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fix(receivers): make AwsLambdaReceiver.toHandler() promise-based (Node.js 24+) #2970

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fix(receivers): make AwsLambdaReceiver.toHandler() promise-based (Node.js 24+) #2970

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17 changes: 15 additions & 2 deletions src/receivers/AwsLambdaReceiver.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -46,6 +46,15 @@ export interface AwsEventV2 {
version: string;
}

/**
* Legacy callback signature retained for backwards compatibility with consumer
* code. `AwsHandler` is now a 2-arg promise-based signature; this callback is
* no longer invoked by `AwsLambdaReceiver`.
*
* @deprecated Lambda's Node.js 24+ runtimes reject callback-style handlers
* (`Runtime.CallbackHandlerDeprecated`). Return a `Promise<AwsResponse>`
* directly instead.
*/
// biome-ignore lint/suspicious/noExplicitAny: userland function results can be anything
export type AwsCallback = (error?: Error | string | null, result?: any) => void;

Expand All @@ -70,7 +79,7 @@ export interface AwsResponse {
}

// biome-ignore lint/suspicious/noExplicitAny: request context can be anything
export type AwsHandler = (event: AwsEvent, context: any, callback: AwsCallback) => Promise<AwsResponse>;
export type AwsHandler = (event: AwsEvent, context: any) => Promise<AwsResponse>;

@mwbrooks mwbrooks Jun 11, 2026

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praise: Good call dropping the callback from the exported type rather than just the implementation.


export interface AwsLambdaReceiverOptions {
/**
Expand Down Expand Up @@ -191,8 +200,12 @@ export default class AwsLambdaReceiver implements Receiver {
}

public toHandler(): AwsHandler {
// Returns a 2-arg promise-based handler so the resulting function has
// `length === 2`. Lambda's Node.js 24+ runtimes reject 3-arg handlers
// up front with `Runtime.CallbackHandlerDeprecated` before the handler
// is invoked.
// biome-ignore lint/suspicious/noExplicitAny: request context can be anything
return async (awsEvent: AwsEvent, _awsContext: any, _awsCallback: AwsCallback): Promise<AwsResponse> => {
return async (awsEvent: AwsEvent, _awsContext: any): Promise<AwsResponse> => {
this.logger.debug(`AWS event: ${JSON.stringify(awsEvent, null, 2)}`);

const rawBody = this.getRawBody(awsEvent);
Expand Down
43 changes: 28 additions & 15 deletions test/unit/receivers/AwsLambdaReceiver.spec.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -66,6 +66,19 @@ describe('AwsLambdaReceiver', () => {
await awsReceiver.stop();
});

// Regression for slackapi/bolt-js#2761: Lambda's Node.js 24+ runtime rejects
// handlers whose `.length` is > 2 with `Runtime.CallbackHandlerDeprecated`
// before the handler is ever invoked. `toHandler()` must return a 2-arg
// promise-based function.
it('should return a 2-arg promise-based handler (Node.js 24+ Lambda runtime)', () => {
const awsReceiver = new AwsLambdaReceiver({
signingSecret: 'my-secret',
logger: noopLogger,
});
const handler = awsReceiver.toHandler();

@mwbrooks mwbrooks Jun 11, 2026

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praise: Thanks for assert.equal(handler.length, 2) to pin the handler, since that is the exact value Lambda runtime reads.

assert.equal(handler.length, 2);
});

it('should return a 404 if app has no registered handlers for an incoming event, and return a 200 if app does have registered handlers', async () => {
const awsReceiver = new AwsLambdaReceiver({
signingSecret: 'my-secret',
Expand All @@ -76,15 +89,15 @@ describe('AwsLambdaReceiver', () => {
const args = createDummyAppMentionEventMiddlewareArgs();
const body = JSON.stringify(args.body);
const awsEvent = createDummyAWSPayload(body, timestamp);
const response1 = await handler(awsEvent, {}, (_error, _result) => {});
const response1 = await handler(awsEvent, {});
assert.equal(response1.statusCode, 404);
const App = importApp(appOverrides);
const app = new App({
token: 'xoxb-',
receiver: awsReceiver,
});
app.event('app_mention', noopVoid);
const response2 = await handler(awsEvent, {}, (_error, _result) => {});
const response2 = await handler(awsEvent, {});
assert.equal(response2.statusCode, 200);
});

Expand All @@ -106,15 +119,15 @@ describe('AwsLambdaReceiver', () => {
'x-slack-request-timestamp': `${timestamp}`,
'x-slack-signature': `v0=${signature}`,
});
const response1 = await handler(awsEvent, {}, (_error, _result) => {});
const response1 = await handler(awsEvent, {});
assert.equal(response1.statusCode, 404);
const App = importApp(appOverrides);
const app = new App({
token: 'xoxb-',
receiver: awsReceiver,
});
app.event('app_mention', noopVoid);
const response2 = await handler(awsEvent, {}, (_error, _result) => {});
const response2 = await handler(awsEvent, {});
assert.equal(response2.statusCode, 200);
});

Expand All @@ -136,7 +149,7 @@ describe('AwsLambdaReceiver', () => {
'X-Slack-Request-Timestamp': `${timestamp}`,
'X-Slack-Signature': `v0=${signature}`,
});
const response1 = await handler(awsEvent, {}, (_error, _result) => {});
const response1 = await handler(awsEvent, {});
assert.equal(response1.statusCode, 404);
const App = importApp(appOverrides);
const app = new App({
Expand All @@ -146,7 +159,7 @@ describe('AwsLambdaReceiver', () => {
app.shortcut('bolt-js-aws-lambda-shortcut', async ({ ack }) => {
await ack();
});
const response2 = await handler(awsEvent, {}, (_error, _result) => {});
const response2 = await handler(awsEvent, {});
assert.equal(response2.statusCode, 200);
});

Expand All @@ -168,7 +181,7 @@ describe('AwsLambdaReceiver', () => {
'X-Slack-Request-Timestamp': `${timestamp}`,
'X-Slack-Signature': `v0=${signature}`,
});
const response1 = await handler(awsEvent, {}, (_error, _result) => {});
const response1 = await handler(awsEvent, {});
assert.equal(response1.statusCode, 404);
const App = importApp(appOverrides);
const app = new App({
Expand All @@ -178,7 +191,7 @@ describe('AwsLambdaReceiver', () => {
app.command('/hello-bolt-js', async ({ ack }) => {
await ack();
});
const response2 = await handler(awsEvent, {}, (_error, _result) => {});
const response2 = await handler(awsEvent, {});
assert.equal(response2.statusCode, 200);
});

Expand All @@ -192,7 +205,7 @@ describe('AwsLambdaReceiver', () => {
const args = createDummyAppMentionEventMiddlewareArgs();
const body = JSON.stringify(args.body);
const awsEvent = createDummyAWSPayload(body, timestamp, undefined, true);
const response1 = await handler(awsEvent, {}, (_error, _result) => {});
const response1 = await handler(awsEvent, {});
assert.equal(response1.statusCode, 404);
});

Expand All @@ -213,7 +226,7 @@ describe('AwsLambdaReceiver', () => {
'X-Slack-Request-Timestamp': `${timestamp}`,
'X-Slack-Signature': `v0=${signature}`,
});
const response = await handler(awsEvent, {}, (_error, _result) => {});
const response = await handler(awsEvent, {});
assert.equal(response.statusCode, 200);
});

Expand All @@ -231,7 +244,7 @@ describe('AwsLambdaReceiver', () => {
});
const handler = awsReceiver.toHandler();
const awsEvent = createDummyAWSPayload(urlVerificationBody, timestamp);
const response = await handler(awsEvent, {}, (_error, _result) => {});
const response = await handler(awsEvent, {});
assert.equal(response.statusCode, 200);
});

Expand Down Expand Up @@ -269,7 +282,7 @@ describe('AwsLambdaReceiver', () => {
body: urlVerificationBody,
isBase64Encoded: false,
};
const response = await handler(awsEvent, {}, (_error, _result) => {});
const response = await handler(awsEvent, {});
assert.equal(response.statusCode, 401);
assert(spy.calledOnce);
});
Expand All @@ -282,7 +295,7 @@ describe('AwsLambdaReceiver', () => {
const handler = awsReceiver.toHandler();
const timestamp = Math.floor(Date.now() / 1000) - 600; // 10 minutes ago
const awsEvent = createDummyAWSPayload(urlVerificationBody, timestamp);
const response = await handler(awsEvent, {}, (_error, _result) => {});
const response = await handler(awsEvent, {});
assert.equal(response.statusCode, 401);
});

Expand All @@ -294,7 +307,7 @@ describe('AwsLambdaReceiver', () => {
});
const handler = awsReceiver.toHandler();
const awsEvent = createDummyAWSPayload(urlVerificationBody);
const response = await handler(awsEvent, {}, (_error, _result) => {});
const response = await handler(awsEvent, {});
assert.equal(response.statusCode, 200);
});

Expand All @@ -311,7 +324,7 @@ describe('AwsLambdaReceiver', () => {
const args = createDummyAppMentionEventMiddlewareArgs();
const body = JSON.stringify(args.body);
const awsEvent = createDummyAWSPayload(body, timestamp);
const response1 = await handler(awsEvent, {}, (_error, _result) => {});
const response1 = await handler(awsEvent, {});
assert.equal(response1.statusCode, 404);
await new Promise((res) => {
setTimeout(res, delay + 2);
Expand Down