Read and write every text field on the wire as latin1

AGENTS.md

@@ -292,6 +292,7 @@ the file.
 - A string body is written in the alphabet its own `data_coding` names, and one that alphabet cannot
   carry is refused by the codec — `message_payload` on the same terms as `short_message`.
 - A GSM 03.38 message declares `data_coding` 0x00, and an inbound 0x01 is still read as GSM.
+- Every text field on the wire is latin1, and a character past `U+00FF` is refused.
 
 ### [The session's life](docs/decisions.md#the-sessions-life)
 

CHANGELOG.md

@@ -6,6 +6,11 @@
   one that expires as an ordinary connect failure, so `reconnect` retries it on its usual backoff.
   A connect previously waited the operating system out, around 130 s on Linux against a host that
   drops SYNs. `connectTimeout` retunes the bound, and `connectTimeout: false` restores the old wait.
+- Addresses, ids and every other text field on the wire are read and written as latin1. A
+  `source_addr` of `Kaffeé` previously reached the application as `Kaffei`, because the codec wrote
+  the octet and then masked bit 7 reading it back; `destination_addr`, `system_id`, `message_id`,
+  `service_type` and the C-Octet String TLVs were affected the same way. A character past `U+00FF`
+  in one of those fields is now refused, where it used to go out as its low octet.
 
 ## 0.5.0
 

README.md

@@ -288,7 +288,8 @@ await session.sendSms({
 ```
 
 **Addresses.** `sourceAddrTon` and `destinationAddrTon` default to 5 for an alphanumeric address
-and 1 for a numeric one; the NPI fields default to 0.
+and 1 for a numeric one; the NPI fields default to 0. An address is latin1, so `Kaffeé` goes out and
+comes back as its own octets; a character past `U+00FF` is refused rather than sent truncated.
 
 **Encoding.**
 

docs/decisions.md

@@ -443,8 +443,8 @@ rule and an index of the titles below.
   the guard would be a second spelling that disagrees about correctness. Rejected: refusing a string
   `message_payload` outright and demanding
   octets, which contradicts `short_message` on the same PDU. Rejected: guarding every string-valued
-  field, which `data_coding` says nothing about — an address is a C-Octet String and ASCII by 3.4's
-  own definition.
+  field against `data_coding`, which says nothing about them — a text field on the wire has an
+  alphabet of its own.
 
 - **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
@@ -478,6 +478,24 @@ rule and an index of the titles below.
   concatenation reference rather than on `data_coding` — so a receipt or a segment that crossed the
   change reads exactly as it did.
 
+- **Every text field on the wire is latin1, and a character past `U+00FF` is refused.** Maintainer's
+  call, 2026-09-21, from the 0.6.0 correctness list: the codec read these fields with
+  `toString('ascii')`, which masks bit 7, and wrote them with `write(text, 'ascii')`, which does not,
+  so an inbound `source_addr` of `Kaffeé` reached the application as `Kaffei` and
+  `objToPdu(pduToObj(x))` was idempotent for none of `source_addr`, `destination_addr`, `system_id`,
+  `message_id`, `service_type` or the cstring TLVs. Goal 3 settles the read: 3.4 calls these fields
+  ASCII, and an operator routing an alphanumeric sender through the upper half is traffic to keep.
+  The write is named latin1 to match, which is what makes the round trip idempotent and is already
+  the octets Node put on the wire, so no peer sees a change. `wantText()` is the single place that
+  says so — which is why the `dest_address` and `unsuccess_sme` structures write their embedded
+  addresses through `cstring.write()` rather than reaching past it — and a character past `U+00FF`
+  is refused there rather than truncated to its low octet: a `size()` that agreed with a `write()`
+  that dropped half a character is a wrong answer about what went out (goal 2), and it is how an
+  alphabet that cannot carry a message body is already handled. Rejected: reading latin1 and leaving
+  the write spelled ASCII, which leaves two halves agreeing only by accident. Rejected: refusing the
+  upper half on send to stay strict to 3.4's ASCII, which would drop exactly the traffic the read
+  keeps.
+
 ## The session's life
 
 - **A close arriving after our own `unbind` is a clean unbind, not an error.** Maintainer's call,

src/defs/types.ts

@@ -28,7 +28,7 @@ export type WireType<T extends ParamValue = ParamValue> = {
 export function paramText(value: ParamValue | undefined): string {
 	if (typeof value === 'string') return value;
 	if (typeof value === 'number') return value.toString();
-	if (Buffer.isBuffer(value)) return value.toString('ascii');
+	if (Buffer.isBuffer(value)) return value.toString('latin1');
 
 	return '';
 }
@@ -79,11 +79,25 @@ function writeInt32(value: ParamValue, buf: Buffer, offset: number): VoidResult
 	return {};
 }
 
-function wantText(value: ParamValue): Result<{ text: string }> {
-	if (typeof value === 'string') return { text: value };
-	if (typeof value === 'number') return { text: value.toString() };
+function pastLatin1(text: string): { err: Error } | undefined {
+	const index = text.search(/[\u0100-\uFFFF]/);
 
-	return { err: new Error(`Expected a string, got ${typeof value}`) };
+	if (index === -1) return undefined;
+
+	const code = text.charCodeAt(index).toString(16).toUpperCase().padStart(4, '0');
+
+	return {
+		err: new Error(
+			`Character U+${code} at index ${String(index)} is past latin1, which every text field on the wire is written in`,
+		),
+	};
+}
+
+function wantText(value: ParamValue): Result<{ text: string }> {
+	if (typeof value === 'number') return { text: value.toString() };
+	if (typeof value !== 'string') return { err: new Error(`Expected a string, got ${typeof value}`) };
+
+	return pastLatin1(value) ?? { text: value };
 }
 
 function wantBytes(value: ParamValue): Result<{ bytes: Buffer }> {
@@ -91,7 +105,7 @@ function wantBytes(value: ParamValue): Result<{ bytes: Buffer }> {
 
 	const { err, text } = wantText(value);
 
-	return err ? { err } : { bytes: Buffer.from(text, 'ascii') };
+	return err ? { err } : { bytes: Buffer.from(text, 'latin1') };
 }
 
 function isDestAddress(value: unknown): value is DestAddress {
@@ -167,7 +181,7 @@ function readCstring(buffer: Buffer, offset: number): Result<{ bytesRead: number
 		}
 	}
 
-	return { bytesRead: length + 1, value: buffer.toString('ascii', offset, offset + length) };
+	return { bytesRead: length + 1, value: buffer.toString('latin1', offset, offset + length) };
 }
 
 function writeCstring(text: string, buffer: Buffer, offset: number): VoidResult {
@@ -175,7 +189,7 @@ function writeCstring(text: string, buffer: Buffer, offset: number): VoidResult
 
 	if (err) return { err };
 
-	buffer.write(text, offset, 'ascii');
+	buffer.write(text, offset, 'latin1');
 	buffer[offset + text.length] = 0;
 
 	return {};
@@ -248,7 +262,7 @@ export const string: WireType<string> = {
 
 		if (err) return { err };
 
-		return { bytesRead: length + 1, value: buffer.toString('ascii', offset + 1, offset + 1 + length) };
+		return { bytesRead: length + 1, value: buffer.toString('latin1', offset + 1, offset + 1 + length) };
 	},
 	size(value) {
 		const { err, text } = wantText(value);
@@ -271,7 +285,7 @@ export const string: WireType<string> = {
 		if (rangeErr) return { err: rangeErr };
 
 		buffer.writeUInt8(text.length, offset);
-		buffer.write(text, offset + 1, 'ascii');
+		buffer.write(text, offset + 1, 'latin1');
 
 		return {};
 	},
@@ -401,7 +415,7 @@ export const dest_address_array: WireType<DestAddress[]> = {
 			if ('dl_name' in dest) {
 				buf.writeUInt8(2, offset++);
 
-				const name = writeCstring(dest.dl_name, buf, offset);
+				const name = cstring.write(dest.dl_name, buf, offset);
 
 				if (name.err) return { err: name.err };
 
@@ -417,7 +431,7 @@ export const dest_address_array: WireType<DestAddress[]> = {
 
 				if (npi.err) return { err: npi.err };
 
-				const addr = writeCstring(dest.destination_addr, buf, offset);
+				const addr = cstring.write(dest.destination_addr, buf, offset);
 
 				if (addr.err) return { err: addr.err };
 
@@ -505,7 +519,7 @@ export const unsuccess_sme_array: WireType<UnsuccessSme[]> = {
 
 			if (npi.err) return { err: npi.err };
 
-			const addr = writeCstring(sme.destination_addr, buf, offset);
+			const addr = cstring.write(sme.destination_addr, buf, offset);
 
 			if (addr.err) return { err: addr.err };
 
@@ -538,7 +552,7 @@ export const tlv = {
 				? offset + length
 				: terminator;
 
-			return { bytesRead: length, value: buf.toString('ascii', offset, end) };
+			return { bytesRead: length, value: buf.toString('latin1', offset, end) };
 		},
 		size(value: ParamValue) {
 			const { err, text } = wantText(value);
@@ -559,7 +573,7 @@ export const tlv = {
 		read(buf: Buffer, offset: number, length = 0) {
 			const err = outOfRange(buf, offset, length);
 
-			return err ? { err } : { bytesRead: length, value: buf.toString('ascii', offset, offset + length) };
+			return err ? { err } : { bytesRead: length, value: buf.toString('latin1', offset, offset + length) };
 		},
 		size(value: ParamValue) {
 			const { err, text } = wantText(value);
@@ -575,7 +589,7 @@ export const tlv = {
 
 			if (rangeErr) return { err: rangeErr };
 
-			buf.write(text, offset, 'ascii');
+			buf.write(text, offset, 'latin1');
 
 			return {};
 		},

test/pdu.test.ts

@@ -127,6 +127,24 @@ describe('parsing real PDUs', () => {
 		assert.equal(pduObj.params.short_message, 'hej 一');
 		assert.equal(pduObj.tlvs.message_state?.tagValue, 2);
 	});
+
+	// 'ascii' masks bit 7 on the way in, so this address used to come back Kaffei.
+	test('keeps an alphanumeric sender whole through the wire and back', () => {
+		const pdu = encode({
+			cmdName: 'deliver_sm',
+			params: {
+				destination_addr: '46709771337',
+				short_message: 'hej',
+				source_addr: 'Kaffeé',
+				source_addr_ton: 5,
+			},
+			seqNr: 9,
+		});
+
+		assert.ok(pdu.includes(Buffer.from('Kaffeé', 'latin1')));
+		assert.equal(decode(pdu).params.source_addr, 'Kaffeé');
+		assert.ok(objToPdu({ cmdName: 'deliver_sm', params: { source_addr: '一' } }).err instanceof Error);
+	});
 });
 
 describe('encoding submit_sm', () => {

test/types.test.ts

@@ -1,8 +1,8 @@
 import assert from 'node:assert/strict';
 import test, { describe } from 'node:test';
 import type { DestAddress, UnsuccessSme } from '../src/defs/types.ts';
+import { paramText, types } from '../src/defs/types.ts';
 import { tlvs } from '../src/defs/tlvs.ts';
-import { types } from '../src/defs/types.ts';
 
 describe('integers', () => {
 	test('int8 reads, sizes and writes one octet', () => {
@@ -69,6 +69,20 @@ describe('string (Octet String)', () => {
 
 		assert.deepEqual(target, encoded);
 	});
+
+	test('carries every latin1 octet, and refuses a character past it', () => {
+		const target = Buffer.alloc(4);
+
+		assert.deepEqual(types.string.read(Buffer.from([3, 0xE9, 0x80, 0xFF]), 0), {
+			bytesRead: 4,
+			value: 'é\u0080ÿ',
+		});
+		assert.deepEqual(types.string.write('é\u0080ÿ', target, 0), {});
+		assert.deepEqual(target, Buffer.from([3, 0xE9, 0x80, 0xFF]));
+
+		assert.ok(types.string.size('一').err instanceof Error);
+		assert.ok(types.string.write('一', Buffer.alloc(4), 0).err instanceof Error);
+	});
 });
 
 describe('cstring (C-Octet String)', () => {
@@ -100,6 +114,20 @@ describe('cstring (C-Octet String)', () => {
 		assert.deepEqual(types.cstring.size(123), { size: 4 });
 	});
 
+	// 'ascii' masks bit 7 on the way in and keeps it on the way out, so an address a peer wrote as
+	// Kaffeé came back Kaffei and objToPdu(pduToObj(x)) stopped being idempotent.
+	test('carries every latin1 octet, and refuses a character past it', () => {
+		const address = Buffer.from([0x4B, 0x61, 0x66, 0x66, 0x65, 0xE9, 0x00]);
+		const target = Buffer.alloc(7);
+
+		assert.deepEqual(types.cstring.read(address, 0), { bytesRead: 7, value: 'Kaffeé' });
+		assert.deepEqual(types.cstring.write('Kaffeé', target, 0), {});
+		assert.deepEqual(target, address);
+
+		assert.ok(types.cstring.size('一').err instanceof Error);
+		assert.ok(types.cstring.write('一', Buffer.alloc(4), 0).err instanceof Error);
+	});
+
 	test('refuses a string with no terminator rather than running off the end', () => {
 		assert.ok(types.cstring.read(Buffer.from('abcd'), 0).err instanceof Error);
 	});
@@ -142,6 +170,32 @@ describe('integer TLVs', () => {
 	});
 });
 
+describe('text TLVs', () => {
+	const encoded = Buffer.from([0xE9, 0x80, 0xFF]);
+
+	test('carry every latin1 octet, and refuse a character past it', () => {
+		const target = Buffer.alloc(3);
+
+		assert.deepEqual(types.tlv.string.read(encoded, 0, 3), { bytesRead: 3, value: 'é\u0080ÿ' });
+		assert.deepEqual(types.tlv.string.write('é\u0080ÿ', target, 0), {});
+		assert.deepEqual(target, encoded);
+
+		assert.ok(types.tlv.string.size('一').err instanceof Error);
+		assert.ok(types.tlv.string.write('一', Buffer.alloc(3), 0).err instanceof Error);
+	});
+
+	test('carry them through a cstring tag too, terminator or none', () => {
+		const target = Buffer.alloc(4);
+
+		assert.deepEqual(types.tlv.cstring.read(encoded, 0, 3), { bytesRead: 3, value: 'é\u0080ÿ' });
+		assert.deepEqual(types.tlv.cstring.write('é\u0080ÿ', target, 0), {});
+		assert.deepEqual(target, Buffer.from([0xE9, 0x80, 0xFF, 0x00]));
+
+		assert.ok(types.tlv.cstring.size('一').err instanceof Error);
+		assert.ok(types.tlv.cstring.write('一', Buffer.alloc(4), 0).err instanceof Error);
+	});
+});
+
 describe('buffer', () => {
 	const expected = Buffer.from('abcd1234');
 
@@ -170,6 +224,22 @@ describe('buffer', () => {
 
 		assert.deepEqual(target, expected);
 	});
+
+	test('takes a string as the latin1 octets it stands for', () => {
+		const target = Buffer.alloc(3);
+
+		assert.deepEqual(types.buffer.size('é\u0080ÿ'), { size: 3 });
+		assert.deepEqual(types.buffer.write('é\u0080ÿ', target, 0), {});
+		assert.deepEqual(target, Buffer.from([0xE9, 0x80, 0xFF]));
+	});
+});
+
+describe('paramText()', () => {
+	// The one reader of a receipt body that arrived with no octets of its own, so masking bit 7
+	// here loses the same characters the wire types used to.
+	test('renders a Buffer parameter as the latin1 text its octets spell', () => {
+		assert.equal(paramText(Buffer.from([0x4B, 0x61, 0x66, 0x66, 0x65, 0xE9])), 'Kaffeé');
+	});
 });
 
 describe('dest_address_array', () => {

todo.md

@@ -189,15 +189,6 @@ and is also what the panel ranked hardest — two methods, one answer.
 
 ### Correctness, ahead of everything below
 
-- [ ] **Read C-Octet Strings as `latin1`, so an address survives the wire.** `defs/types.ts` reads
-      with `toString('ascii')` at four sites and writes with `write(text, 'ascii')` at three. Node
-      masks bit 7 when decoding and not when encoding, so the codec writes `0xE9` and reads back
-      `0x69`: an inbound `source_addr` of `Kaffeé` reaches the application as `Kaffei`, with no raw
-      escape hatch as `short_message` has in `shortMessageOctets`. Affects `source_addr`,
-      `destination_addr`, `system_id`, `message_id`, `service_type` and the cstring TLVs, and makes
-      `objToPdu(pduToObj(x))` non-idempotent for them. Goals 1 and 3. Verified in the container:
-      `Buffer.from([0xE9]).toString('ascii')` is `'i'`.
-
 - [ ] **Answer `alert_notification` and `outbind` by not answering them.** Both are response-less in
       SMPP 3.4, both fall through `route()`'s default into `unhandled()`, which calls
       `sendReturn(pduObj, 'ESME_RINVCMDID')`; `pduReturn()` then finds no response command, and the