fix(engine): evaluate 'attacks alone' relative to the attacker, not the ability source

crates/engine/tests/integration/issue_5328_attacks_alone_observer.rs

@@ -0,0 +1,74 @@
+//! Issue #5328: the "attacks alone" mechanic must be evaluated relative to the
+//! creature that attacked, not the ability's source. When the observer (here
+//! "Agent 13, Sharon Carter") stays back and a DIFFERENT creature you control
+//! attacks alone, the trigger must still fire; when two creatures attack, it
+//! must not.
+//!
+//! https://github.com/phase-rs/phase/issues/5328
+//!
+//! Root cause: `TriggerCondition::MinCoAttackers` excluded the ability SOURCE
+//! from the co-attacker tally. For a non-attacking observer, `source_id` is not
+//! in combat, so the lone attacker stayed counted and `Not(MinCoAttackers{1})`
+//! evaluated false — the "attacks alone" ability silently failed to fire.
+//! CR 506.5 + CR 702.83b: a creature attacks alone iff it is the only creature
+//! declared as an attacker.
+
+use engine::types::phase::Phase;
+
+use super::rules::{run_combat, GameRunner, GameScenario, P0};
+
+const ORACLE: &str = "Whenever a creature you control attacks alone, investigate.";
+
+fn count_clues(runner: &GameRunner) -> usize {
+    runner
+        .state()
+        .battlefield
+        .iter()
+        .filter(|id| {
+            runner
+                .state()
+                .objects
+                .get(id)
+                .is_some_and(|obj| obj.name.eq_ignore_ascii_case("Clue"))
+        })
+        .count()
+}
+
+/// Clues produced with the observer on the battlefield and `num_others`
+/// non-observer attackers (the observer never attacks).
+fn clues_when_others_attack(num_others: usize) -> usize {
+    let mut scenario = GameScenario::new();
+    scenario.at_phase(Phase::PreCombatMain);
+    // The observer stays home; only the "other" creatures attack.
+    scenario.add_creature_from_oracle(P0, "Agent 13, Sharon Carter", 2, 2, ORACLE);
+    let attackers: Vec<_> = (0..num_others)
+        .map(|i| scenario.add_creature(P0, &format!("Bear{i}"), 2, 2).id())
+        .collect();
+    let mut runner = scenario.build();
+
+    run_combat(&mut runner, attackers, vec![]);
+    runner.advance_until_stack_empty();
+    count_clues(&runner)
+}
+
+#[test]
+fn other_creature_attacks_alone_fires_even_when_observer_stays_back() {
+    // The bug: without the fix this is 0 — the lone attacker is wrongly counted
+    // as a co-attacker of the non-attacking observer, so the trigger never fires.
+    assert_eq!(
+        clues_when_others_attack(1),
+        1,
+        "CR 506.5: a single non-observer attacker attacks alone → investigate once"
+    );
+}
+
+#[test]
+fn two_other_creatures_attacking_do_not_fire_attacks_alone() {
+    // Positive control on the opposite side: two attackers is NOT attacking
+    // alone, so the ability must stay silent.
+    assert_eq!(
+        clues_when_others_attack(2),
+        0,
+        "CR 506.5: two attackers is not 'attacks alone' → no investigate"
+    );
+}

crates/engine/tests/integration/main.rs

@@ -429,6 +429,7 @@ mod issue_4921_skullscorch_unless_deal_damage;
 mod issue_4962_volo_guide_to_monsters;
 mod issue_5145_violent_eruption_choose_target_distribution;
 mod issue_5159_attacks_alone_investigate;
+mod issue_5328_attacks_alone_observer;
 mod issue_5335_stonehoof_mass_attack;
 mod issue_5336_kodama_mana_value_filter;
 mod issue_5339_armory_automaton;