diff --git a/acp-model/api/acp-model.api b/acp-model/api/acp-model.api
index f7af3f2..3c52cc3 100644
--- a/acp-model/api/acp-model.api
+++ b/acp-model/api/acp-model.api
@@ -6351,17 +6351,19 @@ public abstract class com/agentclientprotocol/model/SessionUpdate {
 	public static final field Companion Lcom/agentclientprotocol/model/SessionUpdate$Companion;
 }
 
-public final class com/agentclientprotocol/model/SessionUpdate$AgentMessageChunk : com/agentclientprotocol/model/SessionUpdate {
+public final class com/agentclientprotocol/model/SessionUpdate$AgentMessageChunk : com/agentclientprotocol/model/SessionUpdate, com/agentclientprotocol/model/AcpWithMeta {
 	public static final field Companion Lcom/agentclientprotocol/model/SessionUpdate$AgentMessageChunk$Companion;
-	public synthetic fun <init> (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
-	public synthetic fun <init> (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;Lkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public synthetic fun <init> (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public synthetic fun <init> (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;Lkotlin/jvm/internal/DefaultConstructorMarker;)V
 	public final fun component1 ()Lcom/agentclientprotocol/model/ContentBlock;
 	public final fun component2-f2XpNSU ()Ljava/lang/String;
-	public final fun copy-eVVbJIE (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;)Lcom/agentclientprotocol/model/SessionUpdate$AgentMessageChunk;
-	public static synthetic fun copy-eVVbJIE$default (Lcom/agentclientprotocol/model/SessionUpdate$AgentMessageChunk;Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;ILjava/lang/Object;)Lcom/agentclientprotocol/model/SessionUpdate$AgentMessageChunk;
+	public final fun component3 ()Lkotlinx/serialization/json/JsonElement;
+	public final fun copy-5MJPFCQ (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;)Lcom/agentclientprotocol/model/SessionUpdate$AgentMessageChunk;
+	public static synthetic fun copy-5MJPFCQ$default (Lcom/agentclientprotocol/model/SessionUpdate$AgentMessageChunk;Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;ILjava/lang/Object;)Lcom/agentclientprotocol/model/SessionUpdate$AgentMessageChunk;
 	public fun equals (Ljava/lang/Object;)Z
 	public final fun getContent ()Lcom/agentclientprotocol/model/ContentBlock;
 	public final fun getMessageId-f2XpNSU ()Ljava/lang/String;
+	public fun get_meta ()Lkotlinx/serialization/json/JsonElement;
 	public fun hashCode ()I
 	public fun toString ()Ljava/lang/String;
 }
@@ -6381,17 +6383,19 @@ public final class com/agentclientprotocol/model/SessionUpdate$AgentMessageChunk
 	public final fun serializer ()Lkotlinx/serialization/KSerializer;
 }
 
-public final class com/agentclientprotocol/model/SessionUpdate$AgentThoughtChunk : com/agentclientprotocol/model/SessionUpdate {
+public final class com/agentclientprotocol/model/SessionUpdate$AgentThoughtChunk : com/agentclientprotocol/model/SessionUpdate, com/agentclientprotocol/model/AcpWithMeta {
 	public static final field Companion Lcom/agentclientprotocol/model/SessionUpdate$AgentThoughtChunk$Companion;
-	public synthetic fun <init> (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
-	public synthetic fun <init> (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;Lkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public synthetic fun <init> (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public synthetic fun <init> (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;Lkotlin/jvm/internal/DefaultConstructorMarker;)V
 	public final fun component1 ()Lcom/agentclientprotocol/model/ContentBlock;
 	public final fun component2-f2XpNSU ()Ljava/lang/String;
-	public final fun copy-eVVbJIE (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;)Lcom/agentclientprotocol/model/SessionUpdate$AgentThoughtChunk;
-	public static synthetic fun copy-eVVbJIE$default (Lcom/agentclientprotocol/model/SessionUpdate$AgentThoughtChunk;Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;ILjava/lang/Object;)Lcom/agentclientprotocol/model/SessionUpdate$AgentThoughtChunk;
+	public final fun component3 ()Lkotlinx/serialization/json/JsonElement;
+	public final fun copy-5MJPFCQ (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;)Lcom/agentclientprotocol/model/SessionUpdate$AgentThoughtChunk;
+	public static synthetic fun copy-5MJPFCQ$default (Lcom/agentclientprotocol/model/SessionUpdate$AgentThoughtChunk;Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;ILjava/lang/Object;)Lcom/agentclientprotocol/model/SessionUpdate$AgentThoughtChunk;
 	public fun equals (Ljava/lang/Object;)Z
 	public final fun getContent ()Lcom/agentclientprotocol/model/ContentBlock;
 	public final fun getMessageId-f2XpNSU ()Ljava/lang/String;
+	public fun get_meta ()Lkotlinx/serialization/json/JsonElement;
 	public fun hashCode ()I
 	public fun toString ()Ljava/lang/String;
 }
@@ -6776,17 +6780,19 @@ public final class com/agentclientprotocol/model/SessionUpdate$UsageUpdate$Compa
 	public final fun serializer ()Lkotlinx/serialization/KSerializer;
 }
 
-public final class com/agentclientprotocol/model/SessionUpdate$UserMessageChunk : com/agentclientprotocol/model/SessionUpdate {
+public final class com/agentclientprotocol/model/SessionUpdate$UserMessageChunk : com/agentclientprotocol/model/SessionUpdate, com/agentclientprotocol/model/AcpWithMeta {
 	public static final field Companion Lcom/agentclientprotocol/model/SessionUpdate$UserMessageChunk$Companion;
-	public synthetic fun <init> (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
-	public synthetic fun <init> (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;Lkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public synthetic fun <init> (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public synthetic fun <init> (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;Lkotlin/jvm/internal/DefaultConstructorMarker;)V
 	public final fun component1 ()Lcom/agentclientprotocol/model/ContentBlock;
 	public final fun component2-f2XpNSU ()Ljava/lang/String;
-	public final fun copy-eVVbJIE (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;)Lcom/agentclientprotocol/model/SessionUpdate$UserMessageChunk;
-	public static synthetic fun copy-eVVbJIE$default (Lcom/agentclientprotocol/model/SessionUpdate$UserMessageChunk;Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;ILjava/lang/Object;)Lcom/agentclientprotocol/model/SessionUpdate$UserMessageChunk;
+	public final fun component3 ()Lkotlinx/serialization/json/JsonElement;
+	public final fun copy-5MJPFCQ (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;)Lcom/agentclientprotocol/model/SessionUpdate$UserMessageChunk;
+	public static synthetic fun copy-5MJPFCQ$default (Lcom/agentclientprotocol/model/SessionUpdate$UserMessageChunk;Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;ILjava/lang/Object;)Lcom/agentclientprotocol/model/SessionUpdate$UserMessageChunk;
 	public fun equals (Ljava/lang/Object;)Z
 	public final fun getContent ()Lcom/agentclientprotocol/model/ContentBlock;
 	public final fun getMessageId-f2XpNSU ()Ljava/lang/String;
+	public fun get_meta ()Lkotlinx/serialization/json/JsonElement;
 	public fun hashCode ()I
 	public fun toString ()Ljava/lang/String;
 }
diff --git a/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/SessionUpdate.kt b/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/SessionUpdate.kt
index e4a58a4..b641a01 100644
--- a/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/SessionUpdate.kt
+++ b/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/SessionUpdate.kt
@@ -93,8 +93,9 @@ public sealed class SessionUpdate {
     public data class UserMessageChunk(
         val content: ContentBlock,
         @property:UnstableApi
-        val messageId: MessageId? = null
-    ) : SessionUpdate()
+        val messageId: MessageId? = null,
+        override val _meta: JsonElement? = null
+    ) : SessionUpdate(), AcpWithMeta
 
     /**
      * A chunk of the agent's response being streamed.
@@ -105,8 +106,9 @@ public sealed class SessionUpdate {
     public data class AgentMessageChunk(
         val content: ContentBlock,
         @property:UnstableApi
-        val messageId: MessageId? = null
-    ) : SessionUpdate()
+        val messageId: MessageId? = null,
+        override val _meta: JsonElement? = null
+    ) : SessionUpdate(), AcpWithMeta
 
     /**
      * A chunk of the agent's internal reasoning being streamed.
@@ -117,8 +119,9 @@ public sealed class SessionUpdate {
     public data class AgentThoughtChunk(
         val content: ContentBlock,
         @property:UnstableApi
-        val messageId: MessageId? = null
-    ) : SessionUpdate()
+        val messageId: MessageId? = null,
+        override val _meta: JsonElement? = null
+    ) : SessionUpdate(), AcpWithMeta
 
     /**
      * Notification that a new tool call has been initiated.
@@ -470,4 +473,4 @@ internal object SessionUpdateSerializer : KSerializer<SessionUpdate> {
             }
         }
     }
-}
\ No newline at end of file
+}
diff --git a/acp-model/src/commonTest/kotlin/com/agentclientprotocol/model/SessionUpdateSerializerTest.kt b/acp-model/src/commonTest/kotlin/com/agentclientprotocol/model/SessionUpdateSerializerTest.kt
index 285e5ed..96dac4c 100644
--- a/acp-model/src/commonTest/kotlin/com/agentclientprotocol/model/SessionUpdateSerializerTest.kt
+++ b/acp-model/src/commonTest/kotlin/com/agentclientprotocol/model/SessionUpdateSerializerTest.kt
@@ -2,6 +2,10 @@ package com.agentclientprotocol.model
 
 import com.agentclientprotocol.annotations.UnstableApi
 import com.agentclientprotocol.rpc.ACPJson
+import kotlinx.serialization.json.buildJsonObject
+import kotlinx.serialization.json.jsonObject
+import kotlinx.serialization.json.jsonPrimitive
+import kotlinx.serialization.json.put
 import kotlin.test.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertNotNull
@@ -129,6 +133,52 @@ class SessionUpdateSerializerTest {
         assertEquals("Test message", (decoded.content as ContentBlock.Text).text)
     }
 
+    @Test
+    fun `message chunks preserve metadata`() {
+        val metadata = buildJsonObject {
+            put("provider", buildJsonObject { put("phase", "working") })
+        }
+        val updates = listOf(
+            SessionUpdate.UserMessageChunk(ContentBlock.Text("User"), _meta = metadata),
+            SessionUpdate.AgentMessageChunk(ContentBlock.Text("Agent"), _meta = metadata),
+            SessionUpdate.AgentThoughtChunk(ContentBlock.Text("Thought"), _meta = metadata),
+        )
+
+        for (original in updates) {
+            val encoded = ACPJson.encodeToString(SessionUpdate.serializer(), original)
+            val decoded = ACPJson.decodeFromString(SessionUpdate.serializer(), encoded)
+
+            assertEquals(metadata, (decoded as AcpWithMeta)._meta)
+        }
+    }
+
+    @Test
+    fun `agent message metadata is preserved inside session notification`() {
+        val payload = """
+            {
+                "sessionId": "session-123",
+                "update": {
+                    "sessionUpdate": "agent_message_chunk",
+                    "messageId": "commentary-message",
+                    "content": {
+                        "type": "text",
+                        "text": "Checking the relevant event mapping."
+                    },
+                    "_meta": {
+                        "codex": {
+                            "phase": "commentary"
+                        }
+                    }
+                }
+            }
+        """.trimIndent()
+
+        val notification = ACPJson.decodeFromString(SessionNotification.serializer(), payload)
+        val update = notification.update as SessionUpdate.AgentMessageChunk
+
+        assertEquals("commentary", update._meta?.jsonObject?.get("codex")?.jsonObject?.get("phase")?.jsonPrimitive?.content)
+    }
+
     @Test
     fun `round-trip serialization for UsageUpdate`() {
         val original = SessionUpdate.UsageUpdate(