diff --git a/includes/AbuseFilter.php b/includes/AbuseFilter.php
index 14a1d2b1f..1af2a6571 100644
--- a/includes/AbuseFilter.php
+++ b/includes/AbuseFilter.php
@@ -682,7 +682,7 @@ class AbuseFilter {
 	 * @param AbuseFilterVariableHolder $vars
 	 * @param bool $global
 	 *
-	 * @return int|null
+	 * @return int The insert ID.
 	 */
 	public static function storeVarDump( AbuseFilterVariableHolder $vars, $global = false ) {
 		global $wgCompressRevisions;
@@ -692,8 +692,8 @@ class AbuseFilter {
 		$vars = $vars->dumpAllVars( [ 'old_wikitext', 'new_wikitext' ] );
 
 		// Vars is an array with native PHP data types (non-objects) now
-		$text = serialize( $vars );
-		$flags = [ 'nativeDataArray', 'utf-8' ];
+		$text = FormatJson::encode( $vars );
+		$flags = [ 'utf-8' ];
 
 		if ( $wgCompressRevisions && function_exists( 'gzdeflate' ) ) {
 			$text = gzdeflate( $text );
@@ -708,12 +708,8 @@ class AbuseFilter {
 			} else {
 				$text = ExternalStore::insertToDefault( $text );
 			}
-			$flags[] = 'external';
 
-			if ( !$text ) {
-				// Not mission-critical, just return nothing
-				return null;
-			}
+			$flags[] = 'external';
 		}
 
 		// Store to text table
@@ -738,16 +734,27 @@ class AbuseFilter {
 	 *
 	 * @param string $stored_dump
 	 *
-	 * @return array|object|AbuseFilterVariableHolder|bool
+	 * @return AbuseFilterVariableHolder|array
 	 */
 	public static function loadVarDump( $stored_dump ) {
-		// Backward compatibility
-		if ( substr( $stored_dump, 0, strlen( 'stored-text:' ) ) !== 'stored-text:' ) {
+		// Backward compatibility for (old) blobs stored in the abuse_filter_log table
+		if ( !is_numeric( $stored_dump ) &&
+			substr( $stored_dump, 0, strlen( 'stored-text:' ) ) !== 'stored-text:' &&
+			substr( $stored_dump, 0, strlen( 'tt:' ) ) !== 'tt:'
+		) {
 			$data = unserialize( $stored_dump );
 			return is_array( $data ) ? AbuseFilterVariableHolder::newFromArray( $data ) : $data;
 		}
 
-		$text_id = substr( $stored_dump, strlen( 'stored-text:' ) );
+		if ( is_numeric( $stored_dump ) ) {
+			$text_id = (int)$stored_dump;
+		} elseif ( strpos( $stored_dump, 'stored-text:' ) !== false ) {
+			$text_id = (int)str_replace( 'stored-text:', '', $stored_dump );
+		} elseif ( strpos( $stored_dump, 'tt:' ) !== false ) {
+			$text_id = (int)str_replace( 'tt:', '', $stored_dump );
+		} else {
+			throw new LogicException( "Cannot understand format: $stored_dump" );
+		}
 
 		$dbr = wfGetDB( DB_REPLICA );
 
@@ -759,10 +766,12 @@ class AbuseFilter {
 		);
 
 		if ( !$text_row ) {
+			$logger = LoggerFactory::getInstance( 'AbuseFilter' );
+			$logger->warning( __METHOD__ . ": no text row found for input $stored_dump." );
 			return new AbuseFilterVariableHolder;
 		}
 
-		$flags = explode( ',', $text_row->old_flags );
+		$flags = $text_row->old_flags === '' ? [] : explode( ',', $text_row->old_flags );
 		$text = $text_row->old_text;
 
 		if ( in_array( 'external', $flags ) ) {
@@ -773,9 +782,17 @@ class AbuseFilter {
 			$text = gzinflate( $text );
 		}
 
-		$obj = unserialize( $text );
+		$obj = FormatJson::decode( $text, true );
+		if ( $obj === null ) {
+			// Temporary code until all rows will be JSON-encoded
+			$obj = unserialize( $text );
+		}
 
-		if ( in_array( 'nativeDataArray', $flags ) ) {
+		if ( in_array( 'nativeDataArray', $flags ) ||
+			// Temporary condition: we don't add the flag anymore, but the updateVarDump
+			// script could be still running and we cannot assume that this branch is the default.
+			( is_array( $obj ) && array_key_exists( 'action', $obj ) )
+		) {
 			$vars = $obj;
 			$obj = AbuseFilterVariableHolder::newFromArray( $vars );
 			// If old variable names are used, make sure to keep them
diff --git a/includes/AbuseFilterRunner.php b/includes/AbuseFilterRunner.php
index 0aba118f2..24fc4b3da 100644
--- a/includes/AbuseFilterRunner.php
+++ b/includes/AbuseFilterRunner.php
@@ -1208,8 +1208,7 @@ class AbuseFilterRunner {
 
 		// Only store the var dump if we're actually going to add log rows.
 		$varDump = AbuseFilter::storeVarDump( $this->vars );
-		// To distinguish from stuff stored directly
-		$varDump = "stored-text:$varDump";
+		$varDump = "tt:$varDump";
 
 		$localLogIDs = [];
 		global $wgAbuseFilterNotifications, $wgAbuseFilterNotificationsPrivate;
@@ -1272,7 +1271,7 @@ class AbuseFilterRunner {
 		if ( count( $loggedGlobalFilters ) ) {
 			$this->vars->computeDBVars();
 			$globalVarDump = AbuseFilter::storeVarDump( $this->vars, true );
-			$globalVarDump = "stored-text:$globalVarDump";
+			$globalVarDump = "tt:$globalVarDump";
 			foreach ( $centralLogRows as $index => $data ) {
 				$centralLogRows[$index]['afl_var_dump'] = $globalVarDump;
 			}
diff --git a/tests/phpunit/AbuseFilterDBTest.php b/tests/phpunit/AbuseFilterDBTest.php
index 0bd61fcb6..b37b44902 100644
--- a/tests/phpunit/AbuseFilterDBTest.php
+++ b/tests/phpunit/AbuseFilterDBTest.php
@@ -248,10 +248,7 @@ class AbuseFilterDBTest extends MediaWikiTestCase {
 	public function testVarDump( $variables ) {
 		global $wgCompressRevisions, $wgDefaultExternalStore;
 
-		$holder = new AbuseFilterVariableHolder();
-		foreach ( $variables as $name => $value ) {
-			$holder->setVar( $name, $value );
-		}
+		$holder = AbuseFilterVariableHolder::newFromArray( $variables );
 		if ( array_intersect_key( AbuseFilter::getDeprecatedVariables(), $variables ) ) {
 			$holder->mVarsVersion = 1;
 		}
@@ -266,9 +263,9 @@ class AbuseFilterDBTest extends MediaWikiTestCase {
 			[ 'ORDER BY' => 'old_id DESC' ]
 		);
 		$this->assertNotFalse( $flags, 'The var dump has not been saved.' );
-		$flags = explode( ',', $flags );
+		$flags = $flags === '' ? [] : explode( ',', $flags );
 
-		$expectedFlags = [ 'nativeDataArray', 'utf-8' ];
+		$expectedFlags = [ 'utf-8' ];
 		if ( $wgCompressRevisions ) {
 			$expectedFlags[] = 'gzip';
 		}
@@ -291,12 +288,14 @@ class AbuseFilterDBTest extends MediaWikiTestCase {
 		return [
 			'Only basic variables' => [
 				[
+					'action' => 'edit',
 					'old_wikitext' => 'Old text',
 					'new_wikitext' => 'New text'
 				]
 			],
-			[
+			'Normal case' => [
 				[
+					'action' => 'edit',
 					'old_wikitext' => 'Old text',
 					'new_wikitext' => 'New text',
 					'user_editcount' => 15,
@@ -305,22 +304,16 @@ class AbuseFilterDBTest extends MediaWikiTestCase {
 			],
 			'Deprecated variables' => [
 				[
+					'action' => 'edit',
 					'old_wikitext' => 'Old text',
 					'new_wikitext' => 'New text',
 					'article_articleid' => 11745,
 					'article_first_contributor' => 'Good guy'
 				]
 			],
-			[
-				[
-					'old_wikitext' => 'Old text',
-					'new_wikitext' => 'New text',
-					'page_title' => 'Some title',
-					'summary' => 'Fooooo'
-				]
-			],
-			[
+			'Move action' => [
 				[
+					'action' => 'move',
 					'old_wikitext' => 'Old text',
 					'new_wikitext' => 'New text',
 					'all_links' => [ 'https://en.wikipedia.org' ],
@@ -329,7 +322,7 @@ class AbuseFilterDBTest extends MediaWikiTestCase {
 					'new_content_model' => CONTENT_MODEL_JAVASCRIPT
 				]
 			],
-			[
+			'Delete action' => [
 				[
 					'old_wikitext' => 'Old text',
 					'new_wikitext' => 'New text',
@@ -338,21 +331,20 @@ class AbuseFilterDBTest extends MediaWikiTestCase {
 					'page_namespace' => 114
 				]
 			],
-			[
-				[
-					'old_wikitext' => 'Old text',
-					'new_wikitext' => 'New text',
-					'new_html' => 'Foo <small>bar</small> <s>lol</s>.',
-					'new_pst' => '[[Link|link]] test {{blah}}.'
-				]
-			],
 			'Disabled vars' => [
 				[
+					'action' => 'edit',
 					'old_wikitext' => 'Old text',
 					'new_wikitext' => 'New text',
 					'old_html' => 'Foo <small>bar</small> <s>lol</s>.',
 					'old_text' => 'Foobar'
 				]
+			],
+			'Account creation' => [
+				[
+					'action' => 'createaccount',
+					'accountname' => 'XXX'
+				]
 			]
 		];
 	}