feat: deprecated logger in client & add Logger in ClientOption (#738)

Fix #708

1. deprecated logger in client
2. add Logger in ClientOption
3. add unit test for Logger

mcp/client.go

@@ -18,6 +18,7 @@ import (
 	"time"
 
 	"github.com/google/jsonschema-go/jsonschema"
+
 	"github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2"
 	"github.com/modelcontextprotocol/go-sdk/jsonrpc"
 )
@@ -25,8 +26,9 @@ import (
 // A Client is an MCP client, which may be connected to an MCP server
 // using the [Client.Connect] method.
 type Client struct {
-	impl                    *Implementation
+	impl *Implementation
-	opts                    ClientOptions
+	opts ClientOptions
+	// Deprecated
 	logger                  *slog.Logger // TODO: file proposal to export this
 	mu                      sync.Mutex
 	roots                   *featureSet[*Root]
@@ -42,25 +44,34 @@ type Client struct {
 // The first argument must not be nil.
 //
 // If non-nil, the provided options configure the Client.
-func NewClient(impl *Implementation, opts *ClientOptions) *Client {
+func NewClient(impl *Implementation, options *ClientOptions) *Client {
 	if impl == nil {
 		panic("nil Implementation")
 	}
-	c := &Client{
+	var opts ClientOptions
+	if options != nil {
+		opts = *options
+	}
+	options = nil // prevent reuse
+
+	if opts.Logger == nil { // ensure we have a logger
+		opts.Logger = ensureLogger(nil)
+	}
+
+	return &Client{
 		impl:                    impl,
+		opts:                    opts,
 		logger:                  ensureLogger(nil), // ensure we have a logger
 		roots:                   newFeatureSet(func(r *Root) string { return r.URI }),
 		sendingMethodHandler_:   defaultSendingMethodHandler,
 		receivingMethodHandler_: defaultReceivingMethodHandler[*ClientSession],
 	}
-	if opts != nil {
-		c.opts = *opts
-	}
-	return c
 }
 
 // ClientOptions configures the behavior of the client.
 type ClientOptions struct {
+	// Logger may be set to a non-nil value to enable logging of client activity.
+	Logger *slog.Logger
 	// CreateMessageHandler handles incoming requests for sampling/createMessage.
 	//
 	// Setting CreateMessageHandler to a non-nil value automatically causes the
@@ -407,7 +418,7 @@ func changeAndNotify[P Params](c *Client, notification string, params P, change
 		}
 	}
 	c.mu.Unlock()
-	notifySessions(sessions, notification, params, c.logger)
+	notifySessions(sessions, notification, params, c.opts.Logger)
 }
 
 // shouldSendListChangedNotification checks if the client's capabilities allow

mcp/client_test.go

@@ -7,6 +7,7 @@ package mcp
 import (
 	"context"
 	"fmt"
+	"log/slog"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -165,6 +166,23 @@ func TestClientPaginateBasic(t *testing.T) {
 	}
 }
 
+func TestClientLogger(t *testing.T) {
+	// Case 1: No logger provided
+	c1 := NewClient(&Implementation{Name: "test", Version: "1.0"}, nil)
+	if c1.opts.Logger == nil {
+		t.Error("expected default logger, got nil")
+	}
+
+	// Case 2: Logger provided
+	logger := slog.Default()
+	c2 := NewClient(&Implementation{Name: "test", Version: "1.0"}, &ClientOptions{
+		Logger: logger,
+	})
+	if c2.opts.Logger != logger {
+		t.Error("expected provided logger, got different one")
+	}
+}
+
 func TestClientPaginateVariousPageSizes(t *testing.T) {
 	ctx := context.Background()
 	for i := 1; i < len(allItems)+1; i++ {

mcp/server.go

@@ -58,7 +58,7 @@ type Server struct {
 type ServerOptions struct {
 	// Optional instructions for connected clients.
 	Instructions string
-	// If non-nil, log server activity.
+	// Logger may be set to a non-nil value to enable logging of server activity.
 	Logger *slog.Logger
 	// If non-nil, called when "notifications/initialized" is received.
 	InitializedHandler func(context.Context, *InitializedRequest)