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0031: Cost changes ​

Status: Accepted. Item 6 amended by decision 0033 (no hybrid or Phyrexian symbols in cost values); item 2 clarified by decision 0032. Free casts settled by decision 0036.

Context ​

The format records what a player pays for a spell ("PlayerA" pay {B} by cost:#2) but not what decided that amount. Effects change costs all the time in constructed play: Thalia, Guardian of Thraben ("Noncreature spells cost {1} more to cast"), Trinisphere ("each spell that would cost less than three mana to cast costs three mana to cast"), and cost reducers such as Goblin Electromancer. Under Trinisphere, Thoughtseize is paid with three mana, and nothing in the file says why. A reader has to know every card on the battlefield and the order the rules apply them in to explain the payment, which is the "record why, not just what" gap the format exists to close.

Static abilities in general are not recorded yet (spec section 15). Power and toughness were the first exception (decision 0027), because their results matter to every reader. Costs are the second, for the same reason.

Decision ​

  1. Each effect that changes the mana component of a spell's or ability's total cost is one line: OBJREF cost OLD -> NEW CAUSE. This is the same shape as pt OLD -> NEW and life OLD -> NEW: a value before, a value after, and what changed it.
  2. The lines come after the object's choices and targets and before its pay lines, in the order the effects were applied: increases, then reductions in the order the player chose, then effects that apply to the total cost, such as Trinisphere.
  3. The first OLD is the mana cost or alternative cost plus any additional costs chosen; each later OLD is the previous NEW.
  4. The cause is the object whose ability changed the cost.
  5. When no effect changes a cost, nothing is written. The pay lines are enough.
  6. Costs use cost notation (CR 107.4), which has generic amounts, unlike MANA, which lists the six types of mana actually spent (decision 0006). The generic amount comes first as one number, then {W}{U}{B}{R}{G}{C}, then other symbols in the order the mana cost lists them. X is replaced by its chosen value. A cost reduced to nothing is {0}.
  7. Changes to non-mana costs (paying life, sacrificing) stay in "Not yet covered" until a recorded game needs them.

Rules basis ​

Checked against the Comprehensive Rules effective 2026-09-25.

  • 601.2f: "The total cost is the mana cost or alternative cost (as determined in rule 601.2b), plus all additional costs and cost increases, and minus all cost reductions. If multiple cost reductions apply, the player may apply them in any order. ... Once the total cost is determined, any effects that directly affect the total cost are applied. Then the resulting total cost becomes 'locked in.'" This gives the order of the lines, and puts Trinisphere last.
  • 601.2c, 601.2h: targets are chosen before the total cost is determined, and the total cost is paid after it. This gives the lines' place.
  • 118.7: "What a player actually needs to do to pay a cost may be changed or reduced by effects. If the mana component of a cost is reduced to nothing by cost reduction effects, it's considered to be {0}. Paying a cost changed or reduced by an effect counts as paying the original cost."
  • 118.9d: "If an alternative cost is being paid to cast a spell, any additional costs, cost increases, and cost reductions that affect that spell are applied to that alternative cost." This is why the first OLD can be an alternative cost.
  • 107.4, 107.4b: the cost symbols; "Numerical symbols (such as {1}) and variable symbols (such as {X}) represent generic mana in costs."
  • 602.2b: activating an ability follows rules 601.2b to 601.2i, so the same lines apply to abilities.

Alternatives rejected ​

  • Writing only the final total cost (cost {2}{B}). Rejected: it says what was paid but not why. With Thalia and Trinisphere both out, a reader could not tell which of them mattered.
  • Writing the cost on every spell. Rejected: most spells' costs are never changed, and their pay lines already show it. A line only when something changes keeps files short and every line meaningful.
  • Putting the cause on the pay line (pay {U}{B}{B} by cost:#2 by #31). Rejected: one payment can be shaped by several effects, and a payment's cause is the cost it pays.
  • Recording the static abilities themselves as effects. Rejected for now: that is the general static-ability design still listed in section 15. Recording their result where it matters, as with power and toughness, needs no rules engine in the writer.

Consequences ​

  • The spec defines the line in section 7.6, with grammar for cost notation (section 16).
  • examples/more-situations.md shows Thalia and Trinisphere together.
  • Casting a spell "without paying its mana cost" (cascade, suspend) is an alternative cost of {0} that Trinisphere raises to {3}. How such casts are recorded is a separate question, not settled here.