Port Vale are winless in their opening five League One matches this season
Port Vale ended Birmingham's unbeaten start to the season, inflicting a narrow defeat to dump them out of the Carabao Cup in the second round.
Jaheim Headley headed in the only goal of the game from a Ryan Croasdale cross shortly before half-time, after the Valiants had a string of chances from set-pieces.
Ronan Curtis missed a golden chance to make it 2-0 and wrap up victory at their Championship opponents, but his strike rolled fractionally past the post.
League One Port Vale will now go into the bag for the third-round draw on Wednesday for only the third time since 2009-10.
Blues have picked up seven points from their opening three games following their return to the second tier.
They also beat Sheffield United in the EFL Cup first round, while Darren Moore's Valiants are yet to win in the league this season.
Birmingham boss Chris Davies made 10 changes to the side which beat Oxford United on Saturday and it was two debutants who created the best early chance.
Marvin Ducksch linked up with Lewis Koumas and the on-loan Liverpool teenager shot from a tight angle past the near post, before seeing another effort go wide.
Cameron Humphreys had a shot deflected over the bar at the other end from a free-kick as Port Vale started to push forward. It was from set-pieces where they looked most threatening, coming close from a number of corners.
Blues keeper James Beadle made two diving saves to deny Humphreys' header and one from Mo Faal while Croasdale also had a chance that went wide, before Headley finally found the net.
The ball bounced out of the box and when it was crossed in a second time by Croasdale, Headley's diving header from close range this time did not miss.
Forward Jay Stansfield was brought on as one of three changes at the break for Birmingham but they were unable to muster a shot on target in the whole game.
Curtis was found in space as the best chance again fell to Port Vale, and though he sent Beadle the wrong way his effort was off target.
Birmingham piled on the pressure late on as expected, but Port Vale were able to close out an impressive win.
'We looked disjointed' - Davies
Birmingham manager Chris Davies told BBC Radio WM:
"There's disappointment there, that was well below our normal level.
"I made 10 changes to the team, a lot of new faces, a few debuts so there was a lot going on. We looked quite disjointed, we weren't quite as fluid as we normally are.
"We know we can play better than that and they can play better than that although there was a lot of change. Some of them haven't played a lot and we're trying to bring it all together with three games in a week to make sure we have enough energy for all three.
"We couldn't quite find that rhythm that we normally have and that continued into the second half despite a few substitutions. We couldn't find that normal presence around their box and threat on their goal."
Davies: 'Well below our normal level'
Port Vale manager Darren Moore told BBC Radio Stoke:
"Really pleased, I said to every single one of them what a performance tonight.
"Both in and out of possession of the ball, the team done really well, the structure was good, the organisation was good but within that they've got to do their jobs and they did that.
"Once we got to about 20, 25 minutes we had the pace of the game and all of a sudden we started putting our foot on the ball and gaps started to appear and more room to manoeuvre.
"All evening from set plays we looked really dangerous and the winning goal came from the second phase of a set play."
Moore: 'A real solid togetherness performance'
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