diff --git a/server/routes/download-tasks.integration.test.ts b/server/routes/download-tasks.integration.test.ts
index 8367196f..32ef5af0 100644
--- a/server/routes/download-tasks.integration.test.ts
+++ b/server/routes/download-tasks.integration.test.ts
@@ -423,6 +423,48 @@ describe('Download tasks API integration', () => {
     })
   })
 
+  it('returns storage failure details when multipart upload completion fails', async () => {
+    vi.mocked(S3Service.prototype.completeMultipartUpload).mockRejectedValueOnce(new Error('InvalidPart: part missing'))
+    const { app, db } = await createTestApp()
+    await insertStorage(db)
+    const headers = {
+      ...(await authedHeaders(app, 'multipart-complete-user@example.com')),
+      'Content-Type': 'application/json',
+    }
+
+    const createObjectRes = await app.request('/api/objects', {
+      method: 'POST',
+      headers,
+      body: JSON.stringify({
+        name: 'fixture.bin',
+        type: 'application/octet-stream',
+        size: 6 * 1024 * 1024 * 1024,
+        parent: '',
+      }),
+    })
+    expect(createObjectRes.status).toBe(201)
+    const object = (await createObjectRes.json()) as { id: string }
+
+    const sessionRes = await app.request(`/api/objects/${object.id}/uploads`, {
+      method: 'POST',
+      headers,
+      body: JSON.stringify({ partSize: 64 * 1024 * 1024 }),
+    })
+    expect(sessionRes.status).toBe(201)
+    const session = (await sessionRes.json()) as { id: string }
+
+    const completeRes = await app.request(`/api/objects/${object.id}/uploads/${session.id}`, {
+      method: 'PATCH',
+      headers,
+      body: JSON.stringify({ action: 'complete', parts: [{ partNumber: 1, etag: '"etag-1"' }] }),
+    })
+
+    expect(completeRes.status).toBe(502)
+    await expect(completeRes.json()).resolves.toEqual({
+      error: 'Storage multipart upload complete failed: InvalidPart: part missing',
+    })
+  })
+
   it('submits user task actions through downloader polling state', async () => {
     const { app, db } = await createTestApp({ DOWNLOAD_TOKEN_SECRET: 'test-download-token-secret' })
     await insertStorage(db)
diff --git a/server/services/object-upload-sessions.ts b/server/services/object-upload-sessions.ts
index 04051ee4..9d352f0b 100644
--- a/server/services/object-upload-sessions.ts
+++ b/server/services/object-upload-sessions.ts
@@ -114,13 +114,27 @@ export async function patchObjectUploadSession(
   if (row.status !== 'active') throw new ObjectUploadSessionError('invalid_state')
   const now = new Date()
   if (params.input.action === 'complete') {
-    await s3.completeMultipartUpload(params.storage, row.storageKey, row.uploadId, params.input.parts)
+    try {
+      await s3.completeMultipartUpload(params.storage, row.storageKey, row.uploadId, params.input.parts)
+    } catch (error) {
+      throw new ObjectUploadSessionError(
+        'storage_failure',
+        `Storage multipart upload complete failed: ${(error as Error).message}`,
+      )
+    }
     await db
       .update(objectUploadSessions)
       .set({ status: 'completed', updatedAt: now })
       .where(eq(objectUploadSessions.id, row.id))
   } else {
-    await s3.abortMultipartUpload(params.storage, row.storageKey, row.uploadId)
+    try {
+      await s3.abortMultipartUpload(params.storage, row.storageKey, row.uploadId)
+    } catch (error) {
+      throw new ObjectUploadSessionError(
+        'storage_failure',
+        `Storage multipart upload abort failed: ${(error as Error).message}`,
+      )
+    }
     await db
       .update(objectUploadSessions)
       .set({ status: 'aborted', updatedAt: now })
diff --git a/server/services/s3.test.ts b/server/services/s3.test.ts
index ffb2a15e..78b90478 100644
--- a/server/services/s3.test.ts
+++ b/server/services/s3.test.ts
@@ -289,6 +289,21 @@ describe('S3Service', () => {
       expect(mockSend).not.toHaveBeenCalled()
     })
 
+    it('rejects embedded S3 complete errors returned with HTTP 200', async () => {
+      vi.stubGlobal(
+        'fetch',
+        vi
+          .fn()
+          .mockResolvedValueOnce(
+            new Response('<Error><Code>InvalidPart</Code><Message>part does not exist</Message></Error>'),
+          ),
+      )
+
+      await expect(
+        service.completeMultipartUpload(storage, 'video.mp4', 'upload-1', [{ partNumber: 1, etag: '"etag-1"' }]),
+      ).rejects.toThrow('InvalidPart')
+    })
+
     it('aborts multipart uploads through a presigned DELETE', async () => {
       const fetchMock = vi.fn().mockResolvedValueOnce(new Response(null, { status: 204 }))
       vi.stubGlobal('fetch', fetchMock)
diff --git a/server/services/s3.ts b/server/services/s3.ts
index f1160f0c..026a1aa7 100644
--- a/server/services/s3.ts
+++ b/server/services/s3.ts
@@ -116,8 +116,11 @@ export class S3Service {
       headers: { 'Content-Type': 'application/xml' },
       body: multipartCompleteXml(sortedParts),
     })
+    const body = await response.text()
     if (!response.ok) {
-      const body = await response.text()
+      throw new Error(`S3 multipart upload complete failed: ${response.status}: ${body.trim()}`)
+    }
+    if (xmlTag(body, 'Error') !== null || xmlTag(body, 'Code') !== null) {
       throw new Error(`S3 multipart upload complete failed: ${response.status}: ${body.trim()}`)
     }
   }