S3Client PutObjectCommand prepending XML declaration to documents

[dmcmullen]

Checkboxes for prior research

• I've gone through Developer Guide and API reference
• I've checked AWS Forums and StackOverflow.
• I've searched for previous similar issues and didn't find any solution.

Describe the bug

Starting in version 3.931.0 of @aws-sdk/client-s3, the PutObjectCommand is unexpectedly prepending <?xml version="1.0" encoding="UTF-8"?> to documents when the ContentType is set to application/xml.

Regression Issue

• Select this option if this issue appears to be a regression.

SDK version number

@aws-sdk/client-s3@3.931.0

Which JavaScript Runtime is this issue in?

Node.js

Details of the browser/Node.js/ReactNative version

v24.11.0

Reproduction Steps

import { S3Client, PutObjectCommand } from "@aws-sdk/client-s3";

const client = new S3Client({ region: "us-east-1" });

const xmlContent = `<?xml version="1.0" encoding="UTF-8"?><root><element>test</element></root>`;

const command = new PutObjectCommand({
  Bucket: "my-bucket",
  Key: "test.xml",
  Body: xmlContent,
  ContentType: "application/xml",
});

await client.send(command);
// Downloaded content will have additional XML declaration prepended

Observed Behavior

The uploaded document has <?xml version="1.0" encoding="UTF-8"?> prepended to the original content.

Expected Behavior

The uploaded document should contain exactly the content provided, without any XML declaration prepended.

Possible Solution

No response

Additional Information/Context

No response

[kuhe]

I made a PR with a fix in #7551, and we will try to release that today.

As a workaround, you can downgrade to v3.930.0 or wrap your XML string in a Buffer.

await client.putObject({
  Bucket: "...",
  Key: "test.xml",
  Body: Buffer.from(xmlContent), // or stream such as a file stream
  ContentType: "application/xml",
});

[kuhe]

The fix was released in https://github.com/aws/aws-sdk-js-v3/releases/tag/v3.946.0.

[dmcmullen]

Thank you. I had moved to the previous version over the weekend, 3.930.0, and this morning I was able to restore it to latest, 3.946.0, and it is working correctly