Kostornaia is the third woman after teammate Elizaveta Tuktamysheva and Rika Kihira of Japan to attempt and land the maximum number of triple jumps allowed in one senior international competition: four in the short program and eight in the free skate (see Zayak rule).

[12] Kostornaia's international debut was in early October 2017, at a 2017–18 ISU Junior Grand Prix (JGP) competition in Gdańsk, Poland; ranked first in the short program and second in the free skate, she won the gold medal by a margin of 1.36 points over the silver medalist, her training partner Daria Panenkova.

[citation needed] Kostornaia learned to perform a triple Axel in the preceding year, and originally planned to introduce it in competition.

She was ranked first in both the short program and the free skate and won the gold medal by a margin of more than 11 points over the silver medalist, her teammate Alena Kanysheva.

[17] With two JGP victories, she qualified for the 2018–19 Junior Grand Prix Final, where she won the gold medal after placing first in both the short program and the free skate.

However, on 4 March, the first day of the event, Kostornaia withdrew from the competition due to a medical condition, subsequently revealed by choreographer Daniil Gleikhengauz to be leg inflammation that required four weeks away from training.

[23] Kostornaia debuted on the senior international level at the 2019 CS Finlandia Trophy, where she placed first in both the short program and free skate.

She incorporated two triple Axels in her free skate, becoming the tenth woman in history to land the jump in an international competition, and earned a total combined score of 234.84, 22.31 points ahead of silver medalist Elizaveta Tuktamysheva.

[24] Making her senior Grand Prix debut at the 2019 Internationaux de France, Kostornaia ranked first in the short program, incorporating a triple Axel into it for the first time.

[25] She then became the fourth woman ever, after Mao Asada, Rika Kihira and Elizaveta Tuktamysheva, to land two fully rotated triple Axels in a free skate, placing first in the segment with a personal best score of 159.45 points.

However, her lead from the short program was enough to not only win the title by almost 7 points over Shcherbakova, but also to break the world record in the combined total score, previously held by Alexandra Trusova.

[29] Kostornaia competed next at the 2020 Russian National championships, where she again won the short program, setting a ten-point lead over second-place Anna Shcherbakova.

[32] She placed second in the free skate behind Shcherbakova, receiving a deduction for falling on her last jumping pass, a triple Lutz, but still scored enough to claim the European title by three points.

[33] After the 2020 World Championships were cancelled over concern about the COVID-19 pandemic, Kostornaia officially ended her season undefeated at every international competition of her senior debut.

Kostornaia opened her competitive season at the fourth stage of the domestic Russian Cup series held in Kazan, Russia on 8 November.

She won the short program by a small margin over teammate Alexandra Trusova with a score of 78.15, despite a step-out on her triple flip-triple toe loop combination and a loss of control on her Biellmann spin.

[40] She dropped to the silver medal position behind Tuktamysheva in the free skate, after underrotating three triple jumps and losing levels on several non-jump elements.

[46] In addition to her new free skate to "My Way" and "Yellow Moon" by Luca D'Alberto, Kostornaia also debuted a new short program to Antonio Vivaldi's The Four Seasons: Winter at the 2021 Russian Cup Final.

The Final was widely perceived as a contest between Kostornaia and Elizaveta Tuktamysheva for the third spot on the Russian ladies' team for the 2021 World Championships in Stockholm.

Sports.ru also reported on the situation with the additional detail that Tutberidze was hesitant to accept due to doubts that Kostornaia would be able to regain her previous form after struggling with a back injury and a stress fracture on her leg.

At her first international assignment of the season, the 2021 CS Finlandia Trophy, Kostornaia did not attempt the triple Axel in the short program, opting instead to skate cleanly under the advisement of her coaches,[54] and placed second in the segment behind Russian teammate Elizaveta Tuktamysheva.

In the free program, Kostornaia struggled with her technical elements, placing fourth in the segment, but managed to remain on the podium, winning the bronze medal behind training mates Kamila Valieva and Tuktamysheva.

On 26 October, it was announced by Match TV that Kostornaia had chosen to change both of her programs in advance of Skate Canada, switching to a short program set to "New York, New York", and revisiting the free skate choreographer Daniil Gleikhengauz had originally set for her for the 2020–21 season to three variations of Billie Eilish and Khalid's "Lovely.

Kostornaia later confirmed in a video on her YouTube channel that the injury was the result of a bad fall on a failed triple Axel attempt.

Kostornaia/Kunitsa appeared at the 2024 Russian test skates, but subsequently withdrew from the rest of the season due to an unspecified illness contracted by Kostornaia.

She is often compared to her training-mates Anna Shcherbakova and Alexandra Trusova as the three entered the international junior and senior competitive circuits together and for many seasons were considered one another's primary rivals by both fans and specialists.

Of the three (sometimes referred to as the 3A as their names all begin with the letter A), Kostornaia is widely considered the most artistic, with Trusova being her technical counterpart and Shcherbakova falling somewhere in the middle.

At the same time, she does not forget about basics such as balance which viewers of the sport admire as well as her presentation of fine emotional qualities in the execution of her programs.

Silver Doe awards are presented annually by the Federation of Sports Journalists of Russia in recognition of distinguished athletic achievement.