diff --git a/apps/api/app/routes/session.ts b/apps/api/app/routes/session.ts
index afb1065f..eed46f48 100644
--- a/apps/api/app/routes/session.ts
+++ b/apps/api/app/routes/session.ts
@@ -137,18 +137,27 @@ export namespace SessionApi {
 					);
 				}
 
-				// A run launches as a Steam account that has to own the game, so
-				// a game outside the caller's library is a box that starts, tries
-				// to launch, and fails minutes later with nothing to point at.
-				// Refusing here is the same answer sooner.
+				// A game nobody has synced for this person is a box that starts,
+				// tries to launch and fails minutes later with nothing to point
+				// at. Refusing here is the same answer sooner.
 				//
 				// Told apart from a game that does not exist rather than hidden:
 				// the catalog is public, so there is nothing to hide, and "you do
-				// not own this" is the sentence a person can act on.
+				// not own this" is a sentence a person can act on.
 				//
-				// The library is a synced copy, so this refuses a game bought
-				// since the last sync. That is a staleness bug in the sync and
-				// not a reason to launch runs that cannot work.
+				// **This is a weaker check than the one that matters.** A run
+				// launches as one account, but a library entry records only the
+				// person, so what is verified is "somebody this person has linked
+				// owns it" and not "the account playing owns it". For the one
+				// linked account most people have those are the same sentence;
+				// for two they are not, and the second account can ask for a game
+				// only the first owns. Answering the real question needs the
+				// account recorded on the library entry, which is a decision
+				// about what a library *is* and not something to infer here.
+				//
+				// The library is also a synced copy, so this refuses a game
+				// bought since the last sync. Both gaps let a launch fail late;
+				// neither is a reason to start runs already known to fail.
 				const owned = await Library.findByUserAndGame({ userId, gameId: game.id });
 				if (!owned) {
 					throw new VisibleError(
diff --git a/apps/api/test/session.test.ts b/apps/api/test/session.test.ts
index b151e32d..c63e12a2 100644
--- a/apps/api/test/session.test.ts
+++ b/apps/api/test/session.test.ts
@@ -9,6 +9,7 @@ import { Identifier } from '@nestri/core/id';
 import { Machine } from '@nestri/core/machine/index';
 import { Session } from '@nestri/core/session/index';
 import { Library } from '@nestri/core/user/library';
+import { LinkedAccount } from '@nestri/core/user/linked-account';
 
 import { app } from '../app/index';
 import './setup';
@@ -265,6 +266,39 @@ describe('POST /session', () => {
 		expect(await Session.listByBox(s.box.id)).toHaveLength(0);
 	});
 
+	test('the library check is per person, not per account it plays as', async () => {
+		const s = await scene('route-multilink', 5562);
+		// A second Steam account on the same person. The unique index is on
+		// (provider, providerAccountId) and is global rather than per user, so
+		// nothing stops this — but a fixed id here would collide with its own
+		// previous run, hence one shaped like a SteamID64 and unique per run.
+		const second = await LinkedAccount.create({
+			id: Identifier.ascending('linkedAccount'),
+			userId: s.owner.userId,
+			provider: 'steam',
+			providerAccountId: `7656119${Date.now()}`.slice(0, 17),
+			profile: null
+		});
+
+		const res = await app.request('/session', {
+			method: 'POST',
+			headers: s.user,
+			body: JSON.stringify({
+				boxId: s.box.id,
+				gameId: s.gameId,
+				linkedAccountId: second
+			})
+		});
+
+		// Accepted, and this pins a known gap rather than asserting it is
+		// right: a library entry records the person and not the account the
+		// games came from, so "the account playing owns this" cannot be asked.
+		// The run will fail at launch exactly as it did before the check
+		// existed. Closing it means recording the account on the library
+		// entry, which changes what a library is and what the sync must send.
+		expect(res.status).toBe(201);
+	});
+
 	test('an unknown game is a 404 and not a foreign key crash', async () => {
 		const s = await scene('route-nogame', 5507);
 		const res = await app.request('/session', {