Combat: wire pet procs into the turn-based engine

Pet attacks were never resolved in turn-based fights. Roll the proc once
at fight start (a per-round roll would make a proc near-certain over a
long manual fight), persist it on the session so suspend/resume and
reaper auto-play honor the same outcome, and land a single pet hit on
the player's first acting turn.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/plugin/combat_cmd.go

@@ -91,8 +91,11 @@ func (p *AdventurePlugin) handleFightCmd(ctx MessageContext) error {
 	}
 
 	// Carry fight-start one-shot resources (Abjuration Arcane Ward, etc.) onto
-	// the session so they survive the turn engine's resume/commit cycle.
+	// the session so they survive the turn engine's resume/commit cycle, and
-	if seedCombatSessionOneShots(sess, player.Mods) {
+	// make the one-and-only per-fight pet-attack roll.
+	seeded := seedCombatSessionOneShots(sess, player.Mods)
+	pet := rollCombatSessionPetProc(sess, player.Mods)
+	if seeded || pet {
 		if err := saveCombatSession(sess); err != nil {
 			slog.Error("combat: seed session one-shots", "user", ctx.Sender, "err", err)
 		}

internal/plugin/combat_engine.go

@@ -267,6 +267,10 @@ type combatState struct {
 	// the enemy would otherwise attack).
 	enemySkipFirst bool
 
+	// Phase 13 turn-based — pet attack decided once at fight start; the pet
+	// strikes once on the player's first acting turn, which clears this.
+	petProcReady bool
+
 	// Phase 10 SUB2a-ii first-attack one-shots.
 	firstAttackBonusUsed  bool
 	assassinateRerollUsed bool

internal/plugin/combat_session.go

@@ -65,6 +65,14 @@ type CombatStatuses struct {
 	// combatState, so the flag must survive the commit between them.
 	EnemySkipNext bool `json:"enemy_skip_next,omitempty"`
 
+	// PetProcReady is the per-fight pet-attack outcome. Auto-resolve rolls the
+	// pet proc every round; a manual fight can run many rounds, so the roll is
+	// decided once at fight start (rollCombatSessionPetProc) and parked here.
+	// The pet then lands a single hit on the player's first acting turn, which
+	// clears the flag — persisted so a suspend/resume or reaper auto-play sees
+	// the same outcome.
+	PetProcReady bool `json:"pet_proc_ready,omitempty"`
+
 	// Fight-scoped depleting resources — mirror the combatState charges that
 	// genuinely carry round-to-round. Seeded at fight start (Arcane Ward) or by
 	// a mid-fight !cast / !consume, restored into combatState on every resume,

internal/plugin/combat_session_test.go

@@ -425,6 +425,72 @@ func TestTurnEngine_OneShotsRoundTrip(t *testing.T) {
 	}
 }
 
+// ── Pet proc (per-fight) ───────────────────────────────────────────────────
+
+func TestRollCombatSessionPetProc(t *testing.T) {
+	// No pet proc on the player → never rolls true, never touches statuses.
+	none := &CombatSession{SessionID: "no-pet"}
+	if rollCombatSessionPetProc(none, CombatModifiers{}) || none.Statuses.PetProcReady {
+		t.Error("zero PetAttackProc should not arm a pet strike")
+	}
+	// A guaranteed proc arms the flag; the draw is deterministic per session id.
+	sure := &CombatSession{SessionID: "pet-fight"}
+	if !rollCombatSessionPetProc(sure, CombatModifiers{PetAttackProc: 1.0}) || !sure.Statuses.PetProcReady {
+		t.Error("PetAttackProc 1.0 should always arm a pet strike")
+	}
+	again := &CombatSession{SessionID: "pet-fight"}
+	rollCombatSessionPetProc(again, CombatModifiers{PetAttackProc: 1.0})
+	if again.Statuses.PetProcReady != sure.Statuses.PetProcReady {
+		t.Error("same session id should roll the pet proc deterministically")
+	}
+}
+
+// TestTurnEngine_PetStrike confirms an armed pet lands exactly one hit on the
+// player's first acting turn, then never again — the per-fight roll model.
+func TestTurnEngine_PetStrike(t *testing.T) {
+	// Pools huge so no phase can end the fight; isolate the pet behaviour.
+	sess := turnSession(CombatPhasePlayerTurn, 100000, 100000)
+	sess.Statuses.PetProcReady = true
+	player, enemy := basePlayer(), baseEnemy()
+	player.Mods.PetAttackDmg = 8
+
+	events, err := stepEngine(sess, &player, &enemy, PlayerAction{Kind: ActionAttack})
+	if err != nil {
+		t.Fatal(err)
+	}
+	petHits := 0
+	for _, e := range events {
+		if e.Action == "pet_attack" {
+			petHits++
+			if e.Damage <= 0 {
+				t.Errorf("pet_attack damage = %d, want > 0", e.Damage)
+			}
+		}
+	}
+	if petHits != 1 {
+		t.Fatalf("pet_attack events = %d on first player turn, want 1", petHits)
+	}
+	if sess.Statuses.PetProcReady {
+		t.Error("PetProcReady should clear after the pet strikes")
+	}
+
+	// Cycle back to the next player turn — the pet must not strike again.
+	for sess.Phase != CombatPhasePlayerTurn {
+		if _, err := stepEngine(sess, &player, &enemy, PlayerAction{}); err != nil {
+			t.Fatal(err)
+		}
+	}
+	events, err = stepEngine(sess, &player, &enemy, PlayerAction{Kind: ActionAttack})
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, e := range events {
+		if e.Action == "pet_attack" {
+			t.Error("pet struck twice — the roll is per fight, not per round")
+		}
+	}
+}
+
 func TestCombatSessionRNG_DeterministicPerPhase(t *testing.T) {
 	sess := &CombatSession{SessionID: "abc123", Round: 2, Phase: CombatPhaseEnemyTurn}
 	a := combatSessionRNG(sess)

internal/plugin/combat_turn_engine.go

@@ -116,6 +116,7 @@ func resumeTurnEngine(sess *CombatSession, player, enemy *Combatant, rng *rand.R
 		armorBroken:    sess.Statuses.ArmorBroken,
 		armorBreakAmt:  sess.Statuses.ArmorBreakAmt,
 		enemySkipFirst: sess.Statuses.EnemySkipNext,
+		petProcReady:   sess.Statuses.PetProcReady,
 		// Fight-scoped depleting resources + once-per-fight one-shots: restored
 		// from the persisted statuses so a charge or "already used" flag can't
 		// reset across a suspend/resume. commit writes the updated values back.
@@ -186,9 +187,61 @@ func (te *turnEngine) stepPlayerTurn(action PlayerAction) {
 		te.finish(CombatStatusWon)
 		return
 	}
+	if te.petStrike() {
+		te.finish(CombatStatusWon)
+		return
+	}
 	te.sess.Phase = CombatPhaseEnemyTurn
 }
 
+// petStrike resolves the player's pet attack for a turn-based fight. Whether
+// the pet lands a hit was decided once at fight start (rollCombatSessionPetProc)
+// and parked on the session; the pet then strikes a single time on the player's
+// first acting turn — this clears the flag so it never repeats. Damage reuses
+// the auto-resolve formula (PetAttackDmg + d5), and PetAttackDmg already carries
+// any mid-fight buff delta via applySessionBuffs. Returns true if the strike
+// dropped the enemy.
+func (te *turnEngine) petStrike() bool {
+	st := te.st
+	if !st.petProcReady {
+		return false
+	}
+	st.petProcReady = false
+	petDmg := te.player.Mods.PetAttackDmg + st.roll(5)
+	st.enemyHP = max(0, st.enemyHP-petDmg)
+	st.events = append(st.events, CombatEvent{
+		Round: st.round, Phase: turnCombatPhase.Name, Actor: "pet", Action: "pet_attack",
+		Damage: petDmg, PlayerHP: st.playerHP, EnemyHP: st.enemyHP,
+	})
+	return st.enemyHP <= 0
+}
+
+// rollCombatSessionPetProc makes the one-and-only per-fight pet-attack roll and
+// parks the result on the session. Called once at fight start. The draw is
+// deterministic — seeded off the session id on a stream distinct from the
+// per-(round,phase) combat streams — so a reaper auto-play of an abandoned
+// fight reproduces the same outcome. Returns true if the pet will attack (so
+// the caller can decide whether the session needs persisting).
+//
+// Note: only the base PetAttackProc (class/race/subclass passives) is rolled
+// here — a pet-proc buff cast mid-fight gets no fresh roll, consistent with the
+// per-fight rule. Such a buff still raises PetAttackDmg if the pet does strike.
+func rollCombatSessionPetProc(sess *CombatSession, playerMods CombatModifiers) bool {
+	if playerMods.PetAttackProc <= 0 {
+		return false
+	}
+	var seed uint64 = 1469598103934665603
+	for _, c := range sess.SessionID {
+		seed = (seed ^ uint64(c)) * 1099511628211
+	}
+	rng := rand.New(rand.NewPCG(seed, 0x9E3779B97F4A7C15))
+	if rngFloat(rng) < playerMods.PetAttackProc {
+		sess.Statuses.PetProcReady = true
+		return true
+	}
+	return false
+}
+
 // stepPlayerActionEffect resolves a !cast / !consume turn: the command handler
 // has already rolled the spell / picked the item and spent the resource, so the
 // engine only applies the HP deltas and emits the event before handing off to
@@ -219,6 +272,10 @@ func (te *turnEngine) stepPlayerActionEffect(eff *turnActionEffect) {
 		te.finish(CombatStatusWon)
 		return
 	}
+	if te.petStrike() {
+		te.finish(CombatStatusWon)
+		return
+	}
 	if eff.EnemySkip {
 		st.enemySkipFirst = true
 	}
@@ -351,6 +408,7 @@ func (te *turnEngine) commit() {
 	s.ArmorBroken = st.armorBroken
 	s.ArmorBreakAmt = st.armorBreakAmt
 	s.EnemySkipNext = st.enemySkipFirst
+	s.PetProcReady = st.petProcReady
 	s.WardCharges = st.wardCharges
 	s.SporeRounds = st.sporeRounds
 	s.ReflectFrac = st.reflectFrac