diff --git a/lib/Service/AiIntegrations/AiIntegrationsService.php b/lib/Service/AiIntegrations/AiIntegrationsService.php
index a17e78c3aa..c97997c170 100644
--- a/lib/Service/AiIntegrations/AiIntegrationsService.php
+++ b/lib/Service/AiIntegrations/AiIntegrationsService.php
@@ -218,7 +218,12 @@ public function getSmartReply(Account $account, Mailbox $mailbox, Message $messa
 		if (in_array(FreePromptTaskType::class, $this->textProcessingManager->getAvailableTaskTypes(), true)) {
 			$cachedReplies = $this->cache->getValue('smartReplies_' . $message->getId());
 			if ($cachedReplies) {
-				return json_decode($cachedReplies, true, 512);
+				try {
+					return json_decode($cachedReplies, true, 512, JSON_THROW_ON_ERROR);
+				} catch (JsonException $e) {
+					$this->cache->remove('smartReplies_' . $message->getId());
+					throw new ServiceException('Failed to decode smart replies JSON output', previous: $e);
+				}
 			}
 			$client = $this->clientFactory->getClient($account);
 			try {
diff --git a/lib/Service/AiIntegrations/Cache.php b/lib/Service/AiIntegrations/Cache.php
index c4905f61b9..dbbbcb2f73 100644
--- a/lib/Service/AiIntegrations/Cache.php
+++ b/lib/Service/AiIntegrations/Cache.php
@@ -58,5 +58,13 @@ public function addValue(string $key, ?string $value): void {
 		$this->cache->set($key, $value ?? false, self::CACHE_TTL);
 	}
 
+	/**
+	 * @param string $key
+	 *
+	 * @return void
+	 */
+	public function remove(string $key): void {
+		$this->cache->remove($key);
+	}
 
 }
diff --git a/tests/Unit/Controller/MessagesControllerTest.php b/tests/Unit/Controller/MessagesControllerTest.php
index a4eff2ae05..8a2c51b358 100644
--- a/tests/Unit/Controller/MessagesControllerTest.php
+++ b/tests/Unit/Controller/MessagesControllerTest.php
@@ -1398,4 +1398,239 @@ public function testRestrictLimit(?int $limit, int $expectedLimit): void {
 
 		$this->controller->index(100, null, null, $limit);
 	}
+
+	public function testSmartReplyNoUser(): void {
+		$controller = new MessagesController(
+			$this->appName,
+			$this->request,
+			$this->accountService,
+			$this->mailManager,
+			$this->mailSearch,
+			$this->itineraryService,
+			null,
+			$this->userFolder,
+			$this->logger,
+			$this->l10n,
+			$this->mimeTypeDetector,
+			$this->urlGenerator,
+			$this->nonceManager,
+			$this->trustedSenderService,
+			$this->mailTransmission,
+			$this->smimeService,
+			$this->clientFactory,
+			$this->dkimService,
+			$this->userPreferences,
+			$this->snoozeService,
+			$this->aiIntegrationsService,
+			$this->cacheFactory,
+		);
+
+		$actualResponse = $controller->smartReply(100);
+		$expectedResponse = new JSONResponse([], Http::STATUS_UNAUTHORIZED);
+		$this->assertEquals($expectedResponse, $actualResponse);
+	}
+
+	public function testSmartReplyNoMessage(): void {
+		$this->mailManager->expects($this->once())
+			->method('getMessage')
+			->with($this->userId, 100)
+			->willThrowException(new DoesNotExistException(''));
+
+		$actualResponse = $this->controller->smartReply(100);
+		$expectedResponse = new JSONResponse([], Http::STATUS_FORBIDDEN);
+		$this->assertEquals($expectedResponse, $actualResponse);
+	}
+
+	public function testSmartReplyNoMailbox(): void {
+		$message = new \OCA\Mail\Db\Message();
+		$message->setId(100);
+		$message->setMailboxId(1);
+
+		$this->mailManager->expects($this->once())
+			->method('getMessage')
+			->with($this->userId, 100)
+			->willReturn($message);
+
+		$this->mailManager->expects($this->once())
+			->method('getMailbox')
+			->with($this->userId, $message->getMailboxId())
+			->willThrowException(new DoesNotExistException(''));
+
+		$actualResponse = $this->controller->smartReply(100);
+		$expectedResponse = new JSONResponse([], Http::STATUS_FORBIDDEN);
+		$this->assertEquals($expectedResponse, $actualResponse);
+	}
+
+	public function testSmartReplyNoAccount(): void {
+		$message = new \OCA\Mail\Db\Message();
+		$message->setId(100);
+		$message->setMailboxId(1);
+		$mailbox = new Mailbox();
+		$mailbox->setId(1);
+		$mailbox->setAccountId(1);
+
+		$this->mailManager->expects($this->once())
+			->method('getMessage')
+			->with($this->userId, 100)
+			->willReturn($message);
+
+		$this->mailManager->expects($this->once())
+			->method('getMailbox')
+			->with($this->userId, $message->getMailboxId())
+			->willReturn($mailbox);
+
+		$this->accountService->expects($this->once())
+			->method('find')
+			->with($this->userId, $mailbox->getAccountId())
+			->willThrowException(new DoesNotExistException(''));
+
+		$actualResponse = $this->controller->smartReply(100);
+		$expectedResponse = new JSONResponse([], Http::STATUS_FORBIDDEN);
+		$this->assertEquals($expectedResponse, $actualResponse);
+	}
+
+	public function testSmartReplyServiceException(): void {
+		$message = new \OCA\Mail\Db\Message();
+		$message->setId(100);
+		$message->setMailboxId(1);
+		$mailbox = new Mailbox();
+		$mailbox->setId(1);
+		$mailbox->setAccountId(1);
+
+		$this->mailManager->expects($this->once())
+			->method('getMessage')
+			->with($this->userId, 100)
+			->willReturn($message);
+
+		$this->mailManager->expects($this->once())
+			->method('getMailbox')
+			->with($this->userId, $message->getMailboxId())
+			->willReturn($mailbox);
+
+		$this->accountService->expects($this->once())
+			->method('find')
+			->with($this->userId, $mailbox->getAccountId())
+			->willReturn(new Account(new MailAccount()));
+
+		$this->aiIntegrationsService->expects($this->once())
+			->method('getSmartReply')
+			->with($this->anything(), $this->anything(), $this->anything(), $this->userId)
+			->willThrowException(new ServiceException('AI service error'));
+
+		$this->logger->expects($this->once())
+			->method('error');
+
+		$actualResponse = $this->controller->smartReply(100);
+		$expectedResponse = new JSONResponse([], Http::STATUS_NO_CONTENT);
+		$this->assertEquals($expectedResponse, $actualResponse);
+	}
+
+	public function testSmartReplySuccessful(): void {
+		$message = new \OCA\Mail\Db\Message();
+		$message->setId(100);
+		$message->setMailboxId(1);
+		$mailbox = new Mailbox();
+		$mailbox->setId(1);
+		$mailbox->setAccountId(1);
+
+		$this->mailManager->expects($this->once())
+			->method('getMessage')
+			->with($this->userId, 100)
+			->willReturn($message);
+
+		$this->mailManager->expects($this->once())
+			->method('getMailbox')
+			->with($this->userId, $message->getMailboxId())
+			->willReturn($mailbox);
+
+		$this->accountService->expects($this->once())
+			->method('find')
+			->with($this->userId, $mailbox->getAccountId())
+			->willReturn(new Account(new MailAccount()));
+
+		$replies = ['reply1' => 'OK thanks', 'reply2' => 'Sounds good'];
+		$this->aiIntegrationsService->expects($this->once())
+			->method('getSmartReply')
+			->with($this->anything(), $this->anything(), $this->anything(), $this->userId)
+			->willReturn($replies);
+
+		$actualResponse = $this->controller->smartReply(100);
+		$expectedResponse = new JSONResponse(array_values($replies));
+		$this->assertEquals($expectedResponse, $actualResponse);
+	}
+
+	public function testSmartReplyEmptyReplies(): void {
+		$message = new \OCA\Mail\Db\Message();
+		$message->setId(100);
+		$message->setMailboxId(1);
+		$mailbox = new Mailbox();
+		$mailbox->setId(1);
+		$mailbox->setAccountId(1);
+
+		$this->mailManager->expects($this->once())
+			->method('getMessage')
+			->with($this->userId, 100)
+			->willReturn($message);
+
+		$this->mailManager->expects($this->once())
+			->method('getMailbox')
+			->with($this->userId, $message->getMailboxId())
+			->willReturn($mailbox);
+
+		$this->accountService->expects($this->once())
+			->method('find')
+			->with($this->userId, $mailbox->getAccountId())
+			->willReturn(new Account(new MailAccount()));
+
+		$this->aiIntegrationsService->expects($this->once())
+			->method('getSmartReply')
+			->with($this->anything(), $this->anything(), $this->anything(), $this->userId)
+			->willReturn([]);
+
+		$actualResponse = $this->controller->smartReply(100);
+		$expectedResponse = new JSONResponse([]);
+		$this->assertEquals($expectedResponse, $actualResponse);
+	}
+
+	public function testSmartReplyWithCachedInvalidJson(): void {
+		// This test verifies that when getSmartReply() encounters corrupted cache
+		// (which would cause json_decode to fail), it throws ServiceException
+		// and the controller properly handles it by returning NO_CONTENT.
+		// This prevents the TypeError: array_values(): Argument #1 ($array) must be of type array, null given
+
+		$message = new \OCA\Mail\Db\Message();
+		$message->setId(100);
+		$message->setMailboxId(1);
+		$mailbox = new Mailbox();
+		$mailbox->setId(1);
+		$mailbox->setAccountId(1);
+
+		$this->mailManager->expects($this->once())
+			->method('getMessage')
+			->with($this->userId, 100)
+			->willReturn($message);
+
+		$this->mailManager->expects($this->once())
+			->method('getMailbox')
+			->with($this->userId, $message->getMailboxId())
+			->willReturn($mailbox);
+
+		$this->accountService->expects($this->once())
+			->method('find')
+			->with($this->userId, $mailbox->getAccountId())
+			->willReturn(new Account(new MailAccount()));
+
+		// Simulate the AI service throwing ServiceException due to corrupted cache
+		$this->aiIntegrationsService->expects($this->once())
+			->method('getSmartReply')
+			->with($this->anything(), $this->anything(), $this->anything(), $this->userId)
+			->willThrowException(new ServiceException('Failed to decode smart replies JSON output'));
+
+		$this->logger->expects($this->once())
+			->method('error');
+
+		$actualResponse = $this->controller->smartReply(100);
+		$expectedResponse = new JSONResponse([], Http::STATUS_NO_CONTENT);
+		$this->assertEquals($expectedResponse, $actualResponse);
+	}
 }