diff --git a/cloudflare/src/index.ts b/cloudflare/src/index.ts
index f04a28c..f79ee73 100644
--- a/cloudflare/src/index.ts
+++ b/cloudflare/src/index.ts
@@ -41,6 +41,14 @@ import {
   SyncPushRequestError,
   syncPushDocument,
 } from "./sync_push.js";
+import {
+  SyncSnapshotConflictError,
+  SyncSnapshotNotFoundError,
+  SyncSnapshotPersistenceError,
+  SyncSnapshotRequestError,
+  syncSnapshotDownloadDocument,
+  syncSnapshotUploadDocument,
+} from "./sync_snapshot.js";
 import { SyncRequestError, SyncSchemaError, syncPullDocument } from "./sync_pull.js";
 import { jsonResponse } from "./responses.js";
 
@@ -245,6 +253,56 @@ export async function handleRequest(request: Request, env: Env): Promise<Respons
       },
     );
   }
+  if (url.pathname === "/api/sync/snapshot") {
+    return withApprovedDeviceApiControls(
+      request,
+      env,
+      "sync.snapshot",
+      ["GET", "POST"],
+      async (context) => {
+        try {
+          if (request.method === "POST") {
+            return jsonResponse(await syncSnapshotUploadDocument(request, env, context), 201, {
+              "Cache-Control": "no-store",
+            });
+          }
+          return jsonResponse(await syncSnapshotDownloadDocument(url, env, context), 200, {
+            "Cache-Control": "no-store",
+          });
+        } catch (error) {
+          if (error instanceof SyncSnapshotRequestError) {
+            return jsonResponse(
+              { error: "invalid_sync_snapshot" },
+              400,
+              { "Cache-Control": "no-store" },
+            );
+          }
+          if (error instanceof SyncSnapshotNotFoundError) {
+            return jsonResponse(
+              { error: "sync_snapshot_not_found" },
+              404,
+              { "Cache-Control": "no-store" },
+            );
+          }
+          if (error instanceof SyncSnapshotConflictError) {
+            return jsonResponse(
+              { error: "sync_snapshot_conflict" },
+              409,
+              { "Cache-Control": "no-store" },
+            );
+          }
+          if (error instanceof SyncSnapshotPersistenceError) {
+            return jsonResponse(
+              { error: "sync_snapshot_failed" },
+              500,
+              { "Cache-Control": "no-store" },
+            );
+          }
+          throw error;
+        }
+      },
+    );
+  }
   if (url.pathname === "/api/plugins/signing-keys") {
     return withPublicApiControls(request, env, "plugins.signing_keys", ["GET"], () =>
       handlePublicSigningKeys(env),
diff --git a/cloudflare/src/sync_snapshot.ts b/cloudflare/src/sync_snapshot.ts
new file mode 100644
index 0000000..84917e3
--- /dev/null
+++ b/cloudflare/src/sync_snapshot.ts
@@ -0,0 +1,493 @@
+import type { AuthContext } from "./auth.js";
+import type { Env } from "./bindings.js";
+import { StorageObjectError, getVerifiedObject, putVerifiedObject, syncSnapshotKey } from "./storage.js";
+
+const MAX_SNAPSHOT_BYTES = 10 * 1024 * 1024;
+const SNAPSHOT_ID_PATTERN = /^[a-z0-9][a-z0-9._-]{0,127}$/;
+const DEVICE_ID_PATTERN = /^[a-zA-Z0-9._:-]{3,128}$/;
+const SHA256_HEX = /^[a-f0-9]{64}$/;
+const BASE64 = /^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$/;
+const REGION = /^[a-z0-9][a-z0-9-]{1,31}$/;
+const SYNC_SNAPSHOT_BY_ID_QUERY = `
+  SELECT
+    snapshot_id,
+    r2_key,
+    payload_hash,
+    schema_rev,
+    logical_clock,
+    device_id,
+    size_bytes,
+    created_at
+  FROM sync_snapshots
+  WHERE user_id = ? AND snapshot_id = ?
+`;
+const SYNC_SNAPSHOT_UPSERT_QUERY = `
+  INSERT INTO sync_snapshots (
+    user_id,
+    snapshot_id,
+    r2_key,
+    payload_hash,
+    schema_rev,
+    logical_clock,
+    device_id,
+    size_bytes,
+    created_at
+  ) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)
+  ON CONFLICT(user_id, snapshot_id) DO UPDATE SET
+    r2_key = excluded.r2_key,
+    payload_hash = excluded.payload_hash,
+    schema_rev = excluded.schema_rev,
+    logical_clock = excluded.logical_clock,
+    device_id = excluded.device_id,
+    size_bytes = excluded.size_bytes,
+    created_at = excluded.created_at
+  WHERE excluded.logical_clock > sync_snapshots.logical_clock
+    OR (
+      excluded.logical_clock = sync_snapshots.logical_clock
+      AND sync_snapshots.r2_key = excluded.r2_key
+      AND sync_snapshots.payload_hash = excluded.payload_hash
+      AND sync_snapshots.schema_rev = excluded.schema_rev
+      AND sync_snapshots.device_id = excluded.device_id
+      AND sync_snapshots.size_bytes = excluded.size_bytes
+    )
+`;
+
+export interface SyncSnapshotUploadDocument {
+  version: 1;
+  user_id: string;
+  device_id: string;
+  snapshot: SyncSnapshotDocument;
+}
+
+export interface SyncSnapshotDownloadDocument extends SyncSnapshotUploadDocument {
+  data_base64: string;
+}
+
+export interface SyncSnapshotDocument {
+  snapshot_id: string;
+  r2_key: string;
+  payload_hash: string;
+  schema_rev: number;
+  logical_clock: number;
+  device_id: string;
+  size_bytes: number;
+  created_at: number;
+}
+
+interface SyncSnapshotUploadRequest {
+  snapshotId: string;
+  r2Key: string;
+  payloadHash: string;
+  schemaRev: number;
+  logicalClock: number;
+  bytes: ArrayBuffer;
+}
+
+interface SyncSnapshotRow {
+  snapshot_id: unknown;
+  r2_key: unknown;
+  payload_hash: unknown;
+  schema_rev: unknown;
+  logical_clock: unknown;
+  device_id: unknown;
+  size_bytes: unknown;
+  created_at: unknown;
+}
+
+type RequestBody = Record<string, unknown>;
+
+export class SyncSnapshotRequestError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = "SyncSnapshotRequestError";
+  }
+}
+
+export class SyncSnapshotConflictError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = "SyncSnapshotConflictError";
+  }
+}
+
+export class SyncSnapshotNotFoundError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = "SyncSnapshotNotFoundError";
+  }
+}
+
+export class SyncSnapshotPersistenceError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = "SyncSnapshotPersistenceError";
+  }
+}
+
+export async function syncSnapshotUploadDocument(
+  request: Request,
+  env: Env,
+  context: AuthContext,
+  nowSeconds = Math.floor(Date.now() / 1000),
+): Promise<SyncSnapshotUploadDocument> {
+  const deviceId = currentDeviceId(context);
+  const snapshot = await syncSnapshotUploadRequest(request, context.userId);
+  const existingRow = await snapshotRow(env, context.userId, snapshot.snapshotId);
+  if (existingRow !== null) {
+    assertSnapshotCanReplaceExisting(snapshot, deviceId, syncSnapshotDocumentFromRow(existingRow));
+  }
+
+  await persistSnapshot(env, snapshot);
+  await env.ELY_DB.batch([
+    env.ELY_DB.prepare(SYNC_SNAPSHOT_UPSERT_QUERY).bind(
+      context.userId,
+      snapshot.snapshotId,
+      snapshot.r2Key,
+      snapshot.payloadHash,
+      snapshot.schemaRev,
+      snapshot.logicalClock,
+      deviceId,
+      snapshot.bytes.byteLength,
+      nowSeconds,
+    ),
+  ]);
+
+  const savedRow = await snapshotRow(env, context.userId, snapshot.snapshotId);
+  if (savedRow === null) {
+    throw new SyncSnapshotPersistenceError("sync_snapshot_missing");
+  }
+  const savedSnapshot = syncSnapshotDocumentFromRow(savedRow);
+  assertSavedSnapshotMatchesUpload(snapshot, deviceId, savedSnapshot);
+
+  return {
+    version: 1,
+    user_id: context.userId,
+    device_id: deviceId,
+    snapshot: savedSnapshot,
+  };
+}
+
+export async function syncSnapshotDownloadDocument(
+  url: URL,
+  env: Env,
+  context: AuthContext,
+): Promise<SyncSnapshotDownloadDocument> {
+  const deviceId = currentDeviceId(context);
+  const snapshotId = syncSnapshotDownloadQuery(url);
+  const row = await snapshotRow(env, context.userId, snapshotId);
+  if (row === null) {
+    throw new SyncSnapshotNotFoundError("sync_snapshot_missing");
+  }
+
+  const snapshot = syncSnapshotDocumentFromRow(row);
+  let payload: ArrayBuffer | null;
+  try {
+    payload = await getVerifiedObject(env.ELY_STORAGE, snapshot.r2_key, snapshot.payload_hash);
+  } catch (error) {
+    if (error instanceof StorageObjectError) {
+      throw new SyncSnapshotPersistenceError(error.message);
+    }
+    throw error;
+  }
+  if (payload === null) {
+    throw new SyncSnapshotPersistenceError("sync_snapshot_payload_missing");
+  }
+
+  return {
+    version: 1,
+    user_id: context.userId,
+    device_id: deviceId,
+    snapshot,
+    data_base64: base64FromBytes(payload),
+  };
+}
+
+async function syncSnapshotUploadRequest(
+  request: Request,
+  userId: string,
+): Promise<SyncSnapshotUploadRequest> {
+  const body = await requestBody(request);
+  assertOnlyFields(body, [
+    "version",
+    "snapshot_id",
+    "region",
+    "payload_hash",
+    "schema_rev",
+    "logical_clock",
+    "data_base64",
+  ]);
+  if (body.version !== 1) {
+    throw new SyncSnapshotRequestError("version_invalid");
+  }
+
+  const snapshotId = snapshotIdValue(body.snapshot_id);
+  const region = regionValue(body.region);
+  const payloadHash = sha256HexValue(body.payload_hash, "payload_hash");
+  const bytes = payloadBytes(body.data_base64, "data_base64", MAX_SNAPSHOT_BYTES);
+  await assertPayloadHash(bytes, payloadHash);
+  return {
+    snapshotId,
+    r2Key: await snapshotStorageKey(region, userId, snapshotId),
+    payloadHash,
+    schemaRev: integer(body.schema_rev, "schema_rev", 1, Number.MAX_SAFE_INTEGER),
+    logicalClock: integer(body.logical_clock, "logical_clock", 0, Number.MAX_SAFE_INTEGER),
+    bytes,
+  };
+}
+
+function syncSnapshotDownloadQuery(url: URL): string {
+  assertOnlyQueryParams(url, ["snapshot_id"]);
+  return snapshotIdValue(url.searchParams.get("snapshot_id"));
+}
+
+async function snapshotStorageKey(
+  region: string,
+  userId: string,
+  snapshotId: string,
+): Promise<string> {
+  try {
+    return syncSnapshotKey({
+      region,
+      userHash: await sha256Hex(arrayBufferFromBytes(new TextEncoder().encode(userId))),
+      snapshotId,
+    });
+  } catch (error) {
+    if (error instanceof StorageObjectError) {
+      throw new SyncSnapshotRequestError(error.message);
+    }
+    throw error;
+  }
+}
+
+async function snapshotRow(
+  env: Env,
+  userId: string,
+  snapshotId: string,
+): Promise<SyncSnapshotRow | null> {
+  return env.ELY_DB.prepare(SYNC_SNAPSHOT_BY_ID_QUERY).bind(userId, snapshotId).first();
+}
+
+function syncSnapshotDocumentFromRow(row: SyncSnapshotRow): SyncSnapshotDocument {
+  try {
+    return syncSnapshotDocument(row);
+  } catch (error) {
+    if (error instanceof SyncSnapshotRequestError) {
+      throw new SyncSnapshotPersistenceError(error.message);
+    }
+    throw error;
+  }
+}
+
+function syncSnapshotDocument(row: SyncSnapshotRow): SyncSnapshotDocument {
+  return {
+    snapshot_id: snapshotIdValue(row.snapshot_id),
+    r2_key: text(row.r2_key, "r2_key"),
+    payload_hash: sha256HexValue(row.payload_hash, "payload_hash"),
+    schema_rev: integer(row.schema_rev, "schema_rev", 1, Number.MAX_SAFE_INTEGER),
+    logical_clock: integer(row.logical_clock, "logical_clock", 0, Number.MAX_SAFE_INTEGER),
+    device_id: deviceIdValue(row.device_id),
+    size_bytes: integer(row.size_bytes, "size_bytes", 1, MAX_SNAPSHOT_BYTES),
+    created_at: integer(row.created_at, "created_at", 0, Number.MAX_SAFE_INTEGER),
+  };
+}
+
+function assertSnapshotCanReplaceExisting(
+  snapshot: SyncSnapshotUploadRequest,
+  deviceId: string,
+  existing: SyncSnapshotDocument,
+): void {
+  if (existing.logical_clock > snapshot.logicalClock) {
+    throw new SyncSnapshotConflictError("logical_clock_stale");
+  }
+  if (existing.logical_clock < snapshot.logicalClock) {
+    return;
+  }
+  if (
+    existing.payload_hash !== snapshot.payloadHash ||
+    existing.schema_rev !== snapshot.schemaRev ||
+    existing.device_id !== deviceId ||
+    existing.size_bytes !== snapshot.bytes.byteLength
+  ) {
+    throw new SyncSnapshotConflictError("logical_clock_conflict");
+  }
+}
+
+function assertSavedSnapshotMatchesUpload(
+  upload: SyncSnapshotUploadRequest,
+  deviceId: string,
+  snapshot: SyncSnapshotDocument,
+): void {
+  if (snapshot.logical_clock > upload.logicalClock) {
+    throw new SyncSnapshotConflictError("logical_clock_stale");
+  }
+  if (
+    snapshot.logical_clock === upload.logicalClock &&
+    (snapshot.r2_key !== upload.r2Key ||
+      snapshot.payload_hash !== upload.payloadHash ||
+      snapshot.schema_rev !== upload.schemaRev ||
+      snapshot.device_id !== deviceId ||
+      snapshot.size_bytes !== upload.bytes.byteLength)
+  ) {
+    throw new SyncSnapshotConflictError("logical_clock_conflict");
+  }
+  if (
+    snapshot.snapshot_id !== upload.snapshotId ||
+    snapshot.r2_key !== upload.r2Key ||
+    snapshot.payload_hash !== upload.payloadHash ||
+    snapshot.schema_rev !== upload.schemaRev ||
+    snapshot.logical_clock !== upload.logicalClock ||
+    snapshot.device_id !== deviceId ||
+    snapshot.size_bytes !== upload.bytes.byteLength
+  ) {
+    throw new SyncSnapshotPersistenceError("sync_snapshot_mismatch");
+  }
+}
+
+async function persistSnapshot(env: Env, snapshot: SyncSnapshotUploadRequest): Promise<void> {
+  try {
+    await putVerifiedObject(
+      env.ELY_STORAGE,
+      snapshot.r2Key,
+      snapshot.bytes,
+      snapshot.payloadHash,
+      "application/octet-stream",
+    );
+  } catch (error) {
+    if (error instanceof StorageObjectError) {
+      throw new SyncSnapshotRequestError(error.message);
+    }
+    throw error;
+  }
+}
+
+function currentDeviceId(context: AuthContext): string {
+  if (context.deviceId === undefined) {
+    throw new SyncSnapshotRequestError("device_context_required");
+  }
+  return context.deviceId;
+}
+
+async function requestBody(request: Request): Promise<RequestBody> {
+  let value: unknown;
+  try {
+    value = await request.json();
+  } catch {
+    throw new SyncSnapshotRequestError("json_invalid");
+  }
+  return record(value, "body");
+}
+
+function record(value: unknown, label: string): RequestBody {
+  if (typeof value !== "object" || value === null || Array.isArray(value)) {
+    throw new SyncSnapshotRequestError(`${label}_invalid`);
+  }
+  return value as RequestBody;
+}
+
+function assertOnlyFields(value: RequestBody, fields: string[]): void {
+  const allowed = new Set(fields);
+  for (const field of Object.keys(value)) {
+    if (!allowed.has(field)) {
+      throw new SyncSnapshotRequestError(`unexpected_field:${field}`);
+    }
+  }
+}
+
+function assertOnlyQueryParams(url: URL, fields: string[]): void {
+  const allowed = new Set(fields);
+  for (const field of url.searchParams.keys()) {
+    if (!allowed.has(field)) {
+      throw new SyncSnapshotRequestError(`unexpected_query:${field}`);
+    }
+  }
+}
+
+function snapshotIdValue(value: unknown): string {
+  if (typeof value !== "string" || !SNAPSHOT_ID_PATTERN.test(value)) {
+    throw new SyncSnapshotRequestError("snapshot_id_invalid");
+  }
+  return value;
+}
+
+function deviceIdValue(value: unknown): string {
+  if (typeof value !== "string" || !DEVICE_ID_PATTERN.test(value)) {
+    throw new SyncSnapshotRequestError("device_id_invalid");
+  }
+  return value;
+}
+
+function regionValue(value: unknown): string {
+  if (typeof value !== "string" || !REGION.test(value)) {
+    throw new SyncSnapshotRequestError("region_invalid");
+  }
+  return value;
+}
+
+function sha256HexValue(value: unknown, label: string): string {
+  if (typeof value !== "string" || !SHA256_HEX.test(value)) {
+    throw new SyncSnapshotRequestError(`${label}_invalid`);
+  }
+  return value;
+}
+
+function integer(value: unknown, label: string, min: number, max: number): number {
+  if (typeof value !== "number" || !Number.isSafeInteger(value) || value < min || value > max) {
+    throw new SyncSnapshotRequestError(`${label}_invalid`);
+  }
+  return value;
+}
+
+function text(value: unknown, label: string): string {
+  if (typeof value !== "string" || value.length === 0) {
+    throw new SyncSnapshotRequestError(`${label}_invalid`);
+  }
+  return value;
+}
+
+function payloadBytes(value: unknown, label: string, maxBytes: number): ArrayBuffer {
+  const encoded = text(value, label);
+  if (!BASE64.test(encoded)) {
+    throw new SyncSnapshotRequestError(`${label}_invalid`);
+  }
+  const bytes = bytesFromBase64(encoded);
+  if (bytes.byteLength === 0 || bytes.byteLength > maxBytes) {
+    throw new SyncSnapshotRequestError(`${label}_size_invalid`);
+  }
+  return bytes;
+}
+
+function bytesFromBase64(value: string): ArrayBuffer {
+  const binary = atob(value);
+  const bytes = new Uint8Array(binary.length);
+  for (let index = 0; index < binary.length; index += 1) {
+    bytes[index] = binary.charCodeAt(index);
+  }
+  return bytes.buffer;
+}
+
+function base64FromBytes(payload: ArrayBuffer): string {
+  const bytes = new Uint8Array(payload);
+  const parts: string[] = [];
+  for (let offset = 0; offset < bytes.length; offset += 0x8000) {
+    parts.push(String.fromCharCode(...bytes.subarray(offset, offset + 0x8000)));
+  }
+  return btoa(parts.join(""));
+}
+
+function arrayBufferFromBytes(bytes: Uint8Array): ArrayBuffer {
+  const copy = new Uint8Array(bytes.byteLength);
+  copy.set(bytes);
+  return copy.buffer;
+}
+
+async function assertPayloadHash(payload: ArrayBuffer, expectedHash: string): Promise<void> {
+  const actualHash = await sha256Hex(payload);
+  if (actualHash !== expectedHash) {
+    throw new SyncSnapshotRequestError("payload_hash_mismatch");
+  }
+}
+
+async function sha256Hex(payload: ArrayBuffer): Promise<string> {
+  const digest = await crypto.subtle.digest("SHA-256", payload);
+  return [...new Uint8Array(digest)].map((byte) => byte.toString(16).padStart(2, "0")).join("");
+}
diff --git a/cloudflare/tests/devices_test_support.ts b/cloudflare/tests/devices_test_support.ts
index 29c3ad8..a72b3ce 100644
--- a/cloudflare/tests/devices_test_support.ts
+++ b/cloudflare/tests/devices_test_support.ts
@@ -14,6 +14,8 @@ export interface TestEnvOptions {
   d1?: RecordedD1Database;
   kvEntries?: [string, string][];
   kvReads?: string[];
+  r2Gets?: string[];
+  r2Objects?: [string, ArrayBuffer][];
   r2Puts?: RecordedR2Put[];
 }
 
@@ -45,7 +47,7 @@ export function testEnv(options: TestEnvOptions): Env {
         return Promise.resolve(values.get(key) ?? null);
       },
     },
-    ELY_STORAGE: testR2Bucket(options.r2Puts),
+    ELY_STORAGE: testR2Bucket(options.r2Puts, options.r2Objects, options.r2Gets),
     ELY_RATE_LIMITER: {
       limit(): Promise<{ success: boolean }> {
         return Promise.resolve({ success: true });
@@ -119,13 +121,28 @@ function testD1PreparedStatement(
   };
 }
 
-function testR2Bucket(puts: RecordedR2Put[] = []): Env["ELY_STORAGE"] {
+function testR2Bucket(
+  puts: RecordedR2Put[] = [],
+  objects: [string, ArrayBuffer][] = [],
+  gets: string[] = [],
+): Env["ELY_STORAGE"] {
+  const values = new Map(objects);
   return {
-    get() {
-      return Promise.resolve(null);
+    get(key: string) {
+      gets.push(key);
+      const value = values.get(key);
+      if (value === undefined) {
+        return Promise.resolve(null);
+      }
+      return Promise.resolve({
+        arrayBuffer() {
+          return Promise.resolve(value);
+        },
+      });
     },
     put(key: string, value: ArrayBuffer, options: ElyR2PutOptions = {}) {
       puts.push({ key, payload: value, options });
+      values.set(key, value);
       return Promise.resolve({
         arrayBuffer() {
           return Promise.resolve(value);
diff --git a/cloudflare/tests/sync_snapshot_routes.test.ts b/cloudflare/tests/sync_snapshot_routes.test.ts
new file mode 100644
index 0000000..bd8fa48
--- /dev/null
+++ b/cloudflare/tests/sync_snapshot_routes.test.ts
@@ -0,0 +1,321 @@
+import assert from "node:assert/strict";
+import { createHash } from "node:crypto";
+import { describe, it } from "node:test";
+
+import { authSessionCacheKvKey, authTokenHash } from "../src/auth.js";
+import { handleRequest } from "../src/index.js";
+import {
+  ACCESS_TOKEN,
+  type RecordedR2Put,
+  sessionDocument,
+  testD1Database,
+  testEnv,
+} from "./devices_test_support.js";
+
+const USER_ID = "user-01";
+const DEVICE_ID = "device-01";
+const SNAPSHOT_ID = "snapshot-01";
+const REGION = "us-east";
+
+describe("sync snapshot routes", () => {
+  it("uploads an encrypted snapshot from an approved current device", async () => {
+    const payload = bytes("encrypted snapshot payload");
+    const payloadHash = sha256(payload);
+    const key = snapshotKey(payloadHash);
+    const r2Puts: RecordedR2Put[] = [];
+    const tokenHash = await authTokenHash(ACCESS_TOKEN);
+    const d1 = testD1Database({
+      firstRows: [
+        { device_id: DEVICE_ID },
+        null,
+        snapshotRow({ r2_key: key, payload_hash: payloadHash, size_bytes: payload.byteLength }),
+      ],
+    });
+
+    const response = await handleRequest(
+      syncSnapshotPostRequest(
+        syncSnapshotBody({ payload_hash: payloadHash, data_base64: base64(payload) }),
+      ),
+      testEnv({
+        d1,
+        r2Puts,
+        kvEntries: [[authSessionCacheKvKey("local", tokenHash), sessionDocument(DEVICE_ID)]],
+      }),
+    );
+
+    assert.equal(response.status, 201);
+    assert.equal(response.headers.get("cache-control"), "no-store");
+    assert.deepEqual(await response.json(), {
+      version: 1,
+      user_id: USER_ID,
+      device_id: DEVICE_ID,
+      snapshot: snapshotDocument({
+        r2_key: key,
+        payload_hash: payloadHash,
+        size_bytes: payload.byteLength,
+      }),
+    });
+    assert.equal(r2Puts.length, 1);
+    assert.equal(r2Puts[0]?.key, key);
+    assert.deepEqual(new Uint8Array(r2Puts[0]?.payload ?? new ArrayBuffer(0)), new Uint8Array(payload));
+    assert.equal(r2Puts[0]?.options.customMetadata?.sha256, payloadHash);
+    assert.equal(d1.batches[0], 1);
+    assert.ok(d1.queries[1]?.includes("FROM sync_snapshots"));
+    assert.ok(d1.queries[2]?.includes("INSERT INTO sync_snapshots"));
+    assert.deepEqual(d1.binds[2]?.slice(0, 4), [USER_ID, SNAPSHOT_ID, key, payloadHash]);
+  });
+
+  it("downloads an encrypted snapshot with R2 checksum verification", async () => {
+    const payload = bytes("encrypted snapshot payload");
+    const payloadHash = sha256(payload);
+    const key = snapshotKey(payloadHash);
+    const r2Gets: string[] = [];
+    const tokenHash = await authTokenHash(ACCESS_TOKEN);
+    const d1 = testD1Database({
+      firstRows: [
+        { device_id: DEVICE_ID },
+        snapshotRow({ r2_key: key, payload_hash: payloadHash, size_bytes: payload.byteLength }),
+      ],
+    });
+
+    const response = await handleRequest(
+      syncSnapshotGetRequest(),
+      testEnv({
+        d1,
+        r2Gets,
+        r2Objects: [[key, payload]],
+        kvEntries: [[authSessionCacheKvKey("local", tokenHash), sessionDocument(DEVICE_ID)]],
+      }),
+    );
+
+    assert.equal(response.status, 200);
+    assert.deepEqual(await response.json(), {
+      version: 1,
+      user_id: USER_ID,
+      device_id: DEVICE_ID,
+      snapshot: snapshotDocument({
+        r2_key: key,
+        payload_hash: payloadHash,
+        size_bytes: payload.byteLength,
+      }),
+      data_base64: base64(payload),
+    });
+    assert.deepEqual(r2Gets, [key]);
+  });
+
+  it("returns not found for missing snapshot indexes", async () => {
+    const r2Gets: string[] = [];
+    const tokenHash = await authTokenHash(ACCESS_TOKEN);
+    const d1 = testD1Database({ firstRows: [{ device_id: DEVICE_ID }, null] });
+
+    const response = await handleRequest(
+      syncSnapshotGetRequest(),
+      testEnv({
+        d1,
+        r2Gets,
+        kvEntries: [[authSessionCacheKvKey("local", tokenHash), sessionDocument(DEVICE_ID)]],
+      }),
+    );
+
+    assert.equal(response.status, 404);
+    assert.deepEqual(await response.json(), { error: "sync_snapshot_not_found" });
+    assert.deepEqual(r2Gets, []);
+  });
+
+  it("rejects snapshot checksum mismatches before D1 writes", async () => {
+    const payload = bytes("encrypted snapshot payload");
+    const r2Puts: RecordedR2Put[] = [];
+    const tokenHash = await authTokenHash(ACCESS_TOKEN);
+    const d1 = testD1Database({ firstRows: [{ device_id: DEVICE_ID }] });
+
+    const response = await handleRequest(
+      syncSnapshotPostRequest(
+        syncSnapshotBody({ payload_hash: "c".repeat(64), data_base64: base64(payload) }),
+      ),
+      testEnv({
+        d1,
+        r2Puts,
+        kvEntries: [[authSessionCacheKvKey("local", tokenHash), sessionDocument(DEVICE_ID)]],
+      }),
+    );
+
+    assert.equal(response.status, 400);
+    assert.deepEqual(await response.json(), { error: "invalid_sync_snapshot" });
+    assert.equal(r2Puts.length, 0);
+    assert.equal(d1.queries.length, 1);
+    assert.deepEqual(d1.batches, []);
+  });
+
+  it("rejects stale snapshot clocks before R2 writes", async () => {
+    const payload = bytes("encrypted snapshot payload");
+    const payloadHash = sha256(payload);
+    const r2Puts: RecordedR2Put[] = [];
+    const tokenHash = await authTokenHash(ACCESS_TOKEN);
+    const d1 = testD1Database({
+      firstRows: [
+        { device_id: DEVICE_ID },
+        snapshotRow({ payload_hash: "d".repeat(64), logical_clock: 43 }),
+      ],
+    });
+
+    const response = await handleRequest(
+      syncSnapshotPostRequest(
+        syncSnapshotBody({
+          payload_hash: payloadHash,
+          logical_clock: 42,
+          data_base64: base64(payload),
+        }),
+      ),
+      testEnv({
+        d1,
+        r2Puts,
+        kvEntries: [[authSessionCacheKvKey("local", tokenHash), sessionDocument(DEVICE_ID)]],
+      }),
+    );
+
+    assert.equal(response.status, 409);
+    assert.deepEqual(await response.json(), { error: "sync_snapshot_conflict" });
+    assert.equal(r2Puts.length, 0);
+    assert.deepEqual(d1.batches, []);
+  });
+
+  it("rejects same-clock snapshot write races after D1 persistence", async () => {
+    const payload = bytes("encrypted snapshot payload");
+    const payloadHash = sha256(payload);
+    const key = snapshotKey(payloadHash);
+    const r2Puts: RecordedR2Put[] = [];
+    const tokenHash = await authTokenHash(ACCESS_TOKEN);
+    const d1 = testD1Database({
+      firstRows: [
+        { device_id: DEVICE_ID },
+        null,
+        snapshotRow({ r2_key: key, payload_hash: "e".repeat(64), size_bytes: payload.byteLength }),
+      ],
+    });
+
+    const response = await handleRequest(
+      syncSnapshotPostRequest(
+        syncSnapshotBody({ payload_hash: payloadHash, data_base64: base64(payload) }),
+      ),
+      testEnv({
+        d1,
+        r2Puts,
+        kvEntries: [[authSessionCacheKvKey("local", tokenHash), sessionDocument(DEVICE_ID)]],
+      }),
+    );
+
+    assert.equal(response.status, 409);
+    assert.deepEqual(await response.json(), { error: "sync_snapshot_conflict" });
+    assert.equal(r2Puts.length, 1);
+    assert.equal(d1.batches[0], 1);
+  });
+
+  it("rejects revoked devices before reading snapshot payloads", async () => {
+    const tokenHash = await authTokenHash(ACCESS_TOKEN);
+    const d1 = testD1Database({ firstRows: [null] });
+
+    const response = await handleRequest(
+      syncSnapshotPostRequest(syncSnapshotBody()),
+      testEnv({
+        d1,
+        kvEntries: [[authSessionCacheKvKey("local", tokenHash), sessionDocument(DEVICE_ID)]],
+      }),
+    );
+
+    assert.equal(response.status, 403);
+    assert.deepEqual(await response.json(), { error: "device_not_approved" });
+    assert.equal(d1.queries.length, 1);
+    assert.deepEqual(d1.batches, []);
+  });
+
+  it("fails closed when a stored snapshot payload fails checksum verification", async () => {
+    const payload = bytes("encrypted snapshot payload");
+    const payloadHash = sha256(payload);
+    const key = snapshotKey(payloadHash);
+    const tokenHash = await authTokenHash(ACCESS_TOKEN);
+    const d1 = testD1Database({
+      firstRows: [
+        { device_id: DEVICE_ID },
+        snapshotRow({ r2_key: key, payload_hash: payloadHash, size_bytes: payload.byteLength }),
+      ],
+    });
+
+    const response = await handleRequest(
+      syncSnapshotGetRequest(),
+      testEnv({
+        d1,
+        r2Objects: [[key, bytes("corrupt snapshot payload")]],
+        kvEntries: [[authSessionCacheKvKey("local", tokenHash), sessionDocument(DEVICE_ID)]],
+      }),
+    );
+
+    assert.equal(response.status, 500);
+    assert.deepEqual(await response.json(), { error: "sync_snapshot_failed" });
+  });
+});
+
+function syncSnapshotPostRequest(body: Record<string, unknown>): Request {
+  return new Request("https://elydora.test/api/sync/snapshot", {
+    method: "POST",
+    headers: {
+      authorization: `Bearer ${ACCESS_TOKEN}`,
+      "content-type": "application/json",
+    },
+    body: JSON.stringify(body),
+  });
+}
+
+function syncSnapshotGetRequest(): Request {
+  return new Request(`https://elydora.test/api/sync/snapshot?snapshot_id=${SNAPSHOT_ID}`, {
+    headers: { authorization: `Bearer ${ACCESS_TOKEN}` },
+  });
+}
+
+function syncSnapshotBody(overrides: Record<string, unknown> = {}): Record<string, unknown> {
+  const payload = bytes("encrypted snapshot payload");
+  const payloadHash = sha256(payload);
+  return {
+    version: 1,
+    snapshot_id: SNAPSHOT_ID,
+    region: REGION,
+    payload_hash: payloadHash,
+    schema_rev: 1,
+    logical_clock: 42,
+    data_base64: base64(payload),
+    ...overrides,
+  };
+}
+
+function snapshotRow(overrides: Record<string, unknown> = {}): Record<string, unknown> {
+  return {
+    snapshot_id: SNAPSHOT_ID,
+    r2_key: snapshotKey("a".repeat(64)),
+    payload_hash: "a".repeat(64),
+    schema_rev: 1,
+    logical_clock: 42,
+    device_id: DEVICE_ID,
+    size_bytes: 26,
+    created_at: 1_780_000_900,
+    ...overrides,
+  };
+}
+
+function snapshotDocument(overrides: Record<string, unknown> = {}): Record<string, unknown> {
+  return snapshotRow(overrides);
+}
+
+function snapshotKey(_payloadHash: string): string {
+  return `sync-snapshots/${REGION}/${sha256(bytes(USER_ID))}/${SNAPSHOT_ID}.bin`;
+}
+
+function bytes(value: string): ArrayBuffer {
+  return new TextEncoder().encode(value).buffer;
+}
+
+function base64(payload: ArrayBuffer): string {
+  return Buffer.from(payload).toString("base64");
+}
+
+function sha256(payload: ArrayBuffer): string {
+  return createHash("sha256").update(new Uint8Array(payload)).digest("hex");
+}