[codex] add developer instructions (#5897)

we are using developer instructions for code reviews, we need to pass
them in cli as well.

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

@@ -58,6 +58,18 @@ fn assert_message_role(request_body: &serde_json::Value, role: &str) {
     assert_eq!(request_body["role"].as_str().unwrap(), role);
 }
 
+#[expect(clippy::expect_used)]
+fn assert_message_equals(request_body: &serde_json::Value, text: &str) {
+    let content = request_body["content"][0]["text"]
+        .as_str()
+        .expect("invalid message content");
+
+    assert_eq!(
+        content, text,
+        "expected message content '{content}' to equal '{text}'"
+    );
+}
+
 #[expect(clippy::expect_used)]
 fn assert_message_starts_with(request_body: &serde_json::Value, text: &str) {
     let content = request_body["content"][0]["text"]
@@ -608,6 +620,64 @@ async fn includes_user_instructions_message_in_request() {
     assert_message_ends_with(&request_body["input"][1], "</environment_context>");
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn includes_developer_instructions_message_in_request() {
+    skip_if_no_network!();
+    let server = MockServer::start().await;
+
+    let resp_mock =
+        responses::mount_sse_once_match(&server, path("/v1/responses"), sse_completed("resp1"))
+            .await;
+
+    let model_provider = ModelProviderInfo {
+        base_url: Some(format!("{}/v1", server.uri())),
+        ..built_in_model_providers()["openai"].clone()
+    };
+
+    let codex_home = TempDir::new().unwrap();
+    let mut config = load_default_config_for_test(&codex_home);
+    config.model_provider = model_provider;
+    config.user_instructions = Some("be nice".to_string());
+    config.developer_instructions = Some("be useful".to_string());
+
+    let conversation_manager =
+        ConversationManager::with_auth(CodexAuth::from_api_key("Test API Key"));
+    let codex = conversation_manager
+        .new_conversation(config)
+        .await
+        .expect("create new conversation")
+        .conversation;
+
+    codex
+        .submit(Op::UserInput {
+            items: vec![UserInput::Text {
+                text: "hello".into(),
+            }],
+        })
+        .await
+        .unwrap();
+
+    wait_for_event(&codex, |ev| matches!(ev, EventMsg::TaskComplete(_))).await;
+
+    let request = resp_mock.single_request();
+    let request_body = request.body_json();
+
+    assert!(
+        !request_body["instructions"]
+            .as_str()
+            .unwrap()
+            .contains("be nice")
+    );
+    assert_message_role(&request_body["input"][0], "developer");
+    assert_message_equals(&request_body["input"][0], "be useful");
+    assert_message_role(&request_body["input"][1], "user");
+    assert_message_starts_with(&request_body["input"][1], "<user_instructions>");
+    assert_message_ends_with(&request_body["input"][1], "</user_instructions>");
+    assert_message_role(&request_body["input"][2], "user");
+    assert_message_starts_with(&request_body["input"][2], "<environment_context>");
+    assert_message_ends_with(&request_body["input"][2], "</environment_context>");
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn azure_responses_request_includes_store_and_reasoning_ids() {
     skip_if_no_network!();