diff --git a/AGENTS.md b/AGENTS.md
index b5904d0..c7b6769 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -298,19 +298,20 @@ Grouped by what each one constrains.
   second time in `DlrMerger`, so the library cannot answer the application one way and conclude the
   other. Not every peer marks a transient report 0x20 — an ordinary receipt carrying `stat:ENROUTE`
   is common — so the state test is what the marker test cannot replace. `message_state` 0 is 5.0's
-  `SCHEDULED` and undefined in 3.4; a peer that writes it is read as
-  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.
+  `SCHEDULED` and undefined in 3.4; a peer that writes it is read as 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.
+  Rejected: 0x04 for every state, for the sake of a peer that classifies on the marker — the cost
+  accepted here is that such a peer stops recognising a transient report as a report at all and hands
+  its application receipt text as an inbound message, where under 0x04 it would have read the state
+  from `stat:` and been right. A transient state also carries `err:000`, since a message still on its
+  way has not failed.
 
 - **`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
diff --git a/src/dlr.ts b/src/dlr.ts
index 1e855fe..7b6a417 100644
--- a/src/dlr.ts
+++ b/src/dlr.ts
@@ -131,10 +131,10 @@ export function parseReceipt(message: string): Receipt {
 type MessageType = 'intermediate' | 'other' | 'receipt' | 'unmarked';
 
 /** SMPP 3.4 Appendix B lists every other receipt state as final. */
-export const transientStates: MessageState[] = ['ENROUTE', 'SCHEDULED'];
+export const transientStates: readonly MessageState[] = ['ENROUTE', 'SCHEDULED'];
 
 /** Written by the far-end SME, not by the MC reporting on a message we submitted. */
-const smeMessageTypes: number[] = [
+const smeMessageTypes: readonly number[] = [
 	consts.ESM_CLASS.CONVERSATION_ABORT,
 	consts.ESM_CLASS.DELIVERY_ACKNOWLEDGEMENT,
 	consts.ESM_CLASS.USER_ACKNOWLEDGEMENT,
diff --git a/src/sms.ts b/src/sms.ts
index 533c23a..4f4d655 100644
--- a/src/sms.ts
+++ b/src/sms.ts
@@ -128,6 +128,7 @@ async function sendResp(
 /** The receipt as text, which is all of it a peer below SMPP 3.4 is allowed to be sent. */
 function receiptText(sms: Sms, smsId: string, status: MessageState): string {
 	const delivered = status === 'DELIVERED';
+	const failed = !delivered && !transientStates.includes(status);
 
 	return [
 		`id:${smsId}`,
@@ -136,7 +137,7 @@ function receiptText(sms: Sms, smsId: string, status: MessageState): string {
 		`submit date:${smppDate(sms.submitTime)}`,
 		`done date:${smppDate(new Date())}`,
 		`stat:${receiptCodes[status]}`,
-		`err:${delivered ? '000' : '001'}`,
+		`err:${failed ? '001' : '000'}`,
 		'text:',
 	].join(' ');
 }
diff --git a/test/session-extras.test.ts b/test/session-extras.test.ts
index 9499f30..5b1f322 100644
--- a/test/session-extras.test.ts
+++ b/test/session-extras.test.ts
@@ -178,11 +178,11 @@ describe('merged delivery reports', () => {
 		assert.ok(session);
 
 		const merged = once<MessageDlr>(resolve => { session.on('messageDlr', resolve); });
-		const perSegment: boolean[] = [];
+		const reports: Dlr[] = [];
 		const markers: (number | undefined)[] = [];
 
 		session.on('dlr', (dlr, pduObj) => {
-			perSegment.push(dlr.intermediate);
+			reports.push(dlr);
 			markers.push(paramNumber(pduObj.params.esm_class, 0));
 		});
 
@@ -208,15 +208,20 @@ describe('merged delivery reports', () => {
 		assert.equal(report.smsId, 'en-route');
 		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(reports.map(one => one.intermediate), [true, true, true, false, false, false]);
 		assert.deepEqual(
 			markers,
 			[notification, notification, notification, receipt, receipt, receipt],
 			'a transient state goes out under the marker the spec gives it',
 		);
+		assert.deepEqual(
+			reports.map(one => one.errorCode),
+			['000', '000', '000', '000', '000', '000'],
+			'a message still on its way has not failed',
+		);
 	});
 
 	test('reports the worst status across the segments', async t => {
diff --git a/todo.md b/todo.md
index 7487ce6..8ccd36a 100644
--- a/todo.md
+++ b/todo.md
@@ -133,6 +133,12 @@ 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.
 
+- [ ] **`err:` on a receipt for a state that neither delivered nor failed.** `receiptText()` now
+      writes `err:000` for `DELIVERED` and for the two transient states, and `err:001` for every
+      other — so `ACCEPTED`, `SKIPPED`, `UNKNOWN` and `DELETED` still announce an error code the SMSC
+      never had. Which of those are failures is the open half. Raised by review, 2026-09-03; needs a
+      decision.
+
 - [ ] **`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