Does a static ability that reduces spell costs apply to spells with alternative casting costs?

When you cast a spell, you first determine its total cost, which may include an alternative cost in place of the mana cost. Static abilities that reduce costs apply during this cost-determination step, regardless of whether you're paying the normal mana cost or an alternative cost (CR 601.2f, 601.2h).

The key rule is CR 601.2f: after choosing the alternative cost (if any), you then apply any applicable cost reductions. The reduction applies to the total cost as assembled — it doesn't care whether the base you started from was the printed mana cost or an alternative cost. So if an effect says 'spells cost {1} less,' that reduction applies even when you're using an alternative cost.

However, some cost-reduction abilities are worded to specifically target the mana cost or have other restrictions. For example, an ability that says 'the mana cost of the next spell you cast is reduced by {2}' could be interpreted as only applying when you pay the mana cost, not an alternative cost. Always read the exact wording carefully (CR 601.2f).

Concrete example: You control Gobgoblin Warchief (spells cost {1} less) and cast Ancestral Vision using suspend (its alternative cost of {0} when it resolves). The {1} reduction still applies to the {0} alternative cost — but since the result can't go below zero, there's no practical effect. If the alternative cost were {2}, it would be reduced to {1}.

In summary: cost reductions generally apply to whatever total cost you end up paying, including alternative costs, unless the reduction ability's text specifically limits it (CR 601.2f, 601.2h).