diff --git a/acp-model/api/acp-model.api b/acp-model/api/acp-model.api
index eefd011..3dd270d 100644
--- a/acp-model/api/acp-model.api
+++ b/acp-model/api/acp-model.api
@@ -1660,6 +1660,36 @@ public final class com/agentclientprotocol/model/McpServer$Stdio$Companion {
 	public final fun serializer ()Lkotlinx/serialization/KSerializer;
 }
 
+public final class com/agentclientprotocol/model/MessageId {
+	public static final field Companion Lcom/agentclientprotocol/model/MessageId$Companion;
+	public static final synthetic fun box-impl (Ljava/lang/String;)Lcom/agentclientprotocol/model/MessageId;
+	public static fun constructor-impl (Ljava/lang/String;)Ljava/lang/String;
+	public fun equals (Ljava/lang/Object;)Z
+	public static fun equals-impl (Ljava/lang/String;Ljava/lang/Object;)Z
+	public static final fun equals-impl0 (Ljava/lang/String;Ljava/lang/String;)Z
+	public final fun getValue ()Ljava/lang/String;
+	public fun hashCode ()I
+	public static fun hashCode-impl (Ljava/lang/String;)I
+	public fun toString ()Ljava/lang/String;
+	public static fun toString-impl (Ljava/lang/String;)Ljava/lang/String;
+	public final synthetic fun unbox-impl ()Ljava/lang/String;
+}
+
+public final synthetic class com/agentclientprotocol/model/MessageId$$serializer : kotlinx/serialization/internal/GeneratedSerializer {
+	public static final field INSTANCE Lcom/agentclientprotocol/model/MessageId$$serializer;
+	public final fun childSerializers ()[Lkotlinx/serialization/KSerializer;
+	public synthetic fun deserialize (Lkotlinx/serialization/encoding/Decoder;)Ljava/lang/Object;
+	public final fun deserialize-D4v84t0 (Lkotlinx/serialization/encoding/Decoder;)Ljava/lang/String;
+	public final fun getDescriptor ()Lkotlinx/serialization/descriptors/SerialDescriptor;
+	public synthetic fun serialize (Lkotlinx/serialization/encoding/Encoder;Ljava/lang/Object;)V
+	public final fun serialize-XmIeEkI (Lkotlinx/serialization/encoding/Encoder;Ljava/lang/String;)V
+	public fun typeParametersSerializers ()[Lkotlinx/serialization/KSerializer;
+}
+
+public final class com/agentclientprotocol/model/MessageId$Companion {
+	public final fun serializer ()Lkotlinx/serialization/KSerializer;
+}
+
 public final class com/agentclientprotocol/model/ModelId {
 	public static final field Companion Lcom/agentclientprotocol/model/ModelId$Companion;
 	public static final synthetic fun box-impl (Ljava/lang/String;)Lcom/agentclientprotocol/model/ModelId;
@@ -2000,14 +2030,16 @@ public final class com/agentclientprotocol/model/PromptCapabilities$Companion {
 
 public final class com/agentclientprotocol/model/PromptRequest : com/agentclientprotocol/model/AcpRequest, com/agentclientprotocol/model/AcpWithSessionId {
 	public static final field Companion Lcom/agentclientprotocol/model/PromptRequest$Companion;
-	public synthetic fun <init> (Ljava/lang/String;Ljava/util/List;Lkotlinx/serialization/json/JsonElement;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
-	public synthetic fun <init> (Ljava/lang/String;Ljava/util/List;Lkotlinx/serialization/json/JsonElement;Lkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public synthetic fun <init> (Ljava/lang/String;Ljava/util/List;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public synthetic fun <init> (Ljava/lang/String;Ljava/util/List;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;Lkotlin/jvm/internal/DefaultConstructorMarker;)V
 	public final fun component1-7EW-EgU ()Ljava/lang/String;
 	public final fun component2 ()Ljava/util/List;
-	public final fun component3 ()Lkotlinx/serialization/json/JsonElement;
-	public final fun copy-nk3TnMc (Ljava/lang/String;Ljava/util/List;Lkotlinx/serialization/json/JsonElement;)Lcom/agentclientprotocol/model/PromptRequest;
-	public static synthetic fun copy-nk3TnMc$default (Lcom/agentclientprotocol/model/PromptRequest;Ljava/lang/String;Ljava/util/List;Lkotlinx/serialization/json/JsonElement;ILjava/lang/Object;)Lcom/agentclientprotocol/model/PromptRequest;
+	public final fun component3-f2XpNSU ()Ljava/lang/String;
+	public final fun component4 ()Lkotlinx/serialization/json/JsonElement;
+	public final fun copy-q1vIVlU (Ljava/lang/String;Ljava/util/List;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;)Lcom/agentclientprotocol/model/PromptRequest;
+	public static synthetic fun copy-q1vIVlU$default (Lcom/agentclientprotocol/model/PromptRequest;Ljava/lang/String;Ljava/util/List;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;ILjava/lang/Object;)Lcom/agentclientprotocol/model/PromptRequest;
 	public fun equals (Ljava/lang/Object;)Z
+	public final fun getMessageId-f2XpNSU ()Ljava/lang/String;
 	public final fun getPrompt ()Ljava/util/List;
 	public fun getSessionId-7EW-EgU ()Ljava/lang/String;
 	public fun get_meta ()Lkotlinx/serialization/json/JsonElement;
@@ -2032,14 +2064,16 @@ public final class com/agentclientprotocol/model/PromptRequest$Companion {
 
 public final class com/agentclientprotocol/model/PromptResponse : com/agentclientprotocol/model/AcpResponse {
 	public static final field Companion Lcom/agentclientprotocol/model/PromptResponse$Companion;
-	public fun <init> (Lcom/agentclientprotocol/model/StopReason;Lkotlinx/serialization/json/JsonElement;)V
-	public synthetic fun <init> (Lcom/agentclientprotocol/model/StopReason;Lkotlinx/serialization/json/JsonElement;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public synthetic fun <init> (Lcom/agentclientprotocol/model/StopReason;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public synthetic fun <init> (Lcom/agentclientprotocol/model/StopReason;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;Lkotlin/jvm/internal/DefaultConstructorMarker;)V
 	public final fun component1 ()Lcom/agentclientprotocol/model/StopReason;
-	public final fun component2 ()Lkotlinx/serialization/json/JsonElement;
-	public final fun copy (Lcom/agentclientprotocol/model/StopReason;Lkotlinx/serialization/json/JsonElement;)Lcom/agentclientprotocol/model/PromptResponse;
-	public static synthetic fun copy$default (Lcom/agentclientprotocol/model/PromptResponse;Lcom/agentclientprotocol/model/StopReason;Lkotlinx/serialization/json/JsonElement;ILjava/lang/Object;)Lcom/agentclientprotocol/model/PromptResponse;
+	public final fun component2-f2XpNSU ()Ljava/lang/String;
+	public final fun component3 ()Lkotlinx/serialization/json/JsonElement;
+	public final fun copy-5MJPFCQ (Lcom/agentclientprotocol/model/StopReason;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;)Lcom/agentclientprotocol/model/PromptResponse;
+	public static synthetic fun copy-5MJPFCQ$default (Lcom/agentclientprotocol/model/PromptResponse;Lcom/agentclientprotocol/model/StopReason;Ljava/lang/String;Lkotlinx/serialization/json/JsonElement;ILjava/lang/Object;)Lcom/agentclientprotocol/model/PromptResponse;
 	public fun equals (Ljava/lang/Object;)Z
 	public final fun getStopReason ()Lcom/agentclientprotocol/model/StopReason;
+	public final fun getUserMessageId-f2XpNSU ()Ljava/lang/String;
 	public fun get_meta ()Lkotlinx/serialization/json/JsonElement;
 	public fun hashCode ()I
 	public fun toString ()Ljava/lang/String;
@@ -3125,12 +3159,15 @@ public abstract class com/agentclientprotocol/model/SessionUpdate {
 
 public final class com/agentclientprotocol/model/SessionUpdate$AgentMessageChunk : com/agentclientprotocol/model/SessionUpdate {
 	public static final field Companion Lcom/agentclientprotocol/model/SessionUpdate$AgentMessageChunk$Companion;
-	public fun <init> (Lcom/agentclientprotocol/model/ContentBlock;)V
+	public synthetic fun <init> (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public synthetic fun <init> (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;Lkotlin/jvm/internal/DefaultConstructorMarker;)V
 	public final fun component1 ()Lcom/agentclientprotocol/model/ContentBlock;
-	public final fun copy (Lcom/agentclientprotocol/model/ContentBlock;)Lcom/agentclientprotocol/model/SessionUpdate$AgentMessageChunk;
-	public static synthetic fun copy$default (Lcom/agentclientprotocol/model/SessionUpdate$AgentMessageChunk;Lcom/agentclientprotocol/model/ContentBlock;ILjava/lang/Object;)Lcom/agentclientprotocol/model/SessionUpdate$AgentMessageChunk;
+	public final fun component2-f2XpNSU ()Ljava/lang/String;
+	public final fun copy-eVVbJIE (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;)Lcom/agentclientprotocol/model/SessionUpdate$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$AgentMessageChunk;
 	public fun equals (Ljava/lang/Object;)Z
 	public final fun getContent ()Lcom/agentclientprotocol/model/ContentBlock;
+	public final fun getMessageId-f2XpNSU ()Ljava/lang/String;
 	public fun hashCode ()I
 	public fun toString ()Ljava/lang/String;
 }
@@ -3152,12 +3189,15 @@ public final class com/agentclientprotocol/model/SessionUpdate$AgentMessageChunk
 
 public final class com/agentclientprotocol/model/SessionUpdate$AgentThoughtChunk : com/agentclientprotocol/model/SessionUpdate {
 	public static final field Companion Lcom/agentclientprotocol/model/SessionUpdate$AgentThoughtChunk$Companion;
-	public fun <init> (Lcom/agentclientprotocol/model/ContentBlock;)V
+	public synthetic fun <init> (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public synthetic fun <init> (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;Lkotlin/jvm/internal/DefaultConstructorMarker;)V
 	public final fun component1 ()Lcom/agentclientprotocol/model/ContentBlock;
-	public final fun copy (Lcom/agentclientprotocol/model/ContentBlock;)Lcom/agentclientprotocol/model/SessionUpdate$AgentThoughtChunk;
-	public static synthetic fun copy$default (Lcom/agentclientprotocol/model/SessionUpdate$AgentThoughtChunk;Lcom/agentclientprotocol/model/ContentBlock;ILjava/lang/Object;)Lcom/agentclientprotocol/model/SessionUpdate$AgentThoughtChunk;
+	public final fun component2-f2XpNSU ()Ljava/lang/String;
+	public final fun copy-eVVbJIE (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;)Lcom/agentclientprotocol/model/SessionUpdate$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$AgentThoughtChunk;
 	public fun equals (Ljava/lang/Object;)Z
 	public final fun getContent ()Lcom/agentclientprotocol/model/ContentBlock;
+	public final fun getMessageId-f2XpNSU ()Ljava/lang/String;
 	public fun hashCode ()I
 	public fun toString ()Ljava/lang/String;
 }
@@ -3484,12 +3524,15 @@ public final class com/agentclientprotocol/model/SessionUpdate$UsageUpdate$Compa
 
 public final class com/agentclientprotocol/model/SessionUpdate$UserMessageChunk : com/agentclientprotocol/model/SessionUpdate {
 	public static final field Companion Lcom/agentclientprotocol/model/SessionUpdate$UserMessageChunk$Companion;
-	public fun <init> (Lcom/agentclientprotocol/model/ContentBlock;)V
+	public synthetic fun <init> (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public synthetic fun <init> (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;Lkotlin/jvm/internal/DefaultConstructorMarker;)V
 	public final fun component1 ()Lcom/agentclientprotocol/model/ContentBlock;
-	public final fun copy (Lcom/agentclientprotocol/model/ContentBlock;)Lcom/agentclientprotocol/model/SessionUpdate$UserMessageChunk;
-	public static synthetic fun copy$default (Lcom/agentclientprotocol/model/SessionUpdate$UserMessageChunk;Lcom/agentclientprotocol/model/ContentBlock;ILjava/lang/Object;)Lcom/agentclientprotocol/model/SessionUpdate$UserMessageChunk;
+	public final fun component2-f2XpNSU ()Ljava/lang/String;
+	public final fun copy-eVVbJIE (Lcom/agentclientprotocol/model/ContentBlock;Ljava/lang/String;)Lcom/agentclientprotocol/model/SessionUpdate$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 fun equals (Ljava/lang/Object;)Z
 	public final fun getContent ()Lcom/agentclientprotocol/model/ContentBlock;
+	public final fun getMessageId-f2XpNSU ()Ljava/lang/String;
 	public fun hashCode ()I
 	public fun toString ()Ljava/lang/String;
 }
diff --git a/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/Requests.kt b/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/Requests.kt
index b1b76a8..9be7b04 100644
--- a/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/Requests.kt
+++ b/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/Requests.kt
@@ -398,10 +398,13 @@ public data class LoadSessionRequest(
  *
  * See protocol docs: [User Message](https://agentclientprotocol.com/protocol/prompt-turn#1-user-message)
  */
+@OptIn(UnstableApi::class)
 @Serializable
 public data class PromptRequest(
     override val sessionId: SessionId,
     val prompt: List<ContentBlock>,
+    @property:UnstableApi
+    val messageId: MessageId? = null,
     override val _meta: JsonElement? = null
 ) : AcpRequest, AcpWithSessionId
 
@@ -566,9 +569,12 @@ public data class NewSessionResponse(
  *
  * See protocol docs: [Check for Completion](https://agentclientprotocol.com/protocol/prompt-turn#4-check-for-completion)
  */
+@OptIn(UnstableApi::class)
 @Serializable
 public data class PromptResponse(
     val stopReason: StopReason,
+    @property:UnstableApi
+    val userMessageId: MessageId? = null,
     override val _meta: JsonElement? = null
 ) : AcpResponse
 
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 d300b36..cd8cbbe 100644
--- a/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/SessionUpdate.kt
+++ b/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/SessionUpdate.kt
@@ -87,28 +87,37 @@ public sealed class SessionUpdate {
     /**
      * A chunk of the user's message being streamed.
      */
+    @OptIn(UnstableApi::class)
     @Serializable
     @SerialName("user_message_chunk")
     public data class UserMessageChunk(
-        val content: ContentBlock
+        val content: ContentBlock,
+        @property:UnstableApi
+        val messageId: MessageId? = null
     ) : SessionUpdate()
 
     /**
      * A chunk of the agent's response being streamed.
      */
+    @OptIn(UnstableApi::class)
     @Serializable
     @SerialName("agent_message_chunk")
     public data class AgentMessageChunk(
-        val content: ContentBlock
+        val content: ContentBlock,
+        @property:UnstableApi
+        val messageId: MessageId? = null
     ) : SessionUpdate()
 
     /**
      * A chunk of the agent's internal reasoning being streamed.
      */
+    @OptIn(UnstableApi::class)
     @Serializable
     @SerialName("agent_thought_chunk")
     public data class AgentThoughtChunk(
-        val content: ContentBlock
+        val content: ContentBlock,
+        @property:UnstableApi
+        val messageId: MessageId? = null
     ) : SessionUpdate()
 
     /**
diff --git a/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/Types.kt b/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/Types.kt
index 1cabd21..9ed4673 100644
--- a/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/Types.kt
+++ b/acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/Types.kt
@@ -130,6 +130,24 @@ public value class SessionConfigGroupId(public val value: String) {
     override fun toString(): String = value
 }
 
+/**
+ * **UNSTABLE**
+ *
+ * This capability is not part of the spec yet, and may be removed or changed at any point.
+ *
+ * A unique message identifier for [PromptRequest], session update message chunks, and [PromptResponse].
+ *
+ * If provided by the client, the Agent SHOULD echo this value as `userMessageId` in the
+ * [PromptResponse] to confirm it was recorded.
+ * Both clients and agents MUST use UUID format for message IDs.
+ */
+@UnstableApi
+@JvmInline
+@Serializable
+public value class MessageId(public val value: String) {
+    override fun toString(): String = value
+}
+
 /**
  * The sender or recipient of messages and data in a conversation.
  */
diff --git a/acp/src/commonMain/kotlin/com/agentclientprotocol/client/ClientSessionImpl.kt b/acp/src/commonMain/kotlin/com/agentclientprotocol/client/ClientSessionImpl.kt
index 312f177..17c210a 100644
--- a/acp/src/commonMain/kotlin/com/agentclientprotocol/client/ClientSessionImpl.kt
+++ b/acp/src/commonMain/kotlin/com/agentclientprotocol/client/ClientSessionImpl.kt
@@ -69,7 +69,7 @@ internal class ClientSessionImpl(
         }
         try {
             logger.trace { "Sending prompt request: $content" }
-            val promptResponse = AcpMethod.AgentMethods.SessionPrompt(protocol, PromptRequest(sessionId, content, _meta))
+            val promptResponse = AcpMethod.AgentMethods.SessionPrompt(protocol, PromptRequest(sessionId, content, _meta = _meta))
             logger.trace { "Received prompt response: $promptResponse" }
 
             // after receiving prompt response we immediately close the current prompt channel