Include attributed previews in agent message board notifications (#47075)

## Why

Message board notifications exposed only post and thread IDs, requiring a read to see any post content.

## What changed

Deliver notifications as `agent_message` items with author and recipient attribution. Include a `CHANNEL_POST` payload with the sender, channel, message and thread IDs, and up to 150 characters of post text. Direct agents to `read_post` when the preview is truncated, and retain recognition of legacy user-role notices.

## Testing

Update the model-context scenario to assert the notification's author, recipient, and preview payload, and update the idle-agent scenario to check for the new notification format.

GitOrigin-RevId: e0e2794be82e38bba47f41fb4fc8128eb5e47619

codex-rs/core/src/agent_message_board.rs

@@ -15,7 +15,7 @@ use codex_agent_message_board_extension::AgentMessageBoard;
 use codex_agent_message_board_extension::LocalAgentMessageBoard;
 use codex_agent_message_board_extension::MessageBoardHost;
 use codex_agent_message_board_extension::NotificationDelivery;
-use codex_agent_message_board_extension::PostMetadata;
+use codex_agent_message_board_extension::PostPreview;
 use codex_extension_api::ExtensionRegistryBuilder;
 use codex_features::Feature;
 use codex_protocol::AgentPath;
@@ -24,6 +24,7 @@ use codex_protocol::ThreadId;
 use codex_protocol::error::CodexErr;
 use codex_protocol::error::CodexErrorDetails;
 use codex_protocol::error::Result;
+use codex_protocol::protocol::InterAgentCommunication;
 use futures::future::BoxFuture;
 use std::sync::Arc;
 use std::sync::Weak;
@@ -141,14 +142,14 @@ impl MessageBoardHost for LocalBoardHost {
 
     fn notify(
         &self,
-        recipient: ThreadId,
-        post: PostMetadata,
+        recipient_id: ThreadId,
+        post: PostPreview,
     ) -> BoxFuture<'_, Result<NotificationDelivery>> {
         Box::pin(async move {
             let Some(manager) = self.manager.upgrade() else {
                 return Ok(NotificationDelivery::SkippedInactive);
             };
-            let recipient = match manager.get_thread(recipient).await {
+            let recipient = match manager.get_thread(recipient_id).await {
                 Ok(thread) => thread,
                 Err(error) if matches!(error.details(), CodexErrorDetails::ThreadNotFound(_)) => {
                     return Ok(NotificationDelivery::SkippedInactive);
@@ -160,13 +161,24 @@ impl MessageBoardHost for LocalBoardHost {
                     "notification recipient belongs to another board".into(),
                 ));
             }
-            let notice = AgentMessageBoardNotification {
-                message_id: post.message_id,
-                thread_id: post.thread_id,
-            };
+            let recipient_path = recipient
+                .session
+                .services
+                .agent_control
+                .ensure_agent_known(recipient_id)?
+                .agent_path
+                .ok_or_else(|| CodexErr::InvalidRequest("agent has no tree path".into()))?;
+            let notice = AgentMessageBoardNotification(post);
+            let communication = InterAgentCommunication::new(
+                notice.0.metadata.author.clone(),
+                recipient_path,
+                Vec::new(),
+                notice.render(),
+                /*trigger_turn*/ false,
+            );
             Ok(
                 match recipient
-                    .inject_if_running(vec![ContextualUserFragment::into(notice)])
+                    .inject_if_running(vec![communication.to_model_input_item()])
                     .await
                 {
                     Ok(()) => NotificationDelivery::Accepted,

codex-rs/core/src/context/agent_message_board_notification.rs

@@ -1,34 +1,42 @@
-//! Fixed-size discussion notices. Post text is fetched through bounded tools.
+//! Attributed discussion notices with bounded previews.
 
 use super::ContextualUserFragment;
+use codex_agent_message_board_extension::PostPreview;
 use codex_protocol::models::ContentItemKind;
-use uuid::Uuid;
 
-pub(crate) struct AgentMessageBoardNotification {
-    pub(crate) message_id: Uuid,
-    pub(crate) thread_id: Uuid,
-}
+pub(crate) struct AgentMessageBoardNotification(pub(crate) PostPreview);
 
 impl ContextualUserFragment for AgentMessageBoardNotification {
     fn role(&self) -> &'static str {
-        "user"
+        "assistant"
     }
     fn content_kind(&self) -> ContentItemKind {
         ContentItemKind("agent_message_board.notification".into())
     }
     fn markers(&self) -> (&'static str, &'static str) {
-        Self::type_markers()
+        ("", "")
     }
     fn type_markers() -> (&'static str, &'static str) {
+        // Keep recognizing user-role notices in rollouts written before this format.
         (
             "<agent_message_board_notification>",
             "</agent_message_board_notification>",
         )
     }
     fn body(&self) -> String {
+        let post = &self.0;
+        let suffix = if post.truncated {
+            "\n[Use read_post for the rest.]"
+        } else {
+            ""
+        };
         format!(
-            "\nNew post {} in discussion {}. Use read_post or read_thread to read it.\n",
-            self.message_id, self.thread_id
+            "Message Type: CHANNEL_POST\nSender: {}\nChannel: {}\nMessage ID: {}\nThread ID: {}\nPayload:\n{}{suffix}",
+            post.metadata.author,
+            post.metadata.channel_name,
+            post.metadata.message_id,
+            post.metadata.thread_id,
+            post.text_preview,
         )
     }
 }