Lambda runtime errors are formatted like errors that implementor code returns, but they are returned by the runtime. In the following example, the Lambda runtime fails to deserialize the event into an object and responds with error message, cause and other details.

{
  "errorMessage": "An error occurred during JSON parsing",
  "errorType": "java.lang.RuntimeException",
  "stackTrace": [],
  "cause": {
    "errorMessage": "com.fasterxml.jackson.databind.exc.InvalidFormatException: Can not construct instance of java.lang.Integer from String value '1000,10': not a valid Integer value\n at [Source: lambdainternal.util.NativeMemoryAsInputStream@35fc6dc4; line: 1, column: 1] (through reference chain: java.lang.Object[0])",
    "errorType": "java.io.UncheckedIOException",
    "stackTrace": [],
    "cause": {
      "errorMessage": "Can not construct instance of java.lang.Integer from String value '1000,10': not a valid Integer value\n at [Source: lambdainternal.util.NativeMemoryAsInputStream@35fc6dc4; line: 1, column: 1] (through reference chain: java.lang.Object[0])",
      "errorType": "com.fasterxml.jackson.databind.exc.InvalidFormatException",
      "stackTrace": [
        "com.fasterxml.jackson.databind.exc.InvalidFormatException.from(InvalidFormatException.java:55)",
        "com.fasterxml.jackson.databind.DeserializationContext.weirdStringException(DeserializationContext.java:907)",
        ...
      ]
    }
  }
}

In this case, Sidecar Connector rejects the call and respond to user client with an error message in the format described below. HTTP Status Code is set as 502 in case of function error from AWS Lambda and below message is set as HTTP Response Body. Below message format provides enough details to the user client for the cause but not the entire stack trace. This error response body design is closer in compliance to RFC 7807: Problem Details for HTTP APIs as well as AWS Lambda guidelines.

{
    "code": 502,
    "message": {
        "detail": "An error occurred during JSON parsing",
        "title": "java.lang.RuntimeException"
     }
}