diff --git a/AGENTS.md b/AGENTS.md
index 7277e6b..27de277 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -152,6 +152,7 @@ naming the behaviour.
 | Short segments | `splitMsg` accumulates a full segment then pushes `msgPart.slice(0, -1)`, so every segment is one character short: 152 GSM characters instead of 153, 66 UCS2 instead of 67. Long messages are split into more segments than they need, and each extra segment is billed |
 | DLR month off by one | `smppDate()` uses `getMonth()` (0-based) without `+1`, so January renders as `00` |
 | Non-standard DLR status | Receipts emit `stat:UNDELIVERABLE`; the spec's field is 7 characters (`UNDELIV`) |
+| GSM 03.38 declared as IA5 | `sendSms` resolves its encoding through `consts.ENCODING`, so a GSM body goes out under `data_coding` 0x01 — SMPP 3.4 5.2.19's IA5 (CCITT T.50), where `$` and `@` are STX and NUL |
 | Flash destroys UCS2 | `flash: true` overwrites `data_coding` with 0x10, discarding the UCS2 alphabet, which needs 0x18 |
 | Shared concat reference | The concatenation reference counter is a module-level global shared by every session in the process |
 | `send()` never times out | Each call adds a listener keyed on the sequence number; a peer that never answers leaks it and the promise never settles |
@@ -670,6 +671,38 @@ Grouped by what each one constrains.
   field, which `data_coding` says nothing about — an address is a C-Octet String and ASCII by 3.4's
   own definition.
 
+- **A GSM 03.38 message declares `data_coding` 0x00, and an inbound 0x01 is still read as GSM.**
+  Maintainer's call, 2026-09-09: `dataCodingFor()` and `encodeBody()` both resolved an alphabet
+  through `consts.ENCODING`, so `encoding: 'ASCII'` went out as 0x01 — SMPP 3.4 5.2.19's *IA5 (CCITT
+  T.50)/ASCII* — while the codec writes GSM 03.38, where `$` is 0x02 and `@` is 0x00 against IA5's
+  STX and NUL. Goal 1 owns it, and this library's own reader hid it by resolving both codings to the
+  same codec. `dataCodingByEncoding` is the single answer to which coding an alphabet is written
+  under, as `unencodable()` is to whether one can carry a message: the mirror of
+  `encodingByDataCoding()`, and reached by both the `sendSms()` path and `encodeBody()`'s detected
+  one rather than each spelling the map again, which is what `sendDlr()` inherits it through. It is
+  exported for the reason `unencodable()` is — a caller pairing `encodeMessage()`'s octets with a
+  `data_coding` of its own had only `consts.ENCODING` to reach for, which is the trap. 0x00 is the
+  *SMSC's* default alphabet rather than 03.38 by name, so it is a convention rather than a guarantee;
+  it is also what every peer in `interop-tests/` submits under and what LINK Mobility, Route Mobile
+  and Telesign all publish 03.38 as, where 0x01 names a different alphabet from the one written and
+  so is wrong whatever the peer makes of it. Reading is untouched, goal 3: those same three map 0x01
+  to 03.38 too, and Kaleyra and Route Mobile publish that value as known to cause problems, so no
+  researched peer means IA5 by it. The two tables agree over most of the printable range and part at
+  0x00-0x09, 0x0B-0x0C, 0x0E-0x1A, 0x1C-0x1F, 0x24, 0x40, 0x5B-0x60 and 0x7B-0x7F — line feed,
+  carriage return and escape are common to both — which is where a peer that did mean IA5 is
+  misread. Accepted with it: `consts.ENCODING` loses its `ASCII` alias and keeps `IA5`, the two
+  names 5.2.19 gives 0x01, because that alias was the only name the two tables shared at different
+  values and so the only one a reader could carry from the option's vocabulary into SMPP's flat
+  table; `constsById.ENCODING[0x01]` already read `IA5`, so nothing moves but the forward name.
+  Rejected: moving `consts.ENCODING.ASCII` to 0x00, which would make that table contradict the
+  section it exists to spell — the group is SMPP's flat `data_coding` table, not the `encoding`
+  option's vocabulary, the distinction the `FLASH` removal already drew. Rejected: reading 0x01 as
+  Latin-1, the closest codec here to IA5, which mojibakes every peer that means GSM for one nothing
+  researched has found. Accepted: a message already in flight is unmoved — both codings resolve to
+  the same codec, `messageClassOf()` finds no class in either, and `Reassembler` groups on the
+  concatenation reference rather than on `data_coding` — so a receipt or a segment that crossed the
+  change reads exactly as it did.
+
 ### The session's life
 
 - **A close arriving after our own `unbind` is a clean unbind, not an error.** Maintainer's call,
diff --git a/README.md b/README.md
index b8ee462..a4d62df 100644
--- a/README.md
+++ b/README.md
@@ -599,9 +599,11 @@ how binary payloads, hand-built user data headers and deliberately malformed bod
 
 Composing a message by hand takes the send-side pair: `unencodable(message, encoding)` gives
 `{ char, index }` for the first character an alphabet cannot carry and `undefined` where it carries
-them all, which is the check `sendSms()` makes before it encodes anything; `smppTime.encode(value)`
-returns `{ err, text }` for a `validity_period` or `schedule_delivery_time`, as `smppTime.decode()`
-returns `{ err, date }` for one that arrived.
+them all, which is the check `sendSms()` makes before it encodes anything;
+`dataCodingByEncoding[encoding]` is the `data_coding` this library writes each alphabet under, which
+is what to put beside octets `encodeMessage()` handed back; `smppTime.encode(value)` returns
+`{ err, text }` for a `validity_period` or `schedule_delivery_time`, as `smppTime.decode()` returns
+`{ err, date }` for one that arrived.
 
 The spec tables are exported both individually (`cmds`, `consts`, `encodings`, `errors`, `tlvs`,
 `types`, and the matching `*ById` maps) and grouped as `defs`.
@@ -640,6 +642,9 @@ promises and the rough edges taken off.
 - **The `error` event is `sessionError`** (and `serverError` on the server handle).
 - **`log`** takes any object with `debug`, `error`, `info`, `verbose` and `warn` methods instead of a
   `larvitutils` one, and is silent by default. See [Logging](#logging).
+- **`consts.ENCODING.ASCII` is gone**; the same entry is `consts.ENCODING.IA5`, the other name SMPP
+  3.4 5.2.19 gives 0x01. Reach for `dataCodingByEncoding` where you meant the alphabet `sendSms()`
+  writes — that is 0x00, not this.
 
 ### Behaviour that changed on the wire
 
@@ -659,6 +664,10 @@ have worked around any of these, remove the workaround:
   GSM alphabet on a Latin-1 message that has no `data_coding` at all — that pair is refused now.
   Inbound, only a `data_coding` of exactly 0x10 counted as flash, so a flash UCS2 message and the
   whole 0xF0 coding group arrived as ordinary messages.
+- A GSM 03.38 message declared `data_coding` 0x01, which SMPP 3.4 5.2.19 defines as IA5 rather than
+  the alphabet those octets are in, so `$` and `@` reached a peer honouring the field as STX and NUL.
+  It goes out as 0x00, the SMSC default alphabet, and so does a receipt `sendDlr()` writes. Latin-1
+  and UCS2 are unmoved at 0x03 and 0x08, and an inbound 0x01 is still read as GSM 03.38.
 - 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.
 - Alphanumeric senders were sent with TON 1 (international) instead of TON 5.
diff --git a/interop-tests/smppsim.test.ts b/interop-tests/smppsim.test.ts
index 81628b3..04bb07c 100644
--- a/interop-tests/smppsim.test.ts
+++ b/interop-tests/smppsim.test.ts
@@ -276,7 +276,7 @@ describe('smppsim-transition - C4 intermediate then final', () => {
 		const sent = await session.send({
 			cmdName: 'submit_sm',
 			params: {
-				data_coding: consts.ENCODING.ASCII,
+				data_coding: 0,
 				destination_addr: TO,
 				registered_delivery: 0x11,
 				short_message: Buffer.from('transition test', 'latin1'),
diff --git a/src/defs/constants.ts b/src/defs/constants.ts
index ff89833..0ed155e 100644
--- a/src/defs/constants.ts
+++ b/src/defs/constants.ts
@@ -22,7 +22,6 @@ export const consts = {
 		YEARS: 0x0E,
 	},
 	ENCODING: {
-		ASCII: 0x01,
 		BINARY: 0x04,
 		CYRILLIC: 0x06,
 		EXTENDED_KANJI_JIS: 0x0D,
diff --git a/src/defs/encodings.ts b/src/defs/encodings.ts
index 166a1af..c454b02 100644
--- a/src/defs/encodings.ts
+++ b/src/defs/encodings.ts
@@ -188,3 +188,13 @@ export function encodingByDataCoding(dataCoding: number): EncodingName {
 	// 0x02 and 0x04 are 8-bit binary, 0x03 is Latin-1.
 	return dataCoding >= 0x02 && dataCoding <= 0x04 ? 'LATIN1' : 'ASCII';
 }
+
+/**
+ * The `data_coding` an alphabet is written under, the mirror of `encodingByDataCoding()`. GSM 03.38
+ * takes 0x00, the SMSC default alphabet, rather than SMPP 3.4 5.2.19's 0x01, which is IA5.
+ */
+export const dataCodingByEncoding: Readonly<Record<EncodingName, number>> = {
+	ASCII: 0x00,
+	LATIN1: 0x03,
+	UCS2: 0x08,
+};
diff --git a/src/index.ts b/src/index.ts
index 33f9a9e..eea6081 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -4,7 +4,7 @@ export { Session } from './session.ts';
 
 export { cmds, cmdsById, commandNameById, isCommandName } from './defs/commands.ts';
 export { consts, constsById } from './defs/constants.ts';
-export { detect, encodingByDataCoding, encodings, isEncodingName, messageClassOf, unencodable } from './defs/encodings.ts';
+export { dataCodingByEncoding, detect, encodingByDataCoding, encodings, isEncodingName, messageClassOf, unencodable } from './defs/encodings.ts';
 export { errorNameById, errors, errorsById, isErrorName } from './defs/errors.ts';
 export { tlvs, tlvsById } from './defs/tlvs.ts';
 export { types } from './defs/types.ts';
diff --git a/src/message.ts b/src/message.ts
index d4c5719..0665019 100644
--- a/src/message.ts
+++ b/src/message.ts
@@ -1,7 +1,7 @@
 import type { Result } from './result.ts';
 import type { EncodingName } from './defs/encodings.ts';
-import { detect, encodingByDataCoding, encodings, unencodable, unencodableText } from './defs/encodings.ts';
-import { consts, hasUdh } from './defs/constants.ts';
+import { dataCodingByEncoding, detect, encodingByDataCoding, encodings, unencodable, unencodableText } from './defs/encodings.ts';
+import { hasUdh } from './defs/constants.ts';
 import { udhLength } from './udh.ts';
 
 /** A single SMS carries 1120 bits, whatever the alphabet. */
@@ -35,7 +35,7 @@ export function encodeBody(
 	if (dataCoding === undefined) {
 		const detected = encodeMessage(text);
 
-		return { buffer: detected.buffer, dataCoding: consts.ENCODING[detected.encoding] };
+		return { buffer: detected.buffer, dataCoding: dataCodingByEncoding[detected.encoding] };
 	}
 
 	const encoding = encodingByDataCoding(dataCoding);
diff --git a/src/send-sms.ts b/src/send-sms.ts
index 723cf62..43089b2 100644
--- a/src/send-sms.ts
+++ b/src/send-sms.ts
@@ -7,7 +7,7 @@ import type { SmppLog } from './log.ts';
 import type { SmsIdNotation } from './sms-id.ts';
 import { UnansweredError } from './unanswered-error.ts';
 import { consts, defaultMessagingMode, isMessagingMode, isSubmitMessagingMode, submitMessagingModes } from './defs/constants.ts';
-import { detect, encodingNames, isEncodingName, unencodable, unencodableText } from './defs/encodings.ts';
+import { dataCodingByEncoding, detect, encodingNames, isEncodingName, unencodable, unencodableText } from './defs/encodings.ts';
 import { namedValue } from './error-from.ts';
 import { normaliseSmsId } from './sms-id.ts';
 import { paramText } from './defs/types.ts';
@@ -89,7 +89,7 @@ function addressTon(address: string): number {
 }
 
 function dataCodingFor(encoding: EncodingName, flash: boolean): number {
-	if (!flash) return consts.ENCODING[encoding];
+	if (!flash) return dataCodingByEncoding[encoding];
 
 	// Message class present (0x10) plus the alphabet bits, so flash survives UCS2.
 	return encoding === 'UCS2' ? 0x18 : 0x10;
diff --git a/test/declared-alphabet.test.ts b/test/declared-alphabet.test.ts
new file mode 100644
index 0000000..8132883
--- /dev/null
+++ b/test/declared-alphabet.test.ts
@@ -0,0 +1,195 @@
+import assert from 'node:assert/strict';
+import test, { describe } from 'node:test';
+import { bindToSmsc, dummySmsc } from './dummy-smsc.ts';
+import { client } from '../src/client.ts';
+import { closeAfter } from './teardown.ts';
+import { consts } from '../src/defs/constants.ts';
+import { decodeMessage } from '../src/message.ts';
+import { dlrFromPdu } from '../src/dlr.ts';
+import { encodingByDataCoding, encodings } from '../src/defs/encodings.ts';
+import { objToPdu, pduToObj } from '../src/pdu.ts';
+import { paramNumber } from '../src/defs/types.ts';
+import { server } from '../src/server.ts';
+import type { PduObject } from '../src/pdu.ts';
+
+const from = '46701113311';
+const to = '46709771337';
+
+/** GSM 03.38 puts $ at 0x02 and @ at 0x00, where IA5 has STX and NUL. */
+const bothTables = 'Cost 5$ @home';
+
+/** What SMPP 3.4 5.2.19 assigns each coding, read the way a peer honouring the field reads it. */
+const byTheSpecsTable: Record<number, (octets: Buffer) => string> = {
+	// The SMSC default alphabet, which every peer in interop-tests/ runs as GSM 03.38.
+	0x00: octets => encodings.ASCII.decode(octets),
+	// IA5 (CCITT T.50), whose whole range is what Latin-1 reads below 0x80.
+	[consts.ENCODING.IA5]: octets => octets.toString('latin1'),
+};
+
+/** An event that never fires would otherwise block until the CI job limit, asserting nothing. */
+function once<T>(register: (resolve: (value: T) => void) => void): Promise<T> {
+	return new Promise<T>((resolve, reject) => {
+		const timer = setTimeout(() => {
+			reject(new Error('waited 5000 ms for an event that never fired'));
+		}, 5000);
+
+		register(value => {
+			clearTimeout(timer);
+			resolve(value);
+		});
+	});
+}
+
+function submitted(octets: Buffer[]): PduObject[] {
+	return octets.map(pdu => {
+		const { pduObj } = pduToObj(pdu);
+
+		assert.ok(pduObj);
+
+		return pduObj;
+	});
+}
+
+function declaredBy(pduObj: PduObject): number {
+	return paramNumber(pduObj.params.data_coding, 0);
+}
+
+/** A deliver_sm carrying `body` under `dataCoding`, as a peer answering our own send would write it. */
+function delivered(body: Buffer | string, dataCoding: number, esmClass: number): PduObject {
+	const { buffer } = objToPdu({
+		cmdName: 'deliver_sm',
+		params: {
+			data_coding: dataCoding,
+			destination_addr: from,
+			esm_class: esmClass,
+			short_message: body,
+			source_addr: to,
+		},
+		seqNr: 1,
+	});
+
+	assert.ok(buffer);
+
+	const { pduObj } = pduToObj(buffer);
+
+	assert.ok(pduObj);
+
+	return pduObj;
+}
+
+describe('the alphabet a message declares is the one its octets are written in', () => {
+	test('sends GSM 03.38 under data_coding 0x00, the SMSC default alphabet', async t => {
+		const smsc = await dummySmsc(t, { messageIds: ['01a08779-de97-7caa-9d26-e6d50f5c4888'] });
+		const session = await bindToSmsc(t, smsc.port, { reconnect: false });
+		const sent = await session.sendSms({ from, message: bothTables, to });
+
+		assert.equal(sent.err, undefined);
+		assert.deepEqual(submitted(smsc.octets).map(declaredBy), [0x00]);
+	});
+
+	test('keeps $ and @ for a peer that honours the declaration, where IA5 read STX and NUL', async t => {
+		const smsc = await dummySmsc(t, { messageIds: ['01a08779-de98-7d24-9542-e652e0d3761c'] });
+		const session = await bindToSmsc(t, smsc.port, { reconnect: false });
+
+		assert.equal((await session.sendSms({ from, message: bothTables, to })).err, undefined);
+
+		const [pduObj] = submitted(smsc.octets);
+
+		assert.ok(pduObj);
+
+		const octets = pduObj.shortMessageOctets;
+
+		assert.ok(octets);
+		assert.equal(octets.toString('hex'), '436f73742035022000686f6d65');
+
+		const read = byTheSpecsTable[declaredBy(pduObj)];
+
+		assert.ok(read, 'the coding declared must be one SMPP 3.4 5.2.19 names an alphabet for');
+		assert.equal(read(octets), bothTables);
+	});
+
+	test('leaves Latin-1 at 0x03 and UCS2 at 0x08, the codings those alphabets always had', async t => {
+		const smsc = await dummySmsc(t, {
+			messageIds: ['01a08779-de99-7fa5-bcac-18feef55aeee', '01a08779-de99-72ec-8dbb-9365a00158c3'],
+		});
+		const session = await bindToSmsc(t, smsc.port, { reconnect: false });
+
+		assert.equal((await session.sendSms({ encoding: 'LATIN1', from, message: 'Räksmörgås', to })).err, undefined);
+		assert.equal((await session.sendSms({ from, message: 'あいう', to })).err, undefined);
+		assert.deepEqual(submitted(smsc.octets).map(declaredBy), [0x03, 0x08]);
+	});
+
+	// sendDlr() writes its body as a string with no data_coding, so it takes the detected branch too.
+	test('declares 0x00 on a receipt it writes itself', async t => {
+		const { err, server: smpp } = await server({ port: 0 });
+
+		assert.equal(err, undefined);
+		assert.ok(smpp);
+		closeAfter(t, smpp);
+
+		smpp.on('session', peer => peer.on('sms', async sms => {
+			await sms.sendResp();
+			await sms.sendDlr('DELIVERED');
+		}));
+
+		const connected = await client({ port: smpp.port, reconnect: false });
+
+		assert.equal(connected.err, undefined);
+		assert.ok(connected.session);
+		closeAfter(t, connected.session);
+
+		const session = connected.session;
+		const reported = once<PduObject>(resolve => {
+			session.on('dlr', (_report, pduObj) => { resolve(pduObj); });
+		});
+
+		assert.equal((await session.sendSms({ dlr: true, from, message: bothTables, to })).err, undefined);
+		assert.equal(declaredBy(await reported), 0x00);
+	});
+
+	// The low-level surface settles data_coding off the same detection, so it carried the same defect.
+	test('settles a detected string body at 0x00 where the caller named no data_coding', () => {
+		const built = objToPdu({
+			cmdName: 'submit_sm',
+			params: { destination_addr: to, short_message: bothTables, source_addr: from },
+		});
+
+		assert.ok(built.buffer);
+
+		const { pduObj } = pduToObj(built.buffer);
+
+		assert.ok(pduObj);
+		assert.equal(declaredBy(pduObj), 0x00);
+		assert.equal(pduObj.shortMessageOctets?.toString('hex'), '436f73742035022000686f6d65');
+	});
+});
+
+describe('what a peer declares is read as generously as it was before', () => {
+	test('reads data_coding 0x01 as GSM 03.38, as 0x00 is read', () => {
+		assert.equal(encodingByDataCoding(0x01), 'ASCII');
+
+		const octets = Buffer.from('436f73742035022000686f6d65', 'hex');
+
+		for (const dataCoding of [0x00, 0x01]) {
+			assert.equal(decodeMessage(octets, dataCoding).message, bothTables, String(dataCoding));
+		}
+	});
+
+	test('leaves a receipt and an inbound message reading the same under either coding', () => {
+		const receiptId = '01a08779-de97-7caa-9d26-e6d50f5c4888';
+		const body = `id:${receiptId} sub:001 dlvrd:001 submit date:2509091430 done date:2509091431 stat:DELIVRD err:000 text:${bothTables}`;
+
+		for (const dataCoding of [0x00, 0x01]) {
+			const receipt = dlrFromPdu(delivered(body, dataCoding, consts.ESM_CLASS.MC_DELIVERY_RECEIPT));
+
+			assert.ok(receipt, String(dataCoding));
+			assert.equal(receipt.smsId, receiptId, String(dataCoding));
+			assert.equal(receipt.statusMsg, 'DELIVERED', String(dataCoding));
+
+			const inbound = delivered(Buffer.from('436f73742035022000686f6d65', 'hex'), dataCoding, 0);
+
+			assert.equal(dlrFromPdu(inbound), undefined, String(dataCoding));
+			assert.equal(inbound.params.short_message, bothTables, String(dataCoding));
+		}
+	});
+});
diff --git a/test/encodings.test.ts b/test/encodings.test.ts
index 18d531f..67fc68c 100644
--- a/test/encodings.test.ts
+++ b/test/encodings.test.ts
@@ -1,6 +1,6 @@
 import assert from 'node:assert/strict';
 import test, { describe } from 'node:test';
-import { detect, encodingByDataCoding, encodings, unencodable } from '../src/defs/encodings.ts';
+import { dataCodingByEncoding, detect, encodingByDataCoding, encodings, isEncodingName, unencodable } from '../src/defs/encodings.ts';
 
 describe('ASCII (GSM 03.38)', () => {
 	const samples: [string, number[]][] = [
@@ -188,6 +188,14 @@ describe('encodingByDataCoding()', () => {
 		}
 	});
 
+	test('reads every coding dataCodingByEncoding writes back as the alphabet that wrote it', () => {
+		for (const name of Object.keys(dataCodingByEncoding)) {
+			if (!isEncodingName(name)) return assert.fail(`${name} names no alphabet`);
+
+			assert.equal(encodingByDataCoding(dataCodingByEncoding[name]), name);
+		}
+	});
+
 	test('falls back to ASCII for alphabets it has no codec for', () => {
 		assert.equal(encodingByDataCoding(0x05), 'ASCII');
 		assert.equal(encodingByDataCoding(0x0E), 'ASCII');
diff --git a/test/message-class.test.ts b/test/message-class.test.ts
index 24f099c..e613139 100644
--- a/test/message-class.test.ts
+++ b/test/message-class.test.ts
@@ -171,7 +171,7 @@ describe('sendSms() flash', () => {
 		}
 
 		// 0.4.0 forced 0x10 whatever the alphabet was, which mangled every non-GSM flash message.
-		assert.deepEqual(dataCodingsOf(smsc.octets), [0x01, 0x10, 0x08, 0x18, 0x03]);
+		assert.deepEqual(dataCodingsOf(smsc.octets), [0x00, 0x10, 0x08, 0x18, 0x03]);
 	});
 
 	test('refuses a flash Latin-1 message, which no coding group carrying a class can spell', async () => {
diff --git a/test/messaging-mode.test.ts b/test/messaging-mode.test.ts
index 4e00d76..4ac57ee 100644
--- a/test/messaging-mode.test.ts
+++ b/test/messaging-mode.test.ts
@@ -19,13 +19,13 @@ const to = '46709771337';
 const longMessage = 'Segments of a long message, counted in a user data header. '.repeat(7);
 
 /** A freshly bound session's first submit_sm for `Hello world`: sequence number 2. */
-const singleSegmentOctets = '0000004200000004000000000000000200010034363730313131333331310001003436373039373731333337000000000000000001000b48656c6c6f20776f726c64';
+const singleSegmentOctets = '0000004200000004000000000000000200010034363730313131333331310001003436373039373731333337000000000000000000000b48656c6c6f20776f726c64';
 
 /** The same for `longMessage`: concatenation reference 1, sequence numbers 2 to 4. */
 const threeSegmentOctets = [
-	'000000d600000004000000000000000200010034363730313131333331310001003436373039373731333337004000000000000001009f0500030103015365676d656e7473206f662061206c6f6e67206d6573736167652c20636f756e74656420696e206120757365722064617461206865616465722e205365676d656e7473206f662061206c6f6e67206d6573736167652c20636f756e74656420696e206120757365722064617461206865616465722e205365676d656e7473206f662061206c6f6e67206d6573736167652c20636f756e746564',
-	'000000d600000004000000000000000300010034363730313131333331310001003436373039373731333337004000000000000001009f05000301030220696e206120757365722064617461206865616465722e205365676d656e7473206f662061206c6f6e67206d6573736167652c20636f756e74656420696e206120757365722064617461206865616465722e205365676d656e7473206f662061206c6f6e67206d6573736167652c20636f756e74656420696e206120757365722064617461206865616465722e205365676d656e7473206f66',
-	'000000a80000000400000000000000040001003436373031313133333131000100343637303937373133333700400000000000000100710500030103032061206c6f6e67206d6573736167652c20636f756e74656420696e206120757365722064617461206865616465722e205365676d656e7473206f662061206c6f6e67206d6573736167652c20636f756e74656420696e206120757365722064617461206865616465722e20',
+	'000000d600000004000000000000000200010034363730313131333331310001003436373039373731333337004000000000000000009f0500030103015365676d656e7473206f662061206c6f6e67206d6573736167652c20636f756e74656420696e206120757365722064617461206865616465722e205365676d656e7473206f662061206c6f6e67206d6573736167652c20636f756e74656420696e206120757365722064617461206865616465722e205365676d656e7473206f662061206c6f6e67206d6573736167652c20636f756e746564',
+	'000000d600000004000000000000000300010034363730313131333331310001003436373039373731333337004000000000000000009f05000301030220696e206120757365722064617461206865616465722e205365676d656e7473206f662061206c6f6e67206d6573736167652c20636f756e74656420696e206120757365722064617461206865616465722e205365676d656e7473206f662061206c6f6e67206d6573736167652c20636f756e74656420696e206120757365722064617461206865616465722e205365676d656e7473206f66',
+	'000000a80000000400000000000000040001003436373031313133333131000100343637303937373133333700400000000000000000710500030103032061206c6f6e67206d6573736167652c20636f756e74656420696e206120757365722064617461206865616465722e205365676d656e7473206f662061206c6f6e67206d6573736167652c20636f756e74656420696e206120757365722064617461206865616465722e20',
 ];
 
 type BoundPeer = {
diff --git a/test/pdu.test.ts b/test/pdu.test.ts
index 8b60776..999337b 100644
--- a/test/pdu.test.ts
+++ b/test/pdu.test.ts
@@ -130,7 +130,8 @@ describe('parsing real PDUs', () => {
 });
 
 describe('encoding submit_sm', () => {
-	test('produces the same bytes as 0.4.0 for a GSM message', () => {
+	// 0.4.0 declared IA5 for the GSM octets it wrote; that one field is the whole difference.
+	test('produces the 0.4.0 bytes for a GSM message, under the alphabet those octets are in', () => {
 		const pdu = encode({
 			cmdName: 'submit_sm',
 			cmdStatus: 'ESME_ROK',
@@ -144,7 +145,7 @@ describe('encoding submit_sm', () => {
 
 		assert.equal(
 			pdu.toString('hex'),
-			'0000004200000004000000000000000c00000034363730313131333331310000003436373039373731333337000000000000000001000b48656c6c6f20776f726c64',
+			'0000004200000004000000000000000c00000034363730313131333331310000003436373039373731333337000000000000000000000b48656c6c6f20776f726c64',
 		);
 	});
 
diff --git a/test/unsendable.test.ts b/test/unsendable.test.ts
index 1177b3a..cdcda44 100644
--- a/test/unsendable.test.ts
+++ b/test/unsendable.test.ts
@@ -109,7 +109,7 @@ describe('an alphabet the caller named that cannot carry the message', () => {
 			assert.equal((await session.sendSms({ from, message, to })).err, undefined, message);
 		}
 
-		assert.deepEqual(sentAs(smsc.octets), [[0x01, 'Hello world'], [0x08, 'Åsa naïve'], [0x08, 'あいう']]);
+		assert.deepEqual(sentAs(smsc.octets), [[0x00, 'Hello world'], [0x08, 'Åsa naïve'], [0x08, 'あいう']]);
 	});
 });
 
diff --git a/todo.md b/todo.md
index 5b865ca..e639383 100644
--- a/todo.md
+++ b/todo.md
@@ -120,19 +120,6 @@ session message is a change to every call site.
       another message's in a correlation table. The cost is that every consumer narrows, including
       the majority whose SMSC always names an id. Raised by the phase 11 product review, 2026-09-08.
       **This one is 1.0.0-or-never** — after release it needs a major version.
-- [ ] **A default GSM send declares IA5, not GSM 03.38.** `dataCodingFor()` resolves `encoding`
-      through `consts.ENCODING`, so `'ASCII'` goes out as `data_coding` 0x01 — SMPP 3.4 5.2.19's
-      *IA5 (CCITT T.50)/ASCII* — while the codec writes the GSM 03.38 table, where `@` is 0x00 and
-      `$` is 0x02 against IA5's NUL and STX. `encodingByDataCoding()` reads 0x00 back as GSM 03.38,
-      so this library's writer and reader disagree, and every peer in `interop-tests/` submits GSM
-      text at `data_coding` 0. Nothing asserts what our own default send declares, and the echo
-      tests use characters where the two tables agree, which is why the suite is green.
-      `test/message-class.test.ts` pins the current value. Goal 1 owns it. The fix is a three-entry
-      map in `send-sms.ts` (`ASCII` → 0x00, `LATIN1` → 0x03, `UCS2` → 0x08) rather than any move of
-      a published constant — `consts.ENCODING.ASCII` = 0x01 is a correct name for SMPP's flat-table
-      IA5 entry, and the defect is using that group as the option's vocabulary. **Which value an
-      operator should see is the maintainer's call**, and it wants an `interop-tests/` run. Raised
-      by the architecture review of [#99](https://github.com/larvit/larvitsmpp/pull/99), 2026-09-09.
 - [ ] Tag `v1.0.0` to publish.
 - [ ] `npm deprecate larvitsmpp` pointing at `@larvit/smpp`. Maintainer's call to run it; not
       something CI should do.