Hum, I missed this part: changing the serializer service for all transports.
I don't think that's the correct approach.
Instead, I see two ways:

1. patch the native serializer
2. fix the encoding in AmazonSqsSender, just after $encodedMessage = $this->serializer->encode($envelope);

Option 2. looks the most appropriate to me.

Yeah, I wasn't sure about the best strategy for this.

The solution 1 leads to an impact for all the user of PHPSerializer even if they don't use AmazonSQS. There is no reason to follow AmazonSQS needs if you don't use it (so I tried to follow #49844 (comment))

The solution 2 looks interesting, but if I add an extra base64_encode call after $encodedMessage = $this->serializer->encode($envelope);, how can I be sure it will be decoded by the serializer (since any custom Serializer can be used) ?

So I thought changing the defaut serializer was the right way for AmazonSqs users.

The solution 2 looks interesting, but if I add an extra base64_encode call, how can I be sure it will be decoded by the serializer (since any custom Serializer can be used) ?

If a custom serializer is used, it has to comply with Amazon's requirements, so this extra logic we're talking about shouldn't be triggered.

I could add a Header to tell AmazonSqsReceiver it require an extra base64_decode.

WDYT @nicolas-grekas should I add the commit: https://github.com/VincentLanglet/symfony/compare/amazonSerializer...VincentLanglet:symfony:amazonSerializer-2?expand=1 or should I let the PR like this ?