diff --git a/apps/files_sharing/tests/ShareRecipientUpdaterTest.php b/apps/files_sharing/tests/ShareRecipientUpdaterTest.php
new file mode 100644
index 00000000000..e32f7d88fb4
--- /dev/null
+++ b/apps/files_sharing/tests/ShareRecipientUpdaterTest.php
@@ -0,0 +1,188 @@
+<?php
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+namespace OCA\Files_Sharing\Tests;
+
+use OCA\Files_Sharing\MountProvider;
+use OCA\Files_Sharing\ShareRecipientUpdater;
+use OCA\Files_Sharing\ShareTargetValidator;
+use OCP\Files\Cache\ICacheEntry;
+use OCP\Files\Config\ICachedMountInfo;
+use OCP\Files\Config\IUserMountCache;
+use OCP\Files\Mount\IMountPoint;
+use OCP\Files\Node;
+use OCP\Files\Storage\IStorageFactory;
+use OCP\IUser;
+use OCP\Share\IShare;
+use PHPUnit\Framework\MockObject\MockObject;
+use Test\Traits\UserTrait;
+
+class ShareRecipientUpdaterTest extends \Test\TestCase {
+	use UserTrait;
+
+	private IUserMountCache&MockObject $userMountCache;
+	private MountProvider&MockObject $shareMountProvider;
+	private ShareTargetValidator&MockObject $shareTargetValidator;
+	private IStorageFactory&MockObject $storageFactory;
+	private ShareRecipientUpdater $updater;
+
+	protected function setUp(): void {
+		parent::setUp();
+
+		$this->userMountCache = $this->createMock(IUserMountCache::class);
+		$this->shareMountProvider = $this->createMock(MountProvider::class);
+		$this->shareTargetValidator = $this->createMock(ShareTargetValidator::class);
+		$this->storageFactory = $this->createMock(IStorageFactory::class);
+
+		$this->updater = new ShareRecipientUpdater(
+			$this->userMountCache,
+			$this->shareMountProvider,
+			$this->shareTargetValidator,
+			$this->storageFactory,
+		);
+	}
+
+	public function testUpdateForShare() {
+		$share = $this->createMock(IShare::class);
+		$node = $this->createMock(Node::class);
+		$cacheEntry = $this->createMock(ICacheEntry::class);
+		$share->method('getNode')
+			->willReturn($node);
+		$node->method('getData')
+			->willReturn($cacheEntry);
+		$user1 = $this->createUser('user1', '');
+
+		$this->userMountCache->method('getMountsForUser')
+			->with($user1)
+			->willReturn([]);
+
+		$this->shareTargetValidator->method('verifyMountPoint')
+			->with($user1, $share, [], [$share])
+			->willReturn('/new-target');
+
+		$this->userMountCache->expects($this->exactly(1))
+			->method('addMount')
+			->with($user1, '/user1/files/new-target/', $cacheEntry, MountProvider::class);
+
+		$this->updater->updateForShare($user1, $share);
+	}
+
+	/**
+	 * @param IUser $user
+	 * @param list<array{fileid: int, mount_point: string, provider: string}> $mounts
+	 * @return void
+	 */
+	private function setCachedMounts(IUser $user, array $mounts) {
+		$cachedMounts = array_map(function (array $mount): ICachedMountInfo {
+			$cachedMount = $this->createMock(ICachedMountInfo::class);
+			$cachedMount->method('getRootId')
+				->willReturn($mount['fileid']);
+			$cachedMount->method('getMountPoint')
+				->willReturn($mount['mount_point']);
+			$cachedMount->method('getMountProvider')
+				->willReturn($mount['provider']);
+			return $cachedMount;
+		}, $mounts);
+		$mountKeys = array_map(function (array $mount): string {
+			return $mount['fileid'] . '::' . $mount['mount_point'];
+		}, $mounts);
+
+		$this->userMountCache->method('getMountsForUser')
+			->with($user)
+			->willReturn(array_combine($mountKeys, $cachedMounts));
+	}
+
+	public function testUpdateForUserAddedNoExisting() {
+		$share = $this->createMock(IShare::class);
+		$share->method('getTarget')
+			->willReturn('/target');
+		$share->method('getNodeId')
+			->willReturn(111);
+		$user1 = $this->createUser('user1', '');
+		$newMount = $this->createMock(IMountPoint::class);
+
+		$this->shareMountProvider->method('getSuperSharesForUser')
+			->with($user1, [])
+			->willReturn([[
+				$share,
+				[$share],
+			]]);
+
+		$this->shareMountProvider->method('getMountsFromSuperShares')
+			->with($user1, [[
+				$share,
+				[$share],
+			]], $this->storageFactory)
+			->willReturn([$newMount]);
+
+		$this->setCachedMounts($user1, []);
+
+		$this->shareTargetValidator->method('verifyMountPoint')
+			->with($user1, $share, [], [$share])
+			->willReturn('/new-target');
+
+		$this->userMountCache->expects($this->exactly(1))
+			->method('registerMounts')
+			->with($user1, [$newMount], [MountProvider::class]);
+
+		$this->updater->updateForUser($user1);
+	}
+
+	public function testUpdateForUserNoChanges() {
+		$share = $this->createMock(IShare::class);
+		$share->method('getTarget')
+			->willReturn('/target');
+		$share->method('getNodeId')
+			->willReturn(111);
+		$user1 = $this->createUser('user1', '');
+
+		$this->shareMountProvider->method('getSuperSharesForUser')
+			->with($user1, [])
+			->willReturn([[
+				$share,
+				[$share],
+			]]);
+
+		$this->setCachedMounts($user1, [
+			['fileid' => 111, 'mount_point' => '/user1/files/target/', 'provider' => MountProvider::class],
+		]);
+
+		$this->shareTargetValidator->expects($this->never())
+			->method('verifyMountPoint');
+
+		$this->userMountCache->expects($this->never())
+			->method('registerMounts');
+
+		$this->updater->updateForUser($user1);
+	}
+
+	public function testUpdateForUserRemoved() {
+		$share = $this->createMock(IShare::class);
+		$share->method('getTarget')
+			->willReturn('/target');
+		$share->method('getNodeId')
+			->willReturn(111);
+		$user1 = $this->createUser('user1', '');
+
+		$this->shareMountProvider->method('getSuperSharesForUser')
+			->with($user1, [])
+			->willReturn([]);
+
+		$this->setCachedMounts($user1, [
+			['fileid' => 111, 'mount_point' => '/user1/files/target/', 'provider' => MountProvider::class],
+		]);
+
+		$this->shareTargetValidator->expects($this->never())
+			->method('verifyMountPoint');
+
+		$this->userMountCache->expects($this->exactly(1))
+			->method('registerMounts')
+			->with($user1, [], [MountProvider::class]);
+
+		$this->updater->updateForUser($user1);
+	}
+}