Peace be upon you. Smiling intentionally during prayer does not invalidate the prayer, but loud laughter (giggling) does. If someone laughs unintentionally or without sound, it does not invalidate the prayer. If a person cannot prevent themselves from laughing and their face turns red or their body trembles, their prayer remains valid as long as the essence of the prayer is not lost. According to Ayatollah Khamenei, intentional laughter that includes sound invalidates the prayer, while unintentional laughter does not. Ayatollah Sistani states that laughter that includes sound (giggling) invalidates the prayer, even if it is unintentional, unless the circumstances leading to it were not chosen. However, smiling is permissible. If someone laughs silently but their body shows signs of laughter, there is uncertainty about the validity of their prayer, and it is safer to repeat it. May you be successful in all good.