Is it possible to throw custom error message from API Gateway Lambda Authorizer

Question

I have created an Lambda Authorizer for custom authorization in API Gateway. I need to throw custom error message like this.

{
    errorCode: 'xyz_12#',
    errorType: 'Constraint error',
    errorMessage: 'Need permission to perform this action',
    statusCode: 401
  }

Is it possible  from lambda authorizer?

Answer

Yes.  At the end of the example authorizer in the docs (https://docs.aws.amazon.com/apigateway/latest/developerguide/apigateway-use-lambda-authorizer.html), there is way to output optional data.  Updating with your values, it would be something like this:

```
    // Optional output with custom properties of the String, Number or Boolean type.
    authResponse.context = {
        "errorCode": "xyz_12#",
        "errorType": "Constraint error",
        "errorMessage": "Need permisssion to perform this action"
    };
```
In the body mapping template, you'd access these as follows:

```
{"errorCode":"$context.authorizer.context.errorCode", "errorType": "$context.authorizer.context.errorCode"}
```
The one caveat here is that you can't specify the HTTP error code.  The authorizer has to generate a 'Deny' policy which then results in an HTTP 403.

More discussion is available here:
https://stackoverflow.com/questions/47921803

Is it possible to throw custom error message from API Gateway Lambda Authorizer

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