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

- [x] Add gated instructions for Apps.

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

@@ -16,6 +16,7 @@ use codex_core::auth::AuthCredentialsStoreMode;
 use codex_core::built_in_model_providers;
 use codex_core::default_client::originator;
 use codex_core::error::CodexErr;
+use codex_core::features::Feature;
 use codex_core::models_manager::manager::ModelsManager;
 use codex_core::protocol::EventMsg;
 use codex_core::protocol::Op;
@@ -666,6 +667,77 @@ async fn includes_user_instructions_message_in_request() {
     assert_message_ends_with(&request_body["input"][2], "</environment_context>");
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn includes_apps_guidance_as_developer_message_when_enabled() {
+    skip_if_no_network!();
+    let server = MockServer::start().await;
+
+    let resp_mock = mount_sse_once(
+        &server,
+        sse(vec![ev_response_created("resp1"), ev_completed("resp1")]),
+    )
+    .await;
+
+    let mut builder = test_codex()
+        .with_auth(CodexAuth::from_api_key("Test API Key"))
+        .with_config(|config| {
+            config.features.enable(Feature::Apps);
+        });
+    let codex = builder
+        .build(&server)
+        .await
+        .expect("create new conversation")
+        .codex;
+
+    codex
+        .submit(Op::UserInput {
+            items: vec![UserInput::Text {
+                text: "hello".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+        })
+        .await
+        .unwrap();
+
+    wait_for_event(&codex, |ev| matches!(ev, EventMsg::TurnComplete(_))).await;
+
+    let request = resp_mock.single_request();
+    let request_body = request.body_json();
+    let input = request_body["input"].as_array().expect("input array");
+    let apps_snippet = "Apps are mentioned in the prompt in the format";
+
+    let has_developer_apps_guidance = input.iter().any(|item| {
+        item.get("role").and_then(|value| value.as_str()) == Some("developer")
+            && item
+                .get("content")
+                .and_then(|value| value.as_array())
+                .and_then(|value| value.first())
+                .and_then(|value| value.get("text"))
+                .and_then(|value| value.as_str())
+                .is_some_and(|text| text.contains(apps_snippet))
+    });
+    assert!(
+        has_developer_apps_guidance,
+        "expected apps guidance in a developer message, got {input:#?}"
+    );
+
+    let has_user_apps_guidance = input.iter().any(|item| {
+        item.get("role").and_then(|value| value.as_str()) == Some("user")
+            && item
+                .get("content")
+                .and_then(|value| value.as_array())
+                .and_then(|value| value.first())
+                .and_then(|value| value.get("text"))
+                .and_then(|value| value.as_str())
+                .is_some_and(|text| text.contains(apps_snippet))
+    });
+    assert!(
+        !has_user_apps_guidance,
+        "did not expect apps guidance in user messages, got {input:#?}"
+    );
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn skills_append_to_instructions() {
     skip_if_no_network!();