The debate about whether Flutter is a front-end or backend tool has been ongoing since the framework was first released. While there are arguments to be made for both sides, the majority of evidence seems to suggest that Flutter is a front-end tool.
For starters, Flutter is primarily used for developing user interfaces. This is typically seen as the domain of front-end development. Additionally, the Flutter framework is based on the Dart programming language, which is also primarily used for front-end development.
Of course, this debate is likely to continue until there is a more definitive answer. For now, the evidence seems to suggest that Flutter is a front-end development tool.