diff --git a/apps/match/src/match.app.ts b/apps/match/src/match.app.ts
index 3a0e6e2..6dd3295 100644
--- a/apps/match/src/match.app.ts
+++ b/apps/match/src/match.app.ts
@@ -4,7 +4,12 @@ import { useWorkersLogger } from 'workers-tagged-logger'
 import { withNotFound, withOnError } from '@repo/hono-helpers'
 
 import { validateAndGetAccountId } from './jwt'
-import { createRoomInstance, getJoinableInstance, getRoomInstancesByRoom } from './room-instance-db'
+import {
+	createRoomInstance,
+	getJoinableInstance,
+	getRoomInstancesByRoom,
+	setRoomInstanceInProgress,
+} from './room-instance-db'
 import { getOrCreateDormRoom, getRoomById, getRoomByName } from './rooms-db'
 
 import type { Context } from 'hono'
@@ -488,6 +493,24 @@ const app = new Hono<App>()
 	// ---- Room instance -------------------------------------------------------
 	.post('/roominstance/:id/reportjoinresult', (c) => c.body(null, 200))
 
+	// The room owner flips the instance's in-progress flag once the session starts
+	// (e.g. a game round begins). Body is a form post: `inProgress=True|False`.
+	.put('/roominstance/:id/inprogress', async (c) => {
+		const id = await authedId(c)
+		if (id === null) return unauthorized(c)
+
+		const instanceId = Number.parseInt(c.req.param('id'), 10)
+		if (Number.isNaN(instanceId)) return c.body(null, 404)
+
+		const body = await c.req.parseBody().catch(() => ({}) as Record<string, unknown>)
+		const inProgress =
+			typeof body.inProgress === 'string' && body.inProgress.toLowerCase() === 'true'
+
+		const instance = await setRoomInstanceInProgress(c.env.DB, instanceId, inProgress)
+		if (!instance) return c.body(null, 404)
+		return c.body(null, 200)
+	})
+
 	// Rooms flagged as needing a developer/moderator to spawn in. No such queue
 	// yet → empty list.
 	.get('/rooms/requiring/developer', (c) => c.json([]))
diff --git a/apps/match/src/room-instance-db.ts b/apps/match/src/room-instance-db.ts
index 116778b..3b1fc6a 100644
--- a/apps/match/src/room-instance-db.ts
+++ b/apps/match/src/room-instance-db.ts
@@ -176,10 +176,34 @@ export async function getRoomInstance(db: D1Database, id: number): Promise<RoomI
 	return row ? toDto(parse(row.data)) : null
 }
 
+/**
+ * Flip an instance's `isInProgress` flag, rewriting the JSON blob (the generated
+ * `is_in_progress` column follows it). Returns the updated DTO, or null when the
+ * instance doesn't exist.
+ */
+export async function setRoomInstanceInProgress(
+	db: D1Database,
+	id: number,
+	isInProgress: boolean
+): Promise<RoomInstanceDto | null> {
+	const row = await db
+		.prepare('SELECT data FROM room_instance WHERE id = ?1')
+		.bind(id)
+		.first<{ data: string }>()
+	if (!row) return null
+	const stored = parse(row.data)
+	stored.isInProgress = isInProgress
+	await db
+		.prepare('UPDATE room_instance SET data = ?1 WHERE id = ?2')
+		.bind(JSON.stringify(stored), id)
+		.run()
+	return toDto(stored)
+}
+
 /**
  * The oldest joinable public instance of a room (not private, not full, joins
- * enabled), or null when there's none to join. Used by matchmaking to reuse an
- * existing instance before creating a new one.
+ * enabled, not already in progress), or null when there's none to join. Used by
+ * matchmaking to reuse an existing instance before creating a new one.
  */
 export async function getJoinableInstance(
 	db: D1Database,
@@ -189,6 +213,7 @@ export async function getJoinableInstance(
 		.prepare(
 			`SELECT data FROM room_instance
 			 WHERE room_id = ?1 AND is_private = 0 AND is_full = 0 AND join_disabled = 0
+			   AND is_in_progress = 0
 			 ORDER BY id LIMIT 1`
 		)
 		.bind(roomId)