Combat: persist fight-scoped one-shots + turn-based buffs

CombatStatuses now mirrors every persistent combatState one-shot — depleting resources (ward/spore/reflect/autocrit/arcane-ward/heal- charges), once-per-fight class/race/subclass flags, and accumulated buff stat deltas. resumeTurnEngine restores them; commit writes them back in place. Fixes turn-based bugs where Orc rage, Halfling Lucky reroll, and the Assassin first-attack bonus re-fired every round and Abjuration Arcane Ward did nothing. Buff spells and buff-type consumables (ward/atk/def/crit/spore/reflect/ auto-crit) are now usable mid-fight: a flattened-delta model diffs the reused applySpellBuff/ApplyConsumableMods math against a throwaway combatant, folds the marginal effect into the session, and re-applies the persistent stat deltas onto the rebuilt player each round. Pure- utility spells diff to nothing and are refused before a slot is spent. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
2026-07-15 16:42:41 +00:00 · 2026-05-14 07:12:37 -07:00
parent 5cd343af0c
commit befb44ef03
6 changed files with 434 additions and 52 deletions
--- a/internal/plugin/combat_cmd.go
+++ b/internal/plugin/combat_cmd.go
@@ -71,7 +71,7 @@ func (p *AdventurePlugin) handleFightCmd(ctx MessageContext) error {
 		return p.SendDM(ctx.Sender, "_(No bestiary entry for this encounter — file a bug. `!zone abandon` to bail.)_")
 	}

-	_, enemy, _, err := p.buildZoneCombatants(ctx.Sender, monster, int(zone.Tier), run.DMMood)
+	player, enemy, _, err := p.buildZoneCombatants(ctx.Sender, monster, int(zone.Tier), run.DMMood)
 	if err != nil {
 		return p.SendDM(ctx.Sender, "Couldn't set up the fight: "+err.Error())
 	}
@@ -90,6 +90,14 @@ func (p *AdventurePlugin) handleFightCmd(ctx MessageContext) error {
 		return p.SendDM(ctx.Sender, "Couldn't start the fight: "+err.Error())
 	}

+	// Carry fight-start one-shot resources (Abjuration Arcane Ward, etc.) onto
+	// the session so they survive the turn engine's resume/commit cycle.
+	if seedCombatSessionOneShots(sess, player.Mods) {
+		if err := saveCombatSession(sess); err != nil {
+			slog.Error("combat: seed session one-shots", "user", ctx.Sender, "err", err)
+		}
+	}
+
 	var b strings.Builder
 	if isBoss {
 		if line := composeBossEntry(zone.ID, run.RunID, run.CurrentRoom); line != "" {
@@ -467,38 +475,55 @@ func (p *AdventurePlugin) handleCombatCastCmd(ctx MessageContext, args string) e
 		return p.SendDM(ctx.Sender, "Couldn't rebuild the fight: "+err.Error())
 	}

-	out, supported := resolveTurnSpell(c, spell, slotLevel, &enemy.Stats)
-	if !supported {
-		return p.SendDM(ctx.Sender, fmt.Sprintf(
-			"%s is a buff/utility spell — those aren't usable in turn-based fights yet (they need cross-round tracking). Try a damage, healing, or control spell.", spell.Name))
-	}
-
-	// Material component cost (Revivify, Raise Dead — rare in a fight).
-	if spell.MaterialCost > 0 {
-		if p.euro == nil || !p.euro.Debit(ctx.Sender, float64(spell.MaterialCost), "dnd_spell_component") {
+	var eff *turnActionEffect
+	if spell.Effect == EffectBuffSelf || spell.Effect == EffectBuffAlly {
+		// Buff path — resolve the buff against a throwaway combatant, fold the
+		// marginal effect into the session's persisted state, then rebuild the
+		// pair so the buff is live for this round's enemy turn.
+		as, am := player.Stats, player.Mods
+		applySpellBuff(spell, c, &as, &am, slotLevel)
+		d := diffTurnBuff(player.Stats, as, player.Mods, am)
+		if !d.any() {
 			return p.SendDM(ctx.Sender, fmt.Sprintf(
-				"%s needs a %d-coin diamond component you can't afford.", spell.Name, spell.MaterialCost))
+				"%s has no effect the turn-based engine can apply yet.", spell.Name))
 		}
-	}
-	// Slot spend — audit pattern: debit, run the round, refund on failure.
-	if spell.Level > 0 {
-		ok, serr := consumeSpellSlot(ctx.Sender, slotLevel)
-		if serr != nil {
-			return p.SendDM(ctx.Sender, "Couldn't consume slot: "+serr.Error())
+		if msg := p.chargeSpellCost(ctx.Sender, spell, slotLevel); msg != "" {
+			return p.SendDM(ctx.Sender, msg)
 		}
-		if !ok {
+		sess.Statuses.applyBuffDelta(d)
+		player, enemy, err = p.combatantsForSession(sess)
+		if err != nil {
+			if spell.Level > 0 {
+				_ = refundSpellSlot(ctx.Sender, slotLevel)
+			}
+			return p.SendDM(ctx.Sender, "Couldn't rebuild the fight: "+err.Error())
+		}
+		label := spell.Name + " — active"
+		if d.heal > 0 {
+			label = fmt.Sprintf("%s — +%d HP", spell.Name, d.heal)
+		}
+		eff = &turnActionEffect{
+			Action: "spell_cast", Label: label,
+			PlayerHeal: d.heal, EnemySkip: d.enemySkip,
+		}
+	} else {
+		out, supported := resolveTurnSpell(c, spell, slotLevel, &enemy.Stats)
+		if !supported {
 			return p.SendDM(ctx.Sender, fmt.Sprintf(
-				"No L%d slot available. %s", slotLevel, renderSlotsBrief(ctx.Sender)))
+				"%s is a utility spell — those aren't usable in turn-based fights yet. Try a damage, healing, control, or buff spell.", spell.Name))
+		}
+		if msg := p.chargeSpellCost(ctx.Sender, spell, slotLevel); msg != "" {
+			return p.SendDM(ctx.Sender, msg)
+		}
+		eff = &turnActionEffect{
+			Label:       out.Desc,
+			Action:      "spell_cast",
+			EnemyDamage: out.EnemyDamage,
+			PlayerHeal:  out.PlayerHeal,
+			EnemySkip:   out.EnemySkip,
 		}
 	}

-	eff := &turnActionEffect{
-		Label:       out.Desc,
-		Action:      "spell_cast",
-		EnemyDamage: out.EnemyDamage,
-		PlayerHeal:  out.PlayerHeal,
-		EnemySkip:   out.EnemySkip,
-	}
 	events, err := runCombatRound(sess, &player, &enemy, PlayerAction{Kind: ActionCast, Effect: eff})
 	if err != nil {
 		if spell.Level > 0 {
@@ -509,12 +534,35 @@ func (p *AdventurePlugin) handleCombatCastCmd(ctx MessageContext, args string) e
 	return p.SendDM(ctx.Sender, p.renderRoundResult(ctx.Sender, sess, events, player.Name, enemy))
 }

+// chargeSpellCost debits a spell's material component and leveled slot for a
+// turn-based cast. It returns a non-empty player-facing message on failure; on
+// success the caller owns the slot and must refundSpellSlot if the round itself
+// errors. Material components (rare in a fight) are not refunded if the slot
+// debit then fails — matching the auto-resolve cast path.
+func (p *AdventurePlugin) chargeSpellCost(userID id.UserID, spell SpellDefinition, slotLevel int) string {
+	if spell.MaterialCost > 0 {
+		if p.euro == nil || !p.euro.Debit(userID, float64(spell.MaterialCost), "dnd_spell_component") {
+			return fmt.Sprintf("%s needs a %d-coin diamond component you can't afford.", spell.Name, spell.MaterialCost)
+		}
+	}
+	if spell.Level > 0 {
+		ok, serr := consumeSpellSlot(userID, slotLevel)
+		if serr != nil {
+			return "Couldn't consume slot: " + serr.Error()
+		}
+		if !ok {
+			return fmt.Sprintf("No L%d slot available. %s", slotLevel, renderSlotsBrief(userID))
+		}
+	}
+	return ""
+}
+
 // ── !consume (in-combat) ────────────────────────────────────────────────────
 //
 // !consume <item> spends one combat consumable as the player's turn. Heal and
 // flat-damage items resolve fully within the round; buff-type items (ward,
-// atk/def boost, spore, reflect, auto-crit) need cross-round stat tracking and
-// are refused for now.
+// atk/def boost, spore, reflect, auto-crit) fold into the session's persisted
+// fight-scoped state and carry for the rest of the fight.

 // matchConsumable resolves a player-typed name against the player's consumable
 // inventory: case-insensitive exact match first, then a unique prefix match.
@@ -582,6 +630,11 @@ func (p *AdventurePlugin) handleConsumeCmd(ctx MessageContext, args string) erro
 	}

 	def := item.Def
+	player, enemy, err := p.combatantsForSession(sess)
+	if err != nil {
+		return p.SendDM(ctx.Sender, "Couldn't rebuild the fight: "+err.Error())
+	}
+
 	eff := &turnActionEffect{Action: "use_consumable"}
 	switch def.Effect {
 	case EffectHeal:
@@ -591,13 +644,24 @@ func (p *AdventurePlugin) handleConsumeCmd(ctx MessageContext, args string) erro
 		eff.EnemyDamage = int(def.Value)
 		eff.Label = fmt.Sprintf("%s — %d damage", def.Name, eff.EnemyDamage)
 	default:
-		return p.SendDM(ctx.Sender, fmt.Sprintf(
-			"**%s** is a buff-type consumable — those aren't usable in turn-based fights yet. Healing and damage items (Berry Poultice, Coal Bomb, …) work.", def.Name))
-	}
-
-	player, enemy, err := p.combatantsForSession(sess)
-	if err != nil {
-		return p.SendDM(ctx.Sender, "Couldn't rebuild the fight: "+err.Error())
+		// Buff-type consumable — resolve the marginal effect against a
+		// throwaway combatant, fold it into the session's persisted state, and
+		// rebuild the pair so the buff is live for this round's enemy turn.
+		as, am := player.Stats, player.Mods
+		ApplyConsumableMods(&as, &am, []ConsumableItem{{Def: def}})
+		d := diffTurnBuff(player.Stats, as, player.Mods, am)
+		if !d.any() {
+			return p.SendDM(ctx.Sender, fmt.Sprintf(
+				"**%s** has no effect the turn-based engine can apply yet.", def.Name))
+		}
+		sess.Statuses.applyBuffDelta(d)
+		player, enemy, err = p.combatantsForSession(sess)
+		if err != nil {
+			return p.SendDM(ctx.Sender, "Couldn't rebuild the fight: "+err.Error())
+		}
+		eff.Label = def.Name + " — active"
+		eff.PlayerHeal = d.heal
+		eff.EnemySkip = d.enemySkip
 	}

 	events, err := runCombatRound(sess, &player, &enemy, PlayerAction{Kind: ActionConsume, Effect: eff})
--- a/internal/plugin/combat_session.go
+++ b/internal/plugin/combat_session.go
@@ -42,13 +42,16 @@ const (
 // reaper sweeps it. Matches the started_at + 1h note in the schema.
 const combatSessionTTL = time.Hour

-// CombatStatuses is the serialized between-round effect state for a fight —
-// the subset of combatState fields that genuinely persist round-to-round
-// (monster-ability effects). Fight-scoped one-shots (rage, Lucky reroll,
-// ward/heal charges) are deliberately not carried here: they reset if a fight
-// is resumed across a bot restart. Extending coverage to those one-shots is
-// the command-wiring PR's job, once it reconstructs Combatants from a session.
+// CombatStatuses is the serialized between-round effect state for a fight: the
+// subset of combatState that genuinely persists round-to-round, plus the
+// fight-scoped buffs a mid-fight !cast / !consume layers on. Everything here
+// round-trips combatState -> Statuses -> combatState across every engine step,
+// so a fight resumes from exact mid-state after a suspend or bot restart.
+//
+// All fields are scalar by design — CombatStatuses must stay comparable so the
+// persistence round-trip can be asserted with ==.
 type CombatStatuses struct {
+	// Monster-ability effects — the original between-round persistence set.
 	PoisonTicks   int     `json:"poison_ticks,omitempty"`
 	PoisonDmg     int     `json:"poison_dmg,omitempty"`
 	StunPlayer    bool    `json:"stun_player,omitempty"`
@@ -60,6 +63,86 @@ type CombatStatuses struct {
 	// — those phases resolve as separate engine steps with separate in-memory
 	// combatState, so the flag must survive the commit between them.
 	EnemySkipNext bool `json:"enemy_skip_next,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,
+	// written back on every commit so a depleting charge can't silently reset
+	// when a fight is suspended and resumed.
+	WardCharges     int     `json:"ward_charges,omitempty"`
+	SporeRounds     int     `json:"spore_rounds,omitempty"`
+	ReflectFrac     float64 `json:"reflect_frac,omitempty"`
+	AutoCritFirst   bool    `json:"auto_crit_first,omitempty"`
+	ArcaneWardHP    int     `json:"arcane_ward_hp,omitempty"`
+	HealChargesLeft int     `json:"heal_charges_left,omitempty"`
+
+	// Once-per-fight class/race/subclass one-shots: the "already used" flags.
+	// Without persistence these reset every round on resume, letting a Halfling
+	// reroll a nat 1 or an Orc rage every single round of a turn-based fight.
+	DeathSaveUsed     bool `json:"death_save_used,omitempty"`
+	LuckyUsed         bool `json:"lucky_used,omitempty"`
+	Raged             bool `json:"raged,omitempty"`
+	PendingRage       bool `json:"pending_rage,omitempty"`
+	FirstAtkBonusUsed bool `json:"first_atk_bonus_used,omitempty"`
+	AssassinateReroll bool `json:"assassinate_reroll_used,omitempty"`
+	AssassinateBonus  bool `json:"assassinate_bonus_used,omitempty"`
+
+	// Persistent stat buffs from mid-fight !cast / !consume, accumulated as
+	// deltas against the freshly-rebuilt combatant. applySessionBuffs folds
+	// these back onto the player every round; diffTurnBuff produces them.
+	// BuffDamageReductMul is multiplicative (0 = none / 1.0 neutral).
+	BuffACBonus         int     `json:"buff_ac_bonus,omitempty"`
+	BuffAtkBonus        int     `json:"buff_atk_bonus,omitempty"`
+	BuffSpeedBonus      int     `json:"buff_speed_bonus,omitempty"`
+	BuffPetDmg          int     `json:"buff_pet_dmg,omitempty"`
+	BuffCritRate        float64 `json:"buff_crit_rate,omitempty"`
+	BuffDamageBonus     float64 `json:"buff_damage_bonus,omitempty"`
+	BuffPetProc         float64 `json:"buff_pet_proc,omitempty"`
+	BuffDamageReductMul float64 `json:"buff_damage_reduct_mul,omitempty"`
+}
+
+// applyBuffDelta folds one resolved buff (the result of a !cast / !consume
+// player turn) into the persisted fight-scoped state. Stat components
+// accumulate as deltas; depleting resources add to their running counters.
+func (s *CombatStatuses) applyBuffDelta(d turnBuffDelta) {
+	s.BuffACBonus += d.dAC
+	s.BuffAtkBonus += d.dAtk
+	s.BuffSpeedBonus += d.dSpeed
+	s.BuffPetDmg += d.dPetDmg
+	s.BuffCritRate += d.dCrit
+	s.BuffDamageBonus += d.dDmgBonus
+	s.BuffPetProc += d.dPetProc
+	if d.dReductMul > 0 && d.dReductMul != 1 {
+		if s.BuffDamageReductMul == 0 {
+			s.BuffDamageReductMul = d.dReductMul
+		} else {
+			s.BuffDamageReductMul *= d.dReductMul
+		}
+	}
+	s.WardCharges += d.ward
+	s.SporeRounds += d.spore
+	s.ReflectFrac += d.reflect
+	if d.autoCrit {
+		s.AutoCritFirst = true
+	}
+	s.ArcaneWardHP += d.arcaneWard
+}
+
+// seedCombatSessionOneShots copies the fight-start one-shot resources off the
+// freshly-built player combatant into the session's persisted statuses. Only
+// the Abjuration Arcane Ward is normally non-zero at fight start — the
+// turn-based build deliberately omits pre-combat consumables and queued casts —
+// but the full set is seeded for robustness. Returns true if anything was set.
+func seedCombatSessionOneShots(s *CombatSession, playerMods CombatModifiers) bool {
+	st := &s.Statuses
+	st.WardCharges = playerMods.WardCharges
+	st.SporeRounds = playerMods.SporeCloud
+	st.ReflectFrac = playerMods.ReflectNext
+	st.AutoCritFirst = playerMods.AutoCritFirst
+	st.ArcaneWardHP = playerMods.ArcaneWardHP
+	st.HealChargesLeft = playerMods.HealItemCharges
+	return st.WardCharges != 0 || st.SporeRounds != 0 || st.ReflectFrac != 0 ||
+		st.AutoCritFirst || st.ArcaneWardHP != 0 || st.HealChargesLeft != 0
 }

 // CombatSession is the in-memory shape of a combat_session row.
--- a/internal/plugin/combat_session_build.go
+++ b/internal/plugin/combat_session_build.go
@@ -24,7 +24,9 @@ import (
 // applyPendingCast and the auto-heal consumable setup are intentionally left
 // out of the shared builder — those are one-shot mutations that runZoneCombat
 // applies once before SimulateCombat. Re-applying them on every turn-based
-// round would double-consume. Per-round !cast / !consume is a later sub-phase.
+// round would double-consume. Mid-fight !cast / !consume buffs are instead
+// persisted on the CombatSession and folded back in by combatantsForSession
+// via applySessionBuffs.

 // buildZoneCombatants derives the player/enemy Combatant pair for a zone
 // encounter: the player's full D&D layer (stats + class/race/subclass passives
@@ -121,5 +123,31 @@ func (p *AdventurePlugin) combatantsForSession(sess *CombatSession) (player Comb
 	}
 	zone := zoneOrFallback(run.ZoneID)
 	player, enemy, _, err = p.buildZoneCombatants(id.UserID(sess.UserID), monster, int(zone.Tier), run.DMMood)
+	if err != nil {
+		return player, enemy, err
+	}
+	// Fold any fight-scoped buffs a mid-fight !cast / !consume layered on back
+	// onto the freshly-rebuilt player. The depleting one-shots (ward/spore/…)
+	// live on the session's Statuses and flow through the turn engine's
+	// resume/commit cycle, so only the persistent stat deltas are applied here.
+	applySessionBuffs(&player, sess.Statuses)
 	return player, enemy, err
 }
+
+// applySessionBuffs re-derives the persistent stat effect of every mid-fight
+// buff onto the rebuilt player. The buffs are stored as accumulated deltas
+// (diffTurnBuff produced them against the player's state at cast time), so
+// re-applying them to a deterministic rebuild reproduces the same totals every
+// round without double-counting.
+func applySessionBuffs(player *Combatant, s CombatStatuses) {
+	player.Stats.AC += s.BuffACBonus
+	player.Stats.AttackBonus += s.BuffAtkBonus
+	player.Stats.Speed += s.BuffSpeedBonus
+	player.Stats.CritRate += s.BuffCritRate
+	player.Mods.DamageBonus += s.BuffDamageBonus
+	player.Mods.PetAttackProc += s.BuffPetProc
+	player.Mods.PetAttackDmg += s.BuffPetDmg
+	if s.BuffDamageReductMul > 0 {
+		player.Mods.DamageReduct *= s.BuffDamageReductMul
+	}
+}
--- a/internal/plugin/combat_session_test.go
+++ b/internal/plugin/combat_session_test.go
@@ -285,6 +285,118 @@ func TestTurnEngine_StepRejectsTerminalSession(t *testing.T) {
 	}
 }

+// ── Fight-scoped buff persistence ──────────────────────────────────────────
+
+func TestDiffTurnBuff(t *testing.T) {
+	bs := CombatStats{AC: 14, AttackBonus: 3, MaxHP: 50, Speed: 10, CritRate: 0.05}
+	bm := CombatModifiers{DamageBonus: 0.1, DamageReduct: 1.0}
+
+	// A buff that bumps AC, grants ward charges, reduces damage taken, and
+	// raises max HP (Aid-style → collapses to a heal).
+	as, am := bs, bm
+	as.AC = 16
+	as.MaxHP = 55
+	am.WardCharges = 2
+	am.DamageReduct = 0.8
+	d := diffTurnBuff(bs, as, bm, am)
+	if d.dAC != 2 || d.ward != 2 || d.heal != 5 {
+		t.Errorf("delta = %+v, want dAC 2 / ward 2 / heal 5", d)
+	}
+	if d.dReductMul < 0.79 || d.dReductMul > 0.81 {
+		t.Errorf("dReductMul = %v, want ~0.8", d.dReductMul)
+	}
+	if !d.statComponent() || !d.any() {
+		t.Errorf("buff with AC + reduct should report statComponent and any")
+	}
+
+	// A no-op buff (utility spell with no combat hook) diffs to nothing.
+	none := diffTurnBuff(bs, bs, bm, bm)
+	if none.statComponent() || none.any() {
+		t.Errorf("no-op buff should report neither statComponent nor any: %+v", none)
+	}
+}
+
+func TestApplyBuffDelta(t *testing.T) {
+	var s CombatStatuses
+	s.applyBuffDelta(turnBuffDelta{dAC: 2, ward: 1, dReductMul: 0.8, autoCrit: true})
+	if s.BuffACBonus != 2 || s.WardCharges != 1 || !s.AutoCritFirst {
+		t.Errorf("first delta not folded in: %+v", s)
+	}
+	if s.BuffDamageReductMul < 0.79 || s.BuffDamageReductMul > 0.81 {
+		t.Errorf("BuffDamageReductMul = %v, want ~0.8", s.BuffDamageReductMul)
+	}
+	// A second buff stacks: deltas accumulate, reduction multipliers compound.
+	s.applyBuffDelta(turnBuffDelta{dAC: 1, dReductMul: 0.5})
+	if s.BuffACBonus != 3 {
+		t.Errorf("BuffACBonus = %d, want 3 after stacking", s.BuffACBonus)
+	}
+	if s.BuffDamageReductMul < 0.39 || s.BuffDamageReductMul > 0.41 {
+		t.Errorf("BuffDamageReductMul = %v, want ~0.4 (0.8 * 0.5)", s.BuffDamageReductMul)
+	}
+}
+
+func TestApplySessionBuffs(t *testing.T) {
+	player := basePlayer() // AC 0, AttackBonus 0, CritRate 0.05, Speed 10, DamageReduct 1.0
+	applySessionBuffs(&player, CombatStatuses{
+		BuffACBonus: 3, BuffAtkBonus: 2, BuffSpeedBonus: 5, BuffCritRate: 0.15,
+		BuffDamageBonus: 0.25, BuffDamageReductMul: 0.8, BuffPetProc: 0.5, BuffPetDmg: 6,
+	})
+	if player.Stats.AC != 3 || player.Stats.AttackBonus != 2 || player.Stats.Speed != 15 {
+		t.Errorf("stat deltas wrong: %+v", player.Stats)
+	}
+	if player.Stats.CritRate < 0.19 || player.Stats.CritRate > 0.21 {
+		t.Errorf("CritRate = %v, want ~0.20", player.Stats.CritRate)
+	}
+	if player.Mods.DamageBonus != 0.25 || player.Mods.PetAttackProc != 0.5 || player.Mods.PetAttackDmg != 6 {
+		t.Errorf("mod deltas wrong: %+v", player.Mods)
+	}
+	if player.Mods.DamageReduct < 0.79 || player.Mods.DamageReduct > 0.81 {
+		t.Errorf("DamageReduct = %v, want ~0.8 (1.0 * 0.8)", player.Mods.DamageReduct)
+	}
+}
+
+func TestSeedCombatSessionOneShots(t *testing.T) {
+	// Arcane Ward is the one resource normally live at fight start.
+	s := &CombatSession{}
+	if !seedCombatSessionOneShots(s, CombatModifiers{ArcaneWardHP: 15}) {
+		t.Error("seed should report true when Arcane Ward is present")
+	}
+	if s.Statuses.ArcaneWardHP != 15 {
+		t.Errorf("ArcaneWardHP = %d, want 15", s.Statuses.ArcaneWardHP)
+	}
+	// Nothing to seed → false, statuses untouched.
+	empty := &CombatSession{}
+	if seedCombatSessionOneShots(empty, CombatModifiers{}) {
+		t.Error("seed should report false with no fight-start resources")
+	}
+}
+
+// TestTurnEngine_OneShotsRoundTrip drives a no-op round_end step and confirms
+// every fight-scoped one-shot survives the resume -> combatState -> commit
+// cycle. Without that round-trip a Halfling could reroll a nat 1 — or a player
+// re-bank a depleted ward charge — on every resumed round.
+func TestTurnEngine_OneShotsRoundTrip(t *testing.T) {
+	sess := turnSession(CombatPhaseRoundEnd, 50, 50)
+	sess.Statuses = CombatStatuses{
+		WardCharges: 3, SporeRounds: 2, ReflectFrac: 0.5, AutoCritFirst: true,
+		ArcaneWardHP: 10, HealChargesLeft: 1,
+		Raged: true, LuckyUsed: true, DeathSaveUsed: true, PendingRage: true,
+		FirstAtkBonusUsed: true, AssassinateReroll: true, AssassinateBonus: true,
+		// Buff stat deltas are owned by the command layer — commit must leave
+		// them untouched rather than zeroing them on every step.
+		BuffACBonus: 2, BuffDamageBonus: 0.15,
+	}
+	want := sess.Statuses // round_end with no poison mutates none of these
+	player, enemy := basePlayer(), baseEnemy()
+
+	if _, err := stepEngine(sess, &player, &enemy, PlayerAction{}); err != nil {
+		t.Fatal(err)
+	}
+	if sess.Statuses != want {
+		t.Errorf("one-shots not round-tripped:\n got  %+v\n want %+v", sess.Statuses, want)
+	}
+}
+
 func TestCombatSessionRNG_DeterministicPerPhase(t *testing.T) {
 	sess := &CombatSession{SessionID: "abc123", Round: 2, Phase: CombatPhaseEnemyTurn}
 	a := combatSessionRNG(sess)
--- a/internal/plugin/combat_turn_engine.go
+++ b/internal/plugin/combat_turn_engine.go
@@ -116,7 +116,23 @@ func resumeTurnEngine(sess *CombatSession, player, enemy *Combatant, rng *rand.R
 		armorBroken:    sess.Statuses.ArmorBroken,
 		armorBreakAmt:  sess.Statuses.ArmorBreakAmt,
 		enemySkipFirst: sess.Statuses.EnemySkipNext,
-		rng:            rng,
+		// 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.
+		wardCharges:           sess.Statuses.WardCharges,
+		sporeRounds:           sess.Statuses.SporeRounds,
+		reflectFrac:           sess.Statuses.ReflectFrac,
+		autoCrit:              sess.Statuses.AutoCritFirst,
+		arcaneWardHP:          sess.Statuses.ArcaneWardHP,
+		healChargesLeft:       sess.Statuses.HealChargesLeft,
+		deathSaveUsed:         sess.Statuses.DeathSaveUsed,
+		luckyUsed:             sess.Statuses.LuckyUsed,
+		raged:                 sess.Statuses.Raged,
+		pendingRageAttack:     sess.Statuses.PendingRage,
+		firstAttackBonusUsed:  sess.Statuses.FirstAtkBonusUsed,
+		assassinateRerollUsed: sess.Statuses.AssassinateReroll,
+		assassinateBonusUsed:  sess.Statuses.AssassinateBonus,
+		rng:                   rng,
 	}
 	return &turnEngine{
 		sess:   sess,
@@ -265,20 +281,37 @@ func (te *turnEngine) finish(status string) {
 // commit folds this step's combatState back into the session struct: HP,
 // round, the between-round status snapshot, and the appended event log.
 // Phase / Status were already set by step. saveCombatSession persists it.
+//
+// The Buff* stat deltas on Statuses are NOT combatState fields — they're owned
+// by the command layer (a !cast / !consume folds them in) and applied to the
+// rebuilt combatant by applySessionBuffs — so commit mutates Statuses in place
+// rather than replacing it, leaving those deltas untouched.
 func (te *turnEngine) commit() {
 	st := te.st
 	te.sess.Round = st.round
 	te.sess.PlayerHP = st.playerHP
 	te.sess.EnemyHP = st.enemyHP
-	te.sess.Statuses = CombatStatuses{
-		PoisonTicks:   st.poisonTicks,
-		PoisonDmg:     st.poisonDmg,
-		StunPlayer:    st.stunPlayer,
-		Enraged:       st.enraged,
-		ArmorBroken:   st.armorBroken,
-		ArmorBreakAmt: st.armorBreakAmt,
-		EnemySkipNext: st.enemySkipFirst,
-	}
+	s := &te.sess.Statuses
+	s.PoisonTicks = st.poisonTicks
+	s.PoisonDmg = st.poisonDmg
+	s.StunPlayer = st.stunPlayer
+	s.Enraged = st.enraged
+	s.ArmorBroken = st.armorBroken
+	s.ArmorBreakAmt = st.armorBreakAmt
+	s.EnemySkipNext = st.enemySkipFirst
+	s.WardCharges = st.wardCharges
+	s.SporeRounds = st.sporeRounds
+	s.ReflectFrac = st.reflectFrac
+	s.AutoCritFirst = st.autoCrit
+	s.ArcaneWardHP = st.arcaneWardHP
+	s.HealChargesLeft = st.healChargesLeft
+	s.DeathSaveUsed = st.deathSaveUsed
+	s.LuckyUsed = st.luckyUsed
+	s.Raged = st.raged
+	s.PendingRage = st.pendingRageAttack
+	s.FirstAtkBonusUsed = st.firstAttackBonusUsed
+	s.AssassinateReroll = st.assassinateRerollUsed
+	s.AssassinateBonus = st.assassinateBonusUsed
 	te.sess.TurnLog = append(te.sess.TurnLog, st.events...)
 }

--- a/internal/plugin/dnd_spell_combat.go
+++ b/internal/plugin/dnd_spell_combat.go
@@ -406,3 +406,65 @@ func rollSpellDamageDice(spell SpellDefinition, slot, charLevel int) int {
 	}
 	return total
 }
+
+// ── Phase 13 — turn-based buff resolution ────────────────────────────────────
+
+// turnBuffDelta is the marginal effect of one buff spell or consumable applied
+// as a turn-based player action, diffed against the player's already-buffed
+// combatant. Stat components (dAC, dDmgBonus, …) accumulate as session deltas;
+// depleting resources (ward, spore, …) add to their running counters; heal and
+// enemySkip land within the casting round only.
+type turnBuffDelta struct {
+	dAC, dAtk, dSpeed, dPetDmg int
+	dCrit, dDmgBonus, dPetProc float64
+	dReductMul                 float64 // multiplicative; 1 = no change
+	ward, spore, arcaneWard    int
+	reflect                    float64
+	autoCrit, enemySkip        bool
+	heal                       int // a MaxHP-raise (Aid) collapses to an immediate heal
+}
+
+// statComponent reports whether the buff has a re-applicable persistent stat
+// effect — the part applySessionBuffs folds back onto the player every round.
+func (d turnBuffDelta) statComponent() bool {
+	return d.dAC != 0 || d.dAtk != 0 || d.dSpeed != 0 || d.dPetDmg != 0 ||
+		d.dCrit != 0 || d.dDmgBonus != 0 || d.dPetProc != 0 ||
+		(d.dReductMul > 0 && d.dReductMul != 1)
+}
+
+// any reports whether the buff produced any applicable effect at all. A buff
+// the turn engine can't represent yet (pure utility) diffs to nothing, and the
+// caller refuses it before a slot or item is spent.
+func (d turnBuffDelta) any() bool {
+	return d.statComponent() || d.ward != 0 || d.spore != 0 || d.reflect != 0 ||
+		d.autoCrit || d.arcaneWard != 0 || d.enemySkip || d.heal != 0
+}
+
+// diffTurnBuff computes the marginal effect of a buff: (bs,bm) is the player's
+// state before the buff, (as,am) the throwaway result of applying it. Reusing
+// applySpellBuff / ApplyConsumableMods against a copy keeps turn-based buffs
+// numerically identical to the auto-resolve engine.
+func diffTurnBuff(bs, as CombatStats, bm, am CombatModifiers) turnBuffDelta {
+	d := turnBuffDelta{
+		dAC:        as.AC - bs.AC,
+		dAtk:       as.AttackBonus - bs.AttackBonus,
+		dSpeed:     as.Speed - bs.Speed,
+		dPetDmg:    am.PetAttackDmg - bm.PetAttackDmg,
+		dCrit:      as.CritRate - bs.CritRate,
+		dDmgBonus:  am.DamageBonus - bm.DamageBonus,
+		dPetProc:   am.PetAttackProc - bm.PetAttackProc,
+		ward:       am.WardCharges - bm.WardCharges,
+		spore:      am.SporeCloud - bm.SporeCloud,
+		reflect:    am.ReflectNext - bm.ReflectNext,
+		arcaneWard: am.ArcaneWardHP - bm.ArcaneWardHP,
+		autoCrit:   am.AutoCritFirst && !bm.AutoCritFirst,
+		enemySkip:  am.SpellEnemySkipFirst && !bm.SpellEnemySkipFirst,
+		heal:       as.MaxHP - bs.MaxHP,
+	}
+	if bm.DamageReduct > 0 {
+		d.dReductMul = am.DamageReduct / bm.DamageReduct
+	} else {
+		d.dReductMul = 1
+	}
+	return d
+}