diff --git a/commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/model/privateChats/Chatroom.kt b/commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/model/privateChats/Chatroom.kt
index 3932d5f769..0101161787 100644
--- a/commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/model/privateChats/Chatroom.kt
+++ b/commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/model/privateChats/Chatroom.kt
@@ -78,7 +78,7 @@ class Chatroom : NotesGatherer {
 
                 msg.author?.let { author ->
                     if (author !in activeSenders) {
-                        activeSenders + author
+                        activeSenders = activeSenders + author
                     }
                 }
 
diff --git a/commons/src/commonTest/kotlin/com/vitorpamplona/amethyst/commons/model/privateChats/ChatroomSenderIntersectsTest.kt b/commons/src/commonTest/kotlin/com/vitorpamplona/amethyst/commons/model/privateChats/ChatroomSenderIntersectsTest.kt
new file mode 100644
index 0000000000..f82412b3d2
--- /dev/null
+++ b/commons/src/commonTest/kotlin/com/vitorpamplona/amethyst/commons/model/privateChats/ChatroomSenderIntersectsTest.kt
@@ -0,0 +1,80 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.amethyst.commons.model.privateChats
+
+import com.vitorpamplona.amethyst.commons.model.Note
+import com.vitorpamplona.amethyst.commons.model.User
+import com.vitorpamplona.amethyst.commons.model.UserContext
+import kotlin.test.Test
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
+
+/**
+ * Regression guard for the room "Known vs New Request" classification.
+ *
+ * A chatroom is "Known" when a message sender is someone you follow
+ * ([Chatroom.senderIntersects]) OR you have sent to it. The follow-based path
+ * depends on [Chatroom.activeSenders] being populated as messages arrive. A
+ * dropped-assignment bug (`activeSenders + author` instead of
+ * `activeSenders = activeSenders + author`) left that set permanently empty, so
+ * every incoming DM from a followed contact was misclassified into New Requests
+ * and Known sat on the "Loading Feed" spinner until the much-slower history page
+ * exhausted.
+ */
+class ChatroomSenderIntersectsTest {
+    private val noCtx = UserContext { Note("addr") }
+
+    private fun noteFrom(author: User): Note = Note(author.pubkeyHex + "-msg").apply { this.author = author }
+
+    @Test
+    fun senderIsTrackedAndIntersectsFollows() {
+        val alice = User("a".repeat(64), noCtx)
+        val room = Chatroom()
+
+        room.addMessageSync(noteFrom(alice))
+
+        assertTrue(room.senderIntersects(setOf(alice.pubkeyHex)), "a followed sender must make the room intersect the follow set")
+    }
+
+    @Test
+    fun unrelatedSenderDoesNotIntersect() {
+        val alice = User("a".repeat(64), noCtx)
+        val bob = User("b".repeat(64), noCtx)
+        val room = Chatroom()
+
+        room.addMessageSync(noteFrom(bob))
+
+        assertFalse(room.senderIntersects(setOf(alice.pubkeyHex)), "a non-followed sender must not intersect the follow set")
+    }
+
+    @Test
+    fun multipleSendersAreAllTracked() {
+        val alice = User("a".repeat(64), noCtx)
+        val bob = User("b".repeat(64), noCtx)
+        val room = Chatroom()
+
+        room.addMessageSync(noteFrom(alice))
+        room.addMessageSync(noteFrom(bob))
+
+        assertTrue(room.senderIntersects(setOf(bob.pubkeyHex)))
+        assertTrue(room.senderIntersects(setOf(alice.pubkeyHex)))
+    }
+}