fix: report pre-init metadata errors

crates/rmcp/src/service/server.rs

@@ -483,6 +483,16 @@ pub(crate) fn negotiate_protocol_version(
     }
 }
 
+fn missing_request_metadata_error(missing: &[&str]) -> ErrorData {
+    ErrorData::invalid_params(
+        format!(
+            "request _meta is missing or has malformed required fields: {}",
+            missing.join(", ")
+        ),
+        None,
+    )
+}
+
 async fn serve_server_with_ct_inner<S, T>(
     service: S,
     transport: T,
@@ -528,11 +538,22 @@ where
     let initialize_request = match request {
         ClientRequest::InitializeRequest(request) => request,
         mut request => {
-            if !request
+            let missing_metadata = request
                 .get_meta()
-                .missing_required_keys(&ProtocolVersion::V_2026_07_28)
-                .is_empty()
-            {
+                .missing_required_keys(&ProtocolVersion::V_2026_07_28);
+            if !missing_metadata.is_empty() {
+                transport
+                    .send(ServerJsonRpcMessage::error(
+                        missing_request_metadata_error(&missing_metadata),
+                        Some(id.clone()),
+                    ))
+                    .await
+                    .map_err(|error| {
+                        ServerInitializeError::transport::<T>(
+                            error,
+                            "sending pre-init metadata error response",
+                        )
+                    })?;
                 return Err(ServerInitializeError::ExpectedInitializeRequest(Some(
                     ClientJsonRpcMessage::request(request, id),
                 )));

crates/rmcp/tests/test_stateless_server_requests.rs

@@ -162,7 +162,7 @@ async fn stateless_server_uses_each_requests_client_context() {
 }
 
 #[tokio::test]
-async fn stateless_server_rejects_malformed_metadata_opener() {
+async fn stateless_server_rejects_malformed_metadata_opener_with_error_response() {
     let (server_transport, client_transport) = tokio::io::duplex(4096);
     let server_task = tokio::spawn(async move { StatelessServer.serve(server_transport).await });
     let mut client = IntoTransport::<rmcp::RoleClient, _, _>::into_transport(client_transport);
@@ -186,6 +186,24 @@ async fn stateless_server_rejects_malformed_metadata_opener() {
         ))
         .await
         .expect("send list tools");
+    let Some(ServerJsonRpcMessage::Error(error)) = client.receive().await else {
+        panic!("expected invalid params");
+    };
+    assert_eq!(error.id, Some(RequestId::Number(1)));
+    assert_eq!(error.error.code, ErrorCode::INVALID_PARAMS);
+    assert!(
+        error
+            .error
+            .message
+            .contains("request _meta is missing or has malformed required fields")
+    );
+    assert!(
+        error
+            .error
+            .message
+            .contains("io.modelcontextprotocol/clientCapabilities")
+    );
+
     let Err(error) = server_task.await.expect("server task") else {
         panic!("malformed opener should not start a session");
     };