Add errors_returned counter to MCP server status

Track the number of error responses returned by the MCP server,
providing better visibility into server health and debugging.

Changes:
- Add errors_returned atomic counter to MCPServer class
- Increment counter when response.IsError() in ProcessOneMessage
  and HandleConnection
- Add errors_returned to MCPStatus struct type
- Include in GetStatus() output
- Update CreateMCPStatus helper function
- Update tests with new field in expected output

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: teaguesterling

src/duckdb_mcp_extension.cpp

@@ -462,14 +462,16 @@ static LogicalType GetMCPStatusType() {
     members.push_back({"background", LogicalType::BOOLEAN});
     members.push_back({"requests_received", LogicalType::UBIGINT});
     members.push_back({"responses_sent", LogicalType::UBIGINT});
+    members.push_back({"errors_returned", LogicalType::UBIGINT});
     return LogicalType::STRUCT(members);
 }
 
 // Helper to create MCPStatus struct value
 static Value CreateMCPStatus(bool success, bool running, const string &message,
                               const string &transport = "", const string &listen = "",
                               int port = 0, bool background = false,
-                              uint64_t requests_received = 0, uint64_t responses_sent = 0) {
+                              uint64_t requests_received = 0, uint64_t responses_sent = 0,
+                              uint64_t errors_returned = 0) {
     child_list_t<Value> values;
     values.push_back({"success", Value::BOOLEAN(success)});
     values.push_back({"running", Value::BOOLEAN(running)});
@@ -480,6 +482,7 @@ static Value CreateMCPStatus(bool success, bool running, const string &message,
     values.push_back({"background", Value::BOOLEAN(background)});
     values.push_back({"requests_received", Value::UBIGINT(requests_received)});
     values.push_back({"responses_sent", Value::UBIGINT(responses_sent)});
+    values.push_back({"errors_returned", Value::UBIGINT(errors_returned)});
     return Value::STRUCT(values);
 }
 
@@ -578,11 +581,13 @@ static Value MCPServerStartImpl(ExpressionState &state,
                     server.RunMainLoop(); // Blocks until max_requests or shutdown
                     return CreateMCPStatus(true, false, "MCP server completed",
                                            transport, bind_address, port, false,
-                                           server.GetRequestsReceived(), server.GetResponsesSent());
+                                           server.GetRequestsReceived(), server.GetResponsesSent(),
+                                           server.GetErrorsReturned());
                 } catch (const std::exception &e) {
                     return CreateMCPStatus(false, false, string(e.what()),
                                            transport, bind_address, port, false,
-                                           server.GetRequestsReceived(), server.GetResponsesSent());
+                                           server.GetRequestsReceived(), server.GetResponsesSent(),
+                                           server.GetErrorsReturned());
                 }
             }
         } else if (transport == "memory") {
@@ -723,7 +728,8 @@ static void MCPServerStatusFunction(DataChunk &args, ExpressionState &state, Vec
                 result.SetValue(i, CreateMCPStatus(true, true, server->GetStatus(),
                                                    "", "", 0, true,
                                                    server->GetRequestsReceived(),
-                                                   server->GetResponsesSent()));
+                                                   server->GetResponsesSent(),
+                                                   server->GetErrorsReturned()));
             } else {
                 result.SetValue(i, CreateMCPStatus(false, false, "Server manager inconsistency"));
             }

src/include/server/mcp_server.hpp

@@ -91,6 +91,7 @@ class MCPServer {
     time_t GetUptime() const;
     uint64_t GetRequestsReceived() const { return requests_received.load(); }
     uint64_t GetResponsesSent() const { return responses_sent.load(); }
+    uint64_t GetErrorsReturned() const { return errors_returned.load(); }
 
     // Resource management
     bool PublishResource(const string &uri, unique_ptr<ResourceProvider> provider);
@@ -121,6 +122,7 @@ class MCPServer {
     atomic<uint32_t> active_connections;
     atomic<uint64_t> requests_received;
     atomic<uint64_t> responses_sent;
+    atomic<uint64_t> errors_returned;
     time_t start_time;
 
     ResourceRegistry resource_registry;

src/server/mcp_server.cpp

@@ -85,7 +85,7 @@ bool ToolRegistry::ToolExists(const string &name) const {
 //===--------------------------------------------------------------------===//
 
 MCPServer::MCPServer(const MCPServerConfig &config)
-    : config(config), running(false), active_connections(0), requests_received(0), responses_sent(0) {
+    : config(config), running(false), active_connections(0), requests_received(0), responses_sent(0), errors_returned(0) {
     start_time = time(nullptr);
 }
 
@@ -179,6 +179,7 @@ string MCPServer::GetStatus() const {
     status += "Connections: " + std::to_string(active_connections.load()) + "\n";
     status += "Requests Received: " + std::to_string(requests_received.load()) + "\n";
     status += "Responses Sent: " + std::to_string(responses_sent.load()) + "\n";
+    status += "Errors Returned: " + std::to_string(errors_returned.load()) + "\n";
     status += "Uptime: " + std::to_string(GetUptime()) + " seconds\n";
     status += "Resources: " + std::to_string(resource_registry.ListResources().size()) + "\n";
     status += "Tools: " + std::to_string(tool_registry.ListTools().size());
@@ -271,6 +272,9 @@ bool MCPServer::ProcessOneMessage() {
         auto response = HandleRequest(request);
         test_transport->Send(response);
         responses_sent.fetch_add(1);
+        if (response.IsError()) {
+            errors_returned.fetch_add(1);
+        }
         return true;
     } catch (const std::exception &) {
         return false;
@@ -288,6 +292,9 @@ void MCPServer::HandleConnection(unique_ptr<MCPTransport> transport) {
                 auto response = HandleRequest(request);
                 transport->Send(response);
                 responses_sent.fetch_add(1);
+                if (response.IsError()) {
+                    errors_returned.fetch_add(1);
+                }
 
                 // If this was a shutdown request, break out of the loop
                 if (request.method == MCPMethods::SHUTDOWN) {

test/sql/mcp_memory_transport.test

@@ -23,7 +23,7 @@ SELECT mcp_server_stop(true);
 query I
 SELECT mcp_server_start('memory');
 ----
-{'success': true, 'running': true, 'message': 'MCP server started on memory transport (background mode)', 'transport': memory, 'listen': localhost, 'port': 0, 'background': true, 'requests_received': 0, 'responses_sent': 0}
+{'success': true, 'running': true, 'message': 'MCP server started on memory transport (background mode)', 'transport': memory, 'listen': localhost, 'port': 0, 'background': true, 'requests_received': 0, 'responses_sent': 0, 'errors_returned': 0}
 
 # Test initialize handshake - verify MCP spec compliance
 # See: https://modelcontextprotocol.io/specification/2024-11-05/basic/lifecycle

test/sql/mcp_server_lifecycle.test

@@ -10,36 +10,36 @@ require duckdb_mcp
 query I
 SELECT mcp_server_stop(true);
 ----
-{'success': true, 'running': false, 'message': 'MCP server state cleared (forced)', 'transport': '', 'listen': '', 'port': 0, 'background': false, 'requests_received': 0, 'responses_sent': 0}
+{'success': true, 'running': false, 'message': 'MCP server state cleared (forced)', 'transport': '', 'listen': '', 'port': 0, 'background': false, 'requests_received': 0, 'responses_sent': 0, 'errors_returned': 0}
 
 # ===================================
 # Test 2: Check status shows stopped
 # ===================================
 query I
 SELECT mcp_server_status();
 ----
-{'success': true, 'running': false, 'message': Server is stopped, 'transport': '', 'listen': '', 'port': 0, 'background': false, 'requests_received': 0, 'responses_sent': 0}
+{'success': true, 'running': false, 'message': Server is stopped, 'transport': '', 'listen': '', 'port': 0, 'background': false, 'requests_received': 0, 'responses_sent': 0, 'errors_returned': 0}
 
 # ===================================
 # Test 3: Start server with memory transport
 # ===================================
 query I
 SELECT mcp_server_start('memory');
 ----
-{'success': true, 'running': true, 'message': 'MCP server started on memory transport (background mode)', 'transport': memory, 'listen': localhost, 'port': 0, 'background': true, 'requests_received': 0, 'responses_sent': 0}
+{'success': true, 'running': true, 'message': 'MCP server started on memory transport (background mode)', 'transport': memory, 'listen': localhost, 'port': 0, 'background': true, 'requests_received': 0, 'responses_sent': 0, 'errors_returned': 0}
 
 # ===================================
 # Test 4: Stop server
 # ===================================
 query I
 SELECT mcp_server_stop();
 ----
-{'success': true, 'running': false, 'message': MCP server stopped, 'transport': '', 'listen': '', 'port': 0, 'background': false, 'requests_received': 0, 'responses_sent': 0}
+{'success': true, 'running': false, 'message': MCP server stopped, 'transport': '', 'listen': '', 'port': 0, 'background': false, 'requests_received': 0, 'responses_sent': 0, 'errors_returned': 0}
 
 # ===================================
 # Test 5: Final cleanup
 # ===================================
 query I
 SELECT mcp_server_stop(true);
 ----
-{'success': true, 'running': false, 'message': 'MCP server state cleared (forced)', 'transport': '', 'listen': '', 'port': 0, 'background': false, 'requests_received': 0, 'responses_sent': 0}
+{'success': true, 'running': false, 'message': 'MCP server state cleared (forced)', 'transport': '', 'listen': '', 'port': 0, 'background': false, 'requests_received': 0, 'responses_sent': 0, 'errors_returned': 0}