diff --git a/@xen-orchestra/backups/_runners/_createStreamThrottle.mjs b/@xen-orchestra/backups/_runners/_createStreamThrottle.mjs
index b7f05d8dff..1b11293d1c 100644
--- a/@xen-orchestra/backups/_runners/_createStreamThrottle.mjs
+++ b/@xen-orchestra/backups/_runners/_createStreamThrottle.mjs
@@ -10,6 +10,9 @@ export default function createStreamThrottle(rate) {
   }
   const group = new ThrottleGroup({ rate })
   return function throttleStream(stream) {
-    return pipeline(stream, group.createThrottle(), noop)
+    const throttled = pipeline(stream, group.createThrottle(), noop)
+    throttled.length = stream.length
+    throttled.maxStreamLength = stream.maxStreamLength
+    return throttled
   }
 }
diff --git a/@xen-orchestra/backups/_runners/_createStreamThrottle.test.mjs b/@xen-orchestra/backups/_runners/_createStreamThrottle.test.mjs
new file mode 100644
index 0000000000..456d3380bd
--- /dev/null
+++ b/@xen-orchestra/backups/_runners/_createStreamThrottle.test.mjs
@@ -0,0 +1,58 @@
+import assert from 'node:assert/strict'
+import { describe, it } from 'node:test'
+import { Readable } from 'node:stream'
+
+import createStreamThrottle from './_createStreamThrottle.mjs'
+
+const makeStream = (chunks, props = {}) => {
+  const stream = Readable.from(chunks)
+  Object.assign(stream, props)
+  return stream
+}
+
+const readAll = async stream => {
+  const chunks = []
+  for await (const chunk of stream) {
+    chunks.push(chunk)
+  }
+  return Buffer.concat(chunks.map(c => (typeof c === 'string' ? Buffer.from(c) : c))).toString()
+}
+
+describe('createStreamThrottle', () => {
+  it('rate=0 returns identity, stream data and props intact', async () => {
+    const throttleStream = createStreamThrottle(0)
+    const stream = makeStream(['hello world'], { length: 11, maxStreamLength: 20 })
+    const throttled = throttleStream(stream)
+    assert.strictEqual(throttled, stream)
+    assert.strictEqual(throttled.length, 11)
+    assert.strictEqual(throttled.maxStreamLength, 20)
+    assert.strictEqual(await readAll(throttled), 'hello world')
+  })
+
+  it('rate>0 passes data through and preserves length and maxStreamLength', async () => {
+    const throttleStream = createStreamThrottle(1024 * 1024)
+    const stream = makeStream(['hello world'], { length: 11, maxStreamLength: 20 })
+    const throttled = throttleStream(stream)
+    assert.strictEqual(throttled.length, 11)
+    assert.strictEqual(throttled.maxStreamLength, 20)
+    assert.strictEqual(await readAll(throttled), 'hello world')
+  })
+
+  it('rate>0 handles undefined length and maxStreamLength', async () => {
+    const throttleStream = createStreamThrottle(1024 * 1024)
+    const stream = makeStream(['hello world'])
+    const throttled = throttleStream(stream)
+    assert.strictEqual(throttled.length, undefined)
+    assert.strictEqual(throttled.maxStreamLength, undefined)
+    assert.strictEqual(await readAll(throttled), 'hello world')
+  })
+
+  // regression: maxStreamLength was lost after throttling, causing S3 to ignore it and cap at 50GB
+  it('rate>0 preserves maxStreamLength so S3 can compute correct partSize', async () => {
+    const throttleStream = createStreamThrottle(1024 * 1024)
+    const stream = makeStream(['hello world'], { maxStreamLength: 11 })
+    const throttled = throttleStream(stream)
+    assert.strictEqual(throttled.maxStreamLength, 11)
+    assert.strictEqual(await readAll(throttled), 'hello world')
+  })
+})
diff --git a/CHANGELOG.unreleased.md b/CHANGELOG.unreleased.md
index 925f109c90..d75dcfda3e 100644
--- a/CHANGELOG.unreleased.md
+++ b/CHANGELOG.unreleased.md
@@ -16,6 +16,7 @@
 > Users must be able to say: "I had this issue, happy to know it's fixed"
 
 - [Backups] fix alignment issue in full mirror with disk > 50GB from non encrypted to encrypted remote (PR [#10061](https://github.com/vatesfr/xen-orchestra/pull/10061))
+- [Backups] Add length to throttled stream during transfer size (PR [#10079](https://github.com/vatesfr/xen-orchestra/pull/10079))
 
 ### Packages to release
 
@@ -33,6 +34,7 @@
 
 <!--packages-start-->
 
+- @xen-orchestra/backups patch
 - @xen-orchestra/fs patch
 
 <!--packages-end-->