Is there an existing issue for this?

• I have searched the existing issues

Current Behavior

Localstack returns http 400 for SQS messages larger than 256KiB

Expected Behavior

AWS recently increased the max SQS message and batch size to 1 MiB https://aws.amazon.com/about-aws/whats-new/2025/08/amazon-sqs-max-payload-size-1mib/

How are you starting LocalStack?

With a docker-compose file

Steps To Reproduce

How are you starting localstack (e.g., bin/localstack command, arguments, or docker-compose.yml)

docker run localstack/localstack

Client commands (e.g., AWS SDK code snippet, or sequence of "awslocal" commands)

• Create a test queue
  aws --endpoint-url=http://localhost:4566 sqs create-queue --queue-name test-queue

• Generate a payload slightly over 256KiB
  python3 -c "print('A' * 270000)" > big_message.txt

• Send the message
  aws --endpoint-url=http://localhost:4566 sqs send-message
  --queue-url http://localhost:4566/000000000000/test-queue
  --message-body file://big_message.txt

Environment

- OS:
- LocalStack:
  LocalStack version: 4.7.1.dev17
  LocalStack Docker image sha: e62a727f8
  LocalStack build date: 2025-08-05
  LocalStack build git hash:

Anything else?

I'm working on a PR to address this