diff --git a/codex-rs/rollout/src/recorder.rs b/codex-rs/rollout/src/recorder.rs
index 23b05b9defa..19b2b8aa9a6 100644
--- a/codex-rs/rollout/src/recorder.rs
+++ b/codex-rs/rollout/src/recorder.rs
@@ -4,6 +4,9 @@ use std::collections::HashSet;
 use std::fs;
 use std::fs::File;
 use std::io::Error as IoError;
+use std::io::Seek;
+use std::io::SeekFrom;
+use std::io::Write;
 use std::path::Path;
 use std::path::PathBuf;
 use std::sync::Arc;
@@ -694,17 +697,14 @@ impl RolloutRecorder {
 
                 (None, Some(log_file_info), path, Some(session_meta))
             }
-            RolloutRecorderParams::Resume { path } => (
-                Some(
-                    tokio::fs::OpenOptions::new()
-                        .append(true)
-                        .open(&path)
-                        .await?,
-                ),
-                None,
-                path,
-                None,
-            ),
+            RolloutRecorderParams::Resume { path } => {
+                let mut file = std::fs::OpenOptions::new()
+                    .read(true)
+                    .write(true)
+                    .open(&path)?;
+                repair_final_line(&mut file, &path)?;
+                (Some(tokio::fs::File::from_std(file)), None, path, None)
+            }
         };
 
         // Clone the cwd for the spawned task to collect git info asynchronously
@@ -1362,10 +1362,49 @@ fn open_log_file(path: &Path) -> std::io::Result<File> {
         )));
     };
     fs::create_dir_all(parent)?;
-    std::fs::OpenOptions::new()
-        .append(true)
+    let mut file = std::fs::OpenOptions::new()
+        .read(true)
+        .write(true)
         .create(true)
-        .open(path)
+        .truncate(false)
+        .open(path)?;
+    repair_final_line(&mut file, path)?;
+    Ok(file)
+}
+
+fn repair_final_line(file: &mut File, path: &Path) -> std::io::Result<()> {
+    let bytes = fs::read(path)?;
+    let len = bytes.len();
+    if len == 0 {
+        return Ok(());
+    }
+
+    if bytes.last() == Some(&b'\n') {
+        file.seek(SeekFrom::End(0))?;
+        return Ok(());
+    }
+
+    let truncate_to = bytes
+        .iter()
+        .rposition(|byte| *byte == b'\n')
+        .map_or(0, |index| index + 1);
+    if serde_json::from_slice::<RolloutLine>(&bytes[truncate_to..]).is_ok() {
+        file.seek(SeekFrom::End(0))?;
+        file.write_all(b"\n")?;
+        warn!(
+            "added missing final newline to rollout file: path={}, original_len={len}",
+            path.display()
+        );
+        return Ok(());
+    }
+
+    file.set_len(truncate_to as u64)?;
+    file.seek(SeekFrom::End(0))?;
+    warn!(
+        "truncated incomplete final rollout line: path={}, original_len={len}, truncate_to={truncate_to}",
+        path.display()
+    );
+    Ok(())
 }
 
 /// Mutable state owned by the background rollout writer.
@@ -1468,8 +1507,11 @@ impl RolloutWriterState {
         let message = err.to_string();
         if self.last_logged_error.as_ref() != Some(&message) {
             error!(
-                "rollout writer failed for {}; buffered rollout items will be retried: {err}",
-                self.rollout_path.display()
+                "rollout writer failed for {}; buffered rollout items will be retried: {err}; \
+                 error_kind={:?}; raw_os_error={:?}",
+                self.rollout_path.display(),
+                err.kind(),
+                err.raw_os_error()
             );
         }
         self.last_logged_error = Some(message);
@@ -1648,8 +1690,26 @@ impl JsonlWriter {
     async fn write_line(&mut self, item: &impl serde::Serialize) -> std::io::Result<()> {
         let mut json = serde_json::to_string(item)?;
         json.push('\n');
-        self.file.write_all(json.as_bytes()).await?;
-        self.file.flush().await?;
+        let start_len = self.file.metadata().await?.len();
+        if let Err(err) = async {
+            self.file.write_all(json.as_bytes()).await?;
+            self.file.flush().await
+        }
+        .await
+        {
+            if let Err(rollback_err) = self.file.set_len(start_len).await {
+                error!(
+                    "failed to roll back failed rollout line write: start_len={start_len}, \
+                     bytes_len={}, write_error={err}, write_raw_os_error={:?}, \
+                     rollback_error={rollback_err}, rollback_raw_os_error={:?}",
+                    json.len(),
+                    err.raw_os_error(),
+                    rollback_err.raw_os_error()
+                );
+                return Err(rollback_err);
+            }
+            return Err(err);
+        }
         Ok(())
     }
 }
diff --git a/codex-rs/rollout/src/recorder_tests.rs b/codex-rs/rollout/src/recorder_tests.rs
index 90fe15c3120..43db7a7dd8a 100644
--- a/codex-rs/rollout/src/recorder_tests.rs
+++ b/codex-rs/rollout/src/recorder_tests.rs
@@ -494,8 +494,8 @@ async fn persist_reports_filesystem_error_and_retries_buffered_items() -> std::i
 async fn writer_state_retries_write_error_before_reporting_flush_success() -> std::io::Result<()> {
     let home = TempDir::new().expect("temp dir");
     let rollout_path = home.path().join("rollout.jsonl");
-    File::create(&rollout_path)?;
-    let read_only_file = std::fs::OpenOptions::new().read(true).open(&rollout_path)?;
+    std::fs::write(&rollout_path, br#"{"timestamp":"partial"#)?;
+    let read_only_file = File::open(&rollout_path)?;
     let mut state = RolloutWriterState::new(
         Some(tokio::fs::File::from_std(read_only_file)),
         /*deferred_log_file_info*/ None,
@@ -513,10 +513,29 @@ async fn writer_state_retries_write_error_before_reporting_flush_success() -> st
 
     state.flush().await?;
     let text_after_retry = std::fs::read_to_string(&rollout_path)?;
+    assert!(
+        !text_after_retry.contains("partial"),
+        "retry should remove an incomplete final line before appending"
+    );
     assert!(
         text_after_retry.contains("queued-after-writer-error"),
         "flush should retry after reopening and write buffered items"
     );
+    for line in text_after_retry.lines() {
+        serde_json::from_str::<RolloutLine>(line).expect("line should parse after retry");
+    }
+    Ok(())
+}
+
+#[test]
+fn repair_final_line_preserves_valid_line_without_newline() -> std::io::Result<()> {
+    let home = TempDir::new().expect("temp dir");
+    let rollout_path = home.path().join("rollout.jsonl");
+    let line = r#"{"timestamp":"2026-05-25T00:00:00.000Z","type":"event_msg","payload":{"type":"user_message","message":"valid-without-newline","kind":"plain"}}"#;
+    std::fs::write(&rollout_path, line)?;
+    let mut file = File::options().read(true).write(true).open(&rollout_path)?;
+    repair_final_line(&mut file, &rollout_path)?;
+    assert_eq!(std::fs::read_to_string(&rollout_path)?, format!("{line}\n"));
     Ok(())
 }