@@ -63,7 +63,6 @@
     'OCA\\DAV\\CalDAV\\CalendarRoot' => $baseDir . '/../lib/CalDAV/CalendarRoot.php',
     'OCA\\DAV\\CalDAV\\DefaultCalendarValidator' => $baseDir . '/../lib/CalDAV/DefaultCalendarValidator.php',
     'OCA\\DAV\\CalDAV\\EmbeddedCalDavServer' => $baseDir . '/../lib/CalDAV/EmbeddedCalDavServer.php',
-    'OCA\\DAV\\CalDAV\\EventComparisonService' => $baseDir . '/../lib/CalDAV/EventComparisonService.php',
     'OCA\\DAV\\CalDAV\\EventReader' => $baseDir . '/../lib/CalDAV/EventReader.php',
     'OCA\\DAV\\CalDAV\\EventReaderRDate' => $baseDir . '/../lib/CalDAV/EventReaderRDate.php',
     'OCA\\DAV\\CalDAV\\EventReaderRRule' => $baseDir . '/../lib/CalDAV/EventReaderRRule.php',
 
@@ -78,7 +78,6 @@ class ComposerStaticInitDAV
         'OCA\\DAV\\CalDAV\\CalendarRoot' => __DIR__ . '/..' . '/../lib/CalDAV/CalendarRoot.php',
         'OCA\\DAV\\CalDAV\\DefaultCalendarValidator' => __DIR__ . '/..' . '/../lib/CalDAV/DefaultCalendarValidator.php',
         'OCA\\DAV\\CalDAV\\EmbeddedCalDavServer' => __DIR__ . '/..' . '/../lib/CalDAV/EmbeddedCalDavServer.php',
-        'OCA\\DAV\\CalDAV\\EventComparisonService' => __DIR__ . '/..' . '/../lib/CalDAV/EventComparisonService.php',
         'OCA\\DAV\\CalDAV\\EventReader' => __DIR__ . '/..' . '/../lib/CalDAV/EventReader.php',
         'OCA\\DAV\\CalDAV\\EventReaderRDate' => __DIR__ . '/..' . '/../lib/CalDAV/EventReaderRDate.php',
         'OCA\\DAV\\CalDAV\\EventReaderRRule' => __DIR__ . '/..' . '/../lib/CalDAV/EventReaderRRule.php',
 
@@ -10,7 +10,6 @@
 namespace OCA\DAV\CalDAV\Schedule;
 
 use OCA\DAV\CalDAV\CalendarObject;
-use OCA\DAV\CalDAV\EventComparisonService;
 use OCP\Accounts\IAccountManager;
 use OCP\AppFramework\Utility\ITimeFactory;
 use OCP\Defaults;
@@ -65,7 +64,6 @@ public function __construct(
 		private Defaults $defaults,
 		private IUserSession $userSession,
 		private IMipService $imipService,
-		private EventComparisonService $eventComparisonService,
 		private IMailManager $mailManager,
 		private IEmailValidator $emailValidator,
 		private IAccountManager $accountManager,
@@ -146,29 +144,82 @@ public function schedule(Message $iTipMessage) {
 
 		$recipientName = $iTipMessage->recipientName ? (string)$iTipMessage->recipientName : null;
 
-		$newEvents = $iTipMessage->message;
-		$oldEvents = $this->getVCalendar();
+		$newObjects = $iTipMessage->message;
+		$oldObjects = $this->getVCalendar();
+
+		$method = match (strtolower($iTipMessage->method)) {
+			'reply' => self::METHOD_REPLY,
+			'cancel' => self::METHOD_CANCEL,
+			default => self::METHOD_REQUEST,
+		};
+
+		// For REQUEST method, we need to determine which instances have changed by comparing new and old events
+		if ($method === self::METHOD_REQUEST) {
+			$newEvents = $this->imipService->eventInstances($newObjects);
+			$oldEventsByInstance = [];
+			if ($oldObjects !== null) {
+				foreach ($this->imipService->eventInstances($oldObjects) as $oldEvent) {
+					$oldEventsByInstance[$this->imipService->instanceKey($oldEvent)] = $oldEvent;
+				}
+			}
 
-		$modified = $this->eventComparisonService->findModified($newEvents, $oldEvents);
-		/** @var VEvent $vEvent */
-		$vEvent = array_pop($modified['new']);
-		/** @var VEvent $oldVevent */
-		$oldVevent = !empty($modified['old']) && is_array($modified['old']) ? array_pop($modified['old']) : null;
-		$isModified = isset($oldVevent);
+			$allInstances = array_map(
+				fn (VEvent $newEvent) => ['new' => $newEvent, 'old' => $oldEventsByInstance[$this->imipService->instanceKey($newEvent)] ?? null],
+				$newEvents,
+			);
 
-		// No changed events after all - this shouldn't happen if there is significant change yet here we are
+			$changedInstances = array_values(array_filter(
+				$allInstances,
+				fn (array $pair) => $pair['old'] === null || $this->imipService->diffInstance($pair['new'], $pair['old']) !== [],
+			));
+
+			$modifiedInstances = $changedInstances !== [] ? $changedInstances : $allInstances;
+		}
+		// For CANCEL/REPLY only ever consume the new instance, so there's no old instance to diff against or compare
+		else {
+			$modifiedInstances = array_map(
+				static fn (VEvent $event) => ['new' => $event, 'old' => null],
+				$this->imipService->eventInstances($newObjects),
+			);
+		}
+
+		// No VEvents in the message at all - this shouldn't happen if there is significant change yet here we are
 		// The scheduling status is debatable
-		if (empty($vEvent)) {
-			$this->logger->warning('iTip message said the change was significant but comparison did not detect any updated VEvents');
+		if (empty($modifiedInstances)) {
+			$this->logger->warning('iTip message said the change was significant but the message contained no VEvents');
 			$iTipMessage->scheduleStatus = '1.0;We got the message, but it\'s not significant enough to warrant an email';
 			return;
 		}
 
-		// we (should) have one event component left
-		// as the ITip\Broker creates one iTip message per change
-		// and triggers the "schedule" event once per message
-		// we also might not have an old event as this could be a new
-		// invitation, or a new recurrence exception
+		if (count($modifiedInstances) > 1) {
+			$this->logger->debug('iTip message contains multiple instances; only the primary instance is reflected in the invitation email', [
+				'uid' => $iTipMessage->uid,
+				'instanceCount' => count($modifiedInstances),
+			]);
+		}
+
+		// we (should) have one instance per message, as the ITip\Broker creates
+		// one iTip message per attendee and triggers the "schedule" event once
+		// per message; a message can still bundle several instances (e.g.
+		// primary + overrides edited together), in which case only the primary
+		// instance is reflected in the email for now
+		$primaryInstance = null;
+		foreach ($modifiedInstances as $instance) {
+			if (!isset($instance['new']->{'RECURRENCE-ID'})) {
+				$primaryInstance = $instance;
+				break;
+			}
+		}
+		$primaryInstance ??= $modifiedInstances[0];
+
+		/** @var VEvent $vEvent */
+		$vEvent = $primaryInstance['new'];
+		/** @var VEvent|null $oldVevent */
+		$oldVevent = $primaryInstance['old'];
+		$isModified = $oldVevent !== null;
+
+		// we might not have an old event as this could be a new invitation,
+		// or a new recurrence exception
 		$attendee = $this->imipService->getCurrentAttendee($iTipMessage);
 		if ($attendee === null) {
 			$uid = $vEvent->UID ?? 'no UID found';
@@ -199,18 +250,15 @@ public function schedule(Message $iTipMessage) {
 		}
 
 		$replyingAttendee = null;
-		switch (strtolower($iTipMessage->method)) {
+		switch ($method) {
 			case self::METHOD_REPLY:
-				$method = self::METHOD_REPLY;
 				$data = $this->imipService->buildReplyBodyData($vEvent);
 				$replyingAttendee = $this->imipService->getReplyingAttendee($iTipMessage);
 				break;
 			case self::METHOD_CANCEL:
-				$method = self::METHOD_CANCEL;
 				$data = $this->imipService->buildCancelledBodyData($vEvent);
 				break;
 			default:
-				$method = self::METHOD_REQUEST;
 				$data = $this->imipService->buildBodyData($vEvent, $oldVevent);
 				break;
 		}
 
@@ -40,6 +40,32 @@ class IMipService {
 		'meeting_location' => 'LOCATION'
 	];
 
+	/**
+	 * Properties compared to build an instance's changed-property delta:
+	 * everything the invitation email can show a diff for
+	 * (SUMMARY/DESCRIPTION/LOCATION/URL via STRING_DIFF, DTSTART/DTEND/DURATION
+	 * for the "when" string), plus SEQUENCE/LAST-MODIFIED and the rest of
+	 * iTip's own significantChangeProperties (RRULE/RDATE/EXDATE/STATUS;
+	 * DUE is VTODO-only and not relevant to VEVENT invites).
+	 *
+	 * @var string[]
+	 */
+	private const array INSTANCE_DIFF_PROPERTIES = [
+		'SUMMARY',
+		'DESCRIPTION',
+		'LOCATION',
+		'URL',
+		'DTSTART',
+		'DTEND',
+		'DURATION',
+		'STATUS',
+		'RRULE',
+		'RDATE',
+		'EXDATE',
+		'SEQUENCE',
+		'LAST-MODIFIED',
+	];
+
 	public function __construct(
 		private URLGenerator $urlGenerator,
 		private IDBConnection $db,
@@ -149,6 +175,38 @@ private function linkify(?string $url): ?string {
 		return sprintf('<a href="%1$s">%1$s</a>', htmlspecialchars($url));
 	}
 
+	public function instanceKey(VEvent $event): string {
+		return self::readPropertyWithDefault($event, 'UID', '') . '#' . self::readPropertyWithDefault($event, 'RECURRENCE-ID', '');
+	}
+
+	/**
+	 * Every VEVENT component directly in $calendar, in document order.
+	 *
+	 * @return list<VEvent>
+	 */
+	public function eventInstances(VCalendar $calendar): array {
+		return array_values(array_filter(
+			$calendar->getComponents(),
+			static fn ($component) => $component instanceof VEvent,
+		));
+	}
+
+	/**
+	 * Names of the RFC5545 properties that differ between $newEvent and
+	 * $oldEvent.
+	 *
+	 * @return string[]
+	 */
+	public function diffInstance(VEvent $newEvent, VEvent $oldEvent): array {
+		$changed = [];
+		foreach (self::INSTANCE_DIFF_PROPERTIES as $property) {
+			if (self::readPropertyWithDefault($newEvent, $property, '') !== self::readPropertyWithDefault($oldEvent, $property, '')) {
+				$changed[] = $property;
+			}
+		}
+		return $changed;
+	}
+
 	/**
 	 * @param VEvent $vEvent
 	 * @param VEvent|null $oldVEvent
 
@@ -14,7 +14,6 @@
 use OCA\DAV\CalDAV\BirthdayCalendar\EnablePlugin;
 use OCA\DAV\CalDAV\BirthdayService;
 use OCA\DAV\CalDAV\DefaultCalendarValidator;
-use OCA\DAV\CalDAV\EventComparisonService;
 use OCA\DAV\CalDAV\ICSExportPlugin\ICSExportPlugin;
 use OCA\DAV\CalDAV\Publishing\PublishPlugin;
 use OCA\DAV\CalDAV\Schedule\IMipPlugin;
@@ -357,7 +356,6 @@ public function __construct(
 						\OCP\Server::get(Defaults::class),
 						$userSession,
 						\OCP\Server::get(IMipService::class),
-						\OCP\Server::get(EventComparisonService::class),
 						\OCP\Server::get(\OCP\Mail\Provider\IManager::class),
 						\OCP\Server::get(IEmailValidator::class),
 						\OCP\Server::get(IAccountManager::class),