fix: navigate initialization phase before tools/list request in MCP client (#904)

Apparently the MCP server implemented in JavaScript did not require the
`initialize` handshake before responding to tool list/call, so I missed
this.

Author: bolinfest

codex-rs/core/src/mcp_connection_manager.rs

@@ -13,6 +13,8 @@ use anyhow::Context;
 use anyhow::Result;
 use anyhow::anyhow;
 use codex_mcp_client::McpClient;
+use mcp_types::ClientCapabilities;
+use mcp_types::Implementation;
 use mcp_types::Tool;
 use tokio::task::JoinSet;
 use tracing::info;
@@ -83,7 +85,33 @@ impl McpConnectionManager {
             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)
+                match client_res {
+                    Ok(client) => {
+                        // Initialize the client.
+                        let params = mcp_types::InitializeRequestParams {
+                            capabilities: ClientCapabilities {
+                                experimental: None,
+                                roots: None,
+                                sampling: None,
+                            },
+                            client_info: Implementation {
+                                name: "codex-mcp-client".to_owned(),
+                                version: env!("CARGO_PKG_VERSION").to_owned(),
+                            },
+                            protocol_version: mcp_types::MCP_SCHEMA_VERSION.to_owned(),
+                        };
+                        let initialize_notification_params = None;
+                        let timeout = Some(Duration::from_secs(10));
+                        match client
+                            .initialize(params, initialize_notification_params, timeout)
+                            .await
+                        {
+                            Ok(_response) => (server_name, Ok(client)),
+                            Err(e) => (server_name, Err(e)),
+                        }
+                    }
+                    Err(e) => (server_name, Err(e.into())),
+                }
             });
         }
 
@@ -99,7 +127,7 @@ impl McpConnectionManager {
                     clients.insert(server_name, std::sync::Arc::new(client));
                 }
                 Err(e) => {
-                    errors.insert(server_name, e.into());
+                    errors.insert(server_name, e);
                 }
             }
         }

codex-rs/mcp-client/src/main.rs

@@ -10,10 +10,16 @@
 //! program. The utility connects, issues a `tools/list` request and prints the
 //! server's response as pretty JSON.
 
+use std::time::Duration;
+
 use anyhow::Context;
 use anyhow::Result;
 use codex_mcp_client::McpClient;
+use mcp_types::ClientCapabilities;
+use mcp_types::Implementation;
+use mcp_types::InitializeRequestParams;
 use mcp_types::ListToolsRequestParams;
+use mcp_types::MCP_SCHEMA_VERSION;
 
 #[tokio::main]
 async fn main() -> Result<()> {
@@ -33,6 +39,25 @@ async fn main() -> Result<()> {
         .await
         .with_context(|| format!("failed to spawn subprocess: {original_args:?}"))?;
 
+    let params = InitializeRequestParams {
+        capabilities: ClientCapabilities {
+            experimental: None,
+            roots: None,
+            sampling: None,
+        },
+        client_info: Implementation {
+            name: "codex-mcp-client".to_owned(),
+            version: env!("CARGO_PKG_VERSION").to_owned(),
+        },
+        protocol_version: MCP_SCHEMA_VERSION.to_owned(),
+    };
+    let initialize_notification_params = None;
+    let timeout = Some(Duration::from_secs(10));
+    let response = client
+        .initialize(params, initialize_notification_params, timeout)
+        .await?;
+    eprintln!("initialize response: {response:?}");
+
     // Issue `tools/list` request (no params).
     let timeout = None;
     let tools = client

codex-rs/mcp-client/src/mcp_client.rs

@@ -17,10 +17,14 @@ use std::sync::atomic::AtomicI64;
 use std::sync::atomic::Ordering;
 use std::time::Duration;
 
+use anyhow::Context;
 use anyhow::Result;
 use anyhow::anyhow;
 use mcp_types::CallToolRequest;
 use mcp_types::CallToolRequestParams;
+use mcp_types::InitializeRequest;
+use mcp_types::InitializeRequestParams;
+use mcp_types::InitializedNotification;
 use mcp_types::JSONRPC_VERSION;
 use mcp_types::JSONRPCMessage;
 use mcp_types::JSONRPCNotification;
@@ -29,6 +33,7 @@ use mcp_types::JSONRPCResponse;
 use mcp_types::ListToolsRequest;
 use mcp_types::ListToolsRequestParams;
 use mcp_types::ListToolsResult;
+use mcp_types::ModelContextProtocolNotification;
 use mcp_types::ModelContextProtocolRequest;
 use mcp_types::RequestId;
 use serde::Serialize;
@@ -74,6 +79,8 @@ pub struct McpClient {
 
 impl McpClient {
     /// Spawn the given command and establish an MCP session over its STDIO.
+    /// Caller is responsible for sending the `initialize` request. See
+    /// [`initialize`](Self::initialize) for details.
     pub async fn new_stdio_client(
         program: String,
         args: Vec<String>,
@@ -273,6 +280,52 @@ impl McpClient {
         }
     }
 
+    pub async fn send_notification<N>(&self, params: N::Params) -> Result<()>
+    where
+        N: ModelContextProtocolNotification,
+        N::Params: Serialize,
+    {
+        // Serialize params -> JSON. For many request types `Params` is
+        // `Option<T>` and `None` should be encoded as *absence* of the field.
+        let params_json = serde_json::to_value(&params)?;
+        let params_field = if params_json.is_null() {
+            None
+        } else {
+            Some(params_json)
+        };
+
+        let method = N::METHOD.to_string();
+        let jsonrpc_notification = JSONRPCNotification {
+            jsonrpc: JSONRPC_VERSION.to_string(),
+            method: method.clone(),
+            params: params_field,
+        };
+
+        let notification = JSONRPCMessage::Notification(jsonrpc_notification);
+        self.outgoing_tx
+            .send(notification)
+            .await
+            .with_context(|| format!("failed to send notification `{method}` to writer task"))
+    }
+
+    /// Negotiates the initialization with the MCP server. Sends an `initialize`
+    /// request with the specified `initialize_params` and then the
+    /// `notifications/initialized` notification once the response has been
+    /// received. Returns the response to the `initialize` request.
+    pub async fn initialize(
+        &self,
+        initialize_params: InitializeRequestParams,
+        initialize_notification_params: Option<serde_json::Value>,
+        timeout: Option<Duration>,
+    ) -> Result<mcp_types::InitializeResult> {
+        let response = self
+            .send_request::<InitializeRequest>(initialize_params, timeout)
+            .await?;
+        self.send_notification::<InitializedNotification>(initialize_notification_params)
+            .await?;
+        Ok(response)
+    }
+
     /// Convenience wrapper around `tools/list`.
     pub async fn list_tools(
         &self,