diff --git a/codex-rs/tui/src/session_log.rs b/codex-rs/tui/src/session_log.rs
index b2858e8f210..8d3f11e4334 100644
--- a/codex-rs/tui/src/session_log.rs
+++ b/codex-rs/tui/src/session_log.rs
@@ -1,10 +1,12 @@
-use std::fs::File;
 use std::fs::OpenOptions;
+use std::io::BufWriter;
 use std::io::Write;
 use std::path::PathBuf;
 use std::sync::LazyLock;
-use std::sync::Mutex;
 use std::sync::OnceLock;
+use std::sync::mpsc;
+use std::thread;
+use std::time::Duration;
 
 use codex_core::config::Config;
 use codex_core::protocol::Op;
@@ -14,19 +16,28 @@ use serde_json::json;
 use crate::app_event::AppEvent;
 
 static LOGGER: LazyLock<SessionLogger> = LazyLock::new(SessionLogger::new);
+const FLUSH_INTERVAL: Duration = if cfg!(test) {
+    Duration::from_millis(10)
+} else {
+    Duration::from_millis(500)
+};
 
 struct SessionLogger {
-    file: OnceLock<Mutex<File>>,
+    worker: OnceLock<LoggerWorker>,
 }
 
 impl SessionLogger {
     fn new() -> Self {
         Self {
-            file: OnceLock::new(),
+            worker: OnceLock::new(),
         }
     }
 
     fn open(&self, path: PathBuf) -> std::io::Result<()> {
+        if self.worker.get().is_some() {
+            return Ok(());
+        }
+
         let mut opts = OpenOptions::new();
         opts.create(true).truncate(true).write(true);
 
@@ -37,41 +48,100 @@ impl SessionLogger {
         }
 
         let file = opts.open(path)?;
-        self.file.get_or_init(|| Mutex::new(file));
+        let (tx, rx) = mpsc::channel::<LogEntry>();
+        let handle = thread::Builder::new()
+            .name("tui-session-log".to_string())
+            .spawn(move || {
+                let mut writer = BufWriter::with_capacity(64 * 1024, file);
+                let mut dirty = false;
+                loop {
+                    match rx.recv_timeout(FLUSH_INTERVAL) {
+                        Ok(entry) => match entry {
+                            LogEntry::Line(serialized) => {
+                                if let Err(e) = writer.write_all(serialized.as_bytes()) {
+                                    tracing::warn!("session log write error: {e}");
+                                    continue;
+                                }
+                                if let Err(e) = writer.write_all(b"\n") {
+                                    tracing::warn!("session log write error: {e}");
+                                    continue;
+                                }
+                                dirty = true;
+                            }
+                            LogEntry::Flush(done_tx) => {
+                                if let Err(e) = writer.flush() {
+                                    tracing::warn!("session log flush error: {e}");
+                                } else {
+                                    dirty = false;
+                                }
+                                let _ = done_tx.send(());
+                            }
+                        },
+                        Err(mpsc::RecvTimeoutError::Timeout) => {
+                            if dirty {
+                                if let Err(e) = writer.flush() {
+                                    tracing::warn!("session log flush error: {e}");
+                                } else {
+                                    dirty = false;
+                                }
+                            }
+                        }
+                        Err(mpsc::RecvTimeoutError::Disconnected) => break,
+                    }
+                }
+                if dirty {
+                    let _ = writer.flush();
+                }
+            })?;
+
+        let _ = self.worker.set(LoggerWorker {
+            tx,
+            _handle: handle,
+        });
         Ok(())
     }
 
     fn write_json_line(&self, value: serde_json::Value) {
-        let Some(mutex) = self.file.get() else {
+        let Some(worker) = self.worker.get() else {
             return;
         };
-        let mut guard = match mutex.lock() {
-            Ok(g) => g,
-            Err(poisoned) => poisoned.into_inner(),
-        };
         match serde_json::to_string(&value) {
             Ok(serialized) => {
-                if let Err(e) = guard.write_all(serialized.as_bytes()) {
-                    tracing::warn!("session log write error: {}", e);
-                    return;
-                }
-                if let Err(e) = guard.write_all(b"\n") {
-                    tracing::warn!("session log write error: {}", e);
-                    return;
-                }
-                if let Err(e) = guard.flush() {
-                    tracing::warn!("session log flush error: {}", e);
+                if let Err(e) = worker.tx.send(LogEntry::Line(serialized)) {
+                    tracing::warn!("session log write error: {e}");
                 }
             }
-            Err(e) => tracing::warn!("session log serialize error: {}", e),
+            Err(e) => tracing::warn!("session log serialize error: {e}"),
         }
     }
 
     fn is_enabled(&self) -> bool {
-        self.file.get().is_some()
+        self.worker.get().is_some()
+    }
+
+    fn flush(&self) {
+        let Some(worker) = self.worker.get() else {
+            return;
+        };
+        let (done_tx, done_rx) = mpsc::channel();
+        if let Err(e) = worker.tx.send(LogEntry::Flush(done_tx)) {
+            tracing::warn!("session log flush error: {e}");
+            return;
+        }
+        let _ = done_rx.recv();
     }
 }
 
+struct LoggerWorker {
+    tx: mpsc::Sender<LogEntry>,
+    _handle: thread::JoinHandle<()>,
+}
+
+enum LogEntry {
+    Line(String),
+    Flush(mpsc::Sender<()>),
+}
+
 fn now_ts() -> String {
     // RFC3339 for readability; consumers can parse as needed.
     chrono::Utc::now().to_rfc3339_opts(chrono::SecondsFormat::Millis, true)
@@ -194,6 +264,7 @@ pub(crate) fn log_session_end() {
         "kind": "session_end",
     });
     LOGGER.write_json_line(value);
+    LOGGER.flush();
 }
 
 fn write_record<T>(dir: &str, kind: &str, obj: &T)
@@ -208,3 +279,58 @@ where
     });
     LOGGER.write_json_line(value);
 }
+
+#[cfg(test)]
+mod tests {
+    use pretty_assertions::assert_eq;
+    use serde_json::json;
+    use std::thread;
+    use std::time::Duration;
+    use tempfile::tempdir;
+
+    use super::SessionLogger;
+
+    #[test]
+    fn flush_writes_line_before_returning() {
+        let dir = tempdir().expect("tempdir");
+        let path = dir.path().join("session.jsonl");
+        let logger = SessionLogger::new();
+        logger.open(path.clone()).expect("open session log");
+
+        let record = json!({
+            "ts": "T",
+            "dir": "meta",
+            "kind": "session_end",
+        });
+        logger.write_json_line(record.clone());
+        logger.flush();
+
+        let contents = std::fs::read_to_string(path).expect("read log");
+        let lines: Vec<&str> = contents.lines().collect();
+        assert_eq!(lines.len(), 1);
+        let parsed: serde_json::Value = serde_json::from_str(lines[0]).expect("parse json");
+        assert_eq!(parsed, record);
+    }
+
+    #[test]
+    fn periodic_flush_persists_lines() {
+        let dir = tempdir().expect("tempdir");
+        let path = dir.path().join("session.jsonl");
+        let logger = SessionLogger::new();
+        logger.open(path.clone()).expect("open session log");
+
+        let record = json!({
+            "ts": "T2",
+            "dir": "meta",
+            "kind": "session_start",
+        });
+        logger.write_json_line(record.clone());
+        thread::sleep(Duration::from_millis(50));
+
+        let contents = std::fs::read_to_string(path).expect("read log");
+        let lines: Vec<&str> = contents.lines().collect();
+        assert_eq!(lines.len(), 1);
+        let parsed: serde_json::Value = serde_json::from_str(lines[0]).expect("parse json");
+        assert_eq!(parsed, record);
+    }
+}
diff --git a/codex-rs/tui2/src/session_log.rs b/codex-rs/tui2/src/session_log.rs
index b2858e8f210..8d3f11e4334 100644
--- a/codex-rs/tui2/src/session_log.rs
+++ b/codex-rs/tui2/src/session_log.rs
@@ -1,10 +1,12 @@
-use std::fs::File;
 use std::fs::OpenOptions;
+use std::io::BufWriter;
 use std::io::Write;
 use std::path::PathBuf;
 use std::sync::LazyLock;
-use std::sync::Mutex;
 use std::sync::OnceLock;
+use std::sync::mpsc;
+use std::thread;
+use std::time::Duration;
 
 use codex_core::config::Config;
 use codex_core::protocol::Op;
@@ -14,19 +16,28 @@ use serde_json::json;
 use crate::app_event::AppEvent;
 
 static LOGGER: LazyLock<SessionLogger> = LazyLock::new(SessionLogger::new);
+const FLUSH_INTERVAL: Duration = if cfg!(test) {
+    Duration::from_millis(10)
+} else {
+    Duration::from_millis(500)
+};
 
 struct SessionLogger {
-    file: OnceLock<Mutex<File>>,
+    worker: OnceLock<LoggerWorker>,
 }
 
 impl SessionLogger {
     fn new() -> Self {
         Self {
-            file: OnceLock::new(),
+            worker: OnceLock::new(),
         }
     }
 
     fn open(&self, path: PathBuf) -> std::io::Result<()> {
+        if self.worker.get().is_some() {
+            return Ok(());
+        }
+
         let mut opts = OpenOptions::new();
         opts.create(true).truncate(true).write(true);
 
@@ -37,41 +48,100 @@ impl SessionLogger {
         }
 
         let file = opts.open(path)?;
-        self.file.get_or_init(|| Mutex::new(file));
+        let (tx, rx) = mpsc::channel::<LogEntry>();
+        let handle = thread::Builder::new()
+            .name("tui-session-log".to_string())
+            .spawn(move || {
+                let mut writer = BufWriter::with_capacity(64 * 1024, file);
+                let mut dirty = false;
+                loop {
+                    match rx.recv_timeout(FLUSH_INTERVAL) {
+                        Ok(entry) => match entry {
+                            LogEntry::Line(serialized) => {
+                                if let Err(e) = writer.write_all(serialized.as_bytes()) {
+                                    tracing::warn!("session log write error: {e}");
+                                    continue;
+                                }
+                                if let Err(e) = writer.write_all(b"\n") {
+                                    tracing::warn!("session log write error: {e}");
+                                    continue;
+                                }
+                                dirty = true;
+                            }
+                            LogEntry::Flush(done_tx) => {
+                                if let Err(e) = writer.flush() {
+                                    tracing::warn!("session log flush error: {e}");
+                                } else {
+                                    dirty = false;
+                                }
+                                let _ = done_tx.send(());
+                            }
+                        },
+                        Err(mpsc::RecvTimeoutError::Timeout) => {
+                            if dirty {
+                                if let Err(e) = writer.flush() {
+                                    tracing::warn!("session log flush error: {e}");
+                                } else {
+                                    dirty = false;
+                                }
+                            }
+                        }
+                        Err(mpsc::RecvTimeoutError::Disconnected) => break,
+                    }
+                }
+                if dirty {
+                    let _ = writer.flush();
+                }
+            })?;
+
+        let _ = self.worker.set(LoggerWorker {
+            tx,
+            _handle: handle,
+        });
         Ok(())
     }
 
     fn write_json_line(&self, value: serde_json::Value) {
-        let Some(mutex) = self.file.get() else {
+        let Some(worker) = self.worker.get() else {
             return;
         };
-        let mut guard = match mutex.lock() {
-            Ok(g) => g,
-            Err(poisoned) => poisoned.into_inner(),
-        };
         match serde_json::to_string(&value) {
             Ok(serialized) => {
-                if let Err(e) = guard.write_all(serialized.as_bytes()) {
-                    tracing::warn!("session log write error: {}", e);
-                    return;
-                }
-                if let Err(e) = guard.write_all(b"\n") {
-                    tracing::warn!("session log write error: {}", e);
-                    return;
-                }
-                if let Err(e) = guard.flush() {
-                    tracing::warn!("session log flush error: {}", e);
+                if let Err(e) = worker.tx.send(LogEntry::Line(serialized)) {
+                    tracing::warn!("session log write error: {e}");
                 }
             }
-            Err(e) => tracing::warn!("session log serialize error: {}", e),
+            Err(e) => tracing::warn!("session log serialize error: {e}"),
         }
     }
 
     fn is_enabled(&self) -> bool {
-        self.file.get().is_some()
+        self.worker.get().is_some()
+    }
+
+    fn flush(&self) {
+        let Some(worker) = self.worker.get() else {
+            return;
+        };
+        let (done_tx, done_rx) = mpsc::channel();
+        if let Err(e) = worker.tx.send(LogEntry::Flush(done_tx)) {
+            tracing::warn!("session log flush error: {e}");
+            return;
+        }
+        let _ = done_rx.recv();
     }
 }
 
+struct LoggerWorker {
+    tx: mpsc::Sender<LogEntry>,
+    _handle: thread::JoinHandle<()>,
+}
+
+enum LogEntry {
+    Line(String),
+    Flush(mpsc::Sender<()>),
+}
+
 fn now_ts() -> String {
     // RFC3339 for readability; consumers can parse as needed.
     chrono::Utc::now().to_rfc3339_opts(chrono::SecondsFormat::Millis, true)
@@ -194,6 +264,7 @@ pub(crate) fn log_session_end() {
         "kind": "session_end",
     });
     LOGGER.write_json_line(value);
+    LOGGER.flush();
 }
 
 fn write_record<T>(dir: &str, kind: &str, obj: &T)
@@ -208,3 +279,58 @@ where
     });
     LOGGER.write_json_line(value);
 }
+
+#[cfg(test)]
+mod tests {
+    use pretty_assertions::assert_eq;
+    use serde_json::json;
+    use std::thread;
+    use std::time::Duration;
+    use tempfile::tempdir;
+
+    use super::SessionLogger;
+
+    #[test]
+    fn flush_writes_line_before_returning() {
+        let dir = tempdir().expect("tempdir");
+        let path = dir.path().join("session.jsonl");
+        let logger = SessionLogger::new();
+        logger.open(path.clone()).expect("open session log");
+
+        let record = json!({
+            "ts": "T",
+            "dir": "meta",
+            "kind": "session_end",
+        });
+        logger.write_json_line(record.clone());
+        logger.flush();
+
+        let contents = std::fs::read_to_string(path).expect("read log");
+        let lines: Vec<&str> = contents.lines().collect();
+        assert_eq!(lines.len(), 1);
+        let parsed: serde_json::Value = serde_json::from_str(lines[0]).expect("parse json");
+        assert_eq!(parsed, record);
+    }
+
+    #[test]
+    fn periodic_flush_persists_lines() {
+        let dir = tempdir().expect("tempdir");
+        let path = dir.path().join("session.jsonl");
+        let logger = SessionLogger::new();
+        logger.open(path.clone()).expect("open session log");
+
+        let record = json!({
+            "ts": "T2",
+            "dir": "meta",
+            "kind": "session_start",
+        });
+        logger.write_json_line(record.clone());
+        thread::sleep(Duration::from_millis(50));
+
+        let contents = std::fs::read_to_string(path).expect("read log");
+        let lines: Vec<&str> = contents.lines().collect();
+        assert_eq!(lines.len(), 1);
+        let parsed: serde_json::Value = serde_json::from_str(lines[0]).expect("parse json");
+        assert_eq!(parsed, record);
+    }
+}