The formation and evolution of the first supermassive black holes have been put in the spotlight after the discovery of few hundreds extremely massive quasars at high redshift. An interesting twist in our understanding of the matter was introduced by the discovery of an unusual number of jetted sources: X-ray observations of blazars (i.e. AGN with jets aligned to our LoS) at z>4 prove that the most massive active black holes are preferentially hosted in jetted quasars in the first Gigayear from the Big Bang.

Jets might thus play a crucial role in fast assembling and accreting matter onto supermassive black holes. Investigating their occurrence and activity is not straightforward at high z, since the radio-loudness (i.e. the most commonly used tracer of jet presence) is not reliable anymore. Systematic searches of jets in high-z quasars are therefore paramount ingredients to study this population.

I will present our current knowledge about z>4 blazars, a population that provides a comprehensive view on jet, accretion and mass features of the M>10^9Msun jetted quasars.

I will also dive into both differences and common points between low and high redshift blazar populations, other than into the inconsistencies that arise from low and high-frequency observations: do jet features change across cosmic time? What is their role in the evolution of the first supermassive black holes of our Universe?