request fails with LambdaResponseTooLarge

[johnnikho]

We have logic in our code that will write the response to S3 and redirect the client to the S3 URL if the content length is > 900,000. We have requests that are failing but yet that do not exceed the 900,000. I have added debug logs in the boostrap code as follows:

$totalResponseLength = strlen(print_r($response, true));
printf("Total response length: %d\n", $totalResponseLength);

For the request in question, the output was
Total response length: 892878
which clearly is under 900,000 but yet when the lambda execution finishes, some 100K+ is added to the response length which fails the request with LambdaResponseTooLarge.

So my question is where is the 100K+ of addition bytes coming from