log undeliverable late responses in processResult

internal/jsonrpc2/conn.go

@@ -718,6 +718,9 @@ func (c *Connection) processResult(from any, req *incomingRequest, result any, e
 			if err == nil {
 				err = writeErr
 			}
+			if writeErr != nil && c.onInternalError != nil {
+				c.onInternalError(fmt.Errorf("jsonrpc2: failed to write response for %q: %w", req.Method, writeErr))
+			}
 		} else {
 			err = c.internalErrorf("%#v returned a malformed result for %q: %w", from, req.Method, respErr)
 		}
@@ -728,10 +731,6 @@ func (c *Connection) processResult(from any, req *incomingRequest, result any, e
 			err = fmt.Errorf("%w: %q notification failed: %v", ErrInternal, req.Method, err)
 		}
 	}
-	if err != nil {
-		// TODO: can/should we do anything with this error beyond writing it to the event log?
-		// (Is this the right label to attach to the log?)
-	}
 
 	// Cancel the request to free any associated resources.
 	req.cancel()

internal/jsonrpc2/conn_test.go

@@ -0,0 +1,66 @@
+// Copyright 2020 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package jsonrpc2
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"testing"
+)
+
+type errWriter struct {
+	err error
+}
+
+func (w errWriter) Write(context.Context, Message) error {
+	return w.err
+}
+
+func TestProcessResultWriteFailureReportsInternalError(t *testing.T) {
+	writeErr := errors.New("write failed")
+	var internalErrors []error
+	c := &Connection{
+		done:   make(chan struct{}),
+		writer: errWriter{err: writeErr},
+		onInternalError: func(err error) {
+			internalErrors = append(internalErrors, err)
+		},
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	call, err := NewCall(StringID("1"), "test", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	req := &incomingRequest{
+		Request: call,
+		ctx:     ctx,
+		cancel:  cancel,
+	}
+
+	c.updateInFlight(func(s *inFlightState) {
+		s.incoming = 1
+		s.incomingByID = map[ID]*incomingRequest{call.ID: req}
+	})
+
+	if err := c.processResult("test", req, "ok", nil); err != nil {
+		t.Fatalf("processResult() = %v, want nil", err)
+	}
+
+	if len(internalErrors) != 1 {
+		t.Fatalf("OnInternalError calls = %d, want 1", len(internalErrors))
+	}
+	got := internalErrors[0].Error()
+	if !strings.Contains(got, "failed to write response") {
+		t.Errorf("OnInternalError = %q, want message about write failure", got)
+	}
+	if !errors.Is(internalErrors[0], writeErr) {
+		t.Errorf("OnInternalError error does not wrap write error: %v", internalErrors[0])
+	}
+}