Pre token generation Lambda trigger - Unrecognizable lambda output

Question

Hi

I followed the Node.js example explained [here](https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-lambda-pre-token-generation.html) and I alway get the following exception on Cognito login. 
`{"__type":"InvalidLambdaResponseException","message":"Unrecognizable lambda output"}`

The only time it works is, if I call callback immediately without altering the event response

```
exports.handler = (event, context, callback) => {
    callback(null, event);
};
```

This example here (copy pasted from the guide) does not work and throws the mentioned exception "Unrecognizable lambda output". Even if I simplify it to this, it throws the same exception:

```
exports.handler = (event, context, callback) => {
    event.response = {
        "claimsOverrideDetails": {
            "claimsToAddOrOverride": {
                "attribute_key2": "attribute_value2",
                "attribute_key": "attribute_value"
            },
        }
    };

// Return to Amazon Cognito
    callback(null, event);
};
```

I also tried to stringily the event, not calling the callback but instead return the event, etc. Always the same result.

An idea anyone?

Answer

So finally I got it to work like this. Instead of overriding the whole response, I only overrode the event.response.claimsOverrideDetails:

```
exports.handler = async (event, context) => {
    event.response.claimsOverrideDetails = {
         "claimsToAddOrOverride": {
            "attribute_key2": "attribute_value2",
            "attribute_key": "attribute_value"
        }
    };

return event
};
```

Pre token generation Lambda trigger - Unrecognizable lambda output

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