feat: remote compaction (#6795)

Co-authored-by: pakrym-oai <pakrym@openai.com>

codex-rs/core/src/compact_remote.rs

@@ -0,0 +1,97 @@
+use std::sync::Arc;
+
+use crate::Prompt;
+use crate::codex::Session;
+use crate::codex::TurnContext;
+use crate::error::Result as CodexResult;
+use crate::protocol::AgentMessageEvent;
+use crate::protocol::CompactedItem;
+use crate::protocol::ErrorEvent;
+use crate::protocol::EventMsg;
+use crate::protocol::RolloutItem;
+use crate::protocol::TaskStartedEvent;
+use codex_protocol::models::ResponseInputItem;
+use codex_protocol::models::ResponseItem;
+use codex_protocol::user_input::UserInput;
+
+pub(crate) async fn run_remote_compact_task(
+    sess: Arc<Session>,
+    turn_context: Arc<TurnContext>,
+    input: Vec<UserInput>,
+) -> Option<String> {
+    let start_event = EventMsg::TaskStarted(TaskStartedEvent {
+        model_context_window: turn_context.client.get_model_context_window(),
+    });
+    sess.send_event(&turn_context, start_event).await;
+
+    match run_remote_compact_task_inner(&sess, &turn_context, input).await {
+        Ok(()) => {
+            let event = EventMsg::AgentMessage(AgentMessageEvent {
+                message: "Compact task completed".to_string(),
+            });
+            sess.send_event(&turn_context, event).await;
+        }
+        Err(err) => {
+            let event = EventMsg::Error(ErrorEvent {
+                message: err.to_string(),
+            });
+            sess.send_event(&turn_context, event).await;
+        }
+    }
+
+    None
+}
+
+async fn run_remote_compact_task_inner(
+    sess: &Arc<Session>,
+    turn_context: &Arc<TurnContext>,
+    input: Vec<UserInput>,
+) -> CodexResult<()> {
+    let mut history = sess.clone_history().await;
+    if !input.is_empty() {
+        let initial_input_for_turn: ResponseInputItem = ResponseInputItem::from(input);
+        history.record_items(&[initial_input_for_turn.into()]);
+    }
+
+    let prompt = Prompt {
+        input: history.get_history_for_prompt(),
+        tools: vec![],
+        parallel_tool_calls: false,
+        base_instructions_override: turn_context.base_instructions.clone(),
+        output_schema: None,
+    };
+
+    let mut new_history = turn_context
+        .client
+        .compact_conversation_history(&prompt)
+        .await?;
+    // Required to keep `/undo` available after compaction
+    let ghost_snapshots: Vec<ResponseItem> = history
+        .get_history()
+        .iter()
+        .filter(|item| matches!(item, ResponseItem::GhostSnapshot { .. }))
+        .cloned()
+        .collect();
+
+    if !ghost_snapshots.is_empty() {
+        new_history.extend(ghost_snapshots);
+    }
+    sess.replace_history(new_history.clone()).await;
+
+    if let Some(estimated_tokens) = sess
+        .clone_history()
+        .await
+        .estimate_token_count(turn_context.as_ref())
+    {
+        sess.override_last_token_usage_estimate(turn_context.as_ref(), estimated_tokens)
+            .await;
+    }
+
+    let compacted_item = CompactedItem {
+        message: String::new(),
+        replacement_history: Some(new_history),
+    };
+    sess.persist_rollout_items(&[RolloutItem::Compacted(compacted_item)])
+        .await;
+    Ok(())
+}