Posting from AWS-API Gateway to Lambda

This might not have been available when the OP asked the question, but when invoking a Lambda function using the API Gateway, specific response objects are provided.

As previously noted in the documentation Api Gateway Simple Proxy for Lambda Input Format, the API Gateway wraps the input arguments in a fairly verbose wrapper. It also expects a similarly verbose response object.

However, it is not necessary to create custom request and response objects. The AWS team provides the Amazon.Lambda.APIGatewayEvents library, which is also available on NuGet. This library includes APIGatewayProxyRequest and APIGatewayProxyResponse objects ready-made.

It is still necessary to manually deserialize the Body of the request, as it is a string, not a JSON object. I assume this was done for flexibility?

An example function could look like this. It's a modification of the default function provided by the AWS tools:

    public APIGatewayProxyResponse FunctionHandler(APIGatewayProxyRequest request, ILambdaContext context)
    {
        var bodyString = request?.Body;

        if (!string.IsNullOrEmpty(bodyString))
        {
            dynamic body = JsonConvert.DeserializeObject(bodyString);

            if (body.input != null)
            {
                body.input = body.input?.ToString().ToUpper();

                return new APIGatewayProxyResponse
                {
                    StatusCode = 200,
                    Body = JsonConvert.SerializeObject(body)
                };
            }
        }

        return new APIGatewayProxyResponse
        {
            StatusCode = 200
        };
    }

When using Lambda Proxy Integration in API Gateway, the first parameter to your FunctionHandler is not the body of your POST, but is another API Gateway-created object, which let's call LambdaRequest. Try these changes to your sample code. Add:

public class LambdaRequest
{
   public string body { get; set; }
}

Change your handler prototype to:

public LambdaResponse FunctionHandler(LambdaRequest req, ILambdaContext context)

And inside FunctionHandler add:

ZipRequest input = JsonConvert.DeserializeObject<ZipRequest>(req.Body);

The full LambdaRequest object is documented under Input Format of a Lambda Function for Proxy Integration in the AWS docs, and contains HTTP headers, the HTTP method, the query string, the body, and a few other things.