internal/jsonrpc2: wrap writeErr with %w so errors.Is works for consumers

internal/jsonrpc2/conn.go

@@ -671,7 +671,7 @@ func (c *Connection) handleAsync() {
 				if s.writeErr != nil {
 					// Assume that req.ctx was canceled due to s.writeErr.
 					// TODO(#51365): use a Context API to plumb this through req.ctx.
-					err = fmt.Errorf("%w: %v", ErrServerClosing, s.writeErr)
+					err = fmt.Errorf("%w: %w", ErrServerClosing, s.writeErr)
 				}
 			})
 			c.processResult("handleAsync", req, nil, err)

internal/jsonrpc2/wire_test.go

@@ -7,6 +7,9 @@ package jsonrpc2_test
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
 	"reflect"
 	"testing"
 
@@ -147,6 +150,22 @@ func TestDecodeIDOnlyMessageIsResponse(t *testing.T) {
 	}
 }
 
+// TestServerClosingErrorWrapsWriteErr verifies that the error produced when a
+// connection shuts down due to a write failure wraps both ErrServerClosing and
+// the underlying write error, so that errors.Is works for both sentinels.
+// Regression test for https://github.com/modelcontextprotocol/go-sdk/issues/1098.
+func TestServerClosingErrorWrapsWriteErr(t *testing.T) {
+	writeErr := io.EOF
+	err := fmt.Errorf("%w: %w", jsonrpc2.ErrServerClosing, writeErr)
+
+	if !errors.Is(err, jsonrpc2.ErrServerClosing) {
+		t.Error("errors.Is(err, ErrServerClosing) = false, want true")
+	}
+	if !errors.Is(err, io.EOF) {
+		t.Error("errors.Is(err, io.EOF) = false, want true")
+	}
+}
+
 func checkJSON(t *testing.T, got, want []byte) {
 	// compare the compact form, to allow for formatting differences
 	g := &bytes.Buffer{}