diff --git a/firestore.rules b/firestore.rules
index 2634a784..74c4d74a 100644
--- a/firestore.rules
+++ b/firestore.rules
@@ -309,17 +309,20 @@ service cloud.firestore {
         allow create: if isCouplesMember(coupleId)
           && request.resource.data.startedByUserId == request.auth.uid;
 
-        // Update: only the user who started the session can update it, OR valid status transitions.
-        // startedByUserId is immutable for direct client writes.
+        // Update: any couple member may record session progress/completion.
+        // (Async two-device games mark each player done via `completedByUsers`; the
+        //  session flips active→completed once both are in. The previous rule only
+        //  allowed `status`/`completedAt`, so every `completedByUsers` write was denied
+        //  and finished games never closed — locking the couple out of new games. B-001.)
         allow update: if isCouplesMember(coupleId)
-          // Either the original starter can update
-          && (resource.data.startedByUserId == request.auth.uid
-          // Or status transition is valid: active → completed
-          || (resource.data.status == 'active' && request.resource.data.status == 'completed'))
-          // startedByUserId cannot be changed by clients
+          // startedByUserId is immutable for direct client writes.
           && request.resource.data.startedByUserId == resource.data.startedByUserId
-          // Only a fixed set of fields may change
-          && request.resource.data.diff(resource.data).affectedKeys().hasOnly(['status', 'completedAt']);
+          // Only session progress/completion fields may change.
+          && request.resource.data.diff(resource.data).affectedKeys()
+               .hasOnly(['status', 'completedAt', 'completedByUsers'])
+          // status is monotonic: stay the same, or transition active → completed (never revert).
+          && (request.resource.data.status == resource.data.status
+            || (resource.data.status == 'active' && request.resource.data.status == 'completed'));
 
         // Delete: server-only (admin SDK). Admin SDK bypasses rules.
         allow delete: if false;