diff --git a/codex-rs/codex-api/src/endpoint/realtime_websocket/methods.rs b/codex-rs/codex-api/src/endpoint/realtime_websocket/methods.rs
index fa33f3524b..8c18f7515a 100644
--- a/codex-rs/codex-api/src/endpoint/realtime_websocket/methods.rs
+++ b/codex-rs/codex-api/src/endpoint/realtime_websocket/methods.rs
@@ -55,6 +55,7 @@ use tungstenite::protocol::WebSocketConfig;
 use url::Url;
 
 const REALTIME_WIRE_LOG_TARGET: &str = "codex_api::realtime_websocket::wire";
+const OPENAI_REALTIME_API_BASE_URL: &str = "https://api.openai.com/v1";
 
 struct WsStream {
     tx_command: mpsc::Sender<WsCommand>,
@@ -696,11 +697,21 @@ fn contains_transcript_entry(entries: &[RealtimeTranscriptEntry], role: &str, te
 
 pub struct RealtimeWebsocketClient {
     provider: Provider,
+    webrtc_sideband_base_url: String,
 }
 
 impl RealtimeWebsocketClient {
     pub fn new(provider: Provider) -> Self {
-        Self { provider }
+        Self {
+            provider,
+            webrtc_sideband_base_url: OPENAI_REALTIME_API_BASE_URL.to_string(),
+        }
+    }
+
+    /// Overrides the direct WebRTC sideband URL for local development and tests.
+    pub fn with_webrtc_sideband_base_url(mut self, base_url: String) -> Self {
+        self.webrtc_sideband_base_url = base_url;
+        self
     }
 
     pub async fn connect(
@@ -775,13 +786,7 @@ impl RealtimeWebsocketClient {
     ) -> Result<RealtimeWebsocketConnection, ApiError> {
         // Keep the parser/session query shaping from standalone realtime while replacing the model
         // query with a call_id join onto an existing WebRTC session.
-        let ws_url = websocket_url_from_api_url_for_call(
-            self.provider.base_url.as_str(),
-            self.provider.query_params.as_ref(),
-            config.event_parser,
-            config.session_mode,
-            call_id,
-        )?;
+        let ws_url = self.webrtc_sideband_url(config.event_parser, config.session_mode, call_id)?;
         self.connect_realtime_websocket_url(
             ws_url,
             config,
@@ -792,6 +797,21 @@ impl RealtimeWebsocketClient {
         .await
     }
 
+    fn webrtc_sideband_url(
+        &self,
+        event_parser: RealtimeEventParser,
+        session_mode: RealtimeSessionMode,
+        call_id: &str,
+    ) -> Result<Url, ApiError> {
+        websocket_url_from_api_url_for_call(
+            self.webrtc_sideband_base_url.as_str(),
+            /*query_params*/ None,
+            event_parser,
+            session_mode,
+            call_id,
+        )
+    }
+
     async fn connect_realtime_websocket_url(
         &self,
         ws_url: Url,
@@ -1013,6 +1033,7 @@ fn normalize_realtime_path(url: &mut Url, event_parser: RealtimeEventParser) {
 mod tests {
     use super::*;
     use crate::endpoint::realtime_websocket::protocol::RealtimeTranscriptEntry;
+    use crate::provider::RetryConfig;
     use codex_protocol::protocol::RealtimeHandoffRequested;
     use codex_protocol::protocol::RealtimeInputAudioSpeechStarted;
     use codex_protocol::protocol::RealtimeNoopRequested;
@@ -1780,6 +1801,34 @@ mod tests {
         );
     }
 
+    #[test]
+    fn webrtc_frameless_sideband_ignores_provider_base_url() {
+        let client = RealtimeWebsocketClient::new(Provider {
+            name: "chatgpt".to_string(),
+            base_url: "https://chatgpt.com/backend-api/codex".to_string(),
+            query_params: None,
+            headers: HeaderMap::new(),
+            retry: RetryConfig {
+                max_attempts: 0,
+                base_delay: Duration::ZERO,
+                retry_429: false,
+                retry_5xx: false,
+                retry_transport: false,
+            },
+            stream_idle_timeout: Duration::from_secs(5),
+        });
+
+        let url = client
+            .webrtc_sideband_url(
+                RealtimeEventParser::FramelessBidi,
+                RealtimeSessionMode::Conversational,
+                "rtc_test",
+            )
+            .expect("build ws url");
+
+        assert_eq!(url.as_str(), "wss://api.openai.com/v1/live/rtc_test");
+    }
+
     #[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");
diff --git a/codex-rs/codex-api/tests/realtime_websocket_e2e.rs b/codex-rs/codex-api/tests/realtime_websocket_e2e.rs
index 5d0f8f910c..c3018fa929 100644
--- a/codex-rs/codex-api/tests/realtime_websocket_e2e.rs
+++ b/codex-rs/codex-api/tests/realtime_websocket_e2e.rs
@@ -244,7 +244,8 @@ async fn realtime_ws_connect_webrtc_sideband_retries_join_until_server_is_availa
     provider.retry.max_attempts = 1;
     provider.retry.base_delay = Duration::from_millis(100);
 
-    let client = RealtimeWebsocketClient::new(provider);
+    let client = RealtimeWebsocketClient::new(provider)
+        .with_webrtc_sideband_base_url(format!("http://{addr}"));
     let connection = client
         .connect_webrtc_sideband(
             RealtimeSessionConfig {
diff --git a/codex-rs/core/src/realtime_conversation.rs b/codex-rs/core/src/realtime_conversation.rs
index 198c9f858c..740cd6f98c 100644
--- a/codex-rs/core/src/realtime_conversation.rs
+++ b/codex-rs/core/src/realtime_conversation.rs
@@ -458,6 +458,7 @@ struct ConversationState {
 
 struct RealtimeStart {
     api_provider: ApiProvider,
+    realtime_sideband_base_url: Option<String>,
     extra_headers: Option<HeaderMap>,
     client_managed_handoffs: bool,
     flush_transcript_tail_on_session_end: bool,
@@ -518,6 +519,7 @@ impl RealtimeConversationManager {
     async fn start_inner(&self, start: RealtimeStart) -> CodexResult<RealtimeStartOutput> {
         let RealtimeStart {
             api_provider,
+            realtime_sideband_base_url,
             extra_headers,
             client_managed_handoffs,
             flush_transcript_tail_on_session_end,
@@ -569,6 +571,10 @@ impl RealtimeConversationManager {
         };
 
         let client = RealtimeWebsocketClient::new(api_provider);
+        let client = match realtime_sideband_base_url {
+            Some(base_url) => client.with_webrtc_sideband_base_url(base_url),
+            None => client,
+        };
         let (task, sdp) = if let Some(sdp) = sdp {
             let call = model_client
                 .create_realtime_call_with_headers(
@@ -1090,6 +1096,7 @@ pub(crate) async fn handle_start(
 
 struct PreparedRealtimeConversationStart {
     api_provider: ApiProvider,
+    realtime_sideband_base_url: Option<String>,
     extra_headers: Option<HeaderMap>,
     client_managed_handoffs: bool,
     flush_transcript_tail_on_session_end: bool,
@@ -1127,7 +1134,8 @@ async fn prepare_realtime_start(
         .clone()
         .unwrap_or(ConversationStartTransport::Websocket);
     let mut api_provider = provider.to_api_provider(Some(AuthMode::ApiKey))?;
-    if let Some(realtime_ws_base_url) = &config.experimental_realtime_ws_base_url {
+    let realtime_sideband_base_url = config.experimental_realtime_ws_base_url.clone();
+    if let Some(realtime_ws_base_url) = &realtime_sideband_base_url {
         api_provider.base_url = realtime_ws_base_url.clone();
     }
     let realtime_call_api_provider =
@@ -1182,6 +1190,7 @@ async fn prepare_realtime_start(
     ));
     Ok(PreparedRealtimeConversationStart {
         api_provider,
+        realtime_sideband_base_url,
         extra_headers: Some(extra_headers),
         client_managed_handoffs: params.client_managed_handoffs,
         flush_transcript_tail_on_session_end: params.flush_transcript_tail_on_session_end,
@@ -1381,6 +1390,7 @@ async fn handle_start_inner(
 ) -> CodexResult<()> {
     let PreparedRealtimeConversationStart {
         api_provider,
+        realtime_sideband_base_url,
         extra_headers,
         client_managed_handoffs,
         flush_transcript_tail_on_session_end,
@@ -1401,6 +1411,7 @@ async fn handle_start_inner(
     };
     let start = RealtimeStart {
         api_provider,
+        realtime_sideband_base_url,
         extra_headers,
         client_managed_handoffs,
         flush_transcript_tail_on_session_end,