My take on todays argument:
1. Justice Kagan taking the lead with tough questions of plaintiff's counsel, many of which are focused on issues of congress 'drafting for the future' to allow agencies to address unforeseen questions. (As always, the "who decides" question is at the forefront.) Plaintiff's counsel wasn't particularly effective in his responses IMO, in the sense that "future questions" are the sort of thing that Congress should actually speak on and Chevron gives an off ramp. He sort of reverted to 'courts should just interpret the statutory terms.' Kav's questions relatively focused on distinguishing standards of deference for various types of agency authority/action. Barrett raises spectre of more litigation. KBJ argues Chevron actually 'incents' judges to distinguish between legal and policy questions and thereby helps judges stay in their lane - a very interesting point.
2. SG Prelogar, as always, is a pro. Kav/Gorsuch asking some good questions about how in practice judges use step two to avoid step one conclusions they don't like. Basically, questions about how the line between step one and two is blurry and standardless. (Soto offers open ended opportunity to express that 'standard' affirmatively.) Thomas seems to be arguing APA 706 ("the reviewing court shall decide questions of law") is dispositive and doesn't contemplate chevron step two, but the SG asserts that that language really doesn't speak to standard of review.
3. CJ, Alito, Barrett pretty quiet, though Alito's few comments give a decent feel for where he's coming from.
Frankly, pretty hard to speculate much about outcomes based on the argument