated`);
   if (deleted > 0) parts.push(`${deleted} removed`);
+  if (skipped > 0) parts.push(`${skipped} unchanged`);
   if (parts.length === 0) return `${calendarLabel} is up to date.`;
   return `${calendarLabel} updated (${parts.join(", ")}).`;
 }
@@ -187,6 +232,7 @@ export function buildLessonSyncResult(
   created: number,
   updated: number,
   deleted: number,
+  skipped: number,
   failed: number,
   lastSyncAt: number,
 ): GoogleCalendarSyncResult {
@@ -197,7 +243,7 @@ export function buildLessonSyncResult(
     created,
     updated,
     deleted,
-    skipped: 0,
+    skipped,
     failed,
     lastSyncAt,
     error:
@@ -234,7 +280,13 @@ type UpsertLessonEventsParams<TEvent extends MappedLessonEvent> = {
 
 export async function upsertLessonEvents<TEvent extends MappedLessonEvent>(
   params: UpsertLessonEventsParams<TEvent>,
-): Promise<{ created: number; updated: number; failed: number; accessToken: string }> {
+): Promise<{
+  created: number;
+  updated: number;
+  skipped: number;
+  failed: number;
+  accessToken: string;
+}> {
   const {
     events,
     eventMap,
@@ -254,16 +306,29 @@ export async function upsertLessonEvents<TEvent extends MappedLessonEvent>(
   let accessToken = await getAccessToken();
   let created = 0;
   let updated = 0;
+  let skipped = 0;
   let failed = initialFailed;
 
   for (let i = 0; i < events.length; i++) {
     const event = events[i];
     const key = mapKey(origin, event.seqtaKey);
     const existingId = getStoredEventId(eventMap[key]);
+    const desiredFingerprint = eventFingerprint(event);
     const progressCurrent = staleEntryCount + i + 1;
     const progressMessage = `Syncing events (${i + 1}/${events.length})…`;
 
     try {
+      if (existingId && getStoredFingerprint(eventMap[key]) === desiredFingerprint) {
+        skipped += 1;
+        reportSyncProgress(onProgress, {
+          phase: "upserting",
+          current: progressCurrent,
+          total: totalSteps,
+          message: progressMessage,
+        });
+        continue;
+      }
+
       const remoteId = await upsert(accessToken, existingId, event, async () => {
         accessToken = await getAccessToken();
         return accessToken;
@@ -273,6 +338,7 @@ export async function upsertLessonEvents<TEvent extends MappedLessonEvent>(
       eventMap[key] = {
         id: remoteId,
         date: lessonDateForEvent(event.startDateTime, event.seqtaKey),
+        fingerprint: desiredFingerprint,
       };
 
       if ((i + 1) % EVENT_MAP_PERSIST_EVERY === 0 || i === events.length - 1) {
@@ -291,5 +357,5 @@ export async function upsertLessonEvents<TEvent extends MappedLessonEvent>(
     });
   }
 
-  return { created, updated, failed, accessToken };
+  return { created, updated, skipped, failed, accessToken };
 }
diff --git a/src/seqta/utils/calendarSync/remoteEvents.ts b/src/seqta/utils/calendarSync/remoteEvents.ts
index ba28112d..7be3a39c 100644
--- a/src/seqta/utils/calendarSync/remoteEvents.ts
+++ b/src/seqta/utils/calendarSync/remoteEvents.ts
@@ -1,5 +1,20 @@
-import { GOOGLE_CALENDAR_API } from "@/config/googleCalendar";
-import { OUTLOOK_GRAPH_API } from "@/config/outlookCalendar";
+import { BSPLUS_GOOGLE_CALENDAR_EVENT_PROP, GOOGLE_CALENDAR_API } from "@/config/googleCalendar";
+import {
+  BSPLUS_OUTLOOK_CALENDAR_EVENT_CATEGORY,
+  OUTLOOK_GRAPH_API,
+} from "@/config/outlookCalendar";
+import {
+  eventFingerprint,
+  parseOutlookSeqtaKey,
+} from "@/seqta/utils/calendarSync/eventFingerprint";
+import type { SyncDateRange } from "@/seqta/utils/googleCalendar/syncDateRange";
+
+export type RemoteSyncedEvent = {
+  seqtaKey: string;
+  id: string;
+  fingerprint: string;
+  date: string;
+};
 
 async function authorizedFetch(
   accessToken: string,
@@ -138,3 +153,190 @@ export function deleteOutlookCalendarEvent(
     refreshAccessToken,
   );
 }
+
+function toRfc3339Start(date: string): string {
+  return `${date}T00:00:00Z`;
+}
+
+function toRfc3339EndExclusive(date: string): string {
+  const d = new Date(`${date}T12:00:00Z`);
+  d.setUTCDate(d.getUTCDate() + 1);
+  const y = d.getUTCFullYear();
+  const m = String(d.getUTCMonth() + 1).padStart(2, "0");
+  const day = String(d.getUTCDate()).padStart(2, "0");
+  return `${y}-${m}-${day}T00:00:00Z`;
+}
+
+function dateFromDateTime(value: string | undefined): string {
+  if (!value) return "";
+  return value.slice(0, 10);
+}
+
+type GoogleListItem = {
+  id?: string;
+  summary?: string;
+  location?: string;
+  description?: string;
+  start?: { dateTime?: string; date?: string; timeZone?: string };
+  end?: { dateTime?: string; date?: string; timeZone?: string };
+  extendedProperties?: { private?: Record<string, string> };
+};
+
+function googleItemToRemote(item: GoogleListItem): RemoteSyncedEvent | null {
+  const seqtaKey = item.extendedProperties?.private?.[BSPLUS_GOOGLE_CALENDAR_EVENT_PROP];
+  if (!item.id || !seqtaKey) return null;
+  const startDateTime = item.start?.dateTime ?? (item.start?.date ? `${item.start.date}T00:00:00` : "");
+  const endDateTime = item.end?.dateTime ?? (item.end?.date ? `${item.end.date}T00:00:00` : "");
+  const timeZone = item.start?.timeZone ?? item.end?.timeZone ?? "UTC";
+  return {
+    seqtaKey,
+    id: item.id,
+    date: dateFromDateTime(startDateTime) || item.start?.date || "",
+    fingerprint: eventFingerprint({
+      summary: item.summary ?? "",
+      location: item.location,
+      description: item.description,
+      startDateTime,
+      endDateTime,
+      timeZone,
+    }),
+  };
+}
+
+export async function listGoogleSyncedEvents(
+  accessToken: string,
+  calendarId: string,
+  range: SyncDateRange,
+  refreshAccessToken?: () => Promise<string>,
+): Promise<RemoteSyncedEvent[]> {
+  const encodedCalendar = encodeURIComponent(calendarId);
+  const out: RemoteSyncedEvent[] = [];
+  let pageToken: string | undefined;
+
+  do {
+    const params = new URLSearchParams({
+      singleEvents: "true",
+      orderBy: "startTime",
+      maxResults: "2500",
+      timeMin: toRfc3339Start(range.from),
+      timeMax: toRfc3339EndExclusive(range.until),
+      fields:
+        "nextPageToken,items(id,summary,location,description,start,end,extendedProperties)",
+    });
+    if (pageToken) params.set("pageToken", pageToken);
+
+    const res = await authorizedFetch(
+      accessToken,
+      `${GOOGLE_CALENDAR_API}/calendars/${encodedCalendar}/events?${params}`,
+      { method: "GET" },
+      refreshAccessToken,
+    );
+    const json = (await res.json().catch(() => ({}))) as {
+      items?: GoogleListItem[];
+      nextPageToken?: string;
+      error?: { message?: string };
+    };
+    if (!res.ok) {
+      throw new Error(json?.error?.message ?? `Google Calendar list failed (${res.status})`);
+    }
+
+    for (const item of json.items ?? []) {
+      const mapped = googleItemToRemote(item);
+      if (mapped) out.push(mapped);
+    }
+    pageToken = json.nextPageToken;
+  } while (pageToken);
+
+  return out;
+}
+
+type OutlookListItem = {
+  id?: string;
+  subject?: string;
+  body?: { content?: string; contentType?: string };
+  location?: { displayName?: string };
+  start?: { dateTime?: string; timeZone?: string };
+  end?: { dateTime?: string; timeZone?: string };
+  categories?: string[];
+};
+
+function outlookItemToRemote(item: OutlookListItem): RemoteSyncedEvent | null {
+  if (!item.id) return null;
+  const categories = item.categories ?? [];
+  if (!categories.includes(BSPLUS_OUTLOOK_CALENDAR_EVENT_CATEGORY)) return null;
+  const bodyContent = item.body?.content ?? "";
+  const seqtaKey = parseOutlookSeqtaKey(bodyContent);
+  if (!seqtaKey) return null;
+
+  const startDateTime = (item.start?.dateTime ?? "").replace(/\.\d+$/, "");
+  const endDateTime = (item.end?.dateTime ?? "").replace(/\.\d+$/, "");
+  const timeZone = item.start?.timeZone ?? item.end?.timeZone ?? "UTC";
+
+  return {
+    seqtaKey,
+    id: item.id,
+    date: dateFromDateTime(startDateTime),
+    fingerprint: eventFingerprint({
+      summary: item.subject ?? "",
+      location: item.location?.displayName,
+      description: outlookDescriptionForFingerprint(bodyContent),
+      startDateTime,
+      endDateTime,
+      timeZone,
+    }),
+  };
+}
+
+/** Strip Outlook Key line so fingerprint matches local mapped event description. */
+function outlookDescriptionForFingerprint(bodyContent: string): string {
+  return bodyContent
+    .split("\n")
+    .filter((line) => !/^Key:\s*/.test(line))
+    .join("\n")
+    .trim();
+}
+
+export async function listOutlookSyncedEvents(
+  accessToken: string,
+  range: SyncDateRange,
+  refreshAccessToken?: () => Promise<string>,
+): Promise<RemoteSyncedEvent[]> {
+  const out: RemoteSyncedEvent[] = [];
+  const params = new URLSearchParams({
+    startDateTime: toRfc3339Start(range.from),
+    endDateTime: toRfc3339EndExclusive(range.until),
+    $top: "100",
+    $select: "id,subject,body,location,start,end,categories",
+  });
+
+  let nextUrl: string | undefined =
+    `${OUTLOOK_GRAPH_API}/me/calendarView?${params.toString()}`;
+
+  while (nextUrl) {
+    const res = await authorizedFetch(
+      accessToken,
+      nextUrl,
+      {
+        method: "GET",
+        headers: { Prefer: 'outlook.body-content-type="text"' },
+      },
+      refreshAccessToken,
+    );
+    const json = (await res.json().catch(() => ({}))) as {
+      value?: OutlookListItem[];
+      "@odata.nextLink"?: string;
+      error?: { message?: string };
+    };
+    if (!res.ok) {
+      throw new Error(json?.error?.message ?? `Outlook Calendar list failed (${res.status})`);
+    }
+
+    for (const item of json.value ?? []) {
+      const mapped = outlookItemToRemote(item);
+      if (mapped) out.push(mapped);
+    }
+    nextUrl = json["@odata.nextLink"];
+  }
+
+  return out;
+}
diff --git a/src/seqta/utils/calendarSync/syncEngine.ts b/src/seqta/utils/calendarSync/syncEngine.ts
index 1a55bb29..0dd140ce 100644
--- a/src/seqta/utils/calendarSync/syncEngine.ts
+++ b/src/seqta/utils/calendarSync/syncEngine.ts
@@ -9,13 +9,19 @@ import {
   deleteTrackedLessonEvents,
   emptyLessonsSyncResult,
   entriesToPrune,
+  mergeRemoteEventsIntoMap,
   notConfiguredSyncResult,
   notConnectedSyncResult,
   originEventMapEntries,
+  reconcileRangeForMode,
   reportSyncProgress,
   upsertLessonEvents,
 } from "@/seqta/utils/calendarSync/lessonSyncShared";
-import { googleApiEventBody, mapLessonsToGoogleEvents, outlookGraphEventBody } from "@/seqta/utils/googleCalendar/eventMapper";
+import {
+  googleApiEventBody,
+  mapLessonsToGoogleEvents,
+  outlookGraphEventBody,
+} from "@/seqta/utils/googleCalendar/eventMapper";
 import {
   readGoogleCalendarState,
   writeGoogleCalendarState,
@@ -30,14 +36,23 @@ import type {
 import {
   deleteGoogleCalendarEvent,
   deleteOutlookCalendarEvent,
+  listGoogleSyncedEvents,
+  listOutlookSyncedEvents,
   upsertGoogleCalendarEvent,
   upsertOutlookCalendarEvent,
+  type RemoteSyncedEvent,
 } from "@/seqta/utils/calendarSync/remoteEvents";
 import { ensureGoogleAppCalendar } from "@/seqta/utils/googleCalendar/calendarProvisioning";
+import {
+  syncWindowRange,
+  trailingWeekRange,
+  type SyncDateRange,
+} from "@/seqta/utils/googleCalendar/syncDateRange";
 import {
   readOutlookCalendarState,
   writeOutlookCalendarState,
 } from "@/seqta/utils/outlookCalendar/storage";
+import { verboseLog } from "@/utils/verboseLog";
 
 type CalendarStoredState = {
   refreshToken?: string;
@@ -67,6 +82,11 @@ export type CalendarLessonSyncProvider = {
     body: Record<string, unknown>,
     refreshAccessToken: () => Promise<string>,
   ) => Promise<string>;
+  listSyncedEvents: (
+    accessToken: string,
+    range: SyncDateRange,
+    refreshAccessToken: () => Promise<string>,
+  ) => Promise<RemoteSyncedEvent[]>;
   toApiBody: (event: GoogleCalendarEventInput) => Record<string, unknown>;
 };
 
@@ -101,6 +121,13 @@ export const googleLessonSyncProvider: CalendarLessonSyncProvider = {
       body,
       refreshAccessToken,
     ),
+  listSyncedEvents: async (accessToken, range, refreshAccessToken) =>
+    listGoogleSyncedEvents(
+      accessToken,
+      await getOrProvisionGoogleCalendarId(accessToken),
+      range,
+      refreshAccessToken,
+    ),
   toApiBody: googleApiEventBody,
 };
 
@@ -115,6 +142,8 @@ export const outlookLessonSyncProvider: CalendarLessonSyncProvider = {
     deleteOutlookCalendarEvent(accessToken, eventId, refreshAccessToken),
   upsertEvent: (accessToken, existingId, body, refreshAccessToken) =>
     upsertOutlookCalendarEvent(accessToken, existingId, body, refreshAccessToken),
+  listSyncedEvents: (accessToken, range, refreshAccessToken) =>
+    listOutlookSyncedEvents(accessToken, range, refreshAccessToken),
   toApiBody: outlookGraphEventBody,
 };
 
@@ -151,6 +180,28 @@ export async function syncLessonsToCalendar(
   });
 
   const eventMap = { ...(state.eventMap ?? {}) };
+  let accessToken = await getAccessToken();
+  const refreshAccessToken = async () => {
+    accessToken = await getAccessToken();
+    return accessToken;
+  };
+
+  const reconcileRange = reconcileRangeForMode(mode, weeksAhead, {
+    syncWindowRange,
+    trailingWeekRange,
+  });
+
+  try {
+    const remoteEvents = await provider.listSyncedEvents(
+      accessToken,
+      reconcileRange,
+      refreshAccessToken,
+    );
+    mergeRemoteEventsIntoMap(eventMap, request.origin, remoteEvents, eventMapKey);
+  } catch (err) {
+    verboseLog(`[BetterSEQTA+] ${provider.label} remote event list failed:`, err);
+  }
+
   const currentMapKeys = new Set(events.map((event) => eventMapKey(request.origin, event.seqtaKey)));
   const staleEntries = entriesToPrune(eventMap, request.origin, mode, weeksAhead, currentMapKeys);
   const totalSteps = staleEntries.length + events.length;
@@ -179,13 +230,8 @@ export async function syncLessonsToCalendar(
     initialFailed: staleResult.failed,
     getAccessToken,
     mapKey: eventMapKey,
-    upsert: (accessToken, existingId, event, refreshAccessToken) =>
-      provider.upsertEvent(
-        accessToken,
-        existingId,
-        provider.toApiBody(event),
-        refreshAccessToken,
-      ),
+    upsert: (token, existingId, event, refresh) =>
+      provider.upsertEvent(token, existingId, provider.toApiBody(event), refresh),
     writeState: provider.writeState,
     onProgress: options.onProgress,
     logLabel: provider.label,
@@ -210,6 +256,7 @@ export async function syncLessonsToCalendar(
     upsertResult.created,
     upsertResult.updated,
     staleResult.deleted,
+    upsertResult.skipped,
     upsertResult.failed,
     lastSyncAt,
   );
diff --git a/src/seqta/utils/googleCalendar/eventMapper.test.ts b/src/seqta/utils/googleCalendar/eventMapper.test.ts
index f6e2ef94..7db17c67 100644
--- a/src/seqta/utils/googleCalendar/eventMapper.test.ts
+++ b/src/seqta/utils/googleCalendar/eventMapper.test.ts
@@ -2,6 +2,7 @@ import { describe, expect, it } from "@jest/globals";
 import {
   lessonToGoogleEvent,
   mapLessonsToGoogleEvents,
+  outlookGraphEventBody,
   seqtaLessonKey,
   shouldSyncLesson,
 } from "./eventMapper";
@@ -62,3 +63,14 @@ describe("mapLessonsToGoogleEvents", () => {
     expect(events).toHaveLength(1);
   });
 });
+
+describe("outlookGraphEventBody", () => {
+  it("embeds Key: seqtaKey in the body for remote reconcile", () => {
+    const event = lessonToGoogleEvent(ORIGIN, baseLesson, "Australia/Perth");
+    expect(event).not.toBeNull();
+    const body = outlookGraphEventBody(event!);
+    const content = (body.body as { content: string }).content;
+    expect(content).toContain(`Key: ${ORIGIN}:cal:12345`);
+    expect(content).toContain("Synced by BetterSEQTA+");
+  });
+});
diff --git a/src/seqta/utils/googleCalendar/eventMapper.ts b/src/seqta/utils/googleCalendar/eventMapper.ts
index 03b4d4f2..755110b8 100644
--- a/src/seqta/utils/googleCalendar/eventMapper.ts
+++ b/src/seqta/utils/googleCalendar/eventMapper.ts
@@ -1,5 +1,6 @@
 import { BSPLUS_GOOGLE_CALENDAR_EVENT_PROP } from "@/config/googleCalendar";
 import { BSPLUS_OUTLOOK_CALENDAR_EVENT_CATEGORY } from "@/config/outlookCalendar";
+import { outlookDescriptionWithKey } from "@/seqta/utils/calendarSync/eventFingerprint";
 import type { GoogleCalendarEventInput, SeqtaTimetableLesson } from "./types";
 
 const SKIP_TYPES = new Set(["note", "holiday", "assembly-note"]);
@@ -100,7 +101,7 @@ export function outlookGraphEventBody(event: GoogleCalendarEventInput): Record<s
     subject: event.summary,
     body: {
       contentType: "text",
-      content: event.description ?? "Synced by BetterSEQTA+",
+      content: outlookDescriptionWithKey(event.description, event.seqtaKey),
     },
     start: { dateTime: event.startDateTime, timeZone: event.timeZone },
     end: { dateTime: event.endDateTime, timeZone: event.timeZone },
diff --git a/src/seqta/utils/googleCalendar/syncEngine.test.ts b/src/seqta/utils/googleCalendar/syncEngine.test.ts
index a5352e35..66d46fc3 100644
--- a/src/seqta/utils/googleCalendar/syncEngine.test.ts
+++ b/src/seqta/utils/googleCalendar/syncEngine.test.ts
@@ -3,6 +3,8 @@ import type { SeqtaTimetableLesson } from "./types";
 
 jest.mock("@/config/googleCalendar", () => ({
   isGoogleCalendarConfigured: jest.fn(() => true),
+  BSPLUS_GOOGLE_CALENDAR_EVENT_PROP: "bsplusSeqtaKey",
+  GOOGLE_CALENDAR_API: "https://www.googleapis.com/calendar/v3",
 }));
 
 jest.mock("@/utils/verboseLog", () => ({
@@ -27,14 +29,21 @@ jest.mock("@/seqta/utils/calendarSync/settings", () => ({
 jest.mock("@/seqta/utils/calendarSync/remoteEvents", () => ({
   upsertGoogleCalendarEvent: jest.fn(),
   deleteGoogleCalendarEvent: jest.fn(),
+  listGoogleSyncedEvents: jest.fn(async () => []),
 }));
 
 import { readGoogleCalendarState } from "@/seqta/utils/googleCalendar/storage";
 import {
   deleteGoogleCalendarEvent,
+  listGoogleSyncedEvents,
   upsertGoogleCalendarEvent,
 } from "@/seqta/utils/calendarSync/remoteEvents";
-import { deleteSyncedEventsFromGoogleCalendar, syncLessonsToGoogleCalendar } from "@/seqta/utils/calendarSync/syncEngine";
+import {
+  deleteSyncedEventsFromGoogleCalendar,
+  syncLessonsToGoogleCalendar,
+} from "@/seqta/utils/calendarSync/syncEngine";
+import { eventFingerprint } from "@/seqta/utils/calendarSync/eventFingerprint";
+import { lessonToGoogleEvent } from "@/seqta/utils/googleCalendar/eventMapper";
 
 import { syncWindowRange } from "@/seqta/utils/googleCalendar/syncDateRange";
 
@@ -67,6 +76,7 @@ describe("syncLessonsToGoogleCalendar", () => {
     });
     jest.mocked(upsertGoogleCalendarEvent).mockResolvedValue("google-existing");
     jest.mocked(deleteGoogleCalendarEvent).mockResolvedValue(undefined);
+    jest.mocked(listGoogleSyncedEvents).mockResolvedValue([]);
   });
 
   it("updates existing events and removes stale tracked events on full sync", async () => {
@@ -75,6 +85,7 @@ describe("syncLessonsToGoogleCalendar", () => {
       getAccessToken,
     );
 
+    expect(listGoogleSyncedEvents).toHaveBeenCalled();
     expect(deleteGoogleCalendarEvent).toHaveBeenCalled();
     expect(upsertGoogleCalendarEvent).toHaveBeenCalledTimes(1);
     expect(result).toMatchObject({
@@ -107,6 +118,64 @@ describe("syncLessonsToGoogleCalendar", () => {
     });
   });
 
+  it("recovers remote IDs when the local map is empty and skips unchanged", async () => {
+    const timeZone = Intl.DateTimeFormat().resolvedOptions().timeZone || "UTC";
+    const mapped = lessonToGoogleEvent(ORIGIN, baseLesson, timeZone);
+    expect(mapped).not.toBeNull();
+    const fp = eventFingerprint(mapped!);
+
+    jest.mocked(readGoogleCalendarState).mockResolvedValue({
+      refreshToken: "refresh",
+      calendarId: "app-calendar-id",
+      eventMap: {},
+    });
+    jest.mocked(listGoogleSyncedEvents).mockResolvedValue([
+      {
+        seqtaKey: `${ORIGIN}:cal:12345`,
+        id: "recovered-id",
+        date: syncDate,
+        fingerprint: fp,
+      },
+    ]);
+
+    const result = await syncLessonsToGoogleCalendar(
+      { origin: ORIGIN, lessons: [baseLesson], mode: "full" },
+      getAccessToken,
+    );
+
+    expect(upsertGoogleCalendarEvent).not.toHaveBeenCalled();
+    expect(result).toMatchObject({
+      success: true,
+      created: 0,
+      updated: 0,
+      skipped: 1,
+    });
+  });
+
+  it("deletes cancelled lessons that remain inside the sync window", async () => {
+    jest.mocked(readGoogleCalendarState).mockResolvedValue({
+      refreshToken: "refresh",
+      calendarId: "app-calendar-id",
+      eventMap: {
+        [`${ORIGIN}::${ORIGIN}:cal:12345`]: { id: "keep", date: syncDate },
+        [`${ORIGIN}::${ORIGIN}:cal:cancelled`]: { id: "gone", date: syncDate },
+      },
+    });
+
+    const result = await syncLessonsToGoogleCalendar(
+      { origin: ORIGIN, lessons: [baseLesson], mode: "full" },
+      getAccessToken,
+    );
+
+    expect(deleteGoogleCalendarEvent).toHaveBeenCalledWith(
+      "test-token",
+      "app-calendar-id",
+      "gone",
+      expect.any(Function),
+    );
+    expect(result.deleted).toBeGreaterThanOrEqual(1);
+  });
+
   it("does not delete events during incremental sync", async () => {
     const result = await syncLessonsToGoogleCalendar(
       { origin: ORIGIN, lessons: [baseLesson], mode: "incremental" },
diff --git a/src/seqta/utils/outlookCalendar/upsertEvent.test.ts b/src/seqta/utils/outlookCalendar/upsertEvent.test.ts
index b3f284e1..a4e578a8 100644
--- a/src/seqta/utils/outlookCalendar/upsertEvent.test.ts
+++ b/src/seqta/utils/outlookCalendar/upsertEvent.test.ts
@@ -5,8 +5,12 @@ global.fetch = mockFetch as typeof fetch;
 
 import {
   deleteOutlookCalendarEvent,
+  listGoogleSyncedEvents,
+  listOutlookSyncedEvents,
   upsertOutlookCalendarEvent,
 } from "@/seqta/utils/calendarSync/remoteEvents";
+import { BSPLUS_GOOGLE_CALENDAR_EVENT_PROP } from "@/config/googleCalendar";
+import { BSPLUS_OUTLOOK_CALENDAR_EVENT_CATEGORY } from "@/config/outlookCalendar";
 
 describe("outlook calendar remote events", () => {
   beforeEach(() => {
@@ -28,4 +32,101 @@ describe("outlook calendar remote events", () => {
 
     await expect(deleteOutlookCalendarEvent("token", "evt-1")).resolves.toBeUndefined();
   });
+
+  it("lists Outlook synced events and paginates", async () => {
+    mockFetch
+      .mockResolvedValueOnce(
+        new Response(
+          JSON.stringify({
+            value: [
+              {
+                id: "o1",
+                subject: "Math",
+                body: { content: "Synced by BetterSEQTA+\nKey: https://school.seqta.com.au:cal:1" },
+                start: { dateTime: "2026-07-13T09:00:00", timeZone: "UTC" },
+                end: { dateTime: "2026-07-13T10:00:00", timeZone: "UTC" },
+                categories: [BSPLUS_OUTLOOK_CALENDAR_EVENT_CATEGORY],
+              },
+            ],
+            "@odata.nextLink": "https://graph.microsoft.com/v1.0/me/calendarView?$skiptoken=2",
+          }),
+          { status: 200 },
+        ),
+      )
+      .mockResolvedValueOnce(
+        new Response(
+          JSON.stringify({
+            value: [
+              {
+                id: "o2",
+                subject: "English",
+                body: { content: "Synced by BetterSEQTA+\nKey: https://school.seqta.com.au:cal:2" },
+                start: { dateTime: "2026-07-14T09:00:00", timeZone: "UTC" },
+                end: { dateTime: "2026-07-14T10:00:00", timeZone: "UTC" },
+                categories: [BSPLUS_OUTLOOK_CALENDAR_EVENT_CATEGORY],
+              },
+            ],
+          }),
+          { status: 200 },
+        ),
+      );
+
+    const events = await listOutlookSyncedEvents("token", {
+      from: "2026-07-13",
+      until: "2026-07-20",
+    });
+
+    expect(events).toHaveLength(2);
+    expect(events[0]).toMatchObject({
+      id: "o1",
+      seqtaKey: "https://school.seqta.com.au:cal:1",
+    });
+    expect(mockFetch).toHaveBeenCalledTimes(2);
+  });
+});
+
+describe("google calendar remote list", () => {
+  beforeEach(() => {
+    mockFetch.mockReset();
+  });
+
+  it("lists Google synced events using private extended properties", async () => {
+    mockFetch.mockResolvedValueOnce(
+      new Response(
+        JSON.stringify({
+          items: [
+            {
+              id: "g1",
+              summary: "Math",
+              description: "Synced by BetterSEQTA+",
+              start: { dateTime: "2026-07-13T09:00:00", timeZone: "UTC" },
+              end: { dateTime: "2026-07-13T10:00:00", timeZone: "UTC" },
+              extendedProperties: {
+                private: { [BSPLUS_GOOGLE_CALENDAR_EVENT_PROP]: "https://school.seqta.com.au:cal:1" },
+              },
+            },
+            {
+              id: "g-skip",
+              summary: "Unrelated",
+              start: { dateTime: "2026-07-13T11:00:00", timeZone: "UTC" },
+              end: { dateTime: "2026-07-13T12:00:00", timeZone: "UTC" },
+            },
+          ],
+        }),
+        { status: 200 },
+      ),
+    );
+
+    const events = await listGoogleSyncedEvents("token", "cal-id", {
+      from: "2026-07-13",
+      until: "2026-07-20",
+    });
+
+    expect(events).toHaveLength(1);
+    expect(events[0]).toMatchObject({
+      id: "g1",
+      seqtaKey: "https://school.seqta.com.au:cal:1",
+      date: "2026-07-13",
+    });
+  });
 });