diff --git a/apps/dav/composer/composer/autoload_classmap.php b/apps/dav/composer/composer/autoload_classmap.php
index 2ca5cf66f901f..6178526d7f4ff 100644
--- a/apps/dav/composer/composer/autoload_classmap.php
+++ b/apps/dav/composer/composer/autoload_classmap.php
@@ -396,6 +396,8 @@
     'OCA\\DAV\\Migration\\Version1036Date20251202000000' => $baseDir . '/../lib/Migration/Version1036Date20251202000000.php',
     'OCA\\DAV\\Migration\\Version1038Date20260302000000' => $baseDir . '/../lib/Migration/Version1038Date20260302000000.php',
     'OCA\\DAV\\Migration\\Version1039Date20260408000000' => $baseDir . '/../lib/Migration/Version1039Date20260408000000.php',
+    'OCA\\DAV\\Migration\\Version1040Date20260703120000' => $baseDir . '/../lib/Migration/Version1040Date20260703120000.php',
+    'OCA\\DAV\\Migration\\Version1041Date20260703120100' => $baseDir . '/../lib/Migration/Version1041Date20260703120100.php',
     'OCA\\DAV\\Model\\ExampleEvent' => $baseDir . '/../lib/Model/ExampleEvent.php',
     'OCA\\DAV\\Paginate\\LimitedCopyIterator' => $baseDir . '/../lib/Paginate/LimitedCopyIterator.php',
     'OCA\\DAV\\Paginate\\PaginateCache' => $baseDir . '/../lib/Paginate/PaginateCache.php',
diff --git a/apps/dav/composer/composer/autoload_static.php b/apps/dav/composer/composer/autoload_static.php
index c35dd97c02c0e..69ee65b084647 100644
--- a/apps/dav/composer/composer/autoload_static.php
+++ b/apps/dav/composer/composer/autoload_static.php
@@ -411,6 +411,8 @@ class ComposerStaticInitDAV
         'OCA\\DAV\\Migration\\Version1036Date20251202000000' => __DIR__ . '/..' . '/../lib/Migration/Version1036Date20251202000000.php',
         'OCA\\DAV\\Migration\\Version1038Date20260302000000' => __DIR__ . '/..' . '/../lib/Migration/Version1038Date20260302000000.php',
         'OCA\\DAV\\Migration\\Version1039Date20260408000000' => __DIR__ . '/..' . '/../lib/Migration/Version1039Date20260408000000.php',
+        'OCA\\DAV\\Migration\\Version1040Date20260703120000' => __DIR__ . '/..' . '/../lib/Migration/Version1040Date20260703120000.php',
+        'OCA\\DAV\\Migration\\Version1041Date20260703120100' => __DIR__ . '/..' . '/../lib/Migration/Version1041Date20260703120100.php',
         'OCA\\DAV\\Model\\ExampleEvent' => __DIR__ . '/..' . '/../lib/Model/ExampleEvent.php',
         'OCA\\DAV\\Paginate\\LimitedCopyIterator' => __DIR__ . '/..' . '/../lib/Paginate/LimitedCopyIterator.php',
         'OCA\\DAV\\Paginate\\PaginateCache' => __DIR__ . '/..' . '/../lib/Paginate/PaginateCache.php',
diff --git a/apps/dav/lib/CalDAV/CalDavBackend.php b/apps/dav/lib/CalDAV/CalDavBackend.php
index 9254ba2ad102a..d499060ae5b02 100644
--- a/apps/dav/lib/CalDAV/CalDavBackend.php
+++ b/apps/dav/lib/CalDAV/CalDavBackend.php
@@ -152,8 +152,8 @@ class CalDavBackend extends AbstractBackend implements SyncSupport, Subscription
 		'{http://apple.com/ns/ical/}calendar-order' => ['calendarorder', 'int'],
 		'{http://apple.com/ns/ical/}calendar-color' => ['calendarcolor', 'string'],
 		'{' . \OCA\DAV\DAV\Sharing\Plugin::NS_NEXTCLOUD . '}deleted-at' => ['deleted_at', 'int'],
-		'{' . \OCA\DAV\DAV\Sharing\Plugin::NS_NEXTCLOUD . '}default-alarm-part-day' => ['default_alarm_pday', 'int'],
-		'{' . \OCA\DAV\DAV\Sharing\Plugin::NS_NEXTCLOUD . '}default-alarm-full-day' => ['default_alarm_fday', 'int'],
+		'{' . \OCA\DAV\DAV\Sharing\Plugin::NS_NEXTCLOUD . '}default-alarms-part-day' => ['default_alarms_pday', 'string'],
+		'{' . \OCA\DAV\DAV\Sharing\Plugin::NS_NEXTCLOUD . '}default-alarms-full-day' => ['default_alarms_fday', 'string'],
 	];
 
 	/**
diff --git a/apps/dav/lib/Migration/Version1040Date20260703120000.php b/apps/dav/lib/Migration/Version1040Date20260703120000.php
new file mode 100644
index 0000000000000..24a9bd68014a6
--- /dev/null
+++ b/apps/dav/lib/Migration/Version1040Date20260703120000.php
@@ -0,0 +1,90 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\DAV\Migration;
+
+use Closure;
+use OCP\DB\ISchemaWrapper;
+use OCP\DB\Types;
+use OCP\IDBConnection;
+use OCP\Migration\Attributes\AddColumn;
+use OCP\Migration\Attributes\ColumnType;
+use OCP\Migration\Attributes\DataCleansing;
+use OCP\Migration\IOutput;
+use OCP\Migration\SimpleMigrationStep;
+use Override;
+
+/**
+ * Add JSON default-alarm columns and copy legacy single-int defaults via set-based UPDATEs.
+ */
+#[DataCleansing(table: 'calendars', description: 'Migrate legacy default_alarm_* integers to default_alarms_* JSON')]
+#[AddColumn(table: 'calendars', name: 'default_alarms_pday', type: ColumnType::TEXT)]
+#[AddColumn(table: 'calendars', name: 'default_alarms_fday', type: ColumnType::TEXT)]
+class Version1040Date20260703120000 extends SimpleMigrationStep {
+	public function __construct(
+		private IDBConnection $db,
+	) {
+	}
+
+	#[Override]
+	public function changeSchema(IOutput $output, Closure $schemaClosure, array $options): ?ISchemaWrapper {
+		/** @var ISchemaWrapper $schema */
+		$schema = $schemaClosure();
+
+		$calendarsTable = $schema->getTable('calendars');
+
+		if (!$calendarsTable->hasColumn('default_alarms_pday')) {
+			$calendarsTable->addColumn('default_alarms_pday', Types::TEXT, [
+				'notnull' => false,
+				'default' => null,
+			]);
+		}
+
+		if (!$calendarsTable->hasColumn('default_alarms_fday')) {
+			$calendarsTable->addColumn('default_alarms_fday', Types::TEXT, [
+				'notnull' => false,
+				'default' => null,
+			]);
+		}
+
+		return $schema;
+	}
+
+	#[Override]
+	public function postSchemaChange(IOutput $output, Closure $schemaClosure, array $options): void {
+		/** @var ISchemaWrapper $schema */
+		$schema = $schemaClosure();
+		$calendarsTable = $schema->getTable('calendars');
+
+		if ($calendarsTable->hasColumn('default_alarm_pday') && $calendarsTable->hasColumn('default_alarms_pday')) {
+			$this->migrateLegacyIntColumn('default_alarm_pday', 'default_alarms_pday');
+		}
+
+		if ($calendarsTable->hasColumn('default_alarm_fday') && $calendarsTable->hasColumn('default_alarms_fday')) {
+			$this->migrateLegacyIntColumn('default_alarm_fday', 'default_alarms_fday');
+		}
+	}
+
+	/**
+	 * Encode a single legacy trigger int as [{"trigger":N,"action":"DISPLAY"}]
+	 * for all rows still missing the JSON column value.
+	 */
+	private function migrateLegacyIntColumn(string $legacyColumn, string $jsonColumn): void {
+		$qb = $this->db->getQueryBuilder();
+		$qb->update('calendars')
+			->set($jsonColumn, $qb->func()->concat(
+				$qb->expr()->literal('[{"trigger":'),
+				$legacyColumn,
+				$qb->expr()->literal(',"action":"DISPLAY"}]'),
+			))
+			->where($qb->expr()->isNotNull($legacyColumn))
+			->andWhere($qb->expr()->isNull($jsonColumn));
+		$qb->executeStatement();
+	}
+}
diff --git a/apps/dav/lib/Migration/Version1041Date20260703120100.php b/apps/dav/lib/Migration/Version1041Date20260703120100.php
new file mode 100644
index 0000000000000..8816684b5dc1a
--- /dev/null
+++ b/apps/dav/lib/Migration/Version1041Date20260703120100.php
@@ -0,0 +1,42 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\DAV\Migration;
+
+use Closure;
+use OCP\DB\ISchemaWrapper;
+use OCP\Migration\Attributes\DropColumn;
+use OCP\Migration\IOutput;
+use OCP\Migration\SimpleMigrationStep;
+use Override;
+
+/**
+ * Drop legacy single-int default alarm columns after JSON migration.
+ */
+#[DropColumn(table: 'calendars', name: 'default_alarm_pday', description: 'Replaced by default_alarms_pday JSON')]
+#[DropColumn(table: 'calendars', name: 'default_alarm_fday', description: 'Replaced by default_alarms_fday JSON')]
+class Version1041Date20260703120100 extends SimpleMigrationStep {
+	#[Override]
+	public function changeSchema(IOutput $output, Closure $schemaClosure, array $options): ?ISchemaWrapper {
+		/** @var ISchemaWrapper $schema */
+		$schema = $schemaClosure();
+
+		$calendarsTable = $schema->getTable('calendars');
+
+		if ($calendarsTable->hasColumn('default_alarm_pday')) {
+			$calendarsTable->dropColumn('default_alarm_pday');
+		}
+
+		if ($calendarsTable->hasColumn('default_alarm_fday')) {
+			$calendarsTable->dropColumn('default_alarm_fday');
+		}
+
+		return $schema;
+	}
+}
diff --git a/apps/dav/tests/unit/CalDAV/CalDavBackendTest.php b/apps/dav/tests/unit/CalDAV/CalDavBackendTest.php
index e0f507fbe358d..9faed5a80ca41 100644
--- a/apps/dav/tests/unit/CalDAV/CalDavBackendTest.php
+++ b/apps/dav/tests/unit/CalDAV/CalDavBackendTest.php
@@ -1944,46 +1944,64 @@ public function testUnshare(): void {
 
 	}
 
-	public function testDefaultAlarmProperties(): void {
+	public function testDefaultAlarmsProperties(): void {
 		$calendarId = $this->createTestCalendar();
 
-		// Test setting both default alarm properties
+		$partDayProperty = '{http://nextcloud.com/ns}default-alarms-part-day';
+		$fullDayProperty = '{http://nextcloud.com/ns}default-alarms-full-day';
+
+		$partDayJson = json_encode([
+			['trigger' => -86400, 'action' => 'EMAIL'],
+			['trigger' => -900, 'action' => 'DISPLAY'],
+		], JSON_THROW_ON_ERROR);
+		$fullDayJson = json_encode([
+			['trigger' => -3600, 'action' => 'EMAIL'],
+		], JSON_THROW_ON_ERROR);
+
+		// Set plural default-alarms properties
 		$patch = new PropPatch([
-			'{http://nextcloud.com/ns}default-alarm-part-day' => -900,
-			'{http://nextcloud.com/ns}default-alarm-full-day' => -3600,
+			$partDayProperty => $partDayJson,
+			$fullDayProperty => $fullDayJson,
 		]);
 		$this->backend->updateCalendar($calendarId, $patch);
 		$patch->commit();
 
-		// Verify the properties were set
 		$calendars = $this->backend->getCalendarsForUser(self::UNIT_TEST_USER);
 		$this->assertCount(1, $calendars);
-		$this->assertEquals(-900, $calendars[0]['{http://nextcloud.com/ns}default-alarm-part-day']);
-		$this->assertEquals(-3600, $calendars[0]['{http://nextcloud.com/ns}default-alarm-full-day']);
-
-		// Test updating to different values
+		$this->assertEquals($partDayJson, $calendars[0][$partDayProperty]);
+		$this->assertEquals($fullDayJson, $calendars[0][$fullDayProperty]);
+		$this->assertArrayNotHasKey('{http://nextcloud.com/ns}default-alarm-part-day', $calendars[0]);
+		$this->assertArrayNotHasKey('{http://nextcloud.com/ns}default-alarm-full-day', $calendars[0]);
+
+		// Update to different values
+		$updatedPartDayJson = json_encode([
+			['trigger' => -1800, 'action' => 'DISPLAY'],
+		], JSON_THROW_ON_ERROR);
+		$updatedFullDayJson = json_encode([
+			['trigger' => -43200, 'action' => 'DISPLAY'],
+		], JSON_THROW_ON_ERROR);
 		$patch = new PropPatch([
-			'{http://nextcloud.com/ns}default-alarm-part-day' => -86400,
-			'{http://nextcloud.com/ns}default-alarm-full-day' => -43200,
+			$partDayProperty => $updatedPartDayJson,
+			$fullDayProperty => $updatedFullDayJson,
 		]);
 		$this->backend->updateCalendar($calendarId, $patch);
 		$patch->commit();
 
 		$calendars = $this->backend->getCalendarsForUser(self::UNIT_TEST_USER);
-		$this->assertEquals(-86400, $calendars[0]['{http://nextcloud.com/ns}default-alarm-part-day']);
-		$this->assertEquals(-43200, $calendars[0]['{http://nextcloud.com/ns}default-alarm-full-day']);
+		$this->assertEquals($updatedPartDayJson, $calendars[0][$partDayProperty]);
+		$this->assertEquals($updatedFullDayJson, $calendars[0][$fullDayProperty]);
 
-		// Test setting to null
+		// Clear properties
 		$patch = new PropPatch([
-			'{http://nextcloud.com/ns}default-alarm-part-day' => null,
-			'{http://nextcloud.com/ns}default-alarm-full-day' => null,
+			$partDayProperty => null,
+			$fullDayProperty => null,
 		]);
 		$this->backend->updateCalendar($calendarId, $patch);
 		$patch->commit();
 
 		$calendars = $this->backend->getCalendarsForUser(self::UNIT_TEST_USER);
-		$this->assertNull($calendars[0]['{http://nextcloud.com/ns}default-alarm-part-day']);
-		$this->assertNull($calendars[0]['{http://nextcloud.com/ns}default-alarm-full-day']);
+		$this->assertNull($calendars[0][$partDayProperty]);
+		$this->assertNull($calendars[0][$fullDayProperty]);
 
 		// Clean up
 		$this->backend->deleteCalendar($calendars[0]['id'], true);