diff --git a/multiplayer-todo.md b/multiplayer-todo.md
index 3a7c4d25..0b96a00f 100644
--- a/multiplayer-todo.md
+++ b/multiplayer-todo.md
@@ -1227,7 +1227,7 @@ the local/CI/community transport, not a silent production fallback.
 | # | Task | Acceptance |
 |---|---|---|
 | 8.39 `[D:8.3,8.14,8.17]` | Queue UI: playlist/quality, elapsed and estimated wait, proposal countdown, allocation/connect state, cancel and latency/capacity explanations | Every backend state and terminal failure has a non-stuck visible state; cancel/decline is acknowledged authoritatively |
-| 8.40 `[D:8.3,8.14]` | One authenticated revisioned WebSocket plus REST resync; resume valid queue/assignment after client restart | Missed/duplicate/out-of-order events converge and restart never creates a second ticket |
+| 8.40 `[D:8.3,8.14]` | **IN PROGRESS.** Pure Go revisioned replica reducer rejects gaps for REST resync, makes duplicate/out-of-order events idempotent, and resumes from the authoritative snapshot revision | `server/domain/sync.go` covers gap, snapshot, replay and same-revision conflict behavior; authenticated WebSocket/REST transport, client restart persistence and duplicate-ticket integration remain |
 | 8.41 `[D:7.8,8.9,8.31,8.40]` | After assignment-ready, install SDR relay ticket before connect and send match-scoped join authorisation in `hello`; retain ENet assignments locally | Production connects/reconnects/fences old generation through SDR, never before assignment-ready; allocated/direct ENet and community flows remain compatible |
 | 8.42 `[D:8.22,8.23,8.24,8.40]` | Backend-authoritative provisional/rank/tier/delta, abandon status and season countdown UI | Client performs no rating math and displays the committed revision after reconnect |
 | 8.43 `[D:8.39,8.40,8.41]` | Recovery paths for decline, expiry, startup failure, version mismatch, auth expiry, regional outage and failed reconnect | Automated UI/state tests prove every case returns to a usable queue/menu or resumes the match without a duplicate action |
diff --git a/server/domain/sync.go b/server/domain/sync.go
new file mode 100644
index 00000000..a898ebb6
--- /dev/null
+++ b/server/domain/sync.go
@@ -0,0 +1,81 @@
+package domain
+
+import "fmt"
+
+var (
+	ErrRevisionGap  = fmt.Errorf("revision gap requires resync")
+	ErrSyncConflict = fmt.Errorf("conflicting revisioned event")
+)
+
+type SyncEvent struct {
+	Kind       ResourceKind
+	ResourceID string
+	Revision   uint64
+	State      State
+}
+
+type ReplicaResource struct {
+	Kind        ResourceKind
+	ResourceID  string
+	State       State
+	Revision    uint64
+	NeedsResync bool
+}
+
+func NewReplicaResource(kind ResourceKind, resourceID string, state State) (*ReplicaResource, error) {
+	if resourceID == "" || !validStateForKind(kind, state) {
+		return nil, fmt.Errorf("invalid replica resource")
+	}
+	return &ReplicaResource{Kind: kind, ResourceID: resourceID, State: state}, nil
+}
+
+// ApplyEvent makes duplicate/out-of-order delivery converge. A gap is not
+// guessed through; callers must fetch the authoritative REST snapshot and use
+// ReplaceSnapshot before resuming the event stream.
+func (r *ReplicaResource) ApplyEvent(event SyncEvent) error {
+	if event.Kind != r.Kind || event.ResourceID != r.ResourceID {
+		return ErrSyncConflict
+	}
+	if r.NeedsResync {
+		return ErrRevisionGap
+	}
+	if event.Revision <= r.Revision {
+		if event.Revision == r.Revision && event.State != r.State {
+			return ErrSyncConflict
+		}
+		return nil
+	}
+	if event.Revision != r.Revision+1 {
+		r.NeedsResync = true
+		return ErrRevisionGap
+	}
+	if !legalTransition(r.Kind, r.State, event.State) {
+		return ErrSyncConflict
+	}
+	r.State, r.Revision = event.State, event.Revision
+	return nil
+}
+
+func (r *ReplicaResource) ReplaceSnapshot(revision uint64, state State) error {
+	if revision < r.Revision || !validStateForKind(r.Kind, state) {
+		return ErrSyncConflict
+	}
+	r.State, r.Revision, r.NeedsResync = state, revision, false
+	return nil
+}
+
+func validStateForKind(kind ResourceKind, state State) bool {
+	switch kind {
+	case ResourceQueueTicket:
+		_, ok := queueTransitions[state]
+		return ok || state == Queued
+	case ResourceProposal:
+		_, ok := proposalTransitions[state]
+		return ok || state == Open
+	case ResourceMatch:
+		_, ok := matchTransitions[state]
+		return ok
+	default:
+		return false
+	}
+}
diff --git a/server/domain/sync_test.go b/server/domain/sync_test.go
new file mode 100644
index 00000000..04d9837d
--- /dev/null
+++ b/server/domain/sync_test.go
@@ -0,0 +1,51 @@
+package domain
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestRevisionedReplicaRejectsGapAndConvergesAfterAuthoritativeSnapshot(t *testing.T) {
+	r, err := NewReplicaResource(ResourceQueueTicket, "ticket-1", Queued)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := r.ApplyEvent(SyncEvent{Kind: ResourceQueueTicket, ResourceID: "ticket-1", Revision: 2, State: Accepted}); !errors.Is(err, ErrRevisionGap) || !r.NeedsResync {
+		t.Fatalf("gap = %+v err=%v", r, err)
+	}
+	if err := r.ApplyEvent(SyncEvent{Kind: ResourceQueueTicket, ResourceID: "ticket-1", Revision: 1, State: Proposed}); !errors.Is(err, ErrRevisionGap) {
+		t.Fatalf("event applied while resync required: %v", err)
+	}
+	if err := r.ReplaceSnapshot(2, Accepted); err != nil || r.NeedsResync {
+		t.Fatalf("snapshot = %+v err=%v", r, err)
+	}
+	if err := r.ApplyEvent(SyncEvent{Kind: ResourceQueueTicket, ResourceID: "ticket-1", Revision: 3, State: Allocating}); err != nil || r.Revision != 3 {
+		t.Fatalf("resume = %+v err=%v", r, err)
+	}
+}
+
+func TestReplicaRejectsInvalidInitialState(t *testing.T) {
+	if _, err := NewReplicaResource(ResourceProposal, "proposal-1", Live); err == nil {
+		t.Fatal("invalid proposal state accepted")
+	}
+}
+
+func TestRevisionedReplicaMakesDuplicateAndOutOfOrderEventsIdempotent(t *testing.T) {
+	r, _ := NewReplicaResource(ResourceQueueTicket, "ticket-1", Queued)
+	event := SyncEvent{Kind: ResourceQueueTicket, ResourceID: "ticket-1", Revision: 1, State: Proposed}
+	if err := r.ApplyEvent(event); err != nil {
+		t.Fatal(err)
+	}
+	if err := r.ApplyEvent(event); err != nil {
+		t.Fatal(err)
+	}
+	if err := r.ApplyEvent(SyncEvent{Kind: ResourceQueueTicket, ResourceID: "ticket-1", Revision: 0, State: Queued}); err != nil {
+		t.Fatal(err)
+	}
+	if r.Revision != 1 || r.State != Proposed {
+		t.Fatalf("replay changed state: %+v", r)
+	}
+	if err := r.ApplyEvent(SyncEvent{Kind: ResourceQueueTicket, ResourceID: "ticket-1", Revision: 1, State: Accepted}); !errors.Is(err, ErrSyncConflict) {
+		t.Fatalf("same-revision conflict = %v", err)
+	}
+}