Born in
Alatri, Pirazzi has competed as a professional since 2010, joining from the amateur ranks; his twin brother Roberto is also a cyclist, and was also a member of the Palazzago Rad Logistica team with Stefano in 2009. In May 2017, one day before the start of the
2017 Giro d'Italia, the UCI announced that Pirazzi and team-mate
Nicola Ruffoni had tested positive for
growth hormone-releasing peptides in out-of-competition controls carried out the previous month. Later that month the UCI confirmed that both riders' B-samples had also tested positive, leading Bardiani–CSF to confirm that they would terminate both riders' contracts. In October, Pirazzi was given a four-year ban backdated to the date of the original announcement – as a result, he was unable to compete again until May 2021. Following the completion of his ban, he joined the team, making his first start at the
Mercan'Tour Classic Alpes-Maritimes. ==Major results==