add agent_message_clear

acp-model/src/commonMain/kotlin/com/agentclientprotocol/model/SessionUpdate.kt

@@ -111,6 +111,18 @@ public sealed class SessionUpdate {
         val content: ContentBlock
     ) : SessionUpdate()
 
+    /**
+     * Clears the accumulated streamed content for the current agent message.
+     *
+     * Instructs the client to clear the content built from previous [AgentMessageChunk] updates.
+     * Subsequent [AgentMessageChunk] updates will append from empty again.
+     *
+     * See: [agent_message_clear RFD](https://github.com/agentclientprotocol/agent-client-protocol/pull/465)
+     */
+    @Serializable
+    @SerialName("agent_message_clear")
+    public data class AgentMessageClear() : SessionUpdate()
+
     /**
      * Notification that a new tool call has been initiated.
      */
@@ -276,6 +288,13 @@ internal object SessionUpdateSerializer : KSerializer<SessionUpdate> {
                     base.forEach { (k, v) -> put(k, v) }
                 }
             }
+            is SessionUpdate.AgentMessageClear -> {
+                val base = ACPJson.encodeToJsonElement(SessionUpdate.AgentMessageClear.serializer(), value).jsonObject
+                buildJsonObject {
+                    put(SESSION_UPDATE_DISCRIMINATOR, "agent_message_clear")
+                    base.forEach { (k, v) -> put(k, v) }
+                }
+            }
             is SessionUpdate.ToolCall -> {
                 val base = ACPJson.encodeToJsonElement(SessionUpdate.ToolCall.serializer(), value).jsonObject
                 buildJsonObject {
@@ -364,6 +383,10 @@ internal object SessionUpdateSerializer : KSerializer<SessionUpdate> {
                 SessionUpdate.AgentThoughtChunk.serializer(),
                 jsonObject
             )
+            "agent_message_clear" -> ACPJson.decodeFromJsonElement(
+                SessionUpdate.AgentMessageClear.serializer(),
+                jsonObject
+            )
             "tool_call" -> ACPJson.decodeFromJsonElement(
                 SessionUpdate.ToolCall.serializer(),
                 jsonObject

acp-model/src/commonTest/kotlin/com/agentclientprotocol/model/SessionUpdateSerializerTest.kt

@@ -115,6 +115,19 @@ class SessionUpdateSerializerTest {
 
     // Round-trip serialization tests
 
+    @Test
+    fun `decodes agent_message_clear correctly`() {
+        val payload = """
+            {
+                "sessionUpdate": "agent_message_clear"
+            }
+        """.trimIndent()
+
+        val update = ACPJson.decodeFromString(SessionUpdate.serializer(), payload)
+
+        assertTrue(update is SessionUpdate.AgentMessageClear)
+    }
+
     @Test
     fun `round-trip serialization for known types`() {
         val original = SessionUpdate.AgentMessageChunk(
@@ -129,6 +142,17 @@ class SessionUpdateSerializerTest {
         assertEquals("Test message", (decoded.content as ContentBlock.Text).text)
     }
 
+    @Test
+    fun `round-trip serialization for AgentMessageClear`() {
+        val original = SessionUpdate.AgentMessageClear()
+
+        val encoded = ACPJson.encodeToString(SessionUpdate.serializer(), original)
+        assertTrue(encoded.contains("\"sessionUpdate\":\"agent_message_clear\""))
+
+        val decoded = ACPJson.decodeFromString(SessionUpdate.serializer(), encoded)
+        assertTrue(decoded is SessionUpdate.AgentMessageClear)
+    }
+
     @Test
     fun `round-trip serialization for UsageUpdate`() {
         val original = SessionUpdate.UsageUpdate(
@@ -257,6 +281,7 @@ class SessionUpdateSerializerTest {
             """{"sessionUpdate": "user_message_chunk", "content": {"type": "text", "text": "test"}}""",
             """{"sessionUpdate": "agent_message_chunk", "content": {"type": "text", "text": "test"}}""",
             """{"sessionUpdate": "agent_thought_chunk", "content": {"type": "text", "text": "test"}}""",
+            """{"sessionUpdate": "agent_message_clear"}""",
             """{"sessionUpdate": "tool_call", "toolCallId": "t1", "title": "test"}""",
             """{"sessionUpdate": "tool_call_update", "toolCallId": "t1"}""",
             """{"sessionUpdate": "plan", "entries": []}""",

samples/kotlin-acp-client-sample/src/main/kotlin/com/agentclientprotocol/samples/util.kt

@@ -13,6 +13,10 @@ fun SessionUpdate.render() {
             println("Agent thinks: ${this.content.render()}")
         }
 
+        is SessionUpdate.AgentMessageClear -> {
+            println("Agent message cleared")
+        }
+
         is SessionUpdate.AvailableCommandsUpdate -> {
             println("Available commands updated:")
         }