Add in-process app server and wire up exec to use it (#14005)

This is a subset of PR #13636. See that PR for a full overview of the
architectural change.

This PR implements the in-process app server and modifies the
non-interactive "exec" entry point to use the app server.

---------

Co-authored-by: Felipe Coury <felipe.coury@gmail.com>

codex-rs/app-server-client/src/lib.rs

@@ -0,0 +1,801 @@
+//! Shared in-process app-server client facade for CLI surfaces.
+//!
+//! This crate wraps [`codex_app_server::in_process`] behind a single async API
+//! used by surfaces like TUI and exec. It centralizes:
+//!
+//! - Runtime startup and initialize-capabilities handshake.
+//! - Typed caller-provided startup identity (`SessionSource` + client name).
+//! - Typed and raw request/notification dispatch.
+//! - Server request resolution and rejection.
+//! - Event consumption with backpressure signaling ([`InProcessServerEvent::Lagged`]).
+//! - Bounded graceful shutdown with abort fallback.
+//!
+//! The facade interposes a worker task between the caller and the underlying
+//! [`InProcessClientHandle`](codex_app_server::in_process::InProcessClientHandle),
+//! bridging async `mpsc` channels on both sides. Queues are bounded so overload
+//! surfaces as channel-full errors rather than unbounded memory growth.
+
+use std::error::Error;
+use std::fmt;
+use std::io::Error as IoError;
+use std::io::ErrorKind;
+use std::io::Result as IoResult;
+use std::sync::Arc;
+use std::time::Duration;
+
+pub use codex_app_server::in_process::DEFAULT_IN_PROCESS_CHANNEL_CAPACITY;
+pub use codex_app_server::in_process::InProcessServerEvent;
+use codex_app_server::in_process::InProcessStartArgs;
+use codex_app_server_protocol::ClientInfo;
+use codex_app_server_protocol::ClientNotification;
+use codex_app_server_protocol::ClientRequest;
+use codex_app_server_protocol::ConfigWarningNotification;
+use codex_app_server_protocol::InitializeCapabilities;
+use codex_app_server_protocol::InitializeParams;
+use codex_app_server_protocol::JSONRPCErrorError;
+use codex_app_server_protocol::RequestId;
+use codex_app_server_protocol::Result as JsonRpcResult;
+use codex_arg0::Arg0DispatchPaths;
+use codex_core::config::Config;
+use codex_core::config_loader::CloudRequirementsLoader;
+use codex_core::config_loader::LoaderOverrides;
+use codex_feedback::CodexFeedback;
+use codex_protocol::protocol::SessionSource;
+use serde::de::DeserializeOwned;
+use tokio::sync::mpsc;
+use tokio::sync::oneshot;
+use tokio::time::timeout;
+use toml::Value as TomlValue;
+use tracing::warn;
+
+const SHUTDOWN_TIMEOUT: Duration = Duration::from_secs(5);
+
+/// Raw app-server request result for typed in-process requests.
+///
+/// Even on the in-process path, successful responses still travel back through
+/// the same JSON-RPC result envelope used by socket/stdio transports because
+/// `MessageProcessor` continues to produce that shape internally.
+pub type RequestResult = std::result::Result<JsonRpcResult, JSONRPCErrorError>;
+
+fn event_requires_delivery(event: &InProcessServerEvent) -> bool {
+    // These terminal events drive surface shutdown/completion state. Dropping
+    // them under backpressure can leave exec/TUI waiting forever even though
+    // the underlying turn has already ended.
+    match event {
+        InProcessServerEvent::ServerNotification(
+            codex_app_server_protocol::ServerNotification::TurnCompleted(_),
+        ) => true,
+        InProcessServerEvent::LegacyNotification(notification) => matches!(
+            notification
+                .method
+                .strip_prefix("codex/event/")
+                .unwrap_or(&notification.method),
+            "task_complete" | "turn_aborted" | "shutdown_complete"
+        ),
+        _ => false,
+    }
+}
+
+/// Layered error for [`InProcessAppServerClient::request_typed`].
+///
+/// This keeps transport failures, server-side JSON-RPC failures, and response
+/// decode failures distinct so callers can decide whether to retry, surface a
+/// server error, or treat the response as an internal request/response mismatch.
+#[derive(Debug)]
+pub enum TypedRequestError {
+    Transport {
+        method: String,
+        source: IoError,
+    },
+    Server {
+        method: String,
+        source: JSONRPCErrorError,
+    },
+    Deserialize {
+        method: String,
+        source: serde_json::Error,
+    },
+}
+
+impl fmt::Display for TypedRequestError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::Transport { method, source } => {
+                write!(f, "{method} transport error: {source}")
+            }
+            Self::Server { method, source } => {
+                write!(f, "{method} failed: {}", source.message)
+            }
+            Self::Deserialize { method, source } => {
+                write!(f, "{method} response decode error: {source}")
+            }
+        }
+    }
+}
+
+impl Error for TypedRequestError {
+    fn source(&self) -> Option<&(dyn Error + 'static)> {
+        match self {
+            Self::Transport { source, .. } => Some(source),
+            Self::Server { .. } => None,
+            Self::Deserialize { source, .. } => Some(source),
+        }
+    }
+}
+
+#[derive(Clone)]
+pub struct InProcessClientStartArgs {
+    /// Resolved argv0 dispatch paths used by command execution internals.
+    pub arg0_paths: Arg0DispatchPaths,
+    /// Shared config used to initialize app-server runtime.
+    pub config: Arc<Config>,
+    /// CLI config overrides that are already parsed into TOML values.
+    pub cli_overrides: Vec<(String, TomlValue)>,
+    /// Loader override knobs used by config API paths.
+    pub loader_overrides: LoaderOverrides,
+    /// Preloaded cloud requirements provider.
+    pub cloud_requirements: CloudRequirementsLoader,
+    /// Feedback sink used by app-server/core telemetry and logs.
+    pub feedback: CodexFeedback,
+    /// Startup warnings emitted after initialize succeeds.
+    pub config_warnings: Vec<ConfigWarningNotification>,
+    /// Session source recorded in app-server thread metadata.
+    pub session_source: SessionSource,
+    /// Whether auth loading should honor the `CODEX_API_KEY` environment variable.
+    pub enable_codex_api_key_env: bool,
+    /// Client name reported during initialize.
+    pub client_name: String,
+    /// Client version reported during initialize.
+    pub client_version: String,
+    /// Whether experimental APIs are requested at initialize time.
+    pub experimental_api: bool,
+    /// Notification methods this client opts out of receiving.
+    pub opt_out_notification_methods: Vec<String>,
+    /// Queue capacity for command/event channels (clamped to at least 1).
+    pub channel_capacity: usize,
+}
+
+impl InProcessClientStartArgs {
+    /// Builds initialize params from caller-provided metadata.
+    pub fn initialize_params(&self) -> InitializeParams {
+        let capabilities = InitializeCapabilities {
+            experimental_api: self.experimental_api,
+            opt_out_notification_methods: if self.opt_out_notification_methods.is_empty() {
+                None
+            } else {
+                Some(self.opt_out_notification_methods.clone())
+            },
+        };
+
+        InitializeParams {
+            client_info: ClientInfo {
+                name: self.client_name.clone(),
+                title: None,
+                version: self.client_version.clone(),
+            },
+            capabilities: Some(capabilities),
+        }
+    }
+
+    fn into_runtime_start_args(self) -> InProcessStartArgs {
+        let initialize = self.initialize_params();
+        InProcessStartArgs {
+            arg0_paths: self.arg0_paths,
+            config: self.config,
+            cli_overrides: self.cli_overrides,
+            loader_overrides: self.loader_overrides,
+            cloud_requirements: self.cloud_requirements,
+            feedback: self.feedback,
+            config_warnings: self.config_warnings,
+            session_source: self.session_source,
+            enable_codex_api_key_env: self.enable_codex_api_key_env,
+            initialize,
+            channel_capacity: self.channel_capacity,
+        }
+    }
+}
+
+/// Internal command sent from public facade methods to the worker task.
+///
+/// Each variant carries a oneshot sender so the caller can `await` the
+/// result without holding a mutable reference to the client.
+enum ClientCommand {
+    Request {
+        request: Box<ClientRequest>,
+        response_tx: oneshot::Sender<IoResult<RequestResult>>,
+    },
+    Notify {
+        notification: ClientNotification,
+        response_tx: oneshot::Sender<IoResult<()>>,
+    },
+    ResolveServerRequest {
+        request_id: RequestId,
+        result: JsonRpcResult,
+        response_tx: oneshot::Sender<IoResult<()>>,
+    },
+    RejectServerRequest {
+        request_id: RequestId,
+        error: JSONRPCErrorError,
+        response_tx: oneshot::Sender<IoResult<()>>,
+    },
+    Shutdown {
+        response_tx: oneshot::Sender<IoResult<()>>,
+    },
+}
+
+/// Async facade over the in-process app-server runtime.
+///
+/// This type owns a worker task that bridges between:
+/// - caller-facing async `mpsc` channels used by TUI/exec
+/// - [`codex_app_server::in_process::InProcessClientHandle`], which speaks to
+///   the embedded `MessageProcessor`
+///
+/// The facade intentionally preserves the server's request/notification/event
+/// model instead of exposing direct core runtime handles. That keeps in-process
+/// callers aligned with app-server behavior while still avoiding a process
+/// boundary.
+pub struct InProcessAppServerClient {
+    command_tx: mpsc::Sender<ClientCommand>,
+    event_rx: mpsc::Receiver<InProcessServerEvent>,
+    worker_handle: tokio::task::JoinHandle<()>,
+}
+
+impl InProcessAppServerClient {
+    /// Starts the in-process runtime and facade worker task.
+    ///
+    /// The returned client is ready for requests and event consumption. If the
+    /// internal event queue is saturated later, server requests are rejected
+    /// with overload error instead of being silently dropped.
+    pub async fn start(args: InProcessClientStartArgs) -> IoResult<Self> {
+        let channel_capacity = args.channel_capacity.max(1);
+        let mut handle =
+            codex_app_server::in_process::start(args.into_runtime_start_args()).await?;
+        let request_sender = handle.sender();
+        let (command_tx, mut command_rx) = mpsc::channel::<ClientCommand>(channel_capacity);
+        let (event_tx, event_rx) = mpsc::channel::<InProcessServerEvent>(channel_capacity);
+
+        let worker_handle = tokio::spawn(async move {
+            let mut event_stream_enabled = true;
+            let mut skipped_events = 0usize;
+            loop {
+                tokio::select! {
+                    command = command_rx.recv() => {
+                        match command {
+                            Some(ClientCommand::Request { request, response_tx }) => {
+                                let request_sender = request_sender.clone();
+                                // Request waits happen on a detached task so
+                                // this loop can keep draining runtime events
+                                // while the request is blocked on client input.
+                                tokio::spawn(async move {
+                                    let result = request_sender.request(*request).await;
+                                    let _ = response_tx.send(result);
+                                });
+                            }
+                            Some(ClientCommand::Notify {
+                                notification,
+                                response_tx,
+                            }) => {
+                                let result = request_sender.notify(notification);
+                                let _ = response_tx.send(result);
+                            }
+                            Some(ClientCommand::ResolveServerRequest {
+                                request_id,
+                                result,
+                                response_tx,
+                            }) => {
+                                let send_result =
+                                    request_sender.respond_to_server_request(request_id, result);
+                                let _ = response_tx.send(send_result);
+                            }
+                            Some(ClientCommand::RejectServerRequest {
+                                request_id,
+                                error,
+                                response_tx,
+                            }) => {
+                                let send_result = request_sender.fail_server_request(request_id, error);
+                                let _ = response_tx.send(send_result);
+                            }
+                            Some(ClientCommand::Shutdown { response_tx }) => {
+                                let shutdown_result = handle.shutdown().await;
+                                let _ = response_tx.send(shutdown_result);
+                                break;
+                            }
+                            None => {
+                                let _ = handle.shutdown().await;
+                                break;
+                            }
+                        }
+                    }
+                    event = handle.next_event(), if event_stream_enabled => {
+                        let Some(event) = event else {
+                            break;
+                        };
+
+                        if skipped_events > 0 {
+                            if event_requires_delivery(&event) {
+                                // Surface lag before the terminal event, but
+                                // do not let the lag marker itself cause us to
+                                // drop the completion/abort notification that
+                                // the caller is blocked on.
+                                if event_tx
+                                    .send(InProcessServerEvent::Lagged {
+                                        skipped: skipped_events,
+                                    })
+                                    .await
+                                    .is_err()
+                                {
+                                    event_stream_enabled = false;
+                                    continue;
+                                }
+                                skipped_events = 0;
+                            } else {
+                                match event_tx.try_send(InProcessServerEvent::Lagged {
+                                    skipped: skipped_events,
+                                }) {
+                                    Ok(()) => {
+                                        skipped_events = 0;
+                                    }
+                                    Err(mpsc::error::TrySendError::Full(_)) => {
+                                        skipped_events = skipped_events.saturating_add(1);
+                                        warn!(
+                                            "dropping in-process app-server event because consumer queue is full"
+                                        );
+                                        if let InProcessServerEvent::ServerRequest(request) = event {
+                                            let _ = request_sender.fail_server_request(
+                                                request.id().clone(),
+                                                JSONRPCErrorError {
+                                                    code: -32001,
+                                                    message: "in-process app-server event queue is full".to_string(),
+                                                    data: None,
+                                                },
+                                            );
+                                        }
+                                        continue;
+                                    }
+                                    Err(mpsc::error::TrySendError::Closed(_)) => {
+                                        event_stream_enabled = false;
+                                        continue;
+                                    }
+                                }
+                            }
+                        }
+
+                        if event_requires_delivery(&event) {
+                            // Block until the consumer catches up for
+                            // terminal notifications; this preserves the
+                            // completion signal even when the queue is
+                            // otherwise saturated.
+                            if event_tx.send(event).await.is_err() {
+                                event_stream_enabled = false;
+                            }
+                            continue;
+                        }
+
+                        match event_tx.try_send(event) {
+                            Ok(()) => {}
+                            Err(mpsc::error::TrySendError::Full(event)) => {
+                                skipped_events = skipped_events.saturating_add(1);
+                                warn!("dropping in-process app-server event because consumer queue is full");
+                                if let InProcessServerEvent::ServerRequest(request) = event {
+                                    let _ = request_sender.fail_server_request(
+                                        request.id().clone(),
+                                        JSONRPCErrorError {
+                                            code: -32001,
+                                            message: "in-process app-server event queue is full".to_string(),
+                                            data: None,
+                                        },
+                                    );
+                                }
+                            }
+                            Err(mpsc::error::TrySendError::Closed(_)) => {
+                                event_stream_enabled = false;
+                            }
+                        }
+                    }
+                }
+            }
+        });
+
+        Ok(Self {
+            command_tx,
+            event_rx,
+            worker_handle,
+        })
+    }
+
+    /// Sends a typed client request and returns raw JSON-RPC result.
+    ///
+    /// Callers that expect a concrete response type should usually prefer
+    /// [`request_typed`](Self::request_typed).
+    pub async fn request(&self, request: ClientRequest) -> IoResult<RequestResult> {
+        let (response_tx, response_rx) = oneshot::channel();
+        self.command_tx
+            .send(ClientCommand::Request {
+                request: Box::new(request),
+                response_tx,
+            })
+            .await
+            .map_err(|_| {
+                IoError::new(
+                    ErrorKind::BrokenPipe,
+                    "in-process app-server worker channel is closed",
+                )
+            })?;
+        response_rx.await.map_err(|_| {
+            IoError::new(
+                ErrorKind::BrokenPipe,
+                "in-process app-server request channel is closed",
+            )
+        })?
+    }
+
+    /// Sends a typed client request and decodes the successful response body.
+    ///
+    /// This still deserializes from a JSON value produced by app-server's
+    /// JSON-RPC result envelope. Because the caller chooses `T`, `Deserialize`
+    /// failures indicate an internal request/response mismatch at the call site
+    /// (or an in-process bug), not transport skew from an external client.
+    pub async fn request_typed<T>(&self, request: ClientRequest) -> Result<T, TypedRequestError>
+    where
+        T: DeserializeOwned,
+    {
+        let method = request_method_name(&request);
+        let response =
+            self.request(request)
+                .await
+                .map_err(|source| TypedRequestError::Transport {
+                    method: method.clone(),
+                    source,
+                })?;
+        let result = response.map_err(|source| TypedRequestError::Server {
+            method: method.clone(),
+            source,
+        })?;
+        serde_json::from_value(result)
+            .map_err(|source| TypedRequestError::Deserialize { method, source })
+    }
+
+    /// Sends a typed client notification.
+    pub async fn notify(&self, notification: ClientNotification) -> IoResult<()> {
+        let (response_tx, response_rx) = oneshot::channel();
+        self.command_tx
+            .send(ClientCommand::Notify {
+                notification,
+                response_tx,
+            })
+            .await
+            .map_err(|_| {
+                IoError::new(
+                    ErrorKind::BrokenPipe,
+                    "in-process app-server worker channel is closed",
+                )
+            })?;
+        response_rx.await.map_err(|_| {
+            IoError::new(
+                ErrorKind::BrokenPipe,
+                "in-process app-server notify channel is closed",
+            )
+        })?
+    }
+
+    /// Resolves a pending server request.
+    ///
+    /// This should only be called with request IDs obtained from the current
+    /// client's event stream.
+    pub async fn resolve_server_request(
+        &self,
+        request_id: RequestId,
+        result: JsonRpcResult,
+    ) -> IoResult<()> {
+        let (response_tx, response_rx) = oneshot::channel();
+        self.command_tx
+            .send(ClientCommand::ResolveServerRequest {
+                request_id,
+                result,
+                response_tx,
+            })
+            .await
+            .map_err(|_| {
+                IoError::new(
+                    ErrorKind::BrokenPipe,
+                    "in-process app-server worker channel is closed",
+                )
+            })?;
+        response_rx.await.map_err(|_| {
+            IoError::new(
+                ErrorKind::BrokenPipe,
+                "in-process app-server resolve channel is closed",
+            )
+        })?
+    }
+
+    /// Rejects a pending server request with JSON-RPC error payload.
+    pub async fn reject_server_request(
+        &self,
+        request_id: RequestId,
+        error: JSONRPCErrorError,
+    ) -> IoResult<()> {
+        let (response_tx, response_rx) = oneshot::channel();
+        self.command_tx
+            .send(ClientCommand::RejectServerRequest {
+                request_id,
+                error,
+                response_tx,
+            })
+            .await
+            .map_err(|_| {
+                IoError::new(
+                    ErrorKind::BrokenPipe,
+                    "in-process app-server worker channel is closed",
+                )
+            })?;
+        response_rx.await.map_err(|_| {
+            IoError::new(
+                ErrorKind::BrokenPipe,
+                "in-process app-server reject channel is closed",
+            )
+        })?
+    }
+
+    /// Returns the next in-process event, or `None` when worker exits.
+    ///
+    /// Callers are expected to drain this stream promptly. If they fall behind,
+    /// the worker emits [`InProcessServerEvent::Lagged`] markers and may reject
+    /// pending server requests rather than letting approval flows hang.
+    pub async fn next_event(&mut self) -> Option<InProcessServerEvent> {
+        self.event_rx.recv().await
+    }
+
+    /// Shuts down worker and in-process runtime with bounded wait.
+    ///
+    /// If graceful shutdown exceeds timeout, the worker task is aborted to
+    /// avoid leaking background tasks in embedding callers.
+    pub async fn shutdown(self) -> IoResult<()> {
+        let Self {
+            command_tx,
+            event_rx,
+            worker_handle,
+        } = self;
+        let mut worker_handle = worker_handle;
+        // Drop the caller-facing receiver before asking the worker to shut
+        // down. That unblocks any pending must-deliver `event_tx.send(..)`
+        // so the worker can reach `handle.shutdown()` instead of timing out
+        // and getting aborted with the runtime still attached.
+        drop(event_rx);
+        let (response_tx, response_rx) = oneshot::channel();
+        if command_tx
+            .send(ClientCommand::Shutdown { response_tx })
+            .await
+            .is_ok()
+            && let Ok(command_result) = timeout(SHUTDOWN_TIMEOUT, response_rx).await
+        {
+            command_result.map_err(|_| {
+                IoError::new(
+                    ErrorKind::BrokenPipe,
+                    "in-process app-server shutdown channel is closed",
+                )
+            })??;
+        }
+
+        if let Err(_elapsed) = timeout(SHUTDOWN_TIMEOUT, &mut worker_handle).await {
+            worker_handle.abort();
+            let _ = worker_handle.await;
+        }
+        Ok(())
+    }
+}
+
+/// Extracts the JSON-RPC method name for diagnostics without extending the
+/// protocol crate with in-process-only helpers.
+fn request_method_name(request: &ClientRequest) -> String {
+    serde_json::to_value(request)
+        .ok()
+        .and_then(|value| {
+            value
+                .get("method")
+                .and_then(serde_json::Value::as_str)
+                .map(ToOwned::to_owned)
+        })
+        .unwrap_or_else(|| "<unknown>".to_string())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use codex_app_server_protocol::ConfigRequirementsReadResponse;
+    use codex_app_server_protocol::SessionSource as ApiSessionSource;
+    use codex_app_server_protocol::ThreadStartParams;
+    use codex_app_server_protocol::ThreadStartResponse;
+    use codex_core::config::ConfigBuilder;
+    use pretty_assertions::assert_eq;
+    use tokio::time::Duration;
+    use tokio::time::timeout;
+
+    async fn build_test_config() -> Config {
+        match ConfigBuilder::default().build().await {
+            Ok(config) => config,
+            Err(_) => Config::load_default_with_cli_overrides(Vec::new())
+                .expect("default config should load"),
+        }
+    }
+
+    async fn start_test_client_with_capacity(
+        session_source: SessionSource,
+        channel_capacity: usize,
+    ) -> InProcessAppServerClient {
+        InProcessAppServerClient::start(InProcessClientStartArgs {
+            arg0_paths: Arg0DispatchPaths::default(),
+            config: Arc::new(build_test_config().await),
+            cli_overrides: Vec::new(),
+            loader_overrides: LoaderOverrides::default(),
+            cloud_requirements: CloudRequirementsLoader::default(),
+            feedback: CodexFeedback::new(),
+            config_warnings: Vec::new(),
+            session_source,
+            enable_codex_api_key_env: false,
+            client_name: "codex-app-server-client-test".to_string(),
+            client_version: "0.0.0-test".to_string(),
+            experimental_api: true,
+            opt_out_notification_methods: Vec::new(),
+            channel_capacity,
+        })
+        .await
+        .expect("in-process app-server client should start")
+    }
+
+    async fn start_test_client(session_source: SessionSource) -> InProcessAppServerClient {
+        start_test_client_with_capacity(session_source, DEFAULT_IN_PROCESS_CHANNEL_CAPACITY).await
+    }
+
+    #[tokio::test]
+    async fn typed_request_roundtrip_works() {
+        let client = start_test_client(SessionSource::Exec).await;
+        let _response: ConfigRequirementsReadResponse = client
+            .request_typed(ClientRequest::ConfigRequirementsRead {
+                request_id: RequestId::Integer(1),
+                params: None,
+            })
+            .await
+            .expect("typed request should succeed");
+        client.shutdown().await.expect("shutdown should complete");
+    }
+
+    #[tokio::test]
+    async fn typed_request_reports_json_rpc_errors() {
+        let client = start_test_client(SessionSource::Exec).await;
+        let err = client
+            .request_typed::<ConfigRequirementsReadResponse>(ClientRequest::ThreadRead {
+                request_id: RequestId::Integer(99),
+                params: codex_app_server_protocol::ThreadReadParams {
+                    thread_id: "missing-thread".to_string(),
+                    include_turns: false,
+                },
+            })
+            .await
+            .expect_err("missing thread should return a JSON-RPC error");
+        assert!(
+            err.to_string().starts_with("thread/read failed:"),
+            "expected method-qualified JSON-RPC failure message"
+        );
+        client.shutdown().await.expect("shutdown should complete");
+    }
+
+    #[tokio::test]
+    async fn caller_provided_session_source_is_applied() {
+        for (session_source, expected_source) in [
+            (SessionSource::Exec, ApiSessionSource::Exec),
+            (SessionSource::Cli, ApiSessionSource::Cli),
+        ] {
+            let client = start_test_client(session_source).await;
+            let parsed: ThreadStartResponse = client
+                .request_typed(ClientRequest::ThreadStart {
+                    request_id: RequestId::Integer(2),
+                    params: ThreadStartParams {
+                        ephemeral: Some(true),
+                        ..ThreadStartParams::default()
+                    },
+                })
+                .await
+                .expect("thread/start should succeed");
+            assert_eq!(parsed.thread.source, expected_source);
+            client.shutdown().await.expect("shutdown should complete");
+        }
+    }
+
+    #[tokio::test]
+    async fn tiny_channel_capacity_still_supports_request_roundtrip() {
+        let client = start_test_client_with_capacity(SessionSource::Exec, 1).await;
+        let _response: ConfigRequirementsReadResponse = client
+            .request_typed(ClientRequest::ConfigRequirementsRead {
+                request_id: RequestId::Integer(1),
+                params: None,
+            })
+            .await
+            .expect("typed request should succeed");
+        client.shutdown().await.expect("shutdown should complete");
+    }
+
+    #[test]
+    fn typed_request_error_exposes_sources() {
+        let transport = TypedRequestError::Transport {
+            method: "config/read".to_string(),
+            source: IoError::new(ErrorKind::BrokenPipe, "closed"),
+        };
+        assert_eq!(std::error::Error::source(&transport).is_some(), true);
+
+        let server = TypedRequestError::Server {
+            method: "thread/read".to_string(),
+            source: JSONRPCErrorError {
+                code: -32603,
+                data: None,
+                message: "internal".to_string(),
+            },
+        };
+        assert_eq!(std::error::Error::source(&server).is_some(), false);
+
+        let deserialize = TypedRequestError::Deserialize {
+            method: "thread/start".to_string(),
+            source: serde_json::from_str::<u32>("\"nope\"")
+                .expect_err("invalid integer should return deserialize error"),
+        };
+        assert_eq!(std::error::Error::source(&deserialize).is_some(), true);
+    }
+
+    #[tokio::test]
+    async fn next_event_surfaces_lagged_markers() {
+        let (command_tx, _command_rx) = mpsc::channel(1);
+        let (event_tx, event_rx) = mpsc::channel(1);
+        let worker_handle = tokio::spawn(async {});
+        event_tx
+            .send(InProcessServerEvent::Lagged { skipped: 3 })
+            .await
+            .expect("lagged marker should enqueue");
+        drop(event_tx);
+
+        let mut client = InProcessAppServerClient {
+            command_tx,
+            event_rx,
+            worker_handle,
+        };
+
+        let event = timeout(Duration::from_secs(2), client.next_event())
+            .await
+            .expect("lagged marker should arrive before timeout");
+        assert!(matches!(
+            event,
+            Some(InProcessServerEvent::Lagged { skipped: 3 })
+        ));
+
+        client.shutdown().await.expect("shutdown should complete");
+    }
+
+    #[test]
+    fn event_requires_delivery_marks_terminal_events() {
+        assert!(event_requires_delivery(
+            &InProcessServerEvent::ServerNotification(
+                codex_app_server_protocol::ServerNotification::TurnCompleted(
+                    codex_app_server_protocol::TurnCompletedNotification {
+                        thread_id: "thread".to_string(),
+                        turn: codex_app_server_protocol::Turn {
+                            id: "turn".to_string(),
+                            items: Vec::new(),
+                            status: codex_app_server_protocol::TurnStatus::Completed,
+                            error: None,
+                        },
+                    }
+                )
+            )
+        ));
+        assert!(event_requires_delivery(
+            &InProcessServerEvent::LegacyNotification(
+                codex_app_server_protocol::JSONRPCNotification {
+                    method: "codex/event/turn_aborted".to_string(),
+                    params: None,
+                }
+            )
+        ));
+        assert!(!event_requires_delivery(&InProcessServerEvent::Lagged {
+            skipped: 1
+        }));
+    }
+}