Run code mode exclusively through the standalone host (#36217)

## What changed

- Move the V8 implementation into a dedicated `codex-code-mode-runtime` crate used by `codex-code-mode-host`, removing the embedded runtime fallback from the Codex process.
- Resolve the host executable from the active installation layout and check its availability before selecting tools.
- Fall back to direct tools with a one-time warning when optional code mode is unavailable. Keep `code_mode_only` and `disable_in_process_fallback` configurations fail-closed.

## Testing

- Cover host discovery for standalone and package layouts, including missing hosts and symlinks.
- Verify direct-tool fallback, one-time warnings, and fail-closed code-mode-only behavior.

GitOrigin-RevId: 5aa3c6f1db148b2231fc24089a2ee0e2b00dbddb

codex-rs/code-mode-runtime/src/session_runtime/mod.rs

@@ -0,0 +1,313 @@
+mod types;
+
+use std::collections::HashMap;
+use std::future::Future;
+use std::pin::Pin;
+use std::sync::Arc;
+use std::sync::atomic::AtomicU64;
+use std::sync::atomic::Ordering;
+
+use serde_json::Value as JsonValue;
+use tokio::sync::Mutex;
+use tokio_util::sync::CancellationToken;
+use tokio_util::task::TaskTracker;
+
+pub(crate) use self::types::CellEvent;
+pub(crate) use self::types::CellId;
+pub(crate) use self::types::CreateCellRequest;
+pub(crate) use self::types::Error;
+pub(crate) use self::types::ImageDetail;
+pub(crate) use self::types::NestedToolCall;
+pub(crate) use self::types::ObserveMode;
+pub(crate) use self::types::OutputItem;
+pub(crate) use self::types::SessionRuntimeDelegate;
+pub(crate) use self::types::ToolDefinition;
+pub(crate) use self::types::ToolKind;
+pub(crate) use self::types::ToolName;
+use crate::TaskFailureHandler;
+use crate::cell_actor::CellActor;
+use crate::cell_actor::CellError;
+use crate::cell_actor::CellEventFuture;
+use crate::cell_actor::CellHandle;
+use crate::cell_actor::CellHost;
+use crate::cell_actor::CellState;
+use crate::cell_actor::CellToolCall;
+use crate::cell_actor::CompletionCommit;
+
+type RuntimeEventFuture = Pin<Box<dyn Future<Output = Result<CellEvent, Error>> + Send + 'static>>;
+
+/// Owns all cells and shared state for one transport-neutral code-mode session.
+pub(crate) struct SessionRuntime<D: SessionRuntimeDelegate> {
+    inner: Arc<Inner<D>>,
+}
+
+struct Inner<D: SessionRuntimeDelegate> {
+    stored_values: Mutex<HashMap<String, JsonValue>>,
+    cells: Mutex<HashMap<CellId, CellHandle>>,
+    cell_tasks: TaskTracker,
+    shutdown_token: CancellationToken,
+    delegate: Arc<D>,
+    task_failure_handler: Option<TaskFailureHandler>,
+    next_cell_id: AtomicU64,
+}
+
+impl<D: SessionRuntimeDelegate> SessionRuntime<D> {
+    pub(crate) fn new(delegate: Arc<D>) -> Self {
+        Self::new_with_task_failure_handler(delegate, /*task_failure_handler*/ None)
+    }
+
+    pub(crate) fn new_with_task_failure_handler(
+        delegate: Arc<D>,
+        task_failure_handler: Option<TaskFailureHandler>,
+    ) -> Self {
+        Self {
+            inner: Arc::new(Inner {
+                stored_values: Mutex::new(HashMap::new()),
+                cells: Mutex::new(HashMap::new()),
+                cell_tasks: TaskTracker::new(),
+                shutdown_token: CancellationToken::new(),
+                delegate,
+                task_failure_handler,
+                next_cell_id: AtomicU64::new(1),
+            }),
+        }
+    }
+
+    pub(crate) async fn execute(
+        &self,
+        request: CreateCellRequest,
+        initial_observe_mode: ObserveMode,
+    ) -> Result<StartedCell, Error> {
+        if self.inner.shutdown_token.is_cancelled() {
+            return Err(Error::ShuttingDown);
+        }
+        let cell_id = self.allocate_cell_id()?;
+        let initial_event = self
+            .start_cell(cell_id.clone(), request, initial_observe_mode)
+            .await?;
+        Ok(StartedCell {
+            cell_id,
+            initial_event,
+        })
+    }
+
+    pub(crate) async fn observe(
+        &self,
+        cell_id: &CellId,
+        mode: ObserveMode,
+    ) -> Result<CellEvent, Error> {
+        self.begin_observe(cell_id, mode).await?.event().await
+    }
+
+    pub(crate) async fn begin_observe(
+        &self,
+        cell_id: &CellId,
+        mode: ObserveMode,
+    ) -> Result<PendingEvent, Error> {
+        let handle = self
+            .inner
+            .cells
+            .lock()
+            .await
+            .get(cell_id)
+            .cloned()
+            .ok_or_else(|| Error::MissingCell(cell_id.clone()))?;
+        Ok(PendingEvent {
+            event: map_actor_event(cell_id.clone(), handle.observe(mode)),
+        })
+    }
+
+    pub(crate) async fn terminate(&self, cell_id: &CellId) -> Result<CellEvent, Error> {
+        let handle = self
+            .inner
+            .cells
+            .lock()
+            .await
+            .get(cell_id)
+            .cloned()
+            .ok_or_else(|| Error::MissingCell(cell_id.clone()))?;
+        handle
+            .terminate()
+            .await
+            .map_err(|error| actor_error(cell_id, error))
+    }
+
+    pub(crate) async fn shutdown(&self) -> Result<(), Error> {
+        self.begin_shutdown();
+        // Taking the registry lock ensures every cell that passed the shutdown
+        // check has registered its actor with the tracker before we wait.
+        let cells = self.inner.cells.lock().await;
+        self.inner.cell_tasks.close();
+        drop(cells);
+        self.inner.cell_tasks.wait().await;
+        Ok(())
+    }
+
+    fn allocate_cell_id(&self) -> Result<CellId, Error> {
+        self.inner
+            .next_cell_id
+            .fetch_update(Ordering::Relaxed, Ordering::Relaxed, |next_cell_id| {
+                next_cell_id.checked_add(1)
+            })
+            .map(|cell_id| CellId::new(cell_id.to_string()))
+            .map_err(|_| Error::CellIdSpaceExhausted)
+    }
+
+    async fn start_cell(
+        &self,
+        cell_id: CellId,
+        request: CreateCellRequest,
+        initial_observe_mode: ObserveMode,
+    ) -> Result<RuntimeEventFuture, Error> {
+        let stored_values = self.inner.stored_values.lock().await.clone();
+        let host = Arc::new(RuntimeCellHost {
+            cell_id: cell_id.clone(),
+            inner: Arc::clone(&self.inner),
+        });
+        let mut cells = self.inner.cells.lock().await;
+        if self.inner.shutdown_token.is_cancelled() {
+            return Err(Error::ShuttingDown);
+        }
+        if cells.contains_key(&cell_id) {
+            return Err(Error::DuplicateCell(cell_id));
+        }
+        let cell_state = Arc::new(CellState::new(self.inner.shutdown_token.child_token()));
+        let (handle, initial_event, task) = CellActor::prepare(
+            request,
+            stored_values,
+            host,
+            initial_observe_mode,
+            cell_state,
+            self.inner.task_failure_handler.clone(),
+        )
+        .map_err(Error::Runtime)?;
+        cells.insert(cell_id.clone(), handle);
+        let task = self.inner.cell_tasks.spawn(task);
+        if let Some(task_failure_handler) = self.inner.task_failure_handler.clone() {
+            let failed_cell_id = cell_id.clone();
+            let _failure_watcher = self.inner.cell_tasks.spawn(async move {
+                if let Err(err) = task.await {
+                    task_failure_handler(format!(
+                        "code-mode cell {failed_cell_id} task failed: {err}"
+                    ));
+                }
+            });
+        }
+        drop(cells);
+        Ok(map_actor_event(cell_id, initial_event))
+    }
+
+    fn begin_shutdown(&self) {
+        self.inner.shutdown_token.cancel();
+        self.inner.cell_tasks.close();
+    }
+}
+
+impl<D: SessionRuntimeDelegate> Drop for SessionRuntime<D> {
+    fn drop(&mut self) {
+        self.begin_shutdown();
+    }
+}
+
+/// A cell admitted by [`SessionRuntime::execute`].
+pub(crate) struct StartedCell {
+    pub(crate) cell_id: CellId,
+    initial_event: RuntimeEventFuture,
+}
+
+impl StartedCell {
+    pub(crate) async fn initial_event(self) -> Result<CellEvent, Error> {
+        self.initial_event.await
+    }
+}
+
+/// An admitted observation that has not reached its requested frontier yet.
+pub(crate) struct PendingEvent {
+    event: RuntimeEventFuture,
+}
+
+impl PendingEvent {
+    pub(crate) async fn event(self) -> Result<CellEvent, Error> {
+        self.event.await
+    }
+}
+
+struct RuntimeCellHost<D: SessionRuntimeDelegate> {
+    cell_id: CellId,
+    inner: Arc<Inner<D>>,
+}
+
+impl<D: SessionRuntimeDelegate> CellHost for RuntimeCellHost<D> {
+    async fn invoke_tool(
+        &self,
+        invocation: CellToolCall,
+        cancellation_token: CancellationToken,
+    ) -> Result<JsonValue, String> {
+        self.inner
+            .delegate
+            .invoke_tool(
+                NestedToolCall {
+                    cell_id: self.cell_id.clone(),
+                    runtime_tool_call_id: invocation.id,
+                    tool_name: invocation.name,
+                    tool_kind: invocation.kind,
+                    input: invocation.input,
+                },
+                cancellation_token,
+            )
+            .await
+    }
+
+    async fn notify(
+        &self,
+        call_id: String,
+        text: String,
+        cancellation_token: CancellationToken,
+    ) -> Result<(), String> {
+        self.inner
+            .delegate
+            .notify(call_id, self.cell_id.clone(), text, cancellation_token)
+            .await
+    }
+
+    async fn commit_completion(
+        &self,
+        stored_value_writes: HashMap<String, JsonValue>,
+        event: CellEvent,
+        pending_initial_yield_items: Option<Vec<OutputItem>>,
+        cell_state: Arc<CellState>,
+    ) -> CompletionCommit {
+        let cancellation_token = cell_state.cancellation_token();
+        let mut stored_values = tokio::select! {
+            biased;
+            _ = cancellation_token.cancelled() => {
+                return CompletionCommit::Rejected(event);
+            }
+            stored_values = self.inner.stored_values.lock() => stored_values,
+        };
+        cell_state.commit_completion(event, pending_initial_yield_items, || {
+            stored_values.extend(stored_value_writes);
+        })
+    }
+
+    async fn closed(&self) {
+        self.inner.cells.lock().await.remove(&self.cell_id);
+        self.inner.delegate.cell_closed(&self.cell_id);
+    }
+}
+
+fn map_actor_event(cell_id: CellId, event: CellEventFuture) -> RuntimeEventFuture {
+    Box::pin(async move { event.await.map_err(|error| actor_error(&cell_id, error)) })
+}
+
+fn actor_error(cell_id: &CellId, error: CellError) -> Error {
+    match error {
+        CellError::Busy => Error::BusyObserver(cell_id.clone()),
+        CellError::AlreadyTerminating => Error::AlreadyTerminating(cell_id.clone()),
+        CellError::Closed => Error::ClosedCell(cell_id.clone()),
+    }
+}
+
+#[cfg(test)]
+#[path = "tests.rs"]
+mod tests;

codex-rs/code-mode-runtime/src/session_runtime/tests.rs

@@ -0,0 +1,265 @@
+use std::collections::HashMap;
+use std::future::Future;
+use std::sync::Arc;
+use std::task::Context;
+use std::task::Poll;
+use std::task::Waker;
+use std::time::Duration;
+
+use pretty_assertions::assert_eq;
+use serde_json::Value as JsonValue;
+use tokio_util::sync::CancellationToken;
+
+use super::*;
+use crate::cell_actor::CompletionCommit;
+
+struct RecordingDelegate;
+
+struct PanickingClosedDelegate;
+
+impl SessionRuntimeDelegate for RecordingDelegate {
+    async fn invoke_tool(
+        &self,
+        _invocation: NestedToolCall,
+        _cancellation_token: CancellationToken,
+    ) -> Result<JsonValue, String> {
+        Ok(JsonValue::Null)
+    }
+
+    async fn notify(
+        &self,
+        _call_id: String,
+        _cell_id: CellId,
+        _text: String,
+        _cancellation_token: CancellationToken,
+    ) -> Result<(), String> {
+        Ok(())
+    }
+
+    fn cell_closed(&self, _cell_id: &CellId) {}
+}
+
+impl SessionRuntimeDelegate for PanickingClosedDelegate {
+    async fn invoke_tool(
+        &self,
+        _invocation: NestedToolCall,
+        _cancellation_token: CancellationToken,
+    ) -> Result<JsonValue, String> {
+        Ok(JsonValue::Null)
+    }
+
+    async fn notify(
+        &self,
+        _call_id: String,
+        _cell_id: CellId,
+        _text: String,
+        _cancellation_token: CancellationToken,
+    ) -> Result<(), String> {
+        Ok(())
+    }
+
+    fn cell_closed(&self, _cell_id: &CellId) {
+        panic!("cell close panic probe");
+    }
+}
+
+#[tokio::test]
+async fn reports_cell_actor_panics_to_the_owner() {
+    let (failure_tx, mut failure_rx) = tokio::sync::mpsc::unbounded_channel();
+    let runtime = SessionRuntime::new_with_task_failure_handler(
+        Arc::new(PanickingClosedDelegate),
+        Some(Arc::new(move |reason| {
+            let _ = failure_tx.send(reason);
+        })),
+    );
+    let started = runtime
+        .execute(
+            execute_request(r#"text("done");"#),
+            ObserveMode::YieldAfter(Duration::from_secs(1)),
+        )
+        .await
+        .expect("start cell");
+    assert_eq!(
+        started.initial_event().await,
+        Ok(CellEvent::Completed {
+            content_items: vec![OutputItem::Text {
+                text: "done".to_string(),
+            }],
+            error_text: None,
+        })
+    );
+    runtime.shutdown().await.expect("shutdown runtime");
+    let failure = failure_rx
+        .try_recv()
+        .expect("shutdown should wait for the cell failure watcher");
+    assert!(failure.contains("code-mode cell 1 task failed"));
+}
+
+#[tokio::test]
+async fn termination_rejects_a_waiting_store_commit_before_the_next_cell_can_load_it() {
+    let runtime = SessionRuntime::new(Arc::new(RecordingDelegate));
+    let cell_state = Arc::new(CellState::new(CancellationToken::new()));
+    let host = RuntimeCellHost {
+        cell_id: CellId::new("terminating-writer"),
+        inner: Arc::clone(&runtime.inner),
+    };
+    let completion = CellEvent::Completed {
+        content_items: vec![OutputItem::Text {
+            text: "uncommitted output".to_string(),
+        }],
+        error_text: None,
+    };
+
+    let stored_values = runtime.inner.stored_values.lock().await;
+    let commit = host.commit_completion(
+        HashMap::from([(
+            "candidate".to_string(),
+            JsonValue::String("lost".to_string()),
+        )]),
+        completion.clone(),
+        /*pending_initial_yield_items*/ None,
+        Arc::clone(&cell_state),
+    );
+    tokio::pin!(commit);
+    let waker = Waker::noop();
+    let mut context = Context::from_waker(waker);
+    assert!(matches!(commit.as_mut().poll(&mut context), Poll::Pending));
+
+    let termination = cell_state.request_termination();
+    drop(stored_values);
+    assert_eq!(commit.await, CompletionCommit::Rejected(completion));
+    let terminated = CellEvent::Terminated {
+        content_items: Vec::new(),
+    };
+    assert_eq!(
+        cell_state.finish_termination(terminated.clone()),
+        Some(terminated.clone())
+    );
+    assert_eq!(termination.await, Ok(terminated));
+    assert!(
+        !runtime
+            .inner
+            .stored_values
+            .lock()
+            .await
+            .contains_key("candidate")
+    );
+
+    let reader = runtime
+        .execute(
+            CreateCellRequest {
+                tool_call_id: "reader".to_string(),
+                enabled_tools: Vec::new(),
+                source: r#"text(String(load("candidate")));"#.to_string(),
+            },
+            ObserveMode::YieldAfter(Duration::from_secs(1)),
+        )
+        .await
+        .unwrap();
+    assert_eq!(
+        reader.initial_event().await,
+        Ok(CellEvent::Completed {
+            content_items: vec![OutputItem::Text {
+                text: "undefined".to_string(),
+            }],
+            error_text: None,
+        })
+    );
+    runtime.shutdown().await.unwrap();
+}
+
+fn execute_request(source: &str) -> CreateCellRequest {
+    CreateCellRequest {
+        tool_call_id: "call-1".to_string(),
+        enabled_tools: Vec::new(),
+        source: source.to_string(),
+    }
+}
+
+#[tokio::test]
+async fn cell_id_allocation_fails_before_wrapping() {
+    let runtime = SessionRuntime::new(Arc::new(RecordingDelegate));
+    runtime
+        .inner
+        .next_cell_id
+        .store(u64::MAX, Ordering::Relaxed);
+
+    assert_eq!(
+        runtime
+            .execute(
+                execute_request(r#"text("unreachable");"#),
+                ObserveMode::YieldAfter(Duration::from_secs(1)),
+            )
+            .await
+            .err(),
+        Some(Error::CellIdSpaceExhausted)
+    );
+}
+
+#[tokio::test]
+#[expect(
+    clippy::await_holding_invalid_type,
+    reason = "test holds the registry lock to force admission ahead of shutdown"
+)]
+async fn shutdown_rejects_cell_admission_queued_before_the_registry_lock() {
+    let runtime = Arc::new(SessionRuntime::new(Arc::new(RecordingDelegate)));
+    let cells = runtime.inner.cells.lock().await;
+
+    let execution = runtime.execute(
+        execute_request("while (true) {}"),
+        ObserveMode::YieldAfter(Duration::from_millis(/*millis*/ 1)),
+    );
+    tokio::pin!(execution);
+    std::future::poll_fn(|context| match execution.as_mut().poll(context) {
+        Poll::Pending => Poll::Ready(()),
+        Poll::Ready(Ok(_)) => panic!("execution completed before the registry lock was released"),
+        Poll::Ready(Err(error)) => {
+            panic!("execution failed before the registry lock was released: {error}")
+        }
+    })
+    .await;
+
+    let shutdown = runtime.shutdown();
+    tokio::pin!(shutdown);
+    std::future::poll_fn(|context| match shutdown.as_mut().poll(context) {
+        Poll::Pending => Poll::Ready(()),
+        Poll::Ready(Ok(())) => panic!("shutdown completed before acquiring the registry lock"),
+        Poll::Ready(Err(error)) => {
+            panic!("shutdown failed before acquiring the registry lock: {error}")
+        }
+    })
+    .await;
+
+    drop(cells);
+    assert!(matches!(execution.await, Err(Error::ShuttingDown)));
+    assert_eq!(shutdown.await, Ok(()));
+}
+
+#[tokio::test]
+async fn drop_terminates_cells_when_the_registry_is_locked() {
+    let runtime = SessionRuntime::new(Arc::new(RecordingDelegate));
+    let started = runtime
+        .execute(
+            execute_request("while (true) {}"),
+            ObserveMode::YieldAfter(Duration::from_millis(/*millis*/ 1)),
+        )
+        .await
+        .unwrap();
+    assert_eq!(started.cell_id, CellId::new("1"));
+    assert_eq!(
+        started.initial_event().await,
+        Ok(CellEvent::Yielded {
+            content_items: Vec::new(),
+        })
+    );
+
+    let inner = Arc::clone(&runtime.inner);
+    let cells = inner.cells.lock().await;
+    drop(runtime);
+    drop(cells);
+
+    tokio::time::timeout(Duration::from_secs(/*secs*/ 1), inner.cell_tasks.wait())
+        .await
+        .unwrap();
+    assert!(inner.cell_tasks.is_empty());
+}

codex-rs/code-mode-runtime/src/session_runtime/types.rs

@@ -0,0 +1,179 @@
+use std::fmt;
+use std::future::Future;
+use std::time::Duration;
+
+use serde_json::Value as JsonValue;
+use tokio_util::sync::CancellationToken;
+
+/// Identifies one execution cell within a session runtime.
+#[derive(Clone, Debug, Eq, Hash, PartialEq)]
+pub(crate) struct CellId(String);
+
+impl CellId {
+    pub(crate) fn new(value: impl Into<String>) -> Self {
+        Self(value.into())
+    }
+
+    pub(crate) fn as_str(&self) -> &str {
+        &self.0
+    }
+}
+
+impl fmt::Display for CellId {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        formatter.write_str(self.as_str())
+    }
+}
+
+/// Selects the next observable frontier for a running cell.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub(crate) enum ObserveMode {
+    YieldAfter(Duration),
+    PendingFrontier,
+}
+
+/// An observable cell lifecycle event.
+#[derive(Clone, Debug, PartialEq)]
+pub(crate) enum CellEvent {
+    Yielded {
+        content_items: Vec<OutputItem>,
+    },
+    Pending {
+        content_items: Vec<OutputItem>,
+        pending_tool_call_ids: Vec<String>,
+    },
+    Completed {
+        content_items: Vec<OutputItem>,
+        error_text: Option<String>,
+    },
+    Terminated {
+        content_items: Vec<OutputItem>,
+    },
+}
+
+/// Output emitted by a cell since its preceding observation.
+#[derive(Clone, Debug, PartialEq)]
+pub(crate) enum OutputItem {
+    Text {
+        text: String,
+    },
+    Image {
+        image_url: String,
+        detail: Option<ImageDetail>,
+    },
+    Audio {
+        audio_url: String,
+    },
+}
+
+/// Requested image fidelity for an output image.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub(crate) enum ImageDetail {
+    Auto,
+    Low,
+    High,
+    Original,
+}
+
+/// Transport-neutral input for creating a cell.
+///
+/// The owning session assigns the cell ID when it admits the request.
+pub(crate) struct CreateCellRequest {
+    pub(crate) tool_call_id: String,
+    pub(crate) enabled_tools: Vec<ToolDefinition>,
+    pub(crate) source: String,
+}
+
+/// Tool metadata exposed to code running inside a cell.
+pub(crate) struct ToolDefinition {
+    pub(crate) name: String,
+    pub(crate) tool_name: ToolName,
+    pub(crate) description: String,
+    pub(crate) kind: ToolKind,
+}
+
+/// A tool name with an optional namespace.
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub(crate) struct ToolName {
+    pub(crate) name: String,
+    pub(crate) namespace: Option<String>,
+}
+
+/// The JavaScript calling convention for a tool.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub(crate) enum ToolKind {
+    Function,
+    Freeform,
+}
+
+/// A nested tool request emitted by a running cell.
+pub(crate) struct NestedToolCall {
+    pub(crate) cell_id: CellId,
+    pub(crate) runtime_tool_call_id: String,
+    pub(crate) tool_name: ToolName,
+    pub(crate) tool_kind: ToolKind,
+    pub(crate) input: Option<JsonValue>,
+}
+
+/// Host callbacks used by cells owned by a [`super::SessionRuntime`].
+///
+/// Implementations must honor cancellation tokens. `cell_closed` is called
+/// after the runtime has stopped routing requests to the cell.
+pub(crate) trait SessionRuntimeDelegate: Send + Sync + 'static {
+    fn invoke_tool(
+        &self,
+        invocation: NestedToolCall,
+        cancellation_token: CancellationToken,
+    ) -> impl Future<Output = Result<JsonValue, String>> + Send;
+
+    fn notify(
+        &self,
+        call_id: String,
+        cell_id: CellId,
+        text: String,
+        cancellation_token: CancellationToken,
+    ) -> impl Future<Output = Result<(), String>> + Send;
+
+    fn cell_closed(&self, cell_id: &CellId);
+}
+
+/// A failure reported by a session runtime operation.
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub(crate) enum Error {
+    ShuttingDown,
+    CellIdSpaceExhausted,
+    DuplicateCell(CellId),
+    MissingCell(CellId),
+    BusyObserver(CellId),
+    AlreadyTerminating(CellId),
+    ClosedCell(CellId),
+    Runtime(String),
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::ShuttingDown => formatter.write_str("code mode session is shutting down"),
+            Self::CellIdSpaceExhausted => {
+                formatter.write_str("code mode session exhausted its cell ID space")
+            }
+            Self::DuplicateCell(cell_id) => write!(formatter, "exec cell {cell_id} already exists"),
+            Self::MissingCell(cell_id) => write!(formatter, "exec cell {cell_id} not found"),
+            Self::BusyObserver(cell_id) => {
+                write!(
+                    formatter,
+                    "exec cell {cell_id} already has an active observer"
+                )
+            }
+            Self::AlreadyTerminating(cell_id) => {
+                write!(formatter, "exec cell {cell_id} is already terminating")
+            }
+            Self::ClosedCell(cell_id) => {
+                write!(formatter, "exec cell {cell_id} closed unexpectedly")
+            }
+            Self::Runtime(error_text) => formatter.write_str(error_text),
+        }
+    }
+}
+
+impl std::error::Error for Error {}