diff --git a/apps/rooms/src/openapi.ts b/apps/rooms/src/openapi.ts
index 7afbd57..1ebcecb 100644
--- a/apps/rooms/src/openapi.ts
+++ b/apps/rooms/src/openapi.ts
@@ -324,7 +324,12 @@ export const RoomDto = z.object({
 	RankingContext: z.unknown().nullable(),
 	IsDorm: z.boolean().describe('Auto-provisioned personal room; excluded from every feed'),
 	IsPlacePlay: z.boolean(),
-	MaxPlayerCalculationMode: z.int(),
+	MaxPlayerCalculationMode: z
+		.int()
+		.describe(
+			'0 AllSubrooms, 1 OnlyEntrySubrooms — set by `PUT …/max_player_calculation_mode`. A ' +
+				'stored setting; `MaxPlayers` is not derived from it here'
+		),
 	MaxPlayers: z.int(),
 	CloningAllowed: z.boolean().describe('False blocks `POST /rooms/{roomId}/clone`'),
 	DisableMicAutoMute: z.boolean(),
@@ -828,19 +833,29 @@ export const MoveSubRoomRequest = z.object({
 })
 
 /**
- * What the subroom move answers: the SOURCE room (which no longer lists the subroom) under
- * `Value`, in the PascalCase `{ Value, Success, Error, error_id }` envelope — the same mixed
+ * A room in the PascalCase `{ Value, Success, Error, error_id }` envelope — the same mixed
  * casing the unprefixed isBanned check has ({@link IsBannedPascalEnvelope}), NOT the
- * lowercase `{ success, error, value }` every other subroom mutation answers. Kept apart
- * deliberately: the client decodes this one with a different reader.
+ * lowercase `{ success, error, value }` most room mutations answer. The newer settings
+ * routes (the subroom move, the max-player calculation mode) answer this one; the client
+ * decodes them with a different reader, so the two are kept apart deliberately.
  */
-export const MoveSubRoomEnvelope = z.object({
-	Value: RoomDto.nullable().describe('The source room as it now stands; null on a rejection'),
+export const RoomPascalEnvelope = z.object({
+	Value: RoomDto.nullable().describe('The room as it now stands; null on a rejection'),
 	Success: z.boolean(),
 	Error: z.string().nullable().describe('The message shown on a rejection; null on success'),
 	error_id: z.string().nullable().describe('Null. Lowercase, unlike its siblings'),
 })
 
+/**
+ * `PUT /rooms/{roomId}/max_player_calculation_mode` — how the room's player cap is meant to
+ * be read. The client sends the enum NAME; the ordinal is accepted alongside it.
+ */
+export const MaxPlayerCalculationModeRequest = z.object({
+	maxPlayerCalculationMode: z
+		.string()
+		.describe('`AllSubrooms` (0) or `OnlyEntrySubrooms` (1), case-insensitive, or the ordinal'),
+})
+
 /** `POST /rooms/{roomId}/subrooms`. */
 export const CreateSubRoomRequest = z.object({
 	name: z.string().describe('The new subroom’s name'),
diff --git a/apps/rooms/src/rooms.app.ts b/apps/rooms/src/rooms.app.ts
index bdf34a4..6e45472 100644
--- a/apps/rooms/src/rooms.app.ts
+++ b/apps/rooms/src/rooms.app.ts
@@ -115,9 +115,9 @@ import {
 	LeaderboardRequest,
 	LeaderboardResultEnvelope,
 	LoadScreenRequest,
+	MaxPlayerCalculationModeRequest,
 	MissingLookupParam,
 	ModifySubRoomRequest,
-	MoveSubRoomEnvelope,
 	MoveSubRoomRequest,
 	NameRequest,
 	NOT_FRIENDS_RESPONSE,
@@ -140,6 +140,7 @@ import {
 	RoomExperiencePlayer,
 	roomIdParam,
 	RoomLookup,
+	RoomPascalEnvelope,
 	RoomResultEnvelope,
 	RoomRoleDto,
 	RoomSaveEnvelope,
@@ -509,6 +510,32 @@ const RESTRICTION_FIELDS: Record<string, string> = {
  * Hub failures are logged and swallowed — the room write has already committed,
  * so a hub hiccup must not fail the request.
  */
+/**
+ * The room's `MaxPlayerCalculationMode` enum: how its player cap is meant to be read.
+ * `AllSubrooms` (0) and `OnlyEntrySubrooms` (1). A stored setting — nothing here derives
+ * `MaxPlayers` from it.
+ */
+const MAX_PLAYER_CALCULATION_MODES: Readonly<Record<string, number>> = {
+	allsubrooms: 0,
+	onlyentrysubrooms: 1,
+}
+
+/**
+ * Parse a `maxPlayerCalculationMode` field into its ordinal. The client sends the enum
+ * NAME (`OnlyEntrySubrooms`), matched case-insensitively; the ordinal is accepted too.
+ * Undefined when it names nothing in the enum, which the route refuses rather than
+ * defaulting — a silently-wrong mode would misreport a room's capacity.
+ */
+function parseMaxPlayerCalculationMode(value: unknown): number | undefined {
+	if (typeof value !== 'string') return undefined
+	const raw = value.trim()
+	const named = MAX_PLAYER_CALCULATION_MODES[raw.toLowerCase()]
+	if (named !== undefined) return named
+	if (!/^\d+$/.test(raw)) return undefined
+	const ordinal = Number.parseInt(raw, 10)
+	return Object.values(MAX_PLAYER_CALCULATION_MODES).includes(ordinal) ? ordinal : undefined
+}
+
 async function pushRoomUpdate(
 	c: Context<App>,
 	playerId: number,
@@ -3124,6 +3151,58 @@ const app = new Hono<App>()
 		}
 	)
 
+	// Set a room's MaxPlayerCalculationMode (form body `maxPlayerCalculationMode`, the enum
+	// NAME). Auth-gated (401) and owner/co-owner-only. A stored setting only: it does not
+	// touch `MaxPlayers`. Answers the room in the PascalCase `{ Value, Success, Error,
+	// error_id }` envelope, like the subroom move — not the lowercase one its neighbours use.
+	.put(
+		'/rooms/:roomId{[0-9]+}/max_player_calculation_mode',
+		describeRoute({
+			tags: ['Room settings'],
+			summary: 'Set a room’s max-player calculation mode',
+			description: [
+				'Sets `MaxPlayerCalculationMode`: `AllSubrooms` (0) or `OnlyEntrySubrooms` (1). The',
+				'client sends the enum name (`maxPlayerCalculationMode=OnlyEntrySubrooms`); the',
+				'ordinal is accepted too, and anything else is rejected. It is a stored setting:',
+				'`MaxPlayers` is not recomputed from the subrooms here.',
+				'',
+				'Owner or co-owner only. Answers the updated room under `Value` in the PascalCase',
+				'`{ Value, Success, Error, error_id }` envelope — NOT the lowercase',
+				'`{ success, error, value }` the other room-settings routes answer. A rejection is',
+				'HTTP 200 with `Success: false`, `Value: null` and the message in `Error`; only a',
+				'missing token is a 401.',
+			].join('\n'),
+			security: AUTHED,
+			parameters: [roomIdParam],
+			requestBody: form(MaxPlayerCalculationModeRequest, 'The mode'),
+			responses: {
+				200: json(RoomPascalEnvelope, 'The updated room, or a rejection with `Success: false`'),
+				401: { description: 'No bearer token (empty body)' },
+			},
+		}),
+		async (c) => {
+			const accountId = await authedAccountId(c)
+			if (accountId === null) return c.body(null, 401)
+			const refuse = (error: string) =>
+				c.json({ Value: null, Success: false, Error: error, error_id: null })
+
+			const roomId = Number.parseInt(c.req.param('roomId'), 10)
+			const room = await getRoomById(c.env.DB, roomId)
+			if (!room) return refuse('This room does not exist!')
+			if (!canManageRoom(room, accountId)) return refuse('You are not the owner of this room!')
+
+			const body = (await c.req.parseBody().catch(() => ({}))) as Record<string, unknown>
+			const mode = parseMaxPlayerCalculationMode(body.maxPlayerCalculationMode)
+			if (mode === undefined) return refuse('You must provide a valid maxPlayerCalculationMode!')
+
+			const updated = await updateRoomFields(c.env.DB, roomId, room, {
+				MaxPlayerCalculationMode: mode,
+			})
+			await pushRoomUpdate(c, accountId, updated)
+			return c.json({ Value: updated, Success: true, Error: null, error_id: null })
+		}
+	)
+
 	// Set a room's platform/movement support flags (its `Supports*` restrictions).
 	// Auth-gated (401) and owner/co-owner-only (403). Body is a form of
 	// `supports*=True|False` fields (see RESTRICTION_FIELDS); only the fields present
@@ -3914,7 +3993,7 @@ const app = new Hono<App>()
 			parameters: [roomIdParam, subRoomIdParam],
 			requestBody: form(MoveSubRoomRequest, 'The destination room'),
 			responses: {
-				200: json(MoveSubRoomEnvelope, 'The source room, or a rejection with `Success: false`'),
+				200: json(RoomPascalEnvelope, 'The source room, or a rejection with `Success: false`'),
 				401: { description: 'No bearer token (empty body)' },
 			},
 		}),
diff --git a/apps/rooms/src/test/integration/api.test.ts b/apps/rooms/src/test/integration/api.test.ts
index b5d01ab..8cb26ad 100644
--- a/apps/rooms/src/test/integration/api.test.ts
+++ b/apps/rooms/src/test/integration/api.test.ts
@@ -4819,6 +4819,68 @@ describe('rooms endpoints', () => {
 		})
 	})
 
+	it('PUT /rooms/:id/max_player_calculation_mode stores the mode without touching MaxPlayers', async () => {
+		await seedRoomWithSubRooms(env.DB, {
+			RoomId: 720,
+			Name: 'CapMode',
+			CreatorAccountId: 5,
+			MaxPlayerCalculationMode: 0,
+			MaxPlayers: 12,
+			Roles: [{ AccountId: 1, Role: 30, LastChangedByAccountId: null, InvitedRole: 0 }],
+			SubRooms: [
+				{ SubRoomId: 920, UnitySceneId: 'x', MaxPlayers: 12 },
+				{ SubRoomId: 921, UnitySceneId: 'x', MaxPlayers: 4 },
+			],
+		})
+		const put = async (roomId: number, mode: string, sub?: string) =>
+			SELF.fetch(`${ORIGIN}/rooms/${roomId}/max_player_calculation_mode`, {
+				method: 'PUT',
+				headers: {
+					...(sub ? await bearer(sub) : {}),
+					'Content-Type': 'application/x-www-form-urlencoded',
+				},
+				body: new URLSearchParams({ maxPlayerCalculationMode: mode }).toString(),
+			})
+		type Envelope = {
+			Value: { RoomId: number; MaxPlayerCalculationMode: number; MaxPlayers: number } | null
+			Success: boolean
+			Error: string | null
+			error_id: null
+		}
+		const envelope = async (res: Response) => (await res.json()) as Envelope
+
+		// No token → 401; every other refusal is a 200 with Success:false and Value:null.
+		expect((await put(720, 'OnlyEntrySubrooms')).status).toBe(401)
+		for (const res of [
+			await put(720, 'OnlyEntrySubrooms', '999'), // not on the room
+			await put(720, 'Sideways', '1'), // not a mode
+			await put(720, '7', '1'), // not an ordinal in the enum
+			await put(424242, 'OnlyEntrySubrooms', '1'), // no such room
+		]) {
+			expect(res.status).toBe(200)
+			expect(await envelope(res)).toMatchObject({ Value: null, Success: false, error_id: null })
+		}
+
+		// A co-owner sets it by NAME (what the client sends). Stored as the ordinal, and
+		// MaxPlayers is left exactly as it was — the mode is a setting, not a recompute.
+		const res = await put(720, 'OnlyEntrySubrooms', '1')
+		expect(res.status).toBe(200)
+		const body = await envelope(res)
+		expect(body).toMatchObject({ Success: true, Error: null, error_id: null })
+		expect(body.Value).toMatchObject({ RoomId: 720, MaxPlayerCalculationMode: 1, MaxPlayers: 12 })
+		const stored = (await (await SELF.fetch(`${ORIGIN}/rooms/720`)).json()) as {
+			MaxPlayerCalculationMode: number
+			MaxPlayers: number
+		}
+		expect(stored).toMatchObject({ MaxPlayerCalculationMode: 1, MaxPlayers: 12 })
+
+		// Case-insensitive names and the bare ordinal are accepted too.
+		expect(
+			(await envelope(await put(720, 'allsubrooms', '5'))).Value?.MaxPlayerCalculationMode
+		).toBe(0)
+		expect((await envelope(await put(720, '1', '5'))).Value?.MaxPlayerCalculationMode).toBe(1)
+	})
+
 	it('POST /rooms/:id/subrooms/:sid/move re-parents a subroom when the caller manages both rooms', async () => {
 		// Source 710 is owned by 1 with two subrooms; target 711 is owned by 5 with 1 as a
 		// CO-OWNER (the move gate is manage, not own); 712 is somebody else's entirely.
@@ -5313,6 +5375,7 @@ describe('rooms endpoints', () => {
 			'PUT /rooms/{roomId}/interactionby/me/cheer',
 			'PUT /rooms/{roomId}/interactionby/me/favorite',
 			'PUT /rooms/{roomId}/loadscreen',
+			'PUT /rooms/{roomId}/max_player_calculation_mode',
 			'PUT /rooms/{roomId}/name',
 			'PUT /rooms/{roomId}/restrictions',
 			'PUT /rooms/{roomId}/roles/{accountId}',