Add warning on compact (#6052)

This PR introduces the ability for `core` to send `warnings` as it can
send `errors. It also sends a warning on compaction.

<img width="811" height="187" alt="image"
src="https://github.com/user-attachments/assets/0947a42d-b720-420d-b7fd-115f8a65a46a"
/>

codex-rs/core/tests/suite/compact.rs

@@ -8,6 +8,7 @@ use codex_core::protocol::EventMsg;
 use codex_core::protocol::Op;
 use codex_core::protocol::RolloutItem;
 use codex_core::protocol::RolloutLine;
+use codex_core::protocol::WarningEvent;
 use codex_protocol::user_input::UserInput;
 use core_test_support::load_default_config_for_test;
 use core_test_support::skip_if_no_network;
@@ -45,6 +46,7 @@ const CONTEXT_LIMIT_MESSAGE: &str =
 const DUMMY_FUNCTION_NAME: &str = "unsupported_tool";
 const DUMMY_CALL_ID: &str = "call-multi-auto";
 const FUNCTION_CALL_LIMIT_MSG: &str = "function call limit push";
+pub(super) const COMPACT_WARNING_MESSAGE: &str = "Heads up: Long conversations and multiple compactions can cause the model to be less accurate. Start new a new conversation when possible to keep conversations small and targeted.";
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn summarize_context_three_requests_and_instructions() {
@@ -118,6 +120,11 @@ async fn summarize_context_three_requests_and_instructions() {
 
     // 2) Summarize – second hit should include the summarization prompt.
     codex.submit(Op::Compact).await.unwrap();
+    let warning_event = wait_for_event(&codex, |ev| matches!(ev, EventMsg::Warning(_))).await;
+    let EventMsg::Warning(WarningEvent { message }) = warning_event else {
+        panic!("expected warning event after compact");
+    };
+    assert_eq!(message, COMPACT_WARNING_MESSAGE);
     wait_for_event(&codex, |ev| matches!(ev, EventMsg::TaskComplete(_))).await;
 
     // 3) Next user input – third hit; history should include only the summary.
@@ -288,6 +295,11 @@ async fn manual_compact_uses_custom_prompt() {
         .conversation;
 
     codex.submit(Op::Compact).await.expect("trigger compact");
+    let warning_event = wait_for_event(&codex, |ev| matches!(ev, EventMsg::Warning(_))).await;
+    let EventMsg::Warning(WarningEvent { message }) = warning_event else {
+        panic!("expected warning event after compact");
+    };
+    assert_eq!(message, COMPACT_WARNING_MESSAGE);
     wait_for_event(&codex, |ev| matches!(ev, EventMsg::TaskComplete(_))).await;
 
     let requests = server.received_requests().await.expect("collect requests");
@@ -742,7 +754,6 @@ async fn manual_compact_retries_after_context_window_error() {
     wait_for_event(&codex, |ev| matches!(ev, EventMsg::TaskComplete(_))).await;
 
     codex.submit(Op::Compact).await.unwrap();
-
     let EventMsg::BackgroundEvent(event) =
         wait_for_event(&codex, |ev| matches!(ev, EventMsg::BackgroundEvent(_))).await
     else {
@@ -753,6 +764,11 @@ async fn manual_compact_retries_after_context_window_error() {
         "background event should mention trimmed item count: {}",
         event.message
     );
+    let warning_event = wait_for_event(&codex, |ev| matches!(ev, EventMsg::Warning(_))).await;
+    let EventMsg::Warning(WarningEvent { message }) = warning_event else {
+        panic!("expected warning event after compact retry");
+    };
+    assert_eq!(message, COMPACT_WARNING_MESSAGE);
     wait_for_event(&codex, |ev| matches!(ev, EventMsg::TaskComplete(_))).await;
 
     let requests = request_log.requests();

codex-rs/core/tests/suite/compact_resume_fork.rs

@@ -7,6 +7,7 @@
 //! request payload that Codex would send to the model and assert that the
 //! model-visible history matches the expected sequence of messages.
 
+use super::compact::COMPACT_WARNING_MESSAGE;
 use super::compact::FIRST_REPLY;
 use super::compact::SUMMARY_TEXT;
 use codex_core::CodexAuth;
@@ -20,6 +21,7 @@ use codex_core::config::Config;
 use codex_core::config::OPENAI_DEFAULT_MODEL;
 use codex_core::protocol::EventMsg;
 use codex_core::protocol::Op;
+use codex_core::protocol::WarningEvent;
 use codex_core::spawn::CODEX_SANDBOX_NETWORK_DISABLED_ENV_VAR;
 use codex_protocol::user_input::UserInput;
 use core_test_support::load_default_config_for_test;
@@ -813,6 +815,11 @@ async fn compact_conversation(conversation: &Arc<CodexConversation>) {
         .submit(Op::Compact)
         .await
         .expect("compact conversation");
+    let warning_event = wait_for_event(conversation, |ev| matches!(ev, EventMsg::Warning(_))).await;
+    let EventMsg::Warning(WarningEvent { message }) = warning_event else {
+        panic!("expected warning event after compact");
+    };
+    assert_eq!(message, COMPACT_WARNING_MESSAGE);
     wait_for_event(conversation, |ev| matches!(ev, EventMsg::TaskComplete(_))).await;
 }