Write a transient state as an intermediate delivery notification

AGENTS.md

@@ -302,6 +302,16 @@ Grouped by what each one constrains.
   transient rather than as saying nothing, maintainer's call, 2026-09-03, since the codec refuses a
   zero-length integer TLV and so an absent one cannot land there.
 
+- **A transient state goes out as an intermediate delivery notification (0x20), every other state as
+  a delivery receipt (0x04).** Appendix B makes a receipt's `stat` the message's final status, so
+  0x04 over `ENROUTE` emits the two disagreeing spellings of finality the reading side above has to
+  reconcile, and goal 3 has our own senders write the marker 3.4 defines. `sendDlr()` takes the list
+  from `transientStates` in `dlr.ts`, the same one the reader uses, so the two cannot drift.
+  Rejected: 0x04 for every state, for the sake of a peer keyed to that marker alone — such a peer
+  then reads a transient report as a final one, which is goal 2's wrong answer handed to the peer
+  instead of to the application. A 0.4.0 client is unaffected either way: its bind declares 0x00, so
+  it is sent no TLVs and refuses the receipt regardless.
+
 - **`smsIdFormat` names a notation per place, and normalisation never reaches inside a `<base>-<n>`
   id.** An SMSC may answer `submit_sm_resp` in hex and write the receipt's `id:` in decimal, so one
   transform over both sides cannot make them equal. `submitResp` covers the `receipted_message_id`

README.md

@@ -232,7 +232,8 @@ await smpp.close();      // stop listening, then drain and close every live sess
 ```
 
 `sendDlr` accepts `SCHEDULED`, `ENROUTE`, `DELIVERED`, `EXPIRED`, `DELETED`, `UNDELIVERABLE`,
-`ACCEPTED`, `UNKNOWN`, `REJECTED` and `SKIPPED`.
+`ACCEPTED`, `UNKNOWN`, `REJECTED` and `SKIPPED`. `SCHEDULED` and `ENROUTE` go out as intermediate
+delivery notifications (`esm_class` 0x20), the rest as delivery receipts (0x04).
 
 A message whose `data_coding` says 8-bit binary arrives as Latin-1, so `Buffer.from(sms.message,
 'latin1')` gives you back the original octets.
@@ -429,6 +430,9 @@ have worked around any of these, remove the workaround:
 - LATIN1 (`data_coding` 0x03) was silently decoded as ASCII, corrupting the message.
 - Delivery receipt dates were a month off, and the status field read `UNDELIVERABLE` where the spec
   defines the 7-character `UNDELIV`.
+- Every receipt went out as `esm_class` 0x04, which SMPP 3.4 defines as the report of a message's
+  final state. A receipt for a transient state — `sendDlr('ENROUTE')` — is now marked 0x20, the
+  intermediate delivery notification.
 - `flash: true` discarded UCS2, mangling flash messages containing non-GSM characters.
 - The multipart reference counter was shared by every session in the process.
 - `tls: true` never performed a handshake, so the connection was not actually encrypted.

src/dlr.ts

@@ -131,7 +131,7 @@ export function parseReceipt(message: string): Receipt {
 type MessageType = 'intermediate' | 'other' | 'receipt' | 'unmarked';
 
 /** SMPP 3.4 Appendix B lists every other receipt state as final. */
-const transientStates: MessageState[] = ['ENROUTE', 'SCHEDULED'];
+export const transientStates: MessageState[] = ['ENROUTE', 'SCHEDULED'];
 
 /** Written by the far-end SME, not by the MC reporting on a message we submitted. */
 const smeMessageTypes: number[] = [

src/sms.ts

@@ -5,7 +5,7 @@ import type { Result, VoidResult } from './result.ts';
 import type { Session } from './session.ts';
 import { UnansweredError } from './unanswered-error.ts';
 import { consts } from './defs/constants.ts';
-import { receiptCodes } from './dlr.ts';
+import { receiptCodes, transientStates } from './dlr.ts';
 import { smppDate } from './message.ts';
 import { uuidv7 } from './uuid.ts';
 
@@ -192,7 +192,9 @@ async function sendDlr(
 			cmdName: 'deliver_sm',
 			params: {
 				destination_addr: sms.from,
-				esm_class: consts.ESM_CLASS.MC_DELIVERY_RECEIPT,
+				esm_class: transientStates.includes(status)
+					? consts.ESM_CLASS.INTERMEDIATE_DELIVERY
+					: consts.ESM_CLASS.MC_DELIVERY_RECEIPT,
 				short_message: receiptText(sms, smsId, status),
 				source_addr: sms.to,
 			},

test/session-extras.test.ts

@@ -26,7 +26,7 @@ import { closeAfter, closeListenerAfter } from './teardown.ts';
 import { consts } from '../src/defs/constants.ts';
 import { errors } from '../src/defs/errors.ts';
 import { objToPdu } from '../src/pdu.ts';
-import { paramText } from '../src/defs/types.ts';
+import { paramNumber, paramText } from '../src/defs/types.ts';
 import { server } from '../src/server.ts';
 import { silentLog } from '../src/log.ts';
 import { submitSms } from '../src/send-sms.ts';
@@ -179,8 +179,12 @@ describe('merged delivery reports', () => {
 
 		const merged = once<MessageDlr>(resolve => { session.on('messageDlr', resolve); });
 		const perSegment: boolean[] = [];
+		const markers: (number | undefined)[] = [];
 
-		session.on('dlr', dlr => perSegment.push(dlr.intermediate));
+		session.on('dlr', (dlr, pduObj) => {
+			perSegment.push(dlr.intermediate);
+			markers.push(paramNumber(pduObj.params.esm_class, 0));
+		});
 
 		const [sms] = await Promise.all([
 			incoming.then(async received => {
@@ -205,6 +209,14 @@ describe('merged delivery reports', () => {
 		assert.equal(report.statusMsg, 'DELIVERED');
 		assert.equal(report.segments.length, 3);
 		assert.deepEqual(perSegment, [true, true, true, false, false, false]);
+		const notification = consts.ESM_CLASS.INTERMEDIATE_DELIVERY;
+		const receipt = consts.ESM_CLASS.MC_DELIVERY_RECEIPT;
+
+		assert.deepEqual(
+			markers,
+			[notification, notification, notification, receipt, receipt, receipt],
+			'a transient state goes out under the marker the spec gives it',
+		);
 	});
 
 	test('reports the worst status across the segments', async t => {

todo.md

@@ -65,6 +65,7 @@ Rules the API follows:
 | Every runnable README example | `test/readme.test.ts` |
 | Receipt-versus-message classification by `esm_class` | `test/dlr.test.ts`, `test/session.test.ts` |
 | An intermediate delivery notification read as a report marked `intermediate`, as is a receipt reporting `ENROUTE` or `SCHEDULED`, and never counted into a merge | `test/dlr.test.ts`, `test/session.test.ts`, `test/session-extras.test.ts` |
+| A transient state sent under the marker the spec gives it, off the same list the reader uses | `test/session-extras.test.ts` |
 | A listener that throws, or rejects, reaching `sessionError`/`serverError` rather than the process | `test/session.test.ts`, `test/error-from.test.ts` |
 | Cross-checked against node-smpp both ways and over a live session | `test/interop.test.ts` |
 | CI on Node 18/20/22/24, Renovate, tag-triggered publish | `.github/workflows/` |
@@ -132,13 +133,6 @@ session message is a change to every call site.
       and applies it to the other is wrong. A budget type both take would close it. Raised by review,
       2026-09-01.
 
-- [ ] **Write a transient state as an intermediate delivery notification.** `sendDlr()` marks every
-      receipt `esm_class` 0x04, so `sendDlr('ENROUTE')` announces a non-final state with the marker
-      SMPP 3.4 Appendix B reserves for the final one — the two disagreeing spellings of finality the
-      reading side was taught to reconcile on 2026-09-03. Decided the same day: fix it, off
-      `transientStates` in `dlr.ts` so the writer and the reader cannot drift, in a PR of its own.
-      Costs a bullet in README's "Behaviour that changed on the wire".
-
 - [ ] **`once()` is copied into four test files, and two copies never give up.**
       `session-extras.test.ts` and `readme.test.ts` reject after 5000 ms; `session.test.ts` and
       `tls.test.ts` wait forever, so an event that never fires still hangs the run the way an