Nahki Wells grabbed the only goal as Bradford ground out a victory at Northampton to stay in the play-offs.
City bossed the first half but only went ahead after the break when Wells latched on to Danny East's poor header and slotted in.
Home goalkeeper Lee Nicholls parried Craig Forsyth's fierce drive, but James Hanson could only blast the rebound over when well placed to make it 2-0.
Louis Moult went close to an equaliser when his header was tipped over.
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Northampton Town manager Aidy Boothroyd tells BBC Radio Northampton:
"We knew it was going to be a tough, physical game. We matched them and then we gave away a really poor goal.
"We're not talking about being bottom of the league. We've a few injuries and we're not getting the rub of the green.
"After Barnet, we were shell-shocked. But the over-riding emotion is disappointment.
"I know it's only the second game we've lost at home but we've got to make sure we take points out of games like this."