Move default realtime prompt into core (#17165)

- Adds a core-owned realtime backend prompt template and preparation
path.
- Makes omitted realtime start prompts use the core default, while null
or empty prompts intentionally send empty instructions.
- Covers the core realtime path and app-server v2 path with integration
coverage.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/lib.rs

@@ -13,6 +13,7 @@ mod client_common;
 pub(crate) mod codex;
 mod realtime_context;
 mod realtime_conversation;
+mod realtime_prompt;
 pub use codex::SteerInputError;
 mod codex_thread;
 mod compact_remote;

codex-rs/core/src/realtime_conversation.rs

@@ -1,6 +1,7 @@
 use crate::client::ModelClient;
 use crate::codex::Session;
 use crate::realtime_context::build_realtime_startup_context;
+use crate::realtime_prompt::prepare_realtime_backend_prompt;
 use async_channel::Receiver;
 use async_channel::Sender;
 use async_channel::TrySendError;
@@ -546,14 +547,14 @@ async fn prepare_realtime_start(
 
 pub(crate) async fn build_realtime_session_config(
     sess: &Arc<Session>,
-    prompt: String,
+    prompt: Option<Option<String>>,
     session_id: Option<String>,
 ) -> CodexResult<RealtimeSessionConfig> {
     let config = sess.get_config().await;
-    let prompt = config
-        .experimental_realtime_ws_backend_prompt
-        .clone()
-        .unwrap_or(prompt);
+    let prompt = prepare_realtime_backend_prompt(
+        prompt,
+        config.experimental_realtime_ws_backend_prompt.clone(),
+    );
     let startup_context = match config.experimental_realtime_ws_startup_context.clone() {
         Some(startup_context) => startup_context,
         None => {
@@ -562,10 +563,11 @@ pub(crate) async fn build_realtime_session_config(
                 .unwrap_or_default()
         }
     };
-    let prompt = if startup_context.is_empty() {
-        prompt
-    } else {
-        format!("{prompt}\n\n{startup_context}")
+    let prompt = match (prompt.is_empty(), startup_context.is_empty()) {
+        (true, true) => String::new(),
+        (true, false) => startup_context,
+        (false, true) => prompt,
+        (false, false) => format!("{prompt}\n\n{startup_context}"),
     };
     let model = config.experimental_realtime_ws_model.clone();
     let event_parser = match config.realtime.version {

codex-rs/core/src/realtime_prompt.rs

@@ -0,0 +1,81 @@
+const BACKEND_PROMPT: &str = include_str!("../templates/realtime/backend_prompt.md");
+const DEFAULT_USER_FIRST_NAME: &str = "there";
+const USER_FIRST_NAME_PLACEHOLDER: &str = "{{ user_first_name }}";
+
+pub(crate) fn prepare_realtime_backend_prompt(
+    prompt: Option<Option<String>>,
+    config_prompt: Option<String>,
+) -> String {
+    if let Some(config_prompt) = config_prompt
+        && !config_prompt.trim().is_empty()
+    {
+        return config_prompt;
+    }
+
+    match prompt {
+        Some(Some(prompt)) => return prompt,
+        Some(None) => return String::new(),
+        None => {}
+    }
+
+    BACKEND_PROMPT
+        .trim_end()
+        .replace(USER_FIRST_NAME_PLACEHOLDER, &current_user_first_name())
+}
+
+fn current_user_first_name() -> String {
+    [whoami::realname(), whoami::username()]
+        .into_iter()
+        .filter_map(|name| name.split_whitespace().next().map(str::to_string))
+        .find(|name| !name.is_empty())
+        .unwrap_or_else(|| DEFAULT_USER_FIRST_NAME.to_string())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::prepare_realtime_backend_prompt;
+
+    #[test]
+    fn prepare_realtime_backend_prompt_prefers_config_override() {
+        assert_eq!(
+            prepare_realtime_backend_prompt(
+                Some(Some("prompt from request".to_string())),
+                Some("prompt from config".to_string()),
+            ),
+            "prompt from config"
+        );
+    }
+
+    #[test]
+    fn prepare_realtime_backend_prompt_uses_request_prompt() {
+        assert_eq!(
+            prepare_realtime_backend_prompt(
+                Some(Some("prompt from request".to_string())),
+                /*config_prompt*/ None,
+            ),
+            "prompt from request"
+        );
+    }
+
+    #[test]
+    fn prepare_realtime_backend_prompt_preserves_empty_request_prompt() {
+        assert_eq!(
+            prepare_realtime_backend_prompt(Some(Some(String::new())), /*config_prompt*/ None),
+            ""
+        );
+        assert_eq!(
+            prepare_realtime_backend_prompt(Some(None), /*config_prompt*/ None),
+            ""
+        );
+    }
+
+    #[test]
+    fn prepare_realtime_backend_prompt_renders_default() {
+        let prompt =
+            prepare_realtime_backend_prompt(/*prompt*/ None, /*config_prompt*/ None);
+
+        assert!(prompt.starts_with("You are **Codex**"));
+        assert!(prompt.contains("The user's name is "));
+        assert!(!prompt.contains("{{ user_first_name }}"));
+    }
+}