diff --git a/xftp-web/src/client.ts b/xftp-web/src/client.ts
index 1384147fb..3d1f823e5 100644
--- a/xftp-web/src/client.ts
+++ b/xftp-web/src/client.ts
@@ -16,7 +16,7 @@ import {
 import {verifyIdentityProof} from "./crypto/identity.js"
 import {generateX25519KeyPair, encodePubKeyX25519, dh} from "./crypto/keys.js"
 import {
-  encodeFNEW, encodeFADD, encodeFPUT, encodeFGET, encodeFDEL, encodePING,
+  encodeFNEW, encodeFADD, encodeFPUT, encodeFGET, encodeFACK, encodeFDEL, encodePING,
   decodeResponse, type FileResponse, type FileInfo, type XFTPErrorType
 } from "./protocol/commands.js"
 import {decryptReceivedChunk} from "./download.js"
@@ -430,6 +430,13 @@ export async function deleteXFTPChunk(
   if (response.type !== "FROk") throw new Error("unexpected response: " + response.type)
 }
 
+export async function ackXFTPChunk(
+  agent: XFTPClientAgent, server: XFTPServer, rpKey: Uint8Array, rId: Uint8Array
+): Promise<void> {
+  const {response} = await sendXFTPCommand(agent, server, rpKey, rId, encodeFACK())
+  if (response.type !== "FROk") throw new Error("unexpected response: " + response.type)
+}
+
 export async function pingXFTP(agent: XFTPClientAgent, server: XFTPServer): Promise<void> {
   const client = await getXFTPServerClient(agent, server)
   const corrId = new Uint8Array(0)
diff --git a/xftp-web/src/protocol/commands.ts b/xftp-web/src/protocol/commands.ts
index 3ca43541f..ede1fbde1 100644
--- a/xftp-web/src/protocol/commands.ts
+++ b/xftp-web/src/protocol/commands.ts
@@ -73,6 +73,8 @@ export function encodeFPUT(): Uint8Array { return ascii("FPUT") }
 
 export function encodeFDEL(): Uint8Array { return ascii("FDEL") }
 
+export function encodeFACK(): Uint8Array { return ascii("FACK") }
+
 export function encodeFGET(rcvDhKey: Uint8Array): Uint8Array {
   return concatBytes(ascii("FGET"), SPACE, encodeBytes(rcvDhKey))
 }