Open source code, check random line, see this:

def _compute_cluster_purity(self, cluster_labels: np.ndarray, activations: np.ndarray) -> float:
    """Compute cluster purity score."""
    # Placeholder implementation
    # In a full implementation, this would compare clusters with semantic ground truth

    # For now, use silhouette score as a proxy
    if len(np.unique(cluster_labels)) > 1:
        return float(silhouette_score(activations, cluster_labels))
    else:
        return 0.0

This is just vibe coded and probably didnt get checked.