Add experimental realtime websocket backend prompt override (#12418)

- add top-level `experimental_realtime_ws_backend_prompt` config key
(experimental / do not use) and include it in config schema
- apply the override only to `Op::RealtimeConversation` websocket
`backend_prompt`, with config + realtime tests

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

@@ -405,7 +405,10 @@ pub struct Config {
     /// websocket transport base URL (the `Op::RealtimeConversation` `/ws`
     /// connection) without changing normal provider HTTP requests.
     pub experimental_realtime_ws_base_url: Option<String>,
-
+    /// Experimental / do not use. Overrides only the realtime conversation
+    /// websocket transport backend prompt (the `Op::RealtimeConversation`
+    /// `/ws` session.create backend_prompt) without changing normal prompts.
+    pub experimental_realtime_ws_backend_prompt: Option<String>,
     /// When set, restricts ChatGPT login to a specific workspace identifier.
     pub forced_chatgpt_workspace_id: Option<String>,
 
@@ -1136,7 +1139,10 @@ pub struct ConfigToml {
     /// websocket transport base URL (the `Op::RealtimeConversation` `/ws`
     /// connection) without changing normal provider HTTP requests.
     pub experimental_realtime_ws_base_url: Option<String>,
-
+    /// Experimental / do not use. Overrides only the realtime conversation
+    /// websocket transport backend prompt (the `Op::RealtimeConversation`
+    /// `/ws` session.create backend_prompt) without changing normal prompts.
+    pub experimental_realtime_ws_backend_prompt: Option<String>,
     pub projects: Option<HashMap<String, ProjectConfig>>,
 
     /// Controls the web search tool mode: disabled, cached, or live.
@@ -2065,6 +2071,7 @@ impl Config {
                 .or(cfg.chatgpt_base_url)
                 .unwrap_or("https://chatgpt.com/backend-api/".to_string()),
             experimental_realtime_ws_base_url: cfg.experimental_realtime_ws_base_url,
+            experimental_realtime_ws_backend_prompt: cfg.experimental_realtime_ws_backend_prompt,
             forced_chatgpt_workspace_id,
             forced_login_method,
             include_apply_patch_tool: include_apply_patch_tool_flag,
@@ -4607,6 +4614,7 @@ model_verbosity = "high"
                 personality: Some(Personality::Pragmatic),
                 chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
                 experimental_realtime_ws_base_url: None,
+                experimental_realtime_ws_backend_prompt: None,
                 base_instructions: None,
                 developer_instructions: None,
                 compact_prompt: None,
@@ -4728,6 +4736,7 @@ model_verbosity = "high"
             personality: Some(Personality::Pragmatic),
             chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
             experimental_realtime_ws_base_url: None,
+            experimental_realtime_ws_backend_prompt: None,
             base_instructions: None,
             developer_instructions: None,
             compact_prompt: None,
@@ -4847,6 +4856,7 @@ model_verbosity = "high"
             personality: Some(Personality::Pragmatic),
             chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
             experimental_realtime_ws_base_url: None,
+            experimental_realtime_ws_backend_prompt: None,
             base_instructions: None,
             developer_instructions: None,
             compact_prompt: None,
@@ -4952,6 +4962,7 @@ model_verbosity = "high"
             personality: Some(Personality::Pragmatic),
             chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
             experimental_realtime_ws_base_url: None,
+            experimental_realtime_ws_backend_prompt: None,
             base_instructions: None,
             developer_instructions: None,
             compact_prompt: None,
@@ -5738,7 +5749,6 @@ trust_level = "untrusted"
         );
         Ok(())
     }
-
     #[test]
     fn experimental_realtime_ws_base_url_loads_from_config_toml() -> std::io::Result<()> {
         let cfg: ConfigToml = toml::from_str(
@@ -5766,6 +5776,34 @@ experimental_realtime_ws_base_url = "http://127.0.0.1:8011"
         );
         Ok(())
     }
+
+    #[test]
+    fn experimental_realtime_ws_backend_prompt_loads_from_config_toml() -> std::io::Result<()> {
+        let cfg: ConfigToml = toml::from_str(
+            r#"
+experimental_realtime_ws_backend_prompt = "prompt from config"
+"#,
+        )
+        .expect("TOML deserialization should succeed");
+
+        assert_eq!(
+            cfg.experimental_realtime_ws_backend_prompt.as_deref(),
+            Some("prompt from config")
+        );
+
+        let codex_home = TempDir::new()?;
+        let config = Config::load_from_base_config_with_overrides(
+            cfg,
+            ConfigOverrides::default(),
+            codex_home.path().to_path_buf(),
+        )?;
+
+        assert_eq!(
+            config.experimental_realtime_ws_backend_prompt.as_deref(),
+            Some("prompt from config")
+        );
+        Ok(())
+    }
 }
 
 #[cfg(test)]

codex-rs/core/src/realtime_conversation.rs

@@ -174,18 +174,17 @@ pub(crate) async fn handle_start(
     if let Some(realtime_ws_base_url) = &config.experimental_realtime_ws_base_url {
         api_provider.base_url = realtime_ws_base_url.clone();
     }
+    let prompt = config
+        .experimental_realtime_ws_backend_prompt
+        .clone()
+        .unwrap_or(params.prompt);
 
     let requested_session_id = params
         .session_id
         .or_else(|| Some(sess.conversation_id.to_string()));
     let events_rx = match sess
         .conversation
-        .start(
-            api_provider,
-            None,
-            params.prompt,
-            requested_session_id.clone(),
-        )
+        .start(api_provider, None, prompt, requested_session_id.clone())
         .await
     {
         Ok(events_rx) => events_rx,