Szczesny, elevated to Arsenal's first choice at just 20 years of age following injury to his fellow Pole Lukasz Fabianski and Manuel Almunia's loss of form, was at the heart of the defensive mix-up that led to Obafemi Martins scoring Birmingham's winner two minutes from time.
Foster, who used an I-Pod to remind himself of how Tottenham Hotspur players took penalties before helping Manchester United win the League Cup two years ago in a penalty shootout, was named man of the match.
"He was absolutely brilliant," his manager Alex McLeish told reporters. "He showed how good he is even saving balls with his face.
"He has been awesome, he was solid, he's a very good goalkeeper.
"His kicking tactically was excellent today - we worked on some things, and he kicked very accurately which was part of the plan," said the Scot, adding that it was probably the greatest achievement of his career.
"Relatively speaking it is," he said. "For me to come to England and for a small club like Birmingham to beat mighty Arsenal, well no-one gave us a prayer," he said.
OUTSTANDING SAVES
Foster made three outstanding saves from Samir Nasri and stopped what looked like certain goals from Andrei Arshavin and Nicklas Bendtner but he had no chance of stopping Robin van Persie's excellent equaliser.
Szczesny was at fault as Arsenal failed to end their six-year wait for honours.
He was lucky to avoid a red card and conceding a penalty in only the second minute when he clattered into Lee Bowyer as the Birmingham midfielder advanced on goal.
A moment before Szczesny brought Bowyer down, the linesman wrongly flagged that Bowyer was offside in the build-up but TV replays showed he was well onside.
But while Szczesny escaped then, he could not stop the towering Nikola Zigic using his full 6ft 8ins height to head Birmingham ahead after 28 minutes.
He saved his worst moment for the 89th minute when both he and Laurent Koscielny failed to clear a tame header from Zigic and Martins swept the loose ball into the empty net.
Arsenal manager Arsene Wenger rued the error.
"My view is that there was a lack of communication and determination because when the ball is in no-man's land, someone has to take responsibility and go for it really," Wenger said.
"What was amazing is that no-one from Birmingham was going for it either."
