TYPO3 Core

Currently it is not possible to manually modify the "canonical" tag from within an uncached extbase controller action. Canonical can only be modified when action is cached by setting "$GLOBALS['TSFE']->page['canonical_link']". The only possibility to modify the tag even when in an uncached context is to use the "ModifyUrlForCanonicalTagEvent" (EXT:seo) like this:

class FilterSeminarEventsCanonicalEventListener
{

    public function __invoke(ModifyUrlForCanonicalTagEvent $event): void
    {
        if($params = $this->getServerRequest()->getQueryParams()['tx_myext_seminar']) {
            if($params['action'] == 'filterSeminarEvents') {
                $seminarUid = (int) $params['seminar'];
                $locationUid = (int) $params['location'];
                $eventUid = (int) $params['event'];
                if($seminarUid > 0 && ($locationUid > 0 || $eventUid > 0)) {
                    /** @var Site $site */
                    $site = $this->getServerRequest()->getAttribute('site');
                    $parameters = [
                        'tx_myext_seminar' => [
                            'action' => 'show',
                            'controller' => 'Seminars\Seminar',
                            'seminar' => $seminarUid
                        ]
                    ];
                    $url = (string) $site->getRouter()->generateUri($this->getTypoScriptFrontendController()->id, $parameters);
                    $event->setUrl($url);
                }
            }
        }
    }

    /**
     * Returns the typoScriptFrontendController
     *
     * @return TypoScriptFrontendController
     */
    private function getTypoScriptFrontendController()
    {
        return $GLOBALS['TSFE'];
    }

    /**
     * Returns serverRequest
     *
     * @return ServerRequest
     */
    private function getServerRequest()
    {
        return $GLOBALS['TYPO3_REQUEST'];
    }
}

But in addition, there should be an alternate and easier way to modify the canonical directly from e.g. a controller action. Perhaps any implementation similar to PageTitle API or MetaTag API could be added.