Add session loading support to Codex (#1602)

## Summary
- extend rollout format to store all session data in JSON
- add resume/write helpers for rollouts
- track session state after each conversation
- support `LoadSession` op to resume a previous rollout
- allow starting Codex with an existing session via
`experimental_resume` config variable

We need a way later for exploring the available sessions in a user
friendly way.

## Testing
- `cargo test --no-run` *(fails: `cargo: command not found`)*

------
https://chatgpt.com/codex/tasks/task_i_68792a29dd5c832190bf6930d3466fba

This video is outdated. you should use `-c experimental_resume:<full
path>` instead of `--resume <full path>`


https://github.com/user-attachments/assets/7a9975c7-aa04-4f4e-899a-9e87defd947a

codex-rs/common/src/config_override.rs

@@ -64,7 +64,11 @@ impl CliConfigOverrides {
                 // `-c model=o3` without the quotes.
                 let value: Value = match parse_toml_value(value_str) {
                     Ok(v) => v,
-                    Err(_) => Value::String(value_str.to_string()),
+                    Err(_) => {
+                        // Strip leading/trailing quotes if present
+                        let trimmed = value_str.trim().trim_matches(|c| c == '"' || c == '\'');
+                        Value::String(trimmed.to_string())
+                    }
                 };
 
                 Ok((key.to_string(), value))

codex-rs/core/src/codex.rs

@@ -102,6 +102,9 @@ impl Codex {
     /// of `Codex` and the ID of the `SessionInitialized` event that was
     /// submitted to start the session.
     pub async fn spawn(config: Config, ctrl_c: Arc<Notify>) -> CodexResult<(Codex, String)> {
+        // experimental resume path (undocumented)
+        let resume_path = config.experimental_resume.clone();
+        info!("resume_path: {resume_path:?}");
         let (tx_sub, rx_sub) = async_channel::bounded(64);
         let (tx_event, rx_event) = async_channel::bounded(1600);
 
@@ -117,6 +120,7 @@ impl Codex {
             disable_response_storage: config.disable_response_storage,
             notify: config.notify.clone(),
             cwd: config.cwd.clone(),
+            resume_path: resume_path.clone(),
         };
 
         let config = Arc::new(config);
@@ -306,24 +310,30 @@ impl Session {
     /// transcript, if enabled.
     async fn record_conversation_items(&self, items: &[ResponseItem]) {
         debug!("Recording items for conversation: {items:?}");
-        self.record_rollout_items(items).await;
+        self.record_state_snapshot(items).await;
 
         if let Some(transcript) = self.state.lock().unwrap().zdr_transcript.as_mut() {
             transcript.record_items(items);
         }
     }
 
-    /// Append the given items to the session's rollout transcript (if enabled)
+    async fn record_state_snapshot(&self, items: &[ResponseItem]) {
-    /// and persist them to disk.
+        let snapshot = {
-    async fn record_rollout_items(&self, items: &[ResponseItem]) {
+            let state = self.state.lock().unwrap();
-        // Clone the recorder outside of the mutex so we don't hold the lock
+            crate::rollout::SessionStateSnapshot {
-        // across an await point (MutexGuard is not Send).
+                previous_response_id: state.previous_response_id.clone(),
+            }
+        };
+
         let recorder = {
             let guard = self.rollout.lock().unwrap();
             guard.as_ref().cloned()
         };
 
         if let Some(rec) = recorder {
+            if let Err(e) = rec.record_state(snapshot).await {
+                error!("failed to record rollout state: {e:#}");
+            }
             if let Err(e) = rec.record_items(items).await {
                 error!("failed to record rollout items: {e:#}");
             }
@@ -517,7 +527,7 @@ async fn submission_loop(
     ctrl_c: Arc<Notify>,
 ) {
     // Generate a unique ID for the lifetime of this Codex session.
-    let session_id = Uuid::new_v4();
+    let mut session_id = Uuid::new_v4();
 
     let mut sess: Option<Arc<Session>> = None;
     // shorthand - send an event when there is no active session
@@ -570,8 +580,11 @@ async fn submission_loop(
                 disable_response_storage,
                 notify,
                 cwd,
+                resume_path,
             } => {
-                info!("Configuring session: model={model}; provider={provider:?}");
+                info!(
+                    "Configuring session: model={model}; provider={provider:?}; resume={resume_path:?}"
+                );
                 if !cwd.is_absolute() {
                     let message = format!("cwd is not absolute: {cwd:?}");
                     error!(message);
@@ -584,6 +597,41 @@ async fn submission_loop(
                     }
                     return;
                 }
+                // Optionally resume an existing rollout.
+                let mut restored_items: Option<Vec<ResponseItem>> = None;
+                let mut restored_prev_id: Option<String> = None;
+                let rollout_recorder: Option<RolloutRecorder> =
+                    if let Some(path) = resume_path.as_ref() {
+                        match RolloutRecorder::resume(path).await {
+                            Ok((rec, saved)) => {
+                                session_id = saved.session_id;
+                                restored_prev_id = saved.state.previous_response_id;
+                                if !saved.items.is_empty() {
+                                    restored_items = Some(saved.items);
+                                }
+                                Some(rec)
+                            }
+                            Err(e) => {
+                                warn!("failed to resume rollout from {path:?}: {e}");
+                                None
+                            }
+                        }
+                    } else {
+                        None
+                    };
+
+                let rollout_recorder = match rollout_recorder {
+                    Some(rec) => Some(rec),
+                    None => match RolloutRecorder::new(&config, session_id, instructions.clone())
+                        .await
+                    {
+                        Ok(r) => Some(r),
+                        Err(e) => {
+                            warn!("failed to initialise rollout recorder: {e}");
+                            None
+                        }
+                    },
+                };
 
                 let client = ModelClient::new(
                     config.clone(),
@@ -644,21 +692,6 @@ async fn submission_loop(
                         });
                     }
                 }
-
-                // Attempt to create a RolloutRecorder *before* moving the
-                // `instructions` value into the Session struct.
-                // TODO: if ConfigureSession is sent twice, we will create an
-                // overlapping rollout file. Consider passing RolloutRecorder
-                // from above.
-                let rollout_recorder =
-                    match RolloutRecorder::new(&config, session_id, instructions.clone()).await {
-                        Ok(r) => Some(r),
-                        Err(e) => {
-                            warn!("failed to initialise rollout recorder: {e}");
-                            None
-                        }
-                    };
-
                 sess = Some(Arc::new(Session {
                     client,
                     tx_event: tx_event.clone(),
@@ -676,6 +709,19 @@ async fn submission_loop(
                     codex_linux_sandbox_exe: config.codex_linux_sandbox_exe.clone(),
                 }));
 
+                // Patch restored state into the newly created session.
+                if let Some(sess_arc) = &sess {
+                    if restored_prev_id.is_some() || restored_items.is_some() {
+                        let mut st = sess_arc.state.lock().unwrap();
+                        st.previous_response_id = restored_prev_id;
+                        if let (Some(hist), Some(items)) =
+                            (st.zdr_transcript.as_mut(), restored_items.as_ref())
+                        {
+                            hist.record_items(items.iter());
+                        }
+                    }
+                }
+
                 // Gather history metadata for SessionConfiguredEvent.
                 let (history_log_id, history_entry_count) =
                     crate::message_history::history_metadata(&config).await;
@@ -744,6 +790,8 @@ async fn submission_loop(
                 }
             }
             Op::AddToHistory { text } => {
+                // TODO: What should we do if we got AddToHistory before ConfigureSession?
+                // currently, if ConfigureSession has resume path, this history will be ignored
                 let id = session_id;
                 let config = config.clone();
                 tokio::spawn(async move {

codex-rs/core/src/config.rs

@@ -137,6 +137,9 @@ pub struct Config {
 
     /// Base URL for requests to ChatGPT (as opposed to the OpenAI API).
     pub chatgpt_base_url: String,
+
+    /// Experimental rollout resume path (absolute path to .jsonl; undocumented).
+    pub experimental_resume: Option<PathBuf>,
 }
 
 impl Config {
@@ -321,6 +324,9 @@ pub struct ConfigToml {
 
     /// Base URL for requests to ChatGPT (as opposed to the OpenAI API).
     pub chatgpt_base_url: Option<String>,
+
+    /// Experimental rollout resume path (absolute path to .jsonl; undocumented).
+    pub experimental_resume: Option<PathBuf>,
 }
 
 impl ConfigToml {
@@ -448,6 +454,9 @@ impl Config {
                 .as_ref()
                 .map(|info| info.max_output_tokens)
         });
+
+        let experimental_resume = cfg.experimental_resume;
+
         let config = Self {
             model,
             model_context_window,
@@ -494,6 +503,8 @@ impl Config {
                 .chatgpt_base_url
                 .or(cfg.chatgpt_base_url)
                 .unwrap_or("https://chatgpt.com/backend-api/".to_string()),
+
+            experimental_resume,
         };
         Ok(config)
     }
@@ -806,6 +817,7 @@ disable_response_storage = true
                 model_reasoning_summary: ReasoningSummary::Detailed,
                 model_supports_reasoning_summaries: false,
                 chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
+                experimental_resume: None,
             },
             o3_profile_config
         );
@@ -852,6 +864,7 @@ disable_response_storage = true
             model_reasoning_summary: ReasoningSummary::default(),
             model_supports_reasoning_summaries: false,
             chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
+            experimental_resume: None,
         };
 
         assert_eq!(expected_gpt3_profile_config, gpt3_profile_config);
@@ -913,6 +926,7 @@ disable_response_storage = true
             model_reasoning_summary: ReasoningSummary::default(),
             model_supports_reasoning_summaries: false,
             chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
+            experimental_resume: None,
         };
 
         assert_eq!(expected_zdr_profile_config, zdr_profile_config);

codex-rs/core/src/protocol.rs

@@ -69,6 +69,10 @@ pub enum Op {
         /// `ConfigureSession` operation so that the business-logic layer can
         /// operate deterministically.
         cwd: std::path::PathBuf,
+
+        /// Path to a rollout file to resume from.
+        #[serde(skip_serializing_if = "Option::is_none")]
+        resume_path: Option<std::path::PathBuf>,
     },
 
     /// Abort current task.

codex-rs/core/src/rollout.rs

@@ -1,33 +1,47 @@
-//! Functionality to persist a Codex conversation *rollout* – a linear list of
+//! Persist Codex session rollouts (.jsonl) so sessions can be replayed or inspected later.
-//! [`ResponseItem`] objects exchanged during a session – to disk so that
-//! sessions can be replayed or inspected later (mirrors the behaviour of the
-//! upstream TypeScript implementation).
 
 use std::fs::File;
 use std::fs::{self};
 use std::io::Error as IoError;
+use std::path::Path;
 
+use serde::Deserialize;
 use serde::Serialize;
+use serde_json::Value;
 use time::OffsetDateTime;
 use time::format_description::FormatItem;
 use time::macros::format_description;
 use tokio::io::AsyncWriteExt;
 use tokio::sync::mpsc::Sender;
 use tokio::sync::mpsc::{self};
+use tracing::info;
 use uuid::Uuid;
 
 use crate::config::Config;
 use crate::models::ResponseItem;
 
-/// Folder inside `~/.codex` that holds saved rollouts.
 const SESSIONS_SUBDIR: &str = "sessions";
 
-#[derive(Serialize)]
+#[derive(Serialize, Deserialize, Clone, Default)]
-struct SessionMeta {
+pub struct SessionMeta {
-    id: String,
+    pub id: Uuid,
-    timestamp: String,
+    pub timestamp: String,
-    #[serde(skip_serializing_if = "Option::is_none")]
+    pub instructions: Option<String>,
-    instructions: Option<String>,
+}
+
+#[derive(Serialize, Deserialize, Default, Clone)]
+pub struct SessionStateSnapshot {
+    pub previous_response_id: Option<String>,
+}
+
+#[derive(Serialize, Deserialize, Default, Clone)]
+pub struct SavedSession {
+    pub session: SessionMeta,
+    #[serde(default)]
+    pub items: Vec<ResponseItem>,
+    #[serde(default)]
+    pub state: SessionStateSnapshot,
+    pub session_id: Uuid,
 }
 
 /// Records all [`ResponseItem`]s for a session and flushes them to disk after
@@ -41,7 +55,13 @@ struct SessionMeta {
 /// ```
 #[derive(Clone)]
 pub(crate) struct RolloutRecorder {
-    tx: Sender<String>,
+    tx: Sender<RolloutCmd>,
+}
+
+#[derive(Clone)]
+enum RolloutCmd {
+    AddItems(Vec<ResponseItem>),
+    UpdateState(SessionStateSnapshot),
 }
 
 impl RolloutRecorder {
@@ -59,7 +79,6 @@ impl RolloutRecorder {
             timestamp,
         } = create_log_file(config, uuid)?;
 
-        // Build the static session metadata JSON first.
         let timestamp_format: &[FormatItem] = format_description!(
             "[year]-[month]-[day]T[hour]:[minute]:[second].[subsecond digits:3]Z"
         );
@@ -69,46 +88,29 @@ impl RolloutRecorder {
 
         let meta = SessionMeta {
             timestamp,
-            id: session_id.to_string(),
+            id: session_id,
             instructions,
         };
 
         // A reasonably-sized bounded channel. If the buffer fills up the send
         // future will yield, which is fine – we only need to ensure we do not
         // perform *blocking* I/O on the caller’s thread.
-        let (tx, mut rx) = mpsc::channel::<String>(256);
+        let (tx, rx) = mpsc::channel::<RolloutCmd>(256);
 
         // Spawn a Tokio task that owns the file handle and performs async
         // writes. Using `tokio::fs::File` keeps everything on the async I/O
         // driver instead of blocking the runtime.
-        tokio::task::spawn(async move {
+        tokio::task::spawn(rollout_writer(
-            let mut file = tokio::fs::File::from_std(file);
+            tokio::fs::File::from_std(file),
+            rx,
+            Some(meta),
+        ));
 
-            while let Some(line) = rx.recv().await {
+        Ok(Self { tx })
-                // Write line + newline, then flush to disk.
-                if let Err(e) = file.write_all(line.as_bytes()).await {
-                    tracing::warn!("rollout writer: failed to write line: {e}");
-                    break;
-                }
-                if let Err(e) = file.write_all(b"\n").await {
-                    tracing::warn!("rollout writer: failed to write newline: {e}");
-                    break;
-                }
-                if let Err(e) = file.flush().await {
-                    tracing::warn!("rollout writer: failed to flush: {e}");
-                    break;
-                }
-            }
-        });
-
-        let recorder = Self { tx };
-        // Ensure SessionMeta is the first item in the file.
-        recorder.record_item(&meta).await?;
-        Ok(recorder)
     }
 
-    /// Append `items` to the rollout file.
     pub(crate) async fn record_items(&self, items: &[ResponseItem]) -> std::io::Result<()> {
+        let mut filtered = Vec::new();
         for item in items {
             match item {
                 // Note that function calls may look a bit strange if they are
@@ -117,27 +119,86 @@ impl RolloutRecorder {
                 ResponseItem::Message { .. }
                 | ResponseItem::LocalShellCall { .. }
                 | ResponseItem::FunctionCall { .. }
-                | ResponseItem::FunctionCallOutput { .. } => {}
+                | ResponseItem::FunctionCallOutput { .. } => filtered.push(item.clone()),
                 ResponseItem::Reasoning { .. } | ResponseItem::Other => {
                     // These should never be serialized.
                     continue;
                 }
             }
-            self.record_item(item).await?;
         }
-        Ok(())
+        if filtered.is_empty() {
+            return Ok(());
+        }
+        self.tx
+            .send(RolloutCmd::AddItems(filtered))
+            .await
+            .map_err(|e| IoError::other(format!("failed to queue rollout items: {e}")))
     }
 
-    async fn record_item(&self, item: &impl Serialize) -> std::io::Result<()> {
+    pub(crate) async fn record_state(&self, state: SessionStateSnapshot) -> std::io::Result<()> {
-        // Serialize the item to JSON first so that the writer thread only has
-        // to perform the actual write.
-        let json = serde_json::to_string(item)
-            .map_err(|e| IoError::other(format!("failed to serialize response items: {e}")))?;
-
         self.tx
-            .send(json)
+            .send(RolloutCmd::UpdateState(state))
             .await
-            .map_err(|e| IoError::other(format!("failed to queue rollout item: {e}")))
+            .map_err(|e| IoError::other(format!("failed to queue rollout state: {e}")))
+    }
+
+    pub async fn resume(path: &Path) -> std::io::Result<(Self, SavedSession)> {
+        info!("Resuming rollout from {path:?}");
+        let text = tokio::fs::read_to_string(path).await?;
+        let mut lines = text.lines();
+        let meta_line = lines
+            .next()
+            .ok_or_else(|| IoError::other("empty session file"))?;
+        let session: SessionMeta = serde_json::from_str(meta_line)
+            .map_err(|e| IoError::other(format!("failed to parse session meta: {e}")))?;
+        let mut items = Vec::new();
+        let mut state = SessionStateSnapshot::default();
+
+        for line in lines {
+            if line.trim().is_empty() {
+                continue;
+            }
+            let v: Value = match serde_json::from_str(line) {
+                Ok(v) => v,
+                Err(_) => continue,
+            };
+            if v.get("record_type")
+                .and_then(|rt| rt.as_str())
+                .map(|s| s == "state")
+                .unwrap_or(false)
+            {
+                if let Ok(s) = serde_json::from_value::<SessionStateSnapshot>(v.clone()) {
+                    state = s
+                }
+                continue;
+            }
+            if let Ok(item) = serde_json::from_value::<ResponseItem>(v.clone()) {
+                match item {
+                    ResponseItem::Message { .. }
+                    | ResponseItem::LocalShellCall { .. }
+                    | ResponseItem::FunctionCall { .. }
+                    | ResponseItem::FunctionCallOutput { .. } => items.push(item),
+                    ResponseItem::Reasoning { .. } | ResponseItem::Other => {}
+                }
+            }
+        }
+
+        let saved = SavedSession {
+            session: session.clone(),
+            items: items.clone(),
+            state: state.clone(),
+            session_id: session.id,
+        };
+
+        let file = std::fs::OpenOptions::new()
+            .append(true)
+            .read(true)
+            .open(path)?;
+
+        let (tx, rx) = mpsc::channel::<RolloutCmd>(256);
+        tokio::task::spawn(rollout_writer(tokio::fs::File::from_std(file), rx, None));
+        info!("Resumed rollout successfully from {path:?}");
+        Ok((Self { tx }, saved))
     }
 }
 
@@ -185,3 +246,54 @@ fn create_log_file(config: &Config, session_id: Uuid) -> std::io::Result<LogFile
         timestamp,
     })
 }
+
+async fn rollout_writer(
+    mut file: tokio::fs::File,
+    mut rx: mpsc::Receiver<RolloutCmd>,
+    meta: Option<SessionMeta>,
+) {
+    if let Some(meta) = meta {
+        if let Ok(json) = serde_json::to_string(&meta) {
+            let _ = file.write_all(json.as_bytes()).await;
+            let _ = file.write_all(b"\n").await;
+            let _ = file.flush().await;
+        }
+    }
+    while let Some(cmd) = rx.recv().await {
+        match cmd {
+            RolloutCmd::AddItems(items) => {
+                for item in items {
+                    match item {
+                        ResponseItem::Message { .. }
+                        | ResponseItem::LocalShellCall { .. }
+                        | ResponseItem::FunctionCall { .. }
+                        | ResponseItem::FunctionCallOutput { .. } => {
+                            if let Ok(json) = serde_json::to_string(&item) {
+                                let _ = file.write_all(json.as_bytes()).await;
+                                let _ = file.write_all(b"\n").await;
+                            }
+                        }
+                        ResponseItem::Reasoning { .. } | ResponseItem::Other => {}
+                    }
+                }
+                let _ = file.flush().await;
+            }
+            RolloutCmd::UpdateState(state) => {
+                #[derive(Serialize)]
+                struct StateLine<'a> {
+                    record_type: &'static str,
+                    #[serde(flatten)]
+                    state: &'a SessionStateSnapshot,
+                }
+                if let Ok(json) = serde_json::to_string(&StateLine {
+                    record_type: "state",
+                    state: &state,
+                }) {
+                    let _ = file.write_all(json.as_bytes()).await;
+                    let _ = file.write_all(b"\n").await;
+                    let _ = file.flush().await;
+                }
+            }
+        }
+    }
+}

codex-rs/core/tests/cli_stream.rs

@@ -2,7 +2,6 @@
 
 use assert_cmd::Command as AssertCommand;
 use codex_core::exec::CODEX_SANDBOX_NETWORK_DISABLED_ENV_VAR;
-use serde_json::Value;
 use std::time::Duration;
 use std::time::Instant;
 use tempfile::TempDir;
@@ -123,6 +122,7 @@ async fn responses_api_stream_cli() {
     assert!(stdout.contains("fixture hello"));
 }
 
+/// End-to-end: create a session (writes rollout), verify the file, then resume and confirm append.
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn integration_creates_and_checks_session_file() {
     // Honor sandbox network restrictions for CI parity with the other tests.
@@ -170,45 +170,66 @@ async fn integration_creates_and_checks_session_file() {
         String::from_utf8_lossy(&output.stderr)
     );
 
-    // 5. Sessions are written asynchronously; wait briefly for the directory to appear.
+    // Wait for sessions dir to appear.
     let sessions_dir = home.path().join("sessions");
-    let start = Instant::now();
+    let dir_deadline = Instant::now() + Duration::from_secs(5);
-    while !sessions_dir.exists() && start.elapsed() < Duration::from_secs(3) {
+    while !sessions_dir.exists() && Instant::now() < dir_deadline {
         std::thread::sleep(Duration::from_millis(50));
     }
+    assert!(sessions_dir.exists(), "sessions directory never appeared");
 
-    // 6. Scan all session files and find the one that contains our marker.
+    // Find the session file that contains `marker`.
-    let mut matching_files = vec![];
+    let deadline = Instant::now() + Duration::from_secs(10);
-    for entry in WalkDir::new(&sessions_dir) {
+    let mut matching_path: Option<std::path::PathBuf> = None;
-        let entry = entry.unwrap();
+    while Instant::now() < deadline && matching_path.is_none() {
-        if entry.file_type().is_file() && entry.file_name().to_string_lossy().ends_with(".jsonl") {
+        for entry in WalkDir::new(&sessions_dir) {
+            let entry = match entry {
+                Ok(e) => e,
+                Err(_) => continue,
+            };
+            if !entry.file_type().is_file() {
+                continue;
+            }
+            if !entry.file_name().to_string_lossy().ends_with(".jsonl") {
+                continue;
+            }
             let path = entry.path();
-            let content = std::fs::read_to_string(path).unwrap();
+            let Ok(content) = std::fs::read_to_string(path) else {
+                continue;
+            };
             let mut lines = content.lines();
-            // Skip SessionMeta (first line)
+            if lines.next().is_none() {
-            let _ = lines.next();
+                continue;
+            }
             for line in lines {
-                let item: Value = serde_json::from_str(line).unwrap();
+                if line.trim().is_empty() {
-                if let Some("message") = item.get("type").and_then(|t| t.as_str()) {
+                    continue;
-                    if let Some(content) = item.get("content") {
+                }
-                        if content.to_string().contains(&marker) {
+                let item: serde_json::Value = match serde_json::from_str(line) {
-                            matching_files.push(path.to_owned());
+                    Ok(v) => v,
+                    Err(_) => continue,
+                };
+                if item.get("type").and_then(|t| t.as_str()) == Some("message") {
+                    if let Some(c) = item.get("content") {
+                        if c.to_string().contains(&marker) {
+                            matching_path = Some(path.to_path_buf());
                             break;
                         }
                     }
                 }
             }
         }
+        if matching_path.is_none() {
+            std::thread::sleep(Duration::from_millis(50));
+        }
     }
-    assert_eq!(
-        matching_files.len(),
-        1,
-        "Expected exactly one session file containing the marker, found {}",
-        matching_files.len()
-    );
-    let path = &matching_files[0];
 
-    // 7. Verify directory structure: sessions/YYYY/MM/DD/filename.jsonl
+    let path = match matching_path {
+        Some(p) => p,
+        None => panic!("No session file containing the marker was found"),
+    };
+
+    // Basic sanity checks on location and metadata.
     let rel = match path.strip_prefix(&sessions_dir) {
         Ok(r) => r,
         Err(_) => panic!("session file should live under sessions/"),
@@ -237,7 +258,6 @@ async fn integration_creates_and_checks_session_file() {
         day.len() == 2 && day.chars().all(|c| c.is_ascii_digit()),
         "Day dir not zero-padded 2-digit numeric: {day}"
     );
-    // Range checks (best-effort; won't fail on leading zeros)
     if let Ok(m) = month.parse::<u8>() {
         assert!((1..=12).contains(&m), "Month out of range: {m}");
     }
@@ -245,23 +265,32 @@ async fn integration_creates_and_checks_session_file() {
         assert!((1..=31).contains(&d), "Day out of range: {d}");
     }
 
-    // 8. Parse SessionMeta line and basic sanity checks.
+    let content =
-    let content = std::fs::read_to_string(path).unwrap();
+        std::fs::read_to_string(&path).unwrap_or_else(|_| panic!("Failed to read session file"));
     let mut lines = content.lines();
-    let meta: Value = serde_json::from_str(lines.next().unwrap()).unwrap();
+    let meta_line = lines
+        .next()
+        .ok_or("missing session meta line")
+        .unwrap_or_else(|_| panic!("missing session meta line"));
+    let meta: serde_json::Value = serde_json::from_str(meta_line)
+        .unwrap_or_else(|_| panic!("Failed to parse session meta line as JSON"));
     assert!(meta.get("id").is_some(), "SessionMeta missing id");
     assert!(
         meta.get("timestamp").is_some(),
         "SessionMeta missing timestamp"
     );
 
-    // 9. Confirm at least one message contains the marker.
     let mut found_message = false;
     for line in lines {
-        let item: Value = serde_json::from_str(line).unwrap();
+        if line.trim().is_empty() {
-        if item.get("type").map(|t| t == "message").unwrap_or(false) {
+            continue;
-            if let Some(content) = item.get("content") {
+        }
-                if content.to_string().contains(&marker) {
+        let Ok(item) = serde_json::from_str::<serde_json::Value>(line) else {
+            continue;
+        };
+        if item.get("type").and_then(|t| t.as_str()) == Some("message") {
+            if let Some(c) = item.get("content") {
+                if c.to_string().contains(&marker) {
                     found_message = true;
                     break;
                 }
@@ -272,4 +301,61 @@ async fn integration_creates_and_checks_session_file() {
         found_message,
         "No message found in session file containing the marker"
     );
+
+    // Second run: resume and append.
+    let orig_len = content.lines().count();
+    let marker2 = format!("integration-resume-{}", Uuid::new_v4());
+    let prompt2 = format!("echo {marker2}");
+    // Cross‑platform safe resume override.  On Windows, backslashes in a TOML string must be escaped
+    // or the parse will fail and the raw literal (including quotes) may be preserved all the way down
+    // to Config, which in turn breaks resume because the path is invalid. Normalize to forward slashes
+    // to sidestep the issue.
+    let resume_path_str = path.to_string_lossy().replace('\\', "/");
+    let resume_override = format!("experimental_resume=\"{resume_path_str}\"");
+    let mut cmd2 = AssertCommand::new("cargo");
+    cmd2.arg("run")
+        .arg("-p")
+        .arg("codex-cli")
+        .arg("--quiet")
+        .arg("--")
+        .arg("exec")
+        .arg("--skip-git-repo-check")
+        .arg("-c")
+        .arg(&resume_override)
+        .arg("-C")
+        .arg(env!("CARGO_MANIFEST_DIR"))
+        .arg(&prompt2);
+    cmd2.env("CODEX_HOME", home.path())
+        .env("OPENAI_API_KEY", "dummy")
+        .env("CODEX_RS_SSE_FIXTURE", &fixture)
+        .env("OPENAI_BASE_URL", "http://unused.local");
+    let output2 = cmd2.output().unwrap();
+    assert!(output2.status.success(), "resume codex-cli run failed");
+
+    // The rollout writer runs on a background async task; give it a moment to flush.
+    let mut new_len = orig_len;
+    let deadline = Instant::now() + Duration::from_secs(5);
+    let mut content2 = String::new();
+    while Instant::now() < deadline {
+        if let Ok(c) = std::fs::read_to_string(&path) {
+            let count = c.lines().count();
+            if count > orig_len {
+                content2 = c;
+                new_len = count;
+                break;
+            }
+        }
+        std::thread::sleep(Duration::from_millis(50));
+    }
+    if content2.is_empty() {
+        // last attempt
+        content2 = std::fs::read_to_string(&path).unwrap();
+        new_len = content2.lines().count();
+    }
+    assert!(new_len > orig_len, "rollout file did not grow after resume");
+    assert!(content2.contains(&marker), "rollout lost original marker");
+    assert!(
+        content2.contains(&marker2),
+        "rollout missing resumed marker"
+    );
 }