diff --git a/src/reassembly.ts b/src/reassembly.ts
index c3b1b92..417f103 100644
--- a/src/reassembly.ts
+++ b/src/reassembly.ts
@@ -132,7 +132,20 @@ export class Reassembler {
 		}
 
 		const key = groupKey(pduObj, concat.reference);
-		const group = this.groups.get(key) ?? this.open(key, concat.total);
+		const existing = this.groups.get(key);
+
+		// Parts 1/2 and 2/3 would otherwise complete the stored two-part group as a truncated message.
+		if (existing && existing.total !== concat.total) {
+			this.log.warn('reassembler - dropping a segment with an inconsistent UDH total', {
+				existingTotal: existing.total,
+				part: concat.part,
+				total: concat.total,
+			});
+
+			return undefined;
+		}
+
+		const group = existing ?? this.open(key, concat.total);
 		const replaced = group.parts.get(concat.part);
 		const segment = detach(pduObj);
 		const delta = octetsOf(segment) - (replaced === undefined ? 0 : octetsOf(replaced));
diff --git a/src/session.ts b/src/session.ts
index 6699d90..3b2c6db 100644
--- a/src/session.ts
+++ b/src/session.ts
@@ -127,6 +127,11 @@ export class Session extends EventEmitter<SessionEvents> {
 
 		if (this.closed) return { err: new Error('Session is closed') };
 
+		// 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') };
+		}
+
 		await this.window.acquire();
 
 		try {
diff --git a/test/session-extras.test.ts b/test/session-extras.test.ts
index 52bb76d..0a85a71 100644
--- a/test/session-extras.test.ts
+++ b/test/session-extras.test.ts
@@ -365,6 +365,17 @@ describe('reassembly bounds', () => {
 		assert.equal(reassembler.size, 0);
 	});
 
+	// Parts 1/2 then 2/3 would otherwise complete the stored two-part group, truncating the message.
+	test('refuses a segment that renumbers how many parts the message has', () => {
+		const reassembler = new Reassembler({ log: silentLog, max: 10, now: () => 0, timeout: 60_000 });
+
+		assert.equal(collect(reassembler, 9, 1, 2), undefined);
+		assert.equal(collect(reassembler, 9, 2, 3), undefined);
+		assert.equal(reassembler.size, 1);
+
+		reassembler.clear();
+	});
+
 	// 0.4.0 held incomplete groups without limit and swept them only when other traffic arrived.
 	test('drops the oldest incomplete message once the cap is reached', () => {
 		const reassembler = new Reassembler({ log: silentLog, max: 2, now: () => 0, timeout: 60_000 });
diff --git a/test/session.test.ts b/test/session.test.ts
index 493c6e6..c5b68a7 100644
--- a/test/session.test.ts
+++ b/test/session.test.ts
@@ -1000,6 +1000,38 @@ describe('robustness', () => {
 		assert.deepEqual(seen, []);
 	});
 
+	// The guard sits before the send window, or a full window makes the aborted call queue first.
+	test('does not wait for a send window slot it will never use', async t => {
+		const smpp = await startServer();
+
+		t.after(() => smpp.close());
+
+		// The peer answers nothing, so the one slot stays held for the whole test.
+		smpp.on('session', session => session.on('sms', () => undefined));
+
+		const { session } = await connect(smpp, { maxOutstanding: 1, responseTimeout: 5000 });
+
+		assert.ok(session);
+		t.after(() => { session.close(); });
+
+		const held = session.sendSms({ from: '46701113311', message: 'holds the slot', to: '46709771337' });
+		const controller = new AbortController();
+
+		controller.abort();
+
+		const aborted = await raceWithin(500, session.sendSms({
+			from: '46701113311',
+			message: 'must not queue behind the held one',
+			to: '46709771337',
+		}, { signal: controller.signal }));
+
+		assert.notEqual(aborted, false, 'an aborted send should not wait for the window');
+		assert.ok(aborted !== false && aborted.err instanceof Error);
+
+		session.close();
+		await held;
+	});
+
 	// A socket the loop opened and never handed over is one leaked per retry, forever.
 	test('leaves no socket open when coming back up fails', async () => {
 		const opened: net.Socket[] = [];
diff --git a/todo.md b/todo.md
index ddaa1bb..6a30e66 100644
--- a/todo.md
+++ b/todo.md
@@ -5,7 +5,7 @@ rules there constrain every item below.
 
 ## Status
 
-The rewrite is **feature complete and green**: 208 tests, lint and typecheck clean, verified on Node
+The rewrite is **feature complete and green**: 210 tests, lint and typecheck clean, verified on Node
 18, 20, 22 and 24. What is left is release work and a few things worth adding before or after 1.0.0.
 
 ```bash