Speaking in the official FIA press conference in Mexico, Tost confirmed that former driver Daniil Kvyat has officially been dropped by Red Bull, and that going forwards the Italian team is looking to proceed with its current driver pairing.
"We want to test them for the rest of the season as there is a high possibility this will be the driver line-up for 2018,” said Tost.
“Both are Red Bull drivers, both are highly skilled drivers.”
While Gasly made his debut in Malaysia having already driven a 2017-spec F1 car, Hartley went into his first race – in the United States last weekend – without any contemporary experience, going on to finish 13th.
Tost said the New Zealander’s composed performance was indicative of his future potential:
“Brendon is a very high-skilled driver, he is very competitive.”
“I am really happy that he is back and I can tell you we give him a competitive car, he will be there and he will fight in F1 for success.
“I hope next year we will bring together a competitive package that he can fight for victories and other positions [with].”
Red Bull boss Christian Horner added that Hartley and Gasly “represent two exciting prospects for Red Bull Racing further down the line” and hailed Hartley’s “determination and tenacity” to fight his way to the F1 grid having been dropped from the team’s junior programme in 2007.
On the subject of Kvyat, who scored five points in 15 races this season, Tost said the Russian was now “free to do whatever he wants to do”, adding “he is not any more with Red Bull or Toro Rosso.”