jsonrpc2: wrap write error in server-closing error (#1186)

The "server is closing" error produced when a connection shuts down
because its write side failed formatted the underlying write error with
`%v`, so the cause - typically `io.EOF` when a stdio host closes its
pipe - was not part of the error chain. A caller that wants to
distinguish a clean host disconnect from a real failure via
`errors.Is(err, io.EOF)` therefore always got `false`.

This changes both `s.writeErr` wrappings to use `%w`, preserving the
write error in the error chain while keeping the same message text
("server is closing: EOF").

A regression test exercises the write-failure branch of `shuttingDown`
and asserts both `errors.Is(err, ErrServerClosing)` and `errors.Is(err,
io.EOF)`.

Fixes #1098

---------

Co-authored-by: Bo Zhang <187063395+Tethys0@users.noreply.github.com>
Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>

internal/jsonrpc2/conn.go

@@ -158,12 +158,12 @@ func (s *inFlightState) shuttingDown(errClosing error) error {
 	if s.readErr != nil {
 		// If the read side of the connection is broken, we cannot read new call
 		// requests, and cannot read responses to our outgoing calls.
-		return fmt.Errorf("%w: %v", errClosing, s.readErr)
+		return fmt.Errorf("%w: %w", errClosing, s.readErr)
 	}
 	if s.writeErr != nil {
 		// If the write side of the connection is broken, we cannot write responses
 		// for incoming calls, and cannot write requests for outgoing calls.
-		return fmt.Errorf("%w: %v", errClosing, s.writeErr)
+		return fmt.Errorf("%w: %w", errClosing, s.writeErr)
 	}
 	return nil
 }

internal/jsonrpc2/conn_test.go

@@ -0,0 +1,38 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by the license
+// that can be found in the LICENSE file.
+
+package jsonrpc2
+
+import (
+	"errors"
+	"io"
+	"testing"
+)
+
+// TestShuttingDownWrapsWriteError verifies that when a connection shuts down
+// because its write side failed, the returned error preserves the underlying
+// write error in its chain so callers can classify it with errors.Is (for
+// example, distinguishing io.EOF from a clean host disconnect versus a real
+// failure).
+func TestShuttingDownWrapsWriteError(t *testing.T) {
+	s := &inFlightState{writeErr: io.EOF}
+	err := s.shuttingDown(ErrServerClosing)
+	if !errors.Is(err, ErrServerClosing) {
+		t.Errorf("shuttingDown() error = %v, want it to wrap ErrServerClosing", err)
+	}
+	if !errors.Is(err, io.EOF) {
+		t.Errorf("shuttingDown() error = %v, want it to wrap io.EOF", err)
+	}
+}
+
+func TestShuttingDownWrapsReadError(t *testing.T) {
+	s := &inFlightState{readErr: io.EOF}
+	err := s.shuttingDown(ErrServerClosing)
+	if !errors.Is(err, ErrServerClosing) {
+		t.Errorf("shuttingDown() error = %v, want it to wrap ErrServerClosing", err)
+	}
+	if !errors.Is(err, io.EOF) {
+		t.Errorf("shuttingDown() error = %v, want it to wrap io.EOF", err)
+	}
+}