I've been musing about Kaveh and Alhaitham's behaviour towards each other in the recent AQ, and why their interactions seemed just as fractious and acrimonious as their initial introduction together in the first Sumeru AQ and before the epilogue of A Parade of the Providence. It's a noticeable contrast to the quieter moments of shared camaraderie and harmony shown between them in Cyno's SQ2, the Simulanka WQ An Odd Textual Mystery, and even their teasing (but mostly good-humoured) banters in the Ruins Exploration event—so it wouldn't be odd to assume they were on marginally better terms with each other by now; that their relationship had become less turbulent over time.
In this AQ however, they were much more argumentative and pedantic of each other's habits (specifically during the first part of Act 10). We see them flit between their usual snippy quibbles over mundane things to a growing hostile vexation, before finally shifting to a more attentive, professional seriousness and scholarly rigour while they worked together to decipher the ancient desert scripts encoded in Aaru's technology and to rewrite the code for reintegration with the old Akasha system. It was only after they had completed the first phase of the project and during the conversation with Tighnari that we see underlying tensions have arisen between them again due to having to work closely with each other. The subject matter of the current project being all-too-similar to their old research collaboration must have also dredged up memories (and wounds) of their old falling out, and likely contributed to those tensions as well.
In fact, they didn't even try to hide their irritation and displeasure from each other—with Kaveh openly grousing about having to deal with Alhaitham's difficult personality (in CN, he says "脾气 / pí qì", i.e. temperament) and how "some people never change", right before Alhaitham cuts in to agree and then retort back that the same applies to Kaveh, because he too retains his bad habit of complaining about Alhaitham to their friends while Alhaitham is still within listening distance.
This might look like a stagnation, a regression of their relationship—how they can never get past a sentence or two before the conversation inevitably spirals into yet another counter-productive debate peppered with ad hominem attacks on each others' moral character and personality defects. And perhaps they did regress in a sense. Still, I do think that's also done by intention of the narrative. That they were only much more quarrelsome, more dismissive of each other here due to apprehension and the highly-stressful uncertainties of the situation: They've all had to leave their homes and escape into the desert due to the threat of mass psychosis inflicting the populace from the corruption in the Ley Lines; their Archon is in a literal coma at this point, and they have yet to hear of her current condition even with Apep's help; they'd had to fight off berserk Jinns/monsters from attacking civilians just days(?) before they reached the Temple of Silence and the relative safety of King Deshret's Mausoleum; Sumeru is isolated from the rest of the world, poised precariously at the edge of destruction from Irminsul's spreading corruption.
And this is mostly conjecture on my end—but up till this point, I imagine Kaveh and Alhaitham had also actively avoided getting involved in any academic collaboration together since they began living with each other. Perhaps they were wary of doing any serious form of joint research together again—it's easier to stick to arguing over mundane domestic things like ugly wood cravings, or the noise from late-night hammering projects, or whose turn it is to do a grocery run and refill the coffee beans. Annoyances born of daily trivialities tend to be fleeting, after all, and seemed less ruinous to their tentative truce and reconciliation than scholarly contentions have proven to be.
Tighnari probably also figured out the source of their irritability and hostility at each other (since he'd pointed out earlier that he thought they had already "worked things out" between them before, so that makes me think that he'd noticed their relationship did improve slightly over time) and tried to offer a solution by inviting one(?) of them to join his forest patrols instead, to put some distance and help reduce the friction between them.
But instead of jumping at the opportunity to avoid working with each other, both Alhaitham and Kaveh declined because their "differences of opinions will always exist"/"there's zero chance of us ever resolving our differences", and that changing their jobs to avoid a compromise won't just magically make these differences disappear. As strange as it sounds, I think this actually shows how much they have changed from their time as students. Sure, they still debate and criticise each others' views, and they still get annoyed by each other's habits and methods of approach.
Still, at the end of the day, time has changed them too, with the maturity of being older and wiser to the follies of youthful pride. They realise that they can't change the other and have accepted each other as they are, bad habits and all. And as scholars, they also realise the importance of the Aaru project and how much the Akademiya (and the whole of Teyvat, really) relied on them to find a breakthrough together, hence why Kaveh eventually concludes with "Even so, the work must be done", and decides to stop the bickering by extending an olive branch and inviting Alhaitham to get coffee together.
And Alhaitham, being infuriatingly cheeky, still can't help but lightly tease Kaveh for being tsundere "making progress" and basically asking him out for a coffee date, even though Kaveh had been complaining about him just moments before lmfao.
Senpai, just admit you actually like hanging out with him and find his presence and "difficult" personality comforting... :")
**Note: For the record, I also do acknowledge that Kaveh and Alhaitham's "bickering old married/divorced couple" act can sometimes come across as character regression/stagnation because hyv is lazy with their writing likes to play up certain tropes/gags for comic relief and fanservice (and Kavetham aren't even the only characters who suffer from this tbh), so there's that too. But despite that, I still do think Kaveh and Alhaitham's character writing and relationship progression was done well enough, considering all things. Otherwise, I wouldn't be keysmashing my tl;dr feels here and writing fics lol.
