AwsLambdaReceiver is incompatible with AWS Lambda Node.js 24 runtime (callback handlers deprecated)

[HAKASHUN]

Summary

When running a Bolt app on AWS Lambda with the Node.js 24 runtime, functions exported via AwsLambdaReceiver.toHandler() fail at startup with the following error:

Runtime.CallbackHandlerDeprecated

This appears to be related to the fact that the Node.js 24 Lambda runtime no longer supports callback-style handlers.

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Environment

• @slack/bolt: 4.6.0
• AWS Lambda runtime: Node.js 24
• Deployment: API Gateway → Lambda
• Receiver: AwsLambdaReceiver

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Observed behavior

AwsLambdaReceiver.toHandler() currently returns a handler typed as:

(event, context, callback) => Promise<AwsResponse>

When this function is exported directly as the Lambda entry point, the runtime fails before invocation on Node.js 24 with Runtime.CallbackHandlerDeprecated.

This does not occur on earlier Lambda runtimes (e.g. Node.js 18 / 20).

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Expected behavior

Bolt applications using AwsLambdaReceiver should be able to run on the Node.js 24 Lambda runtime without requiring callback-style handlers.

A promise-based handler signature such as:

(event, context) => Promise<AwsResponse>

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Workaround

As a temporary solution, we wrapped the handler and exported a 2-argument async
function:

const boltHandler = receiver.toHandler();

export const handler = async (event, context) => {
  // @ts-expect-error Node.js 24 Lambda does not support callback-style handlers
  return boltHandler(event, context);
};

This resolves the runtime error, but requires suppressing a TypeScript type mismatch because the current Bolt typings still expect a callback parameter.

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It may be helpful to update AwsLambdaReceiver.toHandler() (and its typings) to support promise-based Lambda handlers, ensuring compatibility with the Node.js 24 runtime.

We’re happy to provide additional details or test changes if that’s useful.

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Requirements

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[WilliamBergamin]

Hi @HAKASHUN thanks for opening this issue 💯 Huge thanks for sharing your workaround 🚀

The typescript documentation for aws does state

Callback-based function handlers are only supported up to Node.js 22. Starting from Node.js 24, asynchronous tasks should be implemented using async function handlers.

🤔 Not sure how we could detect this type of change going forward, at this time the types for the aws-lambda package sill include the callback in the handler definition

Ideally we would like to fix this without introducing a breaking change. As the toHandler method does not use the callback parameter, we might be able to get away with making the callback optional in our AwsHandler type

We may need more exploration before fixing this issue