This matchup is a race against a fast, damage-based combo-aggro deck, so maximize cheap interaction that answers their red spells and creatures directly. Blue Elemental Blast and Hydroblast can counter or destroy key threats like Guttersnipe and Kessig Flamebreather, or counter burn spells like Fiery Temper and Lightning Bolt outright. Cut slower card-selection pieces like Abandon Attachments, Spell Pierce (too easy to play around when they're tapped out on cheap spells), and Plunder the Trollshaws since they don't affect the board fast enough. Keep Snuff Out and Counterspell as premium answers to their threats.

Dimir Control plays very few artifacts (mostly none), so Steel Sabotage is largely a dead card here and should stay in the sideboard; reconsider this plan. Against a mirror-like control deck full of countermagic and removal, the game is decided by who generates card advantage and resolves threats, so Thorn of the Black Rose is excellent since becoming the monarch provides a persistent card advantage engine they'll have to attack you to remove. Abandon Attachments and Spell Pierce are the weakest links against a deck with plenty of mana and its own counterspells to play around, so trim them for Thorn of the Black Rose and an extra card-advantage piece.

This mirror-ish matchup is a race to fuel Tolarian Terror and Cryptic Serpent while disrupting the opponent's mill-cantrip engine. Nihil Spellbomb lets us exile their graveyard to strip discounts from Tolarian Terror and Cryptic Serpent, undercutting their whole plan, so bring in 2 copies over the clunkier Abandon Attachments. Keep the core Counterspell and Snuff Out suite intact since we need to answer their Serpents and Terrors while protecting our own.