fix: make McpConnectionManager tolerant of MCPs that fail to start (#854)

I added a typo in my `config.toml` such that the `command` for one of my
`mcp_servers` did not exist and I verified that the error was surfaced
in the TUI (and that I was still able to use Codex).


![image](https://github.com/user-attachments/assets/f13cc08c-f4c6-40ec-9ab4-a9d75e03152f)

codex-rs/core/src/codex.rs

@@ -561,15 +561,35 @@ async fn submission_loop(
 
                 let writable_roots = Mutex::new(get_writable_roots(&cwd));
 
-                let mcp_connection_manager =
+                // Error messages to dispatch after SessionConfigured is sent.
+                let mut mcp_connection_errors = Vec::<Event>::new();
+                let (mcp_connection_manager, failed_clients) =
                     match McpConnectionManager::new(config.mcp_servers.clone()).await {
-                        Ok(mgr) => mgr,
+                        Ok((mgr, failures)) => (mgr, failures),
                         Err(e) => {
-                            error!("Failed to create MCP connection manager: {e:#}");
-                            McpConnectionManager::default()
+                            let message = format!("Failed to create MCP connection manager: {e:#}");
+                            error!("{message}");
+                            mcp_connection_errors.push(Event {
+                                id: sub.id.clone(),
+                                msg: EventMsg::Error { message },
+                            });
+                            (McpConnectionManager::default(), Default::default())
                         }
                     };
 
+                // Surface individual client start-up failures to the user.
+                if !failed_clients.is_empty() {
+                    for (server_name, err) in failed_clients {
+                        let message =
+                            format!("MCP client for `{server_name}` failed to start: {err:#}");
+                        error!("{message}");
+                        mcp_connection_errors.push(Event {
+                            id: sub.id.clone(),
+                            msg: EventMsg::Error { message },
+                        });
+                    }
+                }
+
                 // Attempt to create a RolloutRecorder *before* moving the
                 // `instructions` value into the Session struct.
                 let rollout_recorder = match RolloutRecorder::new(instructions.clone()).await {
@@ -596,12 +616,15 @@ async fn submission_loop(
                 }));
 
                 // ack
-                let event = Event {
-                    id: sub.id,
+                let events = std::iter::once(Event {
+                    id: sub.id.clone(),
                     msg: EventMsg::SessionConfigured { model },
-                };
-                if tx_event.send(event).await.is_err() {
-                    return;
+                })
+                .chain(mcp_connection_errors.into_iter());
+                for event in events {
+                    if let Err(e) = tx_event.send(event).await {
+                        error!("failed to send event: {e:?}");
+                    }
                 }
             }
             Op::UserInput { items } => {

codex-rs/core/src/mcp_connection_manager.rs

@@ -29,6 +29,10 @@ const MCP_TOOL_NAME_DELIMITER: &str = "__OAI_CODEX_MCP__";
 /// Timeout for the `tools/list` request.
 const LIST_TOOLS_TIMEOUT: Duration = Duration::from_secs(10);
 
+/// Map that holds a startup error for every MCP server that could **not** be
+/// spawned successfully.
+pub type ClientStartErrors = HashMap<String, anyhow::Error>;
+
 fn fully_qualified_tool_name(server: &str, tool: &str) -> String {
     format!("{server}{MCP_TOOL_NAME_DELIMITER}{tool}")
 }
@@ -60,40 +64,49 @@ impl McpConnectionManager {
     /// * `mcp_servers` – Map loaded from the user configuration where *keys*
     ///   are human-readable server identifiers and *values* are the spawn
     ///   instructions.
-    pub async fn new(mcp_servers: HashMap<String, McpServerConfig>) -> Result<Self> {
+    ///
+    /// Servers that fail to start are reported in `ClientStartErrors`: the
+    /// user should be informed about these errors.
+    pub async fn new(
+        mcp_servers: HashMap<String, McpServerConfig>,
+    ) -> Result<(Self, ClientStartErrors)> {
         // Early exit if no servers are configured.
         if mcp_servers.is_empty() {
-            return Ok(Self::default());
+            return Ok((Self::default(), ClientStartErrors::default()));
         }
 
-        // Spin up all servers concurrently.
+        // Launch all configured servers concurrently.
         let mut join_set = JoinSet::new();
 
-        // Spawn tasks to launch each server.
         for (server_name, cfg) in mcp_servers {
             // TODO: Verify server name: require `^[a-zA-Z0-9_-]+$`?
             join_set.spawn(async move {
                 let McpServerConfig { command, args, env } = cfg;
                 let client_res = McpClient::new_stdio_client(command, args, env).await;
-
                 (server_name, client_res)
             });
         }
 
         let mut clients: HashMap<String, std::sync::Arc<McpClient>> =
             HashMap::with_capacity(join_set.len());
+        let mut errors = ClientStartErrors::new();
+
         while let Some(res) = join_set.join_next().await {
-            let (server_name, client_res) = res?;
+            let (server_name, client_res) = res?; // JoinError propagation
 
-            let client = client_res
-                .with_context(|| format!("failed to spawn MCP server `{server_name}`"))?;
-
-            clients.insert(server_name, std::sync::Arc::new(client));
+            match client_res {
+                Ok(client) => {
+                    clients.insert(server_name, std::sync::Arc::new(client));
+                }
+                Err(e) => {
+                    errors.insert(server_name, e.into());
+                }
+            }
         }
 
         let tools = list_all_tools(&clients).await?;
 
-        Ok(Self { clients, tools })
+        Ok((Self { clients, tools }, errors))
     }
 
     /// Returns a single map that contains **all** tools. Each key is the