fix(engine): Metamorphic Alteration as-enters choose + host copy (#6013)

client/src/adapter/types.ts

@@ -1639,7 +1639,7 @@ export type WaitingFor =
   | { type: "GameOver"; data: { winner: PlayerId | null } }
   | { type: "ReplacementChoice"; data: { player: PlayerId; candidate_count: number; candidates?: ReplacementCandidateSummary[] } }
   | { type: "OrderTriggers"; data: { player: PlayerId; triggers: PendingTriggerSummary[] } }
-  | { type: "CopyTargetChoice"; data: { player: PlayerId; source_id: ObjectId; valid_targets: ObjectId[]; max_mana_value?: number | null } }
+  | { type: "CopyTargetChoice"; data: { player: PlayerId; source_id: ObjectId; valid_targets: ObjectId[]; max_mana_value?: number | null; purpose?: { type: "BecomeCopy" | "PersistChosenAttribute" } } }
   | { type: "ExploreChoice"; data: { player: PlayerId; source_id: ObjectId; choosable: ObjectId[]; remaining: ObjectId[]; pending_effect: unknown } }
   | { type: "ReturnAsAuraTarget"; data: { player: PlayerId; source_id: ObjectId; returned_id: ObjectId; legal_targets: TargetRef[]; pending_effect: unknown } }
   | { type: "EquipTarget"; data: { player: PlayerId; equipment_id: ObjectId; valid_targets: ObjectId[] } }

crates/engine/src/analysis/ability_graph.rs

@@ -884,6 +884,9 @@ fn effect_projection(effect: &Effect) -> Projection {
         | Effect::HideawayConceal { .. }
         | Effect::CopyTokenBlockingAttacker { .. }
         | Effect::BecomeCopy { .. }
+        // CR 707.2c (Metamorphic Alteration): as-enters copy choice — no repeatable
+        // modeled axis at the candidate stage.
+        | Effect::ChoosePermanent { .. }
         | Effect::GainActivatedAbilitiesOfTarget { .. }
         | Effect::ChooseCard { .. }
         | Effect::DoublePT { .. }

crates/engine/src/game/ability_rw.rs

@@ -2710,6 +2710,9 @@ fn legacy_continuous_modification(m: &ContinuousModification) -> bool {
             legacy_quantity_expr(value)
         }
         ContinuousModification::CopyValues { .. }
+        // CR 707.2c (Metamorphic Alteration): inert parse-time copy marker — no
+        // frozen event-context tag.
+        | ContinuousModification::CopyChosen
         | ContinuousModification::SetName { .. }
         | ContinuousModification::SetTextName { .. }
         | ContinuousModification::AddPower { .. }
@@ -3159,6 +3162,9 @@ fn legacy_effect(x: &Effect) -> bool {
         | Effect::CastFromZone {
             target, duration, ..
         } => legacy_target_filter(target) || odur(duration),
+        // CR 707.2c (Metamorphic Alteration): the copy-source choice pool is a
+        // target filter; walk it for legacy event-refs like every other filter.
+        Effect::ChoosePermanent { filter } => legacy_target_filter(filter),
         Effect::GenericEffect {
             duration,
             target,
@@ -5482,6 +5488,11 @@ fn rw_effect(
         // and may add counters from a live property read. Fail closed until the
         // copy/counter sub-steps have a precise profile.
         | Effect::EachPlayerCopyChosen { .. }
+        // CR 707.2c (Metamorphic Alteration): raised only from the Aura-ETB
+        // replacement path; on answer it installs a Layer-1 copy on the Aura's
+        // host. Never resolves through the intra-ability chain — fail-closed
+        // conservative keeps the exhaustive match honest.
+        | Effect::ChoosePermanent { .. }
         | Effect::Myriad
         | Effect::Encore
         | Effect::CombineHost { .. }

crates/engine/src/game/ability_scan.rs

@@ -890,6 +890,13 @@ fn scan_effect(x: &Effect, mode: ScanMode) -> Axes {
             acc
         }
         Effect::BecomeCopy { .. } => Axes::CONSERVATIVE,
+        // CR 707.2c: the chosen creature's copiable values are latched onto the
+        // Aura's host at the answer — a copy-family continuous effect, same
+        // conservative classification as `BecomeCopy`. `filter` scans no
+        // per-source projected resource (it just bounds the choice pool).
+        Effect::ChoosePermanent { filter } => {
+            scan_target_filter(filter, target_ctx, mode).or(Axes::CONSERVATIVE)
+        }
         Effect::GainActivatedAbilitiesOfTarget {
             target,
             recipient,
@@ -5002,6 +5009,10 @@ fn scan_continuous_modification(m: &ContinuousModification, mode: ScanMode) -> A
         // documented fail-safe (over-veto = missed offer). A full `TriggerDefinition`
         // walker is a follow-up.
         ContinuousModification::CopyValues { .. }
+        // CR 707.2c (Metamorphic Alteration): the copy-marker stands in for a
+        // copy that grants the donor's whole ability set — fail-closed alongside
+        // its `CopyValues` sibling (the real grant is the installed TCE).
+        | ContinuousModification::CopyChosen
         | ContinuousModification::GrantTrigger { .. }
         | ContinuousModification::GrantAllActivatedAbilitiesOf { .. }
         | ContinuousModification::GrantAllTriggeredAbilitiesOf { .. }
@@ -5174,6 +5185,10 @@ fn effect_target_ctx(e: &Effect, mode: ScanMode) -> FilterReadContext {
         //     recipient set ("Shards you control", CR 707.2).
         | Effect::GainActivatedAbilitiesOfTarget { .. }
         | Effect::BecomeCopy { .. }
+        //   CR 707.2c (Metamorphic Alteration): the copy target is chosen from a
+        //     battlefield-reading filter pool; defense-in-depth parity with
+        //     BecomeCopy (fail-closed census — over-vetoes the single-host shortcut).
+        | Effect::ChoosePermanent { .. }
         //   CR 708.2 / CR 708.2a (face-down permanents): `resolved_battlefield_object_
         //     ids` (effects/mod.rs) falls through to a battlefield mass scan for a
         //     non-targeted "turn each matching creature face up/down" (Illithid
@@ -5485,7 +5500,7 @@ enum CensusRole {
 #[cfg(test)]
 fn effect_census_role(e: &Effect) -> CensusRole {
     match e {
-        // -- CENSUS (28): verbatim mirror of `effect_target_ctx`'s LiveBoardCensus
+        // -- CENSUS (29): verbatim mirror of `effect_target_ctx`'s LiveBoardCensus
         // arm - mass battlefield population reads that scale with growth.
         Effect::EachSourceDealsDamage { .. }
         | Effect::EachDealsDamageEqualToPower { .. }
@@ -5527,6 +5542,9 @@ fn effect_census_role(e: &Effect) -> CensusRole {
         | Effect::PhaseIn { .. }
         | Effect::GainActivatedAbilitiesOfTarget { .. }
         | Effect::BecomeCopy { .. }
+        // CR 707.2c (Metamorphic Alteration): parity with the `effect_target_ctx`
+        // LiveBoardCensus member added above.
+        | Effect::ChoosePermanent { .. }
         | Effect::TurnFaceUp { .. }
         | Effect::TurnFaceDown { .. }
         | Effect::MultiplyCounter { .. }
@@ -5920,6 +5938,10 @@ fn effect_resolution_choice_freedom(e: &Effect) -> ResolutionChoiceFreedom {
         | Effect::HideawayConceal { .. }
         | Effect::CopyTokenBlockingAttacker { .. }
         | Effect::BecomeCopy { .. }
+        // CR 707.2c: raises `WaitingFor::CopyTargetChoice` — prompts, fail-closed
+        // MayPrompt (never resolved through the normal chain, but classified here
+        // to keep the match exhaustive).
+        | Effect::ChoosePermanent { .. }
         | Effect::GainActivatedAbilitiesOfTarget { .. }
         | Effect::ChooseCard { .. }
         | Effect::PutCounter { .. }
@@ -6187,6 +6209,8 @@ pub(crate) fn effect_is_randomness_bearing(e: &Effect) -> bool {
         | Effect::HideawayConceal { .. }
         | Effect::CopyTokenBlockingAttacker { .. }
         | Effect::BecomeCopy { .. }
+        // CR 707.2c: choosing a permanent draws on no game randomness.
+        | Effect::ChoosePermanent { .. }
         | Effect::GainActivatedAbilitiesOfTarget { .. }
         | Effect::ChooseCard { .. }
         | Effect::PutCounter { .. }
@@ -6874,7 +6898,7 @@ mod tests {
     }
 
     /// guard#3 (mitigation #3): the `LiveBoardCensus` tag set of `effect_target_ctx`
-    /// == EXACTLY the enumeration-derived MASS-POPULATION set (28). Source-scanned, not
+    /// == EXACTLY the enumeration-derived MASS-POPULATION set (29). Source-scanned, not
     /// hand-counted (the hand-count is what produced the earlier "relax=4" miss). Under
     /// B's SnapshotOrEvent default this is the primary false-certificate gate: only a
     /// census tag vetoes a mass read that ESCALATES over inert token growth (which
@@ -6910,6 +6934,7 @@ mod tests {
             "ChangeZoneAll",
             "ChooseAndSacrificeRest",
             "ChooseObjectsIntoTrackedSet",
+            "ChoosePermanent",
             "CounterAll",
             "DamageAll",
             "DamageEachPlayer",
@@ -6949,7 +6974,7 @@ mod tests {
             got, want,
             "census tag set drifted from the enumeration-derived mass-population set"
         );
-        assert_eq!(got.len(), 28, "exactly 28 mass-population census tags");
+        assert_eq!(got.len(), 29, "exactly 29 mass-population census tags");
     }
 
     /// A7' (mitigation #4, replaces the void census-default A7): with SnapshotOrEvent the
@@ -7060,7 +7085,7 @@ mod tests {
     /// with `effect_target_ctx` on the Census/Relax boundary, closing the F1 gap where a
     /// census-ROLE slot silently in the generic relax `|`-chain (exactly R1's Suspect{All})
     /// is invisible to the census-arm-only guards. Structural: both functions' `Census`
-    /// name-sets are source-scanned and asserted IDENTICAL (== the 28). Behavioral: the
+    /// name-sets are source-scanned and asserted IDENTICAL (== the 29). Behavioral: the
     /// two oracles agree on every discriminator, incl. BOTH Suspect/Unsuspect scopes.
     ///
     /// REVERT-PROBE (discrimination proof): moving `Suspect{All}` out of the census arm of
@@ -7075,7 +7100,7 @@ mod tests {
         use crate::types::ability::{EffectScope, TapStateChange};
         use ScanMode::LoopFirewall;
 
-        // -- Structural: the two census name-sets are byte-identical (and == 28).
+        // -- Structural: the two census name-sets are byte-identical (and == 29).
         fn census_names(fnsrc: &str, terminator: &str) -> Vec<String> {
             let end = fnsrc.find(terminator).expect("census terminator");
             let block = &fnsrc[..end];
@@ -7104,7 +7129,7 @@ mod tests {
             etc_census, ecr_census,
             "effect_census_role Census set diverged from effect_target_ctx"
         );
-        assert_eq!(ecr_census.len(), 28, "exactly 28 census members");
+        assert_eq!(ecr_census.len(), 29, "exactly 29 census members");
 
         // -- Behavioral: the two oracles agree on the Census/Relax boundary for every
         // discriminator. `census(e, true)` requires BOTH `effect_census_role == Census`

crates/engine/src/game/coverage.rs

@@ -2333,6 +2333,10 @@ fn effect_details(effect: &Effect) -> Vec<(String, String)> {
         } => {
             d.push(("target".into(), fmt_target(target)));
         }
+        // CR 707.2c (Metamorphic Alteration): report the copy-source choice pool.
+        Effect::ChoosePermanent { filter } => {
+            d.push(("choose".into(), fmt_target(filter)));
+        }
         Effect::Destroy { target, .. }
         | Effect::Sacrifice { target, .. }
         | Effect::GainControl { target }
@@ -4165,6 +4169,9 @@ fn fmt_modification(m: &crate::types::ability::ContinuousModification) -> String
     use crate::types::ability::ContinuousModification;
     match m {
         ContinuousModification::CopyValues { .. } => "copy values".into(),
+        // CR 707.2c (Metamorphic Alteration): parse-time marker for the enchanted
+        // host's copy — the runtime copy is the latched `CopyValues` TCE.
+        ContinuousModification::CopyChosen => "copy chosen".into(),
         ContinuousModification::SetName { name } => format!("set name {name}"),
         ContinuousModification::SetTextName { name } => format!("set text name {name}"),
         ContinuousModification::AddPower { value } => format!("power {:+}", value),

crates/engine/src/game/effects/mod.rs

@@ -3886,6 +3886,20 @@ pub fn resolve_effect(
             copy_token_blocking::resolve(state, ability, events)
         }
         Effect::BecomeCopy { .. } => become_copy::resolve(state, ability, events),
+        // CR 614.12a + CR 707.2c (Metamorphic Alteration): `ChoosePermanent` is an
+        // as-enters replacement-effect choice, NOT a stack-resolution effect. It is
+        // raised by `apply_post_replacement_effect` and answered by
+        // `handle_copy_target_choice` (PersistChosenAttribute). It never enters the
+        // normal resolution chain, so reaching this dispatch arm is a bug.
+        Effect::ChoosePermanent { .. } => {
+            debug_assert!(
+                false,
+                "Effect::ChoosePermanent must be handled via the Aura-ETB \
+                 replacement path (apply_post_replacement_effect), never resolved \
+                 as a stack effect"
+            );
+            Ok(())
+        }
         Effect::GainActivatedAbilitiesOfTarget { .. } => {
             gain_activated_abilities::resolve(state, ability, events)
         }

crates/engine/src/game/engine_phase_trigger_regression_tests.rs

@@ -3448,6 +3448,7 @@ fn copy_target_choice_resolves_become_copy() {
         source_id: clone_id,
         valid_targets: vec![target_id],
         max_mana_value: None,
+        purpose: crate::types::ability::CopyTargetPurpose::BecomeCopy,
     };
 
     // Player chooses to copy Grizzly Bears
@@ -3542,6 +3543,7 @@ fn copy_target_choice_applies_copied_enter_with_counters_replacement_before_sba(
         source_id: assassin,
         valid_targets: vec![ghave],
         max_mana_value: None,
+        purpose: crate::types::ability::CopyTargetPurpose::BecomeCopy,
     };
 
     apply_as_current(
@@ -3615,6 +3617,7 @@ fn echoing_deeps_copying_sunken_citadel_prompts_for_the_copied_color_choice() {
         source_id: deeps,
         valid_targets: vec![citadel],
         max_mana_value: None,
+        purpose: crate::types::ability::CopyTargetPurpose::BecomeCopy,
     };
 
     let result = apply_as_current(
@@ -3922,6 +3925,7 @@ fn copy_target_choice_fires_granted_etb_trigger_against_deferred_entry_event() {
         source_id: assassin,
         valid_targets: vec![bear],
         max_mana_value: None,
+        purpose: crate::types::ability::CopyTargetPurpose::BecomeCopy,
     };
 
     apply_as_current(
@@ -4103,6 +4107,7 @@ fn copy_target_choice_surfaces_interactive_trigger_prompt_for_deferred_entry() {
         source_id: assassin,
         valid_targets: vec![bear],
         max_mana_value: None,
+        purpose: crate::types::ability::CopyTargetPurpose::BecomeCopy,
     };
 
     let _waiting = apply_as_current(
@@ -4182,6 +4187,7 @@ fn copy_target_choice_rejects_invalid_target() {
         source_id: clone_id,
         valid_targets: vec![valid_id], // Bird is NOT in valid targets
         max_mana_value: None,
+        purpose: crate::types::ability::CopyTargetPurpose::BecomeCopy,
     };
 
     // Try to choose invalid target
@@ -4332,6 +4338,7 @@ fn superior_spider_man_full_copy_flow_copies_graveyard_card_and_exiles_it() {
         source_id: spidey,
         valid_targets: vec![elesh],
         max_mana_value: None,
+        purpose: crate::types::ability::CopyTargetPurpose::BecomeCopy,
     };
 
     let result = apply_as_current(
@@ -4508,6 +4515,7 @@ fn reflexive_when_you_do_fires_after_become_copy_replacement() {
         source_id: cloner,
         valid_targets: vec![source_card],
         max_mana_value: None,
+        purpose: crate::types::ability::CopyTargetPurpose::BecomeCopy,
     };
 
     // Accumulate events across the full resolution so we can count