casting-stack-conditions

Casting, Stack & Condition Systems

Hard rules — casting, stack, and priority are the most rules-dense areas of MTG (see CLAUDE.md § Design Principles):

1. CR-correctness is non-negotiable. Casting (CR 601), activated abilities (CR 602), triggered abilities (CR 603), static abilities (CR 604), mana abilities (CR 605), stack resolution (CR 608), and priority (CR 117) are precisely specified. Every implementation must be verified against the relevant CR section and annotated. If you cannot cite the CR that governs a behavior, you have not validated it.
2. Build for the class, not the card. Payment helpers (pay_mana_cost, pay_ability_cost), targeting, and condition checks are shared building blocks used by every spell and ability. Changes here must work for the entire class — never special-case a single card's casting behavior.
3. Test the exact rules behavior. Every new WaitingFor state, casting step, or condition check needs tests that verify the CR-specified behavior, including edge cases (e.g., fizzle on illegal targets, intervening-if re-check at resolution). Test the building block, not one card's interaction.

Reference for the spell casting pipeline, stack resolution, sub_ability chaining, and all condition systems. Use this to understand how spells go from hand → stack → resolution, and where to extend for new casting features.

Before you start: Trace Ward for a casting-time interaction, or TriggerCondition::LifeGainedThisTurn for a condition that's checked at both trigger and resolution time.

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Casting Flow — crates/engine/src/game/casting.rs

Entry point: handle_cast_spell(state, player, card_id, events)

CastSpell action
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1. Find card in hand (or command zone for Commander)
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2. Get ability: obj.abilities[0].clone()
   (vanilla permanents get placeholder Unimplemented)
   NOTE: Modal spells (obj.modal.is_some()) skip to step 2b
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2b. Modal detection → WaitingFor::ModeChoice
   Player selects modes → handle_select_modes() builds chained ResolvedAbility
   → proceeds to targeting (walks sub_ability chain for first target filter)
  │
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3. Casting prohibition checks (prepare_spell_cast)
   ├─ Zone castability (hand, command, exile w/ permission, graveyard w/ escape/permission)
   ├─ CantCastFrom statics (Grafdigger's Cage) — is_blocked_from_casting_from_zone()
   ├─ CantCastDuring statics (Teferi) — is_blocked_by_cant_cast_during()
   ├─ PerTurnCastLimit statics (Rule of Law) — is_blocked_by_per_turn_cast_limit()
   │  └─ Uses ProhibitionScope (Controller/Opponents/AllPlayers) + optional spell_filter
   └─ Temporary zone restrictions — is_blocked_by_cast_only_from_zones()
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4. Validate timing
   ├─ Instant / Flash → anytime
   └─ Sorcery-speed → main phase + empty stack + active player
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5. Commander color identity check (Commander only)
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6. Calculate mana cost (base + commander tax if from command zone)
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7. Build ResolvedAbility from AbilityDefinition
   └─ Recursively converts sub_ability chain via build_resolved_from_def()
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8. Handle targeting
   ├─ Auras: extract Enchant keyword filter
   └─ Others: extract_target_filter_from_effect()
       ├─ 0 legal targets → error
       ├─ 1 legal target → auto-assign, proceed
       └─ >1 → WaitingFor::TargetSelection
  │
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9. pay_and_push()
   ├─ X in cost → WaitingFor::ManaPayment
   ├─ pay_mana_cost() — shared mana payment building block (see below)
   ├─ Move card to Zone::Stack
   ├─ Record commander cast if applicable
   └─ stack::push_to_stack() creates StackEntry
  │
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10. Return WaitingFor::Priority