diff --git a/multiplayer-todo.md b/multiplayer-todo.md
index 509b7542..672394df 100644
--- a/multiplayer-todo.md
+++ b/multiplayer-todo.md
@@ -1194,7 +1194,7 @@ the local/CI/community transport, not a silent production fallback.
 | 8.14 `[D:8.4,8.5,8.8]` | **IN PROGRESS.** Pure Go queue domain enforces one active ticket per verified player, 10 s heartbeat/30 s expiry, retry-safe create/heartbeat/cancel and deterministic candidate projection | `server/domain/queue.go` has adversarial ownership/expiry/idempotency tests; PostgreSQL transaction adapter, Redis candidate index and cache-loss repair remain |
 | 8.15 `[D:7.8,8.3]` | **IN PROGRESS.** Pure Go probe validation treats Steam location as opaque, requires nonce/freshness/region and server-computed RTT, and implements discrepancy quarantine/release | `server/domain/probes.go` and adversarial fixtures cover stale/wrong/forged evidence, the 25 ms/30% threshold, three-sample quarantine and five-clean release; Steam coordinator and regional probe adapters remain |
 | 8.16 `[D:8.14,8.15]` | **IN PROGRESS.** Pure Go candidate/team selection implements the <=100 ms ceiling, pairwise widening tolerance, anchor inclusion, deterministic set/region scoring and balanced team partitioning | `server/domain/matcher.go`, `teams.go` and adversarial fixtures cover no-common-region, tolerance boundaries, lexical ties, mean-rating balance and input-order independence; queue-backed candidate loading and full population fixtures remain |
-| 8.17 `[D:8.14,8.16]` | Ten-second proposal to **every selected human**: ranked 6; casual largest compatible 6→2 after 60 s with disclosed teams/bots; apply exact decline/timeout/no-show cooldown and queue-precedence rules | Allocation starts only after selected humans accept; 2–5-human casual is reachable; accepter timestamps restore exactly; ranked pre-match no-show has cooldown but no rating loss |
+| 8.17 `[D:8.14,8.16]` | **IN PROGRESS.** Pure Go proposal policy sends a 10-second response window to every selected human, requires unanimous acceptance, applies exact decline/timeout cooldowns and ranked escalation | `server/domain/proposal.go` and adversarial fixtures cover partial/unanimous response, expiry, replay/conflict and ranked six-player/cooldown rules; casual 6→2 composition, queue precedence and allocation integration remain |
 | 8.18 `[D:8.5,8.14,8.17]` | Horizontally replicated matcher: Redis candidates, then PostgreSQL serializable proposal/participant fence, then cache cleanup/repair | Forced loss of the last acknowledged Redis write, retries, worker death and failover cannot claim a player into two proposals/matches |
 | 8.19 `[D:8.18]` | Casual policy: proposal composition above; >=1 human/team, exhaustive rating-balanced teams, bots after 60 s, opt-in kickoff-only backfill, 30 s reconnect and defined backfill/casual penalties | Every 2–6-human shape is tested; no mid-play replacement; declined/backfill participant gets no excluded rating/cooldown; original leaver gets only documented outcome/cooldown |
 | 8.20 `[D:8.18]` | Ranked policy: exactly six verified solo humans, no bots/backfill, only `ArenaRegistry.random`; define initial no-show, proposal timeout and reconnect/abandon transitions | Ranked rejects parties/bots/backfill/elevated arenas; every pre-live failure returns five innocent players with original precedence and applies no rating |
diff --git a/server/domain/proposal.go b/server/domain/proposal.go
new file mode 100644
index 00000000..0a992567
--- /dev/null
+++ b/server/domain/proposal.go
@@ -0,0 +1,130 @@
+package domain
+
+import (
+	"crypto/sha256"
+	"errors"
+	"fmt"
+	"sort"
+	"time"
+)
+
+const ProposalWindow = 10 * time.Second
+
+var (
+	ErrProposalClosed = errors.New("proposal is no longer open")
+	ErrNotParticipant = errors.New("player is not a proposal participant")
+)
+
+type Playlist string
+
+const (
+	Casual Playlist = "casual"
+	Ranked Playlist = "ranked"
+)
+
+type Response string
+
+const (
+	Pending Response = "PENDING"
+	AcceptedResponse Response = "ACCEPTED"
+	DeclinedResponse Response = "DECLINED"
+	TimedOutResponse Response = "TIMED_OUT"
+)
+
+type ProposalParticipant struct {
+	PlayerID string
+	Response Response
+}
+
+type Proposal struct {
+	ProposalID string
+	Playlist Playlist
+	Participants []ProposalParticipant
+	State State
+	Revision uint64
+	ExpiresAt time.Time
+	idempotent map[string]proposalMutation
+}
+
+type proposalMutation struct {
+	digest [32]byte
+	proposal Proposal
+}
+
+func NewProposal(id string, playlist Playlist, playerIDs []string, now time.Time) (Proposal, error) {
+	if id == "" || (playlist != Casual && playlist != Ranked) { return Proposal{}, fmt.Errorf("%w: invalid proposal", ErrConflict) }
+	if playlist == Ranked && len(playerIDs) != 6 { return Proposal{}, fmt.Errorf("%w: ranked requires exactly six players", ErrConflict) }
+	if len(playerIDs) < 2 || len(playerIDs) > 6 { return Proposal{}, fmt.Errorf("%w: proposal requires 2 through 6 players", ErrConflict) }
+	seen := make(map[string]bool, len(playerIDs))
+	participants := make([]ProposalParticipant, 0, len(playerIDs))
+	for _, playerID := range playerIDs {
+		if playerID == "" || seen[playerID] { return Proposal{}, fmt.Errorf("%w: duplicate or empty player", ErrConflict) }
+		seen[playerID] = true
+		participants = append(participants, ProposalParticipant{PlayerID: playerID, Response: Pending})
+	}
+	sort.Slice(participants, func(i, j int) bool { return participants[i].PlayerID < participants[j].PlayerID })
+	return Proposal{ProposalID: id, Playlist: playlist, Participants: participants, State: Open, ExpiresAt: now.Add(ProposalWindow), idempotent: make(map[string]proposalMutation)}, nil
+}
+
+func (p *Proposal) Respond(playerID, idempotencyKey string, accept bool, expectedRevision uint64, now time.Time) (Proposal, error) {
+	digest := sha256.Sum256([]byte(fmt.Sprintf("%s:%t:%d", playerID, accept, expectedRevision)))
+	if prior, ok := p.idempotent[idempotencyKey]; ok {
+		if prior.digest != digest { return Proposal{}, fmt.Errorf("%w: proposal response payload changed", ErrConflict) }
+		return prior.proposal, nil
+	}
+	if idempotencyKey == "" { return Proposal{}, fmt.Errorf("%w: empty proposal response key", ErrConflict) }
+	if p.State != Open || !now.Before(p.ExpiresAt) { return Proposal{}, ErrProposalClosed }
+	if p.Revision != expectedRevision { return Proposal{}, ErrStaleRevision }
+	index := p.participantIndex(playerID)
+	if index < 0 { return Proposal{}, ErrNotParticipant }
+	if p.Participants[index].Response != Pending { return Proposal{}, fmt.Errorf("%w: participant already responded", ErrConflict) }
+	if accept { p.Participants[index].Response = AcceptedResponse } else { p.Participants[index].Response = DeclinedResponse; p.State = Declined }
+	if accept && p.allAccepted() { p.State = Accepted }
+	p.Revision++
+	p.idempotent[idempotencyKey] = proposalMutation{digest: digest, proposal: p.copy()}
+	return p.copy(), nil
+}
+
+func (p *Proposal) Expire(now time.Time) bool {
+	if p.State != Open || now.Before(p.ExpiresAt) { return false }
+	for i := range p.Participants { if p.Participants[i].Response == Pending { p.Participants[i].Response = TimedOutResponse } }
+	p.State = Expired
+	p.Revision++
+	return true
+}
+
+func (p *Proposal) participantIndex(playerID string) int {
+	for i, participant := range p.Participants { if participant.PlayerID == playerID { return i } }
+	return -1
+}
+
+func (p *Proposal) allAccepted() bool {
+	for _, participant := range p.Participants { if participant.Response != AcceptedResponse { return false } }
+	return true
+}
+
+func (p *Proposal) copy() Proposal {
+	clone := *p
+	clone.Participants = append([]ProposalParticipant(nil), p.Participants...)
+	clone.idempotent = nil
+	return clone
+}
+
+type CooldownEvent struct { At time.Time; Playlist Playlist; Kind Response }
+
+func CooldownUntil(events []CooldownEvent, playlist Playlist, now time.Time) time.Time {
+	window := 30 * time.Minute
+	cutoff := now.Add(-window)
+	filtered := make([]CooldownEvent, 0, len(events))
+	for _, event := range events { if event.Playlist == playlist && !event.At.Before(cutoff) { filtered = append(filtered, event) } }
+	sort.Slice(filtered, func(i, j int) bool { return filtered[i].At.Before(filtered[j].At) })
+	var duration time.Duration
+	if len(filtered) > 0 {
+		if playlist == Casual { if filtered[len(filtered)-1].Kind == DeclinedResponse { duration = 30 * time.Second } else { duration = 60 * time.Second } } else {
+			if filtered[len(filtered)-1].Kind == DeclinedResponse { duration = 2 * time.Minute } else { duration = 5 * time.Minute }
+			if len(filtered) >= 3 { duration = 15 * time.Minute }
+		}
+	}
+	if duration == 0 { return time.Time{} }
+	return filtered[len(filtered)-1].At.Add(duration)
+}
diff --git a/server/domain/proposal_test.go b/server/domain/proposal_test.go
new file mode 100644
index 00000000..c4394b91
--- /dev/null
+++ b/server/domain/proposal_test.go
@@ -0,0 +1,45 @@
+package domain
+
+import (
+	"errors"
+	"testing"
+	"time"
+)
+
+func TestProposalRequiresUnanimousAcceptance(t *testing.T) {
+	now := time.Unix(1000, 0)
+	p, err := NewProposal("proposal-123456789", Casual, []string{"b", "a"}, now)
+	if err != nil { t.Fatal(err) }
+	if _, err = p.Respond("a", "response-a-123456", true, 0, now); err != nil { t.Fatal(err) }
+	if p.State != Open || p.Revision != 1 { t.Fatalf("partial acceptance closed proposal: %+v", p) }
+	if _, err = p.Respond("b", "response-b-123456", true, 1, now); err != nil { t.Fatal(err) }
+	if p.State != Accepted || p.Revision != 2 { t.Fatalf("unanimous acceptance not committed: %+v", p) }
+}
+
+func TestProposalResponseReplayIsStableAndPayloadReuseConflicts(t *testing.T) {
+	now := time.Unix(1000, 0)
+	p, err := NewProposal("proposal-123456789", Casual, []string{"a", "b"}, now)
+	if err != nil { t.Fatal(err) }
+	first, err := p.Respond("a", "response-a-123456", true, 0, now)
+	if err != nil { t.Fatal(err) }
+	replay, err := p.Respond("a", "response-a-123456", true, 0, now.Add(20*time.Second))
+	if err != nil || replay.Revision != first.Revision { t.Fatalf("replay = %+v, %v", replay, err) }
+	if _, err = p.Respond("a", "response-a-123456", false, 1, now); !errors.Is(err, ErrConflict) { t.Fatalf("payload reuse error = %v", err) }
+}
+
+func TestProposalExpiryTimesOutPendingParticipantsAndClosesRace(t *testing.T) {
+	now := time.Unix(1000, 0)
+	p, err := NewProposal("proposal-123456789", Ranked, []string{"a", "b", "c", "d", "e", "f"}, now)
+	if err != nil { t.Fatal(err) }
+	if !p.Expire(now.Add(ProposalWindow)) || p.State != Expired || p.Revision != 1 { t.Fatalf("expiry failed: %+v", p) }
+	for _, participant := range p.Participants { if participant.Response != TimedOutResponse { t.Fatalf("pending participant not timed out: %+v", participant) } }
+	if _, err = p.Respond("a", "late-response-123", true, 1, now.Add(ProposalWindow)); !errors.Is(err, ErrProposalClosed) { t.Fatalf("late response error = %v", err) }
+}
+
+func TestRankedProposalAndCooldownEscalation(t *testing.T) {
+	now := time.Unix(1000, 0)
+	if _, err := NewProposal("proposal-123456789", Ranked, []string{"a", "b"}, now); err == nil { t.Fatal("ranked proposal accepted fewer than six") }
+	if got := CooldownUntil([]CooldownEvent{{At: now, Playlist: Ranked, Kind: DeclinedResponse}}, Ranked, now); !got.Equal(now.Add(2*time.Minute)) { t.Fatalf("ranked decline cooldown = %v", got) }
+	events := []CooldownEvent{{At: now, Playlist: Ranked, Kind: TimedOutResponse}, {At: now.Add(time.Minute), Playlist: Ranked, Kind: DeclinedResponse}, {At: now.Add(2*time.Minute), Playlist: Ranked, Kind: TimedOutResponse}}
+	if got := CooldownUntil(events, Ranked, now.Add(2*time.Minute)); !got.Equal(now.Add(17*time.Minute)) { t.Fatalf("ranked escalation cooldown = %v", got) }
+}