Refuse a body the PDU's own data_coding cannot carry (#98)

* Regression tests for a body the PDU's own data_coding cannot carry

* Refuse a body the PDU's own data_coding cannot carry, in the codec both send paths build through

* Record the codec's body refusal, and drop two todo entries it and the interop run close

* Regression tests for the aliased body TLV and the data_coding short_message settles

* Find the body TLV by tag id, leave data_coding to the field that will be read, and correct the record

* Record the two architecture findings this change does not close

* Regression tests for a duplicated body tag and a short_message the wire never carries

* Resolve every body TLV against the field that will be read, and move TLV writing to its table

* Regression tests for a body only the TLV carries and an empty short_message

* Let only octets the command writes settle the alphabet, and stop shadowing the TLV table

AGENTS.md

@@ -109,7 +109,7 @@ src/
 		constants.ts     consts + constsById, and the SMPP version constants
 		encodings.ts     GSM 03.38, LATIN1, UCS2, detection, data_coding resolution
 		errors.ts        errors + errorsById (ESME_*)
-		tlvs.ts          TLV definitions, tlvsById
+		tlvs.ts          TLV definitions, tlvsById, the input shape, and writing a TLV stream
 		types.ts         Wire types: int8/int16/int32/string/cstring/buffer/arrays
 ```
 
@@ -622,6 +622,42 @@ Grouped by what each one constrains.
   the one input a per-character reading passes and the encoder then writes as the extended character
   — the only composition in any of the three codecs, and not a character a message is written in.
 
+- **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`.**
+  Maintainer's call, 2026-09-09, from the architecture review of
+  [#97](https://github.com/larvit/larvitsmpp/pull/97): `objToPdu()` took the codec off the caller's
+  own `data_coding` and encoded with it whatever the text was, so `data_coding` 3 beside `あいう`
+  returned `42 44 46` — `"BDF"` — reported as built, while a string `message_payload` was cut to its
+  low octets whatever `data_coding` said. Goal 2 owns it, as it owns the `sendSms()` guard above.
+  The line falls at the string: a `Buffer` is octets the caller already chose and goes out as given
+  under any `data_coding`, which is what keeps goal 6's escape hatch open — the raw UDH, 8-bit binary
+  and deliberately malformed bodies `interop-tests/` builds are all still buildable — and a string
+  with no `data_coding` is untouched, detection carrying every character it was picked for. The
+  guard is `unencodable()` again rather than a second reading, and `unencodableText()` is the
+  character, its code point and its index said once for both refusals. It is reached through
+  `encodeBody()` in `message.ts`, which is where the `data_coding`-to-text pair already lives:
+  `encodeBody(text, dataCoding)` is `decodeMessage(buffer, dataCoding)`'s mirror and resolves the
+  alphabet through the same `encodingByDataCoding()`. `send()` and `sendReturn()` inherit it,
+  since both build through `buildPdu()`; `sendSms()` does not, and keeps its own guard, because
+  `splitMessage()` hands the codec a Buffer with nothing left to refuse and the index a segment
+  could name is not the one in the message. The TLV is encoded rather than merely checked because
+  `data_coding` names the alphabet of the body wherever it is carried — that is how
+  `messageOctets()` and `decodeMessage()` read one back, and a `data_sm` has nowhere else to put one
+  — so refusing what Latin-1 cannot hold while still writing UCS-2 text as Latin-1 octets would
+  close half of it. `short_message` settles the `data_coding` wherever it carries octets at all, the
+  order `messageOctets()` reads the two in, so the alphabet a PDU declares is the one its body will
+  be read under — and a `short_message` on a command whose table declares none is ignored here as
+  `writeParams()` ignores it, so an empty one, an absent one and one the wire cannot carry are the
+  same input rather than three. A `data_coding` on a command that declares no such field is honoured
+  the other way round, since it is `replace_sm`'s only way to name the alphabet its octets are in.
+  Every entry carrying the payload tag is resolved, by tag id rather
+  than by record key, since `tagIdOf()` lets a caller name it anything and a spelling that escaped
+  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.
+
 ### The session's life
 
 - **A close arriving after our own `unbind` is a clean unbind, not an error.** Maintainer's call,

README.md

@@ -577,6 +577,13 @@ same for the GSM 03.38 message class: `0` for the flash class `sms.flash` alread
 and `3` for the ME-, SIM- and TE-specific ones, and `undefined` where that `data_coding`'s coding
 group carries no class at all.
 
+Building a PDU is the mirror: a string `short_message`, or a string `message_payload`, is encoded in
+the alphabet the PDU's own `data_coding` names, detected from the text where you name none, and a
+`data_coding` whose alphabet cannot carry one of its characters is refused, naming the character, its
+code point and where it is. A `Buffer` goes out exactly as given under any `data_coding`, which is
+how binary payloads, hand-built user data headers and deliberately malformed bodies are sent.
+`session.send()` and `session.sendReturn()` build through the same codec and refuse the same bodies.
+
 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)`

src/defs/encodings.ts

@@ -123,6 +123,13 @@ export function detect(value: string): EncodingName {
 
 export type Unencodable = { char: string; index: number };
 
+/** What an alphabet could not carry, said the one way: the character, its code point and its index. */
+export function unencodableText(at: Unencodable): string {
+	const point = (at.char.codePointAt(0) ?? 0).toString(16).toUpperCase().padStart(4, '0');
+
+	return `${JSON.stringify(at.char)} (U+${point}) at index ${String(at.index)}`;
+}
+
 /** The first character `encoding` cannot carry, or undefined where it carries every one of them. */
 export function unencodable(message: string, encoding: EncodingName): Unencodable | undefined {
 	const codec = encodings[encoding];

src/defs/tlvs.ts

@@ -1,4 +1,5 @@
 import type { ParamValue, WireType } from './types.ts';
+import type { Result } from '../result.ts';
 import { tlv } from './types.ts';
 
 export type TlvDefinition = {
@@ -104,3 +105,60 @@ export type Tlv = {
 	tagName: string | undefined;
 	tagValue: ParamValue;
 };
+
+export type TlvInput = {
+	/** Resolved from the record key; pass it for a tag the TLV table does not define. */
+	tagId?: number | undefined;
+	tagValue: ParamValue;
+};
+
+export function tagIdOf(name: string, input: TlvInput): Result<{ tagId: number }> {
+	const tagId = input.tagId ?? tlvs[name]?.id;
+
+	if (tagId === undefined) {
+		return { err: new Error(`TLV "${name}": unknown tag name, give it a tagId`) };
+	}
+
+	if (!Number.isInteger(tagId) || tagId < 0 || tagId > 0xFFFF) {
+		return { err: new Error(`TLV "${name}": tagId ${String(tagId)} out of range 0-65535`) };
+	}
+
+	return { tagId };
+}
+
+/** Each TLV as its four octet header and the value the tag's own wire type writes. */
+export function writeTlvs(inputs: Record<string, TlvInput> | undefined): Result<{ chunks: Buffer[] }> {
+	const chunks: Buffer[] = [];
+
+	for (const [name, input] of Object.entries(inputs ?? {})) {
+		const tag = tagIdOf(name, input);
+
+		if (tag.err) return { err: tag.err };
+
+		const type = tlvsById[tag.tagId]?.type ?? tlvDefault;
+		const sized = type.size(input.tagValue);
+
+		if (sized.err) {
+			return { err: new Error(`TLV "${name}": ${sized.err.message}`) };
+		}
+
+		if (sized.size > 0xffff) {
+			return { err: new Error(`TLV "${name}": ${String(sized.size)} octets overflow the two octet length`) };
+		}
+
+		const chunk = Buffer.alloc(sized.size + 4);
+
+		chunk.writeUInt16BE(tag.tagId, 0);
+		chunk.writeUInt16BE(sized.size, 2);
+
+		const written = type.write(input.tagValue, chunk, 4);
+
+		if (written.err) {
+			return { err: new Error(`TLV "${name}": ${written.err.message}`) };
+		}
+
+		chunks.push(chunk);
+	}
+
+	return { chunks };
+}

src/message.ts

@@ -1,7 +1,7 @@
 import type { Result } from './result.ts';
 import type { EncodingName } from './defs/encodings.ts';
-import { detect, encodingByDataCoding, encodings } from './defs/encodings.ts';
+import { detect, encodingByDataCoding, encodings, unencodable, unencodableText } from './defs/encodings.ts';
-import { hasUdh } from './defs/constants.ts';
+import { consts, hasUdh } from './defs/constants.ts';
 import { udhLength } from './udh.ts';
 
 /** A single SMS carries 1120 bits, whatever the alphabet. */
@@ -27,6 +27,25 @@ export function encodeMessage(
 	return { buffer: encodings[resolved].encode(message), encoding: resolved };
 }
 
+/** A message body as octets, under the alphabet `dataCoding` resolves to, or one detected for it. */
+export function encodeBody(
+	text: string,
+	dataCoding: number | undefined,
+): Result<{ buffer: Buffer; dataCoding: number }> {
+	if (dataCoding === undefined) {
+		const detected = encodeMessage(text);
+
+		return { buffer: detected.buffer, dataCoding: consts.ENCODING[detected.encoding] };
+	}
+
+	const encoding = encodingByDataCoding(dataCoding);
+	const lost = unencodable(text, encoding);
+
+	return lost
+		? { err: new Error(`data_coding ${String(dataCoding)} resolves to ${encoding}, which cannot carry ${unencodableText(lost)}; pass a Buffer of octets, or a data_coding whose alphabet carries them`) }
+		: { buffer: encodings[encoding].encode(text), dataCoding };
+}
+
 export function decodeMessage(
 	buffer: Buffer,
 	dataCoding: number,

src/pdu.ts

@@ -3,15 +3,14 @@ import type { ErrorName } from './defs/errors.ts';
 import type { ParamValue } from './defs/types.ts';
 import type { PduHeader } from './pdu-refusal.ts';
 import type { Result, VoidResult } from './result.ts';
-import type { Tlv } from './defs/tlvs.ts';
+import type { Tlv, TlvInput } from './defs/tlvs.ts';
 import { PduRefusedError, framingRefusal } from './pdu-refusal.ts';
 import { cmds, commandNameById, respNameFor } from './defs/commands.ts';
-import { consts, hasUdh } from './defs/constants.ts';
+import { hasUdh } from './defs/constants.ts';
-import { decodeMessage, encodeMessage } from './message.ts';
+import { decodeMessage, encodeBody } from './message.ts';
-import { detect, encodingByDataCoding } from './defs/encodings.ts';
 import { errorNameById, errors, isErrorName } from './defs/errors.ts';
 import { paramNumber } from './defs/types.ts';
-import { tlvDefault, tlvs, tlvsById } from './defs/tlvs.ts';
+import { tagIdOf, tlvDefault, tlvs, tlvsById, writeTlvs } from './defs/tlvs.ts';
 
 /** The highest sequence number this library hands out; SMPP 3.4 4.7.1 reserves 0x7fffffff. */
 export const maxSeqNr = 2147483646;
@@ -19,12 +18,6 @@ export const maxSeqNr = 2147483646;
 /** What the field holds. Read and echoed in full, because peers do write above the spec's range. */
 const maxWireSeqNr = 0xFFFFFFFF;
 
-export type TlvInput = {
-	/** Resolved from the record key; pass it for a tag the TLV table does not define. */
-	tagId?: number | undefined;
-	tagValue: ParamValue;
-};
-
 export type PduObjectInput<C extends CommandName = CommandName> = {
 	cmdName: C;
 	cmdStatus?: ErrorName;
@@ -53,6 +46,8 @@ export type PduObject = {
 	tlvs: Record<string, Tlv>;
 };
 
+export type { TlvInput };
+
 const respBit = 0x80000000;
 
 export function isResp(pduObj: Pick<PduObject, 'cmdId'>): boolean {
@@ -71,44 +66,98 @@ export function isCommand<C extends CommandName>(
 	return pduObj.cmdName === cmdName;
 }
 
-function tagIdOf(name: string, input: TlvInput): Result<{ tagId: number }> {
+type ResolvedBody = {
-	const tagId = input.tagId ?? tlvs[name]?.id;
+	params: Record<string, ParamValue | undefined>;
-
+	tlvs: Record<string, TlvInput> | undefined;
-	if (tagId === undefined) {
-		return { err: new Error(`TLV "${name}": unknown tag name, give it a tagId`) };
-	}
-
-	if (!Number.isInteger(tagId) || tagId < 0 || tagId > 0xFFFF) {
-		return { err: new Error(`TLV "${name}": tagId ${String(tagId)} out of range 0-65535`) };
-	}
-
-	return { tagId };
-}
-
-/** Encoding a string short_message settles data_coding and sm_length; a buffer settles sm_length. */
-function resolveShortMessage(
-	params: Record<string, ParamValue | undefined>,
-): Record<string, ParamValue | undefined> {
-	const message = params.short_message;
-
-	if (Buffer.isBuffer(message)) {
-		return params.sm_length === undefined
-			? { ...params, sm_length: message.length }
-			: { ...params };
-	}
-
-	if (typeof message !== 'string') return { ...params };
-
-	const dataCoding = params.data_coding;
-	const encoding = typeof dataCoding === 'number' ? encodingByDataCoding(dataCoding) : detect(message);
-	const encoded = encodeMessage(message, encoding);
-
-	return {
-		...params,
-		data_coding: typeof dataCoding === 'number' ? dataCoding : consts.ENCODING[encoded.encoding],
-		short_message: encoded.buffer,
-		sm_length: encoded.buffer.length,
 };
+
+function codingOf(params: Record<string, ParamValue | undefined>): number | undefined {
+	return typeof params.data_coding === 'number' ? params.data_coding : undefined;
+}
+
+type CarriedBody = { name: string; text: string; tlv: TlvInput };
+
+/** Every entry carrying body text, under whatever names their tagIds are keyed to. */
+function carriedBodies(input: Record<string, TlvInput> | undefined): CarriedBody[] {
+	const carried: CarriedBody[] = [];
+
+	for (const [name, tlv] of Object.entries(input ?? {})) {
+		const tag = tagIdOf(name, tlv);
+
+		if (!tag.err && tag.tagId === tlvs.message_payload.id && typeof tlv.tagValue === 'string') {
+			carried.push({ name, text: tlv.tagValue, tlv });
+		}
+	}
+
+	return carried;
+}
+
+/** messageOctets() reads short_message wherever it holds an octet, and the TLV only where it does not. */
+function carriesOctets(value: ParamValue | undefined): boolean {
+	return Buffer.isBuffer(value) && value.length > 0;
+}
+
+/** The short_message the command's own table will write, since writeParams() ignores any other. */
+function writtenBody(definition: CommandDefinition, value: ParamValue | undefined): ParamValue | undefined {
+	return definition.params?.short_message === undefined ? undefined : value;
+}
+
+/** Encoded in place, settling data_coding where `settles` says no mandatory field will carry it. */
+function resolveCarried(
+	resolved: ResolvedBody,
+	inputs: Record<string, TlvInput> | undefined,
+	dataCoding: number | undefined,
+	settles: boolean,
+): VoidResult {
+	for (const carried of carriedBodies(inputs)) {
+		const encoded = encodeBody(carried.text, dataCoding);
+
+		if (encoded.err) return { err: new Error(`TLV "${carried.name}": ${encoded.err.message}`) };
+
+		if (settles) resolved.params.data_coding = encoded.dataCoding;
+
+		resolved.tlvs = { ...resolved.tlvs, [carried.name]: { ...carried.tlv, tagValue: encoded.buffer } };
+	}
+
+	return {};
+}
+
+/** data_coding names the alphabet of the body, and short_message settles it where it carries octets. */
+function resolveBody(
+	params: Record<string, ParamValue | undefined>,
+	tlvs: Record<string, TlvInput> | undefined,
+	definition: CommandDefinition,
+): Result<ResolvedBody> {
+	const message = writtenBody(definition, params.short_message);
+	const resolved: ResolvedBody = { params: { ...params }, tlvs };
+
+	if (Buffer.isBuffer(message) && params.sm_length === undefined) {
+		resolved.params.sm_length = message.length;
+	}
+
+	if (typeof message === 'string') {
+		const encoded = encodeBody(message, codingOf(params));
+
+		if (encoded.err) {
+			return { err: new Error(`Parameter "short_message" of "${definition.command}": ${encoded.err.message}`) };
+		}
+
+		if (carriesOctets(encoded.buffer)) resolved.params.data_coding = encoded.dataCoding;
+
+		resolved.params.short_message = encoded.buffer;
+		resolved.params.sm_length = encoded.buffer.length;
+	}
+
+	// Only octets the command's own table will write can settle the alphabet the PDU declares.
+	const settles = !carriesOctets(writtenBody(definition, resolved.params.short_message));
+	const carried = resolveCarried(
+		resolved,
+		tlvs,
+		settles ? codingOf(params) : codingOf(resolved.params),
+		settles,
+	);
+
+	return carried.err ? { err: carried.err } : resolved;
 }
 
 function writeParams(
@@ -139,42 +188,6 @@ function writeParams(
 	return { chunks };
 }
 
-function writeTlvs(tlvs: Record<string, TlvInput> | undefined): Result<{ chunks: Buffer[] }> {
-	const chunks: Buffer[] = [];
-
-	for (const [name, tlv] of Object.entries(tlvs ?? {})) {
-		const tag = tagIdOf(name, tlv);
-
-		if (tag.err) return { err: tag.err };
-
-		const type = tlvsById[tag.tagId]?.type ?? tlvDefault;
-		const sized = type.size(tlv.tagValue);
-
-		if (sized.err) {
-			return { err: new Error(`TLV "${name}": ${sized.err.message}`) };
-		}
-
-		if (sized.size > 0xffff) {
-			return { err: new Error(`TLV "${name}": ${String(sized.size)} octets overflow the two octet length`) };
-		}
-
-		const chunk = Buffer.alloc(sized.size + 4);
-
-		chunk.writeUInt16BE(tag.tagId, 0);
-		chunk.writeUInt16BE(sized.size, 2);
-
-		const written = type.write(tlv.tagValue, chunk, 4);
-
-		if (written.err) {
-			return { err: new Error(`TLV "${name}": ${written.err.message}`) };
-		}
-
-		chunks.push(chunk);
-	}
-
-	return { chunks };
-}
-
 /**
  * SMPP 3.4: a response reporting a failure carries no body, so its fields are not "unused but
  * present" — they are absent, and a peer that reads them anyway reads past the end of the PDU.
@@ -188,11 +201,15 @@ function buildBody(
 ): Result<{ body: Buffer }> {
 	if (errors[cmdStatus] !== 0 && definition.id >= respBit) return { body: Buffer.alloc(0) };
 
-	const written = writeParams(definition, resolveShortMessage(params), cmdName);
+	const body = resolveBody(params, tlvs, definition);
+
+	if (body.err) return { err: body.err };
+
+	const written = writeParams(definition, body.params, cmdName);
 
 	if (written.err) return { err: written.err };
 
-	const writtenTlvs = writeTlvs(tlvs);
+	const writtenTlvs = writeTlvs(body.tlvs);
 
 	if (writtenTlvs.err) return { err: writtenTlvs.err };
 

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 } from './defs/encodings.ts';
+import { 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';
@@ -168,9 +168,7 @@ function refusedEncoding(encoding: unknown): Error {
 }
 
 function refusedText(encoding: EncodingName, at: Unencodable): Error {
-	const point = (at.char.codePointAt(0) ?? 0).toString(16).toUpperCase().padStart(4, '0');
+	return new Error(`encoding ${encoding} cannot carry ${unencodableText(at)}; name UCS2 or leave encoding out`);
-
-	return new Error(`encoding ${encoding} cannot carry ${JSON.stringify(at.char)} (U+${point}) at index ${String(at.index)}; name UCS2 or leave encoding out`);
 }
 
 /** An alphabet the caller named has to carry the message; the one detect() picks always does. */

test/unsendable.test.ts

@@ -4,8 +4,9 @@ import type { PduObjectInput } from '../src/pdu.ts';
 import type { SendSmsDeps, SendSmsInput } from '../src/send-sms.ts';
 import { bindToSmsc, dummySmsc } from './dummy-smsc.ts';
 import { decodeMessage } from '../src/message.ts';
+import { messageOctets } from '../src/message-body.ts';
+import { objToPdu, pduToObj } from '../src/pdu.ts';
 import { paramNumber, paramText } from '../src/defs/types.ts';
-import { pduToObj } from '../src/pdu.ts';
 import { silentLog } from '../src/log.ts';
 import { submitSms } from '../src/send-sms.ts';
 
@@ -112,6 +113,238 @@ describe('an alphabet the caller named that cannot carry the message', () => {
 	});
 });
 
+describe('a body the PDU\'s own data_coding cannot carry', () => {
+	/** The octets a built PDU carries as its body, wherever the caller put them. */
+	function bodyOf(built: ReturnType<typeof objToPdu>): Buffer | undefined {
+		assert.equal(built.err, undefined);
+		assert.ok(built.buffer);
+
+		const { pduObj } = pduToObj(built.buffer);
+
+		assert.ok(pduObj);
+
+		return messageOctets(pduObj);
+	}
+
+	// Latin-1 takes the low octet of every code point, so あ (U+3042) went out as 0x42 — the letter B.
+	test('refuses a string short_message the named alphabet cannot carry, naming the character', () => {
+		const built = objToPdu({
+			cmdName: 'submit_sm',
+			params: { data_coding: 0x03, destination_addr: to, short_message: 'あいう', source_addr: from },
+		});
+
+		assert.ok(built.err instanceof Error);
+		assert.equal(built.buffer, undefined);
+		assert.match(built.err.message, /short_message/);
+		assert.match(built.err.message, /LATIN1/);
+		assert.match(built.err.message, /"あ"/);
+		assert.match(built.err.message, /U\+3042/);
+		assert.match(built.err.message, /index 0/);
+	});
+
+	// Å is in the GSM table at 0x0E; ï is the one the encoder flattened to a space.
+	test('refuses a string short_message GSM 03.38 has no code for, naming that one', () => {
+		for (const dataCoding of [0x00, 0x01]) {
+			const built = objToPdu({
+				cmdName: 'submit_sm',
+				params: { data_coding: dataCoding, destination_addr: to, short_message: 'Åsa naïve', source_addr: from },
+			});
+
+			assert.ok(built.err instanceof Error, String(dataCoding));
+			assert.match(built.err.message, /ASCII/);
+			assert.match(built.err.message, /"ï"/);
+			assert.match(built.err.message, /U\+00EF/);
+			assert.match(built.err.message, /index 6/);
+		}
+	});
+
+	test('refuses a string message_payload on the same terms as short_message', () => {
+		const built = objToPdu({
+			cmdName: 'data_sm',
+			params: { data_coding: 0x03, destination_addr: to, source_addr: from },
+			tlvs: { message_payload: { tagValue: 'あいう' } },
+		});
+
+		assert.ok(built.err instanceof Error);
+		assert.equal(built.buffer, undefined);
+		assert.match(built.err.message, /message_payload/);
+		assert.match(built.err.message, /LATIN1/);
+		assert.match(built.err.message, /"あ"/);
+		assert.match(built.err.message, /U\+3042/);
+	});
+
+	test('refuses the body TLV under whatever name the caller keyed its tagId to', () => {
+		const built = objToPdu({
+			cmdName: 'data_sm',
+			params: { data_coding: 0x03, destination_addr: to, source_addr: from },
+			tlvs: { body: { tagId: 0x0424, tagValue: 'あいう' } },
+		});
+
+		assert.ok(built.err instanceof Error);
+		assert.equal(built.buffer, undefined);
+		assert.match(built.err.message, /"body"/);
+		assert.match(built.err.message, /LATIN1/);
+		assert.match(built.err.message, /U\+3042/);
+	});
+
+	test('leaves data_coding to short_message wherever it carries octets, as messageOctets() reads it', () => {
+		const short = Buffer.from([0xDE, 0xAD]);
+		const built = objToPdu({
+			cmdName: 'deliver_sm',
+			params: { destination_addr: to, short_message: short, source_addr: from },
+			tlvs: { message_payload: { tagValue: 'あいう' } },
+		});
+
+		assert.equal(built.err, undefined);
+		assert.ok(built.buffer);
+
+		const { pduObj } = pduToObj(built.buffer);
+
+		assert.ok(pduObj);
+		assert.equal(pduObj.params.data_coding, 0, 'the TLV must not name an alphabet for octets nothing reads it as');
+		assert.deepEqual(messageOctets(pduObj), short);
+	});
+
+	test('encodes every entry carrying the body tag, so a second one cannot go out truncated', () => {
+		const built = objToPdu({
+			cmdName: 'data_sm',
+			params: { data_coding: 0x08, destination_addr: to, source_addr: from },
+			tlvs: { alias: { tagId: 0x0424, tagValue: 'あいう' }, message_payload: { tagValue: 'あいう' } },
+		});
+
+		assert.equal(built.err, undefined);
+		assert.ok(built.buffer);
+
+		const hex = built.buffer.toString('hex');
+
+		assert.equal(hex.split('304230443046').length - 1, 2, 'both entries carry the UCS2 octets');
+		assert.ok(!hex.includes('424446'), 'no entry goes out as the low octets of its code points');
+	});
+
+	test('leaves the alphabet to the body TLV wherever short_message carries no octets', () => {
+		for (const short of [undefined, '', Buffer.alloc(0)]) {
+			const built = objToPdu({
+				cmdName: 'deliver_sm',
+				params: {
+					destination_addr: to,
+					source_addr: from,
+					...(short === undefined ? {} : { short_message: short }),
+				},
+				tlvs: { message_payload: { tagValue: 'あいう' } },
+			});
+			const name = short === undefined ? 'absent' : JSON.stringify(short);
+
+			assert.equal(built.err, undefined, name);
+			assert.ok(built.buffer);
+
+			const { pduObj } = pduToObj(built.buffer);
+
+			assert.ok(pduObj);
+			assert.equal(pduObj.params.data_coding, 0x08, name);
+			assert.equal(messageOctets(pduObj)?.toString('hex'), '304230443046', name);
+		}
+	});
+
+	test('ignores a short_message on a command that declares none, which the wire never carries', () => {
+		for (const short of ['x', Buffer.from([0xDE, 0xAD])]) {
+			const input: PduObjectInput = {
+				cmdName: 'data_sm',
+				params: { destination_addr: to, short_message: short, source_addr: from },
+				tlvs: { message_payload: { tagValue: 'あいう' } },
+			};
+			const built = objToPdu(input);
+
+			assert.equal(built.err, undefined);
+			assert.ok(built.buffer);
+
+			const { pduObj } = pduToObj(built.buffer);
+
+			assert.ok(pduObj);
+			assert.equal(pduObj.params.data_coding, 0x08, JSON.stringify(short));
+			assert.equal(messageOctets(pduObj)?.toString('hex'), '304230443046');
+		}
+	});
+
+	test('leaves data_coding at its default where an empty short_message is the whole body', () => {
+		for (const short of ['', Buffer.alloc(0)]) {
+			const built = objToPdu({
+				cmdName: 'submit_sm',
+				params: { destination_addr: to, short_message: short, source_addr: from },
+			});
+
+			assert.equal(built.err, undefined);
+			assert.ok(built.buffer);
+
+			const { pduObj } = pduToObj(built.buffer);
+
+			assert.ok(pduObj);
+			assert.equal(pduObj.params.data_coding, 0, JSON.stringify(short));
+		}
+	});
+
+	test('never refuses a Buffer body, whatever the coding, because that is how binary is sent', () => {
+		const payload = Buffer.from([0x30, 0x42, 0xC3, 0x28, 0xFF, 0x00]);
+
+		for (const dataCoding of [0x00, 0x01, 0x03, 0x04, 0x08, 0xF0]) {
+			const params = { data_coding: dataCoding, destination_addr: to, source_addr: from };
+			const short = objToPdu({ cmdName: 'submit_sm', params: { ...params, short_message: payload } });
+			const carried = objToPdu({
+				cmdName: 'data_sm',
+				params,
+				tlvs: { message_payload: { tagValue: payload } },
+			});
+
+			assert.deepEqual(bodyOf(short), payload, `short_message under data_coding ${String(dataCoding)}`);
+			assert.deepEqual(bodyOf(carried), payload, `message_payload under data_coding ${String(dataCoding)}`);
+		}
+	});
+
+	test('never refuses a string the caller named no data_coding for, since detection always fits', () => {
+		for (const message of ['Hello world', 'Åsa naïve', 'あいう', '😀 beyond the basic plane']) {
+			const built = objToPdu({
+				cmdName: 'submit_sm',
+				params: { destination_addr: to, short_message: message, source_addr: from },
+			});
+
+			assert.equal(built.err, undefined, message);
+		}
+	});
+
+	test('writes a body the coding does carry as the octets that alphabet spells it in', () => {
+		const bodies: [number, string, string][] = [
+			[0x00, 'Hello world', '48656c6c6f20776f726c64'],
+			[0x03, 'Räksmörgås', '52e46b736df67267e573'],
+			[0x08, 'あいう', '304230443046'],
+		];
+
+		for (const [dataCoding, message, hex] of bodies) {
+			const params = { data_coding: dataCoding, destination_addr: to, source_addr: from };
+			const short = objToPdu({ cmdName: 'submit_sm', params: { ...params, short_message: message } });
+			const carried = objToPdu({
+				cmdName: 'data_sm',
+				params,
+				tlvs: { message_payload: { tagValue: message } },
+			});
+
+			assert.equal(bodyOf(short)?.toString('hex'), hex, `short_message: ${message}`);
+			assert.equal(bodyOf(carried)?.toString('hex'), hex, `message_payload: ${message}`);
+		}
+	});
+
+	test('refuses the same body through session.send(), with nothing reaching the socket', async t => {
+		const smsc = await dummySmsc(t);
+		const session = await bindToSmsc(t, smsc.port, { reconnect: false });
+		const sent = await session.send({
+			cmdName: 'submit_sm',
+			params: { data_coding: 0x03, destination_addr: to, short_message: 'あいう', source_addr: from },
+		});
+
+		assert.ok(sent.err instanceof Error);
+		assert.match(sent.err.message, /LATIN1/);
+		assert.deepEqual(smsc.octets, [], 'a body the named alphabet cannot carry never reaches the socket');
+	});
+});
+
 describe('a time no peer can read', () => {
 	const invalid = new Date('nope');
 

todo.md

@@ -120,11 +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.
-- [ ] **The interop suite still asserts the defect [#95](https://github.com/larvit/larvitsmpp/pull/95)
-      fixed.** `interop-tests/smppsim.test.ts:621`, `a raw submit_sm with data_coding 0xF0 is not
-      read as flash`, fails on the next `./interop-tests/run.py smppsim`; nothing in CI runs that
-      suite, so it fails silently until someone does. The test name and its assertion both need
-      inverting, and the C17 row in `findings/02-smppsim.md` names that sub-test, so it follows.
 - [ ] Tag `v1.0.0` to publish.
 - [ ] `npm deprecate larvitsmpp` pointing at `@larvit/smpp`. Maintainer's call to run it; not
       something CI should do.
@@ -141,16 +136,20 @@ session message is a change to every call site.
 
 ## Worth doing, not blocking
 
-- [ ] **`objToPdu()` corrupts a string body the same way `sendSms()` used to.**
+- [ ] **A send the codec will refuse waits for a link and a window slot first.** `refuse()` in
-      `resolveShortMessage()` in `pdu.ts` picks the codec off the caller's own `data_coding` and
+      `outgoing-requests.ts` runs `misuse()` and the abort check before the wait, precisely so a call
-      encodes a string `short_message` with it, so
+      that can never go out does not queue for what it will never use; a body `objToPdu()` refuses on
-      `objToPdu({ params: { data_coding: 3, short_message: 'あいう' } })` returns `42 44 46` —
+      every attempt is the same case, and #98 made it a common one. On a down link the caller waits
-      `"BDF"` — with no error, and `data_coding` 0 flattens to spaces. Same goal 2 defect as the one
+      `responseTimeout` and is told the link failed rather than that the body could not be built —
-      the `unencodable()` guard closed at `sendSms()`, one layer down and on the published codec. The
+      goal 2's wrong answer about what happened. The cheap fix builds the PDU twice, so the shape is
-      guard is the same call on the branch where `data_coding` is a number and the body is a string;
+      the open half. Raised by the architecture review of
-      the `detect()` branch needs none. Held back only because a refusal there may fail
+      [#98](https://github.com/larvit/larvitsmpp/pull/98), 2026-09-09.
-      `interop-tests/`, which is being edited elsewhere and cannot be verified from here. Raised by
+
-      the architecture review of [#97](https://github.com/larvit/larvitsmpp/pull/97), 2026-09-09.
+- [ ] **`message.ts` answers two questions.** Message coding and the SMPP time format (`smppDate`,
+      `smppTime`) share the file, which the architecture map in AGENTS.md already spells out as four
+      concerns. Nothing is wrong today; if the file has to move for another reason, `smpp-time.ts` is
+      the split. Raised by the architecture review of
+      [#98](https://github.com/larvit/larvitsmpp/pull/98), 2026-09-09.
 
 - [ ] **A gate that refuses a floating version anywhere in the repo.** Maintainer's ask on
       [#71](https://github.com/larvit/larvitsmpp/pull/71), 2026-09-06, on the `release.yaml`