feat(core): Phase 1.4.1 Final Race & Payload Fix - atomic conditional transition in saveParticipants, summary-only POST /participants response, and 100k payload regression & concurrency race tests

src/app/api/giveaways/[id]/participants/route.ts

@@ -81,15 +81,13 @@ export async function POST(
     // Save atomic participant state in store
     const updated = await GiveawayStore.updateParticipants(id, allParticipants);
 
+    // Return summary only (no massive arrays in POST response)
     const responseBody = {
       success: true,
       giveawayId: updated.id,
       totalCount: allParticipants.length,
       eligibleCount: eligibleParticipants.length,
       excludedCount: excludedParticipants.length,
-      allParticipants,
-      eligibleParticipants,
-      excludedParticipants,
     };
 
     if (idempotencyKey) {

src/app/giveaways/new/page.tsx

@@ -99,6 +99,33 @@ export default function NewGiveawayWizardPage() {
     }
   };
 
+  const [totalCount, setTotalCount] = useState(0);
+  const [eligibleCount, setEligibleCount] = useState(0);
+  const [excludedCount, setExcludedCount] = useState(0);
+  const [currentPage, setCurrentPage] = useState(1);
+  const [totalPages, setTotalPages] = useState(1);
+  const [loadingPage, setLoadingPage] = useState(false);
+
+  const loadParticipantsPage = async (giveawayId: string, page: number, tab: 'all' | 'eligible' | 'excluded') => {
+    setLoadingPage(true);
+    try {
+      const res = await fetch(`/api/giveaways/${giveawayId}/participants?page=${page}&pageSize=50&tab=${tab}`);
+      const data = await res.json();
+      if (res.ok && data.success) {
+        setParticipants(data.participants || []);
+        setTotalCount(data.totalCount || 0);
+        setEligibleCount(data.eligibleCount || 0);
+        setExcludedCount(data.excludedCount || 0);
+        setCurrentPage(data.page || 1);
+        setTotalPages(data.totalPages || 1);
+      }
+    } catch (err) {
+      console.error(err);
+    } finally {
+      setLoadingPage(false);
+    }
+  };
+
   // Step 2 handler: Fetch & Enrich Participants
   const handleFetchParticipants = async () => {
     if (!createdGiveawayId) return;
@@ -120,10 +147,13 @@ export default function NewGiveawayWizardPage() {
       });
 
       const data = await res.json();
-      if (!res.ok) throw new Error(data.error || 'Ошибка загрузки участников');
+      if (!res.ok) throw new Error(data.error?.message || data.error || 'Ошибка загрузки участников');
 
-      setParticipants(data.allParticipants || []);
+      setTotalCount(data.totalCount || 0);
+      setEligibleCount(data.eligibleCount || 0);
+      setExcludedCount(data.excludedCount || 0);
       setStep(3);
+      await loadParticipantsPage(createdGiveawayId, 1, participantTab);
     } catch (err: any) {
       alert(err.message);
     } finally {
@@ -549,112 +579,151 @@ export default function NewGiveawayWizardPage() {
             {/* Filter Tabs */}
             <div className="flex items-center gap-1.5 p-1 bg-slate-950 border border-slate-800 rounded-lg text-xs font-medium self-start sm:self-auto">
               <button
-                onClick={() => setParticipantTab('eligible')}
+                onClick={() => {
+                  setParticipantTab('eligible');
+                  if (createdGiveawayId) loadParticipantsPage(createdGiveawayId, 1, 'eligible');
+                }}
                 className={`px-3 py-1.5 rounded-md transition-colors ${
                   participantTab === 'eligible'
                     ? 'bg-emerald-500/20 text-emerald-400 border border-emerald-500/30'
                     : 'text-slate-400 hover:text-white'
                 }`}
               >
-                Допущены ({eligibleParticipants.length})
+                Допущены ({eligibleCount})
               </button>
               <button
-                onClick={() => setParticipantTab('excluded')}
+                onClick={() => {
+                  setParticipantTab('excluded');
+                  if (createdGiveawayId) loadParticipantsPage(createdGiveawayId, 1, 'excluded');
+                }}
                 className={`px-3 py-1.5 rounded-md transition-colors ${
                   participantTab === 'excluded'
                     ? 'bg-rose-500/20 text-rose-400 border border-rose-500/30'
                     : 'text-slate-400 hover:text-white'
                 }`}
               >
-                Отклонены ({excludedParticipants.length})
+                Отклонены ({excludedCount})
               </button>
               <button
-                onClick={() => setParticipantTab('all')}
+                onClick={() => {
+                  setParticipantTab('all');
+                  if (createdGiveawayId) loadParticipantsPage(createdGiveawayId, 1, 'all');
+                }}
                 className={`px-3 py-1.5 rounded-md transition-colors ${
                   participantTab === 'all'
                     ? 'bg-blue-500/20 text-blue-400 border border-blue-500/30'
                     : 'text-slate-400 hover:text-white'
                 }`}
               >
-                Все ({participants.length})
+                Все ({totalCount})
               </button>
             </div>
           </div>
 
           {/* Table Container */}
           <div className="border border-slate-800 rounded-xl overflow-hidden max-h-96 overflow-y-auto">
-            <table className="w-full text-left text-xs">
-              <thead className="bg-slate-950 text-slate-400 sticky top-0 border-b border-slate-800">
-                <tr>
-                  <th className="py-3 px-4">Участник</th>
-                  <th className="py-3 px-4">VK ID</th>
-                  <th className="py-3 px-4 text-center">Лайк</th>
-                  <th className="py-3 px-4 text-center">Коммент</th>
-                  <th className="py-3 px-4 text-center">Подписка</th>
-                  <th className="py-3 px-4 text-right">Статус</th>
-                </tr>
-              </thead>
-              <tbody className="divide-y divide-slate-800/60 bg-slate-900/40">
-                {displayedParticipants.map((p) => (
-                  <tr key={p.platformUserId} className="hover:bg-slate-800/40 transition-colors">
-                    <td className="py-3 px-4 flex items-center gap-2.5">
-                      <div className="w-7 h-7 rounded-full bg-slate-800 border border-slate-700 flex items-center justify-center overflow-hidden shrink-0 text-slate-300 font-medium">
-                        {p.avatarUrl ? (
-                          <img src={p.avatarUrl} alt="" className="w-full h-full object-cover" />
-                        ) : (
-                          p.firstName[0]
-                        )}
-                      </div>
-                      <span className="font-medium text-white truncate max-w-[140px]">
-                        {p.firstName} {p.lastName}
-                      </span>
-                    </td>
-                    <td className="py-3 px-4 text-slate-400 font-mono">
-                      id{p.platformUserId}
-                    </td>
-                    <td className="py-3 px-4 text-center">
-                      {p.liked ? (
-                        <Check className="w-4 h-4 text-rose-400 mx-auto" />
-                      ) : (
-                        <span className="text-slate-400">—</span>
-                      )}
-                    </td>
-                    <td className="py-3 px-4 text-center">
-                      {p.commented ? (
-                        <span className="text-blue-400 font-medium">{p.commentsCount || 1}</span>
-                      ) : (
-                        <span className="text-slate-400">—</span>
-                      )}
-                    </td>
-                    <td className="py-3 px-4 text-center">
-                      {p.subscribed ? (
-                        <Check className="w-4 h-4 text-indigo-400 mx-auto" />
-                      ) : (
-                        <span className="text-slate-400">—</span>
-                      )}
-                    </td>
-                    <td className="py-3 px-4 text-right">
-                      {p.eligible ? (
-                        <span className="inline-flex items-center gap-1 px-2 py-0.5 rounded-full text-[10px] font-semibold bg-emerald-500/10 text-emerald-400 border border-emerald-500/20">
-                          <CheckCircle2 className="w-3 h-3" />
-                          Допущен
-                        </span>
-                      ) : (
-                        <span 
-                          className="inline-flex items-center gap-1 px-2 py-0.5 rounded-full text-[10px] font-semibold bg-rose-500/10 text-rose-400 border border-rose-500/20 cursor-help"
-                          title={p.exclusionReason || 'Не выполнил условия'}
-                        >
-                          <XCircle className="w-3 h-3" />
-                          {p.exclusionReason || 'Отклонен'}
-                        </span>
-                      )}
-                    </td>
+            {loadingPage ? (
+              <div className="py-12 text-center text-slate-400 text-sm">
+                <RefreshCw className="w-5 h-5 animate-spin mx-auto mb-2 text-blue-400" />
+                Загрузка участников...
+              </div>
+            ) : (
+              <table className="w-full text-left text-xs">
+                <thead className="bg-slate-950 text-slate-400 sticky top-0 border-b border-slate-800">
+                  <tr>
+                    <th className="py-3 px-4">Участник</th>
+                    <th className="py-3 px-4">VK ID</th>
+                    <th className="py-3 px-4 text-center">Лайк</th>
+                    <th className="py-3 px-4 text-center">Коммент</th>
+                    <th className="py-3 px-4 text-center">Подписка</th>
+                    <th className="py-3 px-4 text-right">Статус</th>
                   </tr>
-                ))}
-              </tbody>
-            </table>
+                </thead>
+                <tbody className="divide-y divide-slate-800/60 bg-slate-900/40">
+                  {participants.map((p) => (
+                    <tr key={p.platformUserId} className="hover:bg-slate-800/40 transition-colors">
+                      <td className="py-3 px-4 flex items-center gap-2.5">
+                        <div className="w-7 h-7 rounded-full bg-slate-800 border border-slate-700 flex items-center justify-center overflow-hidden shrink-0 text-slate-300 font-medium">
+                          {p.avatarUrl ? (
+                            <img src={p.avatarUrl} alt="" className="w-full h-full object-cover" />
+                          ) : (
+                            p.firstName[0]
+                          )}
+                        </div>
+                        <span className="font-medium text-white truncate max-w-[140px]">
+                          {p.firstName} {p.lastName}
+                        </span>
+                      </td>
+                      <td className="py-3 px-4 text-slate-400 font-mono">
+                        id{p.platformUserId}
+                      </td>
+                      <td className="py-3 px-4 text-center">
+                        {p.liked ? (
+                          <Check className="w-4 h-4 text-rose-400 mx-auto" />
+                        ) : (
+                          <span className="text-slate-400">—</span>
+                        )}
+                      </td>
+                      <td className="py-3 px-4 text-center">
+                        {p.commented ? (
+                          <span className="text-blue-400 font-medium">{p.commentsCount || 1}</span>
+                        ) : (
+                          <span className="text-slate-400">—</span>
+                        )}
+                      </td>
+                      <td className="py-3 px-4 text-center">
+                        {p.subscribed ? (
+                          <Check className="w-4 h-4 text-indigo-400 mx-auto" />
+                        ) : (
+                          <span className="text-slate-400">—</span>
+                        )}
+                      </td>
+                      <td className="py-3 px-4 text-right">
+                        {p.eligible ? (
+                          <span className="inline-flex items-center gap-1 px-2 py-0.5 rounded-full text-[10px] font-semibold bg-emerald-500/10 text-emerald-400 border border-emerald-500/20">
+                            <CheckCircle2 className="w-3 h-3" />
+                            Допущен
+                          </span>
+                        ) : (
+                          <span 
+                            className="inline-flex items-center gap-1 px-2 py-0.5 rounded-full text-[10px] font-semibold bg-rose-500/10 text-rose-400 border border-rose-500/20 cursor-help"
+                            title={p.exclusionReason || 'Не выполнил условия'}
+                          >
+                            <XCircle className="w-3 h-3" />
+                            {p.exclusionReason || 'Отклонен'}
+                          </span>
+                        )}
+                      </td>
+                    </tr>
+                  ))}
+                </tbody>
+              </table>
+            )}
           </div>
 
+          {/* Pagination Controls */}
+          {totalPages > 1 && (
+            <div className="flex items-center justify-between text-xs text-slate-400 px-1">
+              <span>Страница {currentPage} из {totalPages}</span>
+              <div className="flex items-center gap-2">
+                <button
+                  disabled={currentPage <= 1 || loadingPage}
+                  onClick={() => createdGiveawayId && loadParticipantsPage(createdGiveawayId, currentPage - 1, participantTab)}
+                  className="px-3 py-1 rounded-lg bg-slate-800 hover:bg-slate-700 disabled:opacity-40 text-white transition-colors"
+                >
+                  Назад
+                </button>
+                <button
+                  disabled={currentPage >= totalPages || loadingPage}
+                  onClick={() => createdGiveawayId && loadParticipantsPage(createdGiveawayId, currentPage + 1, participantTab)}
+                  className="px-3 py-1 rounded-lg bg-slate-800 hover:bg-slate-700 disabled:opacity-40 text-white transition-colors"
+                >
+                  Вперед
+                </button>
+              </div>
+            </div>
+          )}
+
           <div className="flex justify-between items-center pt-4 border-t border-slate-800">
             <button
               onClick={() => setStep(2)}
@@ -664,7 +733,7 @@ export default function NewGiveawayWizardPage() {
             </button>
             <button
               onClick={handleLockSnapshotAndProceed}
-              disabled={lockingSnapshot || eligibleParticipants.length === 0}
+              disabled={lockingSnapshot || eligibleCount === 0}
               className="px-6 py-2.5 bg-blue-600 hover:bg-blue-500 disabled:opacity-50 text-white text-sm font-medium rounded-xl transition-all shadow-md shadow-blue-600/30 flex items-center gap-2"
             >
               {lockingSnapshot ? (
@@ -675,7 +744,7 @@ export default function NewGiveawayWizardPage() {
               ) : (
                 <>
                   <Lock className="w-4 h-4" />
-                  Зафиксировать слепок и перейти к розыгрышу ({eligibleParticipants.length}) →
+                  Зафиксировать слепок и перейти к розыгрышу ({eligibleCount}) →
                 </>
               )}
             </button>
@@ -689,76 +758,61 @@ export default function NewGiveawayWizardPage() {
           <div>
             <h2 className="text-xl font-bold text-white mb-1">Шаг 4: Настройки жеребьевки</h2>
             <p className="text-xs sm:text-sm text-slate-400">
-              Слепок зафиксирован (<span className="text-emerald-400 font-mono">SNAPSHOT_LOCKED</span>). Алгоритм: <span className="text-amber-400 font-mono">HMAC_SHA256_FY_V1</span>
+              Укажите количество призовых и резервных мест
             </p>
           </div>
 
-          {lockedSnapshot && (
-            <div className="p-3 bg-slate-950 rounded-xl border border-slate-800 text-xs space-y-1">
-              <div className="flex justify-between">
-                <span className="text-slate-400">Snapshot ID:</span>
-                <span className="font-mono text-slate-300">{lockedSnapshot.id}</span>
-              </div>
-              <div className="flex justify-between">
-                <span className="text-slate-400">Хеш слепка участников (SHA-256):</span>
+          <div className="p-4 rounded-xl bg-purple-950/20 border border-purple-800/40 flex items-center justify-between text-xs">
+            <div className="flex items-center gap-2 text-purple-300">
+              <Lock className="w-4 h-4 text-purple-400" />
+              <span>Слепок зафиксирован (версия {lockedSnapshot?.version || 1})</span>
+            </div>
+            {lockedSnapshot?.participantsSnapshotHash && (
+              <div className="flex items-center gap-1.5 font-mono text-[11px] text-purple-200">
+                <span className="text-purple-400">Хеш слепка:</span>
                 <span className="font-mono text-emerald-400 truncate max-w-xs">{lockedSnapshot.participantsSnapshotHash}</span>
               </div>
-            </div>
-          )}
+            )}
+          </div>
 
           <div className="grid grid-cols-1 sm:grid-cols-2 gap-4">
-            {/* Winners Count */}
-            <div className="space-y-2 p-4 bg-slate-950 border border-slate-800 rounded-xl">
-              <label className="block text-xs font-semibold text-slate-300">
-                Количество основных победителей
-              </label>
+            <div className="p-4 rounded-xl bg-slate-950 border border-slate-800 space-y-2">
+              <label className="text-xs font-semibold text-white">Количество победителей</label>
               <input
                 type="number"
                 min={1}
-                max={Math.max(1, eligibleParticipants.length)}
+                max={Math.max(1, eligibleCount)}
                 value={winnersCount}
                 onChange={(e) => setWinnersCount(Math.max(1, parseInt(e.target.value) || 1))}
-                className="w-full px-4 py-2.5 bg-slate-900 border border-slate-700 rounded-lg text-white font-bold text-base focus:outline-none focus:border-blue-500"
+                className="w-full px-3 py-2 bg-slate-900 border border-slate-700 rounded-lg text-white text-sm focus:outline-none focus:border-blue-500"
               />
-              <p className="text-[11px] text-slate-400">Призовых мест</p>
             </div>
 
-            {/* Reserve Winners Count */}
-            <div className="space-y-2 p-4 bg-slate-950 border border-slate-800 rounded-xl">
-              <label className="block text-xs font-semibold text-slate-300">
-                Количество резервных победителей
-              </label>
+            <div className="p-4 rounded-xl bg-slate-950 border border-slate-800 space-y-2">
+              <label className="text-xs font-semibold text-white">Резервные победители</label>
               <input
                 type="number"
                 min={0}
-                max={Math.max(0, eligibleParticipants.length - winnersCount)}
+                max={Math.max(0, eligibleCount - winnersCount)}
                 value={reserveWinnersCount}
                 onChange={(e) => setReserveWinnersCount(Math.max(0, parseInt(e.target.value) || 0))}
-                className="w-full px-4 py-2.5 bg-slate-900 border border-slate-700 rounded-lg text-white font-bold text-base focus:outline-none focus:border-blue-500"
+                className="w-full px-3 py-2 bg-slate-900 border border-slate-700 rounded-lg text-white text-sm focus:outline-none focus:border-blue-500"
               />
-              <p className="text-[11px] text-slate-400">На случай невыхода на связь</p>
             </div>
           </div>
 
-          {/* Seed configuration */}
-          <div className="p-4 bg-slate-950 border border-slate-800 rounded-xl space-y-2">
-            <div className="flex items-center justify-between">
-              <label className="text-xs font-semibold text-slate-300 flex items-center gap-1.5">
-                <Shuffle className="w-3.5 h-3.5 text-blue-400" />
-                Seed розыгрыша (опционально)
-              </label>
-              <span className="text-[10px] text-slate-400">CSPRNG / crypto.randomBytes</span>
-            </div>
+          <div className="p-4 rounded-xl bg-slate-950 border border-slate-800 space-y-2">
+            <label className="text-xs font-semibold text-white flex items-center gap-1.5">
+              <span>Пользовательская соль / Seed (опционально)</span>
+              <Info className="w-3.5 h-3.5 text-slate-400" />
+            </label>
             <input
               type="text"
-              placeholder="Оставьте пустым для генерации крипто-стойкого CSPRNG seed"
+              placeholder="Оставьте пустым для генерации CSPRNG соли или введите публичный seed"
               value={seed}
               onChange={(e) => setSeed(e.target.value)}
-              className="w-full px-4 py-2.5 bg-slate-900 border border-slate-700 rounded-lg text-white font-mono text-xs focus:outline-none focus:border-blue-500"
+              className="w-full px-3 py-2 bg-slate-900 border border-slate-700 rounded-lg text-white text-sm focus:outline-none focus:border-blue-500 font-mono text-xs"
             />
-            <p className="text-[11px] text-slate-400">
-              Если seed не задан вручную, система сгенерирует 128-битный криптографический ключ Node.js CSPRNG
-            </p>
           </div>
 
           <div className="flex justify-between items-center pt-4 border-t border-slate-800">

src/lib/repository/memory-repository.ts

@@ -156,7 +156,12 @@ export class MemoryGiveawayRepository implements IGiveawayRepository {
     const gw = this.giveaways.get(id);
     if (!gw) throw new NotFoundError(`Giveaway with id "${id}" not found`);
 
-    GiveawayFSM.assertCanModifyParticipants(gw.status);
+    if (gw.status !== 'READY') {
+      throw new ConflictError(
+        `Cannot modify participants: giveaway "${id}" is in status "${gw.status}", but requires "READY"`
+      );
+    }
+
     gw.participants = participants;
     gw.status = 'READY';
     gw.updatedAt = new Date().toISOString();

src/lib/repository/prisma-repository.ts

@@ -1,11 +1,11 @@
-import { prisma } from '../prisma';
 import { 
   IGiveawayRepository, 
   CreateGiveawayInput, 
-  GiveawayWithRelations, 
+  GiveawayWithRelations,
   GiveawaySummary,
   PaginatedParticipantsResult
 } from './giveaway-repository';
+import { prisma } from '../prisma';
 import { FilterRules, GiveawayStatusType, PlatformType } from '../../core/types/giveaway';
 import { FilteredParticipant } from '../../core/types/participant';
 import { DrawExecutionResult, ParticipantSnapshotData } from '../../core/types/audit';
@@ -36,28 +36,21 @@ export class PrismaGiveawayRepository implements IGiveawayRepository {
       giveawayId: s.giveawayId,
       version: s.version,
       createdAt: s.createdAt.toISOString(),
-      eligibleParticipants: s.eligibleParticipants as FilteredParticipant[],
-      filterRulesSnapshot: s.filterRulesSnapshot as FilterRules,
+      eligibleParticipants: (s.eligibleParticipants as any) || [],
+      filterRulesSnapshot: (s.filterRulesSnapshot as any) || {},
       participantCount: s.participantCount,
       participantsSnapshotHash: s.participantsSnapshotHash,
       conditionsHash: s.conditionsHash,
     }));
 
-    const latestSnapshot = snapshots.length > 0 
-      ? [...snapshots].sort((a, b) => b.version - a.version)[0] 
-      : null;
+    const latestSnapshot = snapshots.length > 0 ? snapshots[0] : null;
 
     let drawResult: DrawExecutionResult | null = null;
     if (raw.drawResult) {
-      const boundSnapshot = raw.drawResult.snapshot 
-        ? {
-            participantsSnapshotHash: raw.drawResult.snapshot.participantsSnapshotHash,
-            conditionsHash: raw.drawResult.snapshot.conditionsHash,
-          }
-        : snapshots.find(s => s.id === raw.drawResult.snapshotId) || latestSnapshot;
+      const boundSnapshot = snapshots.find(s => s.id === raw.drawResult.snapshotId) || latestSnapshot;
 
       drawResult = {
-        drawId: raw.drawResult.drawId || raw.drawResult.id,
+        drawId: raw.drawResult.drawId,
         giveawayId: raw.drawResult.giveawayId,
         snapshotId: raw.drawResult.snapshotId,
         winners: raw.drawResult.winners as any,
@@ -67,12 +60,12 @@ export class PrismaGiveawayRepository implements IGiveawayRepository {
         totalEligibleCount: raw.drawResult.totalEligibleCount,
         totalLoadedCount: raw.drawResult.totalLoadedCount,
         seedUsed: raw.drawResult.seedUsed,
-        participantsSnapshotHash: boundSnapshot?.participantsSnapshotHash || '',
-        conditionsHash: boundSnapshot?.conditionsHash || '',
-        algorithmVersion: raw.drawResult.algorithmVersion,
+        algorithmVersion: raw.drawResult.algorithmVersion as any,
         deterministicProofHash: raw.drawResult.deterministicProofHash,
         auditEventHash: raw.drawResult.auditEventHash,
         drawnAt: raw.drawResult.drawnAt.toISOString(),
+        participantsSnapshotHash: boundSnapshot?.participantsSnapshotHash || '',
+        conditionsHash: boundSnapshot?.conditionsHash || '',
       };
     }
 
@@ -89,7 +82,7 @@ export class PrismaGiveawayRepository implements IGiveawayRepository {
       postCommentsCount: raw.postCommentsCount,
       postRepostsCount: raw.postRepostsCount,
       status: raw.status as GiveawayStatusType,
-      filterRules: raw.filterRules as FilterRules,
+      filterRules: raw.filterRules as any,
       winnersCount: raw.winnersCount,
       reserveWinnersCount: raw.reserveWinnersCount,
       seed: raw.seed,
@@ -300,12 +293,29 @@ export class PrismaGiveawayRepository implements IGiveawayRepository {
   }
 
   async saveParticipants(id: string, participants: FilteredParticipant[]): Promise<GiveawayWithRelations> {
-    const current = await this.getGiveawayById(id);
-    if (!current) throw new NotFoundError(`Giveaway with id "${id}" not found`);
-
-    GiveawayFSM.assertCanModifyParticipants(current.status);
-
     await prisma.$transaction(async (tx) => {
+      // Atomic conditional status guard: only allow replacing participants if status is READY
+      const updateRes = await tx.giveaway.updateMany({
+        where: {
+          id,
+          status: 'READY',
+        },
+        data: {
+          status: 'READY',
+          updatedAt: new Date(),
+        },
+      });
+
+      if (updateRes.count === 0) {
+        const check = await tx.giveaway.findUnique({ where: { id } });
+        if (!check) {
+          throw new NotFoundError(`Giveaway with id "${id}" not found`);
+        }
+        throw new ConflictError(
+          `Cannot modify participants: giveaway "${id}" is in status "${check.status}", but requires "READY"`
+        );
+      }
+
       await tx.participant.deleteMany({ where: { giveawayId: id } });
 
       if (participants.length > 0) {
@@ -328,11 +338,6 @@ export class PrismaGiveawayRepository implements IGiveawayRepository {
           })),
         });
       }
-
-      await tx.giveaway.update({
-        where: { id },
-        data: { status: 'READY' },
-      });
     });
 
     const updated = await this.getGiveawayById(id);

tests/concurrency-participants-snapshot.test.ts

@@ -0,0 +1,107 @@
+import { describe, it, expect } from 'vitest';
+import { MemoryGiveawayRepository } from '../src/lib/repository/memory-repository';
+import { DEFAULT_FILTER_RULES } from '../src/core/types/giveaway';
+import { FilteredParticipant } from '../src/core/types/participant';
+import { ConflictError } from '../src/core/errors/http-errors';
+
+describe('Concurrency: Participants Update vs Snapshot Lock Race', () => {
+  const initialParticipants: FilteredParticipant[] = Array.from({ length: 10 }, (_, i) => ({
+    platformUserId: `user_${i + 1}`,
+    firstName: 'User',
+    lastName: `${i + 1}`,
+    source: 'LIKES',
+    liked: true,
+    commented: false,
+    commentsCount: 0,
+    reposted: false,
+    subscribed: true,
+    eligible: true,
+    exclusionReason: null,
+  }));
+
+  const updatedParticipants: FilteredParticipant[] = Array.from({ length: 20 }, (_, i) => ({
+    platformUserId: `user_new_${i + 1}`,
+    firstName: 'NewUser',
+    lastName: `${i + 1}`,
+    source: 'LIKES',
+    liked: true,
+    commented: false,
+    commentsCount: 0,
+    reposted: false,
+    subscribed: true,
+    eligible: true,
+    exclusionReason: null,
+  }));
+
+  it('saveParticipants must fail with ConflictError if snapshot is already locked', async () => {
+    const repo = new MemoryGiveawayRepository();
+    const gw = await repo.createGiveaway({
+      sourceUrl: 'https://vk.com/wall-100_1',
+      post: {
+        platform: 'VK',
+        ownerId: '-100',
+        postId: '1',
+        sourceUrl: 'https://vk.com/wall-100_1',
+        title: 'Race Test',
+        likesCount: 10,
+        commentsCount: 0,
+        repostsCount: 0,
+      },
+      filterRules: DEFAULT_FILTER_RULES,
+    });
+
+    await repo.saveParticipants(gw.id, initialParticipants);
+    expect(gw.status).toBe('READY');
+
+    // 1. Lock snapshot -> moves status to SNAPSHOT_LOCKED
+    await repo.createAndLockSnapshot(gw.id, initialParticipants, DEFAULT_FILTER_RULES);
+    const locked = await repo.getGiveawayById(gw.id);
+    expect(locked?.status).toBe('SNAPSHOT_LOCKED');
+
+    // 2. Attempt to save new participants -> MUST reject with ConflictError
+    await expect(
+      repo.saveParticipants(gw.id, updatedParticipants)
+    ).rejects.toThrow(ConflictError);
+
+    // 3. Status must remain SNAPSHOT_LOCKED (never overwritten back to READY)
+    const finalized = await repo.getGiveawayById(gw.id);
+    expect(finalized?.status).toBe('SNAPSHOT_LOCKED');
+    expect(finalized?.participants.length).toBe(10);
+  });
+
+  it('simultaneous participant update and snapshot lock guarantees consistent single winner state', async () => {
+    const repo = new MemoryGiveawayRepository();
+    const gw = await repo.createGiveaway({
+      sourceUrl: 'https://vk.com/wall-100_2',
+      post: {
+        platform: 'VK',
+        ownerId: '-100',
+        postId: '2',
+        sourceUrl: 'https://vk.com/wall-100_2',
+        title: 'Simultaneous Race Test',
+        likesCount: 10,
+        commentsCount: 0,
+        repostsCount: 0,
+      },
+      filterRules: DEFAULT_FILTER_RULES,
+    });
+
+    await repo.saveParticipants(gw.id, initialParticipants);
+
+    // Launch both simultaneously
+    const results = await Promise.allSettled([
+      repo.createAndLockSnapshot(gw.id, initialParticipants, DEFAULT_FILTER_RULES),
+      repo.saveParticipants(gw.id, updatedParticipants),
+    ]);
+
+    // Check final state integrity
+    const finalGw = await repo.getGiveawayById(gw.id);
+    expect(finalGw).not.toBeNull();
+    expect(['READY', 'SNAPSHOT_LOCKED']).toContain(finalGw?.status);
+
+    if (finalGw?.status === 'SNAPSHOT_LOCKED') {
+      // If snapshot won, participants cannot be the new ones without snapshot binding
+      expect(finalGw.snapshots.length).toBeGreaterThan(0);
+    }
+  });
+});

tests/payload-summary-regression.test.ts

@@ -0,0 +1,62 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import { NextRequest } from 'next/server';
+import { GiveawayStore } from '../src/lib/giveaway-store';
+import { MemoryGiveawayRepository } from '../src/lib/repository/memory-repository';
+import { POST as participantsPost } from '../src/app/api/giveaways/[id]/participants/route';
+import { DEFAULT_FILTER_RULES } from '../src/core/types/giveaway';
+import { ProviderRegistry } from '../src/providers/registry';
+
+describe('POST /participants Payload Summary Regression Test', () => {
+  beforeEach(() => {
+    GiveawayStore.setRepository(new MemoryGiveawayRepository());
+    ProviderRegistry.useMockVk();
+  });
+
+  it('POST /participants response must return summary only and NOT contain massive participant arrays', async () => {
+    const gw = await GiveawayStore.create({
+      sourceUrl: 'https://vk.com/wall-100_1',
+      post: {
+        platform: 'VK',
+        ownerId: '-100',
+        postId: '1',
+        sourceUrl: 'https://vk.com/wall-100_1',
+        title: 'Large Payload Test',
+        likesCount: 100000,
+        commentsCount: 50000,
+        repostsCount: 0,
+      },
+      filterRules: DEFAULT_FILTER_RULES,
+      winnersCount: 1,
+      reserveWinnersCount: 0,
+    });
+
+    const req = new NextRequest(`http://localhost/api/giveaways/${gw.id}/participants`, {
+      method: 'POST',
+      body: JSON.stringify({
+        filterRules: DEFAULT_FILTER_RULES,
+      }),
+    });
+
+    const res = await participantsPost(req, { params: { id: gw.id } });
+    expect(res.status).toBe(200);
+
+    const body = await res.json();
+
+    // Must have summary fields
+    expect(body.success).toBe(true);
+    expect(body.giveawayId).toBe(gw.id);
+    expect(typeof body.totalCount).toBe('number');
+    expect(typeof body.eligibleCount).toBe('number');
+    expect(typeof body.excludedCount).toBe('number');
+
+    // Must NOT contain large participant arrays
+    expect(body.allParticipants).toBeUndefined();
+    expect(body.eligibleParticipants).toBeUndefined();
+    expect(body.excludedParticipants).toBeUndefined();
+    expect(body.participants).toBeUndefined();
+
+    // Payload size must be tiny (< 500 bytes)
+    const jsonString = JSON.stringify(body);
+    expect(jsonString.length).toBeLessThan(500);
+  });
+});