Trim compaction input (#10374)

Two fixes:

1. Include trailing tool output in the total context size calculation.
Otherwise when checking whether compaction should run we ignore newly
added outputs.
2. Trim trailing tool output/tool calls until we can fit the request
into the model context size. Otherwise the compaction endpoint will fail
to compact. We only trim items that can be reproduced again by the model
(tool calls, tool call outputs).

codex-rs/core/src/compact_remote.rs

@@ -3,6 +3,8 @@ use std::sync::Arc;
 use crate::Prompt;
 use crate::codex::Session;
 use crate::codex::TurnContext;
+use crate::context_manager::ContextManager;
+use crate::context_manager::is_codex_generated_item;
 use crate::error::Result as CodexResult;
 use crate::protocol::CompactedItem;
 use crate::protocol::EventMsg;
@@ -11,6 +13,7 @@ use crate::protocol::TurnStartedEvent;
 use codex_protocol::items::ContextCompactionItem;
 use codex_protocol::items::TurnItem;
 use codex_protocol::models::ResponseItem;
+use tracing::info;
 
 pub(crate) async fn run_inline_remote_auto_compact_task(
     sess: Arc<Session>,
@@ -45,7 +48,16 @@ async fn run_remote_compact_task_inner_impl(
     let compaction_item = TurnItem::ContextCompaction(ContextCompactionItem::new());
     sess.emit_turn_item_started(turn_context, &compaction_item)
         .await;
-    let history = sess.clone_history().await;
+    let mut history = sess.clone_history().await;
+    let deleted_items =
+        trim_function_call_history_to_fit_context_window(&mut history, turn_context.as_ref());
+    if deleted_items > 0 {
+        info!(
+            turn_id = %turn_context.sub_id,
+            deleted_items,
+            "trimmed history items before remote compaction"
+        );
+    }
 
     // Required to keep `/undo` available after compaction
     let ghost_snapshots: Vec<ResponseItem> = history
@@ -86,3 +98,31 @@ async fn run_remote_compact_task_inner_impl(
         .await;
     Ok(())
 }
+
+fn trim_function_call_history_to_fit_context_window(
+    history: &mut ContextManager,
+    turn_context: &TurnContext,
+) -> usize {
+    let mut deleted_items = 0usize;
+    let Some(context_window) = turn_context.client.get_model_context_window() else {
+        return deleted_items;
+    };
+
+    while history
+        .estimate_token_count(turn_context)
+        .is_some_and(|estimated_tokens| estimated_tokens > context_window)
+    {
+        let Some(last_item) = history.raw_items().last() else {
+            break;
+        };
+        if !is_codex_generated_item(last_item) {
+            break;
+        }
+        if !history.remove_last_item() {
+            break;
+        }
+        deleted_items += 1;
+    }
+
+    deleted_items
+}