One step forward, two steps back, eh Jezza? On Monday's night TV non-debate he unmistakably aced it in the face of aggressive 'gotcha' questioning from Jeremy Paxman.
But the next morning his stumble on Radio 4's Woman's Hour managed to undo all that hard work in one fell swoop.
BBC presenter/journalist Emma Barnett asked the UK Labour Party leader how much Labour's plans to expand free childcare would cost – a policy the party had announced that morning.
'A lot' was the best he could answer. What follows is an excruciatingly long (well, it certainly feels long) back and forth between the pair in which he attempts to consult his iPad and she calls him out live on air. Watch the footage here:
'It will cost erm. It will obviously cost a lot to do so, we accept that,' said Corbyn, to which Barnett replied: 'I presume you have the figures?'
'Yes, I do. It does cost a lot to do it, the point I'm trying to make is that we're making it universal so that we are in a position to make sure that every child gets it and those that can, at the moment get free places will continue to get them, those that have to pay won't and we'll collect the money through taxation, mainly through corporate taxation,' he replied.
Asked again, Mr Corbyn said: 'I'll give you the figure in a moment.' After being challenged numerous times the Islington MP asked: 'Can we come back to that?
Ms Barnett eventually quoted figures at him from Shadow Education Secretary Angela Rayner, putting the cost at £2.7billion, adding: 'Does that sound about right?
Here's the full transcript:
After the show, however, Barnett received a torrent of online abuse from Corbyn supporters, with many labelling the journalist 'biased' and 'unfair'. One Twitter user also called her a 'Zionist shill.'
Corbyn was quick to condemn the negative comments aimed at Barnett and defended her line of questioning.
'If you don't like what a reporter says or asks of me, or anybody else, understand the question they're asking, we will all do our best to answer those questions,' he said yesterday afternoon at a Labour campaign event in north London.
'But under no circumstances whatsoever should anyone throw personal abuse at anyone else because they're doing the job they they've been employed to do.'
Corbyn also apologised for the cost omission in his earlier radio interview.
He said: 'I didn't have the exact figure in front of me, so I was unable to answer that question, for which obviously I apologise. But I don't apologise for what's in the manifesto and I will explain exactly what the cost is.'
The whole thing is embarrassing, sure, but it also feels widely blown out of proportion. The same 'gaffe' happened to Diane Abott when she forgot how much Labour would have to spend to reverse the government's cuts to policing.
Yes, Corbyn should have had that piece of information to hand, but by asking these questions are commentators simply turning the election into a memory test? Lest we remind them: politicians aren't computers.
The ability to remember a figure isn't particularly relevant criteria for being, y'know, a skilled Prime Minster.
Still, feeling overwhelmed by it all? Here's a video of Corbyn as Stormzy to soothe your mind.