diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/marmot/mls/group/MlsGroup.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/marmot/mls/group/MlsGroup.kt
index 3e290dc63a..a0a3dc8ca4 100644
--- a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/marmot/mls/group/MlsGroup.kt
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/marmot/mls/group/MlsGroup.kt
@@ -3638,7 +3638,9 @@ class MlsGroup private constructor(
              * the MIP-era set; a current-profile group passes
              * [buildCurrentProfileRequiredCapabilitiesExtension].
              */
-            requiredCapabilities: Extension = buildMarmotRequiredCapabilitiesExtension(),
+            requiredCapabilities: Extension? = buildMarmotRequiredCapabilitiesExtension(),
+            /** The group's MLS `group_id`. Null picks 32 random bytes. */
+            groupId: ByteArray? = null,
         ): MlsGroup {
             val sigKp =
                 signingKey?.let { key ->
@@ -3647,7 +3649,7 @@ class MlsGroup private constructor(
                 } ?: Ed25519.generateKeyPair()
 
             val encKp = X25519.generateKeyPair()
-            val groupId = MlsCryptoProvider.randomBytes(32)
+            val groupId = groupId ?: MlsCryptoProvider.randomBytes(32)
 
             val leafNode =
                 buildLeafNode(
@@ -3668,7 +3670,7 @@ class MlsGroup private constructor(
             // bake into epoch 0 (e.g. the MIP-01 MarmotGroupData extension so
             // new peers who join later can see the group name without first
             // decrypting a pre-membership bootstrap commit — see MIP-03).
-            val baseExtensions = listOf(requiredCapabilities)
+            val baseExtensions = listOfNotNull(requiredCapabilities)
             val groupContext =
                 GroupContext(
                     groupId = groupId,
diff --git a/quartz/src/jvmAndroidTest/kotlin/com/vitorpamplona/quartz/marmot/mls/group/GroupCreateOptionsTest.kt b/quartz/src/jvmAndroidTest/kotlin/com/vitorpamplona/quartz/marmot/mls/group/GroupCreateOptionsTest.kt
new file mode 100644
index 0000000000..4e71e5f0c4
--- /dev/null
+++ b/quartz/src/jvmAndroidTest/kotlin/com/vitorpamplona/quartz/marmot/mls/group/GroupCreateOptionsTest.kt
@@ -0,0 +1,53 @@
+/*
+ * 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.quartz.marmot.mls.group
+
+import com.vitorpamplona.quartz.nip01Core.core.hexToByteArray
+import kotlin.test.Test
+import kotlin.test.assertContentEquals
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+class GroupCreateOptionsTest {
+    private val creator = "11".repeat(32).hexToByteArray()
+
+    @Test
+    fun aCallerChosenGroupIdIsUsed() {
+        val groupId = "my-group".encodeToByteArray()
+        val alice = MlsGroup.create(creator, groupId = groupId)
+        assertContentEquals(groupId, alice.groupId)
+
+        val bobBundle = alice.createKeyPackage("22".repeat(32).hexToByteArray(), ByteArray(0))
+        val bob = MlsGroup.processWelcome(alice.addMember(bobBundle.keyPackage.toTlsBytes()).welcomeBytes!!, bobBundle)
+        assertContentEquals(groupId, bob.groupId)
+    }
+
+    @Test
+    fun aGroupWithoutRequiredCapabilitiesWorks() {
+        val alice = MlsGroup.create(creator, requiredCapabilities = null)
+        assertTrue(alice.groupContextExtensionsSnapshot().none { it.extensionType == 0x0003 })
+
+        val bobBundle = alice.createKeyPackage("22".repeat(32).hexToByteArray(), ByteArray(0))
+        val bob = MlsGroup.processWelcome(alice.addMember(bobBundle.keyPackage.toTlsBytes()).welcomeBytes!!, bobBundle)
+        assertEquals(alice.epoch, bob.epoch)
+        assertContentEquals("hi".encodeToByteArray(), bob.decrypt(alice.encrypt("hi".encodeToByteArray())).content)
+    }
+}