test(engine): Riptide per-opponent target slots are scoped to that opponent (#6565)

crates/engine/src/game/ability_utils.rs

@@ -4822,7 +4822,7 @@ fn attach_host_enchant_filter(
     Some((filter, attachment_id, controller))
 }
 
-fn is_per_opponent_target_fanout(ability: &ResolvedAbility) -> bool {
+pub(crate) fn is_per_opponent_target_fanout(ability: &ResolvedAbility) -> bool {
     if ability.target_choice_timing != TargetChoiceTiming::Stack {
         return false;
     }

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

@@ -178,6 +178,21 @@ pub fn resolve(
         expected
     };
 
+    // CR 601.2c + CR 401.4 (issue #6565 / #6836): A per-opponent target fanout
+    // ("for each opponent, put up to one target ... that player controls ...")
+    // pre-selects one target PER opponent at stack time — `multi_target.max =
+    // PlayerCount { Opponent }`, so `collected_targets` already holds every
+    // chosen permanent (one per opponent). The effect's `count` (`Fixed(1)`) is
+    // the PER-OPPONENT cap, NOT the total, so it must never gate a further
+    // "choose `count` of them" prompt over the already-targeted permanents
+    // (which would loop forever and place at most one). Each pre-chosen target
+    // is placed into its own owner's library (CR 400.7, routed by the move).
+    let expected = if crate::game::ability_utils::is_per_opponent_target_fanout(ability) {
+        collected_targets.len()
+    } else {
+        expected
+    };
+
     if collected_targets.is_empty() {
         if expected == 0 {
             events.push(GameEvent::EffectResolved {

crates/engine/tests/integration/riptide_gearhulk_5994.rs

@@ -114,3 +114,72 @@ fn riptide_selects_one_opponent_target_and_declines_the_other() {
     // The declined opponent's permanent is untouched.
     outcome.assert_zone(&[p2_perm], Zone::Battlefield);
 }
+
+/// Issue #6565: "for each opponent, put up to one target nonland permanent THAT
+/// PLAYER controls ..." scopes each per-opponent target to the iterated
+/// opponent's permanents — never the caster's own. Offering the caster's own
+/// permanent as the sole target intent must leave it untouched (it is not a legal
+/// target for the opponent's slot), and the ability still resolves.
+#[test]
+fn riptide_per_opponent_slot_excludes_the_casters_own_permanents() {
+    let mut scenario = GameScenario::new();
+    scenario.at_phase(Phase::PreCombatMain);
+    scenario.with_mana_pool(P0, riptide_mana());
+
+    let my_perm = scenario.add_creature(P0, "My Own Bear", 2, 2).id();
+    let opp_perm = scenario.add_creature(P1, "Opp Bear", 2, 2).id();
+    let riptide = scenario
+        .add_creature_to_hand_from_oracle(P0, "Riptide Gearhulk", 4, 4, RIPTIDE_ORACLE)
+        .id();
+
+    let mut runner = scenario.build();
+
+    // Offer ONLY the caster's own permanent as target intent. It is not a legal
+    // target for the opponent's per-opponent slot, so that slot is declined.
+    let outcome = runner
+        .cast(riptide)
+        .target_players(&[P1])
+        .target_objects(&[my_perm])
+        .resolve();
+
+    assert!(
+        matches!(outcome.final_waiting_for(), WaitingFor::Priority { .. }),
+        "the ETB must resolve to priority, got {:?}",
+        outcome.final_waiting_for()
+    );
+    // The caster's own permanent is not controlled by the opponent, so it can
+    // never be chosen for the opponent's slot — it stays on the battlefield.
+    outcome.assert_zone(&[my_perm], Zone::Battlefield);
+    outcome.assert_zone(&[opp_perm], Zone::Battlefield);
+}
+
+/// Issue #6565: each opponent's own permanent is legal only for that opponent's
+/// per-opponent slot. Selecting both opponents independently moves both targets
+/// to their owners' libraries while the caster's own permanent remains untouched.
+#[test]
+fn riptide_per_opponent_slots_target_each_opponents_permanent() {
+    let mut scenario = GameScenario::new_n_player(3, 42);
+    scenario.at_phase(Phase::PreCombatMain);
+    scenario.with_mana_pool(P0, riptide_mana());
+    scenario.add_card_to_library_top(P1, "P1 Library Card");
+    scenario.add_card_to_library_top(P2, "P2 Library Card");
+
+    let my_perm = scenario.add_creature(P0, "My Own Bear", 2, 2).id();
+    let p1_perm = scenario.add_creature(P1, "Opp1 Bear", 2, 2).id();
+    let p2_perm = scenario.add_creature(P2, "Opp2 Bear", 2, 2).id();
+    let riptide = scenario
+        .add_creature_to_hand_from_oracle(P0, "Riptide Gearhulk", 4, 4, RIPTIDE_ORACLE)
+        .id();
+
+    let mut runner = scenario.build();
+
+    let outcome = runner
+        .cast(riptide)
+        .target_players(&[P1, P2])
+        .target_objects(&[p1_perm, p2_perm])
+        .resolve();
+
+    outcome.assert_zone(&[p1_perm, p2_perm], Zone::Library);
+    outcome.assert_zone(&[my_perm], Zone::Battlefield);
+    outcome.assert_zone(&[riptide], Zone::Battlefield);
+}