mcp/server: enable server logging (#501)

mcp/server.go: implement server‑side logging

- Thread a *slog.Logger through server paths to improve debuggability
- Introduce no‑op discard handler + ensureLogger so loggers are never
nil
- Use ensured loggers in Server
- address todo comments related to adding logging support

All tests are passing and no functionality should change with this PR

Fixes https://github.com/modelcontextprotocol/go-sdk/issues/170

mcp/streamable.go

@@ -68,6 +68,10 @@ type StreamableHTTPOptions struct {
 	//
 	// [§2.1.5]: https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#sending-messages-to-the-server
 	JSONResponse bool
+
+	// Logger specifies the logger to use.
+	// If nil, do not log.
+	Logger *slog.Logger
 }
 
 // NewStreamableHTTPHandler returns a new [StreamableHTTPHandler].
@@ -83,6 +87,11 @@ func NewStreamableHTTPHandler(getServer func(*http.Request) *Server, opts *Strea
 	if opts != nil {
 		h.opts = *opts
 	}
+
+	if h.opts.Logger == nil { // ensure we have a logger
+		h.opts.Logger = ensureLogger(nil)
+	}
+
 	return h
 }
 
@@ -368,6 +377,8 @@ type StreamableServerTransport struct {
 	// StreamableHTTPOptions.JSONResponse is exported.
 	jsonResponse bool
 
+	logger *slog.Logger
+
 	// connection is non-nil if and only if the transport has been connected.
 	connection *streamableServerConn
 }
@@ -382,6 +393,7 @@ func (t *StreamableServerTransport) Connect(ctx context.Context) (Connection, er
 		stateless:      t.Stateless,
 		eventStore:     t.EventStore,
 		jsonResponse:   t.jsonResponse,
+		logger:         t.logger,
 		incoming:       make(chan jsonrpc.Message, 10),
 		done:           make(chan struct{}),
 		streams:        make(map[string]*stream),
@@ -408,6 +420,8 @@ type streamableServerConn struct {
 	jsonResponse bool
 	eventStore   EventStore
 
+	logger *slog.Logger
+
 	incoming chan jsonrpc.Message // messages from the client to the server
 
 	mu sync.Mutex // guards all fields below
@@ -755,7 +769,7 @@ func (c *streamableServerConn) respondSSE(stream *stream, w http.ResponseWriter,
 		}
 		if _, err := writeEvent(w, e); err != nil {
 			// Connection closed or broken.
-			// TODO(#170): log when we add server-side logging.
+			c.logger.Warn("error writing event", "error", err)
 			return false
 		}
 		writes++
@@ -774,7 +788,13 @@ func (c *streamableServerConn) respondSSE(stream *stream, w http.ResponseWriter,
 				// simplify.
 				http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
 			} else {
-				// TODO(#170): log when we add server-side logging
+				if ctx.Err() != nil {
+					// Client disconnected or cancelled the request.
+					c.logger.Error("stream context done", "error", ctx.Err())
+				} else {
+					// Some other error.
+					c.logger.Error("error receiving message", "error", err)
+				}
 			}
 			return
 		}