feat: experimental menu (#8071)

This will automatically render any `Stage::Beta` features.

The change only gets applied to the *next session*. This started as a
bug but actually this is a good thing to prevent out of distribution
push

<img width="986" height="288" alt="Screenshot 2025-12-15 at 15 38 35"
src="https://github.com/user-attachments/assets/78b7a71d-0e43-4828-a118-91c5237909c7"
/>


<img width="509" height="109" alt="Screenshot 2025-12-15 at 17 35 44"
src="https://github.com/user-attachments/assets/6933de52-9b66-4abf-b58b-a5f26d5747e2"
/>

codex-rs/core/src/client.rs

@@ -45,6 +45,7 @@ use crate::config::Config;
 use crate::default_client::build_reqwest_client;
 use crate::error::CodexErr;
 use crate::error::Result;
+use crate::features::FEATURES;
 use crate::flags::CODEX_RS_SSE_FIXTURE;
 use crate::model_provider_info::ModelProviderInfo;
 use crate::model_provider_info::WireApi;
@@ -261,6 +262,7 @@ impl ModelClient {
                 store_override: None,
                 conversation_id: Some(conversation_id.clone()),
                 session_source: Some(session_source.clone()),
+                extra_headers: beta_feature_headers(&self.config),
             };
 
             let stream_result = client
@@ -396,6 +398,27 @@ fn build_api_prompt(prompt: &Prompt, instructions: String, tools_json: Vec<Value
     }
 }
 
+fn beta_feature_headers(config: &Config) -> ApiHeaderMap {
+    let enabled = FEATURES
+        .iter()
+        .filter_map(|spec| {
+            if spec.stage.beta_menu_description().is_some() && config.features.enabled(spec.id) {
+                Some(spec.key)
+            } else {
+                None
+            }
+        })
+        .collect::<Vec<_>>();
+    let value = enabled.join(",");
+    let mut headers = ApiHeaderMap::new();
+    if !value.is_empty()
+        && let Ok(header_value) = HeaderValue::from_str(value.as_str())
+    {
+        headers.insert("x-codex-beta-features", header_value);
+    }
+    headers
+}
+
 fn map_response_stream<S>(api_stream: S, otel_manager: OtelManager) -> ResponseStream
 where
     S: futures::Stream<Item = std::result::Result<ResponseEvent, ApiError>>

codex-rs/core/src/features.rs

@@ -18,12 +18,33 @@ pub(crate) use legacy::LegacyFeatureToggles;
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum Stage {
     Experimental,
-    Beta,
+    Beta {
+        menu_description: &'static str,
+        announcement: &'static str,
+    },
     Stable,
     Deprecated,
     Removed,
 }
 
+impl Stage {
+    pub fn beta_menu_description(self) -> Option<&'static str> {
+        match self {
+            Stage::Beta {
+                menu_description, ..
+            } => Some(menu_description),
+            _ => None,
+        }
+    }
+
+    pub fn beta_announcement(self) -> Option<&'static str> {
+        match self {
+            Stage::Beta { announcement, .. } => Some(announcement),
+            _ => None,
+        }
+    }
+}
+
 /// Unique features toggled via configuration.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub enum Feature {
@@ -292,13 +313,32 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
-    // Unstable features.
+    FeatureSpec {
+        id: Feature::WebSearchRequest,
+        key: "web_search_request",
+        stage: Stage::Stable,
+        default_enabled: false,
+    },
+    // Beta program. Rendered in the `/experimental` menu for users.
+    FeatureSpec {
+        id: Feature::Skills,
+        key: "skills",
+        stage: Stage::Beta {
+            menu_description: "Define new `skills` for the model",
+            announcement: "NEW! Try the new `skills` features. Enable in /experimental!",
+        },
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::UnifiedExec,
         key: "unified_exec",
-        stage: Stage::Experimental,
+        stage: Stage::Beta {
+            menu_description: "Run long-running terminal commands in the background.",
+            announcement: "NEW! Try Background terminals for long running processes. Enable in /experimental!",
+        },
         default_enabled: false,
     },
+    // Unstable features.
     FeatureSpec {
         id: Feature::RmcpClient,
         key: "rmcp_client",
@@ -308,13 +348,7 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::ApplyPatchFreeform,
         key: "apply_patch_freeform",
-        stage: Stage::Beta,
-        default_enabled: false,
-    },
-    FeatureSpec {
-        id: Feature::WebSearchRequest,
-        key: "web_search_request",
-        stage: Stage::Stable,
+        stage: Stage::Experimental,
         default_enabled: false,
     },
     FeatureSpec {
@@ -347,12 +381,6 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Experimental,
         default_enabled: false,
     },
-    FeatureSpec {
-        id: Feature::Skills,
-        key: "skills",
-        stage: Stage::Experimental,
-        default_enabled: false,
-    },
     FeatureSpec {
         id: Feature::ShellSnapshot,
         key: "shell_snapshot",