Terminate stdio MCP servers on shutdown to avoid process leaks (#19753)

## Why

Several bug reports describe thread shutdown (including subagent
threads) leaving stdio MCP server processes behind. These reports all
point at the same lifecycle gap: Codex launches stdio MCP servers, but
the session-level shutdown path does not explicitly close MCP clients or
terminate the server process tree.

Fixes #12491
Fixes #12976
Fixes #18881
Fixes #19469

## History

This is best understood as a regression/coverage gap in MCP session
lifecycle management, not as stdio MCP cleanup being absent all along.
#10710 added process-group cleanup for stdio MCP servers, but that
cleanup only runs when the `RmcpClient`/transport is dropped. The older
reports (#12491 and #12976) came after that cleanup existed, which
suggests the remaining problem was that some higher-level shutdown paths
kept the MCP manager alive or replaced it without explicitly draining
clients. The newer reports (#18881 and #19469) exposed the same family
around manager replacement and shutdown.

## What changed

- Added an explicit stdio MCP process handle in `codex-rmcp-client` so
local MCP servers terminate their process group and executor-backed MCP
servers call the executor process terminator.
- Added `RmcpClient::shutdown()` and manager-level MCP shutdown draining
so session shutdown, channel-close fallback, MCP refresh, and connector
probing stop owned MCP clients.
- Added regression coverage that starts a stdio MCP server, begins an
in-flight blocking tool call, shuts down the client, and asserts the
server process exits.

## Verification

- `cargo test -p codex-rmcp-client`
- `cargo test -p codex-mcp`
- `just fix -p codex-rmcp-client`
- `just fix -p codex-mcp`
- `just fix -p codex-core`

- Manual before/after validation with a temporary repro script:
- Pre-fix binary from `HEAD^` (`fed0a8f4fa`): reproduced the leak with
surviving MCP server and child PIDs, `survivors=[77583, 77592]`,
`leaked=true`.
- Post-fix binary from this branch (`67e318148b`): verified both MCP
processes were gone after interrupting `codex exec`, `survivors=[]`,
`leaked=false`.

codex-rs/codex-mcp/src/connection_manager.rs

@@ -71,6 +71,7 @@ pub struct McpConnectionManager {
     clients: HashMap<String, AsyncManagedClient>,
     server_origins: HashMap<String, String>,
     elicitation_requests: ElicitationRequestManager,
+    startup_cancellation_token: CancellationToken,
 }
 
 impl McpConnectionManager {
@@ -85,6 +86,7 @@ impl McpConnectionManager {
                 approval_policy.value(),
                 permission_profile.get().clone(),
             ),
+            startup_cancellation_token: CancellationToken::new(),
         }
     }
 
@@ -92,6 +94,24 @@ impl McpConnectionManager {
         !self.clients.is_empty()
     }
 
+    /// Drain all MCP clients from this manager and return a future that stops
+    /// them and terminates their stdio server processes.
+    pub fn begin_shutdown(&mut self) -> impl std::future::Future<Output = ()> + Send + 'static {
+        self.startup_cancellation_token.cancel();
+        let clients = std::mem::take(&mut self.clients);
+        self.server_origins.clear();
+        async move {
+            for client in clients.into_values() {
+                client.shutdown().await;
+            }
+        }
+    }
+
+    /// Stop all MCP clients owned by this manager and terminate stdio server processes.
+    pub async fn shutdown(&mut self) {
+        self.begin_shutdown().await;
+    }
+
     pub fn server_origin(&self, server_name: &str) -> Option<&str> {
         self.server_origins.get(server_name).map(String::as_str)
     }
@@ -221,6 +241,7 @@ impl McpConnectionManager {
             clients,
             server_origins,
             elicitation_requests: elicitation_requests.clone(),
+            startup_cancellation_token: cancel_token.clone(),
         };
         tokio::spawn(async move {
             let outcomes = join_set.join_all().await;
@@ -609,6 +630,13 @@ impl McpConnectionManager {
     }
 }
 
+impl Drop for McpConnectionManager {
+    fn drop(&mut self) {
+        self.startup_cancellation_token.cancel();
+        self.clients.clear();
+    }
+}
+
 async fn emit_update(
     submit_id: &str,
     tx_event: &Sender<Event>,

codex-rs/codex-mcp/src/connection_manager_tests.rs

@@ -662,6 +662,7 @@ async fn list_all_tools_uses_startup_snapshot_while_client_is_pending() {
             startup_snapshot: Some(startup_tools),
             startup_complete: Arc::new(std::sync::atomic::AtomicBool::new(false)),
             tool_plugin_provenance: Arc::new(ToolPluginProvenance::default()),
+            cancel_token: CancellationToken::new(),
         },
     );
 
@@ -690,6 +691,7 @@ async fn resolve_tool_info_accepts_canonical_namespaced_tool_names() {
             startup_snapshot: Some(startup_tools),
             startup_complete: Arc::new(std::sync::atomic::AtomicBool::new(false)),
             tool_plugin_provenance: Arc::new(ToolPluginProvenance::default()),
+            cancel_token: CancellationToken::new(),
         },
     );
 
@@ -726,6 +728,7 @@ async fn list_all_tools_blocks_while_client_is_pending_without_startup_snapshot(
             startup_snapshot: None,
             startup_complete: Arc::new(std::sync::atomic::AtomicBool::new(false)),
             tool_plugin_provenance: Arc::new(ToolPluginProvenance::default()),
+            cancel_token: CancellationToken::new(),
         },
     );
 
@@ -750,6 +753,7 @@ async fn list_all_tools_does_not_block_when_startup_snapshot_cache_hit_is_empty(
             startup_snapshot: Some(Vec::new()),
             startup_complete: Arc::new(std::sync::atomic::AtomicBool::new(false)),
             tool_plugin_provenance: Arc::new(ToolPluginProvenance::default()),
+            cancel_token: CancellationToken::new(),
         },
     );
 
@@ -784,6 +788,7 @@ async fn list_all_tools_uses_startup_snapshot_when_client_startup_fails() {
             startup_snapshot: Some(startup_tools),
             startup_complete,
             tool_plugin_provenance: Arc::new(ToolPluginProvenance::default()),
+            cancel_token: CancellationToken::new(),
         },
     );
 

codex-rs/codex-mcp/src/rmcp_client.rs

@@ -60,6 +60,7 @@ use rmcp::model::Implementation;
 use rmcp::model::InitializeRequestParams;
 use rmcp::model::ProtocolVersion;
 use tokio_util::sync::CancellationToken;
+use tracing::warn;
 
 /// MCP server capability indicating that Codex should include [`SandboxState`]
 /// in tool-call request `_meta` under this key.
@@ -115,6 +116,7 @@ pub(crate) struct AsyncManagedClient {
     pub(crate) startup_snapshot: Option<Vec<ToolInfo>>,
     pub(crate) startup_complete: Arc<AtomicBool>,
     pub(crate) tool_plugin_provenance: Arc<ToolPluginProvenance>,
+    pub(crate) cancel_token: CancellationToken,
 }
 
 impl AsyncManagedClient {
@@ -142,8 +144,9 @@ impl AsyncManagedClient {
         let startup_tool_filter = tool_filter;
         let startup_complete = Arc::new(AtomicBool::new(false));
         let startup_complete_for_fut = Arc::clone(&startup_complete);
+        let cancel_token_for_fut = cancel_token.clone();
         let fut = async move {
-            let outcome = async {
+            let outcome = match async {
                 if let Err(error) = validate_mcp_server_name(&server_name) {
                     return Err(error.into());
                 }
@@ -158,7 +161,7 @@ impl AsyncManagedClient {
                     )
                     .await?,
                 );
-                match start_server_task(
+                start_server_task(
                     server_name,
                     client,
                     StartServerTaskParams {
@@ -172,14 +175,14 @@ impl AsyncManagedClient {
                         codex_apps_tools_cache_context,
                     },
                 )
-                .or_cancel(&cancel_token)
+                .await
+            }
+            .or_cancel(&cancel_token_for_fut)
             .await
             {
                 Ok(result) => result,
                 Err(CancelErr::Cancelled) => Err(StartupOutcomeError::Cancelled),
-                }
+            };
-            }
-            .await;
 
             startup_complete_for_fut.store(true, Ordering::Release);
             outcome
@@ -197,6 +200,7 @@ impl AsyncManagedClient {
             startup_snapshot,
             startup_complete,
             tool_plugin_provenance,
+            cancel_token,
         }
     }
 
@@ -204,6 +208,17 @@ impl AsyncManagedClient {
         self.client.clone().await
     }
 
+    pub(crate) async fn shutdown(&self) {
+        self.cancel_token.cancel();
+        match self.client().await {
+            Ok(client) => client.client.shutdown().await,
+            Err(StartupOutcomeError::Cancelled) => {}
+            Err(error) => {
+                warn!("failed to initialize MCP client during shutdown: {error:#}");
+            }
+        }
+    }
+
     fn startup_snapshot_while_initializing(&self) -> Option<Vec<ToolInfo>> {
         if !self.startup_complete.load(Ordering::Acquire) {
             return self.startup_snapshot.clone();

codex-rs/core/src/connectors.rs

@@ -267,7 +267,7 @@ pub async fn list_accessible_connectors_from_mcp_tools_with_environment_manager(
         .default_environment()
         .unwrap_or_else(|| environment_manager.local_environment());
 
-    let (mcp_connection_manager, cancel_token) = McpConnectionManager::new(
+    let (mut mcp_connection_manager, cancel_token) = McpConnectionManager::new(
         &mcp_servers,
         config.mcp_oauth_credentials_store_mode,
         auth_status_entries,
@@ -346,6 +346,7 @@ pub async fn list_accessible_connectors_from_mcp_tools_with_environment_manager(
     }
     let accessible_connectors =
         with_app_plugin_sources(accessible_connectors, &tool_plugin_provenance);
+    mcp_connection_manager.shutdown().await;
     Ok(AccessibleConnectorsStatus {
         connectors: accessible_connectors,
         codex_apps_ready,

codex-rs/core/src/session/handlers.rs

@@ -883,6 +883,11 @@ pub async fn shutdown(sess: &Arc<Session>, sub_id: String) -> bool {
         .unified_exec_manager
         .terminate_all_processes()
         .await;
+    let mcp_shutdown = {
+        let mut manager = sess.services.mcp_connection_manager.write().await;
+        manager.begin_shutdown()
+    };
+    mcp_shutdown.await;
     sess.guardian_review_session.shutdown().await;
     info!("Shutting down Codex instance");
     let history = sess.clone_history().await;
@@ -1166,8 +1171,19 @@ pub(super) async fn submission_loop(
             break;
         }
     }
-    // Also drain cached guardian state if the submission loop exits because
+    // If the submission loop exits because the channel closed without an
-    // the channel closed without receiving an explicit shutdown op.
+    // explicit shutdown op, still run process teardown for child processes
+    // owned by this session.
+    sess.services
+        .unified_exec_manager
+        .terminate_all_processes()
+        .await;
+    let mcp_shutdown = {
+        let mut manager = sess.services.mcp_connection_manager.write().await;
+        manager.begin_shutdown()
+    };
+    mcp_shutdown.await;
+    // Also drain cached guardian state on this implicit shutdown path.
     sess.guardian_review_session.shutdown().await;
     debug!("Agent loop exited");
 }

codex-rs/core/src/session/mcp.rs

@@ -255,8 +255,11 @@ impl Session {
             *guard = cancel_token;
         }
 
+        let mut old_manager = {
             let mut manager = self.services.mcp_connection_manager.write().await;
-        *manager = refreshed_manager;
+            std::mem::replace(&mut *manager, refreshed_manager)
+        };
+        old_manager.shutdown().await;
     }
 
     pub(crate) async fn refresh_mcp_servers_if_requested(&self, turn_context: &TurnContext) {

codex-rs/rmcp-client/src/bin/test_stdio_server.rs

@@ -755,6 +755,9 @@ fn parse_data_url(url: &str) -> Option<(String, String)> {
 #[tokio::main]
 async fn main() -> Result<(), Box<dyn std::error::Error>> {
     eprintln!("starting rmcp test server");
+    if let Ok(pid_file) = std::env::var("MCP_TEST_PID_FILE") {
+        std::fs::write(pid_file, std::process::id().to_string())?;
+    }
     // Run the server with STDIO transport. If the client disconnects we simply
     // bubble up the error so the process exits.
     let service = TestToolServer::new();

codex-rs/rmcp-client/src/rmcp_client.rs

@@ -67,6 +67,7 @@ use crate::oauth::OAuthPersistor;
 use crate::oauth::StoredOAuthTokens;
 use crate::stdio_server_launcher::StdioServerCommand;
 use crate::stdio_server_launcher::StdioServerLauncher;
+use crate::stdio_server_launcher::StdioServerProcessHandle;
 use crate::stdio_server_launcher::StdioServerTransport;
 use crate::utils::apply_default_headers;
 use crate::utils::build_default_headers;
@@ -93,6 +94,7 @@ enum ClientState {
         service: Arc<RunningService<RoleClient, ElicitationClientService>>,
         oauth: Option<OAuthPersistor>,
     },
+    Closed,
 }
 
 #[derive(Clone)]
@@ -265,6 +267,7 @@ pub struct ListToolsWithConnectorIdResult {
 /// https://github.com/modelcontextprotocol/rust-sdk
 pub struct RmcpClient {
     state: Mutex<ClientState>,
+    stdio_process: Option<StdioServerProcessHandle>,
     transport_recipe: TransportRecipe,
     initialize_context: Mutex<Option<InitializeContext>>,
     session_recovery_lock: Semaphore,
@@ -287,11 +290,17 @@ impl RmcpClient {
         let transport = Self::create_pending_transport(&transport_recipe)
             .await
             .map_err(io::Error::other)?;
+        let stdio_process = match &transport {
+            PendingTransport::Stdio { transport } => Some(transport.process_handle()),
+            PendingTransport::StreamableHttp { .. }
+            | PendingTransport::StreamableHttpWithOAuth { .. } => None,
+        };
 
         Ok(Self {
             state: Mutex::new(ClientState::Connecting {
                 transport: Some(transport),
             }),
+            stdio_process,
             transport_recipe,
             initialize_context: Mutex::new(None),
             session_recovery_lock: Semaphore::new(/*permits*/ 1),
@@ -325,6 +334,7 @@ impl RmcpClient {
             state: Mutex::new(ClientState::Connecting {
                 transport: Some(transport),
             }),
+            stdio_process: None,
             transport_recipe,
             initialize_context: Mutex::new(None),
             session_recovery_lock: Semaphore::new(/*permits*/ 1),
@@ -353,6 +363,7 @@ impl RmcpClient {
                     None => return Err(anyhow!("client already initializing")),
                 },
                 ClientState::Ready { .. } => return Err(anyhow!("client already initialized")),
+                ClientState::Closed => return Err(anyhow!("MCP client is shut down")),
             }
         };
 
@@ -376,6 +387,9 @@ impl RmcpClient {
 
         {
             let mut guard = self.state.lock().await;
+            if matches!(*guard, ClientState::Closed) {
+                return Err(anyhow!("MCP client is shut down"));
+            }
             *guard = ClientState::Ready {
                 service,
                 oauth: oauth_persistor.clone(),
@@ -623,6 +637,7 @@ impl RmcpClient {
         match &*guard {
             ClientState::Ready { service, .. } => Ok(Arc::clone(service)),
             ClientState::Connecting { .. } => Err(anyhow!("MCP client not initialized")),
+            ClientState::Closed => Err(anyhow!("MCP client is shut down")),
         }
     }
 
@@ -637,6 +652,22 @@ impl RmcpClient {
         }
     }
 
+    /// Stop the MCP transport and any stdio server process owned by this client.
+    pub async fn shutdown(&self) {
+        let previous_state = {
+            let mut guard = self.state.lock().await;
+            std::mem::replace(&mut *guard, ClientState::Closed)
+        };
+
+        if let Some(process) = &self.stdio_process
+            && let Err(error) = process.terminate().await
+        {
+            warn!("failed to terminate MCP stdio server process: {error}");
+        }
+
+        drop(previous_state);
+    }
+
     /// This should be called after every tool call so that if a given tool call triggered
     /// a refresh of the OAuth tokens, they are persisted.
     async fn persist_oauth_tokens(&self) {
@@ -900,6 +931,9 @@ impl RmcpClient {
                 ClientState::Connecting { .. } => {
                     return Err(anyhow!("MCP client not initialized"));
                 }
+                ClientState::Closed => {
+                    return Err(anyhow!("MCP client is shut down"));
+                }
             }
         }
 
@@ -919,6 +953,9 @@ impl RmcpClient {
 
         {
             let mut guard = self.state.lock().await;
+            if matches!(*guard, ClientState::Closed) {
+                return Err(anyhow!("MCP client is shut down"));
+            }
             *guard = ClientState::Ready {
                 service,
                 oauth: oauth_persistor.clone(),

codex-rs/rmcp-client/src/stdio_server_launcher.rs

@@ -18,6 +18,8 @@ use std::io;
 use std::path::PathBuf;
 use std::process::Stdio;
 use std::sync::Arc;
+use std::sync::atomic::AtomicBool;
+use std::sync::atomic::Ordering;
 #[cfg(unix)]
 use std::thread::sleep;
 #[cfg(unix)]
@@ -31,6 +33,7 @@ use codex_config::types::McpServerEnvVar;
 use codex_exec_server::ExecBackend;
 use codex_exec_server::ExecEnvPolicy;
 use codex_exec_server::ExecParams;
+use codex_exec_server::ExecProcess;
 use codex_protocol::config_types::ShellEnvironmentPolicyInherit;
 #[cfg(unix)]
 use codex_utils_pty::process_group::kill_process_group;
@@ -88,11 +91,7 @@ pub struct StdioServerCommand {
 /// directly to `rmcp::service::serve_client`.
 pub struct StdioServerTransport {
     inner: StdioServerTransportInner,
-    // Local child processes can leave subprocesses behind, so the local
+    process: StdioServerProcessHandle,
-    // variant keeps a process-group guard with the transport. Executor-backed
-    // processes are owned and cleaned up by the executor, so that variant uses
-    // `None`.
-    _process_group_guard: Option<ProcessGroupGuard>,
 }
 
 enum StdioServerTransportInner {
@@ -127,6 +126,7 @@ impl Transport<RoleClient> for StdioServerTransport {
     }
 
     async fn close(&mut self) -> std::result::Result<(), Self::Error> {
+        self.process.terminate().await?;
         match &mut self.inner {
             StdioServerTransportInner::Local(transport) => transport.close().await,
             StdioServerTransportInner::Executor(transport) => transport.close().await,
@@ -134,6 +134,12 @@ impl Transport<RoleClient> for StdioServerTransport {
     }
 }
 
+impl StdioServerTransport {
+    pub(crate) fn process_handle(&self) -> StdioServerProcessHandle {
+        self.process.clone()
+    }
+}
+
 impl StdioServerCommand {
     /// Build the stdio process parameters before choosing where the process
     /// runs.
@@ -192,12 +198,33 @@ impl StdioServerLauncher for LocalStdioServerLauncher {
 const PROCESS_GROUP_TERM_GRACE_PERIOD: Duration = Duration::from_secs(2);
 
 #[cfg(unix)]
-struct ProcessGroupGuard {
+struct LocalProcessTerminator {
     process_group_id: u32,
 }
 
-#[cfg(not(unix))]
+#[cfg(windows)]
-struct ProcessGroupGuard;
+struct LocalProcessTerminator {
+    pid: u32,
+}
+
+#[cfg(not(any(unix, windows)))]
+struct LocalProcessTerminator;
+
+#[derive(Clone)]
+pub(crate) struct StdioServerProcessHandle {
+    inner: Arc<StdioServerProcessHandleInner>,
+}
+
+struct StdioServerProcessHandleInner {
+    program_name: String,
+    kind: StdioServerProcessKind,
+    terminated: AtomicBool,
+}
+
+enum StdioServerProcessKind {
+    Local(Option<LocalProcessTerminator>),
+    Executor(Arc<dyn ExecProcess>),
+}
 
 mod private {
     pub trait Sealed {}
@@ -238,7 +265,10 @@ impl LocalStdioServerLauncher {
         let (transport, stderr) = TokioChildProcess::builder(command)
             .stderr(Stdio::piped())
             .spawn()?;
-        let process_group_guard = transport.id().map(ProcessGroupGuard::new);
+        let process = StdioServerProcessHandle::local(
+            program_name.clone(),
+            transport.id().map(LocalProcessTerminator::new),
+        );
 
         if let Some(stderr) = stderr {
             tokio::spawn(async move {
@@ -260,18 +290,24 @@ impl LocalStdioServerLauncher {
 
         Ok(StdioServerTransport {
             inner: StdioServerTransportInner::Local(transport),
-            _process_group_guard: process_group_guard,
+            process,
         })
     }
 }
 
-impl ProcessGroupGuard {
+impl LocalProcessTerminator {
     fn new(process_group_id: u32) -> Self {
         #[cfg(unix)]
         {
             Self { process_group_id }
         }
-        #[cfg(not(unix))]
+        #[cfg(windows)]
+        {
+            Self {
+                pid: process_group_id,
+            }
+        }
+        #[cfg(not(any(unix, windows)))]
         {
             let _ = process_group_id;
             Self
@@ -279,7 +315,7 @@ impl ProcessGroupGuard {
     }
 
     #[cfg(unix)]
-    fn maybe_terminate_process_group(&self) {
+    fn terminate(&self) {
         let process_group_id = self.process_group_id;
         let should_escalate = match terminate_process_group(process_group_id) {
             Ok(exists) => exists,
@@ -298,14 +334,93 @@ impl ProcessGroupGuard {
         }
     }
 
-    #[cfg(not(unix))]
+    #[cfg(windows)]
-    fn maybe_terminate_process_group(&self) {}
+    fn terminate(&self) {
+        let _ = std::process::Command::new("taskkill")
+            .arg("/PID")
+            .arg(self.pid.to_string())
+            .arg("/T")
+            .arg("/F")
+            .status();
+    }
+
+    #[cfg(not(any(unix, windows)))]
+    fn terminate(&self) {}
 }
 
-impl Drop for ProcessGroupGuard {
+impl StdioServerProcessHandle {
+    fn local(program_name: String, terminator: Option<LocalProcessTerminator>) -> Self {
+        Self {
+            inner: Arc::new(StdioServerProcessHandleInner {
+                program_name,
+                kind: StdioServerProcessKind::Local(terminator),
+                terminated: AtomicBool::new(false),
+            }),
+        }
+    }
+
+    pub(crate) fn executor(program_name: String, process: Arc<dyn ExecProcess>) -> Self {
+        Self {
+            inner: Arc::new(StdioServerProcessHandleInner {
+                program_name,
+                kind: StdioServerProcessKind::Executor(process),
+                terminated: AtomicBool::new(false),
+            }),
+        }
+    }
+
+    pub(crate) async fn terminate(&self) -> io::Result<()> {
+        if self.inner.terminated.swap(true, Ordering::AcqRel) {
+            return Ok(());
+        }
+
+        match &self.inner.kind {
+            StdioServerProcessKind::Local(Some(terminator)) => {
+                terminator.terminate();
+                Ok(())
+            }
+            StdioServerProcessKind::Local(None) => Ok(()),
+            StdioServerProcessKind::Executor(process) => match process.terminate().await {
+                Ok(()) => Ok(()),
+                Err(error) => {
+                    self.inner.terminated.store(false, Ordering::Release);
+                    Err(io::Error::other(error))
+                }
+            },
+        }
+    }
+}
+
+impl Drop for StdioServerProcessHandleInner {
     fn drop(&mut self) {
-        if cfg!(unix) {
+        if self.terminated.swap(true, Ordering::AcqRel) {
-            self.maybe_terminate_process_group();
+            return;
+        }
+
+        match &self.kind {
+            StdioServerProcessKind::Local(Some(terminator)) => {
+                terminator.terminate();
+            }
+            StdioServerProcessKind::Local(None) => {}
+            StdioServerProcessKind::Executor(process) => {
+                let process = Arc::clone(process);
+                let program_name = self.program_name.clone();
+                let Ok(handle) = tokio::runtime::Handle::try_current() else {
+                    warn!(
+                        "Could not schedule remote MCP server process termination on drop ({}): no Tokio runtime is available",
+                        self.program_name
+                    );
+                    return;
+                };
+
+                std::mem::drop(handle.spawn(async move {
+                    if let Err(error) = process.terminate().await {
+                        warn!(
+                            "Failed to terminate remote MCP server process on drop ({program_name}): {error}"
+                        );
+                    }
+                }));
+            }
         }
     }
 }
@@ -392,12 +507,14 @@ impl ExecutorStdioServerLauncher {
             .await
             .map_err(io::Error::other)?;
 
+        let process =
+            StdioServerProcessHandle::executor(program_name.clone(), Arc::clone(&started.process));
         Ok(StdioServerTransport {
             inner: StdioServerTransportInner::Executor(ExecutorProcessTransport::new(
                 started.process,
                 program_name,
             )),
-            _process_group_guard: None,
+            process,
         })
     }
 

codex-rs/rmcp-client/tests/process_group_cleanup.rs

@@ -9,8 +9,43 @@ use std::time::Duration;
 
 use anyhow::Context;
 use anyhow::Result;
+use codex_rmcp_client::ElicitationAction;
+use codex_rmcp_client::ElicitationResponse;
 use codex_rmcp_client::LocalStdioServerLauncher;
 use codex_rmcp_client::RmcpClient;
+use futures::FutureExt as _;
+use rmcp::model::ClientCapabilities;
+use rmcp::model::Implementation;
+use rmcp::model::InitializeRequestParams;
+use rmcp::model::ProtocolVersion;
+use serde_json::json;
+
+fn stdio_server_bin() -> Result<std::path::PathBuf> {
+    codex_utils_cargo_bin::cargo_bin("test_stdio_server").map_err(Into::into)
+}
+
+fn init_params() -> InitializeRequestParams {
+    InitializeRequestParams {
+        meta: None,
+        capabilities: ClientCapabilities {
+            experimental: None,
+            extensions: None,
+            roots: None,
+            sampling: None,
+            elicitation: None,
+            tasks: None,
+        },
+        client_info: Implementation {
+            name: "codex-test".into(),
+            version: "0.0.0-test".into(),
+            title: Some("Codex rmcp shutdown test".into()),
+            description: None,
+            icons: None,
+            website_url: None,
+        },
+        protocol_version: ProtocolVersion::V_2025_06_18,
+    }
+}
 
 fn process_exists(pid: u32) -> bool {
     std::process::Command::new("kill")
@@ -94,3 +129,67 @@ async fn drop_kills_wrapper_process_group() -> Result<()> {
 
     wait_for_process_exit(grandchild_pid).await
 }
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn shutdown_kills_initialized_stdio_server_with_in_flight_operation() -> Result<()> {
+    let temp_dir = tempfile::tempdir()?;
+    let server_pid_file = temp_dir.path().join("server.pid");
+    let server_pid_file_str = server_pid_file.to_string_lossy().into_owned();
+
+    let client = Arc::new(
+        RmcpClient::new_stdio_client(
+            stdio_server_bin()?.into(),
+            Vec::<OsString>::new(),
+            Some(HashMap::from([(
+                OsString::from("MCP_TEST_PID_FILE"),
+                OsString::from(server_pid_file_str),
+            )])),
+            &[],
+            /*cwd*/ None,
+            Arc::new(LocalStdioServerLauncher::new(std::env::current_dir()?)),
+        )
+        .await?,
+    );
+
+    client
+        .initialize(
+            init_params(),
+            Some(Duration::from_secs(5)),
+            Box::new(|_, _| {
+                async {
+                    Ok(ElicitationResponse {
+                        action: ElicitationAction::Accept,
+                        content: Some(json!({})),
+                        meta: None,
+                    })
+                }
+                .boxed()
+            }),
+        )
+        .await?;
+
+    let server_pid = wait_for_pid_file(&server_pid_file).await?;
+    assert!(
+        process_exists(server_pid),
+        "expected MCP server process {server_pid} to be running before shutdown"
+    );
+
+    let call_client = Arc::clone(&client);
+    let call_task = tokio::spawn(async move {
+        call_client
+            .call_tool(
+                "sync".to_string(),
+                Some(json!({ "sleep_after_ms": 300_000 })),
+                /*meta*/ None,
+                Some(Duration::from_secs(300)),
+            )
+            .await
+    });
+    tokio::time::sleep(Duration::from_millis(200)).await;
+
+    client.shutdown().await;
+
+    wait_for_process_exit(server_pid).await?;
+    let _ = tokio::time::timeout(Duration::from_secs(5), call_task).await?;
+    Ok(())
+}