[apps] Add gated instructions for Apps. (#10924)

- [x] Add gated instructions for Apps.

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

@@ -0,0 +1,3 @@
+mod render;
+
+pub(crate) use render::render_apps_section;

codex-rs/core/src/apps/render.rs

@@ -0,0 +1,7 @@
+use crate::mcp::CODEX_APPS_MCP_SERVER_NAME;
+
+pub(crate) fn render_apps_section() -> String {
+    format!(
+        "## Apps\nApps are mentioned in the prompt in the format `[$app-name](apps://{{connector_id}})`.\nAn app is equivalent to a set of MCP tools within the `{CODEX_APPS_MCP_SERVER_NAME}` MCP.\nWhen you see an app mention, the app's MCP tools are either already provided in `{CODEX_APPS_MCP_SERVER_NAME}`, or do not exist because the user did not install it.\nDo not additionally call list_mcp_resources for apps that are already mentioned."
+    )
+}

codex-rs/core/src/codex.rs

@@ -15,6 +15,7 @@ use crate::agent::MAX_THREAD_SPAWN_DEPTH;
 use crate::agent::agent_status_from_event;
 use crate::analytics_client::AnalyticsEventsClient;
 use crate::analytics_client::build_track_events_context;
+use crate::apps::render_apps_section;
 use crate::compact;
 use crate::compact::run_inline_auto_compact_task;
 use crate::compact::should_use_remote_compact_task;
@@ -2280,6 +2281,9 @@ impl Session {
                 );
             }
         }
+        if turn_context.features.enabled(Feature::Apps) {
+            items.push(DeveloperInstructions::new(render_apps_section()).into());
+        }
         if let Some(user_instructions) = turn_context.user_instructions.as_deref() {
             items.push(
                 UserInstructions {

codex-rs/core/src/lib.rs

@@ -8,6 +8,7 @@
 mod analytics_client;
 pub mod api_bridge;
 mod apply_patch;
+mod apps;
 pub mod auth;
 pub mod bash;
 mod client;

codex-rs/core/src/project_doc.rs

@@ -547,6 +547,30 @@ mod tests {
         assert_eq!(res, expected);
     }
 
+    #[tokio::test]
+    async fn apps_feature_does_not_emit_user_instructions_by_itself() {
+        let tmp = tempfile::tempdir().expect("tempdir");
+        let mut cfg = make_config(&tmp, 4096, None).await;
+        cfg.features.enable(Feature::Apps);
+
+        let res = get_user_instructions(&cfg, None).await;
+        assert_eq!(res, None);
+    }
+
+    #[tokio::test]
+    async fn apps_feature_does_not_append_to_project_doc_user_instructions() {
+        let tmp = tempfile::tempdir().expect("tempdir");
+        fs::write(tmp.path().join("AGENTS.md"), "base doc").unwrap();
+
+        let mut cfg = make_config(&tmp, 4096, None).await;
+        cfg.features.enable(Feature::Apps);
+
+        let res = get_user_instructions(&cfg, None)
+            .await
+            .expect("instructions expected");
+        assert_eq!(res, "base doc");
+    }
+
     fn create_skill(codex_home: PathBuf, name: &str, description: &str) {
         let skill_dir = codex_home.join(format!("skills/{name}"));
         fs::create_dir_all(&skill_dir).unwrap();