“Mono-Blue Terror is a tempo-combo hybrid that uses cheap self-mill spells (Thought Scour, Mental Note) to rapidly fill the graveyard, enabling Tolarian Terror and Cryptic Serpent to hit the board as massive threats for very little mana. The deck pairs these threats with a permission suite headlined by Counterspell and Force Spike to protect its board and disrupt the opponent while Delver of Secrets provides an additional early clock. The engine lives and dies on graveyard density, so the mill-cantrip package and the cost-reduction threats are inseparable and must be preserved at all costs.”
Missing sideboard cards? The full 15-card sideboard runs about $45.53.
Buy sideboard on ManapoolMono Red Madness wins by resolving cheap red spells and burning you out before you stabilize, so Hydroblast and Blue Elemental Blast are premium answers that counter or destroy their red spells and creatures like Kessig Flamebreather and Guttersnipe at a mana advantage. Trimming Thought Scour slows our self-mill slightly but we have plenty of redundancy, and cutting Spell Pierce (easily paid around once they're flooding mana into burn) is low-impact since it rarely stops their key spells. Note Spell Pierce isn't actually in the mainboard list as removable beyond its single copy, so only 1 is cut; the extra 3 come from Thought Scour. Keep Counterspell and Dispel in full to answer Fiery Temper and Fireblast, and prioritize killing Guttersnipe/Kessig Flamebreather on sight since they turn every spell into damage.
Dimir Control doesn't lean on artifacts or enchantments heavily (just Bojuka Bog is a land, not a spell target), so Annul and Steel Sabotage are actually weak here — reconsider. This matchup is really a mirror-adjacent grind: they have Counterspell, Snuff Out, and a full suite of card advantage, so the game is won by resolving our mill-cantrip engine faster and protecting Tolarian Terror/Cryptic Serpent with Counterspell and Dispel. Bring in nothing from the sideboard since none of our artifact/enchantment or anti-red tools are live threats here; instead just tighten the maindeck. Shave Deem Inferior for extra copies of Mental Note/Thought Scour is not needed since those are already maxed, so the best plan is a near-zero change: keep the 75 as is, since no sideboard card improves on what's already present.
Affinity is an artifact-dense deck built around Thoughtcast, Cryogen Relic, Refurbished Familiar, and a host of artifact lands, making Annul and Steel Sabotage excellent. Annul counters their artifact spells like Cryogen Relic, Refurbished Familiar, or Utrom Monitor before they resolve (note that Thoughtcast is a sorcery, so Annul can't target it), while Steel Sabotage can counter an artifact spell or bounce a problematic artifact creature like Refurbished Familiar or Utrom Monitor back to hand. Dispel is weak here since Affinity runs very few instants, and Deem Inferior is a slow answer against a low-curve aggressive deck, so both are the easiest cuts. Keep Counterspell up to answer their bigger threats and hold Lorien Revealed/Brainstorm to find Terror/Serpent quickly, since racing their go-wide artifact board is key.