The Lambda function returned invalid JSON: the JSON output is not parsable

February 26, 2021

There could be a number of reasons you’re getting this error message from your CloudFront distribution with a Lambda@Edge configured.

For example, your Lambda function could actually be returning a poorly encoded response. But if you know you’re returning proper JSON, why could you get that error?

Well, it turns out this error message is also a way for AWS to tell you that the (absolutely valid) JSON that your Lambda function returned was too big.

Indeed, a Lambda@Edge is limited to a 40 kB response body, which is not a lot of kB if you ask me.

And instead of returning a meaningful error when this happens (I confirmed in my case, this was the issue), we’re getting an invalid JSON error.

Why? Maybe something in the stack of things involved on AWS between CloudFront and the Lambda function just truncates the response body to 40 kB, and a truncated JSON is indeed invalid JSON.

Regardless, if you’re getting this error on a Lambda@Edge, it’s worth checking your payload size, and it might save you hours of debugging your JSON structure and what could corrupt it!