add agent_message_clear

work with agentclientprotocol/rust-sdk#59 , impls agentclientprotocol#465

Author: steve02081504

schema/schema.json

@@ -2564,6 +2564,17 @@
           "required": ["sessionUpdate"],
           "type": "object"
         },
+        {
+          "description": "Instructs the client to clear the accumulated streamed content for the current agent message. Subsequent agent_message_chunk updates append from empty. Enables non-monotonic streaming (e.g. reformatting, delegation, iterative refinement).",
+          "properties": {
+            "sessionUpdate": {
+              "const": "agent_message_clear",
+              "type": "string"
+            }
+          },
+          "required": ["sessionUpdate"],
+          "type": "object"
+        },
         {
           "allOf": [
             {

src/client.rs

@@ -77,6 +77,11 @@ pub enum SessionUpdate {
     UserMessageChunk(ContentChunk),
     /// A chunk of the agent's response being streamed.
     AgentMessageChunk(ContentChunk),
+    /// Instructs the client to clear the accumulated streamed content for the current agent message.
+    /// Subsequent `agent_message_chunk` updates append from empty. Enables non-monotonic streaming
+    /// (e.g. reformatting, delegation, iterative refinement).
+    /// See protocol docs: [RFD: agent_message_clear](https://github.com/agentclientprotocol/agent-client-protocol/blob/main/docs/rfds/agent_message_clear.md)
+    AgentMessageClear,
     /// A chunk of the agent's internal reasoning being streamed.
     AgentThoughtChunk(ContentChunk),
     /// Notification that a new tool call has been initiated.