fix: accept stringified numeric response IDs

crates/rmcp/src/model.rs

@@ -240,6 +240,22 @@ impl NumberOrString {
             NumberOrString::String(s) => Value::String(s.to_string()),
         }
     }
+
+    pub(crate) fn numeric_string_value(&self) -> Option<i64> {
+        match self {
+            Self::String(id) => id.parse().ok(),
+            Self::Number(_) => None,
+        }
+    }
+
+    pub(crate) fn matches_response_id(&self, response_id: &Self) -> bool {
+        self == response_id
+            || matches!(
+                self,
+                Self::Number(request_id)
+                    if response_id.numeric_string_value() == Some(*request_id)
+            )
+    }
 }
 
 impl std::fmt::Display for NumberOrString {

crates/rmcp/src/service.rs

@@ -305,6 +305,17 @@ pub trait ProgressTokenProvider: Send + Sync + 'static {
 pub type AtomicU32RequestIdProvider = AtomicU32Provider;
 pub type AtomicU32ProgressTokenProvider = AtomicU32Provider;
 
+pub(crate) fn remove_pending_request<T>(
+    pending_requests: &mut HashMap<RequestId, T>,
+    response_id: &RequestId,
+) -> Option<T> {
+    pending_requests.remove(response_id).or_else(|| {
+        response_id
+            .numeric_string_value()
+            .and_then(|id| pending_requests.remove(&RequestId::Number(id)))
+    })
+}
+
 #[derive(Debug, Default)]
 pub struct AtomicU32Provider {
     id: AtomicU64,
@@ -1481,7 +1492,9 @@ where
                     id,
                     ..
                 })) => {
-                    if let Some(responder) = local_responder_pool.remove(&id) {
+                    if let Some(responder) =
+                        remove_pending_request(&mut local_responder_pool, &id)
+                    {
                         let response_result = responder.send(Ok(result));
                         if let Err(_error) = response_result {
                             tracing::warn!(%id, "Error sending response");
@@ -1495,7 +1508,9 @@ where
                         tracing::debug!(?error, "received id-less peer error");
                         continue;
                     };
-                    if let Some(responder) = local_responder_pool.remove(&id) {
+                    if let Some(responder) =
+                        remove_pending_request(&mut local_responder_pool, &id)
+                    {
                         let service_error = if error.is_transport_closed() {
                             ServiceError::TransportClosed
                         } else {

crates/rmcp/src/service/client.rs

@@ -687,7 +687,7 @@ where
     let (response, response_id) =
         expect_response(transport, "initialize response", service, peer.clone()).await?;
 
-    if id != response_id {
+    if !id.matches_response_id(&response_id) {
         return Err(ClientInitializeError::ConflictInitResponseId(
             id,
             response_id,
@@ -753,7 +753,7 @@ where
 
         match expect_response(transport, "discover response", service, peer.clone()).await {
             Ok((ServerResult::DiscoverResult(result), response_id)) => {
-                if response_id != id {
+                if !id.matches_response_id(&response_id) {
                     return Err(ClientInitializeError::ConflictInitResponseId(
                         id,
                         response_id,

crates/rmcp/src/transport/streamable_http_client.rs

@@ -499,8 +499,14 @@ impl<C: StreamableHttpClient> StreamableHttpClientWorker<C> {
         pending_stream_response_ids: &mut HashSet<RequestId>,
         message: &ServerJsonRpcMessage,
     ) {
-        if let Some(id) = Self::server_response_id(message) {
-            pending_stream_response_ids.remove(id);
+        let Some(response_id) = Self::server_response_id(message) else {
+            return;
+        };
+        if pending_stream_response_ids.remove(response_id) {
+            return;
+        }
+        if let Some(id) = response_id.numeric_string_value() {
+            pending_stream_response_ids.remove(&RequestId::Number(id));
         }
     }
 
@@ -1382,7 +1388,10 @@ impl<C: StreamableHttpClient> Worker for StreamableHttpClientWorker<C> {
                 }
                 Event::ServerMessage(mut json_rpc_message) => {
                     if let Some(response_id) = Self::server_response_id(&json_rpc_message)
-                        && let Some(stream_ct) = request_stream_cancellations.remove(response_id)
+                        && let Some(stream_ct) = crate::service::remove_pending_request(
+                            &mut request_stream_cancellations,
+                            response_id,
+                        )
                     {
                         stream_ct.cancel();
                     }
@@ -1848,4 +1857,39 @@ mod tests {
             vec!["legacy"]
         );
     }
+
+    #[cfg(feature = "transport-streamable-http-client-reqwest")]
+    #[test]
+    fn clear_stream_response_pending_accepts_stringified_numeric_id() {
+        let mut pending = HashSet::from([NumberOrString::Number(1)]);
+        let response = ServerJsonRpcMessage::response(
+            ServerResult::ListToolsResult(ListToolsResult::default()),
+            NumberOrString::String("1".into()),
+        );
+
+        StreamableHttpClientWorker::<reqwest::Client>::clear_stream_response_pending(
+            &mut pending,
+            &response,
+        );
+
+        assert!(pending.is_empty());
+    }
+
+    #[cfg(feature = "transport-streamable-http-client-reqwest")]
+    #[test]
+    fn clear_stream_response_pending_prefers_exact_string_id() {
+        let string_id = NumberOrString::String("1".into());
+        let mut pending = HashSet::from([NumberOrString::Number(1), string_id.clone()]);
+        let response = ServerJsonRpcMessage::response(
+            ServerResult::ListToolsResult(ListToolsResult::default()),
+            string_id,
+        );
+
+        StreamableHttpClientWorker::<reqwest::Client>::clear_stream_response_pending(
+            &mut pending,
+            &response,
+        );
+
+        assert_eq!(pending, HashSet::from([NumberOrString::Number(1)]));
+    }
 }

crates/rmcp/tests/test_client_initialization.rs

@@ -9,11 +9,102 @@ use common::handlers::TestClientHandler;
 use rmcp::{
     ServiceExt,
     model::{
-        ErrorCode, ErrorData, JsonRpcError, JsonRpcVersion2_0, RequestId, ServerJsonRpcMessage,
+        ClientJsonRpcMessage, ErrorCode, ErrorData, InitializeResult, JsonRpcError,
+        JsonRpcVersion2_0, RequestId, ServerCapabilities, ServerJsonRpcMessage, ServerResult,
     },
     transport::{IntoTransport, Transport},
 };
 
+fn stringify_numeric_id(id: RequestId) -> RequestId {
+    let RequestId::Number(id) = id else {
+        panic!("expected a numeric request ID");
+    };
+    RequestId::String(id.to_string().into())
+}
+
+#[tokio::test]
+async fn client_initialization_accepts_stringified_numeric_response_id() {
+    let (server_transport, client_transport) = tokio::io::duplex(1024);
+    let mut server = IntoTransport::<rmcp::RoleServer, _, _>::into_transport(server_transport);
+    let server_task = tokio::spawn(async move {
+        let ClientJsonRpcMessage::Request(request) =
+            server.receive().await.expect("expected initialize request")
+        else {
+            panic!("expected initialize request");
+        };
+        server
+            .send(ServerJsonRpcMessage::response(
+                ServerResult::InitializeResult(
+                    InitializeResult::new(ServerCapabilities::default()),
+                ),
+                stringify_numeric_id(request.id),
+            ))
+            .await
+            .expect("send initialize response");
+        assert!(matches!(
+            server.receive().await,
+            Some(ClientJsonRpcMessage::Notification(_))
+        ));
+    });
+
+    let client = TestClientHandler::new(true, true)
+        .serve(client_transport)
+        .await
+        .expect("client should accept stringified initialize response ID");
+    client.cancel().await.expect("cancel client");
+    server_task.await.expect("server task");
+}
+
+#[tokio::test]
+async fn client_correlates_stringified_numeric_response_id() {
+    let (server_transport, client_transport) = tokio::io::duplex(1024);
+    let mut server = IntoTransport::<rmcp::RoleServer, _, _>::into_transport(server_transport);
+    let server_task = tokio::spawn(async move {
+        let ClientJsonRpcMessage::Request(initialize) =
+            server.receive().await.expect("expected initialize request")
+        else {
+            panic!("expected initialize request");
+        };
+        server
+            .send(ServerJsonRpcMessage::response(
+                ServerResult::InitializeResult(
+                    InitializeResult::new(ServerCapabilities::default()),
+                ),
+                initialize.id,
+            ))
+            .await
+            .expect("send initialize response");
+        assert!(matches!(
+            server.receive().await,
+            Some(ClientJsonRpcMessage::Notification(_))
+        ));
+
+        let ClientJsonRpcMessage::Request(request) =
+            server.receive().await.expect("expected tools/list request")
+        else {
+            panic!("expected tools/list request");
+        };
+        server
+            .send(ServerJsonRpcMessage::response(
+                ServerResult::ListToolsResult(Default::default()),
+                stringify_numeric_id(request.id),
+            ))
+            .await
+            .expect("send tools/list response");
+    });
+
+    let client = TestClientHandler::new(true, true)
+        .serve(client_transport)
+        .await
+        .expect("initialize client");
+    client
+        .list_tools(None)
+        .await
+        .expect("client should correlate stringified response ID");
+    client.cancel().await.expect("cancel client");
+    server_task.await.expect("server task");
+}
+
 #[tokio::test]
 async fn test_client_init_handles_jsonrpc_error() {
     let (server_transport, client_transport) = tokio::io::duplex(1024);

crates/rmcp/tests/test_client_lifecycle_modes.rs

@@ -4,7 +4,7 @@ use rmcp::{
     ClientHandler, ClientLifecycleMode, ClientServiceExt, ServerHandler, ServiceExt,
     model::{
         ClientJsonRpcMessage, ClientRequest, DiscoverResult, ErrorCode, ErrorData, GetMeta,
-        Implementation, InitializeResult, ProtocolVersion, ServerCapabilities,
+        Implementation, InitializeResult, ProtocolVersion, RequestId, ServerCapabilities,
         ServerJsonRpcMessage, ServerResult,
     },
     service::PeerRequestOptions,
@@ -21,6 +21,45 @@ struct StatelessServer;
 
 impl ServerHandler for StatelessServer {}
 
+#[tokio::test]
+async fn discover_startup_accepts_stringified_numeric_response_id() {
+    let (server_transport, client_transport) = tokio::io::duplex(4096);
+    let mut server = IntoTransport::<rmcp::RoleServer, _, _>::into_transport(server_transport);
+    let server_task = tokio::spawn(async move {
+        let ClientJsonRpcMessage::Request(request) =
+            server.receive().await.expect("expected discover request")
+        else {
+            panic!("expected discover request");
+        };
+        let RequestId::Number(response_id) = request.id else {
+            panic!("expected a numeric request ID");
+        };
+        server
+            .send(ServerJsonRpcMessage::response(
+                ServerResult::DiscoverResult(DiscoverResult::new(
+                    vec![ProtocolVersion::V_2026_07_28],
+                    ServerCapabilities::default(),
+                    Implementation::new("discover-server", "1.0.0"),
+                )),
+                RequestId::String(response_id.to_string().into()),
+            ))
+            .await
+            .expect("send discover response");
+    });
+
+    let client = DiscoverClient
+        .serve_with_lifecycle(
+            client_transport,
+            ClientLifecycleMode::Discover {
+                preferred_versions: vec![ProtocolVersion::V_2026_07_28],
+            },
+        )
+        .await
+        .expect("client should accept stringified discover response ID");
+    client.cancel().await.expect("cancel client");
+    server_task.await.expect("server task");
+}
+
 #[tokio::test]
 async fn high_level_server_accepts_discover_startup_without_initialize() {
     let (server_transport, client_transport) = tokio::io::duplex(4096);