How to add configuration parameters to AWS lambda with API gateway

Unfortunately, there is no easy way to add configuration parameters to AWS lambdas, there is almost nothing in context that you can configure; there is description, but besides being ambiguous, it is not accessible from function itself. Fortunately, it is possible to set additional values from API gateway, even it is not straightforward.

When you have created API gateway mapping, go to the method screen in gateway console, there is screen with multiple boxes, one of which is called Integration Request, click it and there will be section called Mapping Templates. Add new mapping template for type application/x-www-form-urlencoded, save it, edit value and change from default Input passthrough to Mapping template, then you can transform your original request, by adding additional parameters, it uses velocity for transformation, but there is basic example:

{
  "my-new-configuration" : "my value",
  "body" : "$input.path('$')",
  "params" : "$input.params()",
  "ip" : "$context.identity.sourceIp",
  "user-agent" : "$context.identity.userAgent"
}

Besides possibility to add anything to lambda function it also has access to interesting request data like headers, ip, client type, user agent, country, etc. - all will be available in your map or pojo as parameter for lambda function. And do not forget to click Deploy API after making changes to make them live.