diff --git a/mirai-core/src/commonTest/kotlin/message/protocol/impl/CustomMessageProtocolTest.kt b/mirai-core/src/commonTest/kotlin/message/protocol/impl/CustomMessageProtocolTest.kt
new file mode 100644
index 000000000..41779fd3e
--- /dev/null
+++ b/mirai-core/src/commonTest/kotlin/message/protocol/impl/CustomMessageProtocolTest.kt
@@ -0,0 +1,85 @@
+/*
+ * Copyright 2019-2022 Mamoe Technologies and contributors.
+ *
+ * 此源代码的使用受 GNU AFFERO GENERAL PUBLIC LICENSE version 3 许可证的约束, 可以在以下链接找到该许可证.
+ * Use of this source code is governed by the GNU AGPLv3 license that can be found through the following link.
+ *
+ * https://github.com/mamoe/mirai/blob/dev/LICENSE
+ */
+
+package net.mamoe.mirai.internal.message.protocol.impl
+
+import kotlinx.serialization.Serializable
+import net.mamoe.mirai.contact.MemberPermission
+import net.mamoe.mirai.internal.message.protocol.MessageProtocol
+import net.mamoe.mirai.internal.utils.io.ProtoBuf
+import net.mamoe.mirai.internal.utils.io.serialization.loadAs
+import net.mamoe.mirai.internal.utils.io.serialization.toByteArray
+import net.mamoe.mirai.message.data.CustomMessage
+import net.mamoe.mirai.message.data.CustomMessageMetadata
+import net.mamoe.mirai.utils.hexToBytes
+import org.junit.jupiter.api.BeforeEach
+import org.junit.jupiter.api.Test
+
+internal class CustomMessageProtocolTest : AbstractMessageProtocolTest() {
+    override val protocols: Array<out MessageProtocol> = arrayOf(CustomMessageProtocol(), TextProtocol())
+
+    @BeforeEach
+    fun `init group`() {
+        defaultTarget = bot.addGroup(123, 1230003).apply {
+            addMember(1230003, "user3", MemberPermission.OWNER)
+        }
+    }
+
+    @BeforeEach
+    fun init() {
+        MyCustomMessage(1) // register
+    }
+
+    @Serializable
+    class MyCustomMessage(
+        val int: Int,
+    ) : CustomMessageMetadata(), ProtoBuf {
+        override fun getFactory(): Factory = Factory
+
+        object Factory : CustomMessage.Factory<MyCustomMessage>("MyCustomMessage") {
+            override fun dump(message: MyCustomMessage): ByteArray {
+                return message.toByteArray(serializer())
+            }
+
+            override fun load(input: ByteArray): MyCustomMessage {
+                return input.loadAs(serializer())
+            }
+        }
+    }
+
+    @Test
+    fun `test encode`() {
+        buildChecks {
+            elem(
+                net.mamoe.mirai.internal.network.protocol.data.proto.ImMsgBody.Elem(
+                    customElem = net.mamoe.mirai.internal.network.protocol.data.proto.ImMsgBody.CustomElem(
+                        data = "00 00 00 17 08 01 12 0F 4D 79 43 75 73 74 6F 6D 4D 65 73 73 61 67 65 1A 02 08 01".hexToBytes(),
+                        enumType = 103904510,
+                    ),
+                )
+            )
+            message(MyCustomMessage(1))
+        }.doEncoderChecks()
+    }
+
+    @Test
+    fun `test decode`() {
+        buildChecks {
+            elem(
+                net.mamoe.mirai.internal.network.protocol.data.proto.ImMsgBody.Elem(
+                    customElem = net.mamoe.mirai.internal.network.protocol.data.proto.ImMsgBody.CustomElem(
+                        data = "00 00 00 17 08 01 12 0F 4D 79 43 75 73 74 6F 6D 4D 65 73 73 61 67 65 1A 02 08 01".hexToBytes(),
+                        enumType = 103904510,
+                    ),
+                )
+            )
+            message(MyCustomMessage(1))
+        }.doEncoderChecks()
+    }
+}
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