mcp: log warnings when writing responses to disconnected clients fails

internal/jsonrpc2/conn.go

@@ -54,8 +54,9 @@ type Connection struct {
 	writer  Writer
 	handler Handler
 
-	onInternalError func(error)
-	onDone          func()
+	onInternalError      func(error)
+	onResponseWriteError func(*Request, error)
+	onDone               func()
 }
 
 // inFlightState records the state of the incoming and outgoing calls on a
@@ -219,6 +220,11 @@ type ConnectionConfig struct {
 	OnDone          func()                    // optional
 	OnInternalError func(error)               // optional
 
+	// OnResponseWriteError, if set, is called when the response to an incoming
+	// call could not be written, for example because the peer went away before
+	// the handler finished. The response is lost: the peer never receives it.
+	OnResponseWriteError func(*Request, error) // optional
+
 	// PropagateCancellation controls whether cancellation of the context
 	// passed to [NewConnection] is observable by request handlers.
 	//
@@ -244,11 +250,12 @@ func NewConnection(ctx context.Context, cfg ConnectionConfig) *Connection {
 	}
 
 	c := &Connection{
-		state:           inFlightState{closer: cfg.Closer},
-		done:            make(chan struct{}),
-		writer:          cfg.Writer,
-		onDone:          cfg.OnDone,
-		onInternalError: cfg.OnInternalError,
+		state:                inFlightState{closer: cfg.Closer},
+		done:                 make(chan struct{}),
+		writer:               cfg.Writer,
+		onDone:               cfg.OnDone,
+		onInternalError:      cfg.OnInternalError,
+		onResponseWriteError: cfg.OnResponseWriteError,
 	}
 	c.handler = cfg.Bind(c)
 	c.start(ctx, cfg.Reader, cfg.Preempter)
@@ -759,6 +766,9 @@ func (c *Connection) processResult(from any, req *incomingRequest, result any, e
 			delete(s.incomingByID, req.ID)
 		})
 		writeErr := c.write(notDone{req.ctx}, response)
+		if writeErr != nil && c.onResponseWriteError != nil {
+			c.onResponseWriteError(req.Request, writeErr)
+		}
 		if err == nil {
 			err = writeErr
 		}

mcp/transport.go

@@ -233,6 +233,9 @@ func connect[H handler, State any](ctx context.Context, t Transport, b binder[H,
 		OnInternalError: func(err error) {
 			logger.Error("jsonrpc2 internal error", "error", err)
 		},
+		OnResponseWriteError: func(req *jsonrpc2.Request, err error) {
+			logger.Warn("failed to write response", "method", req.Method, "id", req.ID.Raw(), "error", err)
+		},
 		PropagateCancellation: propagateCancellation,
 	})
 	assert(preempter.conn != nil, "unbound preempter")

mcp/transport_test.go

@@ -11,10 +11,12 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"log/slog"
 	"strings"
 	"testing"
 	"time"
 
+	"github.com/google/jsonschema-go/jsonschema"
 	"github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2"
 	"github.com/modelcontextprotocol/go-sdk/jsonrpc"
 )
@@ -132,6 +134,46 @@ func connectionClosedTests() []connectionClosedTest {
 	}
 }
 
+// TestServerLogsUndeliveredResponse is a regression test for #1147: when the
+// client goes away before a tool handler finishes, the server logs that the
+// response could not be delivered instead of dropping it silently.
+func TestServerLogsUndeliveredResponse(t *testing.T) {
+	ctx := context.Background()
+	var buf bytes.Buffer
+	server := NewServer(testImpl, &ServerOptions{
+		Logger: slog.New(slog.NewTextHandler(&buf, &slog.HandlerOptions{Level: slog.LevelWarn})),
+	})
+	started := make(chan struct{})
+	server.AddTool(&Tool{Name: "slow", InputSchema: &jsonschema.Schema{Type: "object"}},
+		func(ctx context.Context, _ *CallToolRequest) (*CallToolResult, error) {
+			close(started)
+			<-ctx.Done() // cancelled once the client's connection is gone
+			return &CallToolResult{}, nil
+		})
+	serverIn, clientOut := io.Pipe()
+	clientIn, serverOut := io.Pipe()
+	ss, err := server.Connect(ctx, &IOTransport{Reader: serverIn, Writer: serverOut}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	cs, err := NewClient(testImpl, nil).Connect(ctx, &IOTransport{Reader: clientIn, Writer: clientOut}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer cs.Close()
+	go cs.CallTool(ctx, &CallToolParams{Name: "slow"})
+	<-started
+	// The client goes away abruptly, as one that gives up on a slow call does.
+	clientOut.Close()
+	clientIn.Close()
+	ss.Wait()
+
+	got := buf.String()
+	if !strings.Contains(got, `level=WARN msg="failed to write response" method=tools/call id=`) {
+		t.Errorf("server log does not report the undelivered response:\n%s", got)
+	}
+}
+
 func checkConnectionClosedError(t *testing.T, err error, causes ...error) {
 	t.Helper()
 	for _, want := range append([]error{ErrConnectionClosed}, causes...) {