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 58ade3b75f..17ed27f439 100644
--- a/codex-rs/codex-api/src/endpoint/realtime_websocket/methods.rs
+++ b/codex-rs/codex-api/src/endpoint/realtime_websocket/methods.rs
@@ -42,6 +42,7 @@ use std::collections::VecDeque;
 use std::sync::Arc;
 use std::sync::atomic::AtomicBool;
 use std::sync::atomic::Ordering;
+use std::time::Duration;
 use tokio::sync::Mutex;
 use tokio::sync::mpsc;
 use tokio::sync::oneshot;
@@ -908,15 +909,28 @@ impl RealtimeWebsocketClient {
         default_headers: HeaderMap,
         transcript_state: RealtimeTranscriptState,
     ) -> Result<RealtimeWebsocketConnection, ApiError> {
-        self.connect_sideband(
-            config,
-            call_id,
-            extra_headers,
-            default_headers,
-            RealtimeSessionInitialization::ExistingCall,
-            transcript_state,
-        )
-        .await
+        // Instant Connect returns a call ID before ICE activates the call. Its sideband
+        // can briefly return 404 while the client connects media in parallel.
+        let mut activation_delays = [100, 300, 800].into_iter();
+        loop {
+            let result = self
+                .connect_sideband(
+                    config.clone(),
+                    call_id,
+                    extra_headers.clone(),
+                    default_headers.clone(),
+                    RealtimeSessionInitialization::ExistingCall,
+                    transcript_state.clone(),
+                )
+                .await;
+            if matches!(&result, Err(ApiError::Api { status, .. }) if *status == StatusCode::NOT_FOUND)
+                && let Some(delay_ms) = activation_delays.next()
+            {
+                sleep(Duration::from_millis(delay_ms)).await;
+                continue;
+            }
+            return result;
+        }
     }
 
     async fn connect_sideband(
diff --git a/codex-rs/codex-api/tests/realtime_websocket_e2e.rs b/codex-rs/codex-api/tests/realtime_websocket_e2e.rs
index bd6f463852..37a028eef2 100644
--- a/codex-rs/codex-api/tests/realtime_websocket_e2e.rs
+++ b/codex-rs/codex-api/tests/realtime_websocket_e2e.rs
@@ -3,6 +3,7 @@ use std::collections::HashMap;
 use std::future::Future;
 use std::time::Duration;
 
+use codex_api::ApiError;
 use codex_api::Provider;
 use codex_api::RealtimeAudioFrame;
 use codex_api::RealtimeEvent;
@@ -23,11 +24,15 @@ use codex_protocol::protocol::RealtimeVoice;
 use futures::SinkExt;
 use futures::StreamExt;
 use http::HeaderMap;
+use http::StatusCode;
 use serde_json::Value;
 use serde_json::json;
 use tokio::net::TcpListener;
 use tokio_tungstenite::accept_async;
+use tokio_tungstenite::accept_hdr_async;
 use tokio_tungstenite::tungstenite::Message;
+use tokio_tungstenite::tungstenite::handshake::server::Request;
+use tokio_tungstenite::tungstenite::handshake::server::Response;
 
 type RealtimeWsStream = tokio_tungstenite::WebSocketStream<tokio::net::TcpStream>;
 
@@ -287,6 +292,86 @@ async fn realtime_ws_connect_webrtc_sideband_retries_join_until_server_is_availa
     server.await.expect("server task");
 }
 
+#[tokio::test]
+async fn realtime_ws_existing_call_retries_activation_404_with_a_bounded_budget() {
+    for (status, failures, activates) in [
+        (StatusCode::NOT_FOUND, 3, true),
+        (StatusCode::NOT_FOUND, 4, false),
+        (StatusCode::GONE, 1, false),
+    ] {
+        tokio::time::timeout(Duration::from_secs(5), async {
+            let listener = TcpListener::bind("127.0.0.1:0").await.expect("bind");
+            let addr = listener.local_addr().expect("address");
+            let server = tokio::spawn(async move {
+                for _ in 0..failures {
+                    let (stream, _) = listener.accept().await.expect("accept handshake");
+                    accept_hdr_async(stream, move |request: &Request, _: Response| {
+                        assert!(
+                            request
+                                .uri()
+                                .query()
+                                .expect("query")
+                                .contains("call_id=rtc_prepared")
+                        );
+                        Err(http::Response::builder()
+                            .status(status)
+                            .body(None)
+                            .expect("error response"))
+                    })
+                    .await
+                    .expect_err("call is not active");
+                }
+                if activates {
+                    let (stream, _) = listener.accept().await.expect("accept activated call");
+                    let mut ws = accept_async(stream).await.expect("successful handshake");
+                    // Existing-call attachment must not overwrite the prepared session.
+                    assert!(matches!(
+                        ws.next().await.expect("close").expect("frame"),
+                        Message::Close(_)
+                    ));
+                }
+            });
+            let mut provider = test_provider(format!("http://{addr}"));
+            provider.retry.max_attempts = 0;
+            let client =
+                test_client(provider).with_webrtc_sideband_base_url(format!("http://{addr}"));
+            let result = client
+                .connect_existing_call_sideband(
+                    RealtimeSessionConfig {
+                        instructions: "unused existing-call prompt".to_string(),
+                        initial_items: Vec::new(),
+                        delegation_ack_filler: None,
+                        model: None,
+                        session_id: None,
+                        event_parser: RealtimeEventParser::RealtimeV2,
+                        session_mode: RealtimeSessionMode::Conversational,
+                        output_modality: RealtimeOutputModality::Audio,
+                        voice: RealtimeVoice::Marin,
+                    },
+                    "rtc_prepared",
+                    HeaderMap::new(),
+                    HeaderMap::new(),
+                    RealtimeTranscriptState::default(),
+                )
+                .await;
+            if activates {
+                result
+                    .expect("connect after activation")
+                    .close()
+                    .await
+                    .expect("close");
+            } else {
+                assert!(
+                    matches!(result, Err(ApiError::Api { status: actual, .. }) if actual == status)
+                );
+            }
+            server.await.expect("server task");
+        })
+        .await
+        .expect("bounded sideband handshake attempts");
+    }
+}
+
 #[tokio::test]
 async fn realtime_ws_e2e_send_while_next_event_waits() {
     let (addr, server) = spawn_realtime_ws_server(|mut ws: RealtimeWsStream| async move {