[4][5] Costa was arrested in the failed Bakuninist 1874 Bologna insurrection as its main Italian organizer.

[7] In a letter, "To My Friends and to My Adversaries", he defended himself against charges of reformism or anti-revolutionarism but effectively broke from his anarchist past.

[8] The Russian socialist Anna Kulischov, who had met Costa in Paris in 1876 and was another former Bakuninist, is believed to have spurred his transition from anarchism to socialism.

[11] His close friend and masonic brother Giovanni Pascoli wrote the funeral inscription dedicated to him,[4] whom he knew together with Alceste Faggioli when he was a university student.

[12][13] The parents of Benito Mussolini gave him the middle name "Andrea" in Costa's honour, alongside fellow Italian socialist Amilcare Cipriani.