diff --git a/includes/MediaWiki.php b/includes/MediaWiki.php
index 94f8b524762..59fd8a70c21 100644
--- a/includes/MediaWiki.php
+++ b/includes/MediaWiki.php
@@ -42,9 +42,6 @@ class MediaWiki {
 	private $context;
 	/** @var Config */
 	private $config;
-
-	/** @var string|null Cache what action this request is */
-	private $action;
 	/** @var int Class DEFER_* constant; how non-blocking post-response tasks should run */
 	private $postSendStrategy;
 
@@ -173,11 +170,7 @@ class MediaWiki {
 	 * @return string Action
 	 */
 	public function getAction(): string {
-		if ( $this->action === null ) {
-			$this->action = $this->context->getActionName();
-		}
-
-		return $this->action;
+		return $this->context->getActionName();
 	}
 
 	/**
@@ -296,10 +289,11 @@ class MediaWiki {
 							$this->context->setRequest( $derivateRequest );
 							// Do not varnish cache these. May vary even for anons
 							$output->lowerCdnMaxage( 0 );
+							// NOTE: This also clears any action cache.
+							// Action should not have been computed yet, but if it was,
+							// we reset it because special pages only support "view".
 							$this->context->setTitle( $target );
 							$wgTitle = $target;
-							// Reset action type cache. (Special pages have only view)
-							$this->action = null;
 							$title = $target;
 							$output->addJsConfigVars( [
 								'wgInternalRedirectTargetUrl' => $target->getLinkURL( $query ),
@@ -417,8 +411,8 @@ class MediaWiki {
 		$title = $this->context->getTitle();
 		$services = MediaWikiServices::getInstance();
 		if ( $this->context->canUseWikiPage() ) {
-			// Reuse the WikiPage instance from context, as it may already have been initialized
-			// by an earlier this->getAction() call.
+			// Optimization: Reuse the WikiPage instance from context, to avoid
+			// repeat fetching or computation of data already loaded.
 			$page = $this->context->getWikiPage();
 		} else {
 			// This case should not happen, but just in case.
@@ -477,12 +471,14 @@ class MediaWiki {
 						$rarticle->setRedirectedFrom( $title );
 
 						$article = $rarticle;
+						// NOTE: This also clears any action cache
 						$this->context->setTitle( $target );
 						$this->context->setWikiPage( $article->getPage() );
 					}
 				}
 			} else {
 				// Article may have been changed by hook
+				// NOTE: This also clears any action cache
 				$this->context->setTitle( $article->getTitle() );
 				$this->context->setWikiPage( $article->getPage() );
 			}
diff --git a/includes/context/RequestContext.php b/includes/context/RequestContext.php
index b294fbed7c0..7bcbe784790 100644
--- a/includes/context/RequestContext.php
+++ b/includes/context/RequestContext.php
@@ -294,10 +294,9 @@ class RequestContext implements IContextSource, MutableContext {
 		// expensive operations to compute this. The computation involves creation
 		// of Article, WikiPage, and ContentHandler objects (and the various
 		// database queries these classes require to be instantiated), as well
-		// as potentially slow extension hooks at various level in these
-		// classes.
+		// as potentially slow extension hooks in these classes.
 		//
-		// This is value frequently needed in OutputPage and in various
+		// This value is frequently needed in OutputPage and in various
 		// Skin-related methods and classes.
 		if ( $this->action === null ) {
 			$this->action = MediaWikiServices::getInstance()
@@ -310,18 +309,15 @@ class RequestContext implements IContextSource, MutableContext {
 
 	private function clearActionName(): void {
 		if ( $this->action !== null ) {
-			// Log if cleared after something already computed it as that is
-			// likely to cause bugs (given the first caller may be using it for
-			// something), and also because it's an expensive thing to needlessly
-			// compute multiple times even when it produces the same value.
+			// If we're clearing after something else has actually already computed the action,
+			// emit a warning.
 			//
-			// TODO: Once confident we don't rely on this,
-			// change to E_USER_WARNING with trigger_error and silence error
-			// in relevant tests.
-			$logger = LoggerFactory::getInstance( 'Setup' );
-			$logger->warning( 'Changing action after getActionName was already called',
-				[ 'exception' => new Exception ]
-			);
+			// Doing so is unstable, given the first caller got something that turns out to be
+			// incomplete or incorrect. Even if we end up re-creating an instance of the same
+			// class, we may now be acting on a different title/skin/user etc.
+			//
+			// Re-computing the action is expensive and can be a performance problem (T302623).
+			trigger_error( 'Unexpected clearActionName after getActionName already called' );
 			$this->action = null;
 		}
 	}
diff --git a/tests/phpunit/includes/context/RequestContextTest.php b/tests/phpunit/includes/context/RequestContextTest.php
index 97aededa5e6..ab9b99b9930 100644
--- a/tests/phpunit/includes/context/RequestContextTest.php
+++ b/tests/phpunit/includes/context/RequestContextTest.php
@@ -190,7 +190,8 @@ class RequestContextTest extends MediaWikiIntegrationTestCase {
 		$this->assertSame( 'aaa', $context->getActionName(), 'first from factory' );
 		$this->assertSame( 'aaa', $context->getActionName(), 'cached first' );
 
-		$context->setTitle( $this->createMock( Title::class ) );
+		// Ignore warning from clearActionName
+		@$context->setTitle( $this->createMock( Title::class ) );
 		$this->assertSame( 'bbb', $context->getActionName(), 'second from factory' );
 		$this->assertSame( 'bbb', $context->getActionName(), 'cached second' );
 	}