diff --git a/.phpcs.xml b/.phpcs.xml
index fef07e6204a..b60a3aff66e 100644
--- a/.phpcs.xml
+++ b/.phpcs.xml
@@ -189,7 +189,6 @@
 			Whitelist existing violations, but enable the sniff to prevent
 			any new occurrences.
 		-->
-		<exclude-pattern>*/includes/api/ApiErrorFormatter\.php</exclude-pattern>
 		<exclude-pattern>*/includes/parser/Preprocessor_DOM\.php</exclude-pattern>
 		<exclude-pattern>*/includes/parser/Preprocessor_Hash\.php</exclude-pattern>
 		<exclude-pattern>*/includes/parser/Preprocessor\.php</exclude-pattern>
diff --git a/autoload.php b/autoload.php
index 4f41c8af91d..35137ab943c 100644
--- a/autoload.php
+++ b/autoload.php
@@ -39,7 +39,7 @@ $wgAutoloadLocalClasses = [
 	'ApiEditPage' => __DIR__ . '/includes/api/ApiEditPage.php',
 	'ApiEmailUser' => __DIR__ . '/includes/api/ApiEmailUser.php',
 	'ApiErrorFormatter' => __DIR__ . '/includes/api/ApiErrorFormatter.php',
-	'ApiErrorFormatter_BackCompat' => __DIR__ . '/includes/api/ApiErrorFormatter.php',
+	'ApiErrorFormatter_BackCompat' => __DIR__ . '/includes/api/ApiErrorFormatter_BackCompat.php',
 	'ApiExpandTemplates' => __DIR__ . '/includes/api/ApiExpandTemplates.php',
 	'ApiFeedContributions' => __DIR__ . '/includes/api/ApiFeedContributions.php',
 	'ApiFeedRecentChanges' => __DIR__ . '/includes/api/ApiFeedRecentChanges.php',
diff --git a/includes/api/ApiErrorFormatter.php b/includes/api/ApiErrorFormatter.php
index 9669464733d..8049cd84b98 100644
--- a/includes/api/ApiErrorFormatter.php
+++ b/includes/api/ApiErrorFormatter.php
@@ -388,111 +388,3 @@ class ApiErrorFormatter {
 		}
 	}
 }
-
-/**
- * Format errors and warnings in the old style, for backwards compatibility.
- * @since 1.25
- * @deprecated Only for backwards compatibility, do not use
- * @ingroup API
- */
-// phpcs:ignore Squiz.Classes.ValidClassName.NotCamelCaps
-class ApiErrorFormatter_BackCompat extends ApiErrorFormatter {
-
-	/**
-	 * @param ApiResult $result Into which data will be added
-	 */
-	public function __construct( ApiResult $result ) {
-		parent::__construct( $result, Language::factory( 'en' ), 'none', false );
-	}
-
-	public function getFormat() {
-		return 'bc';
-	}
-
-	public function arrayFromStatus( StatusValue $status, $type = 'error', $format = null ) {
-		if ( $status->isGood() || !$status->getErrors() ) {
-			return [];
-		}
-
-		$result = [];
-		foreach ( $status->getErrorsByType( $type ) as $error ) {
-			$msg = ApiMessage::create( $error );
-			$error = [
-				'message' => $msg->getKey(),
-				'params' => $msg->getParams(),
-				'code' => $msg->getApiCode(),
-			] + $error;
-			ApiResult::setIndexedTagName( $error['params'], 'param' );
-			$result[] = $error;
-		}
-		ApiResult::setIndexedTagName( $result, $type );
-
-		return $result;
-	}
-
-	protected function formatMessageInternal( $msg, $format ) {
-		return [
-			'code' => $msg->getApiCode(),
-			'info' => $msg->text(),
-		] + $msg->getApiData();
-	}
-
-	/**
-	 * Format an exception as an array
-	 * @since 1.29
-	 * @param Exception|Throwable $exception
-	 * @param array $options See parent::formatException(), plus
-	 *  - bc: (bool) Return only the string, not an array
-	 * @return array|string
-	 */
-	public function formatException( $exception, array $options = [] ) {
-		$ret = parent::formatException( $exception, $options );
-		return empty( $options['bc'] ) ? $ret : $ret['info'];
-	}
-
-	protected function addWarningOrError( $tag, $modulePath, $msg ) {
-		$value = self::stripMarkup( $msg->text() );
-
-		if ( $tag === 'error' ) {
-			// In BC mode, only one error
-			$existingError = $this->result->getResultData( [ 'error' ] );
-			if ( !is_array( $existingError ) ||
-				!isset( $existingError['code'] ) || !isset( $existingError['info'] )
-			) {
-				$value = [
-					'code' => $msg->getApiCode(),
-					'info' => $value,
-				] + $msg->getApiData();
-				$this->result->addValue( null, 'error', $value,
-					ApiResult::OVERRIDE | ApiResult::ADD_ON_TOP | ApiResult::NO_SIZE_CHECK );
-			}
-		} else {
-			if ( $modulePath === null ) {
-				$moduleName = 'unknown';
-			} else {
-				$i = strrpos( $modulePath, '+' );
-				$moduleName = $i === false ? $modulePath : substr( $modulePath, $i + 1 );
-			}
-
-			// Don't add duplicate warnings
-			$tag .= 's';
-			$path = [ $tag, $moduleName ];
-			$oldWarning = $this->result->getResultData( [ $tag, $moduleName, $tag ] );
-			if ( $oldWarning !== null ) {
-				$warnPos = strpos( $oldWarning, $value );
-				// If $value was found in $oldWarning, check if it starts at 0 or after "\n"
-				if ( $warnPos !== false && ( $warnPos === 0 || $oldWarning[$warnPos - 1] === "\n" ) ) {
-					// Check if $value is followed by "\n" or the end of the $oldWarning
-					$warnPos += strlen( $value );
-					if ( strlen( $oldWarning ) <= $warnPos || $oldWarning[$warnPos] === "\n" ) {
-						return;
-					}
-				}
-				// If there is a warning already, append it to the existing one
-				$value = "$oldWarning\n$value";
-			}
-			$this->result->addContentValue( $path, $tag, $value,
-				ApiResult::OVERRIDE | ApiResult::ADD_ON_TOP | ApiResult::NO_SIZE_CHECK );
-		}
-	}
-}
diff --git a/includes/api/ApiErrorFormatter_BackCompat.php b/includes/api/ApiErrorFormatter_BackCompat.php
new file mode 100644
index 00000000000..6aee4cb2163
--- /dev/null
+++ b/includes/api/ApiErrorFormatter_BackCompat.php
@@ -0,0 +1,127 @@
+<?php
+/**
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ */
+
+/**
+ * Format errors and warnings in the old style, for backwards compatibility.
+ * @since 1.25
+ * @deprecated Only for backwards compatibility, do not use
+ * @ingroup API
+ */
+// phpcs:ignore Squiz.Classes.ValidClassName.NotCamelCaps
+class ApiErrorFormatter_BackCompat extends ApiErrorFormatter {
+
+	/**
+	 * @param ApiResult $result Into which data will be added
+	 */
+	public function __construct( ApiResult $result ) {
+		parent::__construct( $result, Language::factory( 'en' ), 'none', false );
+	}
+
+	public function getFormat() {
+		return 'bc';
+	}
+
+	public function arrayFromStatus( StatusValue $status, $type = 'error', $format = null ) {
+		if ( $status->isGood() || !$status->getErrors() ) {
+			return [];
+		}
+
+		$result = [];
+		foreach ( $status->getErrorsByType( $type ) as $error ) {
+			$msg = ApiMessage::create( $error );
+			$error = [
+				'message' => $msg->getKey(),
+				'params' => $msg->getParams(),
+				'code' => $msg->getApiCode(),
+			] + $error;
+			ApiResult::setIndexedTagName( $error['params'], 'param' );
+			$result[] = $error;
+		}
+		ApiResult::setIndexedTagName( $result, $type );
+
+		return $result;
+	}
+
+	protected function formatMessageInternal( $msg, $format ) {
+		return [
+			'code' => $msg->getApiCode(),
+			'info' => $msg->text(),
+		] + $msg->getApiData();
+	}
+
+	/**
+	 * Format an exception as an array
+	 * @since 1.29
+	 * @param Exception|Throwable $exception
+	 * @param array $options See parent::formatException(), plus
+	 *  - bc: (bool) Return only the string, not an array
+	 * @return array|string
+	 */
+	public function formatException( $exception, array $options = [] ) {
+		$ret = parent::formatException( $exception, $options );
+		return empty( $options['bc'] ) ? $ret : $ret['info'];
+	}
+
+	protected function addWarningOrError( $tag, $modulePath, $msg ) {
+		$value = self::stripMarkup( $msg->text() );
+
+		if ( $tag === 'error' ) {
+			// In BC mode, only one error
+			$existingError = $this->result->getResultData( [ 'error' ] );
+			if ( !is_array( $existingError ) ||
+				!isset( $existingError['code'] ) || !isset( $existingError['info'] )
+			) {
+				$value = [
+					'code' => $msg->getApiCode(),
+					'info' => $value,
+				] + $msg->getApiData();
+				$this->result->addValue( null, 'error', $value,
+					ApiResult::OVERRIDE | ApiResult::ADD_ON_TOP | ApiResult::NO_SIZE_CHECK );
+			}
+		} else {
+			if ( $modulePath === null ) {
+				$moduleName = 'unknown';
+			} else {
+				$i = strrpos( $modulePath, '+' );
+				$moduleName = $i === false ? $modulePath : substr( $modulePath, $i + 1 );
+			}
+
+			// Don't add duplicate warnings
+			$tag .= 's';
+			$path = [ $tag, $moduleName ];
+			$oldWarning = $this->result->getResultData( [ $tag, $moduleName, $tag ] );
+			if ( $oldWarning !== null ) {
+				$warnPos = strpos( $oldWarning, $value );
+				// If $value was found in $oldWarning, check if it starts at 0 or after "\n"
+				if ( $warnPos !== false && ( $warnPos === 0 || $oldWarning[$warnPos - 1] === "\n" ) ) {
+					// Check if $value is followed by "\n" or the end of the $oldWarning
+					$warnPos += strlen( $value );
+					if ( strlen( $oldWarning ) <= $warnPos || $oldWarning[$warnPos] === "\n" ) {
+						return;
+					}
+				}
+				// If there is a warning already, append it to the existing one
+				$value = "$oldWarning\n$value";
+			}
+			$this->result->addContentValue( $path, $tag, $value,
+				ApiResult::OVERRIDE | ApiResult::ADD_ON_TOP | ApiResult::NO_SIZE_CHECK );
+		}
+	}
+}