Cap realtime mirrored user turns (#17685)

Cap mirrored user text sent to realtime with the existing 300-token turn
budget while preserving the full model turn.

Adds integration coverage for capped realtime mirror payloads.

---------

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

codex-rs/core/tests/suite/realtime_conversation.rs

@@ -25,6 +25,7 @@ use codex_protocol::protocol::RealtimeVoice;
 use codex_protocol::protocol::RolloutItem;
 use codex_protocol::protocol::SessionSource;
 use codex_protocol::user_input::UserInput;
+use codex_utils_output_truncation::approx_token_count;
 use core_test_support::responses;
 use core_test_support::responses::WebSocketConnectionConfig;
 use core_test_support::responses::start_mock_server;
@@ -1910,6 +1911,123 @@ async fn conversation_user_text_turn_is_sent_to_realtime_when_active() -> Result
     Ok(())
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn conversation_user_text_turn_is_capped_when_mirrored_to_realtime() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let api_server = start_mock_server().await;
+    let response_mock = responses::mount_sse_once(
+        &api_server,
+        responses::sse(vec![
+            responses::ev_response_created("resp_long_user_text"),
+            responses::ev_assistant_message("msg_long_user_text", "ack"),
+            responses::ev_completed("resp_long_user_text"),
+        ]),
+    )
+    .await;
+
+    let realtime_server = start_websocket_server(vec![vec![
+        vec![json!({
+            "type": "session.updated",
+            "session": { "id": "sess_long_user_text", "instructions": "backend prompt" }
+        })],
+        vec![],
+    ]])
+    .await;
+
+    let mut builder = test_codex().with_config({
+        let realtime_base_url = realtime_server.uri().to_string();
+        move |config| {
+            config.experimental_realtime_ws_base_url = Some(realtime_base_url);
+            config.experimental_realtime_ws_startup_context = Some(String::new());
+        }
+    });
+    let test = builder.build(&api_server).await?;
+
+    // Phase 1: start realtime so the next normal user turn mirrors over the
+    // active WebSocket session.
+    test.codex
+        .submit(Op::RealtimeConversationStart(ConversationStartParams {
+            prompt: Some(Some("backend prompt".to_string())),
+            session_id: None,
+            transport: None,
+            voice: None,
+        }))
+        .await?;
+
+    let session_updated = wait_for_event_match(&test.codex, |msg| match msg {
+        EventMsg::RealtimeConversationRealtime(RealtimeConversationRealtimeEvent {
+            payload: RealtimeEvent::SessionUpdated { session_id, .. },
+        }) => Some(session_id.clone()),
+        _ => None,
+    })
+    .await;
+    assert_eq!(session_updated, "sess_long_user_text");
+
+    // Phase 2: submit one oversized text turn. The model request should keep
+    // the exact user text, while the realtime mirror should get the capped copy.
+    let user_text = format!(
+        "mirror-head {} mirror-middle {} mirror-tail",
+        "alpha ".repeat(900),
+        "omega ".repeat(900),
+    );
+    test.codex
+        .submit(Op::UserInput {
+            items: vec![UserInput::Text {
+                text: user_text.clone(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+            responsesapi_client_metadata: None,
+        })
+        .await?;
+
+    wait_for_event(&test.codex, |event| {
+        matches!(event, EventMsg::TurnComplete(_))
+    })
+    .await;
+
+    // Phase 3: capture the mirrored WebSocket item; the snapshot below records
+    // the capped payload shape.
+    let realtime_text_request = wait_for_matching_websocket_request(
+        &realtime_server,
+        "capped normal user turn text mirrored to realtime",
+        |request| websocket_request_text(request).is_some_and(|text| text.contains("mirror-head")),
+    )
+    .await;
+    let realtime_text =
+        websocket_request_text(&realtime_text_request).expect("realtime request text");
+    let model_user_texts = response_mock.single_request().message_input_texts("user");
+
+    let realtime_request_body = realtime_text_request.body_json();
+    let content = &realtime_request_body["item"]["content"][0];
+
+    // Snapshot the request envelope and capped text together so reviewers can
+    // see the preserved head/tail and truncation marker in one place.
+    let snapshot = format!(
+        "type: {}\nitem.type: {}\nitem.role: {}\ncontent[0].type: {}\nmodel_has_full_user_text: {}\nrealtime_text_equal_full_user_text: {}\nrealtime_text_approx_tokens: {}\ncontent[0].text: {}",
+        realtime_request_body["type"].as_str().unwrap_or_default(),
+        realtime_request_body["item"]["type"]
+            .as_str()
+            .unwrap_or_default(),
+        realtime_request_body["item"]["role"]
+            .as_str()
+            .unwrap_or_default(),
+        content["type"].as_str().unwrap_or_default(),
+        model_user_texts.iter().any(|text| text == &user_text),
+        realtime_text == user_text,
+        approx_token_count(&realtime_text),
+        realtime_text,
+    );
+    insta::assert_snapshot!(
+        "conversation_user_text_turn_is_capped_when_mirrored_to_realtime",
+        snapshot
+    );
+
+    realtime_server.shutdown().await;
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn conversation_mirrors_assistant_message_text_to_realtime_handoff() -> Result<()> {
     skip_if_no_network!(Ok(()));

codex-rs/core/tests/suite/snapshots/all__suite__realtime_conversation__conversation_user_text_turn_is_capped_when_mirrored_to_realtime.snap

@@ -0,0 +1,12 @@
+---
+source: core/tests/suite/realtime_conversation.rs
+expression: snapshot
+---
+type: conversation.item.create
+item.type: message
+item.role: user
+content[0].type: input_text
+model_has_full_user_text: true
+realtime_text_equal_full_user_text: false
+realtime_text_approx_tokens: 300
+content[0].text: mirror-head alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alph…2417 tokens truncated…ega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega  mirror-tail