I can not invoke a Lambda function from AWS Lex

Hi Roberto,

Lex v2 requires a specific response format from the lambda function. https://docs.aws.amazon.com/lexv2/latest/dg/lambda.html#lambda-response-format

Add a response in the lambda code to indicate success, such as:

  intent = event['sessionState']['intent']['name']
  # if success and completed, send fulfilled response
  return {
                "sessionState": {
                    "dialogAction": {
                        "type": "Close"
                    },
                    "intent": {
                        "name": intent,
                        "state": 'Fulfilled'
                    }
                },
                'sessionId': event['sessionId']
            } 

I know this is an older question, but I wanted to put an answer on it for anyone who might hit the same issue in future and come across this post.

The issue you are hitting is likely because you are testing against the TestBotAlias, but in your CloudFormation you only associated the Lambda function with your Prod alias. When you test from the console you are hitting the TestBotAlias and I'm guessing that is where you are doing initial tests.

To associate it with the TestBotAlias you need to use the TestBotAliasSettings in the AWS::Lex::Bot resource.

From the CloudFormation you provided in your question, that would look like this:

  ReHubBot:
    DependsOn: BotRuntimeRole
    Type: AWS::Lex::Bot
    Properties:
      Name: "ReHubBot"
      RoleArn: !GetAtt BotRuntimeRole.Arn
      DataPrivacy:
        ChildDirected: false
      IdleSessionTTLInSeconds: 300
      Description: "Provides voice control to the ReHub app."
      AutoBuildBotLocales: true
      TestBotAliasSettings:
        BotAliasLocaleSettings:
            - LocaleId: es_ES
              BotAliasLocaleSetting:
                Enabled: true
                CodeHookSpecification:
                  LambdaCodeHook:
                    CodeHookInterfaceVersion: "1.0"
                    LambdaArn: !GetAtt TestLexFunction.Arn
    ....

You would also need to update the permissions to allow both the Prod and Test aliases to call the Lambda function. Here is what it would look like to update your CloudFormation to allow ALL aliases from that bot:

  LexInvokeLambdaPermission:
    Type: AWS::Lambda::Permission
    Properties:
      Action: "lambda:InvokeFunction"
      FunctionName: !GetAtt TestLexFunction.Arn
      Principal: "lexv2.amazonaws.com"
      SourceArn:
        !Join
        - '/'
        - - !Sub arn:aws:lex:${AWS::Region}:${AWS::AccountId}:bot-alias
          - !Ref ReHubBot
          - '*'

As the previous poster also pointed out you do need to respond to Lex with the correct response format or it will fail. Here is the most basic version of that, it just does a pass-through delegating back to Lex and allowing it to use the configuration you set up.

import logging
logger = logging.getLogger()
logger.setLevel(logging.DEBUG)
def lambda_handler(event, context):
  logger.debug(event)
  session_state = event['sessionState']
  session_state['dialogAction'] = {
    'type': "Delegate"
  } 
  request_attributes = event.get('requestAttributes', {})
  lex_response = {
    'sessionState': session_state,
    'requestAttributes': request_attributes
  }
  logger.debug(lex_response)
  return lex_response

This code will work with any Lex v2 code hook (although take no action), so it's a good one if you are just testing out initially.