diff --git a/lib/Folder/FolderManager.php b/lib/Folder/FolderManager.php
index 9bc3868c6..4c86f3bc5 100644
--- a/lib/Folder/FolderManager.php
+++ b/lib/Folder/FolderManager.php
@@ -976,7 +976,50 @@ public function setFolderQuota(int $folderId, int $quota): void {
 			->where($query->expr()->eq('folder_id', $query->createNamedParameter($folderId)));
 		$query->executeStatement();
 
-		$this->eventDispatcher->dispatchTyped(new CriticalActionPerformedEvent('The quota for groupfolder with id %d was set to %d bytes', [$folderId, $quota]));
+		$this->eventDispatcher->dispatchTyped(
+			new CriticalActionPerformedEvent(
+				'The quota for groupfolder with id %d was set to %d bytes',
+				[$folderId, $quota]
+			)
+		);
+
+		// Invalidate the root etag of this group folder directly in the filecache
+		// using the root_id already stored in group_folders.  This forces desktop
+		// clients to issue a fresh PROPFIND on the next sync cycle and receive the
+		// updated quota-available-bytes value.
+		//
+		// Direct filecache update is used intentionally:
+		//   - propagateChange() requires a mounted storage (i.e. an active user
+		//     session), which is not guaranteed when an admin changes quota.
+		//   - root_id is always present in group_folders after folder creation,
+		//     so this path works regardless of whether any user is logged in.
+		try {
+			$rootIdQuery = $this->connection->getQueryBuilder();
+			$rootIdQuery->select('root_id')
+				->from('group_folders')
+				->where($rootIdQuery->expr()->eq(
+					'folder_id',
+					$rootIdQuery->createNamedParameter($folderId, IQueryBuilder::PARAM_INT)
+				));
+			$rootIdResult = $rootIdQuery->executeQuery();
+			$raw = $rootIdResult->fetchOne();
+			$rootIdResult->closeCursor();
+			$rootId = is_numeric($raw) ? (int)$raw : 0;
+
+			if ($rootId > 0) {
+				$etagQuery = $this->connection->getQueryBuilder();
+				$etagQuery->update('filecache')
+					->set('etag', $etagQuery->createNamedParameter(uniqid()))
+					->where($etagQuery->expr()->eq(
+						'fileid',
+						$etagQuery->createNamedParameter($rootId, IQueryBuilder::PARAM_INT)
+					));
+				$etagQuery->executeStatement();
+			}
+		} catch (\Throwable) {
+			// Non-fatal: best-effort invalidation.
+			// If this fails the stale etag will correct itself on the next full sync.
+		}
 	}
 
 	/**
diff --git a/tests/Folder/FolderManagerTest.php b/tests/Folder/FolderManagerTest.php
index eda4f24ee..350ebf024 100644
--- a/tests/Folder/FolderManagerTest.php
+++ b/tests/Folder/FolderManagerTest.php
@@ -16,6 +16,7 @@
 use OCA\GroupFolders\Mount\FolderStorageManager;
 use OCA\GroupFolders\ResponseDefinitions;
 use OCP\Constants;
+use OCP\DB\QueryBuilder\IQueryBuilder;
 use OCP\EventDispatcher\IEventDispatcher;
 use OCP\Files\FileInfo;
 use OCP\Files\IMimeTypeLoader;
@@ -73,11 +74,57 @@ protected function setUp(): void {
 	}
 
 	private function clean(): void {
-		$query = Server::get(IDBConnection::class)->getQueryBuilder();
-		$query->delete('group_folders')->executeStatement();
+		$db = Server::get(IDBConnection::class);
+
+		$db->getQueryBuilder()->delete('group_folders')->executeStatement();
+		$db->getQueryBuilder()->delete('group_folders_groups')->executeStatement();
+
+		// Remove any filecache rows seeded by seedFilecacheForFolder().
+		// storage = 0 is used as a sentinel as no real storage is
+		// ever assigned numeric ID 0, so this delete is always safe.
+		$q = $db->getQueryBuilder();
+		$q->delete('filecache')
+			->where($q->expr()->eq('storage', $q->createNamedParameter(0, IQueryBuilder::PARAM_INT)))
+			->executeStatement();
+	}
 
-		$query = Server::get(IDBConnection::class)->getQueryBuilder();
-		$query->delete('group_folders_groups')->executeStatement();
+	/**
+	 * Insert a minimal filecache row so that getFolder()'s INNER JOIN on
+	 * filecache succeeds, then patch group_folders.root_id to point at it.
+	 * storage = 0 as a sentinel value that clean() sweeps unconditionally.
+	 */
+	private function seedFilecacheForFolder(int $folderId): void {
+		$db = Server::get(IDBConnection::class);
+		$now = time();
+
+		$insert = $db->getQueryBuilder();
+		$insert->insert('filecache')
+			->values([
+				'storage' => $insert->createNamedParameter(0, IQueryBuilder::PARAM_INT),
+				'path' => $insert->createNamedParameter(''),
+				'path_hash' => $insert->createNamedParameter(md5('')),
+				'parent' => $insert->createNamedParameter(-1, IQueryBuilder::PARAM_INT),
+				'name' => $insert->createNamedParameter(''),
+				'mimetype' => $insert->createNamedParameter(0, IQueryBuilder::PARAM_INT),
+				'mimepart' => $insert->createNamedParameter(0, IQueryBuilder::PARAM_INT),
+				'size' => $insert->createNamedParameter(0, IQueryBuilder::PARAM_INT),
+				'mtime' => $insert->createNamedParameter($now, IQueryBuilder::PARAM_INT),
+				'storage_mtime' => $insert->createNamedParameter($now, IQueryBuilder::PARAM_INT),
+				'etag' => $insert->createNamedParameter(uniqid()),
+				'encrypted' => $insert->createNamedParameter(0, IQueryBuilder::PARAM_INT),
+				'permissions' => $insert->createNamedParameter(0, IQueryBuilder::PARAM_INT),
+			]);
+		$insert->executeStatement();
+		$filecacheId = $insert->getLastInsertId();
+
+		$update = $db->getQueryBuilder();
+		$update->update('group_folders')
+			->set('root_id', $update->createNamedParameter($filecacheId, IQueryBuilder::PARAM_INT))
+			->where($update->expr()->eq(
+				'folder_id',
+				$update->createNamedParameter($folderId, IQueryBuilder::PARAM_INT)
+			))
+			->executeStatement();
 	}
 
 	/**
@@ -494,6 +541,38 @@ public function testGetFolderPermissionsForUserMerge(): void {
 		$this->assertEquals(0, $permissions);
 	}
 
+	public function testSetFolderQuotaInvalidatesEtag(): void {
+		$this->config->expects($this->any())
+			->method('getSystemValueInt')
+			->with('groupfolders.quota.default', FileInfo::SPACE_UNLIMITED)
+			->willReturn(FileInfo::SPACE_UNLIMITED);
+
+		$folderId = $this->manager->createFolder('quota-etag-test');
+		$this->seedFilecacheForFolder($folderId);
+
+		$folderBefore = $this->manager->getFolder($folderId);
+		$this->assertNotNull($folderBefore);
+		$etagBefore = $folderBefore->rootCacheEntry->getEtag();
+
+		$this->manager->setFolderQuota($folderId, 1024 * 1024 * 1024);
+
+		$folderAfter = $this->manager->getFolder($folderId);
+		$this->assertNotNull($folderAfter);
+		$etagAfter = $folderAfter->rootCacheEntry->getEtag();
+
+		$this->assertNotEquals(
+			$etagBefore,
+			$etagAfter,
+			'Etag must change after quota update so desktop clients re-fetch quota-available-bytes via PROPFIND',
+		);
+
+		$this->assertSame(
+			1024 * 1024 * 1024,
+			$folderAfter->quota,
+			'Quota value must be persisted correctly',
+		);
+	}
+
 	public function testQuotaDefaultValue(): void {
 		$folderId1 = $this->manager->createFolder('foo');