Missing logger customization

[adalisk-emikhaylov]

Currently the logger is hardcoded to print to std::cerr. It would be great to be able to have it run a callback instead.

[Adriatic1]

Came here to report the exact same issue :)
I solved my issue with the code below, feel free to include this into the official project:

--- a/ext/cpp-mcp/include/mcp_logger.h
+++ b/ext/cpp-mcp/include/mcp_logger.h
@@ -12,6 +12,7 @@
 #include <mutex>
 #include <chrono>
 #include <iomanip>
+#include <optional>

 namespace mcp {

@@ -22,6 +23,8 @@ enum class log_level {
     error
 };

+typedef void (*LOG_SINK_CB)(log_level, const char*);
+
 class logger {
 public:
     static logger& instance() {
@@ -34,6 +37,11 @@ public:
         level_ = level;
     }

+    void set_sink(LOG_SINK_CB fn) {
+        std::lock_guard<std::mutex> lock(mutex_);
+        sink_ = fn;
+    }
+
     template<typename... Args>
     void debug(Args&&... args) {
         log(log_level::debug, std::forward<Args>(args)...);
@@ -75,28 +83,30 @@ private:
         }

         std::stringstream ss;
-
-        // Add timestamp
-        auto now = std::chrono::system_clock::now();
-        auto now_c = std::chrono::system_clock::to_time_t(now);
-        auto now_tm = std::localtime(&now_c);
-
-        ss << std::put_time(now_tm, "%Y-%m-%d %H:%M:%S") << " ";
-
-        // Add log level and color
-        switch (level) {
-            case log_level::debug:
-                ss << "\033[36m[DEBUG]\033[0m ";  // Cyan
-                break;
-            case log_level::info:
-                ss << "\033[32m[INFO]\033[0m ";   // Green
-                break;
-            case log_level::warning:
-                ss << "\033[33m[WARNING]\033[0m "; // Yellow
-                break;
-            case log_level::error:
-                ss << "\033[31m[ERROR]\033[0m ";   // Red
-                break;
+
+        if(!sink_) {
+            // Add timestamp
+            auto now = std::chrono::system_clock::now();
+            auto now_c = std::chrono::system_clock::to_time_t(now);
+            auto now_tm = std::localtime(&now_c);
+
+            ss << std::put_time(now_tm, "%Y-%m-%d %H:%M:%S") << " ";
+
+            // Add log level and color
+            switch (level) {
+                case log_level::debug:
+                    ss << "\033[36m[DEBUG]\033[0m ";  // Cyan
+                    break;
+                case log_level::info:
+                    ss << "\033[32m[INFO]\033[0m ";   // Green
+                    break;
+                case log_level::warning:
+                    ss << "\033[33m[WARNING]\033[0m "; // Yellow
+                    break;
+                case log_level::error:
+                    ss << "\033[31m[ERROR]\033[0m ";   // Red
+                    break;
+            }
         }

         // Add log content
@@ -104,9 +114,14 @@ private:

         // Output log
         std::lock_guard<std::mutex> lock(mutex_);
-        std::cerr << ss.str() << std::endl;
+        if(sink_) {
+            (*sink_)(level, ss.str().c_str());
+        } else {
+            std::cerr << ss.str() << std::endl;
+        }
     }

+    std::optional<LOG_SINK_CB> sink_;
     log_level level_;
     std::mutex mutex_;
 };
@@ -120,6 +135,10 @@ inline void set_log_level(log_level level) {
     mcp::logger::instance().set_level(level);
 }

+inline void set_log_callback(LOG_SINK_CB fn) {
+    mcp::logger::instance().set_sink(fn);
+}
+
 } // namespace mcp