diff --git a/appinfo/info.xml b/appinfo/info.xml
index b3884af95..dbc31e7f0 100644
--- a/appinfo/info.xml
+++ b/appinfo/info.xml
@@ -62,7 +62,7 @@ Known providers:
 
 More details on how to set this up in the [admin docs](https://docs.nextcloud.com/server/latest/admin_manual/ai/index.html)
 ]]>	</description>
-	<version>3.4.3</version>
+	<version>3.5.0.-dev.1</version>
 	<licence>agpl</licence>
 	<author>Julien Veyssier</author>
 	<namespace>Assistant</namespace>
diff --git a/lib/AppInfo/Application.php b/lib/AppInfo/Application.php
index b1b2771f3..07296100e 100644
--- a/lib/AppInfo/Application.php
+++ b/lib/AppInfo/Application.php
@@ -23,6 +23,7 @@
 use OCA\Assistant\Listener\TaskSuccessfulListener;
 use OCA\Assistant\Listener\Text2Image\Text2ImageReferenceListener;
 use OCA\Assistant\Listener\Text2Image\Text2StickerListener;
+use OCA\Assistant\Listener\UserDeletedListener;
 use OCA\Assistant\Notification\Notifier;
 use OCA\Assistant\Reference\FreePromptReferenceProvider;
 use OCA\Assistant\Reference\SpeechToTextReferenceProvider;
@@ -47,6 +48,7 @@
 use OCP\TaskProcessing\Events\TaskFailedEvent;
 use OCP\TaskProcessing\Events\TaskSuccessfulEvent;
 use OCP\TaskProcessing\IManager;
+use OCP\User\Events\UserDeletedEvent;
 
 class Application extends App implements IBootstrap {
 
@@ -100,6 +102,8 @@ public function register(IRegistrationContext $context): void {
 
 		$context->registerEventListener(AddContentSecurityPolicyEvent::class, CSPListener::class);
 
+		$context->registerEventListener(UserDeletedEvent::class, UserDeletedListener::class);
+
 		if (class_exists('OCP\\TaskProcessing\\TaskTypes\\AudioToAudioChat')) {
 			$context->registerTaskProcessingProvider(AudioToAudioChatProvider::class);
 		}
diff --git a/lib/Db/ChattyLLM/SessionMapper.php b/lib/Db/ChattyLLM/SessionMapper.php
index 9006bcf63..660543c09 100644
--- a/lib/Db/ChattyLLM/SessionMapper.php
+++ b/lib/Db/ChattyLLM/SessionMapper.php
@@ -180,6 +180,17 @@ public function deleteSession(string $userId, int $sessionId) {
 		$qb->executeStatement();
 	}
 
+	/**
+	 * @throws \OCP\DB\Exception
+	 */
+	public function deleteAllSessionsForUser(string $userId): void {
+		$qb = $this->db->getQueryBuilder();
+		$qb->delete($this->getTableName())
+			->where($qb->expr()->eq('user_id', $qb->createPositionalParameter($userId, IQueryBuilder::PARAM_STR)));
+
+		$qb->executeStatement();
+	}
+
 	public function updateSessionIsRemembered(?string $userId, int $sessionId, bool $is_remembered) {
 		$session = $this->getUserSession($userId, $sessionId);
 		$session->setIsRemembered($is_remembered);
diff --git a/lib/Listener/UserDeletedListener.php b/lib/Listener/UserDeletedListener.php
new file mode 100644
index 000000000..acbc4b799
--- /dev/null
+++ b/lib/Listener/UserDeletedListener.php
@@ -0,0 +1,51 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\Assistant\Listener;
+
+use OCA\Assistant\Service\ChatService;
+use OCA\Assistant\Service\InternalException;
+use OCA\Assistant\Service\SessionSummaryService;
+use OCP\EventDispatcher\Event;
+use OCP\EventDispatcher\IEventListener;
+use OCP\User\Events\UserDeletedEvent;
+use Psr\Log\LoggerInterface;
+
+/**
+ * @template-implements IEventListener<UserDeletedEvent>
+ */
+class UserDeletedListener implements IEventListener {
+
+	public function __construct(
+		private ChatService $chatService,
+		private LoggerInterface $logger,
+		private SessionSummaryService $sessionSummaryService,
+	) {
+	}
+
+	public function handle(Event $event): void {
+		if (!($event instanceof UserDeletedEvent)) {
+			return;
+		}
+
+		$userId = $event->getUid();
+
+		try {
+			$this->chatService->deleteAllUserChatData($userId);
+		} catch (InternalException $e) {
+			$this->logger->error('Error while deleting chat data for user ' . $userId, ['exception' => $e]);
+		}
+
+		try {
+			$this->sessionSummaryService->deleteSummaryJobsForUser($userId);
+		} catch (InternalException $e) {
+			$this->logger->error('Error while deleting summary jobs for user ' . $userId, ['exception' => $e]);
+		}
+	}
+}
diff --git a/lib/Migration/Version030500Date20260715083046.php b/lib/Migration/Version030500Date20260715083046.php
new file mode 100644
index 000000000..60da26478
--- /dev/null
+++ b/lib/Migration/Version030500Date20260715083046.php
@@ -0,0 +1,93 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\Assistant\Migration;
+
+use Closure;
+use OCA\Assistant\Service\ChatService;
+use OCA\Assistant\Service\InternalException;
+use OCA\Assistant\Service\SessionSummaryService;
+use OCP\DB\ISchemaWrapper;
+use OCP\IDBConnection;
+use OCP\IUserManager;
+use OCP\Migration\IOutput;
+use OCP\Migration\SimpleMigrationStep;
+use Override;
+use Psr\Log\LoggerInterface;
+
+class Version030500Date20260715083046 extends SimpleMigrationStep {
+
+	public function __construct(
+		private IDBConnection $db,
+		private IUserManager $userManager,
+		private ChatService $chatService,
+		private SessionSummaryService $sessionSummaryService,
+		private LoggerInterface $logger,
+	) {
+	}
+
+	/**
+	 * Clean up chat and assignment data left behind by users deleted before UserDeletedListener existed.
+	 *
+	 * @param IOutput $output
+	 * @param Closure(): ISchemaWrapper $schemaClosure
+	 * @param array $options
+	 */
+	#[Override]
+	public function postSchemaChange(IOutput $output, Closure $schemaClosure, array $options): void {
+		$userIds = $this->getDistinctUserIds();
+		$cleaned = 0;
+
+		foreach ($userIds as $userId) {
+			if ($this->userManager->userExists($userId)) {
+				continue;
+			}
+
+			try {
+				$this->chatService->deleteAllUserChatData($userId);
+			} catch (InternalException $e) {
+				$this->logger->error('Error while deleting chat data for deleted user ' . $userId, ['exception' => $e]);
+			}
+
+			try {
+				$this->sessionSummaryService->deleteSummaryJobsForUser($userId);
+			} catch (InternalException $e) {
+				$this->logger->error('Error while deleting summary jobs for deleted user ' . $userId, ['exception' => $e]);
+			}
+
+			$cleaned++;
+		}
+
+		if ($cleaned > 0) {
+			$output->info('Cleaned up assistant data for ' . $cleaned . ' deleted user(s)');
+		}
+	}
+
+	/**
+	 * @return array<int, mixed>
+	 */
+	private function getDistinctUserIds(): array {
+		$userIds = [];
+
+		$userIdsChat = [];
+		$userIdsAssignments = [];
+
+		if ($this->db->tableExists('assistant_chat_sns')) {
+			$qb = $this->db->getQueryBuilder();
+			$qb->selectDistinct('user_id')
+				->from('assistant_chat_sns');
+			$result = $qb->executeQuery();
+			$userIdsChat = $result->fetchFirstColumn();
+			$result->closeCursor();
+		}
+
+		$userIds = array_unique(array_merge($userIdsChat, $userIdsAssignments));
+		return $userIds;
+	}
+}
diff --git a/lib/Service/ChatService.php b/lib/Service/ChatService.php
new file mode 100644
index 000000000..97e325c5f
--- /dev/null
+++ b/lib/Service/ChatService.php
@@ -0,0 +1,38 @@
+<?php
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\Assistant\Service;
+
+use OCA\Assistant\Db\ChattyLLM\MessageMapper;
+use OCA\Assistant\Db\ChattyLLM\SessionMapper;
+use OCP\DB\Exception;
+
+class ChatService {
+	public const EMPTY_CONVERSATION_TOKEN = '{}';
+
+	public function __construct(
+		private readonly SessionMapper $sessionMapper,
+		private readonly MessageMapper $messageMapper,
+	) {
+	}
+
+	/**
+	 * @throws InternalException
+	 */
+	public function deleteAllUserChatData(string $userId): void {
+		try {
+			$sessions = $this->sessionMapper->getUserSessions($userId);
+			foreach ($sessions as $session) {
+				$this->messageMapper->deleteMessagesBySession($session->getId());
+			}
+			$this->sessionMapper->deleteAllSessionsForUser($userId);
+		} catch (Exception|\RuntimeException $e) {
+			throw new InternalException(previous: $e);
+		}
+	}
+
+}
diff --git a/lib/Service/InternalException.php b/lib/Service/InternalException.php
new file mode 100644
index 000000000..28cd5c0d1
--- /dev/null
+++ b/lib/Service/InternalException.php
@@ -0,0 +1,11 @@
+<?php
+
+/**
+ * SPDX-FileCopyrightText: 2024 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\Assistant\Service;
+
+class InternalException extends \Exception {
+}
diff --git a/lib/Service/SessionSummaryService.php b/lib/Service/SessionSummaryService.php
index 7c490b488..21c9dc85c 100644
--- a/lib/Service/SessionSummaryService.php
+++ b/lib/Service/SessionSummaryService.php
@@ -41,6 +41,15 @@ public function __construct(
 	) {
 	}
 
+	public function deleteSummaryJobsForUser(string $userId): void {
+		if ($this->jobList->has(GenerateNewChatSummaries::class, ['userId' => $userId])) {
+			$this->jobList->remove(GenerateNewChatSummaries::class, ['userId' => $userId]);
+		}
+		if ($this->jobList->has(RegenerateOutdatedChatSummariesJob::class, ['userId' => $userId])) {
+			$this->jobList->remove(RegenerateOutdatedChatSummariesJob::class, ['userId' => $userId]);
+		}
+	}
+
 	private function generateSummaries(array $sessions): void {
 		foreach ($sessions as $session) {
 			try {