Smithy deserialisation error: Unexpected token 'H', "Hello, world!" is not valid JSON

[Inqnuam]

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

Since 3.929.0 this simple Lambda streaming response is throwing an error when called with InvokeWithResponseStream cmd

export const handler = awslambda.streamifyResponse(async (event, responseStream, context) => {
  responseStream.setContentType("text/plain");
  responseStream.write("Hello, world!");
  responseStream.end();
});

Regression Issue

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

SDK version number

@aws-sdk/client-lambda@3.929.0

Which JavaScript Runtime is this issue in?

Node.js

Details of the browser/Node.js/ReactNative version

v22.21.1

Reproduction Steps

import { Lambda } from "@aws-sdk/client-lambda";

const client = new Lambda();

const res = await client.invokeWithResponseStream({
  FunctionName: "MyFunction",
});

let collectedChunks = Buffer.alloc(0);
for await (const event of res.EventStream!) {
  if (event.PayloadChunk) {
    collectedChunks = Buffer.concat([collectedChunks, event.PayloadChunk.Payload]);
  }
}

Observed Behavior

Unexpected token 'H', "Hello, world!" is not valid JSON
  Deserialization error: to see the raw response, inspect the hidden field {error}.$response on this object.


HttpResponse {
  statusCode: 200,
  reason: 'OK',
  headers: {
    date: 'Wed, 12 Nov 2025 17:33:17 GMT',
    'content-type': 'application/vnd.amazon.eventstream',
    'transfer-encoding': 'chunked',
    connection: 'keep-alive',
    'x-amzn-requestid': '5dfbde6b-533a-416f-b823-b0f28c5b86c4',
    'x-amz-executed-version': '$LATEST',
    'x-amzn-remapped-content-type': 'text/plain'
  },
  body: 'Hello, world!'
}

Expected Behavior

Should not fail

Possible Solution

Chunks shouldn't be parsed when :event-type is PayloadChunk

Additional Information/Context

No response

[kuhe]

I confirmed this regression. I've been working on a fix for this, and will post the release version link here when it's ready.

[kuhe]

The fix for event stream deserialization is in this version: https://www.npmjs.com/package/@smithy/core/v/3.18.3

To upgrade to this version, make a fresh install without the lockfile, or run npm up @smithy/core, or an equivalent command in your package manager.

[Inqnuam]

@kuhe All good on my end, thanks.