diff --git a/apps/api/src/openapi.ts b/apps/api/src/openapi.ts
index 93ada2b..5a93563 100644
--- a/apps/api/src/openapi.ts
+++ b/apps/api/src/openapi.ts
@@ -145,7 +145,13 @@ export const ApiConfigV2 = JsonObject.describe(
 	'The static client config, plus a ShareBaseUrl templated from the deploy domain'
 )
 
-/** `GET /api/versioncheck/v4` — whether the client's `?v=` build matches GAME_VERSION. */
+/**
+ * `GET /api/versioncheck/islandedversions` — builds islanded onto their own matchmaking
+ * pool. Always empty here.
+ */
+export const IslandedVersions = z.array(z.string())
+
+/** `GET /api/versioncheck/v4` — whether the client's `?v=` build is one we serve. */
 export const VersionCheck = z.object({
 	VersionStatus: z.int().describe('0 = current, 1 = client on a different build'),
 	UpdateNotificationStage: z.int(),
diff --git a/apps/api/src/routes/config.ts b/apps/api/src/routes/config.ts
index 1aef822..668a500 100644
--- a/apps/api/src/routes/config.ts
+++ b/apps/api/src/routes/config.ts
@@ -1,7 +1,7 @@
 import { Hono } from 'hono'
 import { describeRoute } from 'hono-openapi'
 
-import { GAME_VERSION } from '@repo/domain'
+import { isSupportedGameVersion } from '@repo/domain'
 
 import apiConfigV2 from '../../static/api-config-v2.json'
 import gameConfigsV1All from '../../static/gameconfigs-v1-all.json'
@@ -10,6 +10,7 @@ import {
 	ApiConfigV2,
 	AzureSpeechConfig,
 	BacktraceConfig,
+	IslandedVersions,
 	json,
 	JsonObject,
 	VersionCheck,
@@ -100,18 +101,33 @@ export const configRoutes = new Hono<App>({ strict: false })
 			summary: 'Client version check',
 			description:
 				'Whether the client build is current. Compares the client’s `?v=` build against ' +
-				'our target `GAME_VERSION`: `VersionStatus` is 0 when they match, 1 when the ' +
-				'client is on a different build.',
+				'the builds we serve (`SUPPORTED_GAME_VERSIONS`): `VersionStatus` is 0 when the ' +
+				'client is on one of them, 1 when it is on some other build.',
 			responses: { 200: json(VersionCheck, 'Version status') },
 		}),
 		(c) =>
 			c.json({
-				VersionStatus: c.req.query('v') === GAME_VERSION ? 0 : 1,
+				VersionStatus: isSupportedGameVersion(c.req.query('v')) ? 0 : 1,
 				UpdateNotificationStage: 0,
 				IsVersionIslanded: false,
 				IsCrossPlayDisabled: false,
 			})
 	)
+	// Islanding splits players onto version-specific matchmaking pools. We serve every
+	// supported build from one pool, so the list is empty — the client reads it as
+	// "nobody is islanded" and matchmakes normally.
+	.get(
+		'/api/versioncheck/islandedversions',
+		describeRoute({
+			tags: ['Config'],
+			summary: 'Islanded client builds',
+			description:
+				'The builds that are islanded off into their own matchmaking pool. This server ' +
+				'never islands a build, so the list is always empty.',
+			responses: { 200: json(IslandedVersions, 'Always an empty list') },
+		}),
+		(c) => c.json([])
+	)
 	.get(
 		'/api/gameconfigs/v1/all',
 		describeRoute({
diff --git a/apps/api/src/test/integration/api.test.ts b/apps/api/src/test/integration/api.test.ts
index 8b6b8c8..595080b 100644
--- a/apps/api/src/test/integration/api.test.ts
+++ b/apps/api/src/test/integration/api.test.ts
@@ -19,6 +19,7 @@ import {
 	ROOM_SCHEMA_DDL,
 	seedRoomWithSubRooms,
 	SUBROOM_SCHEMA_DDL,
+	SUPPORTED_GAME_VERSIONS,
 } from '@repo/domain'
 
 import '../../api.app'
@@ -197,11 +198,29 @@ describe('public endpoints', () => {
 		expect(await res.json()).toMatchObject({ VersionStatus: 0 })
 	})
 
+	test('GET /api/versioncheck/v4 reports current for every supported build', async () => {
+		for (const version of SUPPORTED_GAME_VERSIONS) {
+			const res = await exports.default.fetch(`${ORIGIN}/api/versioncheck/v4?v=${version}`)
+			expect(await res.json(), version).toMatchObject({ VersionStatus: 0 })
+		}
+	})
+
 	test('GET /api/versioncheck/v4 flags a mismatched build', async () => {
 		const res = await exports.default.fetch(`${ORIGIN}/api/versioncheck/v4?v=19990101`)
 		expect(await res.json()).toMatchObject({ VersionStatus: 1 })
 	})
 
+	test('GET /api/versioncheck/v4 flags a client that sends no build', async () => {
+		const res = await exports.default.fetch(`${ORIGIN}/api/versioncheck/v4`)
+		expect(await res.json()).toMatchObject({ VersionStatus: 1 })
+	})
+
+	test('GET /api/versioncheck/islandedversions is empty', async () => {
+		const res = await exports.default.fetch(`${ORIGIN}/api/versioncheck/islandedversions`)
+		expect(res.status).toBe(200)
+		expect(await res.json()).toEqual([])
+	})
+
 	test('GET /api/relationships/v2/get returns empty array for a player with none', async () => {
 		const res = await exports.default.fetch(`${ORIGIN}/api/relationships/v2/get`, {
 			headers: await bearer('99999'),
@@ -3646,6 +3665,7 @@ describe('openapi', () => {
 			'GET /api/roomkeys/v1/mine',
 			'GET /api/roomkeys/v1/room',
 			'GET /api/rooms/v1/filters',
+			'GET /api/versioncheck/islandedversions',
 			'GET /api/versioncheck/v4',
 			'GET /voice/config',
 			'POST /api/PlayerReporting/v1/deviceId',
diff --git a/apps/auth/src/auth.app.ts b/apps/auth/src/auth.app.ts
index dc587ea..31afee7 100644
--- a/apps/auth/src/auth.app.ts
+++ b/apps/auth/src/auth.app.ts
@@ -434,7 +434,14 @@ const app = new Hono<App>()
 	// id, so the client can offer them on the login screen (and post one back as a
 	// cached_login grant). No linked account → [], and the client falls back to a
 	// fresh login / create_account.
-	.get(
+	//
+	// GET or POST: the 2023 build asks with a GET, the 2025 build (20250424.01) POSTs
+	// the same path with a form body — `deviceId`, `platformAuth` (a JSON blob holding
+	// the platform's session ticket and app id) and `time`. The body is READ BY NOTHING
+	// here; both methods answer the same list off the path params, so a newer client
+	// gets its picker. Verifying that ticket is the eventual point of the POST.
+	.on(
+		['GET', 'POST'],
 		'/cachedlogin/forplatformid/:platform/:id',
 		describeRoute({
 			tags: ['Cached login'],
@@ -449,6 +456,10 @@ const app = new Hono<App>()
 				'APKs: with no Meta SDK they have no real identity to ask about and stall on an',
 				'empty picker. It consults nothing and returns one canned, non-redeemable entry',
 				'with `requirePassword: true`, sending the build to username/password login.',
+				'Older clients GET this; the 20250424.01 build POSTs it with a',
+				'`deviceId` / `platformAuth` / `time` form body attesting the platform session.',
+				'That body is accepted and ignored — both methods answer identically from the',
+				'path params.',
 			].join(' '),
 			parameters: [
 				{
diff --git a/apps/auth/src/test/integration/api.test.ts b/apps/auth/src/test/integration/api.test.ts
index c313736..b1ac519 100644
--- a/apps/auth/src/test/integration/api.test.ts
+++ b/apps/auth/src/test/integration/api.test.ts
@@ -420,6 +420,59 @@ describe('auth worker routes', () => {
 		])
 	})
 
+	// The 20250424.01 build POSTs the picker lookup with a platform-attestation form body
+	// instead of GETting it. Nothing reads that body yet, so both methods must answer the
+	// same list — otherwise the newer client's login screen comes up empty.
+	test('POST /cachedlogin/forplatformid answers exactly what the GET answers', async () => {
+		const steamId = '76561197962463211'
+		await env.DB.prepare('INSERT OR IGNORE INTO account (data) VALUES (?1)')
+			.bind(
+				JSON.stringify({
+					accountId: 31381,
+					username: 'SteamPlayer2025',
+					platform: 0,
+					platformId: steamId,
+					lastLoginTime: '2026-08-13T04:21:34.768Z',
+				})
+			)
+			.run()
+		await linkPlatformIdentity(env.DB, 31381, 0, steamId)
+
+		// The body as the live client sends it: device id, the platform session ticket, a
+		// timestamp. All ignored for now.
+		const body = new URLSearchParams({
+			deviceId: '69640e6ae1b54ae5b0ca8eeb4a8872ec6cf8fd88',
+			platformAuth: JSON.stringify({ Ticket: '140000009C5F501B447424FF', AppId: '471710' }),
+			time: '2026-08-13T04:21:34.7684754Z',
+		}).toString()
+		const posted = await exports.default.fetch(`${ORIGIN}/cachedlogin/forplatformid/0/${steamId}`, {
+			method: 'POST',
+			headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+			body,
+		})
+		expect(posted.status).toBe(200)
+		const expected = [
+			{
+				platform: 0,
+				platformId: steamId,
+				accountId: 31381,
+				lastLoginTime: '2026-08-13T04:21:34.768Z',
+				requirePassword: false,
+			},
+		]
+		expect(await posted.json()).toEqual(expected)
+		expect(await cachedLogins(0, steamId)).toEqual(expected)
+	})
+
+	// A POST with no body at all still resolves — the client's body is never consulted.
+	test('POST /cachedlogin/forplatformid/1/1 still returns the canned Oculus entry', async () => {
+		const res = await exports.default.fetch(`${ORIGIN}/cachedlogin/forplatformid/1/1`, {
+			method: 'POST',
+		})
+		expect(res.status).toBe(200)
+		expect(await res.json()).toMatchObject([{ accountId: 1, requirePassword: true }])
+	})
+
 	test('one account, a Steam and a Meta identity: both pickers offer it', async () => {
 		// The point of the link table. The same account is reachable from the PC and from
 		// the headset, and each picker reports the identity IT was asked about — that's
@@ -1122,6 +1175,7 @@ describe('auth worker routes', () => {
 			'GET /role/developer/{id}',
 			'GET /role/moderator/{id}',
 			'POST /account/me/changepassword',
+			'POST /cachedlogin/forplatformid/{platform}/{id}',
 			'POST /cachedlogin/forplatformids',
 			'POST /connect/token',
 		])
diff --git a/packages/domain/src/presence-db.ts b/packages/domain/src/presence-db.ts
index f712073..3ff1a3b 100644
--- a/packages/domain/src/presence-db.ts
+++ b/packages/domain/src/presence-db.ts
@@ -32,6 +32,18 @@ export const PRESENCE_TTL_SECONDS = 900
  */
 export const GAME_VERSION = '20230414'
 
+/**
+ * Client builds this server treats as current. `GAME_VERSION` is the one we report for
+ * ourselves; the rest are additional builds `/api/versioncheck/v4` answers "current"
+ * for, so a player on one of them isn't pushed into an update loop.
+ */
+export const SUPPORTED_GAME_VERSIONS: string[] = [GAME_VERSION, '20250424.01']
+
+/** Whether a client-supplied build (the version check's `?v=`) is one we serve. */
+export function isSupportedGameVersion(version: string | null | undefined): boolean {
+	return version != null && SUPPORTED_GAME_VERSIONS.includes(version)
+}
+
 /** Schema DDL (mirror of migrations/0006_presence.sql). */
 export const PRESENCE_SCHEMA_DDL: string[] = [
 	`CREATE TABLE IF NOT EXISTS presence (