This mirror-adjacent Dimir Midrange matchup is a grindy fight over card advantage and disruption, so extra Thoughtseize helps you win the information and tempo war by stripping their key countermagic or threats like Psychic Frog before they can deploy them. Harbinger of the Seas disrupts their heavy reliance on nonbasic fetches and shocklands, denying clean black mana for Fatal Push and Thoughtseize. Toxic Deluge can reset the board against their Orcish Bowmasters and Psychic Frog if they get ahead on creatures. Basim, Skateboard, and Sink into Stupor are lower-impact in this interaction-heavy mirror, so they're cut for more disruptive, proactive cards that fight through counterspells and discard.