Normalise a listener's rejection reason before routing it to sessionError

AGENTS.md

@@ -218,8 +218,9 @@ exactly 140.
   `[EventEmitter.captureRejectionSymbol]`, which lands a rejected `async` listener on `sessionError`
   or `serverError` beside the synchronous guard in `emit()`. Dispatching `rawListeners()` from
   `emit()` instead needs a cast to call them with the event's argument tuple, which hard rule 4
-  forbids. `Session.on()` still types its listeners as void-returning, because widening that needs
-  the same cast — which is why `test/*.test.ts` turns `no-misused-promises` off.
+  forbids. A rejection reason is `unknown`, so the handler normalises it the way the synchronous
+  guard does. `Session.on()` still types its listeners as void-returning, because widening that needs
+  the same cast — which is why `test/*.test.ts` turns off `no-misused-promises` on arguments.
 
 - **The TLS tests build their own self-signed certificate in DER** (`test/tls.test.ts`) instead of
   adding a devDependency or shelling out to openssl. Maintainer's call, 2026-08-26: the dev image

eslint.config.js

@@ -51,7 +51,9 @@ export default tseslint.config(
 	{
 		// An async event listener is a documented, supported shape; EventEmitter types listeners void.
 		files: ['test/*.test.ts'],
-		rules: { '@typescript-eslint/no-misused-promises': 'off' },
+		rules: {
+			'@typescript-eslint/no-misused-promises': ['error', { checksVoidReturn: { arguments: false } }],
+		},
 	},
 	{
 		files: ['eslint.config.js'],

src/server.ts

@@ -91,10 +91,11 @@ export class SmppServer extends EventEmitter<ServerEvents> {
 
 	/** The same guard for a listener that rejects rather than throws; captureRejections routes here. */
 	override [EventEmitter.captureRejectionSymbol](
-		error: Error,
+		reason: unknown,
 		...args: [event: keyof ServerEvents, ...rest: unknown[]]
 	): void {
 		const [event] = args;
+		const error = reason instanceof Error ? reason : new Error(String(reason));
 
 		this.log.error('server - a listener rejected', { event, message: error.message });
 

src/session.ts

@@ -78,10 +78,11 @@ export class Session extends EventEmitter<SessionEvents> {
 
 	/** The same guard for a listener that rejects rather than throws; captureRejections routes here. */
 	override [EventEmitter.captureRejectionSymbol](
-		error: Error,
+		reason: unknown,
 		...args: [event: keyof SessionEvents, ...rest: unknown[]]
 	): void {
 		const [event] = args;
+		const error = reason instanceof Error ? reason : new Error(String(reason));
 
 		this.log.error('session - a listener rejected', { event, message: error.message });
 

test/session.test.ts

@@ -1291,15 +1291,16 @@ describe('application hooks that throw or reject', () => {
 		await smpp.close();
 	});
 
-	test('turns a rejecting async sms listener into a session error', async () => {
+	test('normalises whatever a rejecting async sms listener threw into a session error', async () => {
 		const smpp = await startServer();
+		const reason: unknown = null;
 		const failed = once<Error>(resolve => {
 			smpp.on('session', session => {
 				session.on('sessionError', resolve);
 				session.on('sms', async sms => {
 					await sms.sendResp();
 
-					throw new Error('listener rejected');
+					throw reason;
 				});
 			});
 		});
@@ -1316,7 +1317,7 @@ describe('application hooks that throw or reject', () => {
 
 		assert.equal(sent.err, undefined);
 		assert.ok(reported instanceof Error, 'a rejecting sms listener should reach the session');
-		assert.equal(reported.message, 'listener rejected');
+		assert.equal(reported.message, 'null');
 
 		session.close();
 		await smpp.close();

todo.md

@@ -111,6 +111,14 @@ session message is a change to every call site.
 
 ## Worth doing, not blocking
 
+- [ ] **Should `on()` accept a promise-returning listener in its types?** A listener that rejects is
+      routed to `sessionError` now, but `EventEmitter` types every listener as void-returning, so the
+      `session.on('sms', async sms => …)` the README documents trips `no-misused-promises` in a
+      consumer's project exactly as it does in this repository's own tests. Widening the parameter
+      needs a cast, which hard rule 4 forbids; a declaration overload over an implementation
+      signature Node's types accept might not. Raised by review, 2026-08-27; worth deciding while
+      the surface is still movable.
+
 - [ ] **In-flight sends across a reconnect.** They currently fail with "Session closed before a
       response arrived" and the caller retries. Re-queueing them automatically would be friendlier
       but risks duplicate delivery, so it needs a decision before it is built.