Can a Barbarian have 26 Con at lv20?

If you picked up a ‘wasted’ half feat whilst levelling that has +1 con in it when you already have 20

If you pick a Con Epic Boon (say Fortitude)

When you get to the +4 con capstone, in what order do things apply? Because really interestingly the capstone specifically says that max Con increases to 25 (which makes sense to not confuse players who just got 21 in con a level ago)…

But epic boons say they can increase a stat up to 30 and so if the max Con has increased, can the previously wasted +1 Con from the Half-Feat apply before the Epic Boon bringing you up to 26 Con?

Which with Tough as the starting feat and Dwarf as race would bring the total to 405hp without magic items.

Nonono, the barbarian capstone feature itself makes the max 25 fo it would be able to go that far and then epic boon increase applys after that

That makes it sound as if once you have recieved  a feat that is in the past. What happens if you no longer meet a prerequistie? (I know in 2014 you temporarily no longer had access)

It seems reasonable that the feat increased your Con outside of the measurable parameters at the time—the 21 was there however you were unable to count/apply it until the upper limit also increased. I doubt you will find any rules supporting this anywhere however I also doubt you will find any specifically forbidding it. Check with your DM. It’s a judgement call and I don’t really see the harm in letting a character get full value for their few precious feats despite the wonky timing. How often do you even get to play at level 20? Might as well make it worthwhile ^.^

But feats say they can't increase an ability score over 20. There is no notion of a maximum, feats just don't let you do that.

Quote from Jurmondur >>

But feats say they can't increase an ability score over 20. There is no notion of a maximum, feats just don't let you do that.

Apologies. I’m still playing 2014 so not very familiar with the new rules, specifically the epic boons. OP seemed to indicate the epic boon lifted that restriction. Upon closer inspection, I see that the epic boons only let the feat that they award increase an ability score up to 30 rather than changing any condition for all ASI’s and feats. You are correct. 

However barbarian capstone does lift the normal limit to 25

Quote from Kaynor12 >>

However barbarian capstone does lift the normal limit to 25

 The text does not say that.

Level 20: Primal Champion

You embody primal power. Your Strength and Constitution scores increase by 4, to a maximum of 25.

 

Just like Monks and Barbs, they both get capstone level 20 features. +4 (to respected abilities). Only Epic Boons (level 19) allows for advancement to 30. 

Here is what the design was: At level 19 you pick an epic boon, add the modifier +1 (str/com or dex/wis) your base becomes 21, then next level add the +4 capstone (25). 
If your base was already greater then 20 due to magic items (book of…) wish, etc then you would still follow the same logic add +1 for epic boon, but the capstone would only be 25 (example your base is 22). 
So, if your base score is 21 or higher at level 19, choose a feat that would increase another useful ability. Otherwise the +4 would be a diminishing value at 20.

The reason AsI 30 for epic boons are for players who want to advance past level 20, which you can select a new boon and get your ASI to 30 in game play.

Each thing which improve your ability scores tells you how those scores are improved. The capstone doesn't mean your maximum is now 25 and general feats can carry you the rest of the way because those general feats still have their own limitations. The capstone is what lets the score improve up to the specified limit.

It's possible to have either Strength or Constitution increased to 21 at level 19, via an Epic Boon feat, and then the capstone allows for an increase up to 25.

We'll gonna see it possible when WotC release the Epic Player Handbook for LVLs 21 to 40. Or that or you'll have to buy 5 Manuals of Bodily Health.

Manual of bodily health's interaction with capstone stat bumps is interesting.

Say a level 19 Barbarian has con 20 (or 21 if they increased con with their epic boon) and then reads the manual to increase their con to 22 (or 23). When they hit level 20 there Con is capped at 25. 

If they do not read the manual until after they reach level 20 their con would go to 26 (or 27)

Quote from Jegpeg >>

Manual of bodily health's interaction with capstone stat bumps is interesting.

Say a level 19 Barbarian has con 20 (or 21 if they increased con with their epic boon) and then reads the manual to increase their con to 22 (or 23). When they hit level 20 there Con is capped at 25. 

If they do not read the manual until after they reach level 20 their con would go to 26 (or 27)

 Yes indeed