Warn users on enabling underdevelopment features (#9954)

<img width="938" height="73" alt="image"
src="https://github.com/user-attachments/assets/a2d5ac46-92c5-4828-b35e-0965c30cdf36"
/>

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

@@ -74,6 +74,7 @@ mod tools;
 mod truncation;
 mod undo;
 mod unified_exec;
+mod unstable_features_warning;
 mod user_notification;
 mod user_shell_cmd;
 mod view_image;

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

@@ -0,0 +1,90 @@
+#![allow(clippy::unwrap_used, clippy::expect_used)]
+
+use codex_core::AuthManager;
+use codex_core::CodexAuth;
+use codex_core::NewThread;
+use codex_core::ThreadManager;
+use codex_core::config::CONFIG_TOML_FILE;
+use codex_core::features::Feature;
+use codex_core::protocol::EventMsg;
+use codex_core::protocol::InitialHistory;
+use codex_core::protocol::WarningEvent;
+use codex_utils_absolute_path::AbsolutePathBuf;
+use core::time::Duration;
+use core_test_support::load_default_config_for_test;
+use core_test_support::wait_for_event;
+use tempfile::TempDir;
+use tokio::time::timeout;
+use toml::toml;
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn emits_warning_when_unstable_features_enabled_via_config() {
+    let home = TempDir::new().expect("tempdir");
+    let mut config = load_default_config_for_test(&home).await;
+    config.features.enable(Feature::ChildAgentsMd);
+    let user_config_path =
+        AbsolutePathBuf::from_absolute_path(config.codex_home.join(CONFIG_TOML_FILE))
+            .expect("absolute user config path");
+    config.config_layer_stack = config.config_layer_stack.with_user_config(
+        &user_config_path,
+        toml! { features = { child_agents_md = true } }.into(),
+    );
+
+    let thread_manager = ThreadManager::with_models_provider(
+        CodexAuth::from_api_key("test"),
+        config.model_provider.clone(),
+    );
+    let auth_manager = AuthManager::from_auth_for_testing(CodexAuth::from_api_key("test"));
+
+    let NewThread {
+        thread: conversation,
+        ..
+    } = thread_manager
+        .resume_thread_with_history(config, InitialHistory::New, auth_manager)
+        .await
+        .expect("spawn conversation");
+
+    let warning = wait_for_event(&conversation, |ev| matches!(ev, EventMsg::Warning(_))).await;
+    let EventMsg::Warning(WarningEvent { message }) = warning else {
+        panic!("expected warning event");
+    };
+    assert!(message.contains("child_agents_md"));
+    assert!(message.contains("Under-development features enabled"));
+    assert!(message.contains("suppress_unstable_features_warning = true"));
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn suppresses_warning_when_configured() {
+    let home = TempDir::new().expect("tempdir");
+    let mut config = load_default_config_for_test(&home).await;
+    config.features.enable(Feature::ChildAgentsMd);
+    config.suppress_unstable_features_warning = true;
+    let user_config_path =
+        AbsolutePathBuf::from_absolute_path(config.codex_home.join(CONFIG_TOML_FILE))
+            .expect("absolute user config path");
+    config.config_layer_stack = config.config_layer_stack.with_user_config(
+        &user_config_path,
+        toml! { features = { child_agents_md = true } }.into(),
+    );
+
+    let thread_manager = ThreadManager::with_models_provider(
+        CodexAuth::from_api_key("test"),
+        config.model_provider.clone(),
+    );
+    let auth_manager = AuthManager::from_auth_for_testing(CodexAuth::from_api_key("test"));
+
+    let NewThread {
+        thread: conversation,
+        ..
+    } = thread_manager
+        .resume_thread_with_history(config, InitialHistory::New, auth_manager)
+        .await
+        .expect("spawn conversation");
+
+    let warning = timeout(
+        Duration::from_millis(150),
+        wait_for_event(&conversation, |ev| matches!(ev, EventMsg::Warning(_))),
+    )
+    .await;
+    assert!(warning.is_err());
+}