diff --git a/internal/db/db.go b/internal/db/db.go
index b37c8c8..e0b461b 100644
--- a/internal/db/db.go
+++ b/internal/db/db.go
@@ -352,6 +352,11 @@ func runMigrations(d *sql.DB) error {
 		// DEFAULT 0 is wrong for in-flight rows — bootstrapRoomsTraversed
 		// backfills them from visited_nodes.
 		`ALTER TABLE dnd_zone_run ADD COLUMN rooms_traversed INTEGER NOT NULL DEFAULT 0`,
+		// N3/P4 party combat. Every pre-existing fight is solo, and solo is
+		// exactly roster_size 1, so the DEFAULT is the correct value for every
+		// row and no bootstrap backfill is needed. The column is a read guard:
+		// loadCombatParticipants is skipped entirely when it reads 1.
+		`ALTER TABLE combat_session ADD COLUMN roster_size INTEGER NOT NULL DEFAULT 1`,
 	}
 	for _, stmt := range columnMigrations {
 		if _, err := d.Exec(stmt); err != nil {
@@ -2080,12 +2085,63 @@ CREATE TABLE IF NOT EXISTS combat_session (
     status          TEXT NOT NULL DEFAULT 'active',      -- active|won|lost|fled|expired
     started_at      DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
     last_action_at  DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
-    expires_at      DATETIME NOT NULL
+    expires_at      DATETIME NOT NULL,
+    -- N3/P4: seated player characters, seat 0 (this row) included. 1 == solo,
+    -- which is what every pre-N3 row is, so the DEFAULT needs no backfill. Read
+    -- as a guard: only a roster_size > 1 makes the loader touch
+    -- combat_participant, keeping the solo fight at one query.
+    roster_size     INTEGER NOT NULL DEFAULT 1
 );
 CREATE INDEX IF NOT EXISTS idx_combat_session_active
     ON combat_session(user_id, status);
 CREATE INDEX IF NOT EXISTS idx_combat_session_expiry
     ON combat_session(status, expires_at);
+
+-- ── N3/P4 — party combat: the seats a session row cannot hold ─────────────
+-- One row per party member from seat 1 up. Seat 0 is the session's own
+-- user_id/player_hp/statuses_json, so a solo fight writes no rows here and
+-- reads none: absent == solo, which is why this table needs no backfill.
+--
+-- seat:           index into the combat roster. 1..N-1; seat 0 is the session.
+-- hp / hp_max:    this member's live pool. Seat 0's lives on combat_session.
+-- statuses_json:  serialized ActorStatuses — the per-character half of the
+--                 effect state (their concentration, their debuffs, their
+--                 once-per-fight one-shots). The enemy's stance and the round
+--                 cursor are fight-scoped and stay on combat_session.
+CREATE TABLE IF NOT EXISTS combat_participant (
+    session_id      TEXT NOT NULL,
+    seat            INTEGER NOT NULL,
+    user_id         TEXT NOT NULL,
+    hp              INTEGER NOT NULL,
+    hp_max          INTEGER NOT NULL,
+    statuses_json   TEXT NOT NULL DEFAULT '{}',
+    PRIMARY KEY (session_id, seat)
+);
+CREATE INDEX IF NOT EXISTS idx_combat_participant_user
+    ON combat_participant(user_id);
+
+-- ── N3/P4 — expedition parties ────────────────────────────────────────────
+-- Membership for a co-op expedition. The leader's dnd_expedition row stays
+-- the single source of truth for the shared clock, threat, and supply pool;
+-- members reference it through here rather than owning a row of their own.
+-- A solo expedition has no rows: absent == solo, so no backfill is needed.
+--
+-- There is deliberately no party_id on dnd_expedition. The expedition_id is
+-- already the party's identity, and a second key would be a second source of
+-- truth for "who is in this party" that could disagree with this table.
+--
+-- role: 'leader' | 'member'. Exactly one leader per expedition, matching
+--       dnd_expedition.user_id; enforced in code, not by constraint (the
+--       same discipline combat_session uses for one-active-per-user).
+CREATE TABLE IF NOT EXISTS expedition_party (
+    expedition_id   TEXT NOT NULL,
+    user_id         TEXT NOT NULL,
+    role            TEXT NOT NULL DEFAULT 'member',
+    joined_at       DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    PRIMARY KEY (expedition_id, user_id)
+);
+CREATE INDEX IF NOT EXISTS idx_expedition_party_user
+    ON expedition_party(user_id);
 `
 
 // SeedSchedulerDefaults inserts default scheduler jobs if they don't exist.
diff --git a/internal/plugin/combat_participant_test.go b/internal/plugin/combat_participant_test.go
new file mode 100644
index 0000000..557fd62
--- /dev/null
+++ b/internal/plugin/combat_participant_test.go
@@ -0,0 +1,331 @@
+package plugin
+
+import (
+	"encoding/json"
+	"math/rand/v2"
+	"testing"
+
+	"gogobee/internal/db"
+	"maunium.net/go/mautrix/id"
+)
+
+// setupEmptyTestDB opens a fresh schema in a temp dir. Unlike setupZoneRunTestDB
+// it copies nothing from data/gogobee.db, so it runs everywhere instead of
+// skipping when the prod db is absent — the session/participant/party tables are
+// the only ones these tests touch, and none of them need seeded characters.
+func setupEmptyTestDB(t *testing.T) {
+	t.Helper()
+	db.Close()
+	if err := db.Init(t.TempDir()); err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(db.Close)
+}
+
+// ── The wire format ─────────────────────────────────────────────────────────
+
+// The ActorStatuses split moved ~25 fields behind an anonymous embed. That is a
+// no-op on the wire only because the embed is untagged: encoding/json flattens
+// it into the parent object. If someone ever gives it a json tag, every
+// in-flight prod session silently loses its poison, its charges, and its
+// once-per-fight one-shots on the next resume. Pin the flattening.
+func TestCombatStatuses_JSONStaysFlat(t *testing.T) {
+	s := CombatStatuses{
+		ActorStatuses: ActorStatuses{PoisonTicks: 2, PoisonDmg: 5, WardCharges: 3},
+		Enraged:       true,
+		EnemyRegen:    4,
+	}
+	raw, err := json.Marshal(s)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var obj map[string]any
+	if err := json.Unmarshal(raw, &obj); err != nil {
+		t.Fatal(err)
+	}
+	for _, key := range []string{"poison_ticks", "poison_dmg", "ward_charges", "enraged", "enemy_regen"} {
+		if _, ok := obj[key]; !ok {
+			t.Errorf("key %q missing — ActorStatuses is nested, not flattened: %s", key, raw)
+		}
+	}
+	if _, nested := obj["ActorStatuses"]; nested {
+		t.Errorf("ActorStatuses serialized as a nested object: %s", raw)
+	}
+}
+
+// A statuses_json blob written before the split must decode unchanged.
+func TestCombatStatuses_DecodesPreSplitRow(t *testing.T) {
+	const preSplit = `{"poison_ticks":3,"poison_dmg":6,"enraged":true,` +
+		`"ward_charges":2,"lucky_used":true,"enemy_retaliate_frac":0.25,` +
+		`"buff_ac_bonus":4,"max_hp_drain":7}`
+	var s CombatStatuses
+	if err := json.Unmarshal([]byte(preSplit), &s); err != nil {
+		t.Fatal(err)
+	}
+	if s.PoisonTicks != 3 || s.PoisonDmg != 6 || s.WardCharges != 2 ||
+		s.LuckyUsed != true || s.BuffACBonus != 4 || s.MaxHPDrain != 7 {
+		t.Errorf("per-actor fields lost: %+v", s.ActorStatuses)
+	}
+	if !s.Enraged || s.EnemyRetaliateFrac != 0.25 {
+		t.Errorf("fight-scoped fields lost: %+v", s)
+	}
+}
+
+// ── Solo stays solo ─────────────────────────────────────────────────────────
+
+// The whole balance corpus rides the solo path. It must write no participant
+// rows, and its roster_size must stay at the DEFAULT so the loader never issues
+// the second query (project_combat_session_cache_deferred: don't make it worse).
+func TestSoloSessionWritesNoParticipantRows(t *testing.T) {
+	setupEmptyTestDB(t)
+	uid := id.UserID("@solo-noparts:example.org")
+	defer cleanupCombatSessions(uid)
+
+	s, err := startCombatSession(uid, "run-solo", "node-1", "owlbear", 40, 40, 60, 60)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := saveCombatSession(s); err != nil {
+		t.Fatal(err)
+	}
+
+	var rows int
+	if err := db.Get().QueryRow(
+		`SELECT COUNT(*) FROM combat_participant WHERE session_id = ?`, s.SessionID,
+	).Scan(&rows); err != nil {
+		t.Fatal(err)
+	}
+	if rows != 0 {
+		t.Errorf("solo fight wrote %d participant rows, want 0", rows)
+	}
+
+	var rosterSize int
+	if err := db.Get().QueryRow(
+		`SELECT roster_size FROM combat_session WHERE session_id = ?`, s.SessionID,
+	).Scan(&rosterSize); err != nil {
+		t.Fatal(err)
+	}
+	if rosterSize != 1 {
+		t.Errorf("roster_size = %d, want 1", rosterSize)
+	}
+
+	got, err := getActiveCombatSession(uid)
+	if err != nil || got == nil {
+		t.Fatalf("getActiveCombatSession: %v / %v", got, err)
+	}
+	if got.IsParty() || got.RosterSize() != 1 || len(got.Participants) != 0 {
+		t.Errorf("solo session reads back as a party: %+v", got.Participants)
+	}
+}
+
+// ── Party seats ─────────────────────────────────────────────────────────────
+
+func seatParty(t *testing.T, s *CombatSession, members ...CombatParticipant) {
+	t.Helper()
+	if err := insertCombatParticipants(s.SessionID, members); err != nil {
+		t.Fatalf("insertCombatParticipants: %v", err)
+	}
+	s.Participants = members
+}
+
+func TestCombatParticipants_RoundTrip(t *testing.T) {
+	setupEmptyTestDB(t)
+	uid := id.UserID("@party-lead:example.org")
+	defer cleanupCombatSessions(uid)
+
+	s, err := startCombatSession(uid, "run-party", "node-1", "owlbear", 40, 40, 200, 200)
+	if err != nil {
+		t.Fatal(err)
+	}
+	seatParty(t, s,
+		CombatParticipant{Seat: 1, UserID: "@b:x", HP: 30, HPMax: 30,
+			Statuses: ActorStatuses{WardCharges: 2}},
+		CombatParticipant{Seat: 2, UserID: "@c:x", HP: 25, HPMax: 25,
+			Statuses: ActorStatuses{ConcentrationDmg: 9, LuckyUsed: true}},
+	)
+
+	got, err := getActiveCombatSession(uid)
+	if err != nil || got == nil {
+		t.Fatalf("getActiveCombatSession: %v / %v", got, err)
+	}
+	if !got.IsParty() || got.RosterSize() != 3 {
+		t.Fatalf("roster = %d, want 3", got.RosterSize())
+	}
+	if got.Participants[0].UserID != "@b:x" || got.Participants[0].Statuses.WardCharges != 2 {
+		t.Errorf("seat 1 round-trip wrong: %+v", got.Participants[0])
+	}
+	if got.Participants[1].Statuses.ConcentrationDmg != 9 || !got.Participants[1].Statuses.LuckyUsed {
+		t.Errorf("seat 2 round-trip wrong: %+v", got.Participants[1])
+	}
+	if want := []string{string(uid), "@b:x", "@c:x"}; !equalStrings(got.SeatUserIDs(), want) {
+		t.Errorf("SeatUserIDs = %v, want %v", got.SeatUserIDs(), want)
+	}
+
+	// Mutable half writes back; hp_max and user_id do not move.
+	got.Participants[0].HP = 11
+	got.Participants[0].Statuses.WardCharges = 0
+	got.Participants[0].Statuses.Raged = true
+	if err := saveCombatSession(got); err != nil {
+		t.Fatal(err)
+	}
+	again, err := getCombatSession(s.SessionID)
+	if err != nil || again == nil {
+		t.Fatalf("getCombatSession: %v / %v", again, err)
+	}
+	p := again.Participants[0]
+	if p.HP != 11 || p.HPMax != 30 || p.Statuses.WardCharges != 0 || !p.Statuses.Raged {
+		t.Errorf("seat 1 save round-trip wrong: %+v", p)
+	}
+}
+
+// A gap in the seat sequence would shift every member down one index, because
+// the engine addresses the roster positionally. Fail the load instead.
+func TestLoadCombatParticipants_RejectsSeatGap(t *testing.T) {
+	setupEmptyTestDB(t)
+	uid := id.UserID("@party-gap:example.org")
+	defer cleanupCombatSessions(uid)
+
+	s, err := startCombatSession(uid, "run-gap", "node-1", "owlbear", 40, 40, 200, 200)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Seat 2 with no seat 1.
+	if err := insertCombatParticipants(s.SessionID, []CombatParticipant{
+		{Seat: 2, UserID: "@c:x", HP: 25, HPMax: 25},
+	}); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := loadCombatParticipants(s.SessionID); err == nil {
+		t.Error("loadCombatParticipants accepted a seat gap")
+	}
+}
+
+// roster_size disagreeing with the persisted seats means a half-written fight.
+// Reading it as solo would silently drop the party mid-combat.
+func TestHydrateCombatParticipants_RejectsRosterMismatch(t *testing.T) {
+	setupEmptyTestDB(t)
+	uid := id.UserID("@party-mismatch:example.org")
+	defer cleanupCombatSessions(uid)
+
+	s, err := startCombatSession(uid, "run-mm", "node-1", "owlbear", 40, 40, 200, 200)
+	if err != nil {
+		t.Fatal(err)
+	}
+	seatParty(t, s, CombatParticipant{Seat: 1, UserID: "@b:x", HP: 30, HPMax: 30})
+	if _, err := db.Get().Exec(
+		`UPDATE combat_session SET roster_size = 3 WHERE session_id = ?`, s.SessionID,
+	); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := getActiveCombatSession(uid); err == nil {
+		t.Error("hydrate accepted roster_size 3 with 2 seats persisted")
+	}
+}
+
+func TestGetActiveCombatSessionForMember(t *testing.T) {
+	setupEmptyTestDB(t)
+	lead := id.UserID("@lead-lookup:example.org")
+	member := id.UserID("@member-lookup:example.org")
+	defer cleanupCombatSessions(lead)
+
+	s, err := startCombatSession(lead, "run-lookup", "node-1", "owlbear", 40, 40, 200, 200)
+	if err != nil {
+		t.Fatal(err)
+	}
+	seatParty(t, s, CombatParticipant{Seat: 1, UserID: string(member), HP: 30, HPMax: 30})
+
+	// The member owns no session row, so the seat-0 lookup must miss them...
+	if got, err := getActiveCombatSession(member); err != nil || got != nil {
+		t.Errorf("getActiveCombatSession found a row for a party member: %v / %v", got, err)
+	}
+	// ...and the member lookup must find the fight, fully hydrated.
+	got, err := getActiveCombatSessionForMember(member)
+	if err != nil || got == nil {
+		t.Fatalf("getActiveCombatSessionForMember: %v / %v", got, err)
+	}
+	if got.SessionID != s.SessionID || got.RosterSize() != 2 {
+		t.Errorf("wrong session for member: %+v", got)
+	}
+	// The leader is not a participant row, so the member lookup must miss them.
+	if got, err := getActiveCombatSessionForMember(lead); err != nil || got != nil {
+		t.Errorf("member lookup matched the leader: %v / %v", got, err)
+	}
+}
+
+// ── The engine writes every seat, not just the cursor ───────────────────────
+
+// P3 shipped with seats 1+ opening fresh from their Mods on every resume: a
+// party member's once-per-fight one-shots rearmed every single engine step. P4
+// is what fixes that, so pin it. A round_end step mutates no one-shot, so an
+// exact round-trip is the assertion.
+func TestTurnEngine_PartySeatStatusesSurviveAStep(t *testing.T) {
+	sess := turnSession(CombatPhaseRoundEnd, 50, 50)
+	sess.Participants = []CombatParticipant{{
+		Seat: 1, UserID: "@b:x", HP: 30, HPMax: 30,
+		Statuses: ActorStatuses{
+			Raged: true, LuckyUsed: true, DeathSaveUsed: true,
+			WardCharges: 2, ArcaneWardHP: 8,
+			BuffACBonus: 3, // command-layer owned; commit must not zero it
+		},
+	}}
+	want := sess.Participants[0].Statuses
+
+	a, b, enemy := basePlayer(), basePlayer(), baseEnemy()
+	rng := rand.New(rand.NewPCG(42, phaseOrdinal(sess.Phase)))
+	te := resumeTurnEngine(sess, []*Combatant{&a, &b}, &enemy, rng)
+	if _, err := te.step(PlayerAction{}); err != nil {
+		t.Fatal(err)
+	}
+	te.commit()
+
+	if got := sess.Participants[0].Statuses; got != want {
+		t.Errorf("seat 1 statuses moved across a step:\n got %+v\nwant %+v", got, want)
+	}
+	if sess.Participants[0].HP != 30 {
+		t.Errorf("seat 1 HP = %d, want 30", sess.Participants[0].HP)
+	}
+}
+
+// commit reads each seat by index, never off the cursor — round_end parks the
+// cursor at seat 0, and the enemy turn parks it on its target. A seat's damage
+// must land on that seat's row.
+func TestTurnEngine_CommitWritesSeatsByIndex(t *testing.T) {
+	sess := turnSession(CombatPhaseRoundEnd, 50, 50)
+	sess.Participants = []CombatParticipant{
+		{Seat: 1, UserID: "@b:x", HP: 30, HPMax: 30, Statuses: ActorStatuses{PoisonTicks: 1, PoisonDmg: 4}},
+		{Seat: 2, UserID: "@c:x", HP: 25, HPMax: 25},
+	}
+	a, b, c, enemy := basePlayer(), basePlayer(), basePlayer(), baseEnemy()
+	rng := rand.New(rand.NewPCG(42, phaseOrdinal(sess.Phase)))
+	te := resumeTurnEngine(sess, []*Combatant{&a, &b, &c}, &enemy, rng)
+	if _, err := te.step(PlayerAction{}); err != nil {
+		t.Fatal(err)
+	}
+	te.commit()
+
+	// Seat 1's poison ticked on seat 1 alone.
+	if sess.Participants[0].HP != 26 {
+		t.Errorf("seat 1 HP = %d, want 26 (30 - 4 poison)", sess.Participants[0].HP)
+	}
+	if sess.Participants[0].Statuses.PoisonTicks != 0 {
+		t.Errorf("seat 1 poison ticks = %d, want 0", sess.Participants[0].Statuses.PoisonTicks)
+	}
+	if sess.Participants[1].HP != 25 {
+		t.Errorf("seat 2 HP = %d, want 25 — seat 1's poison leaked", sess.Participants[1].HP)
+	}
+	if sess.PlayerHP != 50 {
+		t.Errorf("seat 0 HP = %d, want 50 — seat 1's poison leaked", sess.PlayerHP)
+	}
+}
+
+func equalStrings(a, b []string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := range a {
+		if a[i] != b[i] {
+			return false
+		}
+	}
+	return true
+}
diff --git a/internal/plugin/combat_session.go b/internal/plugin/combat_session.go
index d122027..5861ba2 100644
--- a/internal/plugin/combat_session.go
+++ b/internal/plugin/combat_session.go
@@ -8,6 +8,7 @@ import (
 	"errors"
 	"fmt"
 	"log/slog"
+	"strings"
 	"time"
 
 	"gogobee/internal/db"
@@ -43,41 +44,34 @@ 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 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.
+// ActorStatuses is the per-seat half of the persisted effect state: everything
+// that belongs to one character rather than to the fight. It mirrors the fields
+// of `actor` (combat_engine.go) that must survive a suspend/resume.
 //
-// All fields are scalar by design — CombatStatuses must stay comparable so the
+// The split follows P2's rule. A debuff the enemy stacked onto one character
+// (stat_drain / debuff / max_hp_drain), that character's concentration, their
+// depleting charges, and their once-per-fight one-shots are all per-actor — a
+// party of three carries three independent copies. The enemy's own stance and
+// the round cursor are fight-scoped and live on CombatStatuses.
+//
+// Seat 0's copy is embedded in the session row's statuses_json (see
+// CombatStatuses); seats 1+ each get a combat_participant row carrying this
+// struct alone. That asymmetry is deliberate: a solo fight writes exactly the
+// bytes it wrote before parties existed, and no participant rows at all.
+//
+// All fields are scalar by design — ActorStatuses 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"`
-	Enraged       bool    `json:"enraged,omitempty"`
-	ArmorBroken   bool    `json:"armor_broken,omitempty"`
-	ArmorBreakAmt float64 `json:"armor_break_amt,omitempty"`
-	// EnemySkipNext carries a control-spell skip (Hold Person, Sleep) from the
-	// player_turn phase it was cast in to the enemy_turn phase that consumes it
-	// — 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"`
+type ActorStatuses struct {
+	// Monster-ability effects landing on this character.
+	PoisonTicks int  `json:"poison_ticks,omitempty"`
+	PoisonDmg   int  `json:"poison_dmg,omitempty"`
+	StunPlayer  bool `json:"stun_player,omitempty"`
 
-	// TurnIdx is the position within the round's initiative order (see
-	// turnOrder) of whoever acts next. A solo fight's order is the fixed
-	// [player, enemy], so TurnIdx mirrors the phase and the field is dead
-	// weight — omitempty keeps it out of every solo row. Rows written before
-	// this field existed decode as 0 and are reconciled against Phase on
-	// resume, so a mid-fight upgrade lands on the right slot.
-	TurnIdx int `json:"turn_idx,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.
+	// Depleting resources — mirror the actor charges that genuinely carry
+	// round-to-round. Seeded at fight start (Arcane Ward) or by a mid-fight
+	// !cast / !consume, restored into the actor 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"`
@@ -85,7 +79,7 @@ type CombatStatuses struct {
 	ArcaneWardHP    int     `json:"arcane_ward_hp,omitempty"`
 	HealChargesLeft int     `json:"heal_charges_left,omitempty"`
 
-	// ConcentrationDmg is the per-round damage of the player's active
+	// ConcentrationDmg is the per-round damage of this character's active
 	// concentration AOE (Spirit Guardians, Spike Growth, Call Lightning,
 	// Flaming Sphere…). A one-shot !cast lands its burst the casting round,
 	// then this re-ticks the aura at round_end every round until the fight
@@ -105,31 +99,15 @@ type CombatStatuses struct {
 	AssassinateReroll bool `json:"assassinate_reroll_used,omitempty"`
 	AssassinateBonus  bool `json:"assassinate_bonus_used,omitempty"`
 
-	// Slice-3 stateful monster-ability effects — armed by applyAbility, read by
-	// the shared resolution primitives, round-tripped through combatState so a
-	// suspended/resumed fight (or a reaper auto-play) keeps the same effect
-	// state. EnemyEvadeNext is a one-shot; the rest persist for the fight.
-	EnemyEvadeNext     bool    `json:"enemy_evade_next,omitempty"`
-	EnemyBlockUp       bool    `json:"enemy_block_up,omitempty"`
-	EnemyAdvantage     bool    `json:"enemy_advantage,omitempty"`
-	EnemyRetaliateFrac float64 `json:"enemy_retaliate_frac,omitempty"`
-	EnemyRegen         int     `json:"enemy_regen,omitempty"`
-	EnemySurviveArmed  bool    `json:"enemy_survive_armed,omitempty"`
-	PlayerAtkDrain     int     `json:"player_atk_drain,omitempty"`
-	PlayerACDebuff     int     `json:"player_ac_debuff,omitempty"`
-	MaxHPDrain         int     `json:"max_hp_drain,omitempty"`
+	// Debuffs the enemy has stacked onto this character specifically.
+	PlayerAtkDrain int `json:"player_atk_drain,omitempty"`
+	PlayerACDebuff int `json:"player_ac_debuff,omitempty"`
+	MaxHPDrain     int `json:"max_hp_drain,omitempty"`
 
-	// Slice-4 monster-ability effects — the former flavor-only placeholders.
-	// EnemyRevealNext is a one-shot; the other three persist for the fight.
-	EnemySpellResist bool `json:"enemy_spell_resist,omitempty"`
-	EnemyRevealNext  bool `json:"enemy_reveal_next,omitempty"`
-	EnemyFearImmune  bool `json:"enemy_fear_immune,omitempty"`
-	EnemyAtkBuff     int  `json:"enemy_atk_buff,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).
+	// Persistent stat buffs from this character's mid-fight !cast / !consume,
+	// accumulated as deltas against their freshly-rebuilt combatant.
+	// applySessionBuffs folds these back on 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"`
@@ -142,10 +120,65 @@ type CombatStatuses struct {
 	BuffSpiritDmg       int     `json:"buff_spirit_dmg,omitempty"`
 }
 
+// CombatStatuses is the serialized between-round effect state for a fight: the
+// fight-scoped half (the enemy's stance, the round cursor) plus seat 0's
+// ActorStatuses, embedded. 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.
+//
+// ActorStatuses is embedded anonymously and untagged, so it flattens into the
+// same one-level JSON object the field list used to produce. Rows written
+// before the split decode unchanged, and rows written after are byte-compatible
+// with a reader that predates it.
+//
+// All fields are scalar by design — CombatStatuses must stay comparable so the
+// persistence round-trip can be asserted with ==.
+type CombatStatuses struct {
+	// Seat 0's per-character state. Seats 1+ carry their own copy in
+	// combat_participant rows.
+	ActorStatuses
+
+	Enraged       bool    `json:"enraged,omitempty"`
+	ArmorBroken   bool    `json:"armor_broken,omitempty"`
+	ArmorBreakAmt float64 `json:"armor_break_amt,omitempty"`
+	// EnemySkipNext carries a control-spell skip (Hold Person, Sleep) from the
+	// player_turn phase it was cast in to the enemy_turn phase that consumes it
+	// — those phases resolve as separate engine steps with separate in-memory
+	// combatState, so the flag must survive the commit between them. Holding the
+	// enemy holds it for the whole party, hence fight-scoped.
+	EnemySkipNext bool `json:"enemy_skip_next,omitempty"`
+
+	// TurnIdx is the position within the round's initiative order (see
+	// turnOrder) of whoever acts next. A solo fight's order is the fixed
+	// [player, enemy], so TurnIdx mirrors the phase and the field is dead
+	// weight — omitempty keeps it out of every solo row. Rows written before
+	// this field existed decode as 0 and are reconciled against Phase on
+	// resume, so a mid-fight upgrade lands on the right slot.
+	TurnIdx int `json:"turn_idx,omitempty"`
+
+	// Slice-3 stateful monster-ability effects — armed by applyAbility, read by
+	// the shared resolution primitives, round-tripped through combatState so a
+	// suspended/resumed fight (or a reaper auto-play) keeps the same effect
+	// state. EnemyEvadeNext is a one-shot; the rest persist for the fight.
+	EnemyEvadeNext     bool    `json:"enemy_evade_next,omitempty"`
+	EnemyBlockUp       bool    `json:"enemy_block_up,omitempty"`
+	EnemyAdvantage     bool    `json:"enemy_advantage,omitempty"`
+	EnemyRetaliateFrac float64 `json:"enemy_retaliate_frac,omitempty"`
+	EnemyRegen         int     `json:"enemy_regen,omitempty"`
+	EnemySurviveArmed  bool    `json:"enemy_survive_armed,omitempty"`
+
+	// Slice-4 monster-ability effects — the former flavor-only placeholders.
+	// EnemyRevealNext is a one-shot; the other three persist for the fight.
+	EnemySpellResist bool `json:"enemy_spell_resist,omitempty"`
+	EnemyRevealNext  bool `json:"enemy_reveal_next,omitempty"`
+	EnemyFearImmune  bool `json:"enemy_fear_immune,omitempty"`
+	EnemyAtkBuff     int  `json:"enemy_atk_buff,omitempty"`
+}
+
 // applyBuffDelta folds one resolved buff (the result of a !cast / !consume
-// player turn) into the persisted fight-scoped state. Stat components
+// player turn) into that character's persisted state. Stat components
 // accumulate as deltas; depleting resources add to their running counters.
-func (s *CombatStatuses) applyBuffDelta(d turnBuffDelta) {
+func (s *ActorStatuses) applyBuffDelta(d turnBuffDelta) {
 	s.BuffACBonus += d.dAC
 	s.BuffAtkBonus += d.dAtk
 	s.BuffSpeedBonus += d.dSpeed
@@ -188,6 +221,17 @@ func seedCombatSessionOneShots(s *CombatSession, playerMods CombatModifiers) boo
 		st.AutoCritFirst || st.ArcaneWardHP != 0 || st.HealChargesLeft != 0
 }
 
+// CombatParticipant is one party member's seat in a fight, from seat 1 up.
+// Seat 0 is the session row itself (UserID / PlayerHP / Statuses.ActorStatuses)
+// and never appears here.
+type CombatParticipant struct {
+	Seat     int
+	UserID   string
+	HP       int
+	HPMax    int
+	Statuses ActorStatuses
+}
+
 // CombatSession is the in-memory shape of a combat_session row.
 type CombatSession struct {
 	SessionID    string
@@ -207,11 +251,48 @@ type CombatSession struct {
 	StartedAt    time.Time
 	LastActionAt time.Time
 	ExpiresAt    time.Time
+
+	// Participants are the party's seats 1..N-1, ordered by seat and contiguous
+	// (loadCombatParticipants enforces both). Empty for a solo fight — the only
+	// kind that existed before N3 — so nothing in the solo path allocates or
+	// queries for it.
+	Participants []CombatParticipant
+
+	// rosterSize mirrors the combat_session.roster_size column as scanned. It
+	// exists only so a loader can tell "solo, skip the participant query" from
+	// "party, go fetch the seats" in one round trip. In memory Participants is
+	// the single source of truth; every write re-derives the column from it.
+	rosterSize int
 }
 
 // IsActive reports whether the fight is still in flight.
 func (s *CombatSession) IsActive() bool { return s.Status == CombatStatusActive }
 
+// RosterSize is the number of seated player characters: seat 0 plus the party.
+func (s *CombatSession) RosterSize() int { return 1 + len(s.Participants) }
+
+// IsParty reports whether more than one character is seated.
+func (s *CombatSession) IsParty() bool { return len(s.Participants) > 0 }
+
+// SeatUserIDs lists every seated member's user id in seat order.
+func (s *CombatSession) SeatUserIDs() []string {
+	out := make([]string, 0, s.RosterSize())
+	out = append(out, s.UserID)
+	for _, p := range s.Participants {
+		out = append(out, p.UserID)
+	}
+	return out
+}
+
+// actorStatusesForSeat returns the persisted per-character state for a seat.
+// Seat 0 reads through to the session's embedded copy.
+func (s *CombatSession) actorStatusesForSeat(seat int) ActorStatuses {
+	if seat == 0 {
+		return s.Statuses.ActorStatuses
+	}
+	return s.Participants[seat-1].Statuses
+}
+
 // Errors returned by the combat session layer.
 var (
 	ErrCombatSessionAlreadyActive = errors.New("combat session already active for player")
@@ -224,7 +305,7 @@ const combatSessionCols = `
 	session_id, user_id, run_id, encounter_id, enemy_id,
 	round, phase, player_hp, player_hp_max, enemy_hp, enemy_hp_max,
 	statuses_json, turn_log_json, status,
-	started_at, last_action_at, expires_at`
+	started_at, last_action_at, expires_at, roster_size`
 
 // newCombatSessionID — 16-char hex token. Same scheme as zone runs / expeditions.
 func newCombatSessionID() string {
@@ -296,7 +377,10 @@ func getActiveCombatSession(userID id.UserID) (*CombatSession, error) {
 	if errors.Is(err, sql.ErrNoRows) {
 		return nil, nil
 	}
-	return s, err
+	if err != nil {
+		return nil, err
+	}
+	return s, hydrateCombatParticipants(s)
 }
 
 // hasActiveCombatSession reports whether the player is currently locked into a
@@ -324,7 +408,10 @@ func getCombatSession(sessionID string) (*CombatSession, error) {
 	if errors.Is(err, sql.ErrNoRows) {
 		return nil, nil
 	}
-	return s, err
+	if err != nil {
+		return nil, err
+	}
+	return s, hydrateCombatParticipants(s)
 }
 
 // getCombatSessionForEncounter returns the most recent session for a
@@ -341,7 +428,10 @@ func getCombatSessionForEncounter(runID, encounterID string) (*CombatSession, er
 	if errors.Is(err, sql.ErrNoRows) {
 		return nil, nil
 	}
-	return s, err
+	if err != nil {
+		return nil, err
+	}
+	return s, hydrateCombatParticipants(s)
 }
 
 func scanCombatSession(row scanner) (*CombatSession, error) {
@@ -354,7 +444,7 @@ func scanCombatSession(row scanner) (*CombatSession, error) {
 		&s.SessionID, &s.UserID, &s.RunID, &s.EncounterID, &s.EnemyID,
 		&s.Round, &s.Phase, &s.PlayerHP, &s.PlayerHPMax, &s.EnemyHP, &s.EnemyHPMax,
 		&statusesJSON, &logJSON, &s.Status,
-		&s.StartedAt, &s.LastActionAt, &s.ExpiresAt,
+		&s.StartedAt, &s.LastActionAt, &s.ExpiresAt, &s.rosterSize,
 	); err != nil {
 		return nil, err
 	}
@@ -377,32 +467,68 @@ func scanCombatSession(row scanner) (*CombatSession, error) {
 // saveCombatSession persists the mutable fields after a state-machine step.
 // session_id / user_id / run_id / encounter_id / enemy_id / *_hp_max are
 // immutable for a fight's lifetime and are not rewritten.
+//
+// A party fight also writes back its seats, in the same transaction as the
+// session row: a crash between the two would leave seat 0 a round ahead of the
+// rest of the roster. The solo path keeps its single unwrapped UPDATE — there
+// are no seats to desync from, and it is the path the whole balance corpus and
+// every pre-N3 fight take.
+//
+// roster_size is not rewritten here: insertCombatParticipants stamped it at
+// fight start and the roster is fixed for the fight's lifetime (a dead member
+// keeps their seat at 0 HP).
 func saveCombatSession(s *CombatSession) error {
 	statusesJSON, _ := json.Marshal(s.Statuses)
 	logJSON, _ := json.Marshal(s.TurnLog)
 	now := time.Now().UTC()
-	if _, err := db.Get().Exec(`
-		UPDATE combat_session
-		   SET round = ?, phase = ?,
-		       player_hp = ?, enemy_hp = ?,
-		       statuses_json = ?, turn_log_json = ?,
-		       status = ?, last_action_at = ?
-		 WHERE session_id = ?`,
+
+	if len(s.Participants) == 0 {
+		if _, err := db.Get().Exec(saveCombatSessionSQL,
+			s.Round, s.Phase, s.PlayerHP, s.EnemyHP,
+			string(statusesJSON), string(logJSON), s.Status, now, s.SessionID,
+		); err != nil {
+			return err
+		}
+		s.LastActionAt = now
+		return nil
+	}
+
+	tx, err := db.Get().Begin()
+	if err != nil {
+		return err
+	}
+	defer tx.Rollback()
+
+	if _, err := tx.Exec(saveCombatSessionSQL,
 		s.Round, s.Phase, s.PlayerHP, s.EnemyHP,
 		string(statusesJSON), string(logJSON), s.Status, now, s.SessionID,
 	); err != nil {
 		return err
 	}
+	if err := saveCombatParticipantsTx(tx, s.SessionID, s.Participants); err != nil {
+		return err
+	}
+	if err := tx.Commit(); err != nil {
+		return err
+	}
 	s.LastActionAt = now
 	return nil
 }
 
+const saveCombatSessionSQL = `
+	UPDATE combat_session
+	   SET round = ?, phase = ?,
+	       player_hp = ?, enemy_hp = ?,
+	       statuses_json = ?, turn_log_json = ?,
+	       status = ?, last_action_at = ?
+	 WHERE session_id = ?`
+
 // listExpiredCombatSessions returns every active session past its expires_at.
 // The timeout reaper (reapExpiredCombatSessions) auto-plays each of these to a
 // real win/loss from persisted mid-state — per the 2026-05-13 decision, an
 // abandoned fight is finished by the engine, not flatly marked as a retreat.
 func listExpiredCombatSessions() ([]*CombatSession, error) {
-	rows, err := db.Get().Query(`SELECT `+combatSessionCols+`
+	rows, err := db.Get().Query(`SELECT ` + combatSessionCols + `
 		  FROM combat_session
 		 WHERE status = 'active'
 		   AND expires_at <= CURRENT_TIMESTAMP
@@ -420,9 +546,162 @@ func listExpiredCombatSessions() ([]*CombatSession, error) {
 		}
 		out = append(out, s)
 	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	// Hydrate only after the cursor is closed: a nested query while iterating
+	// can stall on a single-connection pool.
+	rows.Close()
+	for _, s := range out {
+		if err := hydrateCombatParticipants(s); err != nil {
+			return nil, err
+		}
+	}
+	return out, nil
+}
+
+// ── Party seats (N3/P4) ──────────────────────────────────────────────────────
+
+// hydrateCombatParticipants fills s.Participants for a party session. A solo
+// session (roster_size 1, which is every row written before N3) returns
+// immediately without touching combat_participant, so the common path stays at
+// the single query it has always been.
+func hydrateCombatParticipants(s *CombatSession) error {
+	if s == nil || s.rosterSize <= 1 {
+		return nil
+	}
+	ps, err := loadCombatParticipants(s.SessionID)
+	if err != nil {
+		return err
+	}
+	if got := 1 + len(ps); got != s.rosterSize {
+		return fmt.Errorf("combat session %s: roster_size %d but %d seats persisted",
+			s.SessionID, s.rosterSize, got)
+	}
+	s.Participants = ps
+	return nil
+}
+
+// loadCombatParticipants returns seats 1..N-1 in seat order, verifying that the
+// seats are contiguous from 1. The engine indexes the roster positionally, so a
+// gap would silently shift every member down a seat — better to fail the load.
+func loadCombatParticipants(sessionID string) ([]CombatParticipant, error) {
+	rows, err := db.Get().Query(`
+		SELECT seat, user_id, hp, hp_max, statuses_json
+		  FROM combat_participant
+		 WHERE session_id = ?
+		 ORDER BY seat ASC`, sessionID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var out []CombatParticipant
+	for rows.Next() {
+		var (
+			p            CombatParticipant
+			statusesJSON string
+		)
+		if err := rows.Scan(&p.Seat, &p.UserID, &p.HP, &p.HPMax, &statusesJSON); err != nil {
+			return nil, err
+		}
+		if statusesJSON != "" {
+			if err := json.Unmarshal([]byte(statusesJSON), &p.Statuses); err != nil {
+				return nil, fmt.Errorf("decode participant statuses (seat %d): %w", p.Seat, err)
+			}
+		}
+		if want := len(out) + 1; p.Seat != want {
+			return nil, fmt.Errorf("combat session %s: seat %d out of order (want %d)",
+				sessionID, p.Seat, want)
+		}
+		out = append(out, p)
+	}
 	return out, rows.Err()
 }
 
+// insertCombatParticipants seats the party at fight start and stamps
+// roster_size so later loads know to come back for them. Seat 0 is the session
+// row's own user and is not written here; callers pass seats 1..N-1.
+func insertCombatParticipants(sessionID string, ps []CombatParticipant) error {
+	if len(ps) == 0 {
+		return nil
+	}
+	tx, err := db.Get().Begin()
+	if err != nil {
+		return err
+	}
+	defer tx.Rollback()
+
+	for _, p := range ps {
+		statusesJSON, _ := json.Marshal(p.Statuses)
+		if _, err := tx.Exec(`
+			INSERT INTO combat_participant (session_id, seat, user_id, hp, hp_max, statuses_json)
+			VALUES (?, ?, ?, ?, ?, ?)`,
+			sessionID, p.Seat, p.UserID, p.HP, p.HPMax, string(statusesJSON),
+		); err != nil {
+			return fmt.Errorf("insert participant seat %d: %w", p.Seat, err)
+		}
+	}
+	if _, err := tx.Exec(
+		`UPDATE combat_session SET roster_size = ? WHERE session_id = ?`,
+		1+len(ps), sessionID,
+	); err != nil {
+		return fmt.Errorf("stamp roster_size: %w", err)
+	}
+	return tx.Commit()
+}
+
+// saveCombatParticipantsTx writes back the mutable half of every seat after an
+// engine step. session_id / seat / user_id / hp_max are immutable for a fight's
+// lifetime and are not rewritten. It runs inside the caller's transaction so
+// the seats commit together with the session row that indexes them.
+func saveCombatParticipantsTx(tx *sql.Tx, sessionID string, ps []CombatParticipant) error {
+	for _, p := range ps {
+		statusesJSON, _ := json.Marshal(p.Statuses)
+		if _, err := tx.Exec(`
+			UPDATE combat_participant
+			   SET hp = ?, statuses_json = ?
+			 WHERE session_id = ? AND seat = ?`,
+			p.HP, string(statusesJSON), sessionID, p.Seat,
+		); err != nil {
+			return fmt.Errorf("save participant seat %d: %w", p.Seat, err)
+		}
+	}
+	return nil
+}
+
+// getActiveCombatSessionForMember finds the in-flight fight a player is seated
+// in at seat 1+, i.e. the one their own user_id does not key. Seat 0's fight is
+// getActiveCombatSession's job; this is the party-member equivalent, and it
+// returns (nil, nil) when there is none.
+func getActiveCombatSessionForMember(userID id.UserID) (*CombatSession, error) {
+	row := db.Get().QueryRow(`SELECT `+prefixCols("cs", combatSessionCols)+`
+		  FROM combat_session cs
+		  JOIN combat_participant cp ON cp.session_id = cs.session_id
+		 WHERE cp.user_id = ? AND cs.status = 'active'
+		 ORDER BY cs.started_at DESC
+		 LIMIT 1`, string(userID))
+	s, err := scanCombatSession(row)
+	if errors.Is(err, sql.ErrNoRows) {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+	return s, hydrateCombatParticipants(s)
+}
+
+// prefixCols qualifies every column in a SELECT list with a table alias, so the
+// shared combatSessionCols can be reused inside a join without ambiguity on the
+// columns both tables carry (session_id, user_id, statuses_json).
+func prefixCols(alias, cols string) string {
+	fields := strings.Split(cols, ",")
+	for i, f := range fields {
+		fields[i] = alias + "." + strings.TrimSpace(f)
+	}
+	return strings.Join(fields, ", ")
+}
+
 // markCombatSessionExpired is the fallback terminal outcome for a stale session
 // the reaper cannot auto-play (zone run gone, enemy missing from the bestiary,
 // or a runaway fight that won't converge). It parks the row in 'expired'/'over'
diff --git a/internal/plugin/combat_session_build.go b/internal/plugin/combat_session_build.go
index 25b643b..7efaeed 100644
--- a/internal/plugin/combat_session_build.go
+++ b/internal/plugin/combat_session_build.go
@@ -132,16 +132,17 @@ func (p *AdventurePlugin) combatantsForSession(sess *CombatSession) (player Comb
 	// 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)
+	applySessionBuffs(&player, sess.Statuses.ActorStatuses)
 	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
+// buff onto one rebuilt character. The buffs are stored as accumulated deltas
+// (diffTurnBuff produced them against that character'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) {
+// round without double-counting. It takes ActorStatuses rather than the whole
+// session because buffs are per-caster: each seat folds in only its own.
+func applySessionBuffs(player *Combatant, s ActorStatuses) {
 	player.Stats.AC += s.BuffACBonus
 	player.Stats.AttackBonus += s.BuffAtkBonus
 	player.Stats.Speed += s.BuffSpeedBonus
diff --git a/internal/plugin/combat_session_test.go b/internal/plugin/combat_session_test.go
index 51554bf..5cf19df 100644
--- a/internal/plugin/combat_session_test.go
+++ b/internal/plugin/combat_session_test.go
@@ -52,7 +52,7 @@ func TestStartCombatSession_RoundTrip(t *testing.T) {
 	got.Phase = CombatPhaseRoundEnd
 	got.PlayerHP = 40
 	got.EnemyHP = 15
-	got.Statuses = CombatStatuses{PoisonTicks: 2, PoisonDmg: 5, Enraged: true}
+	got.Statuses = CombatStatuses{ActorStatuses: ActorStatuses{PoisonTicks: 2, PoisonDmg: 5}, Enraged: true}
 	got.TurnLog = append(got.TurnLog, CombatEvent{Round: 3, Actor: "player", Action: "hit", Damage: 9})
 	if err := saveCombatSession(got); err != nil {
 		t.Fatalf("saveCombatSession: %v", err)
@@ -67,7 +67,7 @@ func TestStartCombatSession_RoundTrip(t *testing.T) {
 	if reloaded.PlayerHP != 40 || reloaded.EnemyHP != 15 {
 		t.Errorf("hp not persisted: %+v", reloaded)
 	}
-	if reloaded.Statuses != (CombatStatuses{PoisonTicks: 2, PoisonDmg: 5, Enraged: true}) {
+	if reloaded.Statuses != (CombatStatuses{ActorStatuses: ActorStatuses{PoisonTicks: 2, PoisonDmg: 5}, Enraged: true}) {
 		t.Errorf("statuses not persisted: %+v", reloaded.Statuses)
 	}
 	if len(reloaded.TurnLog) != 1 || reloaded.TurnLog[0].Action != "hit" {
@@ -223,7 +223,7 @@ func TestTurnEngine_PhaseProgression(t *testing.T) {
 
 func TestTurnEngine_PoisonTickAtRoundEnd(t *testing.T) {
 	sess := turnSession(CombatPhaseRoundEnd, 50, 80)
-	sess.Statuses = CombatStatuses{PoisonTicks: 2, PoisonDmg: 7}
+	sess.Statuses = CombatStatuses{ActorStatuses: ActorStatuses{PoisonTicks: 2, PoisonDmg: 7}}
 	player, enemy := basePlayer(), baseEnemy()
 
 	events, err := stepEngine(sess, &player, &enemy, PlayerAction{})
@@ -246,7 +246,7 @@ func TestTurnEngine_PoisonTickAtRoundEnd(t *testing.T) {
 
 func TestTurnEngine_PoisonTickCanBeLethal(t *testing.T) {
 	sess := turnSession(CombatPhaseRoundEnd, 4, 80)
-	sess.Statuses = CombatStatuses{PoisonTicks: 1, PoisonDmg: 9}
+	sess.Statuses = CombatStatuses{ActorStatuses: ActorStatuses{PoisonTicks: 1, PoisonDmg: 9}}
 	player, enemy := basePlayer(), baseEnemy()
 
 	if _, err := stepEngine(sess, &player, &enemy, PlayerAction{}); err != nil {
@@ -265,7 +265,7 @@ func TestTurnEngine_PoisonTickCanBeLethal(t *testing.T) {
 
 func TestTurnEngine_ConcentrationTickAtRoundEnd(t *testing.T) {
 	sess := turnSession(CombatPhaseRoundEnd, 50, 80)
-	sess.Statuses = CombatStatuses{ConcentrationDmg: 12}
+	sess.Statuses = CombatStatuses{ActorStatuses: ActorStatuses{ConcentrationDmg: 12}}
 	player, enemy := basePlayer(), baseEnemy()
 
 	events, err := stepEngine(sess, &player, &enemy, PlayerAction{})
@@ -288,7 +288,7 @@ func TestTurnEngine_ConcentrationTickAtRoundEnd(t *testing.T) {
 
 func TestTurnEngine_ConcentrationTickCanBeLethal(t *testing.T) {
 	sess := turnSession(CombatPhaseRoundEnd, 50, 9)
-	sess.Statuses = CombatStatuses{ConcentrationDmg: 12}
+	sess.Statuses = CombatStatuses{ActorStatuses: ActorStatuses{ConcentrationDmg: 12}}
 	player, enemy := basePlayer(), baseEnemy()
 
 	if _, err := stepEngine(sess, &player, &enemy, PlayerAction{}); err != nil {
@@ -433,7 +433,7 @@ func TestApplyBuffDelta(t *testing.T) {
 
 func TestApplySessionBuffs(t *testing.T) {
 	player := basePlayer() // AC 0, AttackBonus 0, CritRate 0.05, Speed 10, DamageReduct 1.0
-	applySessionBuffs(&player, CombatStatuses{
+	applySessionBuffs(&player, ActorStatuses{
 		BuffACBonus: 3, BuffAtkBonus: 2, BuffSpeedBonus: 5, BuffCritRate: 0.15,
 		BuffDamageBonus: 0.25, BuffDamageReductMul: 0.8, BuffPetProc: 0.5, BuffPetDmg: 6,
 	})
@@ -473,7 +473,7 @@ func TestSeedCombatSessionOneShots(t *testing.T) {
 // re-bank a depleted ward charge — on every resumed round.
 func TestTurnEngine_OneShotsRoundTrip(t *testing.T) {
 	sess := turnSession(CombatPhaseRoundEnd, 50, 50)
-	sess.Statuses = CombatStatuses{
+	sess.Statuses = CombatStatuses{ActorStatuses: ActorStatuses{
 		WardCharges: 3, SporeRounds: 2, ReflectFrac: 0.5, AutoCritFirst: true,
 		ArcaneWardHP: 10, HealChargesLeft: 1,
 		Raged: true, LuckyUsed: true, DeathSaveUsed: true, PendingRage: true,
@@ -481,7 +481,7 @@ func TestTurnEngine_OneShotsRoundTrip(t *testing.T) {
 		// 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()
 
diff --git a/internal/plugin/combat_turn_engine.go b/internal/plugin/combat_turn_engine.go
index 6282d2e..7bd2342 100644
--- a/internal/plugin/combat_turn_engine.go
+++ b/internal/plugin/combat_turn_engine.go
@@ -215,44 +215,98 @@ func turnIdxForPhase(order []int, idx int, phase string) int {
 // resumeTurnEngine rebuilds the in-memory combatState from a persisted session.
 // rng is the deterministic source for this step (see combatSessionStepRNG).
 //
-// Seat 0 restores the session's persisted per-character statuses. Seats 1+ open
-// fresh from their combatant's modifiers: a party's per-member statuses need the
-// combat_participant rows that P4 adds, and until then no persisted session ever
-// carries more than one seat.
-func resumeTurnEngine(sess *CombatSession, players []*Combatant, enemy *Combatant, rng *rand.Rand) *turnEngine {
-	seat0 := &actor{
-		c:           players[0],
-		playerHP:    sess.PlayerHP,
-		hpMax:       sess.PlayerHPMax,
-		poisonTicks: sess.Statuses.PoisonTicks,
-		poisonDmg:   sess.Statuses.PoisonDmg,
-		stunPlayer:  sess.Statuses.StunPlayer,
-		// 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,
-		concentrationDmg:      sess.Statuses.ConcentrationDmg,
-		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,
+// restoreActor rebuilds one seat from its persisted state: live HP, the pool
+// ceiling, and the per-character effect set. Depleting resources and the
+// once-per-fight "already used" flags are restored rather than rearmed, so a
+// charge can't silently reset across a suspend/resume or a reaper auto-play.
+// snapshotActor is its exact inverse — keep the two field lists in step.
+func restoreActor(c *Combatant, hp, hpMax int, s ActorStatuses) *actor {
+	return &actor{
+		c:        c,
+		playerHP: hp,
+		hpMax:    hpMax,
+
+		poisonTicks: s.PoisonTicks,
+		poisonDmg:   s.PoisonDmg,
+		stunPlayer:  s.StunPlayer,
+
+		wardCharges:     s.WardCharges,
+		sporeRounds:     s.SporeRounds,
+		reflectFrac:     s.ReflectFrac,
+		autoCrit:        s.AutoCritFirst,
+		arcaneWardHP:    s.ArcaneWardHP,
+		healChargesLeft: s.HealChargesLeft,
+
+		concentrationDmg: s.ConcentrationDmg,
+
+		deathSaveUsed:         s.DeathSaveUsed,
+		luckyUsed:             s.LuckyUsed,
+		raged:                 s.Raged,
+		pendingRageAttack:     s.PendingRage,
+		firstAttackBonusUsed:  s.FirstAtkBonusUsed,
+		assassinateRerollUsed: s.AssassinateReroll,
+		assassinateBonusUsed:  s.AssassinateBonus,
+
 		// Enemy debuffs stacked onto this character specifically.
-		playerAtkDrain: sess.Statuses.PlayerAtkDrain,
-		playerACDebuff: sess.Statuses.PlayerACDebuff,
-		maxHPDrain:     sess.Statuses.MaxHPDrain,
+		playerAtkDrain: s.PlayerAtkDrain,
+		playerACDebuff: s.PlayerACDebuff,
+		maxHPDrain:     s.MaxHPDrain,
 	}
+}
+
+// snapshotActor folds one seat's live state back into its persisted form. The
+// Buff* deltas are owned by the command layer (a !cast / !consume folds them in
+// via applyBuffDelta) and are not combatState fields, so they are carried over
+// from the prior snapshot rather than re-derived.
+func snapshotActor(a *actor, prior ActorStatuses) ActorStatuses {
+	s := prior
+	s.PoisonTicks = a.poisonTicks
+	s.PoisonDmg = a.poisonDmg
+	s.StunPlayer = a.stunPlayer
+
+	s.WardCharges = a.wardCharges
+	s.SporeRounds = a.sporeRounds
+	s.ReflectFrac = a.reflectFrac
+	s.AutoCritFirst = a.autoCrit
+	s.ArcaneWardHP = a.arcaneWardHP
+	s.HealChargesLeft = a.healChargesLeft
+
+	s.ConcentrationDmg = a.concentrationDmg
+
+	s.DeathSaveUsed = a.deathSaveUsed
+	s.LuckyUsed = a.luckyUsed
+	s.Raged = a.raged
+	s.PendingRage = a.pendingRageAttack
+	s.FirstAtkBonusUsed = a.firstAttackBonusUsed
+	s.AssassinateReroll = a.assassinateRerollUsed
+	s.AssassinateBonus = a.assassinateBonusUsed
+
+	s.PlayerAtkDrain = a.playerAtkDrain
+	s.PlayerACDebuff = a.playerACDebuff
+	s.MaxHPDrain = a.maxHPDrain
+	return s
+}
+
+// Every seat restores its own persisted per-character statuses: seat 0 from the
+// session row's embedded ActorStatuses, seats 1+ from their combat_participant
+// row. Without that, a party member's once-per-fight one-shots would rearm on
+// every engine step. players[i] must be the combatant for seat i — the caller
+// owns that ordering, and it must match sess.Participants[i-1].UserID.
+func resumeTurnEngine(sess *CombatSession, players []*Combatant, enemy *Combatant, rng *rand.Rand) *turnEngine {
+	seat0 := restoreActor(players[0], sess.PlayerHP, sess.PlayerHPMax, sess.Statuses.ActorStatuses)
 	actors := make([]*actor, len(players))
 	actors[0] = seat0
 	for i := 1; i < len(players); i++ {
-		actors[i] = newActor(players[i])
+		// A roster longer than the persisted seats can only happen if a caller
+		// hands us combatants the session never seated. Open those fresh rather
+		// than panicking mid-fight; hydrateCombatParticipants already rejects
+		// the reverse (a session claiming more seats than it persisted).
+		if i-1 >= len(sess.Participants) {
+			actors[i] = newActor(players[i])
+			continue
+		}
+		p := sess.Participants[i-1]
+		actors[i] = restoreActor(players[i], p.HP, p.HPMax, p.Statuses)
 	}
 	st := &combatState{
 		actor:          seat0,
@@ -712,53 +766,45 @@ func (te *turnEngine) finish(status string) {
 //
 // 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.
-// Per-actor state is read off seat 0 rather than off the cursor, which the
-// enemy turn parks on its target and round_end walks across the roster. Seat 0
-// is the only member a session row can hold; P4's participant rows carry the
-// rest.
+// rebuilt combatant by applySessionBuffs — so snapshotActor carries them over
+// from the prior state rather than re-deriving them.
+//
+// Per-actor state is read off each seat by index, never off the cursor: the
+// enemy turn parks the cursor on its target and round_end walks it across the
+// roster, so whoever it points at when commit runs is an accident of the phase.
+// Seat 0 lands on the session row; seats 1+ on their participant rows.
 func (te *turnEngine) commit() {
 	st := te.st
-	a := st.actors[0]
 	te.sess.Round = st.round
-	te.sess.PlayerHP = a.playerHP
 	te.sess.EnemyHP = st.enemyHP
+
+	seat0 := st.actors[0]
+	te.sess.PlayerHP = seat0.playerHP
 	s := &te.sess.Statuses
-	s.PoisonTicks = a.poisonTicks
-	s.PoisonDmg = a.poisonDmg
-	s.StunPlayer = a.stunPlayer
+	s.ActorStatuses = snapshotActor(seat0, s.ActorStatuses)
+
+	for i := 1; i < len(st.actors) && i-1 < len(te.sess.Participants); i++ {
+		p := &te.sess.Participants[i-1]
+		p.HP = st.actors[i].playerHP
+		p.Statuses = snapshotActor(st.actors[i], p.Statuses)
+	}
+
+	// Fight-scoped: the enemy's own stance, and the debuffs it wears.
 	s.Enraged = st.enraged
 	s.ArmorBroken = st.armorBroken
 	s.ArmorBreakAmt = st.armorBreakAmt
 	s.EnemySkipNext = st.enemySkipFirst
-	s.WardCharges = a.wardCharges
-	s.SporeRounds = a.sporeRounds
-	s.ReflectFrac = a.reflectFrac
-	s.AutoCritFirst = a.autoCrit
-	s.ArcaneWardHP = a.arcaneWardHP
-	s.HealChargesLeft = a.healChargesLeft
-	s.ConcentrationDmg = a.concentrationDmg
-	s.DeathSaveUsed = a.deathSaveUsed
-	s.LuckyUsed = a.luckyUsed
-	s.Raged = a.raged
-	s.PendingRage = a.pendingRageAttack
-	s.FirstAtkBonusUsed = a.firstAttackBonusUsed
-	s.AssassinateReroll = a.assassinateRerollUsed
-	s.AssassinateBonus = a.assassinateBonusUsed
 	s.EnemyEvadeNext = st.enemyEvadeNext
 	s.EnemyBlockUp = st.enemyBlockUp
 	s.EnemyAdvantage = st.enemyAdvantage
 	s.EnemyRetaliateFrac = st.enemyRetaliateFrac
 	s.EnemyRegen = st.enemyRegen
 	s.EnemySurviveArmed = st.enemySurviveArmed
-	s.PlayerAtkDrain = a.playerAtkDrain
-	s.PlayerACDebuff = a.playerACDebuff
-	s.MaxHPDrain = a.maxHPDrain
 	s.EnemySpellResist = st.enemySpellResist
 	s.EnemyRevealNext = st.enemyRevealNext
 	s.EnemyFearImmune = st.enemyFearImmune
 	s.EnemyAtkBuff = st.enemyAtkBuff
+
 	te.sess.TurnLog = append(te.sess.TurnLog, st.events...)
 }
 
diff --git a/internal/plugin/dnd_subclass_combat_test.go b/internal/plugin/dnd_subclass_combat_test.go
index 69665f6..207dee9 100644
--- a/internal/plugin/dnd_subclass_combat_test.go
+++ b/internal/plugin/dnd_subclass_combat_test.go
@@ -1,6 +1,7 @@
 package plugin
 
 import (
+	"math/rand/v2"
 	"testing"
 
 	"maunium.net/go/mautrix/id"
@@ -653,9 +654,12 @@ func TestSimulateCombat_FirstAttackBonusImprovesEarlyHits(t *testing.T) {
 	// Probabilistic: a +4 on the opening swing should noticeably lift the
 	// player's win rate against a stiff enemy. The lift is small (a single
 	// hit's worth), so the trial count needs to be high enough that
-	// variance doesn't swamp the signal — at 1500 trials we were seeing
-	// ~14 wins of difference on bad seeds vs. the 25-win threshold.
-	const trials = 6000
+	// variance doesn't swamp the signal. At 6000 trials it still did: sweeping
+	// 40 seeds, the mean margin was +127 wins but the worst was -42, against a
+	// +50 threshold — so this test failed for roughly one seed in ten and had
+	// been doing so on a clean tree. At 24000 every one of those 40 seeds
+	// clears by at least +267.
+	const trials = 24000
 	hardEnemy := Combatant{
 		Name: "Wall",
 		Stats: CombatStats{
@@ -664,12 +668,17 @@ func TestSimulateCombat_FirstAttackBonusImprovesEarlyHits(t *testing.T) {
 		},
 		Mods: CombatModifiers{DamageReduct: 1.0},
 	}
+	// Seeded, not global: SimulateCombat(nil rng) draws from the package-global
+	// stream, so this test's verdict used to depend on how much randomness every
+	// test declared before it happened to consume. It failed intermittently on a
+	// clean tree and re-flaked whenever an unrelated change shifted the stream.
+	rng := statCompareRNG()
 	plainWins, bmWins := 0, 0
 	for i := 0; i < trials; i++ {
-		if SimulateCombat(plainBMPlayer(), hardEnemy, defaultCombatPhases).PlayerWon {
+		if simulateCombatWithRNG(plainBMPlayer(), hardEnemy, defaultCombatPhases, rng).PlayerWon {
 			plainWins++
 		}
-		if SimulateCombat(bmPrecisionPlayer(), hardEnemy, defaultCombatPhases).PlayerWon {
+		if simulateCombatWithRNG(bmPrecisionPlayer(), hardEnemy, defaultCombatPhases, rng).PlayerWon {
 			bmWins++
 		}
 	}
@@ -678,13 +687,25 @@ func TestSimulateCombat_FirstAttackBonusImprovesEarlyHits(t *testing.T) {
 	}
 }
 
+// statCompareRNG seeds the two statistical A/B subclass tests below. They
+// compare win counts between two builds over thousands of trials with a fixed
+// threshold, which is only meaningful if the stream is theirs alone. The seed
+// is arbitrary — any value where the true effect clears the threshold works;
+// this one does, and pinning it is what makes the tests reproducible.
+func statCompareRNG() *rand.Rand { return rand.New(rand.NewPCG(0x5eed, 0xc0ffee)) }
+
 // Surface check: AssassinateBonusDmg only consumed once.
 func TestResolvePlayerAttack_AssassinateBonusFirstHitOnly(t *testing.T) {
 	// Drive resolvePlayerAttack via a SimulateCombat run and confirm the
 	// total enemy damage taken on hit is the *base* damage on subsequent
 	// hits (i.e. only the first hit got the +bonus). Statistical compare
 	// against an Assassin L5 vs. an identical player without the bonus.
-	const trials = 800
+	//
+	// 800 trials was too few for the "any lift at all" threshold: over 40 seeds
+	// the mean margin was only +12.8 wins and two seeds came out negative. The
+	// bonus is genuinely small — one hit's worth, once per fight — so the trial
+	// count carries the signal. At 6000 the worst of those seeds clears by +68.
+	const trials = 6000
 	build := func(bonus int) Combatant {
 		return Combatant{
 			Name: "Assn", IsPlayer: true,
@@ -707,16 +728,18 @@ func TestResolvePlayerAttack_AssassinateBonusFirstHitOnly(t *testing.T) {
 			Mods: CombatModifiers{DamageReduct: 1.0},
 		}
 	}
+	// Seeded for the same reason as the Precision Attack test above.
+	rng := statCompareRNG()
 	plainWins, assnWins := 0, 0
 	for i := 0; i < trials; i++ {
-		if SimulateCombat(build(0), enemy(), defaultCombatPhases).PlayerWon {
+		if simulateCombatWithRNG(build(0), enemy(), defaultCombatPhases, rng).PlayerWon {
 			plainWins++
 		}
-		if SimulateCombat(build(8), enemy(), defaultCombatPhases).PlayerWon {
+		if simulateCombatWithRNG(build(8), enemy(), defaultCombatPhases, rng).PlayerWon {
 			assnWins++
 		}
 	}
-	if assnWins <= plainWins {
+	if assnWins-plainWins < 25 {
 		t.Errorf("Assassinate bonus damage didn't help: plain=%d assn=%d", plainWins, assnWins)
 	}
 }
diff --git a/internal/plugin/expedition_party.go b/internal/plugin/expedition_party.go
new file mode 100644
index 0000000..ae441d2
--- /dev/null
+++ b/internal/plugin/expedition_party.go
@@ -0,0 +1,281 @@
+package plugin
+
+import (
+	"database/sql"
+	"errors"
+	"fmt"
+	"time"
+
+	"gogobee/internal/db"
+	"maunium.net/go/mautrix/id"
+)
+
+// N3/P4 — expedition party membership.
+//
+// A co-op expedition is one dnd_expedition row plus an expedition_party roster.
+// The leader's row stays the single source of truth for the shared clock, the
+// threat track, and the supply pool; members reference it rather than owning a
+// row of their own. There is no party_id column — expedition_id already
+// identifies the party, and a second key would be a second answer to "who is in
+// this party".
+//
+// Absence means solo. Every expedition that existed before N3 has no rows here,
+// which is exactly the right reading, so nothing needs backfilling. A solo
+// expedition started after N3 writes no rows either: the roster materializes on
+// the first successful invite (P6), and until then partyMembers returns empty.
+
+// Party roles (expedition_party.role).
+const (
+	PartyRoleLeader = "leader"
+	PartyRoleMember = "member"
+)
+
+// expeditionPartyMax is the roster ceiling including the leader. C1 scopes v1
+// to 2–3 players; the combat roster and the supply pool are both sized off this.
+const expeditionPartyMax = 3
+
+// Errors returned by the party layer.
+var (
+	ErrPartyFull           = errors.New("expedition party is full")
+	ErrAlreadyInParty      = errors.New("player is already in this party")
+	ErrNotPartyLeader      = errors.New("only the party leader may do that")
+	ErrPlayerBusyElsewhere = errors.New("player already has an active expedition")
+)
+
+// PartyMember is one seat on an expedition roster.
+type PartyMember struct {
+	ExpeditionID string
+	UserID       string
+	Role         string
+	JoinedAt     time.Time
+}
+
+// IsLeader reports whether this member owns the expedition row.
+func (m PartyMember) IsLeader() bool { return m.Role == PartyRoleLeader }
+
+// partyMembers returns the roster in join order, leader first. A solo
+// expedition has no rows and returns an empty slice — callers should treat
+// len(roster) == 0 and len(roster) == 1 alike, as "one player".
+func partyMembers(expeditionID string) ([]PartyMember, error) {
+	rows, err := db.Get().Query(`
+		SELECT expedition_id, user_id, role, joined_at
+		  FROM expedition_party
+		 WHERE expedition_id = ?
+		 ORDER BY (role <> 'leader'), joined_at ASC`, expeditionID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var out []PartyMember
+	for rows.Next() {
+		var m PartyMember
+		if err := rows.Scan(&m.ExpeditionID, &m.UserID, &m.Role, &m.JoinedAt); err != nil {
+			return nil, err
+		}
+		out = append(out, m)
+	}
+	return out, rows.Err()
+}
+
+// partyMemberIDs is partyMembers reduced to user ids, leader first. It is what
+// the fan-out seams (digest, briefing, recap) want: a solo expedition yields
+// just the owner, so a caller can loop unconditionally.
+func partyMemberIDs(expeditionID, ownerID string) ([]id.UserID, error) {
+	members, err := partyMembers(expeditionID)
+	if err != nil {
+		return nil, err
+	}
+	if len(members) == 0 {
+		return []id.UserID{id.UserID(ownerID)}, nil
+	}
+	out := make([]id.UserID, 0, len(members))
+	for _, m := range members {
+		out = append(out, id.UserID(m.UserID))
+	}
+	return out, nil
+}
+
+// partySize is the number of seated players: 1 for a solo expedition.
+func partySize(expeditionID string) (int, error) {
+	var n int
+	err := db.Get().QueryRow(
+		`SELECT COUNT(*) FROM expedition_party WHERE expedition_id = ?`,
+		expeditionID).Scan(&n)
+	if err != nil {
+		return 0, err
+	}
+	if n == 0 {
+		return 1, nil
+	}
+	return n, nil
+}
+
+// openParty seats the leader, converting a solo expedition row into a party of
+// one. It is idempotent: a second call on the same expedition is a no-op, so
+// the invite path can call it unconditionally before adding a member.
+func openParty(expeditionID string, leaderID id.UserID) error {
+	_, err := db.Get().Exec(`
+		INSERT INTO expedition_party (expedition_id, user_id, role)
+		VALUES (?, ?, 'leader')
+		ON CONFLICT (expedition_id, user_id) DO NOTHING`,
+		expeditionID, string(leaderID))
+	return err
+}
+
+// joinParty seats a member. It refuses a full roster, a duplicate, and a player
+// who already leads or rides an expedition of their own — the "one active
+// expedition per user" rule that startExpedition enforces in code, extended to
+// cover membership. The check and the insert share a transaction so two
+// simultaneous invites cannot both find the last seat free.
+//
+// It seats the leader first if nobody has. A roster whose leader is missing
+// would quietly drop them from every fan-out (partyMemberIDs reads the roster,
+// not the expedition row), so the ordering is enforced here rather than left to
+// each caller to remember.
+func joinParty(expeditionID string, userID id.UserID) error {
+	tx, err := db.Get().Begin()
+	if err != nil {
+		return err
+	}
+	defer tx.Rollback()
+
+	if err := seatLeader(tx, expeditionID); err != nil {
+		return err
+	}
+
+	var n int
+	if err := tx.QueryRow(
+		`SELECT COUNT(*) FROM expedition_party WHERE expedition_id = ?`,
+		expeditionID).Scan(&n); err != nil {
+		return err
+	}
+	if n >= expeditionPartyMax {
+		return ErrPartyFull
+	}
+
+	if err := assertNotAdventuring(tx, expeditionID, userID); err != nil {
+		return err
+	}
+
+	if _, err := tx.Exec(`
+		INSERT INTO expedition_party (expedition_id, user_id, role)
+		VALUES (?, ?, 'member')`,
+		expeditionID, string(userID)); err != nil {
+		return fmt.Errorf("seat %s: %w", userID, err)
+	}
+	return tx.Commit()
+}
+
+// seatLeader is openParty inside a caller's transaction: it reads the owner off
+// the expedition row rather than trusting a passed-in id, so the roster's leader
+// can never disagree with dnd_expedition.user_id.
+func seatLeader(tx *sql.Tx, expeditionID string) error {
+	var owner string
+	err := tx.QueryRow(
+		`SELECT user_id FROM dnd_expedition WHERE expedition_id = ?`,
+		expeditionID).Scan(&owner)
+	if errors.Is(err, sql.ErrNoRows) {
+		return fmt.Errorf("expedition %s not found", expeditionID)
+	}
+	if err != nil {
+		return err
+	}
+	_, err = tx.Exec(`
+		INSERT INTO expedition_party (expedition_id, user_id, role)
+		VALUES (?, ?, 'leader')
+		ON CONFLICT (expedition_id, user_id) DO NOTHING`, expeditionID, owner)
+	return err
+}
+
+// assertNotAdventuring fails if the player is already committed elsewhere:
+// seated on this or any other party, or owning an active expedition row. Runs
+// inside joinParty's transaction so the guard and the insert are atomic.
+func assertNotAdventuring(tx *sql.Tx, expeditionID string, userID id.UserID) error {
+	var seatedIn string
+	err := tx.QueryRow(`
+		SELECT expedition_id FROM expedition_party WHERE user_id = ? LIMIT 1`,
+		string(userID)).Scan(&seatedIn)
+	switch {
+	case err == nil && seatedIn == expeditionID:
+		return ErrAlreadyInParty
+	case err == nil:
+		return ErrPlayerBusyElsewhere
+	case !errors.Is(err, sql.ErrNoRows):
+		return err
+	}
+
+	var owned int
+	if err := tx.QueryRow(`
+		SELECT COUNT(*) FROM dnd_expedition
+		 WHERE user_id = ? AND status IN ('active', 'extracting')`,
+		string(userID)).Scan(&owned); err != nil {
+		return err
+	}
+	if owned > 0 {
+		return ErrPlayerBusyElsewhere
+	}
+	return nil
+}
+
+// leaveParty removes one member. The leader cannot leave — an expedition
+// without its owner has no row to hang the shared clock on; the leader ends the
+// run for everyone with !extract instead.
+func leaveParty(expeditionID string, userID id.UserID) error {
+	res, err := db.Get().Exec(`
+		DELETE FROM expedition_party
+		 WHERE expedition_id = ? AND user_id = ? AND role <> 'leader'`,
+		expeditionID, string(userID))
+	if err != nil {
+		return err
+	}
+	if n, _ := res.RowsAffected(); n == 0 {
+		return ErrNotPartyLeader
+	}
+	return nil
+}
+
+// disbandParty clears the roster. Called when an expedition reaches a terminal
+// status, so every member is free to start their own next run.
+func disbandParty(expeditionID string) error {
+	_, err := db.Get().Exec(
+		`DELETE FROM expedition_party WHERE expedition_id = ?`, expeditionID)
+	return err
+}
+
+// expeditionForMember resolves the expedition a player is seated on but does
+// not own. getActiveExpedition keys on dnd_expedition.user_id and so is blind to
+// members; every player-facing lookup wants both, which is what
+// activeExpeditionFor provides.
+func expeditionForMember(userID id.UserID) (*Expedition, error) {
+	row := db.Get().QueryRow(`
+		SELECT e.expedition_id, e.user_id, e.zone_id, e.run_id, e.status,
+		       e.start_date, e.current_day, e.current_region, e.boss_defeated,
+		       e.supplies_json, e.camp_json, e.threat_level, e.threat_siege,
+		       e.threat_events, e.temporal_stack, e.region_state,
+		       e.xp_earned, e.coins_earned, e.gm_mood,
+		       e.last_briefing_at, e.last_recap_at, e.last_ambient_kind,
+		       e.last_activity, e.completed_at
+		  FROM dnd_expedition e
+		  JOIN expedition_party p ON p.expedition_id = e.expedition_id
+		 WHERE p.user_id = ? AND p.role <> 'leader' AND e.status = 'active'
+		 ORDER BY e.start_date DESC
+		 LIMIT 1`, string(userID))
+	e, err := scanExpedition(row)
+	if errors.Is(err, sql.ErrNoRows) {
+		return nil, nil
+	}
+	return e, err
+}
+
+// activeExpeditionFor returns the expedition a player is on, whether they lead
+// it or ride it, plus whether they lead it. This is the lookup every
+// player-facing command wants; getActiveExpedition alone silently tells a party
+// member they have no expedition.
+func activeExpeditionFor(userID id.UserID) (e *Expedition, isLeader bool, err error) {
+	if e, err = getActiveExpedition(userID); err != nil || e != nil {
+		return e, e != nil, err
+	}
+	e, err = expeditionForMember(userID)
+	return e, false, err
+}
diff --git a/internal/plugin/expedition_party_test.go b/internal/plugin/expedition_party_test.go
new file mode 100644
index 0000000..6576e9c
--- /dev/null
+++ b/internal/plugin/expedition_party_test.go
@@ -0,0 +1,290 @@
+package plugin
+
+import (
+	"errors"
+	"testing"
+	"time"
+
+	"gogobee/internal/db"
+	"maunium.net/go/mautrix/id"
+)
+
+// seedExpedition inserts the minimum dnd_expedition row the party layer reads:
+// the owner, the status, and enough non-null columns for scanExpedition.
+func seedExpedition(t *testing.T, expeditionID string, owner id.UserID, status string) {
+	t.Helper()
+	if _, err := db.Get().Exec(`
+		INSERT INTO dnd_expedition (expedition_id, user_id, zone_id, run_id, status, start_date)
+		VALUES (?, ?, 'goblin_warrens', ?, ?, ?)`,
+		expeditionID, string(owner), "run-"+expeditionID, status, time.Now().UTC(),
+	); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestParty_SoloExpeditionHasNoRoster(t *testing.T) {
+	setupEmptyTestDB(t)
+	owner := id.UserID("@solo:example.org")
+	seedExpedition(t, "exp-solo", owner, "active")
+
+	members, err := partyMembers("exp-solo")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(members) != 0 {
+		t.Errorf("solo expedition has %d roster rows, want 0", len(members))
+	}
+	// Absent must read as "one player", not "zero players" — every fan-out seam
+	// loops over this.
+	if n, err := partySize("exp-solo"); err != nil || n != 1 {
+		t.Errorf("partySize = %d (%v), want 1", n, err)
+	}
+	ids, err := partyMemberIDs("exp-solo", string(owner))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(ids) != 1 || ids[0] != owner {
+		t.Errorf("partyMemberIDs = %v, want [%s]", ids, owner)
+	}
+}
+
+func TestParty_OpenIsIdempotent(t *testing.T) {
+	setupEmptyTestDB(t)
+	owner := id.UserID("@lead:example.org")
+	seedExpedition(t, "exp-1", owner, "active")
+
+	for i := 0; i < 2; i++ {
+		if err := openParty("exp-1", owner); err != nil {
+			t.Fatalf("openParty #%d: %v", i+1, err)
+		}
+	}
+	members, err := partyMembers("exp-1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(members) != 1 || !members[0].IsLeader() {
+		t.Fatalf("roster = %+v, want one leader", members)
+	}
+}
+
+func TestParty_JoinSeatsMembersLeaderFirst(t *testing.T) {
+	setupEmptyTestDB(t)
+	owner := id.UserID("@lead2:example.org")
+	seedExpedition(t, "exp-2", owner, "active")
+	if err := openParty("exp-2", owner); err != nil {
+		t.Fatal(err)
+	}
+	for _, u := range []id.UserID{"@b:x", "@c:x"} {
+		if err := joinParty("exp-2", u); err != nil {
+			t.Fatalf("joinParty %s: %v", u, err)
+		}
+	}
+
+	members, err := partyMembers("exp-2")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(members) != 3 {
+		t.Fatalf("roster = %d, want 3", len(members))
+	}
+	if !members[0].IsLeader() || members[0].UserID != string(owner) {
+		t.Errorf("leader is not first: %+v", members)
+	}
+	if n, err := partySize("exp-2"); err != nil || n != 3 {
+		t.Errorf("partySize = %d (%v), want 3", n, err)
+	}
+}
+
+// An invite that lands before anyone called openParty must still produce a
+// roster with the leader on it — partyMemberIDs reads the roster, not the
+// expedition row, so a leaderless party would silently stop DMing the leader.
+func TestParty_JoinSeatsTheLeaderIfMissing(t *testing.T) {
+	setupEmptyTestDB(t)
+	owner := id.UserID("@lead-implicit:example.org")
+	seedExpedition(t, "exp-implicit", owner, "active")
+
+	// No openParty call at all.
+	if err := joinParty("exp-implicit", "@b:x"); err != nil {
+		t.Fatal(err)
+	}
+	members, err := partyMembers("exp-implicit")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(members) != 2 {
+		t.Fatalf("roster = %+v, want leader + member", members)
+	}
+	if !members[0].IsLeader() || members[0].UserID != string(owner) {
+		t.Errorf("leader missing from roster: %+v", members)
+	}
+	// And the leader cannot then be double-seated as a member.
+	if err := joinParty("exp-implicit", owner); !errors.Is(err, ErrAlreadyInParty) {
+		t.Errorf("leader join: err = %v, want ErrAlreadyInParty", err)
+	}
+}
+
+func TestParty_JoinRefusesFullRoster(t *testing.T) {
+	setupEmptyTestDB(t)
+	owner := id.UserID("@lead3:example.org")
+	seedExpedition(t, "exp-3", owner, "active")
+	if err := openParty("exp-3", owner); err != nil {
+		t.Fatal(err)
+	}
+	for _, u := range []id.UserID{"@b:x", "@c:x"} {
+		if err := joinParty("exp-3", u); err != nil {
+			t.Fatal(err)
+		}
+	}
+	// Leader + 2 == expeditionPartyMax.
+	if err := joinParty("exp-3", "@d:x"); !errors.Is(err, ErrPartyFull) {
+		t.Errorf("4th seat: err = %v, want ErrPartyFull", err)
+	}
+}
+
+func TestParty_JoinRefusesDuplicate(t *testing.T) {
+	setupEmptyTestDB(t)
+	owner := id.UserID("@lead4:example.org")
+	seedExpedition(t, "exp-4", owner, "active")
+	if err := openParty("exp-4", owner); err != nil {
+		t.Fatal(err)
+	}
+	if err := joinParty("exp-4", "@b:x"); err != nil {
+		t.Fatal(err)
+	}
+	if err := joinParty("exp-4", "@b:x"); !errors.Is(err, ErrAlreadyInParty) {
+		t.Errorf("re-join: err = %v, want ErrAlreadyInParty", err)
+	}
+	// The leader is already seated, so inviting them back is a duplicate too.
+	if err := joinParty("exp-4", owner); !errors.Is(err, ErrAlreadyInParty) {
+		t.Errorf("leader re-join: err = %v, want ErrAlreadyInParty", err)
+	}
+}
+
+// "One active expedition per user" is code-enforced for owners
+// (startExpedition). Membership must not be a way around it: a player who owns
+// a run, or already rides someone else's, cannot take a third seat.
+func TestParty_JoinRefusesPlayerBusyElsewhere(t *testing.T) {
+	setupEmptyTestDB(t)
+	seedExpedition(t, "exp-5", "@lead5:example.org", "active")
+	seedExpedition(t, "exp-6", "@lead6:example.org", "active")
+	if err := openParty("exp-5", "@lead5:example.org"); err != nil {
+		t.Fatal(err)
+	}
+	if err := openParty("exp-6", "@lead6:example.org"); err != nil {
+		t.Fatal(err)
+	}
+
+	// Owns their own active expedition.
+	seedExpedition(t, "exp-own", "@busy:example.org", "active")
+	if err := joinParty("exp-5", "@busy:example.org"); !errors.Is(err, ErrPlayerBusyElsewhere) {
+		t.Errorf("owner of another run: err = %v, want ErrPlayerBusyElsewhere", err)
+	}
+
+	// Already seated on another party.
+	if err := joinParty("exp-5", "@rider:example.org"); err != nil {
+		t.Fatal(err)
+	}
+	if err := joinParty("exp-6", "@rider:example.org"); !errors.Is(err, ErrPlayerBusyElsewhere) {
+		t.Errorf("rider of another party: err = %v, want ErrPlayerBusyElsewhere", err)
+	}
+
+	// A leader is seated on their own party, so they are busy too.
+	if err := joinParty("exp-6", "@lead5:example.org"); !errors.Is(err, ErrPlayerBusyElsewhere) {
+		t.Errorf("leader of another party: err = %v, want ErrPlayerBusyElsewhere", err)
+	}
+}
+
+func TestParty_LeaveRemovesMemberButNotLeader(t *testing.T) {
+	setupEmptyTestDB(t)
+	owner := id.UserID("@lead7:example.org")
+	seedExpedition(t, "exp-7", owner, "active")
+	if err := openParty("exp-7", owner); err != nil {
+		t.Fatal(err)
+	}
+	if err := joinParty("exp-7", "@b:x"); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := leaveParty("exp-7", "@b:x"); err != nil {
+		t.Fatalf("leaveParty: %v", err)
+	}
+	if n, _ := partySize("exp-7"); n != 1 {
+		t.Errorf("after leave, partySize = %d, want 1", n)
+	}
+	// The expedition row hangs off the leader; they extract, they don't leave.
+	if err := leaveParty("exp-7", owner); !errors.Is(err, ErrNotPartyLeader) {
+		t.Errorf("leader leave: err = %v, want ErrNotPartyLeader", err)
+	}
+	// Leaving frees the member for their own next run.
+	seedExpedition(t, "exp-7b", "@lead7b:example.org", "active")
+	if err := openParty("exp-7b", "@lead7b:example.org"); err != nil {
+		t.Fatal(err)
+	}
+	if err := joinParty("exp-7b", "@b:x"); err != nil {
+		t.Errorf("departed member could not rejoin elsewhere: %v", err)
+	}
+}
+
+func TestParty_DisbandFreesEveryone(t *testing.T) {
+	setupEmptyTestDB(t)
+	owner := id.UserID("@lead8:example.org")
+	seedExpedition(t, "exp-8", owner, "complete")
+	if err := openParty("exp-8", owner); err != nil {
+		t.Fatal(err)
+	}
+	if err := joinParty("exp-8", "@b:x"); err != nil {
+		t.Fatal(err)
+	}
+	if err := disbandParty("exp-8"); err != nil {
+		t.Fatal(err)
+	}
+	if n, _ := partySize("exp-8"); n != 1 {
+		t.Errorf("after disband, partySize = %d, want 1 (no rows)", n)
+	}
+	seedExpedition(t, "exp-8b", "@lead8b:example.org", "active")
+	if err := openParty("exp-8b", "@lead8b:example.org"); err != nil {
+		t.Fatal(err)
+	}
+	if err := joinParty("exp-8b", "@b:x"); err != nil {
+		t.Errorf("disbanded member still looks busy: %v", err)
+	}
+}
+
+// getActiveExpedition keys on dnd_expedition.user_id, so it is blind to party
+// members. activeExpeditionFor is the lookup every player-facing command wants.
+func TestParty_ActiveExpeditionForResolvesBothRoles(t *testing.T) {
+	setupEmptyTestDB(t)
+	owner := id.UserID("@lead9:example.org")
+	member := id.UserID("@member9:example.org")
+	seedExpedition(t, "exp-9", owner, "active")
+	if err := openParty("exp-9", owner); err != nil {
+		t.Fatal(err)
+	}
+	if err := joinParty("exp-9", member); err != nil {
+		t.Fatal(err)
+	}
+
+	// The member owns no row, so the legacy lookup misses them entirely.
+	if e, err := getActiveExpedition(member); err != nil || e != nil {
+		t.Errorf("getActiveExpedition found a row for a member: %v / %v", e, err)
+	}
+
+	e, isLeader, err := activeExpeditionFor(owner)
+	if err != nil || e == nil || !isLeader || e.ID != "exp-9" {
+		t.Errorf("leader lookup: %+v leader=%v err=%v", e, isLeader, err)
+	}
+	e, isLeader, err = activeExpeditionFor(member)
+	if err != nil || e == nil || isLeader || e.ID != "exp-9" {
+		t.Errorf("member lookup: %+v leader=%v err=%v", e, isLeader, err)
+	}
+	// The member resolves to the leader's expedition, clock and all.
+	if e != nil && e.UserID != string(owner) {
+		t.Errorf("member resolved to expedition owned by %q, want %q", e.UserID, owner)
+	}
+
+	// A stranger is on no expedition at all.
+	e, _, err = activeExpeditionFor("@nobody:example.org")
+	if err != nil || e != nil {
+		t.Errorf("stranger lookup: %+v / %v", e, err)
+	}
+}