diff --git a/server/store/postgres_integration_test.go b/server/store/postgres_integration_test.go
index f45dc4f2..cfd6dd85 100644
--- a/server/store/postgres_integration_test.go
+++ b/server/store/postgres_integration_test.go
@@ -2562,3 +2562,43 @@ func TestPostgreSQLSessionRevocationIsImmediateOnAnotherReplica(t *testing.T) {
 		t.Fatalf("revoking one session invalidated another: %v", err)
 	}
 }
+
+// Reusing an idempotency key with a different payload must be a conflict.
+// Both idempotency paths returned a bare error, which writeDomainError maps
+// to its 422 default, so the API answered 422 where openapi.json and
+// state-transitions.json ("same_key_different_payload":
+// "reject_conflict_without_state_change") both require 409. It is the
+// difference between "your request was malformed" and "that key is taken",
+// and a client acting on the former would rewrite a correct request.
+func TestPostgreSQLIdempotencyKeyReuseIsAConflict(t *testing.T) {
+	db := openIntegrationPostgres(t)
+	applyIntegrationMigrations(t, db)
+
+	ctx := context.Background()
+	now := time.Now().UTC().Truncate(time.Microsecond)
+	if _, err := db.ExecContext(ctx, `INSERT INTO identities (player_id, steam_id) VALUES ('idem-player', 'idem-steam')`); err != nil {
+		t.Fatal(err)
+	}
+	spec := domain.QueueSpec{Playlist: domain.Casual, ClientBuild: "build-1", ProtocolVersion: 1}
+	if _, err := CreateQueueTicket(ctx, db, "idem-ticket-one", "idem-player", "idem-key-00000001", spec, now); err != nil {
+		t.Fatalf("first create: %v", err)
+	}
+
+	// Same key, same payload: replays the original result.
+	replay, err := CreateQueueTicket(ctx, db, "idem-ticket-one", "idem-player", "idem-key-00000001", spec, now.Add(time.Second))
+	if err != nil {
+		t.Fatalf("identical replay must succeed: %v", err)
+	}
+	if replay.TicketID != "idem-ticket-one" {
+		t.Fatalf("replay returned %q", replay.TicketID)
+	}
+
+	// Same key, different payload: conflict, not a validation error.
+	_, err = CreateQueueTicket(ctx, db, "idem-ticket-two", "idem-player", "idem-key-00000001", spec, now.Add(time.Second))
+	if err == nil {
+		t.Fatal("reusing a key with a different ticket was accepted")
+	}
+	if !errors.Is(err, domain.ErrConflict) {
+		t.Fatalf("err = %v; must wrap domain.ErrConflict so the API answers 409, not its 422 default", err)
+	}
+}
diff --git a/server/store/queue_sql.go b/server/store/queue_sql.go
index 2e97937d..57293e91 100644
--- a/server/store/queue_sql.go
+++ b/server/store/queue_sql.go
@@ -178,7 +178,12 @@ func CreateQueueTicket(ctx context.Context, db *sql.DB, ticketID, playerID, idem
 				return err
 			}
 			if !bytes.Equal(priorDigest, digest[:]) {
-				return fmt.Errorf("queue create idempotency conflict")
+				// Must wrap ErrConflict: writeDomainError maps unrecognised
+				// errors to 422, but the contract
+				// (state-transitions.json "same_key_different_payload") and
+				// openapi.json both require 409 for reusing a key with a
+				// different payload.
+				return fmt.Errorf("%w: queue create idempotency conflict", domain.ErrConflict)
 			}
 			var prior queueTicketRecord
 			if err := json.Unmarshal(priorResult, &prior); err != nil {
@@ -321,7 +326,7 @@ func mutateQueueTicket(ctx context.Context, db *sql.DB, playerID, ticketID, idem
 				return err
 			}
 			if !bytes.Equal(priorDigest, digest[:]) {
-				return fmt.Errorf("queue mutation idempotency conflict")
+				return fmt.Errorf("%w: queue mutation idempotency conflict", domain.ErrConflict)
 			}
 			var prior queueTicketRecord
 			if err := json.Unmarshal(priorResult, &prior); err != nil {