diff --git a/includes/Checker/Checks/Plugin_Repo/Core_Library_File_Signature_Checker.php b/includes/Checker/Checks/Plugin_Repo/Core_Library_File_Signature_Checker.php
new file mode 100644
index 000000000..54ff42276
--- /dev/null
+++ b/includes/Checker/Checks/Plugin_Repo/Core_Library_File_Signature_Checker.php
@@ -0,0 +1,522 @@
+<?php
+/**
+ * Class Core_Library_File_Signature_Checker.
+ *
+ * @package plugin-check
+ */
+
+namespace WordPress\Plugin_Check\Checker\Checks\Plugin_Repo;
+
+/**
+ * Helper for confirming whether a filename candidate is a bundled WordPress core library.
+ *
+ * @since n.e.x.t
+ */
+class Core_Library_File_Signature_Checker {
+
+	/**
+	 * Checks whether a file path and content match a known WordPress core library.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param string $file          Absolute file path.
+	 * @param string $relative_file File path relative to the plugin root.
+	 * @return bool True when the file is likely a bundled WordPress core library, false otherwise.
+	 */
+	public static function file_matches_core_library_signature( $file, $relative_file ) {
+		foreach ( self::get_core_library_file_signatures() as $library ) {
+			if ( preg_match( $library['file_pattern'], $relative_file ) && self::file_contains_core_library_signature( $file, $library ) ) {
+				return true;
+			}
+		}
+
+		return false;
+	}
+
+	/**
+	 * Gets WordPress core library filename candidates and required content signatures.
+	 *
+	 * The filename patterns intentionally match full basenames only. A matching
+	 * filename is only a candidate; the file must also contain library-specific
+	 * symbols, package names, global classes, or namespaced classes before the
+	 * check reports it as a bundled copy of a WordPress core library.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @return array[] Core library file signature definitions.
+	 */
+	private static function get_core_library_file_signatures() {
+		return array_merge(
+			self::get_jquery_core_library_file_signatures(),
+			self::get_javascript_core_library_file_signatures(),
+			self::get_php_core_library_file_signatures()
+		);
+	}
+
+	/**
+	 * Gets WordPress core jQuery-family library filename candidates and required content signatures.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @return array[] Core library file signature definitions.
+	 */
+	private static function get_jquery_core_library_file_signatures() {
+		return array(
+			array(
+				'file_pattern'     => '~(?:^|/)jquery(?:-[0-9.]+)?(?:\.slim)?(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~jQuery JavaScript Library~i', '~\bwindow\.jQuery\b~', '~\bjQuery\.fn\b~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)jquery-ui(?:-[0-9.]+)?(?:\.slim)?(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~jQuery UI~i', '~\bjQuery\.ui\b~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)jquery\.color(?:\.slim)?(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~jQuery Color~i', '~\bjQuery\.Color\b~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)jquery\.ui\.touch-punch$~i',
+				'content_patterns' => array( '~jQuery UI Touch Punch~i' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)jquery\.hoverintent$~i',
+				'content_patterns' => array( '~hoverIntent~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)jquery\.imgareaselect$~i',
+				'content_patterns' => array( '~imgAreaSelect~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)jquery\.hotkeys$~i',
+				'content_patterns' => array( '~jQuery Hotkeys~i' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)jquery\.ba-serializeobject$~i',
+				'content_patterns' => array( '~serializeObject~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)jquery\.query-object$~i',
+				'content_patterns' => array( '~jQuery\.query~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)jquery\.suggest$~i',
+				'content_patterns' => array( '~jQuery Suggest~i' ),
+			),
+		);
+	}
+
+	/**
+	 * Gets WordPress core JavaScript library filename candidates and required content signatures.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @return array[] Core library file signature definitions.
+	 */
+	private static function get_javascript_core_library_file_signatures() {
+		return array(
+			array(
+				'file_pattern'     => '~(?:^|/)polyfill(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~wp-polyfill~i', '~\bcore-js\b~i', '~\bregeneratorRuntime\b~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)iris(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~\bIris\b~', '~\.iris\s*\(~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)backbone(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~Backbone\.js~i', '~\bBackbone\.VERSION\b~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)clipboard(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~clipboard\.js~i', '~\bClipboardJS\b~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)closest(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~Element\.prototype\.closest~', '~element-closest~i' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)codemirror(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~\bCodeMirror\b~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)formdata(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~FormData\.prototype~', '~formdata-polyfill~i' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)json2(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~json2\.js~i', '~JSON\.stringify~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)lodash(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~lodash~i', '~\b_\.VERSION\b~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)masonry\.pkgd(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~masonry\.pkgd~i', '~\bMasonry\b~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)mediaelement-and-player(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~MediaElementPlayer~', '~\bmejs\b~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)moment(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~moment\.js~i', '~hooks\.version~', '~moment\.version~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)plupload\.full(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~\bplupload\b~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)thickbox(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~\btb_show\b~', '~thickbox~i' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)twemoji(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~\btwemoji\b~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)underscore(?:[.-]min)?\.js$~i',
+				'content_patterns' => array( '~Underscore\.js~i', '~\b_\.VERSION\b~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)moxie(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~mOxie~' ),
+			),
+			array(
+				'file_pattern'     => '~(?:^|/)zxcvbn(?:\.min)?\.js$~i',
+				'content_patterns' => array( '~\bzxcvbn\b~' ),
+			),
+		);
+	}
+
+	/**
+	 * Gets WordPress core PHP library filename candidates and required class signatures.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @return array[] Core library file signature definitions.
+	 */
+	private static function get_php_core_library_file_signatures() {
+		return array(
+			array(
+				'file_pattern' => '~(?:^|/)getid3\.php$~i',
+				'php_classes'  => array( 'getID3' ),
+			),
+			array(
+				'file_pattern' => '~(?:^|/)pclzip\.lib\.php$~i',
+				'php_classes'  => array( 'PclZip' ),
+			),
+			array(
+				'file_pattern' => '~(?:^|/)PasswordHash\.php$~i',
+				'php_classes'  => array( 'PasswordHash' ),
+			),
+			array(
+				'file_pattern' => '~(?:^|/)PHPMailer\.php$~i',
+				'php_classes'  => array( 'PHPMailer', 'PHPMailer\PHPMailer\PHPMailer' ),
+			),
+			array(
+				'file_pattern' => '~(?:^|/)SimplePie\.php$~i',
+				'php_classes'  => array( 'SimplePie' ),
+			),
+		);
+	}
+
+	/**
+	 * Checks whether a candidate file contains a known core library signature.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param string $file    Absolute file path.
+	 * @param array  $library Core library signature definition.
+	 * @return bool True when the file contains a matching signature, false otherwise.
+	 */
+	private static function file_contains_core_library_signature( $file, array $library ) {
+		// phpcs:ignore WordPress.WP.AlternativeFunctions.file_get_contents_file_get_contents
+		$contents = file_get_contents( $file );
+		if ( false === $contents ) {
+			return false;
+		}
+
+		if ( isset( $library['php_classes'] ) || isset( $library['php_functions'] ) ) {
+			$symbols = self::get_php_symbols( $contents );
+
+			foreach ( $library['php_classes'] ?? array() as $class_name ) {
+				if ( isset( $symbols['classes'][ self::normalize_php_symbol_name( $class_name ) ] ) ) {
+					return true;
+				}
+			}
+
+			foreach ( $library['php_functions'] ?? array() as $function_name ) {
+				if ( isset( $symbols['functions'][ self::normalize_php_symbol_name( $function_name ) ] ) ) {
+					return true;
+				}
+			}
+		}
+
+		foreach ( $library['content_patterns'] ?? array() as $pattern ) {
+			if ( preg_match( $pattern, $contents ) ) {
+				return true;
+			}
+		}
+
+		return false;
+	}
+
+	/**
+	 * Extracts global and namespaced PHP class and function symbols.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param string $contents PHP file contents.
+	 * @return array Extracted symbols keyed by normalized fully qualified name.
+	 */
+	private static function get_php_symbols( $contents ) {
+		$tokens = token_get_all( $contents );
+		$state  = array(
+			'namespace'           => '',
+			'brace_depth'         => 0,
+			'pending_class_brace' => false,
+			'class_brace_depths'  => array(),
+			'symbols'             => array(
+				'classes'   => array(),
+				'functions' => array(),
+			),
+		);
+
+		foreach ( $tokens as $index => $token ) {
+			if ( self::update_php_symbol_scope( $token, $state ) || ! is_array( $token ) ) {
+				continue;
+			}
+
+			self::collect_php_symbol( $tokens, $index, $state );
+		}
+
+		return $state['symbols'];
+	}
+
+	/**
+	 * Updates PHP parser state for brace-delimited scopes.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param mixed $token PHP token or token text.
+	 * @param array $state PHP symbol parser state.
+	 * @return bool True when the token was handled as a scope token, false otherwise.
+	 */
+	private static function update_php_symbol_scope( $token, array &$state ) {
+		if ( '{' === $token ) {
+			++$state['brace_depth'];
+			if ( $state['pending_class_brace'] ) {
+				$state['class_brace_depths'][] = $state['brace_depth'];
+				$state['pending_class_brace']  = false;
+			}
+			return true;
+		}
+
+		if ( '}' === $token ) {
+			self::leave_php_symbol_scope( $state );
+			return true;
+		}
+
+		return false;
+	}
+
+	/**
+	 * Leaves the current PHP symbol parser scope.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param array $state PHP symbol parser state.
+	 */
+	private static function leave_php_symbol_scope( array &$state ) {
+		while ( ! empty( $state['class_brace_depths'] ) && end( $state['class_brace_depths'] ) === $state['brace_depth'] ) {
+			array_pop( $state['class_brace_depths'] );
+		}
+		--$state['brace_depth'];
+	}
+
+	/**
+	 * Collects a PHP class, interface, trait, function, or namespace symbol.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param array $tokens Token stream from token_get_all().
+	 * @param int   $index  Current token index.
+	 * @param array $state  PHP symbol parser state.
+	 */
+	private static function collect_php_symbol( array $tokens, $index, array &$state ) {
+		$token = $tokens[ $index ];
+
+		if ( T_NAMESPACE === $token[0] ) {
+			$state['namespace'] = self::parse_php_namespace( $tokens, $index );
+			return;
+		}
+
+		if ( self::is_php_class_declaration_token( $tokens, $index ) ) {
+			self::add_php_class_symbol( $tokens, $index, $state );
+			return;
+		}
+
+		if ( T_FUNCTION === $token[0] && empty( $state['class_brace_depths'] ) ) {
+			self::add_php_function_symbol( $tokens, $index, $state );
+		}
+	}
+
+	/**
+	 * Checks whether the token is a class, interface, or trait declaration.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param array $tokens Token stream from token_get_all().
+	 * @param int   $index  Current token index.
+	 * @return bool True when the token declares a PHP type, false otherwise.
+	 */
+	private static function is_php_class_declaration_token( array $tokens, $index ) {
+		$token = $tokens[ $index ];
+		if ( ! in_array( $token[0], array( T_CLASS, T_INTERFACE, T_TRAIT ), true ) ) {
+			return false;
+		}
+
+		return T_CLASS !== $token[0] || ! self::previous_php_token_is( $tokens, $index, T_DOUBLE_COLON );
+	}
+
+	/**
+	 * Adds a PHP class, interface, or trait symbol to parser state.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param array $tokens Token stream from token_get_all().
+	 * @param int   $index  Current token index.
+	 * @param array $state  PHP symbol parser state.
+	 */
+	private static function add_php_class_symbol( array $tokens, $index, array &$state ) {
+		$class_name = self::next_php_string_token( $tokens, $index );
+		if ( '' !== $class_name ) {
+			$symbol                                 = self::normalize_php_symbol_name( $state['namespace'] . '\\' . $class_name );
+			$state['symbols']['classes'][ $symbol ] = true;
+		}
+		$state['pending_class_brace'] = true;
+	}
+
+	/**
+	 * Adds a PHP function symbol to parser state.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param array $tokens Token stream from token_get_all().
+	 * @param int   $index  Current token index.
+	 * @param array $state  PHP symbol parser state.
+	 */
+	private static function add_php_function_symbol( array $tokens, $index, array &$state ) {
+		$function_name = self::next_php_string_token( $tokens, $index );
+		if ( '' === $function_name ) {
+			return;
+		}
+
+		$symbol                                   = self::normalize_php_symbol_name( $state['namespace'] . '\\' . $function_name );
+		$state['symbols']['functions'][ $symbol ] = true;
+	}
+
+	/**
+	 * Parses a namespace declaration from a PHP token stream.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param array $tokens          Token stream from token_get_all().
+	 * @param int   $namespace_index Index of the T_NAMESPACE token.
+	 * @return string Namespace name, or an empty string for the global namespace.
+	 */
+	private static function parse_php_namespace( array $tokens, $namespace_index ) {
+		$namespace = '';
+
+		for ( $index = $namespace_index + 1; isset( $tokens[ $index ] ); ++$index ) {
+			$token = $tokens[ $index ];
+			if ( ';' === $token || '{' === $token ) {
+				break;
+			}
+
+			if ( is_array( $token ) && self::is_php_name_token( $token ) ) {
+				$namespace .= $token[1];
+			}
+		}
+
+		return trim( $namespace, '\\' );
+	}
+
+	/**
+	 * Checks whether a token can be part of a PHP qualified name.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param array $token Token from token_get_all().
+	 * @return bool True when the token is a PHP name token, false otherwise.
+	 */
+	private static function is_php_name_token( array $token ) {
+		$name_token_ids = array( T_STRING, T_NS_SEPARATOR );
+		if ( defined( 'T_NAME_QUALIFIED' ) ) {
+			$name_token_ids[] = constant( 'T_NAME_QUALIFIED' );
+		}
+		if ( defined( 'T_NAME_FULLY_QUALIFIED' ) ) {
+			$name_token_ids[] = constant( 'T_NAME_FULLY_QUALIFIED' );
+		}
+
+		return in_array( $token[0], $name_token_ids, true );
+	}
+
+	/**
+	 * Gets the next PHP T_STRING token after a given token index.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param array $tokens Token stream from token_get_all().
+	 * @param int   $index  Token index to start after.
+	 * @return string Token value, or an empty string when none is found.
+	 */
+	private static function next_php_string_token( array $tokens, $index ) {
+		for ( $next_index = $index + 1; isset( $tokens[ $next_index ] ); ++$next_index ) {
+			$token = $tokens[ $next_index ];
+			if ( is_array( $token ) && T_WHITESPACE === $token[0] ) {
+				continue;
+			}
+
+			return is_array( $token ) && T_STRING === $token[0] ? $token[1] : '';
+		}
+
+		return '';
+	}
+
+	/**
+	 * Checks whether the previous non-whitespace PHP token has the given token ID.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param array $tokens   Token stream from token_get_all().
+	 * @param int   $index    Token index to start before.
+	 * @param int   $token_id Token ID to check for.
+	 * @return bool True when the previous meaningful token matches, false otherwise.
+	 */
+	private static function previous_php_token_is( array $tokens, $index, $token_id ) {
+		for ( $previous_index = $index - 1; $previous_index >= 0; --$previous_index ) {
+			$token = $tokens[ $previous_index ];
+			if ( is_array( $token ) && T_WHITESPACE === $token[0] ) {
+				continue;
+			}
+
+			return is_array( $token ) && $token_id === $token[0];
+		}
+
+		return false;
+	}
+
+	/**
+	 * Normalizes a PHP symbol name for case-insensitive lookup.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param string $symbol_name PHP symbol name.
+	 * @return string Normalized PHP symbol name.
+	 */
+	private static function normalize_php_symbol_name( $symbol_name ) {
+		return strtolower( trim( $symbol_name, '\\' ) );
+	}
+}
diff --git a/includes/Checker/Checks/Plugin_Repo/File_Type_Check.php b/includes/Checker/Checks/Plugin_Repo/File_Type_Check.php
index f470b8dd5..f9972469a 100644
--- a/includes/Checker/Checks/Plugin_Repo/File_Type_Check.php
+++ b/includes/Checker/Checks/Plugin_Repo/File_Type_Check.php
@@ -375,62 +375,20 @@ function ( $file ) use ( $plugin_path ) {
 	 * @param array        $files  List of absolute file paths.
 	 */
 	protected function look_for_library_core_files( Check_Result $result, array $files ) {
-		// Known libraries that are part of WordPress core.
-		// https://meta.trac.wordpress.org/browser/sites/trunk/api.wordpress.org/public_html/core/credits/wp-59.php#L739 .
-		$look_known_libraries_core_services = array(
-			'(?<![\.|-])jquery(-[0-9|\.]*)?(\.slim)?(\.min)?\.js(?!\/)',
-			'jquery-ui(-[0-9|\.]*)?(\.slim)?(\.min)?\.js(?!\/)',
-			'jquery.color(\.slim)?(\.min)?\.js(?!\/)',
-			'jquery.ui.touch-punch(?!\/)',
-			'jquery.hoverintent(?!\/)',
-			'jquery.imgareaselect(?!\/)',
-			'jquery.hotkeys(?!\/)',
-			'jquery.ba-serializeobject(?!\/)',
-			'jquery.query-object(?!\/)',
-			'jquery.suggest(?!\/)',
-			'polyfill(\.min)?\.js(?!\/)',
-			'iris(\.min)?\.js(?!\/)',
-			'backbone(\.min)?\.js(?!\/)',
-			'clipboard(\.min)?\.js(?!\/)',
-			'closest(\.min)?\.js(?!\/)',
-			'codemirror(\.min)?\.js(?!\/)',
-			'formdata(\.min)?\.js(?!\/)',
-			'json2(\.min)?\.js(?!\/)',
-			'lodash(\.min)?\.js(?!\/)',
-			'masonry(\.pkgd)(\.min)?\.js(?!\/)',
-			'mediaelement-and-player(\.min)?\.js(?!\/)',
-			'moment(\.min)?\.js(?!\/)',
-			'plupload(\.full)(\.min)?\.js(?!\/)',
-			'thickbox(\.min)?\.js(?!\/)',
-			'twemoji(\.min)?\.js(?!\/)',
-			'underscore([\.|-]min)?\.js(?!\/)',
-			'moxie(\.min)?\.js(?!\/)',
-			'zxcvbn(\.min)?\.js(?!\/)',
-			'getid3\.php(?!\/)',
-			'pclzip\.lib\.php(?!\/)',
-			'PasswordHash\.php(?!\/)',
-			'PHPMailer\.php(?!\/)',
-			'SimplePie\.php(?!\/)',
-		);
-
-		$combined_pattern = '/(' . implode( ')|(', $look_known_libraries_core_services ) . ')/i';
-
 		$plugin_path = $result->plugin()->path();
 
-		$files = array_map(
-			function ( $file ) use ( $plugin_path ) {
-				return str_replace( $plugin_path, '', $file );
-			},
-			$files
-		);
-
+		$relative_files = array();
 		foreach ( $files as $file ) {
-			if ( preg_match( $combined_pattern, $file ) ) {
+			$relative_files[ $file ] = str_replace( $plugin_path, '', $file );
+		}
+
+		foreach ( $relative_files as $file => $relative_file ) {
+			if ( Core_Library_File_Signature_Checker::file_matches_core_library_signature( $file, $relative_file ) ) {
 				$this->add_result_error_for_file(
 					$result,
 					__( 'Library files that are already in the WordPress core are not permitted.', 'plugin-check' ),
 					'library_core_files',
-					$file,
+					$relative_file,
 					0,
 					0,
 					'',
diff --git a/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-errors/PHPMailer.php b/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-errors/PHPMailer.php
index b3d9bbc7f..d3747bc6f 100644
--- a/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-errors/PHPMailer.php
+++ b/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-errors/PHPMailer.php
@@ -1 +1,5 @@
 <?php
+
+namespace PHPMailer\PHPMailer;
+
+class PHPMailer {}
diff --git a/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-errors/clipboard.js b/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-errors/clipboard.js
new file mode 100644
index 000000000..4a1e631ab
--- /dev/null
+++ b/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-errors/clipboard.js
@@ -0,0 +1,2 @@
+/*! clipboard.js v2.0.11 */
+window.ClipboardJS = function ClipboardJS() {};
diff --git a/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-errors/jquery.js b/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-errors/jquery.js
index d22803be9..ee2640722 100644
--- a/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-errors/jquery.js
+++ b/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-errors/jquery.js
@@ -1 +1,2 @@
-// For testing purposes, this file is empty.
+/*! jQuery JavaScript Library v3.7.1 | jquery.org/license */
+window.jQuery = window.jQuery || {};
diff --git a/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-without-errors/assets/polyfill.js b/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-without-errors/assets/polyfill.js
new file mode 100644
index 000000000..a811eb966
--- /dev/null
+++ b/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-without-errors/assets/polyfill.js
@@ -0,0 +1,5 @@
+/**
+ * Custom feature detection helper that happens to use a generic polyfill.js
+ * filename without bundling WordPress core's registered polyfill script.
+ */
+window.examplePluginPolyfill = true;
diff --git a/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-without-errors/includes/PHPMailer.php b/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-without-errors/includes/PHPMailer.php
new file mode 100644
index 000000000..20eedb82a
--- /dev/null
+++ b/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-without-errors/includes/PHPMailer.php
@@ -0,0 +1,5 @@
+<?php
+
+namespace ExamplePlugin\Mail;
+
+class PHPMailer {}
diff --git a/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-without-errors/load.php b/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-without-errors/load.php
new file mode 100644
index 000000000..9bac1ec56
--- /dev/null
+++ b/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-without-errors/load.php
@@ -0,0 +1,18 @@
+<?php
+/**
+ * Plugin Name: Test Plugin File Type Library Core Files Without Errors
+ * Plugin URI: https://github.com/WordPress/plugin-check
+ * Description: Some plugin description.
+ * Author: WordPress Performance Team
+ * Author URI: https://make.wordpress.org/plugins/
+ * License: GPLv2 or later
+ * License URI: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html
+ * Text Domain: test-plugin-file-type-library-core-without-errors
+ *
+ * @package test-plugin-file-type-library-core-without-errors
+ */
+
+/**
+ * Plugin folder contains files with core-library-like names that are not
+ * bundled copies of WordPress core libraries.
+ */
diff --git a/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-without-errors/src/utils/clipboard.js b/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-without-errors/src/utils/clipboard.js
new file mode 100644
index 000000000..3f716809c
--- /dev/null
+++ b/tests/phpunit/testdata/plugins/test-plugin-file-type-library-core-without-errors/src/utils/clipboard.js
@@ -0,0 +1,8 @@
+/**
+ * Copies text to the clipboard using browser APIs.
+ *
+ * This is a custom helper and not a bundled copy of a third-party package.
+ */
+export function copyToClipboard( text ) {
+	return navigator.clipboard.writeText( text );
+}
diff --git a/tests/phpunit/tests/Checker/Checks/File_Type_Check_Tests.php b/tests/phpunit/tests/Checker/Checks/File_Type_Check_Tests.php
index 92c20c246..d4a36d592 100644
--- a/tests/phpunit/tests/Checker/Checks/File_Type_Check_Tests.php
+++ b/tests/phpunit/tests/Checker/Checks/File_Type_Check_Tests.php
@@ -211,7 +211,7 @@ public function test_run_with_library_core_errors() {
 		$errors = $check_result->get_errors();
 
 		$this->assertNotEmpty( $errors );
-		$this->assertEquals( 2, $check_result->get_error_count() );
+		$this->assertEquals( 3, $check_result->get_error_count() );
 
 		// Check for core PHPMailer.
 		$this->assertArrayHasKey( 0, $errors['PHPMailer.php'] );
@@ -222,6 +222,22 @@ public function test_run_with_library_core_errors() {
 		$this->assertArrayHasKey( 0, $errors['jquery.js'] );
 		$this->assertArrayHasKey( 0, $errors['jquery.js'][0] );
 		$this->assertCount( 1, wp_list_filter( $errors['jquery.js'][0][0], array( 'code' => 'library_core_files' ) ) );
+
+		// Check for core ClipboardJS.
+		$this->assertArrayHasKey( 0, $errors['clipboard.js'] );
+		$this->assertArrayHasKey( 0, $errors['clipboard.js'][0] );
+		$this->assertCount( 1, wp_list_filter( $errors['clipboard.js'][0][0], array( 'code' => 'library_core_files' ) ) );
+	}
+
+	public function test_run_with_library_core_filename_false_positives() {
+		$check_context = new Check_Context( UNIT_TESTS_PLUGIN_DIR . 'test-plugin-file-type-library-core-without-errors/load.php' );
+		$check_result  = new Check_Result( $check_context );
+
+		$check = new File_Type_Check( File_Type_Check::TYPE_LIBRARY_CORE );
+		$check->run( $check_result );
+
+		$this->assertSame( 0, $check_result->get_error_count() );
+		$this->assertEmpty( $check_result->get_errors() );
 	}
 
 	public function test_run_with_composer_errors() {