WordPress at a glance
filter is not described

http_headers_useragent filter-hook . WP 2.7.0

Filters the user agent value sent with an HTTP request.

Usage

add_filter( 'http_headers_useragent', 'filter_function_name_4867' );
function filter_function_name_4867( $user_agent ){
	// filter...

	return $user_agent;
}
$user_agent(string)
WordPress user agent string.

Where the hook is called

WP_Http::request()
wp_xmlrpc_server::pingback_ping()
wp-includes/class-http.php 183
'user-agent' => apply_filters( 'http_headers_useragent', 'WordPress/' . get_bloginfo( 'version' ) . '; ' . get_bloginfo( 'url' ) ),
wp-includes/class-wp-xmlrpc-server.php 6409
$user_agent = apply_filters( 'http_headers_useragent', 'WordPress/' . get_bloginfo( 'version' ) . '; ' . get_bloginfo( 'url' ) );

Where the hook is used (in WP core)

Does not used.