diff --git a/xftp-web/src/agent.ts b/xftp-web/src/agent.ts
index 3205afdc7..db3d605d5 100644
--- a/xftp-web/src/agent.ts
+++ b/xftp-web/src/agent.ts
@@ -16,10 +16,11 @@ import {
 import type {FileInfo} from "./protocol/commands.js"
 import {
   createXFTPChunk, uploadXFTPChunk, downloadXFTPChunk, downloadXFTPChunkRaw,
-  deleteXFTPChunk, type XFTPClientAgent
+  deleteXFTPChunk, ackXFTPChunk, type XFTPClientAgent
 } from "./client.js"
 export {newXFTPAgent, closeXFTPAgent, type XFTPClientAgent, type TransportConfig,
-  XFTPRetriableError, XFTPPermanentError, isRetriable, categorizeError, humanReadableMessage} from "./client.js"
+  XFTPRetriableError, XFTPPermanentError, isRetriable, categorizeError, humanReadableMessage,
+  ackXFTPChunk} from "./client.js"
 import {processDownloadedFile, decryptReceivedChunk} from "./download.js"
 import type {XFTPServer} from "./protocol/address.js"
 import {formatXFTPServer, parseXFTPServer} from "./protocol/address.js"
@@ -31,8 +32,7 @@ interface SentChunk {
   chunkNo: number
   senderId: Uint8Array
   senderKey: Uint8Array      // 64B libsodium Ed25519 private key
-  recipientId: Uint8Array
-  recipientKey: Uint8Array   // 64B libsodium Ed25519 private key
+  recipients: {recipientId: Uint8Array, recipientKey: Uint8Array}[]
   chunkSize: number
   digest: Uint8Array         // SHA-256
   server: XFTPServer
@@ -50,7 +50,7 @@ export interface EncryptedFileInfo extends EncryptedFileMetadata {
 }
 
 export interface UploadResult {
-  rcvDescription: FileDescription
+  rcvDescriptions: FileDescription[]
   sndDescription: FileDescription
   uri: string                 // base64url-encoded compressed YAML (no leading #)
 }
@@ -101,6 +101,8 @@ export interface UploadOptions {
   onProgress?: (uploaded: number, total: number) => void
   redirectThreshold?: number
   readChunk?: (offset: number, size: number) => Promise<Uint8Array>
+  auth?: Uint8Array
+  numRecipients?: number
 }
 
 export async function uploadFile(
@@ -110,7 +112,7 @@ export async function uploadFile(
   options?: UploadOptions
 ): Promise<UploadResult> {
   if (servers.length === 0) throw new Error("uploadFile: servers list is empty")
-  const {onProgress, redirectThreshold, readChunk: readChunkOpt} = options ?? {}
+  const {onProgress, redirectThreshold, readChunk: readChunkOpt, auth, numRecipients = 1} = options ?? {}
   const readChunk: (offset: number, size: number) => Promise<Uint8Array> = readChunkOpt
     ? readChunkOpt
     : ('encData' in encrypted
@@ -118,49 +120,71 @@ export async function uploadFile(
         : () => { throw new Error("uploadFile: readChunk required when encData is absent") })
   const total = encrypted.chunkSizes.reduce((a, b) => a + b, 0)
   const specs = prepareChunkSpecs(encrypted.chunkSizes)
-  const sentChunks: SentChunk[] = []
-  let uploaded = 0
-  for (let i = 0; i < specs.length; i++) {
-    const spec = specs[i]
-    const chunkNo = i + 1
-    const server = servers[Math.floor(Math.random() * servers.length)]
-    const sndKp = generateEd25519KeyPair()
-    const rcvKp = generateEd25519KeyPair()
-    const chunkData = await readChunk(spec.chunkOffset, spec.chunkSize)
-    const chunkDigest = getChunkDigest(chunkData)
-    console.log(`[AGENT-DBG] upload chunk=${chunkNo} offset=${spec.chunkOffset} size=${spec.chunkSize} digest=${_dbgHex(chunkDigest, 32)} data[0..8]=${_dbgHex(chunkData)} data[-8..]=${_dbgHex(chunkData.slice(-8))}`)
-    const fileInfo: FileInfo = {
-      sndKey: encodePubKeyEd25519(sndKp.publicKey),
-      size: spec.chunkSize,
-      digest: chunkDigest
-    }
-    const rcvKeysForChunk = [encodePubKeyEd25519(rcvKp.publicKey)]
-    const {senderId, recipientIds} = await createXFTPChunk(
-      agent, server, sndKp.privateKey, fileInfo, rcvKeysForChunk
-    )
-    await uploadXFTPChunk(agent, server, sndKp.privateKey, senderId, chunkData)
-    sentChunks.push({
-      chunkNo, senderId, senderKey: sndKp.privateKey,
-      recipientId: recipientIds[0], recipientKey: rcvKp.privateKey,
-      chunkSize: spec.chunkSize, digest: chunkDigest, server
-    })
-    uploaded += spec.chunkSize
-    onProgress?.(uploaded, total)
+
+  // Pre-assign servers and group by server (matching Haskell groupAllOn)
+  const chunkJobs = specs.map((spec, i) => ({
+    index: i,
+    spec,
+    server: servers[Math.floor(Math.random() * servers.length)]
+  }))
+  const byServer = new Map<string, typeof chunkJobs>()
+  for (const job of chunkJobs) {
+    const key = formatXFTPServer(job.server)
+    if (!byServer.has(key)) byServer.set(key, [])
+    byServer.get(key)!.push(job)
   }
-  const rcvDescription = buildDescription("recipient", encrypted, sentChunks)
-  const sndDescription = buildDescription("sender", encrypted, sentChunks)
-  let uri = encodeDescriptionURI(rcvDescription)
-  let finalRcvDescription = rcvDescription
+
+  // Upload groups in parallel, sequential within each group
+  const sentChunks: SentChunk[] = new Array(specs.length)
+  let uploaded = 0
+  await Promise.all([...byServer.values()].map(async (jobs) => {
+    for (const {index, spec, server} of jobs) {
+      const chunkNo = index + 1
+      const sndKp = generateEd25519KeyPair()
+      const rcvKps = Array.from({length: numRecipients}, () => generateEd25519KeyPair())
+      const chunkData = await readChunk(spec.chunkOffset, spec.chunkSize)
+      const chunkDigest = getChunkDigest(chunkData)
+      console.log(`[AGENT-DBG] upload chunk=${chunkNo} offset=${spec.chunkOffset} size=${spec.chunkSize} digest=${_dbgHex(chunkDigest, 32)} data[0..8]=${_dbgHex(chunkData)} data[-8..]=${_dbgHex(chunkData.slice(-8))}`)
+      const fileInfo: FileInfo = {
+        sndKey: encodePubKeyEd25519(sndKp.publicKey),
+        size: spec.chunkSize,
+        digest: chunkDigest
+      }
+      const rcvKeysForChunk = rcvKps.map(kp => encodePubKeyEd25519(kp.publicKey))
+      const {senderId, recipientIds} = await createXFTPChunk(
+        agent, server, sndKp.privateKey, fileInfo, rcvKeysForChunk, auth ?? null
+      )
+      await uploadXFTPChunk(agent, server, sndKp.privateKey, senderId, chunkData)
+      sentChunks[index] = {
+        chunkNo, senderId, senderKey: sndKp.privateKey,
+        recipients: recipientIds.map((rid, ri) => ({
+          recipientId: rid, recipientKey: rcvKps[ri].privateKey
+        })),
+        chunkSize: spec.chunkSize, digest: chunkDigest, server
+      }
+      uploaded += spec.chunkSize
+      onProgress?.(uploaded, total)
+    }
+  }))
+
+  const rcvDescriptions = Array.from({length: numRecipients}, (_, ri) =>
+    buildDescription("recipient", ri, encrypted, sentChunks)
+  )
+  const sndDescription = buildDescription("sender", 0, encrypted, sentChunks)
+  let uri = encodeDescriptionURI(rcvDescriptions[0])
+  let finalRcvDescriptions = rcvDescriptions
   const threshold = redirectThreshold ?? DEFAULT_REDIRECT_THRESHOLD
   if (uri.length > threshold && sentChunks.length > 1) {
-    finalRcvDescription = await uploadRedirectDescription(agent, servers, rcvDescription)
-    uri = encodeDescriptionURI(finalRcvDescription)
+    const redirected = await uploadRedirectDescription(agent, servers, rcvDescriptions[0], auth)
+    finalRcvDescriptions = [redirected, ...rcvDescriptions.slice(1)]
+    uri = encodeDescriptionURI(redirected)
   }
-  return {rcvDescription: finalRcvDescription, sndDescription, uri}
+  return {rcvDescriptions: finalRcvDescriptions, sndDescription, uri}
 }
 
 function buildDescription(
   party: "recipient" | "sender",
+  recipientIndex: number,
   enc: EncryptedFileMetadata,
   chunks: SentChunk[]
 ): FileDescription {
@@ -178,8 +202,8 @@ function buildDescription(
       digest: c.digest,
       replicas: [{
         server: formatXFTPServer(c.server),
-        replicaId: party === "recipient" ? c.recipientId : c.senderId,
-        replicaKey: encodePrivKeyEd25519(party === "recipient" ? c.recipientKey : c.senderKey)
+        replicaId: party === "recipient" ? c.recipients[recipientIndex].recipientId : c.senderId,
+        replicaKey: encodePrivKeyEd25519(party === "recipient" ? c.recipients[recipientIndex].recipientKey : c.senderKey)
       }]
     })),
     redirect: null
@@ -189,37 +213,52 @@ function buildDescription(
 async function uploadRedirectDescription(
   agent: XFTPClientAgent,
   servers: XFTPServer[],
-  innerFd: FileDescription
+  innerFd: FileDescription,
+  auth?: Uint8Array
 ): Promise<FileDescription> {
   const yaml = encodeFileDescription(innerFd)
   const yamlBytes = new TextEncoder().encode(yaml)
   const enc = encryptFileForUpload(yamlBytes, "")
   const specs = prepareChunkSpecs(enc.chunkSizes)
-  const sentChunks: SentChunk[] = []
-  for (let i = 0; i < specs.length; i++) {
-    const spec = specs[i]
-    const chunkNo = i + 1
-    const server = servers[Math.floor(Math.random() * servers.length)]
-    const sndKp = generateEd25519KeyPair()
-    const rcvKp = generateEd25519KeyPair()
-    const chunkData = enc.encData.subarray(spec.chunkOffset, spec.chunkOffset + spec.chunkSize)
-    const chunkDigest = getChunkDigest(chunkData)
-    const fileInfo: FileInfo = {
-      sndKey: encodePubKeyEd25519(sndKp.publicKey),
-      size: spec.chunkSize,
-      digest: chunkDigest
-    }
-    const rcvKeysForChunk = [encodePubKeyEd25519(rcvKp.publicKey)]
-    const {senderId, recipientIds} = await createXFTPChunk(
-      agent, server, sndKp.privateKey, fileInfo, rcvKeysForChunk
-    )
-    await uploadXFTPChunk(agent, server, sndKp.privateKey, senderId, chunkData)
-    sentChunks.push({
-      chunkNo, senderId, senderKey: sndKp.privateKey,
-      recipientId: recipientIds[0], recipientKey: rcvKp.privateKey,
-      chunkSize: spec.chunkSize, digest: chunkDigest, server
-    })
+
+  const chunkJobs = specs.map((spec, i) => ({
+    index: i,
+    spec,
+    server: servers[Math.floor(Math.random() * servers.length)]
+  }))
+  const byServer = new Map<string, typeof chunkJobs>()
+  for (const job of chunkJobs) {
+    const key = formatXFTPServer(job.server)
+    if (!byServer.has(key)) byServer.set(key, [])
+    byServer.get(key)!.push(job)
   }
+
+  const sentChunks: SentChunk[] = new Array(specs.length)
+  await Promise.all([...byServer.values()].map(async (jobs) => {
+    for (const {index, spec, server} of jobs) {
+      const chunkNo = index + 1
+      const sndKp = generateEd25519KeyPair()
+      const rcvKp = generateEd25519KeyPair()
+      const chunkData = enc.encData.subarray(spec.chunkOffset, spec.chunkOffset + spec.chunkSize)
+      const chunkDigest = getChunkDigest(chunkData)
+      const fileInfo: FileInfo = {
+        sndKey: encodePubKeyEd25519(sndKp.publicKey),
+        size: spec.chunkSize,
+        digest: chunkDigest
+      }
+      const rcvKeysForChunk = [encodePubKeyEd25519(rcvKp.publicKey)]
+      const {senderId, recipientIds} = await createXFTPChunk(
+        agent, server, sndKp.privateKey, fileInfo, rcvKeysForChunk, auth ?? null
+      )
+      await uploadXFTPChunk(agent, server, sndKp.privateKey, senderId, chunkData)
+      sentChunks[index] = {
+        chunkNo, senderId, senderKey: sndKp.privateKey,
+        recipients: [{recipientId: recipientIds[0], recipientKey: rcvKp.privateKey}],
+        chunkSize: spec.chunkSize, digest: chunkDigest, server
+      }
+    }
+  }))
+
   return {
     party: "recipient",
     size: enc.chunkSizes.reduce((a, b) => a + b, 0),
@@ -233,8 +272,8 @@ async function uploadRedirectDescription(
       digest: c.digest,
       replicas: [{
         server: formatXFTPServer(c.server),
-        replicaId: c.recipientId,
-        replicaKey: encodePrivKeyEd25519(c.recipientKey)
+        replicaId: c.recipients[0].recipientId,
+        replicaKey: encodePrivKeyEd25519(c.recipients[0].recipientKey)
       }]
     })),
     redirect: {size: innerFd.size, digest: innerFd.digest}
@@ -253,7 +292,6 @@ export interface RawDownloadedChunk {
 
 export interface DownloadRawOptions {
   onProgress?: (downloaded: number, total: number) => void
-  concurrency?: number
 }
 
 export async function downloadFileRaw(
@@ -264,7 +302,7 @@ export async function downloadFileRaw(
 ): Promise<FileDescription> {
   const err = validateFileDescription(fd)
   if (err) throw new Error("downloadFileRaw: " + err)
-  const {onProgress, concurrency = 1} = options ?? {}
+  const {onProgress} = options ?? {}
   // Resolve redirect on main thread (redirect data is small)
   if (fd.redirect !== null) {
     console.log(`[AGENT-DBG] resolving redirect: outer size=${fd.size} chunks=${fd.chunks.length}`)
@@ -296,6 +334,7 @@ export async function downloadFileRaw(
         body: raw.body,
         digest: chunk.digest
       })
+      await ackXFTPChunk(agent, server, kp.privateKey, replica.replicaId)
       downloaded += chunk.chunkSize
       onProgress?.(downloaded, resolvedFd.size)
     }
diff --git a/xftp-web/web/upload.ts b/xftp-web/web/upload.ts
index 0faa70c0a..cf63ba379 100644
--- a/xftp-web/web/upload.ts
+++ b/xftp-web/web/upload.ts
@@ -3,9 +3,8 @@ import {getServers} from './servers.js'
 import {createProgressRing} from './progress.js'
 import {
   newXFTPAgent, closeXFTPAgent, uploadFile, encodeDescriptionURI,
-  type EncryptedFileMetadata
+  XFTPPermanentError, type EncryptedFileMetadata
 } from '../src/agent.js'
-import {XFTPPermanentError} from '../src/client.js'
 
 const MAX_SIZE = 100 * 1024 * 1024