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/config.schema.json

@@ -1465,6 +1465,10 @@
       ],
       "description": "User-level skill config entries keyed by SKILL.md path."
     },
+    "suppress_unstable_features_warning": {
+      "description": "Suppress warnings about unstable (under development) features.",
+      "type": "boolean"
+    },
     "tool_output_token_limit": {
       "description": "Token budget applied when storing tool/function outputs in the context manager.",
       "format": "uint",

codex-rs/core/src/codex.rs

@@ -22,6 +22,7 @@ use crate::connectors;
 use crate::exec_policy::ExecPolicyManager;
 use crate::features::Feature;
 use crate::features::Features;
+use crate::features::maybe_push_unstable_features_warning;
 use crate::models_manager::manager::ModelsManager;
 use crate::parse_command::parse_command;
 use crate::parse_turn_item;
@@ -754,6 +755,7 @@ impl Session {
             });
         }
         maybe_push_chat_wire_api_deprecation(&config, &mut post_session_configured_events);
+        maybe_push_unstable_features_warning(&config, &mut post_session_configured_events);
 
         let auth = auth.as_ref();
         let otel_manager = OtelManager::new(

codex-rs/core/src/config/mod.rs

@@ -316,6 +316,9 @@ pub struct Config {
     /// Centralized feature flags; source of truth for feature gating.
     pub features: Features,
 
+    /// When `true`, suppress warnings about unstable (under development) features.
+    pub suppress_unstable_features_warning: bool,
+
     /// The active profile name used to derive this `Config` (if any).
     pub active_profile: Option<String>,
 
@@ -906,6 +909,9 @@ pub struct ConfigToml {
     #[schemars(schema_with = "crate::config::schema::features_schema")]
     pub features: Option<FeaturesToml>,
 
+    /// Suppress warnings about unstable (under development) features.
+    pub suppress_unstable_features_warning: Option<bool>,
+
     /// Settings for ghost snapshots (used for undo).
     #[serde(default)]
     pub ghost_snapshot: Option<GhostSnapshotToml>,
@@ -1564,6 +1570,9 @@ impl Config {
             use_experimental_unified_exec_tool,
             ghost_snapshot,
             features,
+            suppress_unstable_features_warning: cfg
+                .suppress_unstable_features_warning
+                .unwrap_or(false),
             active_profile: active_profile_name,
             active_project,
             windows_wsl_setup_acknowledged: cfg.windows_wsl_setup_acknowledged.unwrap_or(false),
@@ -3732,6 +3741,7 @@ model_verbosity = "high"
                 use_experimental_unified_exec_tool: false,
                 ghost_snapshot: GhostSnapshotConfig::default(),
                 features: Features::with_defaults(),
+                suppress_unstable_features_warning: false,
                 active_profile: Some("o3".to_string()),
                 active_project: ProjectConfig { trust_level: None },
                 windows_wsl_setup_acknowledged: false,
@@ -3814,6 +3824,7 @@ model_verbosity = "high"
             use_experimental_unified_exec_tool: false,
             ghost_snapshot: GhostSnapshotConfig::default(),
             features: Features::with_defaults(),
+            suppress_unstable_features_warning: false,
             active_profile: Some("gpt3".to_string()),
             active_project: ProjectConfig { trust_level: None },
             windows_wsl_setup_acknowledged: false,
@@ -3911,6 +3922,7 @@ model_verbosity = "high"
             use_experimental_unified_exec_tool: false,
             ghost_snapshot: GhostSnapshotConfig::default(),
             features: Features::with_defaults(),
+            suppress_unstable_features_warning: false,
             active_profile: Some("zdr".to_string()),
             active_project: ProjectConfig { trust_level: None },
             windows_wsl_setup_acknowledged: false,
@@ -3994,6 +4006,7 @@ model_verbosity = "high"
             use_experimental_unified_exec_tool: false,
             ghost_snapshot: GhostSnapshotConfig::default(),
             features: Features::with_defaults(),
+            suppress_unstable_features_warning: false,
             active_profile: Some("gpt5".to_string()),
             active_project: ProjectConfig { trust_level: None },
             windows_wsl_setup_acknowledged: false,

codex-rs/core/src/features.rs

@@ -5,14 +5,20 @@
 //! booleans through multiple types, call sites consult a single `Features`
 //! container attached to `Config`.
 
+use crate::config::CONFIG_TOML_FILE;
+use crate::config::Config;
 use crate::config::ConfigToml;
 use crate::config::profile::ConfigProfile;
+use crate::protocol::Event;
+use crate::protocol::EventMsg;
+use crate::protocol::WarningEvent;
 use codex_otel::OtelManager;
 use schemars::JsonSchema;
 use serde::Deserialize;
 use serde::Serialize;
 use std::collections::BTreeMap;
 use std::collections::BTreeSet;
+use toml::Value as TomlValue;
 
 mod legacy;
 pub(crate) use legacy::LegacyFeatureToggles;
@@ -466,3 +472,54 @@ pub const FEATURES: &[FeatureSpec] = &[
         default_enabled: false,
     },
 ];
+
+/// Push a warning event if any under-development features are enabled.
+pub fn maybe_push_unstable_features_warning(
+    config: &Config,
+    post_session_configured_events: &mut Vec<Event>,
+) {
+    if config.suppress_unstable_features_warning {
+        return;
+    }
+
+    let mut under_development_feature_keys = Vec::new();
+    if let Some(table) = config
+        .config_layer_stack
+        .effective_config()
+        .get("features")
+        .and_then(TomlValue::as_table)
+    {
+        for (key, value) in table {
+            if value.as_bool() != Some(true) {
+                continue;
+            }
+            let Some(spec) = FEATURES.iter().find(|spec| spec.key == key.as_str()) else {
+                continue;
+            };
+            if !config.features.enabled(spec.id) {
+                continue;
+            }
+            if matches!(spec.stage, Stage::UnderDevelopment) {
+                under_development_feature_keys.push(spec.key.to_string());
+            }
+        }
+    }
+
+    if under_development_feature_keys.is_empty() {
+        return;
+    }
+
+    let under_development_feature_keys = under_development_feature_keys.join(", ");
+    let config_path = config
+        .codex_home
+        .join(CONFIG_TOML_FILE)
+        .display()
+        .to_string();
+    let message = format!(
+        "Under-development features enabled: {under_development_feature_keys}. Under-development features are incomplete and may behave unpredictably. To suppress this warning, set `suppress_unstable_features_warning = true` in {config_path}."
+    );
+    post_session_configured_events.push(Event {
+        id: "".to_owned(),
+        msg: EventMsg::Warning(WarningEvent { message }),
+    });
+}

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());
+}