Add realtime transcription mode for websocket sessions (#14556)

- add experimental_realtime_ws_mode (conversational/transcription) and
plumb it into realtime conversation session config
- switch realtime websocket intent and session.update payload shape
based on mode
- update config schema and realtime/config tests

---------

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

codex-rs/codex-api/src/endpoint/realtime_websocket/methods.rs

@@ -7,6 +7,7 @@ use crate::endpoint::realtime_websocket::protocol::RealtimeEvent;
 use crate::endpoint::realtime_websocket::protocol::RealtimeEventParser;
 use crate::endpoint::realtime_websocket::protocol::RealtimeOutboundMessage;
 use crate::endpoint::realtime_websocket::protocol::RealtimeSessionConfig;
+use crate::endpoint::realtime_websocket::protocol::RealtimeSessionMode;
 use crate::endpoint::realtime_websocket::protocol::RealtimeTranscriptDelta;
 use crate::endpoint::realtime_websocket::protocol::RealtimeTranscriptEntry;
 use crate::endpoint::realtime_websocket::protocol::SessionAudio;
@@ -52,6 +53,16 @@ const REALTIME_V2_SESSION_TYPE: &str = "realtime";
 const REALTIME_V2_CODEX_TOOL_NAME: &str = "codex";
 const REALTIME_V2_CODEX_TOOL_DESCRIPTION: &str = "Delegate work to Codex and return the result.";
 
+fn normalized_session_mode(
+    event_parser: RealtimeEventParser,
+    session_mode: RealtimeSessionMode,
+) -> RealtimeSessionMode {
+    match event_parser {
+        RealtimeEventParser::V1 => RealtimeSessionMode::Conversational,
+        RealtimeEventParser::RealtimeV2 => session_mode,
+    }
+}
+
 struct WsStream {
     tx_command: mpsc::Sender<WsCommand>,
     pump_task: tokio::task::JoinHandle<()>,
@@ -289,12 +300,16 @@ impl RealtimeWebsocketWriter {
     }
 
     pub async fn send_conversation_item_create(&self, text: String) -> Result<(), ApiError> {
+        let content_kind = match self.event_parser {
+            RealtimeEventParser::V1 => "text",
+            RealtimeEventParser::RealtimeV2 => "input_text",
+        };
         self.send_json(RealtimeOutboundMessage::ConversationItemCreate {
             item: ConversationItemPayload::Message(ConversationMessageItem {
                 kind: "message".to_string(),
                 role: "user".to_string(),
                 content: vec![ConversationItemContent {
-                    kind: "text".to_string(),
+                    kind: content_kind.to_string(),
                     text,
                 }],
             }),
@@ -326,34 +341,51 @@ impl RealtimeWebsocketWriter {
         self.send_json(message).await
     }
 
-    pub async fn send_session_update(&self, instructions: String) -> Result<(), ApiError> {
+    pub async fn send_session_update(
-        let (session_kind, tools) = match self.event_parser {
+        &self,
-            RealtimeEventParser::V1 => (REALTIME_V1_SESSION_TYPE.to_string(), None),
+        instructions: String,
-            RealtimeEventParser::RealtimeV2 => (
+        session_mode: RealtimeSessionMode,
-                REALTIME_V2_SESSION_TYPE.to_string(),
+    ) -> Result<(), ApiError> {
-                Some(vec![SessionFunctionTool {
+        let session_mode = normalized_session_mode(self.event_parser, session_mode);
-                    kind: "function".to_string(),
+        let (session_kind, session_instructions, output_audio) = match session_mode {
-                    name: REALTIME_V2_CODEX_TOOL_NAME.to_string(),
+            RealtimeSessionMode::Conversational => {
-                    description: REALTIME_V2_CODEX_TOOL_DESCRIPTION.to_string(),
+                let kind = match self.event_parser {
-                    parameters: json!({
+                    RealtimeEventParser::V1 => REALTIME_V1_SESSION_TYPE.to_string(),
-                        "type": "object",
+                    RealtimeEventParser::RealtimeV2 => REALTIME_V2_SESSION_TYPE.to_string(),
-                        "properties": {
+                };
-                            "prompt": {
+                (
-                                "type": "string",
+                    kind,
-                                "description": "Prompt text for the delegated Codex task."
+                    Some(instructions),
-                            }
+                    Some(SessionAudioOutput {
-                        },
+                        voice: REALTIME_AUDIO_VOICE.to_string(),
-                        "required": ["prompt"],
-                        "additionalProperties": false
                     }),
-                }]),
+                )
-            ),
+            }
+            RealtimeSessionMode::Transcription => ("transcription".to_string(), None, None),
+        };
+        let tools = match self.event_parser {
+            RealtimeEventParser::RealtimeV2 => Some(vec![SessionFunctionTool {
+                kind: "function".to_string(),
+                name: REALTIME_V2_CODEX_TOOL_NAME.to_string(),
+                description: REALTIME_V2_CODEX_TOOL_DESCRIPTION.to_string(),
+                parameters: json!({
+                    "type": "object",
+                    "properties": {
+                        "prompt": {
+                            "type": "string",
+                            "description": "Prompt text for the delegated Codex task."
+                        }
+                    },
+                    "required": ["prompt"],
+                    "additionalProperties": false
+                }),
+            }]),
+            RealtimeEventParser::V1 => None,
         };
-
         self.send_json(RealtimeOutboundMessage::SessionUpdate {
             session: SessionUpdateSession {
                 kind: session_kind,
-                instructions,
+                instructions: session_instructions,
                 audio: SessionAudio {
                     input: SessionAudioInput {
                         format: SessionAudioFormat {
@@ -361,9 +393,7 @@ impl RealtimeWebsocketWriter {
                             rate: REALTIME_AUDIO_SAMPLE_RATE,
                         },
                     },
-                    output: SessionAudioOutput {
+                    output: output_audio,
-                        voice: REALTIME_AUDIO_VOICE.to_string(),
-                    },
                 },
                 tools,
             },
@@ -514,6 +544,8 @@ impl RealtimeWebsocketClient {
             self.provider.base_url.as_str(),
             self.provider.query_params.as_ref(),
             config.model.as_deref(),
+            config.event_parser,
+            config.session_mode,
         )?;
 
         let mut request = ws_url
@@ -555,7 +587,7 @@ impl RealtimeWebsocketClient {
         );
         connection
             .writer
-            .send_session_update(config.instructions)
+            .send_session_update(config.instructions, config.session_mode)
             .await?;
         Ok(connection)
     }
@@ -600,6 +632,8 @@ fn websocket_url_from_api_url(
     api_url: &str,
     query_params: Option<&HashMap<String, String>>,
     model: Option<&str>,
+    event_parser: RealtimeEventParser,
+    _session_mode: RealtimeSessionMode,
 ) -> Result<Url, ApiError> {
     let mut url = Url::parse(api_url)
         .map_err(|err| ApiError::Stream(format!("failed to parse realtime api_url: {err}")))?;
@@ -619,9 +653,20 @@ fn websocket_url_from_api_url(
         }
     }
 
-    {
+    let intent = match event_parser {
+        RealtimeEventParser::V1 => Some("quicksilver"),
+        RealtimeEventParser::RealtimeV2 => None,
+    };
+    let has_extra_query_params = query_params.is_some_and(|query_params| {
+        query_params
+            .iter()
+            .any(|(key, _)| key != "intent" && !(key == "model" && model.is_some()))
+    });
+    if intent.is_some() || model.is_some() || has_extra_query_params {
         let mut query = url.query_pairs_mut();
-        query.append_pair("intent", "quicksilver");
+        if let Some(intent) = intent {
+            query.append_pair("intent", intent);
+        }
         if let Some(model) = model {
             query.append_pair("model", model);
         }
@@ -902,8 +947,14 @@ mod tests {
 
     #[test]
     fn websocket_url_from_http_base_defaults_to_ws_path() {
-        let url =
+        let url = websocket_url_from_api_url(
-            websocket_url_from_api_url("http://127.0.0.1:8011", None, None).expect("build ws url");
+            "http://127.0.0.1:8011",
+            None,
+            None,
+            RealtimeEventParser::V1,
+            RealtimeSessionMode::Conversational,
+        )
+        .expect("build ws url");
         assert_eq!(
             url.as_str(),
             "ws://127.0.0.1:8011/v1/realtime?intent=quicksilver"
@@ -912,9 +963,14 @@ mod tests {
 
     #[test]
     fn websocket_url_from_ws_base_defaults_to_ws_path() {
-        let url =
+        let url = websocket_url_from_api_url(
-            websocket_url_from_api_url("wss://example.com", None, Some("realtime-test-model"))
+            "wss://example.com",
-                .expect("build ws url");
+            None,
+            Some("realtime-test-model"),
+            RealtimeEventParser::V1,
+            RealtimeSessionMode::Conversational,
+        )
+        .expect("build ws url");
         assert_eq!(
             url.as_str(),
             "wss://example.com/v1/realtime?intent=quicksilver&model=realtime-test-model"
@@ -923,8 +979,14 @@ mod tests {
 
     #[test]
     fn websocket_url_from_v1_base_appends_realtime_path() {
-        let url = websocket_url_from_api_url("https://api.openai.com/v1", None, Some("snapshot"))
+        let url = websocket_url_from_api_url(
-            .expect("build ws url");
+            "https://api.openai.com/v1",
+            None,
+            Some("snapshot"),
+            RealtimeEventParser::V1,
+            RealtimeSessionMode::Conversational,
+        )
+        .expect("build ws url");
         assert_eq!(
             url.as_str(),
             "wss://api.openai.com/v1/realtime?intent=quicksilver&model=snapshot"
@@ -933,9 +995,14 @@ mod tests {
 
     #[test]
     fn websocket_url_from_nested_v1_base_appends_realtime_path() {
-        let url =
+        let url = websocket_url_from_api_url(
-            websocket_url_from_api_url("https://example.com/openai/v1", None, Some("snapshot"))
+            "https://example.com/openai/v1",
-                .expect("build ws url");
+            None,
+            Some("snapshot"),
+            RealtimeEventParser::V1,
+            RealtimeSessionMode::Conversational,
+        )
+        .expect("build ws url");
         assert_eq!(
             url.as_str(),
             "wss://example.com/openai/v1/realtime?intent=quicksilver&model=snapshot"
@@ -951,6 +1018,8 @@ mod tests {
                 ("intent".to_string(), "ignored".to_string()),
             ])),
             Some("snapshot"),
+            RealtimeEventParser::V1,
+            RealtimeSessionMode::Conversational,
         )
         .expect("build ws url");
         assert_eq!(
@@ -959,6 +1028,54 @@ mod tests {
         );
     }
 
+    #[test]
+    fn websocket_url_v1_ignores_transcription_mode() {
+        let url = websocket_url_from_api_url(
+            "https://example.com",
+            None,
+            None,
+            RealtimeEventParser::V1,
+            RealtimeSessionMode::Transcription,
+        )
+        .expect("build ws url");
+        assert_eq!(
+            url.as_str(),
+            "wss://example.com/v1/realtime?intent=quicksilver"
+        );
+    }
+
+    #[test]
+    fn websocket_url_omits_intent_for_realtime_v2_conversational_mode() {
+        let url = websocket_url_from_api_url(
+            "https://example.com/v1/realtime?foo=bar",
+            Some(&HashMap::from([
+                ("trace".to_string(), "1".to_string()),
+                ("intent".to_string(), "ignored".to_string()),
+            ])),
+            Some("snapshot"),
+            RealtimeEventParser::RealtimeV2,
+            RealtimeSessionMode::Conversational,
+        )
+        .expect("build ws url");
+        assert_eq!(
+            url.as_str(),
+            "wss://example.com/v1/realtime?foo=bar&model=snapshot&trace=1"
+        );
+    }
+
+    #[test]
+    fn websocket_url_omits_intent_for_realtime_v2_transcription_mode() {
+        let url = websocket_url_from_api_url(
+            "https://example.com",
+            None,
+            None,
+            RealtimeEventParser::RealtimeV2,
+            RealtimeSessionMode::Transcription,
+        )
+        .expect("build ws url");
+        assert_eq!(url.as_str(), "wss://example.com/v1/realtime");
+    }
+
     #[tokio::test]
     async fn e2e_connect_and_exchange_events_against_mock_ws_server() {
         let listener = TcpListener::bind("127.0.0.1:0").await.expect("bind");
@@ -1124,6 +1241,7 @@ mod tests {
                     model: Some("realtime-test-model".to_string()),
                     session_id: Some("conv_1".to_string()),
                     event_parser: RealtimeEventParser::V1,
+                    session_mode: RealtimeSessionMode::Conversational,
                 },
                 HeaderMap::new(),
                 HeaderMap::new(),
@@ -1301,14 +1419,36 @@ mod tests {
             assert_eq!(second_json["type"], "conversation.item.create");
             assert_eq!(
                 second_json["item"]["type"],
+                Value::String("message".to_string())
+            );
+            assert_eq!(
+                second_json["item"]["content"][0]["type"],
+                Value::String("input_text".to_string())
+            );
+            assert_eq!(
+                second_json["item"]["content"][0]["text"],
+                Value::String("delegate this".to_string())
+            );
+
+            let third = ws
+                .next()
+                .await
+                .expect("third msg")
+                .expect("third msg ok")
+                .into_text()
+                .expect("text");
+            let third_json: Value = serde_json::from_str(&third).expect("json");
+            assert_eq!(third_json["type"], "conversation.item.create");
+            assert_eq!(
+                third_json["item"]["type"],
                 Value::String("function_call_output".to_string())
             );
             assert_eq!(
-                second_json["item"]["call_id"],
+                third_json["item"]["call_id"],
                 Value::String("call_1".to_string())
             );
             assert_eq!(
-                second_json["item"]["output"],
+                third_json["item"]["output"],
                 Value::String("delegated result".to_string())
             );
         });
@@ -1335,6 +1475,7 @@ mod tests {
                     model: Some("realtime-test-model".to_string()),
                     session_id: Some("conv_1".to_string()),
                     event_parser: RealtimeEventParser::RealtimeV2,
+                    session_mode: RealtimeSessionMode::Conversational,
                 },
                 HeaderMap::new(),
                 HeaderMap::new(),
@@ -1355,6 +1496,10 @@ mod tests {
             }
         );
 
+        connection
+            .send_conversation_item_create("delegate this".to_string())
+            .await
+            .expect("send text item");
         connection
             .send_conversation_handoff_append("call_1".to_string(), "delegated result".to_string())
             .await
@@ -1364,6 +1509,205 @@ mod tests {
         server.await.expect("server task");
     }
 
+    #[tokio::test]
+    async fn transcription_mode_session_update_omits_output_audio_and_instructions() {
+        let listener = TcpListener::bind("127.0.0.1:0").await.expect("bind");
+        let addr = listener.local_addr().expect("local addr");
+
+        let server = tokio::spawn(async move {
+            let (stream, _) = listener.accept().await.expect("accept");
+            let mut ws = accept_async(stream).await.expect("accept ws");
+
+            let first = ws
+                .next()
+                .await
+                .expect("first msg")
+                .expect("first msg ok")
+                .into_text()
+                .expect("text");
+            let first_json: Value = serde_json::from_str(&first).expect("json");
+            assert_eq!(first_json["type"], "session.update");
+            assert_eq!(
+                first_json["session"]["type"],
+                Value::String("transcription".to_string())
+            );
+            assert!(first_json["session"].get("instructions").is_none());
+            assert!(first_json["session"]["audio"].get("output").is_none());
+            assert_eq!(
+                first_json["session"]["tools"][0]["name"],
+                Value::String("codex".to_string())
+            );
+
+            ws.send(Message::Text(
+                json!({
+                    "type": "session.updated",
+                    "session": {"id": "sess_transcription"}
+                })
+                .to_string()
+                .into(),
+            ))
+            .await
+            .expect("send session.updated");
+
+            let second = ws
+                .next()
+                .await
+                .expect("second msg")
+                .expect("second msg ok")
+                .into_text()
+                .expect("text");
+            let second_json: Value = serde_json::from_str(&second).expect("json");
+            assert_eq!(second_json["type"], "input_audio_buffer.append");
+        });
+
+        let provider = Provider {
+            name: "test".to_string(),
+            base_url: format!("http://{addr}"),
+            query_params: Some(HashMap::new()),
+            headers: HeaderMap::new(),
+            retry: crate::provider::RetryConfig {
+                max_attempts: 1,
+                base_delay: Duration::from_millis(1),
+                retry_429: false,
+                retry_5xx: false,
+                retry_transport: false,
+            },
+            stream_idle_timeout: Duration::from_secs(5),
+        };
+        let client = RealtimeWebsocketClient::new(provider);
+        let connection = client
+            .connect(
+                RealtimeSessionConfig {
+                    instructions: "backend prompt".to_string(),
+                    model: Some("realtime-test-model".to_string()),
+                    session_id: Some("conv_1".to_string()),
+                    event_parser: RealtimeEventParser::RealtimeV2,
+                    session_mode: RealtimeSessionMode::Transcription,
+                },
+                HeaderMap::new(),
+                HeaderMap::new(),
+            )
+            .await
+            .expect("connect");
+
+        let created = connection
+            .next_event()
+            .await
+            .expect("next event")
+            .expect("event");
+        assert_eq!(
+            created,
+            RealtimeEvent::SessionUpdated {
+                session_id: "sess_transcription".to_string(),
+                instructions: None,
+            }
+        );
+
+        connection
+            .send_audio_frame(RealtimeAudioFrame {
+                data: "AQID".to_string(),
+                sample_rate: 24_000,
+                num_channels: 1,
+                samples_per_channel: Some(480),
+            })
+            .await
+            .expect("send audio");
+
+        connection.close().await.expect("close");
+        server.await.expect("server task");
+    }
+
+    #[tokio::test]
+    async fn v1_transcription_mode_is_treated_as_conversational() {
+        let listener = TcpListener::bind("127.0.0.1:0").await.expect("bind");
+        let addr = listener.local_addr().expect("local addr");
+
+        let server = tokio::spawn(async move {
+            let (stream, _) = listener.accept().await.expect("accept");
+            let mut ws = accept_async(stream).await.expect("accept ws");
+
+            let first = ws
+                .next()
+                .await
+                .expect("first msg")
+                .expect("first msg ok")
+                .into_text()
+                .expect("text");
+            let first_json: Value = serde_json::from_str(&first).expect("json");
+            assert_eq!(first_json["type"], "session.update");
+            assert_eq!(
+                first_json["session"]["type"],
+                Value::String("quicksilver".to_string())
+            );
+            assert_eq!(
+                first_json["session"]["instructions"],
+                Value::String("backend prompt".to_string())
+            );
+            assert_eq!(
+                first_json["session"]["audio"]["output"]["voice"],
+                Value::String("fathom".to_string())
+            );
+            assert!(first_json["session"].get("tools").is_none());
+
+            ws.send(Message::Text(
+                json!({
+                    "type": "session.updated",
+                    "session": {"id": "sess_v1_mode"}
+                })
+                .to_string()
+                .into(),
+            ))
+            .await
+            .expect("send session.updated");
+        });
+
+        let provider = Provider {
+            name: "test".to_string(),
+            base_url: format!("http://{addr}"),
+            query_params: Some(HashMap::new()),
+            headers: HeaderMap::new(),
+            retry: crate::provider::RetryConfig {
+                max_attempts: 1,
+                base_delay: Duration::from_millis(1),
+                retry_429: false,
+                retry_5xx: false,
+                retry_transport: false,
+            },
+            stream_idle_timeout: Duration::from_secs(5),
+        };
+        let client = RealtimeWebsocketClient::new(provider);
+        let connection = client
+            .connect(
+                RealtimeSessionConfig {
+                    instructions: "backend prompt".to_string(),
+                    model: Some("realtime-test-model".to_string()),
+                    session_id: Some("conv_1".to_string()),
+                    event_parser: RealtimeEventParser::V1,
+                    session_mode: RealtimeSessionMode::Transcription,
+                },
+                HeaderMap::new(),
+                HeaderMap::new(),
+            )
+            .await
+            .expect("connect");
+
+        let created = connection
+            .next_event()
+            .await
+            .expect("next event")
+            .expect("event");
+        assert_eq!(
+            created,
+            RealtimeEvent::SessionUpdated {
+                session_id: "sess_v1_mode".to_string(),
+                instructions: None,
+            }
+        );
+
+        connection.close().await.expect("close");
+        server.await.expect("server task");
+    }
+
     #[tokio::test]
     async fn send_does_not_block_while_next_event_waits_for_inbound_data() {
         let listener = TcpListener::bind("127.0.0.1:0").await.expect("bind");
@@ -1427,6 +1771,7 @@ mod tests {
                     model: Some("realtime-test-model".to_string()),
                     session_id: Some("conv_1".to_string()),
                     event_parser: RealtimeEventParser::V1,
+                    session_mode: RealtimeSessionMode::Conversational,
                 },
                 HeaderMap::new(),
                 HeaderMap::new(),

codex-rs/codex-api/src/endpoint/realtime_websocket/mod.rs

@@ -12,3 +12,4 @@ pub use methods::RealtimeWebsocketEvents;
 pub use methods::RealtimeWebsocketWriter;
 pub use protocol::RealtimeEventParser;
 pub use protocol::RealtimeSessionConfig;
+pub use protocol::RealtimeSessionMode;

codex-rs/codex-api/src/endpoint/realtime_websocket/protocol.rs

@@ -14,12 +14,19 @@ pub enum RealtimeEventParser {
     RealtimeV2,
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum RealtimeSessionMode {
+    Conversational,
+    Transcription,
+}
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct RealtimeSessionConfig {
     pub instructions: String,
     pub model: Option<String>,
     pub session_id: Option<String>,
     pub event_parser: RealtimeEventParser,
+    pub session_mode: RealtimeSessionMode,
 }
 
 #[derive(Debug, Clone, Serialize)]
@@ -42,7 +49,8 @@ pub(super) enum RealtimeOutboundMessage {
 pub(super) struct SessionUpdateSession {
     #[serde(rename = "type")]
     pub(super) kind: String,
-    pub(super) instructions: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub(super) instructions: Option<String>,
     pub(super) audio: SessionAudio,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub(super) tools: Option<Vec<SessionFunctionTool>>,
@@ -51,7 +59,8 @@ pub(super) struct SessionUpdateSession {
 #[derive(Debug, Clone, Serialize)]
 pub(super) struct SessionAudio {
     pub(super) input: SessionAudioInput,
-    pub(super) output: SessionAudioOutput,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub(super) output: Option<SessionAudioOutput>,
 }
 
 #[derive(Debug, Clone, Serialize)]

codex-rs/codex-api/src/lib.rs

@@ -29,6 +29,7 @@ pub use crate::endpoint::memories::MemoriesClient;
 pub use crate::endpoint::models::ModelsClient;
 pub use crate::endpoint::realtime_websocket::RealtimeEventParser;
 pub use crate::endpoint::realtime_websocket::RealtimeSessionConfig;
+pub use crate::endpoint::realtime_websocket::RealtimeSessionMode;
 pub use crate::endpoint::realtime_websocket::RealtimeWebsocketClient;
 pub use crate::endpoint::realtime_websocket::RealtimeWebsocketConnection;
 pub use crate::endpoint::responses::ResponsesClient;

codex-rs/codex-api/tests/realtime_websocket_e2e.rs

@@ -6,6 +6,7 @@ use codex_api::RealtimeAudioFrame;
 use codex_api::RealtimeEvent;
 use codex_api::RealtimeEventParser;
 use codex_api::RealtimeSessionConfig;
+use codex_api::RealtimeSessionMode;
 use codex_api::RealtimeWebsocketClient;
 use codex_api::provider::Provider;
 use codex_api::provider::RetryConfig;
@@ -142,6 +143,7 @@ async fn realtime_ws_e2e_session_create_and_event_flow() {
                 model: Some("realtime-test-model".to_string()),
                 session_id: Some("conv_123".to_string()),
                 event_parser: RealtimeEventParser::V1,
+                session_mode: RealtimeSessionMode::Conversational,
             },
             HeaderMap::new(),
             HeaderMap::new(),
@@ -235,6 +237,7 @@ async fn realtime_ws_e2e_send_while_next_event_waits() {
                 model: Some("realtime-test-model".to_string()),
                 session_id: Some("conv_123".to_string()),
                 event_parser: RealtimeEventParser::V1,
+                session_mode: RealtimeSessionMode::Conversational,
             },
             HeaderMap::new(),
             HeaderMap::new(),
@@ -299,6 +302,7 @@ async fn realtime_ws_e2e_disconnected_emitted_once() {
                 model: Some("realtime-test-model".to_string()),
                 session_id: Some("conv_123".to_string()),
                 event_parser: RealtimeEventParser::V1,
+                session_mode: RealtimeSessionMode::Conversational,
             },
             HeaderMap::new(),
             HeaderMap::new(),
@@ -360,6 +364,7 @@ async fn realtime_ws_e2e_ignores_unknown_text_events() {
                 model: Some("realtime-test-model".to_string()),
                 session_id: Some("conv_123".to_string()),
                 event_parser: RealtimeEventParser::V1,
+                session_mode: RealtimeSessionMode::Conversational,
             },
             HeaderMap::new(),
             HeaderMap::new(),
@@ -424,6 +429,7 @@ async fn realtime_ws_e2e_realtime_v2_parser_emits_handoff_requested() {
                 model: Some("realtime-test-model".to_string()),
                 session_id: Some("conv_123".to_string()),
                 event_parser: RealtimeEventParser::RealtimeV2,
+                session_mode: RealtimeSessionMode::Conversational,
             },
             HeaderMap::new(),
             HeaderMap::new(),

codex-rs/core/config.schema.json

@@ -1342,6 +1342,13 @@
       },
       "type": "object"
     },
+    "RealtimeWsMode": {
+      "enum": [
+        "conversational",
+        "transcription"
+      ],
+      "type": "string"
+    },
     "ReasoningEffort": {
       "description": "See https://platform.openai.com/docs/guides/reasoning?api-mode=responses#get-started-with-reasoning",
       "enum": [
@@ -1816,6 +1823,14 @@
       "description": "Experimental / do not use. Overrides only the realtime conversation websocket transport base URL (the `Op::RealtimeConversation` `/v1/realtime` connection) without changing normal provider HTTP requests.",
       "type": "string"
     },
+    "experimental_realtime_ws_mode": {
+      "allOf": [
+        {
+          "$ref": "#/definitions/RealtimeWsMode"
+        }
+      ],
+      "description": "Experimental / do not use. Selects the realtime websocket intent mode. `conversational` is speech-to-speech while `transcription` is transcript-only."
+    },
     "experimental_realtime_ws_model": {
       "description": "Experimental / do not use. Selects the realtime websocket model/snapshot used for the `Op::RealtimeConversation` connection.",
       "type": "string"

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

@@ -4129,6 +4129,7 @@ fn test_precedence_fixture_with_o3_profile() -> std::io::Result<()> {
             experimental_realtime_start_instructions: None,
             experimental_realtime_ws_base_url: None,
             experimental_realtime_ws_model: None,
+            experimental_realtime_ws_mode: RealtimeWsMode::Conversational,
             experimental_realtime_ws_backend_prompt: None,
             experimental_realtime_ws_startup_context: None,
             base_instructions: None,
@@ -4265,6 +4266,7 @@ fn test_precedence_fixture_with_gpt3_profile() -> std::io::Result<()> {
         experimental_realtime_start_instructions: None,
         experimental_realtime_ws_base_url: None,
         experimental_realtime_ws_model: None,
+        experimental_realtime_ws_mode: RealtimeWsMode::Conversational,
         experimental_realtime_ws_backend_prompt: None,
         experimental_realtime_ws_startup_context: None,
         base_instructions: None,
@@ -4399,6 +4401,7 @@ fn test_precedence_fixture_with_zdr_profile() -> std::io::Result<()> {
         experimental_realtime_start_instructions: None,
         experimental_realtime_ws_base_url: None,
         experimental_realtime_ws_model: None,
+        experimental_realtime_ws_mode: RealtimeWsMode::Conversational,
         experimental_realtime_ws_backend_prompt: None,
         experimental_realtime_ws_startup_context: None,
         base_instructions: None,
@@ -4519,6 +4522,7 @@ fn test_precedence_fixture_with_gpt5_profile() -> std::io::Result<()> {
         experimental_realtime_start_instructions: None,
         experimental_realtime_ws_base_url: None,
         experimental_realtime_ws_model: None,
+        experimental_realtime_ws_mode: RealtimeWsMode::Conversational,
         experimental_realtime_ws_backend_prompt: None,
         experimental_realtime_ws_startup_context: None,
         base_instructions: None,
@@ -5566,6 +5570,34 @@ experimental_realtime_ws_model = "realtime-test-model"
     Ok(())
 }
 
+#[test]
+fn experimental_realtime_ws_mode_loads_from_config_toml() -> std::io::Result<()> {
+    let cfg: ConfigToml = toml::from_str(
+        r#"
+experimental_realtime_ws_mode = "transcription"
+"#,
+    )
+    .expect("TOML deserialization should succeed");
+
+    assert_eq!(
+        cfg.experimental_realtime_ws_mode,
+        Some(RealtimeWsMode::Transcription)
+    );
+
+    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_mode,
+        RealtimeWsMode::Transcription
+    );
+    Ok(())
+}
+
 #[test]
 fn realtime_audio_loads_from_config_toml() -> std::io::Result<()> {
     let cfg: ConfigToml = toml::from_str(

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

@@ -463,6 +463,9 @@ pub struct Config {
     /// Experimental / do not use. Selects the realtime websocket model/snapshot
     /// used for the `Op::RealtimeConversation` connection.
     pub experimental_realtime_ws_model: Option<String>,
+    /// Experimental / do not use. Selects the realtime websocket intent mode.
+    /// `conversational` is speech-to-speech while `transcription` is transcript-only.
+    pub experimental_realtime_ws_mode: RealtimeWsMode,
     /// Experimental / do not use. Overrides only the realtime conversation
     /// websocket transport instructions (the `Op::RealtimeConversation`
     /// `/ws` session.update instructions) without changing normal prompts.
@@ -1238,6 +1241,9 @@ pub struct ConfigToml {
     /// Experimental / do not use. Selects the realtime websocket model/snapshot
     /// used for the `Op::RealtimeConversation` connection.
     pub experimental_realtime_ws_model: Option<String>,
+    /// Experimental / do not use. Selects the realtime websocket intent mode.
+    /// `conversational` is speech-to-speech while `transcription` is transcript-only.
+    pub experimental_realtime_ws_mode: Option<RealtimeWsMode>,
     /// Experimental / do not use. Overrides only the realtime conversation
     /// websocket transport instructions (the `Op::RealtimeConversation`
     /// `/ws` session.update instructions) without changing normal prompts.
@@ -1383,6 +1389,14 @@ pub struct RealtimeAudioConfig {
     pub speaker: Option<String>,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, Copy, Default, PartialEq, Eq, JsonSchema)]
+#[serde(rename_all = "snake_case")]
+pub enum RealtimeWsMode {
+    #[default]
+    Conversational,
+    Transcription,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, Default, PartialEq, Eq, JsonSchema)]
 #[schemars(deny_unknown_fields)]
 pub struct RealtimeAudioToml {
@@ -2462,6 +2476,7 @@ impl Config {
                 }),
             experimental_realtime_ws_base_url: cfg.experimental_realtime_ws_base_url,
             experimental_realtime_ws_model: cfg.experimental_realtime_ws_model,
+            experimental_realtime_ws_mode: cfg.experimental_realtime_ws_mode.unwrap_or_default(),
             experimental_realtime_ws_backend_prompt: cfg.experimental_realtime_ws_backend_prompt,
             experimental_realtime_ws_startup_context: cfg.experimental_realtime_ws_startup_context,
             experimental_realtime_start_instructions: cfg.experimental_realtime_start_instructions,

codex-rs/core/src/realtime_conversation.rs

@@ -15,6 +15,7 @@ use codex_api::RealtimeAudioFrame;
 use codex_api::RealtimeEvent;
 use codex_api::RealtimeEventParser;
 use codex_api::RealtimeSessionConfig;
+use codex_api::RealtimeSessionMode;
 use codex_api::RealtimeWebsocketClient;
 use codex_api::endpoint::realtime_websocket::RealtimeWebsocketEvents;
 use codex_api::endpoint::realtime_websocket::RealtimeWebsocketWriter;
@@ -116,10 +117,7 @@ impl RealtimeConversationManager {
         &self,
         api_provider: ApiProvider,
         extra_headers: Option<HeaderMap>,
-        prompt: String,
+        session_config: RealtimeSessionConfig,
-        model: Option<String>,
-        session_id: Option<String>,
-        event_parser: RealtimeEventParser,
     ) -> CodexResult<(Receiver<RealtimeEvent>, Arc<AtomicBool>)> {
         let previous_state = {
             let mut guard = self.state.lock().await;
@@ -131,12 +129,6 @@ impl RealtimeConversationManager {
             let _ = state.task.await;
         }
 
-        let session_config = RealtimeSessionConfig {
-            instructions: prompt,
-            model,
-            session_id,
-            event_parser,
-        };
         let client = RealtimeWebsocketClient::new(api_provider);
         let connection = client
             .connect(
@@ -307,23 +299,26 @@ pub(crate) async fn handle_start(
     } else {
         RealtimeEventParser::V1
     };
-
+    let session_mode = match config.experimental_realtime_ws_mode {
+        crate::config::RealtimeWsMode::Conversational => RealtimeSessionMode::Conversational,
+        crate::config::RealtimeWsMode::Transcription => RealtimeSessionMode::Transcription,
+    };
     let requested_session_id = params
         .session_id
         .or_else(|| Some(sess.conversation_id.to_string()));
+    let session_config = RealtimeSessionConfig {
+        instructions: prompt,
+        model,
+        session_id: requested_session_id.clone(),
+        event_parser,
+        session_mode,
+    };
     let extra_headers =
         realtime_request_headers(requested_session_id.as_deref(), realtime_api_key.as_str())?;
     info!("starting realtime conversation");
     let (events_rx, realtime_active) = match sess
         .conversation
-        .start(
+        .start(api_provider, extra_headers, session_config)
-            api_provider,
-            extra_headers,
-            prompt,
-            model,
-            requested_session_id.clone(),
-            event_parser,
-        )
         .await
     {
         Ok(events_rx) => events_rx,