Hold a send with no link for the next one, and count the rest as unanswered

AGENTS.md

@@ -45,6 +45,7 @@ src/
 	error-from.ts        errorFrom(): whatever was thrown or rejected, as an Error
 	expiring-groups.ts   ExpiringGroups: the capped, expiring store both of those share
 	incoming-requests.ts Every request the peer sends: messages, receipts, links, unknown commands
+	link-gate.ts         LinkGate: where a request with no link to go out on waits for the next one
 	link-timers.ts       LinkTimers: the enquire_link heartbeat and the idle timeout
 	log.ts               SmppLog, the logger contract, and silentLog — the default
 	message.ts           Encoding detection, splitting, bit counting, SMPP date formatting
@@ -178,8 +179,8 @@ exactly 140.
 - **The published surface is frozen at what `src/index.ts` exports today.** `Session` is exported and
   publicly constructible, which is why `SessionOptions` and `ReconnectOptions` are public too — that
   is correct, not a leak, and it has been raised twice. The collaborators `session.ts` delegates to
-  (`Reassembler`, `PendingRequests`, `SendWindow`, `ReconnectLoop`, `LinkTimers`, `DlrMerger`,
-  `submitSms`) stay unpublished so they can be reshaped.
+  (`Reassembler`, `PendingRequests`, `SendWindow`, `ReconnectLoop`, `LinkTimers`, `LinkGate`,
+  `DlrMerger`, `submitSms`) stay unpublished so they can be reshaped.
 - **The sub-3.4 optional-parameter rule is a predicate, not a chokepoint.** `acceptsOptionalParams()`
   is consulted by the library's own senders; `session.send({ tlvs })` is passed through as written,
   because silently stripping a caller's explicit TLVs off a deliberately public low-level surface
@@ -345,3 +346,17 @@ exactly 140.
   as no number and so reaches `expect()` and `collect()` unchanged. Normalising the base instead
   would break that pair. The option is on `client()` only — a `server()` session generates its own
   ids and writes its own receipts, so both places are already one notation.
+
+- **A send waits for the next link only if it never reached the socket; one that did is counted, not
+  resent.** Maintainer's call, 2026-09-01: re-queueing everything unanswered would resend a
+  `submit_sm` the SMSC accepted and answered into a dead socket, which is delivered and billed
+  twice, while a request that never left this process can be lost for free. `LinkGate` holds a send
+  that has no link and `comeBackUp()` opens it once the rebind is bound, so a send issued between
+  links and a segment still queued behind a full window when the drop hit both go out on the new
+  one. The hold is bounded by `responseTimeout` rather than an option of its own — that is already
+  the answer to how long one request may take — so the worst case is twice it: the hold, then the
+  answer. It happens before `window.acquire()` rather than inside it, because the rebind's own
+  `bind()` goes through `session.send()` and would deadlock behind slots held by waiting sends.
+  `teardown()` settles what was on the wire with `UnansweredError`, which `collectSent()` counts
+  into `SendSmsResult.unanswered`: `smsIds` alone cannot tell a message that never left from one
+  whose every segment the peer took and answered into a socket that was already gone.

README.md

@@ -100,7 +100,7 @@ Every one is optional.
 | `tls` | `false` | `true` for defaults, or a `tls.ConnectionOptions` object for a private CA or a client certificate. |
 | `enquireLinkInterval` | `20000` | How often to send `enquire_link` on a quiet link. |
 | `idleTimeout` | `2 × enquireLinkInterval` | Give up on a link the peer has stopped answering, and re-bind unless `reconnect` is `false`. |
-| `responseTimeout` | `30000` | How long to wait for a response before giving up on it; `0` waits forever. |
+| `responseTimeout` | `30000` | How long to wait for a response before giving up on it, and how long a send with no link waits for the next one; `0` waits forever. |
 | `shutdownTimeout` | `5000` | How long `close()` and `unbind()` wait for the requests this end already sent; `0` waits forever. |
 | `maxOutstanding` | `10` | Requests allowed on the wire at once; further sends queue. |
 | `smsIdFormat` | — | The notation the SMSC writes message ids in, per place it writes them: `{ receipt: 'decimal', submitResp: 'hex' }`. Only needed where the two disagree. |
@@ -135,13 +135,16 @@ Messages too long for one SMS are split automatically and sent as a concatenated
 one id per segment:
 
 ```javascript
-const { err, pduObjs, smsIds } = await session.sendSms({ from, message, to });
+const { err, pduObjs, smsIds, unanswered } = await session.sendSms({ from, message, to });
 ```
 
 `err` is set when the SMSC refuses a segment, and it names the status it refused with. Because every
 segment goes on the wire together, `pduObjs` and `smsIds` then hold what the SMSC did accept — enough
 to reconcile against a later receipt, not enough to resend the rest, so treat a partial failure as a
-failed message. A message needing more than 255 segments is refused before anything is sent, since
+failed message. `unanswered` counts the segments the link dropped under: the SMSC may have taken
+each of them and lost only the response, so a message with `unanswered` above zero cannot be sent
+again without risking a duplicate, however empty `smsIds` is. A message needing more than 255
+segments is refused before anything is sent, since
 the concatenation header numbers segments in a single octet. `maxSegments` lowers that ceiling:
 most handsets and SMSCs stop well short of 255, and refusing beats a message only half delivered.
 
@@ -341,6 +344,12 @@ const { err, pduObj } = await session.send({
 });
 ```
 
+A send issued while the link is down waits for the reconnect instead of failing, and goes out on the
+new link once it is bound — up to `responseTimeout`, after which it gives up having sent nothing. A
+request already on the wire when the link drops is the other case: the SMSC may have taken it and
+lost only the response, so it fails, and `sendSms()` counts it in `unanswered`. Neither happens with
+`reconnect: false`, where a drop ends the session and every send after it is refused.
+
 `acceptsOptionalParams()` answers whether the peer declared SMPP 3.4 or later, which is the version
 at and above which the spec allows optional parameters to be sent to it; `peerInterfaceVersion` is
 the version it declared, `0x00` if it declared none. The library's own senders consult the first before attaching a TLV — a

src/link-gate.ts

@@ -0,0 +1,85 @@
+import type { VoidResult } from './result.ts';
+
+export type LinkGateOptions = {
+	/** Whether the link can carry nothing right now. */
+	isDown: () => boolean;
+	/** How long a request may wait for a link. 0 waits for as long as one may still arrive. */
+	timeout: number;
+	/** Whether a link that is down will be brought back. */
+	willReturn: () => boolean;
+};
+
+type Waiter = (result: VoidResult) => void;
+
+function expired(): Error {
+	return new Error('The link did not come back in time');
+}
+
+/** Where a request with no link to go out on waits for the next one. */
+export class LinkGate {
+	private readonly options: LinkGateOptions;
+	private readonly waiting = new Set<Waiter>();
+
+	constructor(options: LinkGateOptions) {
+		this.options = options;
+	}
+
+	/** When a hold starting now has to give up. 0 never does. */
+	deadline(): number {
+		return this.options.timeout > 0 ? Date.now() + this.options.timeout : 0;
+	}
+
+	/** Resolves once a link can carry the request, or with the reason none ever will. */
+	wait(deadline: number, signal: AbortSignal | undefined): Promise<VoidResult> {
+		if (!this.options.isDown()) return Promise.resolve({});
+
+		if (!this.options.willReturn()) return Promise.resolve({ err: new Error('Session is closed') });
+
+		const left = deadline === 0 ? 0 : deadline - Date.now();
+
+		if (deadline !== 0 && left <= 0) return Promise.resolve({ err: expired() });
+
+		return this.hold(left, signal);
+	}
+
+	/** A link is up: everything held goes out on it. */
+	open(): void {
+		this.release({});
+	}
+
+	/** No link is coming, and this is why. */
+	shut(err: Error): void {
+		this.release({ err });
+	}
+
+	private hold(left: number, signal: AbortSignal | undefined): Promise<VoidResult> {
+		return new Promise<VoidResult>(resolve => {
+			let timer: NodeJS.Timeout | undefined = undefined;
+			const settle = (result: VoidResult): void => {
+				if (timer) clearTimeout(timer);
+
+				signal?.removeEventListener('abort', onAbort);
+				this.waiting.delete(settle);
+				resolve(result);
+			};
+
+			function onAbort(): void {
+				settle({ err: new Error('Aborted while waiting for a link') });
+			}
+
+			if (left > 0) {
+				timer = setTimeout(() => { settle({ err: expired() }); }, left);
+				timer.unref();
+			}
+
+			signal?.addEventListener('abort', onAbort, { once: true });
+			this.waiting.add(settle);
+		});
+	}
+
+	private release(result: VoidResult): void {
+		for (const settle of [...this.waiting]) {
+			settle(result);
+		}
+	}
+}

src/pending-requests.ts

@@ -12,6 +12,14 @@ type Pending = {
 	settle: (result: Result<{ pduObj: PduObject }>) => void;
 };
 
+/** The request went out and the link died before an answer: the peer may have accepted it. */
+export class UnansweredError extends Error {
+	constructor() {
+		super('The link dropped after the request went out; the peer may have accepted it');
+		this.name = 'UnansweredError';
+	}
+}
+
 /** Hands out sequence numbers and matches responses to the requests waiting for them. */
 export class PendingRequests {
 	private readonly log: SmppLog;
@@ -69,9 +77,10 @@ export class PendingRequests {
 		this.pending.get(seqNr)?.settle(result);
 	}
 
-	settleAll(err: Error): void {
+	/** Everything still on the wire when the link died, each one possibly accepted by the peer. */
+	settleAll(): void {
 		for (const [seqNr] of this.pending) {
-			this.settle(seqNr, { err });
+			this.settle(seqNr, { err: new UnansweredError() });
 		}
 	}
 

src/send-sms.ts

@@ -4,6 +4,7 @@ import type { PduObject, PduObjectInput } from './pdu.ts';
 import type { Result } from './result.ts';
 import type { SmppLog } from './log.ts';
 import type { SmsIdNotation } from './sms-id.ts';
+import { UnansweredError } from './pending-requests.ts';
 import { consts } from './defs/constants.ts';
 import { detect } from './defs/encodings.ts';
 import { normaliseSmsId } from './sms-id.ts';
@@ -28,7 +29,13 @@ export type SendSmsOptions = {
 };
 
 /** Both arrays hold what the peer accepted, so a partial failure names what is already delivered. */
-export type SendSmsResult = { err?: Error; pduObjs: PduObject[]; smsIds: string[] };
+export type SendSmsResult = {
+	err?: Error;
+	pduObjs: PduObject[];
+	smsIds: string[];
+	/** Segments the link dropped under. The peer may have taken them, so sending again may duplicate. */
+	unanswered: number;
+};
 
 /** What sending needs from the session: a concat reference and a way onto the wire. */
 export type SendSmsDeps = {
@@ -107,9 +114,12 @@ function collectSent(
 	const pduObjs: PduObject[] = [];
 	const smsIds: string[] = [];
 	let failure: Error | undefined;
+	let unanswered = 0;
 
 	for (const one of sent) {
 		if (one.err) {
+			if (one.err instanceof UnansweredError) unanswered++;
+
 			failure ??= one.err;
 		} else if (one.pduObj.cmdStatus === 'ESME_ROK') {
 			pduObjs.push(one.pduObj);
@@ -121,7 +131,7 @@ function collectSent(
 		}
 	}
 
-	return failure ? { err: failure, pduObjs, smsIds } : { pduObjs, smsIds };
+	return failure ? { err: failure, pduObjs, smsIds, unanswered } : { pduObjs, smsIds, unanswered };
 }
 
 /** Puts a message on the wire as one submit_sm per segment. */
@@ -131,7 +141,7 @@ export async function submitSms(deps: SendSmsDeps, sms: SendSmsOptions): Promise
 	const segments = splitMessage(sms.message, { encoding, reference: deps.reference });
 	const refused = checkSegments(allowed, segments.length);
 
-	if (refused) return { err: refused, pduObjs: [], smsIds: [] };
+	if (refused) return { err: refused, pduObjs: [], smsIds: [], unanswered: 0 };
 
 	const multipart = segments.length > 1;
 

src/session.ts

@@ -10,6 +10,7 @@ import type { Socket } from 'node:net';
 import { DlrMerger } from './dlr-merger.ts';
 import { EventEmitter } from 'node:events';
 import { IncomingRequests } from './incoming-requests.ts';
+import { LinkGate } from './link-gate.ts';
 import { LinkTimers } from './link-timers.ts';
 import { PduTransport } from './pdu-transport.ts';
 import { PendingRequests } from './pending-requests.ts';
@@ -38,6 +39,9 @@ export { bindCommands, defaultSystemId };
 /** A listener may return a promise: an `async` one that rejects is routed like one that throws. */
 type SessionListener<K extends keyof SessionEvents> = (...args: SessionEvents[K]) => unknown;
 
+/** `unsent` means nothing reached the socket, so the next link can still carry this request. */
+type Attempt = { result: Result<{ pduObj: PduObject }>; unsent: boolean };
+
 export class Session extends EventEmitter<SessionEvents> {
 	declare addListener: <K extends keyof SessionEvents>(event: K, listener: SessionListener<K>) => this;
 	declare off: <K extends keyof SessionEvents>(event: K, listener: SessionListener<K>) => this;
@@ -57,6 +61,7 @@ export class Session extends EventEmitter<SessionEvents> {
 	userData: unknown = undefined;
 
 	private readonly dlrMerger: DlrMerger;
+	private readonly gate: LinkGate;
 	private readonly incoming: IncomingRequests;
 	private readonly options: SessionOptions;
 	private readonly pending: PendingRequests;
@@ -112,6 +117,11 @@ export class Session extends EventEmitter<SessionEvents> {
 			max: defaults.maxDlrMerges,
 			timeout: defaults.dlrMergeTimeout,
 		});
+		this.gate = new LinkGate({
+			isDown: () => this.linkDown(),
+			timeout: options.responseTimeout ?? defaults.responseTimeout,
+			willReturn: () => this.retrying(),
+		});
 		this.incoming = new IncomingRequests({
 			dlrMerger: this.dlrMerger,
 			log: this.log,
@@ -160,28 +170,46 @@ export class Session extends EventEmitter<SessionEvents> {
 		input: PduObjectInput,
 		options: SendOptions = {},
 	): Promise<Result<{ pduObj: PduObject }>> {
-		if (input.cmdName.endsWith('_resp')) {
-			return { err: new Error(`Use sendReturn() for responses, not send(): ${input.cmdName}`) };
-		}
+		const refused = this.refuseSend(input, options);
 
-		if (this.closed) return { err: new Error('Session is closed') };
+		if (refused) return { err: refused };
 
-		if (this.draining) return { err: new Error('Session is shutting down') };
+		const deadline = this.gate.deadline();
 
-		// Before the window, or a full window makes an aborted call wait for a slot it will not use.
-		if (options.signal?.aborted === true) {
-			return { err: new Error('Aborted before the request was sent') };
-		}
+		for (;;) {
+			const held = await this.gate.wait(deadline, options.signal);
+
+			if (held.err) return { err: held.err };
 
 			await this.window.acquire();
 
-		try {
-			return await this.request(input, options);
-		} finally {
-			this.window.release();
+			const attempt = await this.attempt(input, options).finally(() => { this.window.release(); });
+
+			// Nothing reached the socket, so the next link carries it instead of the caller resending.
+			if (!attempt.unsent || !this.linkDown() || !this.retrying()) return attempt.result;
 		}
 	}
 
+	/** Why a request cannot go out at all, as opposed to not yet. */
+	private refuseSend(input: PduObjectInput, options: SendOptions): Error | undefined {
+		if (input.cmdName.endsWith('_resp')) {
+			return new Error(`Use sendReturn() for responses, not send(): ${input.cmdName}`);
+		}
+
+		// A drain on a live link. A link that is down is the gate's answer, which says closed instead.
+		if (this.draining && !this.linkDown()) return new Error('Session is shutting down');
+
+		// Before the gate and the window, or an aborted call waits for a link it will not use.
+		if (options.signal?.aborted === true) return new Error('Aborted before the request was sent');
+
+		return undefined;
+	}
+
+	/** Read through a method: a drop can land while a send is awaiting. */
+	private linkDown(): boolean {
+		return this.closed || this.sock.destroyed;
+	}
+
 	/** Answers a request the peer sent us. Responses are never waited on. */
 	async sendReturn(
 		pdu: PduObject,
@@ -207,7 +235,12 @@ export class Session extends EventEmitter<SessionEvents> {
 
 	async sendSms(sms: SendSmsOptions, options: SendOptions = {}): Promise<SendSmsResult> {
 		if (!this.bindAllows('submit_sm')) {
-			return { err: new Error('A receiver-bound session does not carry submit_sm'), pduObjs: [], smsIds: [] };
+			return {
+				err: new Error('A receiver-bound session does not carry submit_sm'),
+				pduObjs: [],
+				smsIds: [],
+				unanswered: 0,
+			};
 		}
 
 		const sent = await submitSms({
@@ -229,9 +262,9 @@ export class Session extends EventEmitter<SessionEvents> {
 	async unbind(): Promise<VoidResult> {
 		const drained = await this.drain(undefined);
 		const wasOpen = !this.closed;
-		// request(), not send(): the drain gate refuses a send, and the unbind goes out either way.
+		// attempt(), not send(): the drain gate refuses a send, and the unbind goes out either way.
 		const sent = wasOpen
-			? await this.request({ cmdName: 'unbind' }, {})
+			? (await this.attempt({ cmdName: 'unbind' }, {})).result
 			: { err: new Error('Session is closed') };
 		const closedOnUnbind = wasOpen && this.closed;
 
@@ -303,6 +336,7 @@ export class Session extends EventEmitter<SessionEvents> {
 		this.resetTimers();
 		this.log.info('session - reconnected');
 		this.emit('reconnected');
+		this.gate.open();
 
 		return {};
 	}
@@ -312,19 +346,16 @@ export class Session extends EventEmitter<SessionEvents> {
 		this.closed = false;
 	}
 
-	private async request(
-		input: PduObjectInput,
-		options: SendOptions,
-	): Promise<Result<{ pduObj: PduObject }>> {
+	private async attempt(input: PduObjectInput, options: SendOptions): Promise<Attempt> {
 		// pending.wait() alone settles the caller while the request still goes out to the peer.
 		if (options.signal?.aborted === true) {
-			return { err: new Error('Aborted before the request was sent') };
+			return { result: { err: new Error('Aborted before the request was sent') }, unsent: false };
 		}
 
 		const seqNr = this.pending.nextSeqNr();
 		const built = objToPdu({ ...input, seqNr });
 
-		if (built.err) return { err: built.err };
+		if (built.err) return { result: { err: built.err }, unsent: false };
 
 		const response = this.pending.wait(seqNr, {
 			signal: options.signal,
@@ -335,10 +366,10 @@ export class Session extends EventEmitter<SessionEvents> {
 		if (written.err) {
 			this.pending.settle(seqNr, { err: written.err });
 
-			return { err: written.err };
+			return { result: { err: written.err }, unsent: true };
 		}
 
-		return response;
+		return { result: await response, unsent: false };
 	}
 
 	/** Stops new sends and waits out the ones already issued. */
@@ -346,13 +377,13 @@ export class Session extends EventEmitter<SessionEvents> {
 		this.reconnectLoop?.stop();
 		this.draining = true;
 
-		if (this.closed || this.sock.destroyed) return {};
+		if (this.linkDown()) return {};
 
 		const timeout = this.options.shutdownTimeout ?? defaults.shutdownTimeout;
 		const unfinished = await this.window.idle(timeout, signal);
 
 		// The window empties on a teardown too, which settles everything the link was carrying.
-		if (this.isClosed()) return { err: new Error('The session closed before the drain finished') };
+		if (this.linkDown()) return { err: new Error('The session closed before the drain finished') };
 
 		if (unfinished === 0) return {};
 
@@ -361,11 +392,6 @@ export class Session extends EventEmitter<SessionEvents> {
 		return { err: new Error(`Shut down with ${String(unfinished)} request(s) unfinished`) };
 	}
 
-	/** Read through a method: teardown() can land while the drain is awaiting. */
-	private isClosed(): boolean {
-		return this.closed;
-	}
-
 	/** The session is over now, drained or not. Nothing brings it back. */
 	private end(): void {
 		this.reconnectLoop?.stop();
@@ -378,6 +404,7 @@ export class Session extends EventEmitter<SessionEvents> {
 		if (this.ended) return;
 
 		this.ended = true;
+		this.gate.shut(new Error('Session closed before the link came back'));
 		this.emit('close');
 	}
 
@@ -386,7 +413,7 @@ export class Session extends EventEmitter<SessionEvents> {
 
 		this.closed = true;
 		this.timers.clear();
-		this.pending.settleAll(new Error('Session closed before a response arrived'));
+		this.pending.settleAll();
 		this.incoming.clear();
 		this.sock.destroy();
 

test/readme.test.ts

@@ -83,7 +83,7 @@ describe('README: Client', () => {
 		closeAfter(t, session);
 
 		const reported = once<Dlr>(resolve => { session.on('dlr', resolve); });
-		const { err: sendErr, smsIds } = await session.sendSms({
+		const { err: sendErr, smsIds, unanswered } = await session.sendSms({
 			dlr: true,
 			from: '46701113311',
 			message: '«baff»',
@@ -92,6 +92,7 @@ describe('README: Client', () => {
 
 		assert.equal(sendErr, undefined);
 		assert.equal(smsIds.length, 1);
+		assert.equal(unanswered, 0);
 		assert.equal((await reported).smsId, smsIds[0]);
 	});
 

test/session-extras.test.ts

@@ -557,6 +557,182 @@ describe('reconnect', () => {
 	});
 });
 
+describe('sends across a reconnect', () => {
+	/** Answers every message after the first, which is left to hold the send window open. */
+	function answerAfterTheFirst(smpp: SmppServer, arrived: string[]): Gate {
+		const first = gate();
+
+		smpp.on('session', peer => {
+			peer.on('sms', async sms => {
+				arrived.push(sms.message);
+
+				if (arrived.length === 1) first.open();
+				else await sms.sendResp();
+			});
+		});
+
+		return first;
+	}
+
+	test('holds a send issued while the link is down and puts it on the new link', async t => {
+		const smpp = await startServer(t);
+		const arrived: string[] = [];
+
+		smpp.on('session', peer => { peer.on('sms', async sms => { arrived.push(sms.message); await sms.sendResp(); }); });
+
+		const { session } = await connect(t, smpp, { reconnect: { maxDelay: 100, minDelay: 20 } });
+
+		assert.ok(session);
+
+		const down = once<true>(resolve => { session.on('disconnected', () => { resolve(true); }); });
+
+		await peerOf(smpp).close();
+		await down;
+
+		const sent = await session.sendSms({ from: '46701113311', message: 'held', to: '46709771337' });
+
+		assert.equal(sent.err, undefined);
+		assert.equal(sent.smsIds.length, 1);
+		assert.equal(sent.unanswered, 0);
+		assert.deepEqual(arrived, ['held']);
+	});
+
+	test('puts a segment still queued behind a full window on the new link', async t => {
+		const smpp = await startServer(t);
+		const arrived: string[] = [];
+		const first = answerAfterTheFirst(smpp, arrived);
+		const { session } = await connect(t, smpp, {
+			maxOutstanding: 1,
+			reconnect: { maxDelay: 100, minDelay: 20 },
+		});
+
+		assert.ok(session);
+
+		const holding = session.sendSms({ from: '46701113311', message: 'first', to: '46709771337' });
+
+		await first.passed;
+
+		const queued = session.sendSms({ from: '46701113311', message: 'second', to: '46709771337' });
+
+		peerOf(smpp).sock.destroy();
+
+		const [dropped, resent] = await Promise.all([holding, queued]);
+
+		assert.equal(dropped.unanswered, 1);
+		assert.equal(resent.err, undefined, 'a request that never reached the socket is not lost with it');
+		assert.equal(resent.smsIds.length, 1);
+		assert.deepEqual(arrived, ['first', 'second']);
+	});
+
+	test('reports a segment the link dropped under as unanswered, not as never sent', async t => {
+		const smpp = await startServer(t);
+		const arrived = once<Sms>(resolve => { smpp.on('session', peer => peer.on('sms', resolve)); });
+		const { session } = await connect(t, smpp, { reconnect: false });
+
+		assert.ok(session);
+
+		const sending = session.sendSms({ from: '46701113311', message: 'in flight', to: '46709771337' });
+
+		await arrived;
+		peerOf(smpp).sock.destroy();
+
+		const sent = await sending;
+
+		assert.match(sent.err?.message ?? '', /may have accepted/);
+		assert.equal(sent.unanswered, 1, 'the peer may have accepted it, so sending it again would duplicate');
+		assert.deepEqual(sent.smsIds, []);
+	});
+
+	test('gives up a held send after responseTimeout, with nothing put on the wire', async t => {
+		const smpp = await startServer(t);
+		const { session } = await connect(t, smpp, {
+			reconnect: { maxDelay: 10_000, minDelay: 10_000 },
+			responseTimeout: 60,
+		});
+
+		assert.ok(session);
+
+		const down = once<true>(resolve => { session.on('disconnected', () => { resolve(true); }); });
+
+		await peerOf(smpp).close();
+		await down;
+
+		const sent = await session.sendSms({ from: '46701113311', message: 'held', to: '46709771337' });
+
+		assert.match(sent.err?.message ?? '', /did not come back/);
+		assert.equal(sent.unanswered, 0, 'nothing reached the peer, so the message can be sent again');
+	});
+
+	test('fails a held send when the session closes rather than leaving it waiting', async t => {
+		const smpp = await startServer(t);
+		const { session } = await connect(t, smpp, {
+			reconnect: { maxDelay: 10_000, minDelay: 10_000 },
+		});
+
+		assert.ok(session);
+
+		const down = once<true>(resolve => { session.on('disconnected', () => { resolve(true); }); });
+
+		await peerOf(smpp).close();
+		await down;
+
+		const sending = session.sendSms({ from: '46701113311', message: 'held', to: '46709771337' });
+		let settled = false;
+
+		void sending.then(() => { settled = true; });
+		await delay(30);
+
+		assert.equal(settled, false, 'the send waits for a link rather than failing on the spot');
+
+		await session.close();
+
+		const sent = await sending;
+
+		assert.match(sent.err?.message ?? '', /closed/);
+		assert.equal(sent.unanswered, 0);
+	});
+
+	test('aborts a held send instead of making it wait out the link', async t => {
+		const smpp = await startServer(t);
+		const { session } = await connect(t, smpp, {
+			reconnect: { maxDelay: 10_000, minDelay: 10_000 },
+		});
+
+		assert.ok(session);
+
+		const down = once<true>(resolve => { session.on('disconnected', () => { resolve(true); }); });
+
+		await peerOf(smpp).close();
+		await down;
+
+		const controller = new AbortController();
+		const sending = session.sendSms(
+			{ from: '46701113311', message: 'held', to: '46709771337' },
+			{ signal: controller.signal },
+		);
+
+		controller.abort();
+
+		const sent = await sending;
+
+		assert.match(sent.err?.message ?? '', /Aborted while waiting for a link/);
+		assert.equal(sent.unanswered, 0);
+	});
+
+	test('refuses a send outright once the session is over', async t => {
+		const smpp = await startServer(t);
+		const { session } = await connect(t, smpp, { reconnect: false });
+
+		assert.ok(session);
+		await session.close();
+
+		const sent = await session.sendSms({ from: '46701113311', message: 'too late', to: '46709771337' });
+
+		assert.match(sent.err?.message ?? '', /closed/);
+		assert.equal(sent.unanswered, 0);
+	});
+});
+
 describe('reassembly bounds', () => {
 	function segment(reference: number, part: number, total: number): PduObject {
 		const udh = Buffer.from([0x05, 0x00, 0x03, reference, total, part]);
@@ -789,7 +965,8 @@ describe('graceful shutdown', () => {
 		const result = await sent;
 
 		assert.ok(result.err instanceof Error);
-		assert.equal(result.err.message, 'Session closed before a response arrived');
+		assert.match(result.err.message, /may have accepted/);
+		assert.equal(result.unanswered, 1);
 	});
 
 	// The window empties on a drop as well as on an answer, so it cannot be what the result reads.

test/session.test.ts

@@ -236,7 +236,7 @@ describe('bind', () => {
 		const sent = await session.send({ cmdName: 'enquire_link' });
 
 		assert.ok(sent.err instanceof Error);
-		assert.equal(sent.err.message, 'Session closed before a response arrived');
+		assert.match(sent.err.message, /may have accepted/);
 	});
 
 	test('reports an unbind the peer left unanswered on a link that stays up', async t => {

todo.md

@@ -57,6 +57,7 @@ Rules the API follows:
 | Merged multipart DLRs including across a reconnect, reassembly bounds, per-send abort, the segment cap | `test/session-extras.test.ts` |
 | `smsIdFormat`: a peer's `submit_sm_resp` and receipt ids read into one notation before they are compared | `test/dlr.test.ts`, `test/session-extras.test.ts` |
 | A draining `close()` and `unbind()`, bounded by `shutdownTimeout` or an abort | `test/session-extras.test.ts` |
+| A send with no link held for the next one, and one the link dropped under counted as `unanswered` | `test/session-extras.test.ts` |
 | Every runnable README example | `test/readme.test.ts` |
 | Receipt-versus-message classification by `esm_class` | `test/dlr.test.ts`, `test/session.test.ts` |
 | A listener that throws, or rejects, reaching `sessionError`/`serverError` rather than the process | `test/session.test.ts`, `test/error-from.test.ts` |
@@ -113,9 +114,6 @@ session message is a change to every call site.
 
 ## Worth doing, not blocking
 
-- [ ] **In-flight sends across a reconnect.** They currently fail with "Session closed before a
-      response arrived" and the caller retries. Re-queueing them automatically would be friendlier
-      but risks duplicate delivery, so it needs a decision before it is built.
 - [ ] **The drain covers only what this end sent.** `close()` and `unbind()` wait on the send window,
       which `sendReturn()` never enters, so a server session tears down without waiting for the
       application to answer the messages it is holding — the duplicate-on-retry outcome again, in
@@ -126,9 +124,12 @@ session message is a change to every call site.
       answered. Surviving one means exposing the merge state for the application to persist and hand
       back, which is a public-surface decision.
 - [ ] **Group the session's collaborators under `src/session/`.** Only `session.ts` imports
-      `reassembly`, `dlr-merger`, `send-window`, `link-timers`, `reconnect-loop`, `pending-requests`
-      and `send-sms`, so the directory would make that boundary visible. `pdu-transport` joined them
-      on 2026-08-31 without the move being made, so it is a move of its own now.
+      `reassembly`, `dlr-merger`, `send-window`, `link-timers`, `link-gate`, `reconnect-loop`,
+      `pending-requests` and `send-sms`, so the directory would make that boundary visible.
+      `pdu-transport` joined them on 2026-08-31 and `link-gate` on 2026-09-01, both without the move
+      being made, so it is a move of its own now. `session.ts` sits within a couple of lines of the
+      350-line cap again; the outgoing request path — `send()`, `refuseSend()`, `attempt()` and the
+      gate and window they drive — is the next thing that would come out of it.
 - [ ] **Does an intermediate delivery notification deserve to be a `dlr`?** `esm_class` message type
       `INTERMEDIATE_DELIVERY` (0x20) is classified as a message today, so a peer that reports
       non-final states with it hands the application a raw `id:… stat:ENROUTE` text as an inbound