Default realtime startup to v2 model (#17183)

- Default realtime sessions to v2 and gpt-realtime-1.5 when no override
is configured.
- Add Op::RealtimeConversationStart integration coverage and keep
v1-specific tests explicit.

---------

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

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

@@ -6478,6 +6478,35 @@ experimental_realtime_ws_model = "realtime-test-model"
     Ok(())
 }
 
+#[test]
+fn realtime_config_partial_table_uses_realtime_defaults() -> std::io::Result<()> {
+    let cfg: ConfigToml = toml::from_str(
+        r#"
+[realtime]
+voice = "marin"
+"#,
+    )
+    .expect("TOML deserialization should succeed");
+
+    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.realtime,
+        RealtimeConfig {
+            version: RealtimeWsVersion::V2,
+            session_type: RealtimeWsMode::Conversational,
+            transport: RealtimeTransport::Websocket,
+            voice: Some(RealtimeVoice::Marin),
+        }
+    );
+    Ok(())
+}
+
 #[test]
 fn realtime_loads_from_config_toml() -> std::io::Result<()> {
     let cfg: ConfigToml = toml::from_str(

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

@@ -2089,11 +2089,14 @@ impl Config {
             experimental_realtime_ws_model: cfg.experimental_realtime_ws_model,
             realtime: cfg
                 .realtime
-                .map_or_else(RealtimeConfig::default, |realtime| RealtimeConfig {
-                    version: realtime.version.unwrap_or_default(),
-                    session_type: realtime.session_type.unwrap_or_default(),
-                    transport: realtime.transport.unwrap_or_default(),
-                    voice: realtime.voice,
+                .map_or_else(RealtimeConfig::default, |realtime| {
+                    let defaults = RealtimeConfig::default();
+                    RealtimeConfig {
+                        version: realtime.version.unwrap_or(defaults.version),
+                        session_type: realtime.session_type.unwrap_or(defaults.session_type),
+                        transport: realtime.transport.unwrap_or(defaults.transport),
+                        voice: realtime.voice,
+                    }
                 }),
             experimental_realtime_ws_backend_prompt: cfg.experimental_realtime_ws_backend_prompt,
             experimental_realtime_ws_startup_context: cfg.experimental_realtime_ws_startup_context,

codex-rs/core/src/realtime_conversation.rs

@@ -61,6 +61,7 @@ const USER_TEXT_IN_QUEUE_CAPACITY: usize = 64;
 const HANDOFF_OUT_QUEUE_CAPACITY: usize = 64;
 const OUTPUT_EVENTS_QUEUE_CAPACITY: usize = 256;
 const REALTIME_STARTUP_CONTEXT_TOKEN_BUDGET: usize = 5_000;
+const DEFAULT_REALTIME_MODEL: &str = "gpt-realtime-1.5";
 const ACTIVE_RESPONSE_CONFLICT_ERROR_PREFIX: &str =
     "Conversation already has an active response in progress:";
 
@@ -572,7 +573,12 @@ pub(crate) async fn build_realtime_session_config(
         (false, true) => prompt,
         (false, false) => format!("{prompt}\n\n{startup_context}"),
     };
-    let model = config.experimental_realtime_ws_model.clone();
+    let model = Some(
+        config
+            .experimental_realtime_ws_model
+            .clone()
+            .unwrap_or_else(|| DEFAULT_REALTIME_MODEL.to_string()),
+    );
     let event_parser = match config.realtime.version {
         RealtimeWsVersion::V1 => RealtimeEventParser::V1,
         RealtimeWsVersion::V2 => RealtimeEventParser::RealtimeV2,