codex: address PR review feedback (#16803)

Preserve buffered assistant text across non-message item completions so delayed assistant items can still replay early deltas.

codex-rs/core/src/codex.rs

@@ -7374,6 +7374,9 @@ async fn try_run_sampling_request(
     let mut should_emit_turn_diff = false;
     let plan_mode = turn_context.collaboration_mode.mode == ModeKind::Plan;
     let mut assistant_message_stream_parsers = AssistantMessageStreamParsers::new(plan_mode);
+    // Some streams send output text deltas before the assistant message item is announced.
+    // Hold those early bytes so we can replay them once `output_item.added` arrives.
+    let mut pending_output_text_delta = String::new();
     let mut plan_mode_state = plan_mode.then(|| PlanModeStreamState::new(&turn_context.sub_id));
     let receiving_span = trace_span!("receiving_stream");
     let outcome: CodexResult<SamplingRequestResult> = loop {
@@ -7413,6 +7416,8 @@ async fn try_run_sampling_request(
         match event {
             ResponseEvent::Created => {}
             ResponseEvent::OutputItemDone(item) => {
+                let completed_assistant_message =
+                    matches!(&item, ResponseItem::Message { role, .. } if role == "assistant");
                 let previously_active_item = active_item.take();
                 if let Some(previous) = previously_active_item.as_ref()
                     && matches!(previous, TurnItem::AgentMessage(_))
@@ -7470,6 +7475,17 @@ async fn try_run_sampling_request(
                 let output_result = handle_output_item_done(&mut ctx, item, previously_active_item)
                     .instrument(handle_responses)
                     .await?;
+                if completed_assistant_message && !pending_output_text_delta.is_empty() {
+                    // If an assistant message completed while this buffer is still non-empty,
+                    // those bytes could not be matched to that message item. Clear them so they
+                    // don't leak into a later assistant item, but keep the buffer across
+                    // reasoning/tool completions so a delayed message item can still claim it.
+                    warn!(
+                        buffered_len = pending_output_text_delta.len(),
+                        "dropping buffered output text deltas after item completion"
+                    );
+                    pending_output_text_delta.clear();
+                }
                 if let Some(tool_future) = output_result.tool_future {
                     in_flight.push_back(tool_future);
                 }
@@ -7495,26 +7511,46 @@ async fn try_run_sampling_request(
                 .await
                 {
                     let mut turn_item = turn_item;
-                    let mut seeded_parsed: Option<ParsedAssistantTextDelta> = None;
-                    let mut seeded_item_id: Option<String> = None;
-                    if matches!(turn_item, TurnItem::AgentMessage(_))
-                        && let Some(raw_text) = raw_assistant_output_text_from_item(&item)
-                    {
+                    let mut seeded_emit: Option<(String, ParsedAssistantTextDelta)> = None;
+                    let mut pending_emit: Option<(String, ParsedAssistantTextDelta)> = None;
+                    let raw_output_text = matches!(turn_item, TurnItem::AgentMessage(_))
+                        .then(|| raw_assistant_output_text_from_item(&item))
+                        .flatten();
+                    if matches!(turn_item, TurnItem::AgentMessage(_)) {
                         let item_id = turn_item.id();
-                        let mut seeded =
-                            assistant_message_stream_parsers.seed_item_text(&item_id, &raw_text);
-                        if let TurnItem::AgentMessage(agent_message) = &mut turn_item {
-                            agent_message.content =
-                                vec![codex_protocol::items::AgentMessageContent::Text {
-                                    text: if plan_mode {
-                                        String::new()
-                                    } else {
-                                        std::mem::take(&mut seeded.visible_text)
-                                    },
-                                }];
+                        if let Some(raw_text) = raw_output_text.as_deref() {
+                            let mut seeded =
+                                assistant_message_stream_parsers.seed_item_text(&item_id, raw_text);
+                            if let TurnItem::AgentMessage(agent_message) = &mut turn_item {
+                                agent_message.content =
+                                    vec![codex_protocol::items::AgentMessageContent::Text {
+                                        text: if plan_mode {
+                                            String::new()
+                                        } else {
+                                            std::mem::take(&mut seeded.visible_text)
+                                        },
+                                    }];
+                            }
+                            seeded_emit = plan_mode.then_some((item_id.clone(), seeded));
+                        }
+                        if !pending_output_text_delta.is_empty() {
+                            let mut pending_delta = std::mem::take(&mut pending_output_text_delta);
+                            if let Some(raw_text) = raw_output_text.as_deref() {
+                                // The item payload may already contain all or part of the text we
+                                // buffered from pre-item deltas. Trim the overlap so replayed
+                                // deltas don't duplicate visible output.
+                                if pending_delta.starts_with(raw_text) {
+                                    pending_delta.drain(..raw_text.len());
+                                } else if raw_text.starts_with(&pending_delta) {
+                                    pending_delta.clear();
+                                }
+                            }
+                            if !pending_delta.is_empty() {
+                                let parsed = assistant_message_stream_parsers
+                                    .parse_delta(&item_id, &pending_delta);
+                                pending_emit = Some((item_id, parsed));
+                            }
                         }
-                        seeded_parsed = plan_mode.then_some(seeded);
-                        seeded_item_id = Some(item_id);
                     }
                     if let Some(state) = plan_mode_state.as_mut()
                         && matches!(turn_item, TurnItem::AgentMessage(_))
@@ -7526,16 +7562,22 @@ async fn try_run_sampling_request(
                     } else {
                         sess.emit_turn_item_started(&turn_context, &turn_item).await;
                     }
-                    if let (Some(state), Some(item_id), Some(parsed)) = (
-                        plan_mode_state.as_mut(),
-                        seeded_item_id.as_deref(),
-                        seeded_parsed,
-                    ) {
+                    if let Some((item_id, parsed)) = seeded_emit {
                         emit_streamed_assistant_text_delta(
                             &sess,
                             &turn_context,
-                            Some(state),
-                            item_id,
+                            plan_mode_state.as_mut(),
+                            &item_id,
+                            parsed,
+                        )
+                        .await;
+                    }
+                    if let Some((item_id, parsed)) = pending_emit {
+                        emit_streamed_assistant_text_delta(
+                            &sess,
+                            &turn_context,
+                            plan_mode_state.as_mut(),
+                            &item_id,
                             parsed,
                         )
                         .await;
@@ -7612,7 +7654,12 @@ async fn try_run_sampling_request(
                             .await;
                     }
                 } else {
-                    error_or_panic("OutputTextDelta without active item".to_string());
+                    // This event should normally follow `output_item.added`, but tolerate the
+                    // reversed ordering by buffering text until the item metadata arrives.
+                    if pending_output_text_delta.is_empty() {
+                        warn!("buffering OutputTextDelta without active item");
+                    }
+                    pending_output_text_delta.push_str(&delta);
                 }
             }
             ResponseEvent::ReasoningSummaryDelta {
@@ -7630,7 +7677,12 @@ async fn try_run_sampling_request(
                     sess.send_event(&turn_context, EventMsg::ReasoningContentDelta(event))
                         .await;
                 } else {
-                    error_or_panic("ReasoningSummaryDelta without active item".to_string());
+                    // Without an active reasoning item there is nowhere safe to attach this delta.
+                    // Drop the orphan event and rely on the eventual completed item snapshot.
+                    warn!(
+                        summary_index,
+                        "dropping ReasoningSummaryDelta without active item"
+                    );
                 }
             }
             ResponseEvent::ReasoningSummaryPartAdded { summary_index } => {
@@ -7642,7 +7694,12 @@ async fn try_run_sampling_request(
                         });
                     sess.send_event(&turn_context, event).await;
                 } else {
-                    error_or_panic("ReasoningSummaryPartAdded without active item".to_string());
+                    // Keep section-break handling consistent with summary deltas: a break without
+                    // an active reasoning item is orphaned stream state, not a fatal parser error.
+                    warn!(
+                        summary_index,
+                        "dropping ReasoningSummaryPartAdded without active item"
+                    );
                 }
             }
             ResponseEvent::ReasoningContentDelta {
@@ -7660,7 +7717,12 @@ async fn try_run_sampling_request(
                     sess.send_event(&turn_context, EventMsg::ReasoningRawContentDelta(event))
                         .await;
                 } else {
-                    error_or_panic("ReasoningRawContentDelta without active item".to_string());
+                    // Raw reasoning deltas can also arrive before their item in malformed or
+                    // reordered streams. Preserve liveness by dropping the orphan event.
+                    warn!(
+                        content_index,
+                        "dropping ReasoningRawContentDelta without active item"
+                    );
                 }
             }
         }

codex-rs/core/tests/common/responses.rs

@@ -719,6 +719,13 @@ pub fn ev_reasoning_summary_text_delta(delta: &str) -> Value {
     })
 }
 
+pub fn ev_reasoning_summary_part_added(summary_index: i64) -> Value {
+    serde_json::json!({
+        "type": "response.reasoning_summary_part.added",
+        "summary_index": summary_index,
+    })
+}
+
 pub fn ev_reasoning_text_delta(delta: &str) -> Value {
     serde_json::json!({
         "type": "response.reasoning_text.delta",

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

@@ -21,6 +21,7 @@ use core_test_support::responses::ev_message_item_added;
 use core_test_support::responses::ev_output_text_delta;
 use core_test_support::responses::ev_reasoning_item;
 use core_test_support::responses::ev_reasoning_item_added;
+use core_test_support::responses::ev_reasoning_summary_part_added;
 use core_test_support::responses::ev_reasoning_summary_text_delta;
 use core_test_support::responses::ev_reasoning_text_delta;
 use core_test_support::responses::ev_response_created;
@@ -1061,6 +1062,186 @@ async fn plan_mode_handles_missing_plan_close_tag() -> anyhow::Result<()> {
     Ok(())
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn output_text_delta_before_output_item_added_is_buffered() -> anyhow::Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    let TestCodex {
+        codex,
+        session_configured,
+        ..
+    } = test_codex().build(&server).await?;
+
+    let stream = sse(vec![
+        ev_response_created("resp-1"),
+        ev_output_text_delta("Hello "),
+        ev_message_item_added("msg-1", ""),
+        ev_output_text_delta("world"),
+        ev_assistant_message("msg-1", "Hello world"),
+        ev_completed("resp-1"),
+    ]);
+    mount_sse_once(&server, stream).await;
+
+    codex
+        .submit(Op::UserTurn {
+            items: vec![UserInput::Text {
+                text: "hello".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+            cwd: std::env::current_dir()?,
+            approval_policy: codex_protocol::protocol::AskForApproval::Never,
+            approvals_reviewer: None,
+            sandbox_policy: codex_protocol::protocol::SandboxPolicy::DangerFullAccess,
+            model: session_configured.model.clone(),
+            effort: None,
+            summary: None,
+            service_tier: None,
+            collaboration_mode: None,
+            personality: None,
+        })
+        .await?;
+
+    let mut agent_deltas = Vec::new();
+    let mut completed = None;
+    loop {
+        match wait_for_event(&codex, |_| true).await {
+            EventMsg::AgentMessageContentDelta(event) => agent_deltas.push(event.delta),
+            EventMsg::ItemCompleted(ItemCompletedEvent {
+                item: TurnItem::AgentMessage(item),
+                ..
+            }) => completed = Some(item),
+            EventMsg::TurnComplete(_) => break,
+            _ => {}
+        }
+    }
+
+    assert_eq!(agent_deltas.concat(), "Hello world");
+    let completed_text: String = completed
+        .expect("assistant item completion should be emitted")
+        .content
+        .iter()
+        .map(|entry| match entry {
+            AgentMessageContent::Text { text } => text.as_str(),
+        })
+        .collect();
+    assert_eq!(completed_text, "Hello world");
+
+    Ok(())
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn buffered_output_text_survives_intervening_reasoning_item_done() -> anyhow::Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    let TestCodex {
+        codex,
+        session_configured,
+        ..
+    } = test_codex().build(&server).await?;
+
+    let stream = sse(vec![
+        ev_response_created("resp-1"),
+        ev_output_text_delta("Hello "),
+        ev_reasoning_item("reasoning-1", &["thinking"], &[]),
+        ev_message_item_added("msg-1", ""),
+        ev_output_text_delta("world"),
+        ev_assistant_message("msg-1", "Hello world"),
+        ev_completed("resp-1"),
+    ]);
+    mount_sse_once(&server, stream).await;
+
+    codex
+        .submit(Op::UserTurn {
+            items: vec![UserInput::Text {
+                text: "hello".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+            cwd: std::env::current_dir()?,
+            approval_policy: codex_protocol::protocol::AskForApproval::Never,
+            approvals_reviewer: None,
+            sandbox_policy: codex_protocol::protocol::SandboxPolicy::DangerFullAccess,
+            model: session_configured.model.clone(),
+            effort: None,
+            summary: None,
+            service_tier: None,
+            collaboration_mode: None,
+            personality: None,
+        })
+        .await?;
+
+    let mut agent_deltas = Vec::new();
+    let mut completed = None;
+    loop {
+        match wait_for_event(&codex, |_| true).await {
+            EventMsg::AgentMessageContentDelta(event) => agent_deltas.push(event.delta),
+            EventMsg::ItemCompleted(ItemCompletedEvent {
+                item: TurnItem::AgentMessage(item),
+                ..
+            }) => completed = Some(item),
+            EventMsg::TurnComplete(_) => break,
+            _ => {}
+        }
+    }
+
+    assert_eq!(agent_deltas.concat(), "Hello world");
+    let completed_text: String = completed
+        .expect("assistant item completion should be emitted")
+        .content
+        .iter()
+        .map(|entry| match entry {
+            AgentMessageContent::Text { text } => text.as_str(),
+        })
+        .collect();
+    assert_eq!(completed_text, "Hello world");
+
+    Ok(())
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn orphan_reasoning_summary_events_do_not_break_completed_reasoning_item()
+-> anyhow::Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    let TestCodex { codex, .. } = test_codex().build(&server).await?;
+
+    let stream = sse(vec![
+        ev_response_created("resp-1"),
+        ev_reasoning_summary_part_added(/*summary_index*/ 0),
+        ev_reasoning_summary_text_delta("Summary only"),
+        ev_reasoning_item("reasoning-1", &["Summary only"], &[]),
+        ev_completed("resp-1"),
+    ]);
+    mount_sse_once(&server, stream).await;
+
+    codex
+        .submit(Op::UserInput {
+            items: vec![UserInput::Text {
+                text: "think".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+        })
+        .await?;
+
+    let completed = wait_for_event_match(&codex, |ev| match ev {
+        EventMsg::ItemCompleted(ItemCompletedEvent {
+            item: TurnItem::Reasoning(item),
+            ..
+        }) => Some(item.clone()),
+        _ => None,
+    })
+    .await;
+
+    assert_eq!(completed.summary_text, vec!["Summary only".to_string()]);
+
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn reasoning_content_delta_has_item_metadata() -> anyhow::Result<()> {
     skip_if_no_network!(Ok(()));

codex-rs/tui/src/chatwidget.rs

@@ -6016,7 +6016,16 @@ impl ChatWidget {
             ThreadItem::Reasoning {
                 summary, content, ..
             } => {
-                if from_replay {
+                let mut completed_reasoning = summary.join("\n\n");
+                if self.config.show_raw_agent_reasoning {
+                    if !completed_reasoning.is_empty() && !content.is_empty() {
+                        completed_reasoning.push_str("\n\n");
+                    }
+                    completed_reasoning.push_str(&content.join("\n\n"));
+                }
+                let streamed_reasoning =
+                    format!("{}{}", self.full_reasoning_buffer, self.reasoning_buffer);
+                if from_replay || streamed_reasoning.trim().is_empty() {
                     for delta in summary {
                         self.on_agent_reasoning_delta(delta);
                     }
@@ -6025,6 +6034,17 @@ impl ChatWidget {
                             self.on_agent_reasoning_delta(delta);
                         }
                     }
+                } else if !completed_reasoning.is_empty()
+                    && completed_reasoning != streamed_reasoning
+                    && let Some(missing_prefix) = completed_reasoning
+                        .strip_suffix(&streamed_reasoning)
+                        .filter(|missing_prefix| !missing_prefix.is_empty())
+                {
+                    // If early summary deltas were dropped before the item became active, prepend
+                    // the missing prefix from the completed item snapshot and keep the streamed
+                    // suffix so we still avoid double-rendering.
+                    self.full_reasoning_buffer =
+                        format!("{missing_prefix}{}", self.full_reasoning_buffer);
                 }
                 self.on_agent_reasoning_final();
             }

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__live_reasoning_item_completed_backfills_missing_prefix_across_section_breaks.snap

@@ -0,0 +1,7 @@
+---
+source: tui/src/chatwidget/tests/history_replay.rs
+expression: rendered
+---
+• prefix middle
+  
+  tail

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__live_reasoning_item_completed_backfills_missing_prefix_before_streamed_suffix.snap

@@ -0,0 +1,5 @@
+---
+source: tui/src/chatwidget/tests/history_replay.rs
+expression: rendered
+---
+• prefix suffix

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__live_reasoning_item_completed_renders_summary_without_prior_delta.snap

@@ -0,0 +1,5 @@
+---
+source: tui/src/chatwidget/tests/history_replay.rs
+expression: rendered
+---
+• Summary only

codex-rs/tui/src/chatwidget/tests/history_replay.rs

@@ -1,4 +1,5 @@
 use super::*;
+use codex_app_server_protocol::ReasoningSummaryPartAddedNotification;
 use pretty_assertions::assert_eq;
 
 #[tokio::test]
@@ -725,6 +726,183 @@ async fn live_reasoning_summary_is_not_rendered_twice_when_item_completes() {
     assert_eq!(rendered.matches("Summary only").count(), 1);
 }
 
+#[tokio::test]
+async fn live_reasoning_item_completed_renders_summary_without_prior_delta() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
+    chat.show_welcome_banner = false;
+
+    chat.handle_server_notification(
+        ServerNotification::TurnStarted(TurnStartedNotification {
+            thread_id: "thread-1".to_string(),
+            turn: AppServerTurn {
+                id: "turn-1".to_string(),
+                items: Vec::new(),
+                status: AppServerTurnStatus::InProgress,
+                error: None,
+            },
+        }),
+        /*replay_kind*/ None,
+    );
+    let _ = drain_insert_history(&mut rx);
+
+    chat.handle_server_notification(
+        ServerNotification::ItemCompleted(ItemCompletedNotification {
+            thread_id: "thread-1".to_string(),
+            turn_id: "turn-1".to_string(),
+            item: AppServerThreadItem::Reasoning {
+                id: "reasoning-1".to_string(),
+                summary: vec!["Summary only".to_string()],
+                content: Vec::new(),
+            },
+        }),
+        /*replay_kind*/ None,
+    );
+
+    let rendered = match rx.try_recv() {
+        Ok(AppEvent::InsertHistoryCell(cell)) => {
+            lines_to_single_string(&cell.transcript_lines(/*width*/ 80))
+        }
+        other => panic!("expected InsertHistoryCell, got {other:?}"),
+    };
+    assert!(rendered.contains("Summary only"));
+    assert_chatwidget_snapshot!(
+        "live_reasoning_item_completed_renders_summary_without_prior_delta",
+        rendered
+    );
+}
+
+#[tokio::test]
+async fn live_reasoning_item_completed_backfills_missing_prefix_before_streamed_suffix() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
+    chat.show_welcome_banner = false;
+
+    chat.handle_server_notification(
+        ServerNotification::TurnStarted(TurnStartedNotification {
+            thread_id: "thread-1".to_string(),
+            turn: AppServerTurn {
+                id: "turn-1".to_string(),
+                items: Vec::new(),
+                status: AppServerTurnStatus::InProgress,
+                error: None,
+            },
+        }),
+        /*replay_kind*/ None,
+    );
+    let _ = drain_insert_history(&mut rx);
+
+    chat.handle_server_notification(
+        ServerNotification::ReasoningSummaryTextDelta(ReasoningSummaryTextDeltaNotification {
+            thread_id: "thread-1".to_string(),
+            turn_id: "turn-1".to_string(),
+            item_id: "reasoning-1".to_string(),
+            delta: "suffix".to_string(),
+            summary_index: 0,
+        }),
+        /*replay_kind*/ None,
+    );
+
+    chat.handle_server_notification(
+        ServerNotification::ItemCompleted(ItemCompletedNotification {
+            thread_id: "thread-1".to_string(),
+            turn_id: "turn-1".to_string(),
+            item: AppServerThreadItem::Reasoning {
+                id: "reasoning-1".to_string(),
+                summary: vec!["prefix suffix".to_string()],
+                content: Vec::new(),
+            },
+        }),
+        /*replay_kind*/ None,
+    );
+
+    let rendered = match rx.try_recv() {
+        Ok(AppEvent::InsertHistoryCell(cell)) => {
+            lines_to_single_string(&cell.transcript_lines(/*width*/ 80))
+        }
+        other => panic!("expected InsertHistoryCell, got {other:?}"),
+    };
+    assert_eq!(rendered.matches("prefix suffix").count(), 1);
+    assert_chatwidget_snapshot!(
+        "live_reasoning_item_completed_backfills_missing_prefix_before_streamed_suffix",
+        rendered
+    );
+}
+
+#[tokio::test]
+async fn live_reasoning_item_completed_backfills_missing_prefix_across_section_breaks() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
+    chat.show_welcome_banner = false;
+
+    chat.handle_server_notification(
+        ServerNotification::TurnStarted(TurnStartedNotification {
+            thread_id: "thread-1".to_string(),
+            turn: AppServerTurn {
+                id: "turn-1".to_string(),
+                items: Vec::new(),
+                status: AppServerTurnStatus::InProgress,
+                error: None,
+            },
+        }),
+        /*replay_kind*/ None,
+    );
+    let _ = drain_insert_history(&mut rx);
+
+    chat.handle_server_notification(
+        ServerNotification::ReasoningSummaryTextDelta(ReasoningSummaryTextDeltaNotification {
+            thread_id: "thread-1".to_string(),
+            turn_id: "turn-1".to_string(),
+            item_id: "reasoning-1".to_string(),
+            delta: "middle".to_string(),
+            summary_index: 0,
+        }),
+        /*replay_kind*/ None,
+    );
+    chat.handle_server_notification(
+        ServerNotification::ReasoningSummaryPartAdded(ReasoningSummaryPartAddedNotification {
+            thread_id: "thread-1".to_string(),
+            turn_id: "turn-1".to_string(),
+            item_id: "reasoning-1".to_string(),
+            summary_index: 1,
+        }),
+        /*replay_kind*/ None,
+    );
+    chat.handle_server_notification(
+        ServerNotification::ReasoningSummaryTextDelta(ReasoningSummaryTextDeltaNotification {
+            thread_id: "thread-1".to_string(),
+            turn_id: "turn-1".to_string(),
+            item_id: "reasoning-1".to_string(),
+            delta: "tail".to_string(),
+            summary_index: 1,
+        }),
+        /*replay_kind*/ None,
+    );
+
+    chat.handle_server_notification(
+        ServerNotification::ItemCompleted(ItemCompletedNotification {
+            thread_id: "thread-1".to_string(),
+            turn_id: "turn-1".to_string(),
+            item: AppServerThreadItem::Reasoning {
+                id: "reasoning-1".to_string(),
+                summary: vec!["prefix middle".to_string(), "tail".to_string()],
+                content: Vec::new(),
+            },
+        }),
+        /*replay_kind*/ None,
+    );
+
+    let rendered = match rx.try_recv() {
+        Ok(AppEvent::InsertHistoryCell(cell)) => {
+            lines_to_single_string(&cell.transcript_lines(/*width*/ 80))
+        }
+        other => panic!("expected InsertHistoryCell, got {other:?}"),
+    };
+    assert_eq!(rendered.matches("prefix middle").count(), 1);
+    assert!(rendered.contains("tail"));
+    assert_chatwidget_snapshot!(
+        "live_reasoning_item_completed_backfills_missing_prefix_across_section_breaks",
+        rendered
+    );
+}
+
 #[tokio::test]
 async fn replayed_turn_started_does_not_mark_task_running() {
     let (mut chat, _rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;