There was a space in french translation so I missed it
In more information tooltip :

Here's how it looks now :

AI talk is about the eventuality that more hard counters are added
But the proposal itself doesn't affect AI yes

Could also consider to delay reaction of the AI by a multiple of AI difficulty, with 0 for the very hard one

By the first 2 screenshots, i meant overall in this differential. I posted 5 in total.
If unit classes are not in yellow wouldn't it look weird if line is inbetween attack and armor tooltip, with nothing other than digits in white ? The first 2 screenshot show the result if i put it on the top, where i would say white font looks less weird but putting counters here is also not logically pertinent. In the third screenshot the line is after attack tooltip and in white(but with "against" that is unnecessary and removed now)
I didn't find the missing semi-colon.
For effectiveness of petra, true it would reduce it but it can be possible to make him train counters to his opponent units (AoM AI does it IIRC). Of course, that AI train units well doesn't mean it would use those units well. (also i think AI has a big room for improvement)
And now i just noticed i removed sprintf where i shouldn't

What do you prefer ? As done in first 2 screenshots, or in these :

Here is how it looks after moving the line down attack :

Also for the if(bonus.Classes) code, I put it there because before i saw it in DamageBonus.js code in simulation, but didn't understand why it was there, if there are Bonuses in the template there should always be at least a bonus, which should always contain Classes and Multiplier, so maybe it can just be removed, didn't try it.

Now it works like it should. Here are the screenshots (where i didn't bother putting my game in english). First one in vanilla, second one in Delenda Est where we can have multiple counters.

Unrelated with last comment but for the noisiness, perhaps counter could be added after the details of one AttackType, without adding new lines except if it goes out of bounds (which would most of the time).
Something like Melee attack: 3.0 Hack, 2.5 Pierce, Rate: 1.0 Seconds, Counters: 3.0 Cavalry
But the problem is that i don't see how to harmonize it aesthetically, it would certainly look weird if i want to put unit classes in small font yellow letters

Well, indeed it got triggered, now it's template.attack (attack in lowercase).
But after correcting this, it still doesn't work, after I try warn(uneval(template.attack)) I don't find any bonuses even if some units should have them

Well, I added my function name in gui\reference\draw.js, gui\session\selection_details.js and gui\session\selection_panels.js which is where my file search lead me, but it doesn't seem to add the tooltip still. I will search some more.
Currently my code adds a new line for each attack type, but also for each counter contained in that attack type, it would maybe help to regroup counters in a line for each counters that share the same multiplier, but it would still remain quite noisy if someone wanted to regroup all possibilities in a single unit.

Not sure I understood everything but there's what i modified.
I still (temporarily ?) modify the identity tooltip property, while i search for something better.

Indeed i think that ptol barrack cost rebalance was needed