diff --git a/src/services/npdi.ts b/src/services/npdi.ts
index 596e9d1..5af9c89 100644
--- a/src/services/npdi.ts
+++ b/src/services/npdi.ts
@@ -3,6 +3,7 @@ import { restrictHostnames } from '@/middleware/host-limit';
 import { config } from '@/config-manager';
 import { getCDNFileAsStream, streamFileToResponse } from '@/cdn';
 import { getTaskFileByDataID } from '@/database';
+import { handleEtag, sendEtagCacheResponse } from '@/util';
 
 const npdi = express.Router();
 
@@ -18,6 +19,11 @@ npdi.get('/p01/data/1/:bossAppId/:dataId/:fileHash', async (request, response) =
 		return response.sendStatus(404);
 	}
 
+	const { clientHasCached } = handleEtag(request, response, file.hash);
+	if (clientHasCached) {
+		return sendEtagCacheResponse(response);
+	}
+
 	const fileStream = await getCDNFileAsStream('taskFile', file.file_key);
 	if (!fileStream) {
 		throw new Error('File not found in CDN');
diff --git a/src/services/npdl.ts b/src/services/npdl.ts
index e934949..d41a998 100644
--- a/src/services/npdl.ts
+++ b/src/services/npdl.ts
@@ -3,6 +3,7 @@ import { getTaskFile } from '@/database';
 import { config } from '@/config-manager';
 import { restrictHostnames } from '@/middleware/host-limit';
 import { getCDNFileAsStream, streamFileToResponse } from '@/cdn';
+import { handleEtag, sendEtagCacheResponse } from '@/util';
 
 const npdl = express.Router();
 
@@ -29,6 +30,11 @@ npdl.get([
 		return;
 	}
 
+	const { clientHasCached } = handleEtag(request, response, file.hash);
+	if (clientHasCached) {
+		return sendEtagCacheResponse(response);
+	}
+
 	const readStream = await getCDNFileAsStream('taskFile', file.file_key);
 	if (!readStream) {
 		response.sendStatus(404);
diff --git a/src/util.ts b/src/util.ts
index a4e12c0..ebc8bf4 100644
--- a/src/util.ts
+++ b/src/util.ts
@@ -10,7 +10,7 @@ import type { AccountClient } from '@pretendonetwork/grpc/account/account_servic
 import type { GetNEXDataResponse } from '@pretendonetwork/grpc/account/get_nex_data_rpc';
 import type { GetUserDataResponse } from '@pretendonetwork/grpc/account/get_user_data_rpc';
 import type { GetUserFriendPIDsResponse } from '@pretendonetwork/grpc/friends/get_user_friend_pids_rpc';
-import type { Response } from 'express';
+import type { Request, Response } from 'express';
 import type { Stats } from 'node:fs';
 
 const gRPCAccountChannel = createChannel(`${config.grpc.account.address}:${config.grpc.account.port}`);
@@ -148,3 +148,16 @@ export async function getFriends(pid: number): Promise<GetUserFriendPIDsResponse
 		return null;
 	}
 }
+
+export function handleEtag(request: Request, response: Response, hash: string): { clientHasCached: boolean } {
+	const etag = `"${hash}"`;
+	response.setHeader('ETag', etag);
+	if (request.headers['if-none-match'] === etag) {
+		return { clientHasCached: true };
+	}
+	return { clientHasCached: false };
+}
+
+export function sendEtagCacheResponse(response: Response): void {
+	response.sendStatus(304).end();
+}