diff --git a/packages/super-sync-server/src/sync/services/snapshot.service.ts b/packages/super-sync-server/src/sync/services/snapshot.service.ts
index 9a984b22ad..48f21c5c72 100644
--- a/packages/super-sync-server/src/sync/services/snapshot.service.ts
+++ b/packages/super-sync-server/src/sync/services/snapshot.service.ts
@@ -445,9 +445,15 @@ export class SnapshotService {
           }
         }
 
+        // Always validate that the cached base is not poisoned by encrypted
+        // ops before serving it, including on the fast path — a legacy build
+        // that didn't reject encrypted ops could have produced a poisoned
+        // cache, and the fast path would serve it forever. The assertion is
+        // a findFirst + count, which is cheap relative to skipping replay.
+        await this._assertCachedSnapshotBaseReplayable(tx, userId, startSeq);
+
         // Fast path: cached snapshot is already at the latest seq AND in the
-        // current schema version. No replay needed → skip the encrypted-op
-        // probes (they cost a findFirst + count per call).
+        // current schema version. No replay needed.
         if (
           startSeq >= latestSeq &&
           cachedRow?.snapshotData &&
@@ -461,8 +467,6 @@ export class SnapshotService {
           };
         }
 
-        await this._assertCachedSnapshotBaseReplayable(tx, userId, startSeq);
-
         // Migrate snapshot if needed
         if (stateNeedsMigration(snapshotSchemaVersion, CURRENT_SCHEMA_VERSION)) {
           Logger.info(
@@ -534,30 +538,46 @@ export class SnapshotService {
         const generatedAt = Date.now();
 
         // Cache the generated snapshot inline so it matches the returned state.
-        // Upsert: a user that just had a snapshot generated may still have no
-        // userSyncState row (first-time path), in which case `update` throws
-        // RecordNotFound. Capture the byte delta so the storage usage counter
-        // can be updated AFTER the txn commits (see `cacheDelta`).
+        // Race-safe write (same pattern as `cacheSnapshot`): a concurrent
+        // upload may have advanced `lastSnapshotSeq` beyond our `latestSeq`
+        // while we replayed under RepeatableRead. Conditional `updateMany`
+        // only writes when our seq is newer; if no row exists yet (first-time
+        // user), fall back to `create` and swallow P2002 if another writer
+        // beat us to the insert. When the race is lost, `cacheDelta` MUST
+        // stay 0 — otherwise `onCacheDelta` would over-credit storage by
+        // `compressed.length - previousCachedBytes` even though nothing was
+        // written.
         const previousCachedBytes = cachedRow?.snapshotData?.length ?? 0;
         const compressed = await gzipAsync(Buffer.from(JSON.stringify(state), 'utf-8'));
         if (compressed.length <= MAX_SNAPSHOT_SIZE_BYTES) {
-          await tx.userSyncState.upsert({
-            where: { userId },
-            update: {
-              snapshotData: compressed,
-              lastSnapshotSeq: latestSeq,
-              snapshotAt: BigInt(generatedAt),
-              snapshotSchemaVersion: CURRENT_SCHEMA_VERSION,
-            },
-            create: {
+          const cacheData = {
+            snapshotData: compressed,
+            lastSnapshotSeq: latestSeq,
+            snapshotAt: BigInt(generatedAt),
+            snapshotSchemaVersion: CURRENT_SCHEMA_VERSION,
+          };
+          const updateResult = await tx.userSyncState.updateMany({
+            where: {
               userId,
-              snapshotData: compressed,
-              lastSnapshotSeq: latestSeq,
-              snapshotAt: BigInt(generatedAt),
-              snapshotSchemaVersion: CURRENT_SCHEMA_VERSION,
+              OR: [{ lastSnapshotSeq: null }, { lastSnapshotSeq: { lt: latestSeq } }],
             },
+            data: cacheData,
           });
-          cacheDelta = compressed.length - previousCachedBytes;
+          if (updateResult.count > 0) {
+            cacheDelta = compressed.length - previousCachedBytes;
+          } else {
+            // Either no row exists yet (first-time-user path) or a newer
+            // snapshot already won the race. Try a create — if a row was
+            // inserted concurrently, the unique-userId constraint throws
+            // P2002 and we treat that as "newer snapshot won".
+            try {
+              await tx.userSyncState.create({ data: { userId, ...cacheData } });
+              cacheDelta = compressed.length - previousCachedBytes;
+            } catch (err) {
+              if ((err as { code?: string }).code !== 'P2002') throw err;
+              // cacheDelta stays 0: nothing was written.
+            }
+          }
         }
 
         return {
@@ -896,7 +916,19 @@ export class SnapshotService {
           for (const key of Object.keys(state)) {
             delete state[key];
           }
-          Object.assign(state, fullState);
+          // Copy key-by-key (not Object.assign) so a malicious `__proto__`
+          // key in the client-uploaded payload cannot pollute Object's
+          // prototype via the `__proto__` setter. JSON.parse creates
+          // `__proto__` as an own data property (no setter), but
+          // Object.assign would then `state['__proto__'] = …`, which DOES
+          // trigger the setter and pollute the prototype chain.
+          const fullStateRecord = fullState as Record<string, unknown>;
+          for (const key of Object.keys(fullStateRecord)) {
+            if (key === '__proto__' || key === 'constructor' || key === 'prototype') {
+              continue;
+            }
+            state[key] = fullStateRecord[key];
+          }
           continue;
         }
 
diff --git a/packages/super-sync-server/tests/snapshot.service.spec.ts b/packages/super-sync-server/tests/snapshot.service.spec.ts
index a215bf0c6b..42295e61da 100644
--- a/packages/super-sync-server/tests/snapshot.service.spec.ts
+++ b/packages/super-sync-server/tests/snapshot.service.spec.ts
@@ -347,10 +347,11 @@ describe('SnapshotService', () => {
     // Regression test for the RecordNotFound bug: when a user calls
     // generateSnapshot before any uploads, there is no userSyncState row.
     // Previously the inline `tx.userSyncState.update` threw P2025 and the
-    // route returned 500. With upsert the path must succeed and return an
-    // empty snapshot at seq=0.
+    // route returned 500. With the race-safe updateMany+create fallback the
+    // path must succeed and return an empty snapshot at seq=0.
     it('should not throw RecordNotFound when no userSyncState row exists', async () => {
-      const upsertSpy = vi.fn().mockResolvedValue({});
+      const updateManySpy = vi.fn().mockResolvedValue({ count: 0 });
+      const createSpy = vi.fn().mockResolvedValue({});
 
       vi.mocked(prisma.$transaction).mockImplementation(async (fn) => {
         const mockTx = {
@@ -358,7 +359,8 @@ describe('SnapshotService', () => {
             // Both findUnique calls (lastSeq + cachedRow) return null —
             // simulating a brand-new user.
             findUnique: vi.fn().mockResolvedValue(null),
-            upsert: upsertSpy,
+            updateMany: updateManySpy,
+            create: createSpy,
           },
           operation: {
             count: vi.fn().mockResolvedValue(0),
@@ -372,15 +374,13 @@ describe('SnapshotService', () => {
       const result = await service.generateSnapshot(1);
 
       // latestSeq=0, startSeq=0 → no replay needed but no cache either, so
-      // the service runs through replay (empty loop) and still upserts.
+      // the service runs through replay (empty loop) and still writes the
+      // cache row via updateMany+create fallback.
       expect(result.serverSeq).toBe(0);
       expect(result.state).toEqual({});
-      expect(upsertSpy).toHaveBeenCalledWith(
-        expect.objectContaining({
-          where: { userId: 1 },
-          create: expect.objectContaining({ userId: 1, lastSnapshotSeq: 0 }),
-        }),
-      );
+      expect(createSpy).toHaveBeenCalledWith({
+        data: expect.objectContaining({ userId: 1, lastSnapshotSeq: 0 }),
+      });
     });
 
     it('cacheSnapshot via the standalone path should create the row on first-time user', async () => {
@@ -423,7 +423,8 @@ describe('SnapshotService', () => {
                 snapshotAt: null,
                 snapshotSchemaVersion: null,
               }),
-            upsert: vi.fn().mockResolvedValue({}),
+            updateMany: vi.fn().mockResolvedValue({ count: 0 }),
+            create: vi.fn().mockResolvedValue({}),
           },
           operation: { count: vi.fn().mockResolvedValue(0), findMany },
         };
@@ -453,7 +454,8 @@ describe('SnapshotService', () => {
       // MAX_SNAPSHOT_SIZE_BYTES with no quota accounting.
       const previousBytes = 500;
       vi.mocked(prisma.$transaction).mockImplementation(async (fn: any) => {
-        const upsert = vi.fn().mockResolvedValue({});
+        const updateMany = vi.fn().mockResolvedValue({ count: 1 });
+        const create = vi.fn().mockResolvedValue({});
         const mockTx = {
           userSyncState: {
             findUnique: vi
@@ -465,7 +467,8 @@ describe('SnapshotService', () => {
                 snapshotAt: null,
                 snapshotSchemaVersion: 1,
               }),
-            upsert,
+            updateMany,
+            create,
           },
           operation: {
             count: vi.fn().mockResolvedValue(0),
@@ -504,8 +507,14 @@ describe('SnapshotService', () => {
     it('should not invoke onCacheDelta when the cached snapshot is already up to date', async () => {
       // When startSeq >= latestSeq the snapshot service returns the cached
       // state without rewriting snapshotData — no counter update is needed.
+      // The fast path now runs _assertCachedSnapshotBaseReplayable first, so
+      // the operation mocks must answer the findFirst/count probes. Set
+      // snapshotSchemaVersion to CURRENT_SCHEMA_VERSION (2) so the fast
+      // path triggers; v1 would force a migration and bypass it.
       const cachedState = { TASK: { t1: { id: 't1' } } };
       const compressed = zlib.gzipSync(JSON.stringify(cachedState));
+      const updateManySpy = vi.fn().mockResolvedValue({ count: 0 });
+      const createSpy = vi.fn();
       vi.mocked(prisma.$transaction).mockImplementation(async (fn: any) => {
         const mockTx = {
           userSyncState: {
@@ -516,11 +525,16 @@ describe('SnapshotService', () => {
                 snapshotData: compressed,
                 lastSnapshotSeq: 7,
                 snapshotAt: BigInt(1),
-                snapshotSchemaVersion: 1,
+                snapshotSchemaVersion: 2,
               }),
-            upsert: vi.fn(),
+            updateMany: updateManySpy,
+            create: createSpy,
+          },
+          operation: {
+            count: vi.fn().mockResolvedValue(0),
+            findFirst: vi.fn().mockResolvedValue({ serverSeq: 7 }),
+            findMany: vi.fn(),
           },
-          operation: { count: vi.fn(), findFirst: vi.fn(), findMany: vi.fn() },
         };
         return fn(mockTx);
       });
@@ -530,6 +544,50 @@ describe('SnapshotService', () => {
       await service.generateSnapshot(1, onCacheDelta);
 
       expect(onCacheDelta).not.toHaveBeenCalled();
+      // Fast path returns before the cache-write block, so neither write op
+      // is invoked.
+      expect(updateManySpy).not.toHaveBeenCalled();
+      expect(createSpy).not.toHaveBeenCalled();
+    });
+
+    it('should run _assertCachedSnapshotBaseReplayable before the fast path returns the cached snapshot', async () => {
+      // Defense in depth: a legacy server build that failed to reject
+      // encrypted ops could have produced a poisoned cache. The fast path
+      // (startSeq >= latestSeq + schema match) MUST still validate the
+      // cached base, otherwise it would serve poisoned cache forever.
+      const cachedState = { TASK: { t1: { id: 't1' } } };
+      const compressed = zlib.gzipSync(JSON.stringify(cachedState));
+      const findFirstSpy = vi.fn().mockResolvedValue(null);
+      const countSpy = vi.fn().mockResolvedValue(1); // encrypted op present
+      vi.mocked(prisma.$transaction).mockImplementation(async (fn: any) => {
+        const mockTx = {
+          userSyncState: {
+            findUnique: vi
+              .fn()
+              .mockResolvedValueOnce({ lastSeq: 7 })
+              .mockResolvedValueOnce({
+                snapshotData: compressed,
+                lastSnapshotSeq: 7,
+                snapshotAt: BigInt(1),
+                snapshotSchemaVersion: 2,
+              }),
+            updateMany: vi.fn(),
+            create: vi.fn(),
+          },
+          operation: {
+            findFirst: findFirstSpy,
+            count: countSpy,
+            findMany: vi.fn(),
+          },
+        };
+        return fn(mockTx);
+      });
+
+      await expect(service.generateSnapshot(1)).rejects.toThrow(
+        'ENCRYPTED_OPS_NOT_SUPPORTED',
+      );
+      expect(findFirstSpy).toHaveBeenCalled();
+      expect(countSpy).toHaveBeenCalled();
     });
 
     it('should swallow a thrown onCacheDelta to avoid corrupting the snapshot result', async () => {
@@ -547,7 +605,8 @@ describe('SnapshotService', () => {
                 snapshotAt: null,
                 snapshotSchemaVersion: null,
               }),
-            upsert: vi.fn().mockResolvedValue({}),
+            updateMany: vi.fn().mockResolvedValue({ count: 0 }),
+            create: vi.fn().mockResolvedValue({}),
           },
           operation: {
             count: vi.fn().mockResolvedValue(0),
@@ -700,7 +759,8 @@ describe('SnapshotService', () => {
         const mockTx = {
           userSyncState: {
             findUnique: vi.fn().mockResolvedValue({ lastSeq: 0 }),
-            upsert: vi.fn().mockResolvedValue({}),
+            updateMany: vi.fn().mockResolvedValue({ count: 0 }),
+            create: vi.fn().mockResolvedValue({}),
           },
           operation: {
             findMany: vi.fn().mockResolvedValue([]),
@@ -733,7 +793,8 @@ describe('SnapshotService', () => {
         const mockTx = {
           userSyncState: {
             findUnique: vi.fn().mockResolvedValue({ lastSeq: 0 }),
-            upsert: vi.fn().mockResolvedValue({}),
+            updateMany: vi.fn().mockResolvedValue({ count: 0 }),
+            create: vi.fn().mockResolvedValue({}),
           },
           operation: {
             findMany: vi.fn().mockResolvedValue([]),
@@ -790,7 +851,8 @@ describe('SnapshotService', () => {
               .fn()
               .mockResolvedValueOnce({ lastSeq: 1 })
               .mockResolvedValueOnce({ snapshotData: null, lastSnapshotSeq: null }),
-            upsert: vi.fn().mockResolvedValue({}),
+            updateMany: vi.fn().mockResolvedValue({ count: 0 }),
+            create: vi.fn().mockResolvedValue({}),
           },
           operation: {
             count: vi.fn().mockResolvedValue(0),
@@ -849,7 +911,8 @@ describe('SnapshotService', () => {
       // Scenario: after a clean-slate upload (sync.service preserves lastSeq but
       // wipes ops) the next SYNC_IMPORT lands at lastSeq+1 (e.g. 102). Snapshot
       // replay must accept the leading gap because SYNC_IMPORT resets state.
-      const upsertSpy = vi.fn().mockResolvedValue({});
+      const updateManySpy = vi.fn().mockResolvedValue({ count: 0 });
+      const createSpy = vi.fn().mockResolvedValue({});
 
       vi.mocked(prisma.$transaction).mockImplementation(async (fn) => {
         const mockTx = {
@@ -858,7 +921,8 @@ describe('SnapshotService', () => {
               .fn()
               .mockResolvedValueOnce({ lastSeq: 102 })
               .mockResolvedValueOnce({ snapshotData: null, lastSnapshotSeq: null }),
-            upsert: upsertSpy,
+            updateMany: updateManySpy,
+            create: createSpy,
           },
           operation: {
             count: vi.fn().mockResolvedValue(0),
@@ -945,7 +1009,8 @@ describe('SnapshotService', () => {
         const mockTx = {
           userSyncState: {
             findUnique: vi.fn().mockResolvedValue({ lastSeq: 0 }),
-            upsert: vi.fn().mockResolvedValue({}),
+            updateMany: vi.fn().mockResolvedValue({ count: 0 }),
+            create: vi.fn().mockResolvedValue({}),
           },
           operation: {
             findMany: vi.fn().mockResolvedValue([]),
@@ -1461,6 +1526,54 @@ describe('SnapshotService', () => {
       expect(result.TAG).toBeUndefined();
     });
 
+    it('SYNC_IMPORT must not allow prototype pollution via __proto__ in the uploaded payload', () => {
+      // Regression: a malicious client could upload a SYNC_IMPORT payload
+      // whose `appDataComplete` contains a `__proto__` key. The previous
+      // implementation used Object.assign(state, fullState) which triggers
+      // the prototype setter (Object.assign uses [[Set]] semantics) and
+      // pollutes Object.prototype for every plain object in the process —
+      // a server-wide compromise from a single malicious upload.
+      //
+      // Construct a malicious payload via JSON.parse so __proto__ is an
+      // own property (V8 / spec behaviour since 2018).
+      const maliciousPayload = JSON.parse(
+        JSON.stringify({
+          appDataComplete: {
+            TASK: { 'new-task': { id: 'new-task' } },
+          },
+        }).replace('"TASK"', '"__proto__":{"polluted":true},"TASK"'),
+      );
+      // Sanity-check the test fixture itself before relying on it.
+      const inner = maliciousPayload.appDataComplete as Record<string, unknown>;
+      expect(Object.prototype.hasOwnProperty.call(inner, '__proto__')).toBe(true);
+
+      const ops = [
+        {
+          id: 'op-malicious',
+          opType: 'SYNC_IMPORT',
+          entityType: 'FULL_STATE',
+          entityId: null,
+          payload: maliciousPayload,
+          isPayloadEncrypted: false,
+          serverSeq: 1,
+          schemaVersion: 1,
+        },
+      ];
+
+      try {
+        service.replayOpsToState(ops as any);
+        // The benign keys must be copied.
+        // (Object.prototype itself must not gain a `polluted` property.)
+        // Cast through unknown because TS narrows {} to never for arbitrary
+        // index access.
+        expect(({} as { polluted?: unknown }).polluted).toBeUndefined();
+        expect((Object.prototype as { polluted?: unknown }).polluted).toBeUndefined();
+      } finally {
+        // Defensive cleanup in case the assertion fires after pollution.
+        delete (Object.prototype as { polluted?: unknown }).polluted;
+      }
+    });
+
     it('should throw EncryptedOpsNotSupportedError for encrypted operations', () => {
       const ops = [
         {
diff --git a/packages/super-sync-server/tests/sync-operations.spec.ts b/packages/super-sync-server/tests/sync-operations.spec.ts
index 3ba2ce37ef..5e98b4db8a 100644
--- a/packages/super-sync-server/tests/sync-operations.spec.ts
+++ b/packages/super-sync-server/tests/sync-operations.spec.ts
@@ -219,6 +219,33 @@ vi.mock('../src/db', async () => {
         state.userSyncStates.set(args.where.userId, result);
         return result;
       }),
+      // Race-safe writers used by SnapshotService._generateSnapshotImpl:
+      // updateMany honours the `OR: [lastSnapshotSeq null, lastSnapshotSeq < seq]`
+      // guard; create is the first-time-user fallback.
+      updateMany: vi.fn().mockImplementation(async (args: any) => {
+        const existing = state.userSyncStates.get(args.where.userId);
+        if (!existing) return { count: 0 };
+        const seqFilter = args.where?.OR?.find(
+          (clause: any) => clause.lastSnapshotSeq?.lt !== undefined,
+        )?.lastSnapshotSeq?.lt;
+        const cachedSeq = existing.lastSnapshotSeq;
+        const matches =
+          cachedSeq === null ||
+          cachedSeq === undefined ||
+          (seqFilter !== undefined && cachedSeq < seqFilter);
+        if (!matches) return { count: 0 };
+        const updated = { ...existing, ...args.data };
+        state.userSyncStates.set(args.where.userId, updated);
+        return { count: 1 };
+      }),
+      create: vi.fn().mockImplementation(async (args: any) => {
+        if (state.userSyncStates.has(args.data.userId)) {
+          throw Object.assign(new Error('Unique constraint'), { code: 'P2002' });
+        }
+        const result = { ...args.data };
+        state.userSyncStates.set(args.data.userId, result);
+        return result;
+      }),
       update: vi.fn().mockImplementation(async (args: any) => {
         const existing = state.userSyncStates.get(args.where.userId);
         if (existing) {
diff --git a/packages/super-sync-server/tests/time-tracking-operations.spec.ts b/packages/super-sync-server/tests/time-tracking-operations.spec.ts
index 2dd8f341f3..dd7e3deeaf 100644
--- a/packages/super-sync-server/tests/time-tracking-operations.spec.ts
+++ b/packages/super-sync-server/tests/time-tracking-operations.spec.ts
@@ -218,6 +218,33 @@ vi.mock('../src/db', async () => {
         state.userSyncStates.set(args.where.userId, result);
         return result;
       }),
+      // Race-safe writers used by SnapshotService._generateSnapshotImpl:
+      // updateMany honours the `OR: [lastSnapshotSeq null, lastSnapshotSeq < seq]`
+      // guard; create is the first-time-user fallback.
+      updateMany: vi.fn().mockImplementation(async (args: any) => {
+        const existing = state.userSyncStates.get(args.where.userId);
+        if (!existing) return { count: 0 };
+        const seqFilter = args.where?.OR?.find(
+          (clause: any) => clause.lastSnapshotSeq?.lt !== undefined,
+        )?.lastSnapshotSeq?.lt;
+        const cachedSeq = existing.lastSnapshotSeq;
+        const matches =
+          cachedSeq === null ||
+          cachedSeq === undefined ||
+          (seqFilter !== undefined && cachedSeq < seqFilter);
+        if (!matches) return { count: 0 };
+        const updated = { ...existing, ...args.data };
+        state.userSyncStates.set(args.where.userId, updated);
+        return { count: 1 };
+      }),
+      create: vi.fn().mockImplementation(async (args: any) => {
+        if (state.userSyncStates.has(args.data.userId)) {
+          throw Object.assign(new Error('Unique constraint'), { code: 'P2002' });
+        }
+        const result = { ...args.data };
+        state.userSyncStates.set(args.data.userId, result);
+        return result;
+      }),
       update: vi.fn().mockImplementation(async (args: any) => {
         const existing = state.userSyncStates.get(args.where.userId);
         if (existing) {