diff --git a/agents/antigravity/done/TASK-2026-08-20-seed-precommit-atomicity.md b/agents/antigravity/done/TASK-2026-08-20-seed-precommit-atomicity.md
new file mode 100644
index 0000000..9aed010
--- /dev/null
+++ b/agents/antigravity/done/TASK-2026-08-20-seed-precommit-atomicity.md
@@ -0,0 +1,109 @@
+# Phase 2.4.1 — Atomic Snapshot + Seed Commitment Binding Report
+
+**Date:** 2026-08-20  
+**Base Commit SHA:** `78151572bd2ae01645d70a0768c6ece517e2cab0`  
+**Status:** COMPLETED / READY FOR RE-REVIEW  
+**Assigned Agent:** Antigravity (Implementation Orchestrator)
+
+---
+
+## 1. Problem Addressed
+
+В ходе независимого ревью Phase 2.4 было выявлено, что:
+1. `createAndLockSnapshot` в Prisma-репозитории допускал смену статуса из `READY` или `SNAPSHOT_LOCKED` (`status IN ('READY', 'SNAPSHOT_LOCKED')`), а Memory-репозиторий аналогично допускал повторную фиксацию слепков.
+2. В роуте `POST /api/giveaways/[id]/snapshot` фиксация слепка и чтение `giveaway.seed` для вычисления `seedCommitment` выполнялись двумя независимыми операциями (`createAndLockSnapshot` с последующим `getById`). При конкурентных запросах блокировки это могло привести к гонке, когда ответ возвращал слепок A с хешем seed B.
+
+---
+
+## 2. Implemented Solutions
+
+1. **Single Lock Invariant:**
+   - Переход разрешён **только** `READY` → `SNAPSHOT_LOCKED`.
+   - Любая попытка заблокировать слепок, когда статус отличен от `READY` (например, уже `SNAPSHOT_LOCKED` или `DRAWN`), немедленно возвращает `409 CONFLICT`.
+   - При этом новый seed не генерируется, повторный слепок не создаётся, существующий `seedCommitment` остаётся неизменным.
+
+2. **Atomic Return of `{ snapshot, seedCommitment }`:**
+   - Интерфейс `IGiveawayRepository.createAndLockSnapshot` и класс `GiveawayStore.createAndLockSnapshot` теперь возвращают `Promise<LockedSnapshotResult>`:
+     ```typescript
+     export interface LockedSnapshotResult {
+       snapshot: ParticipantSnapshotData;
+       seedCommitment: string;
+     }
+     ```
+   - Генерация CSPRNG seed, вычисление `seedCommitment = sha256(seed)`, сохранение seed в базе данных и создание записи `ParticipantSnapshot` выполняются строго внутри единой атомарной транзакции (в Prisma: `$transaction`; в Memory: синхронная мутация Map).
+
+3. **Elimination of Post-Transaction Query:**
+   - Роут `POST /api/giveaways/[id]/snapshot` использует `seedCommitment`, возвращённый непосредственно из атомарной операции `GiveawayStore.createAndLockSnapshot`. Дополнительный запрос `GiveawayStore.getById(id)` полностью исключён.
+
+4. **Driver Parity (Memory & Prisma):**
+   - `PrismaGiveawayRepository`: `where: { id, status: 'READY' }` внутри `$transaction`.
+   - `MemoryGiveawayRepository`: строгая проверка `if (gw.status !== 'READY') throw new ConflictError(...)`.
+
+5. **Idempotency & Commitment Stability:**
+   - Повторные вызовы с одинаковым `Idempotency-Key` отдают закэшированный ответ 200 со стабильным `seedCommitment` без перегенерации seed.
+   - Повторный вызов с новым ключом после фиксации слепка возвращает `409 CONFLICT`.
+   - До жеребьёвки `giveaway.seed` маскируется (`null`), отдаётся только `seedCommitment`. После жеребьёвки `sha256(drawResult.seedUsed) === seedCommitment`.
+
+---
+
+## 3. Files Changed
+
+| File | Type | Description |
+|------|------|-------------|
+| `src/lib/repository/giveaway-repository.ts` | Interface | Добавлен интерфейс `LockedSnapshotResult`, обновлена сигнатура `createAndLockSnapshot`. |
+| `src/lib/giveaway-store.ts` | Store | Обновлена сигнатура `GiveawayStore.createAndLockSnapshot` (`Promise<LockedSnapshotResult>`). |
+| `src/lib/repository/memory-repository.ts` | Driver | Строгий инвариант `READY` → `SNAPSHOT_LOCKED` (409 на повторы), атомарный возврат `{ snapshot, seedCommitment }`. |
+| `src/lib/repository/prisma-repository.ts` | Driver | Условие `status: 'READY'` внутри `$transaction`, атомарный возврат `{ snapshot, seedCommitment }`. |
+| `src/app/api/giveaways/[id]/snapshot/route.ts` | API Route | Прямое использование возвращённых `{ snapshot, seedCommitment }`, убран redundant `getById`. |
+| `tests/winner-count-contract.test.ts` | Tests | Деструктуризация `{ snapshot }` из вызовов `createAndLockSnapshot`. |
+| `tests/persistence.test.ts` | Tests | Деструктуризация `{ snapshot, seedCommitment }`. |
+| `tests/snapshot-binding.test.ts` | Tests | Деструктуризация `{ snapshot: snapshotV1/V2 }`. |
+| `tests/concurrency-draw.test.ts` | Tests | Деструктуризация `{ snapshot }`. |
+| `tests/concurrency-draw-100.test.ts` | Tests | Деструктуризация `{ snapshot }`. |
+| `tests/seed-precommit-gate.test.ts` | Tests | Деструктуризация `{ snapshot, seedCommitment }`, проверка равенства commitment. |
+| `tests/snapshot-seed-atomicity.test.ts` | Tests (NEW) | Комплексный сьют проверки атомарности, конкуренции и стабильности commitment (4 теста). |
+
+---
+
+## 4. Unchanged Core Algorithms
+
+Все алгоритмы генерации и верификации не изменялись:
+- `HMAC_SHA256_FY_V1`
+- `DeterministicHmacStream`
+- `executeDeterministicDrawV1`
+- `computeParticipantsSnapshotHash`
+- `computeConditionsHash`
+- `computeDeterministicProofHash`
+- `computeAuditEventHash`
+- `verifyDrawResult`
+
+---
+
+## 5. Test & Gate Results
+
+Фактически выполненные команды:
+
+```text
+npx prisma generate  -> EXIT 0 (Prisma Client v5.22.0)
+npm test             -> EXIT 0 (49 test files, 284 passed, 0 failed)
+npm run lint         -> EXIT 0 (Clean)
+npm run build        -> EXIT 0 (Next.js production build succeeded)
+```
+
+### Concurrency Test Evidence:
+1. `tests/snapshot-seed-atomicity.test.ts` (4 теста):
+   - `Memory repository: 20 concurrent createAndLockSnapshot calls yield exactly 1 success and 19 ConflictErrors` → **PASS**
+   - `API route: concurrent snapshot lock requests with different Idempotency-Keys produce exactly 1 200 and remaining 409s` → **PASS**
+   - `idempotency: replaying same key returns cached commitment; new key after lock returns 409` → **PASS**
+   - `commitment stability: commitment is invariant across reads and equals sha256(seedUsed) after draw` → **PASS**
+2. `tests/seed-precommit-gate.test.ts` (7 тестов) → **PASS**
+3. `tests/concurrency-draw-100.test.ts` (2 теста) → **PASS**
+
+---
+
+## 6. Audit & Migration
+
+- **Database migration required:** NO (поле `Giveaway.seed` уже присутствует в схеме Prisma).
+- **CRITICAL/HIGH findings:** 0 open in implementation.
+- **UNVERIFIED claims:** None.
+- **Next step:** Передача на независимое security re-review (Grok/Claude/OpenCode).
diff --git a/agents/antigravity/inbox/TASK-2026-08-20-seed-precommit-atomicity.md b/agents/antigravity/inbox/TASK-2026-08-20-seed-precommit-atomicity.md
new file mode 100644
index 0000000..f953643
--- /dev/null
+++ b/agents/antigravity/inbox/TASK-2026-08-20-seed-precommit-atomicity.md
@@ -0,0 +1,18 @@
+# Task: Phase 2.4.1 — Atomic Snapshot + Seed Commitment Binding
+
+**Assigned to:** Antigravity (Implementation Orchestrator)  
+**Priority:** CRITICAL (fairness / concurrency)  
+**Date:** 2026-08-20  
+**Base SHA:** `78151572bd2ae01645d70a0768c6ece517e2cab0`
+
+## Scope
+1. Enforce strict single-lock invariant: `createAndLockSnapshot` only transitions `READY` → `SNAPSHOT_LOCKED`. Any request on already locked/drawn giveaway yields 409 CONFLICT.
+2. Extend `createAndLockSnapshot` repository interface and implementations (`MemoryGiveawayRepository` and `PrismaGiveawayRepository`) to atomically generate `seed`, compute `seedCommitment = sha256(seed)`, update DB, and return `{ snapshot: ParticipantSnapshotData; seedCommitment: string }`.
+3. Eliminate secondary `getById` read in `POST /api/giveaways/[id]/snapshot`. Use returned `seedCommitment` directly.
+4. Concurrency regression tests (memory & API):
+   - Multiple concurrent snapshot requests: exactly 1 succeeds, others get 409.
+   - Snapshot count strictly 1, seed commitment matches persisted seed.
+   - Idempotency replay returns cached commitment without generating new seed.
+   - Post-draw verification `sha256(seedUsed) === seedCommitment`.
+5. Run full gate: `npx prisma generate`, `npm test`, `npm run lint`, `npm run build`.
+6. Output report in `agents/antigravity/done/TASK-2026-08-20-seed-precommit-atomicity.md`.
diff --git a/src/app/api/giveaways/[id]/snapshot/route.ts b/src/app/api/giveaways/[id]/snapshot/route.ts
index cedbc4a..c4e049a 100644
--- a/src/app/api/giveaways/[id]/snapshot/route.ts
+++ b/src/app/api/giveaways/[id]/snapshot/route.ts
@@ -8,7 +8,6 @@ import { expensiveApiRateLimiter } from '@/lib/rate-limiter';
 import { IdempotencyStore } from '@/lib/idempotency';
 import { resolveClientIp } from '@/lib/client-ip';
 import { requireGiveawayOwner } from '@/lib/auth/auth-guard';
-import { computeSeedCommitment } from '@/core/randomizer/hasher';
 
 export const dynamic = 'force-dynamic';
 
@@ -58,15 +57,12 @@ export async function POST(
     }
 
     // Atomically create and lock snapshot + pre-commit seed in database
-    const snapshot = await GiveawayStore.createAndLockSnapshot(
+    const { snapshot, seedCommitment } = await GiveawayStore.createAndLockSnapshot(
       id,
       eligibleParticipants,
       validated.filterRules
     );
 
-    const updatedGw = await GiveawayStore.getById(id);
-    const seedCommitment = updatedGw?.seed ? computeSeedCommitment(updatedGw.seed) : null;
-
     const responseBody = {
       success: true,
       giveawayId: id,
diff --git a/src/lib/giveaway-store.ts b/src/lib/giveaway-store.ts
index 2bd121a..26b653a 100644
--- a/src/lib/giveaway-store.ts
+++ b/src/lib/giveaway-store.ts
@@ -3,7 +3,8 @@ import {
   GiveawayWithRelations, 
   GiveawaySummary,
   PaginatedParticipantsResult,
-  CreateGiveawayInput 
+  CreateGiveawayInput,
+  LockedSnapshotResult
 } from './repository/giveaway-repository';
 import { PrismaGiveawayRepository } from './repository/prisma-repository';
 import { MemoryGiveawayRepository } from './repository/memory-repository';
@@ -68,7 +69,7 @@ export class GiveawayStore {
     id: string, 
     eligibleParticipants: FilteredParticipant[], 
     rules: FilterRules
-  ): Promise<ParticipantSnapshotData> {
+  ): Promise<LockedSnapshotResult> {
     return await activeRepository.createAndLockSnapshot(id, eligibleParticipants, rules);
   }
 
diff --git a/src/lib/repository/giveaway-repository.ts b/src/lib/repository/giveaway-repository.ts
index 6b38b2b..704d19d 100644
--- a/src/lib/repository/giveaway-repository.ts
+++ b/src/lib/repository/giveaway-repository.ts
@@ -73,6 +73,11 @@ export interface CreateGiveawayInput {
   organizerId: string;
 }
 
+export interface LockedSnapshotResult {
+  snapshot: ParticipantSnapshotData;
+  seedCommitment: string;
+}
+
 export interface IGiveawayRepository {
   createGiveaway(input: CreateGiveawayInput): Promise<GiveawayWithRelations>;
   getGiveawayById(id: string): Promise<GiveawayWithRelations | null>;
@@ -90,7 +95,7 @@ export interface IGiveawayRepository {
     id: string, 
     eligibleParticipants: FilteredParticipant[], 
     rules: FilterRules
-  ): Promise<ParticipantSnapshotData>;
+  ): Promise<LockedSnapshotResult>;
   getLatestSnapshot(giveawayId: string): Promise<ParticipantSnapshotData | null>;
   saveDrawResultAndAudit(
     id: string, 
diff --git a/src/lib/repository/memory-repository.ts b/src/lib/repository/memory-repository.ts
index 34138e0..aaf2a92 100644
--- a/src/lib/repository/memory-repository.ts
+++ b/src/lib/repository/memory-repository.ts
@@ -3,7 +3,8 @@ import {
   CreateGiveawayInput, 
   GiveawayWithRelations,
   GiveawaySummary,
-  PaginatedParticipantsResult
+  PaginatedParticipantsResult,
+  LockedSnapshotResult
 } from './giveaway-repository';
 import { FilterRules, GiveawayStatusType, PlatformType } from '../../core/types/giveaway';
 import { FilteredParticipant } from '../../core/types/participant';
@@ -186,12 +187,15 @@ export class MemoryGiveawayRepository implements IGiveawayRepository {
     id: string, 
     eligibleParticipants: FilteredParticipant[], 
     rules: FilterRules
-  ): Promise<ParticipantSnapshotData> {
+  ): Promise<LockedSnapshotResult> {
     const gw = this.giveaways.get(id);
     if (!gw) throw new NotFoundError(`Giveaway with id "${id}" not found`);
 
-    if (gw.status === 'DRAWN' || gw.status === 'PUBLISHED') {
-      throw new ConflictError(`Cannot lock snapshot in final status "${gw.status}"`);
+    // Strict Single Lock Invariant: Only READY -> SNAPSHOT_LOCKED transition is permitted
+    if (gw.status !== 'READY') {
+      throw new ConflictError(
+        `Cannot lock snapshot: giveaway "${id}" is in status "${gw.status}", but requires "READY"`
+      );
     }
 
     if (eligibleParticipants.length === 0) {
@@ -222,15 +226,19 @@ export class MemoryGiveawayRepository implements IGiveawayRepository {
 
     // Generate and lock cryptographic seed atomically with snapshot creation
     const seed = generateCryptoSecureSeed();
+    const seedCommitment = computeSeedCommitment(seed);
 
     gw.status = 'SNAPSHOT_LOCKED';
     gw.filterRules = rules;
     gw.latestSnapshot = snapshot;
     gw.seed = seed;
-    gw.seedCommitment = computeSeedCommitment(seed);
+    gw.seedCommitment = seedCommitment;
     gw.updatedAt = new Date().toISOString();
 
-    return snapshot;
+    return {
+      snapshot,
+      seedCommitment,
+    };
   }
 
   async getLatestSnapshot(giveawayId: string): Promise<ParticipantSnapshotData | null> {
diff --git a/src/lib/repository/prisma-repository.ts b/src/lib/repository/prisma-repository.ts
index bb8417c..f40ceee 100644
--- a/src/lib/repository/prisma-repository.ts
+++ b/src/lib/repository/prisma-repository.ts
@@ -3,7 +3,8 @@ import {
   CreateGiveawayInput, 
   GiveawayWithRelations,
   GiveawaySummary,
-  PaginatedParticipantsResult
+  PaginatedParticipantsResult,
+  LockedSnapshotResult
 } from './giveaway-repository';
 import { prisma } from '../prisma';
 import { FilterRules, GiveawayStatusType, PlatformType } from '../../core/types/giveaway';
@@ -358,12 +359,15 @@ export class PrismaGiveawayRepository implements IGiveawayRepository {
     id: string, 
     eligibleParticipants: FilteredParticipant[], 
     rules: FilterRules
-  ): Promise<ParticipantSnapshotData> {
+  ): Promise<LockedSnapshotResult> {
     const current = await this.getGiveawayById(id);
     if (!current) throw new NotFoundError(`Giveaway with id "${id}" not found`);
 
-    if (current.status === 'DRAWN' || current.status === 'PUBLISHED') {
-      throw new ConflictError(`Cannot lock snapshot in final status "${current.status}"`);
+    // Strict Single Lock Invariant: Only READY -> SNAPSHOT_LOCKED transition is permitted
+    if (current.status !== 'READY') {
+      throw new ConflictError(
+        `Cannot lock snapshot: giveaway "${id}" is in status "${current.status}", but requires "READY"`
+      );
     }
 
     if (eligibleParticipants.length === 0) {
@@ -375,14 +379,15 @@ export class PrismaGiveawayRepository implements IGiveawayRepository {
 
     try {
       return await prisma.$transaction(async (tx) => {
-        // Generate cryptographic seed for pre-commitment
+        // Generate cryptographic seed and commitment
         const seed = generateCryptoSecureSeed();
+        const seedCommitment = computeSeedCommitment(seed);
 
-        // Atomic status and seed guard
+        // Atomic status and seed guard: ONLY transition from READY
         const updateRes = await tx.giveaway.updateMany({
           where: {
             id,
-            status: { in: ['READY', 'SNAPSHOT_LOCKED'] },
+            status: 'READY',
           },
           data: {
             status: 'SNAPSHOT_LOCKED',
@@ -415,15 +420,18 @@ export class PrismaGiveawayRepository implements IGiveawayRepository {
         });
 
         return {
-          id: snapshot.id,
-          giveawayId: snapshot.giveawayId,
-          version: snapshot.version,
-          createdAt: snapshot.createdAt.toISOString(),
-          eligibleParticipants,
-          filterRulesSnapshot: rules,
-          participantCount: snapshot.participantCount,
-          participantsSnapshotHash: snapshot.participantsSnapshotHash,
-          conditionsHash: snapshot.conditionsHash,
+          snapshot: {
+            id: snapshot.id,
+            giveawayId: snapshot.giveawayId,
+            version: snapshot.version,
+            createdAt: snapshot.createdAt.toISOString(),
+            eligibleParticipants,
+            filterRulesSnapshot: rules,
+            participantCount: snapshot.participantCount,
+            participantsSnapshotHash: snapshot.participantsSnapshotHash,
+            conditionsHash: snapshot.conditionsHash,
+          },
+          seedCommitment,
         };
       });
     } catch (err: any) {
diff --git a/tests/concurrency-draw-100.test.ts b/tests/concurrency-draw-100.test.ts
index ed35bdd..938e4ec 100644
--- a/tests/concurrency-draw-100.test.ts
+++ b/tests/concurrency-draw-100.test.ts
@@ -41,7 +41,7 @@ describe('100-Draw Concurrency Regression & Mixed Race', () => {
       organizerId: 'usr_conc_100',
     });
 
-    const snapshot = await repo.createAndLockSnapshot(gw.id, participants, DEFAULT_FILTER_RULES);
+    const { snapshot } = await repo.createAndLockSnapshot(gw.id, participants, DEFAULT_FILTER_RULES);
 
     // Launch 100 concurrent draw requests
     const drawPromises = Array.from({ length: 100 }, async (_, index) => {
diff --git a/tests/concurrency-draw.test.ts b/tests/concurrency-draw.test.ts
index 0947d50..13604bc 100644
--- a/tests/concurrency-draw.test.ts
+++ b/tests/concurrency-draw.test.ts
@@ -41,7 +41,7 @@ describe('Concurrency Double Draw Protection', () => {
       organizerId: 'usr_conc_draw',
     });
 
-    const snapshot = await repo.createAndLockSnapshot(gw.id, participants, DEFAULT_FILTER_RULES);
+    const { snapshot } = await repo.createAndLockSnapshot(gw.id, participants, DEFAULT_FILTER_RULES);
 
     // 2. Launch 20 concurrent draw attempts
     const concurrentDrawPromises = Array.from({ length: 20 }, async (_, index) => {
diff --git a/tests/persistence.test.ts b/tests/persistence.test.ts
index 0df57b7..4ea3c8d 100644
--- a/tests/persistence.test.ts
+++ b/tests/persistence.test.ts
@@ -78,7 +78,7 @@ describe('Repository Persistence & Lifecycle Scenario', () => {
     expect(updatedGw.participants.length).toBe(3);
 
     // 3. Create & Lock Snapshot
-    const snapshot = await repo.createAndLockSnapshot(
+    const { snapshot, seedCommitment } = await repo.createAndLockSnapshot(
       gw.id, 
       sampleParticipants, 
       DEFAULT_FILTER_RULES
@@ -88,6 +88,7 @@ describe('Repository Persistence & Lifecycle Scenario', () => {
     expect(snapshot.participantCount).toBe(3);
     expect(snapshot.participantsSnapshotHash).toBeDefined();
     expect(snapshot.conditionsHash).toBeDefined();
+    expect(seedCommitment).toBeDefined();
 
     // Verify giveaway status transitioned to SNAPSHOT_LOCKED
     const lockedGw = await repo.getGiveawayById(gw.id);
diff --git a/tests/seed-precommit-gate.test.ts b/tests/seed-precommit-gate.test.ts
index c44747c..cb234e0 100644
--- a/tests/seed-precommit-gate.test.ts
+++ b/tests/seed-precommit-gate.test.ts
@@ -306,14 +306,16 @@ describe('Phase 2.4 — Seed Pre-Commit Gate (Seed Grinding Elimination)', () =>
     expect(gw.seed).toBeNull();
     expect(gw.seedCommitment).toBeNull();
 
-    const snapshot = await repo.createAndLockSnapshot(gw.id, testParticipants.slice(0, 10), DEFAULT_FILTER_RULES);
+    const { snapshot, seedCommitment } = await repo.createAndLockSnapshot(gw.id, testParticipants.slice(0, 10), DEFAULT_FILTER_RULES);
     expect(snapshot.id).toBeDefined();
+    expect(seedCommitment).toBeDefined();
 
     const lockedGw = await repo.getGiveawayById(gw.id);
     expect(lockedGw?.status).toBe('SNAPSHOT_LOCKED');
     expect(lockedGw?.seed).toBeDefined();
     expect(lockedGw?.seed).toHaveLength(32);
     expect(lockedGw?.seedCommitment).toBe(computeSeedCommitment(lockedGw!.seed!));
+    expect(seedCommitment).toBe(lockedGw?.seedCommitment);
   });
 
   // ─── Test 7: Repository Driver Parity (Prisma repository mapping & seed commitment) ───
diff --git a/tests/snapshot-binding.test.ts b/tests/snapshot-binding.test.ts
index d3aea9a..04f0516 100644
--- a/tests/snapshot-binding.test.ts
+++ b/tests/snapshot-binding.test.ts
@@ -73,13 +73,13 @@ describe('DrawResult Snapshot Binding Regression Tests', () => {
     });
 
     // 2. Create snapshot V1
-    const snapshotV1 = await repo.createAndLockSnapshot(gw.id, participantsV1, DEFAULT_FILTER_RULES);
+    const { snapshot: snapshotV1 } = await repo.createAndLockSnapshot(gw.id, participantsV1, DEFAULT_FILTER_RULES);
     expect(snapshotV1.version).toBe(1);
     const hashV1 = snapshotV1.participantsSnapshotHash;
 
     // 3. Unlock / simulate revision and create Snapshot V2
     await repo.updateStatus(gw.id, 'READY');
-    const snapshotV2 = await repo.createAndLockSnapshot(gw.id, participantsV2, {
+    const { snapshot: snapshotV2 } = await repo.createAndLockSnapshot(gw.id, participantsV2, {
       ...DEFAULT_FILTER_RULES,
       requireComment: true,
     });
diff --git a/tests/snapshot-seed-atomicity.test.ts b/tests/snapshot-seed-atomicity.test.ts
new file mode 100644
index 0000000..cd8515f
--- /dev/null
+++ b/tests/snapshot-seed-atomicity.test.ts
@@ -0,0 +1,270 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import { NextRequest } from 'next/server';
+import { createHash } from 'crypto';
+import { POST as snapshotPost } from '../src/app/api/giveaways/[id]/snapshot/route';
+import { POST as drawPost } from '../src/app/api/giveaways/[id]/draw/route';
+import { GET as giveawayDetailGet } from '../src/app/api/giveaways/[id]/route';
+import { GET as verifyGet } from '../src/app/api/giveaways/[id]/verify/route';
+import { GiveawayStore } from '../src/lib/giveaway-store';
+import { MemoryGiveawayRepository } from '../src/lib/repository/memory-repository';
+import { defaultSessionStore, SESSION_COOKIE_NAME } from '../src/lib/auth/session';
+import { DEFAULT_FILTER_RULES } from '../src/core/types/giveaway';
+import { FilteredParticipant } from '../src/core/types/participant';
+import { ConflictError } from '../src/core/errors/http-errors';
+import { computeSeedCommitment } from '../src/core/randomizer/hasher';
+
+describe('Phase 2.4.1 — Atomic Snapshot + Seed Commitment Binding', () => {
+  const organizerUser = { id: 'usr_atomic_snap_organizer', vkUserId: '888222' };
+  let sessionCookie: string;
+
+  const testParticipants: FilteredParticipant[] = Array.from({ length: 50 }, (_, i) => ({
+    platformUserId: `${2000 + i}`,
+    firstName: `User${i}`,
+    lastName: `Atomic${i}`,
+    source: 'LIKES',
+    liked: true,
+    commented: false,
+    commentsCount: 0,
+    reposted: false,
+    subscribed: true,
+    eligible: true,
+    exclusionReason: null,
+  }));
+
+  beforeEach(async () => {
+    GiveawayStore.setRepository(new MemoryGiveawayRepository());
+    defaultSessionStore.clear();
+    const sessionId = await defaultSessionStore.createSession(organizerUser);
+    sessionCookie = `${SESSION_COOKIE_NAME}=${sessionId}`;
+  });
+
+  async function createReadyGiveaway() {
+    const gw = await GiveawayStore.create({
+      sourceUrl: 'https://vk.com/wall-33445566_789',
+      post: {
+        platform: 'VK',
+        ownerId: '-33445566',
+        postId: '789',
+        sourceUrl: 'https://vk.com/wall-33445566_789',
+        title: 'Atomic Snapshot Test Post',
+        likesCount: 50,
+        commentsCount: 0,
+        repostsCount: 0,
+      },
+      filterRules: DEFAULT_FILTER_RULES,
+      organizerId: organizerUser.id,
+    });
+
+    await GiveawayStore.updateParticipants(gw.id, testParticipants);
+    return gw;
+  }
+
+  // ─── Test 1: Memory Repository Single Lock Invariant & Concurrency ───────────
+  it('Memory repository: 20 concurrent createAndLockSnapshot calls yield exactly 1 success and 19 ConflictErrors', async () => {
+    const repo = new MemoryGiveawayRepository();
+    const gw = await repo.createGiveaway({
+      sourceUrl: 'https://vk.com/wall-1_1',
+      post: {
+        platform: 'VK',
+        ownerId: '-1',
+        postId: '1',
+        sourceUrl: 'https://vk.com/wall-1_1',
+        title: 'Parity Test',
+        likesCount: 50,
+        commentsCount: 0,
+        repostsCount: 0,
+      },
+      filterRules: DEFAULT_FILTER_RULES,
+      organizerId: 'org_atomic_mem',
+    });
+
+    await repo.saveParticipants(gw.id, testParticipants);
+
+    const attempts = Array.from({ length: 20 }, async (_, index) => {
+      try {
+        const res = await repo.createAndLockSnapshot(gw.id, testParticipants, {
+          ...DEFAULT_FILTER_RULES,
+          requireLike: index % 2 === 0,
+        });
+        return { success: true, res };
+      } catch (err: any) {
+        return { success: false, error: err };
+      }
+    });
+
+    const results = await Promise.all(attempts);
+    const successes = results.filter(r => r.success);
+    const conflicts = results.filter(r => !r.success);
+
+    expect(successes).toHaveLength(1);
+    expect(conflicts).toHaveLength(19);
+
+    // The winning result has both snapshot and seedCommitment
+    const winning = (successes[0] as any).res;
+    expect(winning.snapshot).toBeDefined();
+    expect(winning.seedCommitment).toBeDefined();
+    expect(winning.seedCommitment).toHaveLength(64);
+
+    // Verify DB state
+    const locked = await repo.getGiveawayById(gw.id);
+    expect(locked?.status).toBe('SNAPSHOT_LOCKED');
+    expect(locked?.snapshots).toHaveLength(1);
+    expect(locked?.seed).toBeDefined();
+    expect(computeSeedCommitment(locked!.seed!)).toBe(winning.seedCommitment);
+
+    // All failed attempts threw ConflictError
+    conflicts.forEach(c => {
+      expect(c.error).toBeInstanceOf(ConflictError);
+    });
+  });
+
+  // ─── Test 2: API Concurrency with different Idempotency keys ─────────────────
+  it('API route: concurrent snapshot lock requests with different Idempotency-Keys produce exactly 1 200 and remaining 409s', async () => {
+    const gw = await createReadyGiveaway();
+
+    const requests = Array.from({ length: 5 }, (_, i) => {
+      const req = new NextRequest(`http://localhost:3000/api/giveaways/${gw.id}/snapshot`, {
+        method: 'POST',
+        headers: {
+          'Content-Type': 'application/json',
+          Cookie: sessionCookie,
+          'Idempotency-Key': `key-diff-${i}-${Date.now()}`,
+        },
+        body: JSON.stringify({ filterRules: DEFAULT_FILTER_RULES }),
+      });
+      return snapshotPost(req, { params: { id: gw.id } });
+    });
+
+    const responses = await Promise.all(requests);
+    const statusCodes = responses.map(r => r.status);
+
+    const count200 = statusCodes.filter(s => s === 200).length;
+    const count409 = statusCodes.filter(s => s === 409).length;
+
+    expect(count200).toBe(1);
+    expect(count409).toBe(4);
+
+    const successRes = responses.find(r => r.status === 200)!;
+    const body = await successRes.json();
+    expect(body.success).toBe(true);
+    expect(body.status).toBe('SNAPSHOT_LOCKED');
+    expect(body.snapshot).toBeDefined();
+    expect(body.seedCommitment).toBeDefined();
+
+    // Verify persisted seed matches returned commitment directly
+    const stored = await GiveawayStore.getById(gw.id);
+    expect(stored?.status).toBe('SNAPSHOT_LOCKED');
+    expect(computeSeedCommitment(stored!.seed!)).toBe(body.seedCommitment);
+  });
+
+  // ─── Test 3: Idempotency Replay with same key vs new key ─────────────────────
+  it('idempotency: replaying same key returns cached commitment; new key after lock returns 409', async () => {
+    const gw = await createReadyGiveaway();
+    const idempotencyKey = `idem-key-stable-${Date.now()}`;
+
+    // 1. Initial lock
+    const req1 = new NextRequest(`http://localhost:3000/api/giveaways/${gw.id}/snapshot`, {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json',
+        Cookie: sessionCookie,
+        'Idempotency-Key': idempotencyKey,
+      },
+      body: JSON.stringify({ filterRules: DEFAULT_FILTER_RULES }),
+    });
+
+    const res1 = await snapshotPost(req1, { params: { id: gw.id } });
+    expect(res1.status).toBe(200);
+    const data1 = await res1.json();
+    const initialCommitment = data1.seedCommitment;
+    expect(initialCommitment).toMatch(/^[a-f0-9]{64}$/);
+
+    // 2. Replay with identical key -> returns cached 200 with identical snapshot & commitment
+    const replayReq = new NextRequest(`http://localhost:3000/api/giveaways/${gw.id}/snapshot`, {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json',
+        Cookie: sessionCookie,
+        'Idempotency-Key': idempotencyKey,
+      },
+      body: JSON.stringify({ filterRules: DEFAULT_FILTER_RULES }),
+    });
+
+    const replayRes = await snapshotPost(replayReq, { params: { id: gw.id } });
+    expect(replayRes.status).toBe(200);
+    const replayData = await replayRes.json();
+    expect(replayData.seedCommitment).toBe(initialCommitment);
+    expect(replayData.snapshot.id).toBe(data1.snapshot.id);
+
+    // 3. New request with different key -> 409 Conflict
+    const newKeyReq = new NextRequest(`http://localhost:3000/api/giveaways/${gw.id}/snapshot`, {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json',
+        Cookie: sessionCookie,
+        'Idempotency-Key': `new-key-${Date.now()}`,
+      },
+      body: JSON.stringify({ filterRules: DEFAULT_FILTER_RULES }),
+    });
+
+    const newKeyRes = await snapshotPost(newKeyReq, { params: { id: gw.id } });
+    expect(newKeyRes.status).toBe(409);
+  });
+
+  // ─── Test 4: Commitment Stability and Provable Draw Verification ────────────
+  it('commitment stability: commitment is invariant across reads and equals sha256(seedUsed) after draw', async () => {
+    const gw = await createReadyGiveaway();
+
+    // 1. Lock snapshot
+    const snapReq = new NextRequest(`http://localhost:3000/api/giveaways/${gw.id}/snapshot`, {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json',
+        Cookie: sessionCookie,
+      },
+      body: JSON.stringify({ filterRules: DEFAULT_FILTER_RULES }),
+    });
+
+    const snapRes = await snapshotPost(snapReq, { params: { id: gw.id } });
+    expect(snapRes.status).toBe(200);
+    const snapData = await snapRes.json();
+    const lockedCommitment = snapData.seedCommitment;
+
+    // 2. Repeated GET before draw
+    for (let i = 0; i < 3; i++) {
+      const getReq = new NextRequest(`http://localhost:3000/api/giveaways/${gw.id}`, {
+        method: 'GET',
+        headers: { Cookie: sessionCookie },
+      });
+      const getRes = await giveawayDetailGet(getReq, { params: { id: gw.id } });
+      const getData = await getRes.json();
+      expect(getData.giveaway.seed).toBeNull(); // Masked before DRAWN
+      expect(getData.giveaway.seedCommitment).toBe(lockedCommitment);
+    }
+
+    // 3. Execute draw
+    const drawReq = new NextRequest(`http://localhost:3000/api/giveaways/${gw.id}/draw`, {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json',
+        Cookie: sessionCookie,
+      },
+      body: JSON.stringify({ winnersCount: 1, reserveWinnersCount: 0 }),
+    });
+
+    const drawRes = await drawPost(drawReq, { params: { id: gw.id } });
+    expect(drawRes.status).toBe(200);
+    const drawData = await drawRes.json();
+
+    const seedUsed = drawData.drawResult.seedUsed;
+    expect(createHash('sha256').update(seedUsed).digest('hex')).toBe(lockedCommitment);
+
+    // 4. Verify public endpoint
+    const verifyReq = new NextRequest(`http://localhost:3000/api/giveaways/${gw.id}/verify`, {
+      method: 'GET',
+    });
+    const verifyRes = await verifyGet(verifyReq, { params: { id: gw.id } });
+    const verifyData = await verifyRes.json();
+    expect(verifyData.verified).toBe(true);
+  });
+});
diff --git a/tests/winner-count-contract.test.ts b/tests/winner-count-contract.test.ts
index db7ed65..87f7d4c 100644
--- a/tests/winner-count-contract.test.ts
+++ b/tests/winner-count-contract.test.ts
@@ -54,7 +54,7 @@ describe('Winner Count Contract & Draw Retry Invariants', () => {
     });
 
     await GiveawayStore.updateParticipants(gw.id, threeParticipants);
-    const snapshot = await GiveawayStore.createAndLockSnapshot(gw.id, threeParticipants, DEFAULT_FILTER_RULES);
+    const { snapshot } = await GiveawayStore.createAndLockSnapshot(gw.id, threeParticipants, DEFAULT_FILTER_RULES);
 
     const result = executeDeterministicDrawV1({
       giveawayId: gw.id,
@@ -90,7 +90,7 @@ describe('Winner Count Contract & Draw Retry Invariants', () => {
     });
 
     await GiveawayStore.updateParticipants(gw.id, threeParticipants);
-    const snapshot = await GiveawayStore.createAndLockSnapshot(gw.id, threeParticipants, DEFAULT_FILTER_RULES);
+    const { snapshot } = await GiveawayStore.createAndLockSnapshot(gw.id, threeParticipants, DEFAULT_FILTER_RULES);
 
     const result = executeDeterministicDrawV1({
       giveawayId: gw.id,
@@ -126,7 +126,7 @@ describe('Winner Count Contract & Draw Retry Invariants', () => {
     });
 
     await GiveawayStore.updateParticipants(gw.id, threeParticipants);
-    const snapshot = await GiveawayStore.createAndLockSnapshot(gw.id, threeParticipants, DEFAULT_FILTER_RULES);
+    const { snapshot } = await GiveawayStore.createAndLockSnapshot(gw.id, threeParticipants, DEFAULT_FILTER_RULES);
 
     expect(() =>
       executeDeterministicDrawV1({