From 52098e1554c074b010c8c447ceb2efd7aeafc52a Mon Sep 17 00:00:00 2001
From: Bo Zhang <187063395+Tethys0@users.noreply.github.com>
Date: Wed, 19 Aug 2026 10:17:57 -0500
Subject: [PATCH 1/2] jsonrpc2: wrap write error in server-closing error

The connection's "server is closing" error formatted the underlying
write error with %v, so the cause - typically io.EOF when a stdio host
closes its pipe - was not in the error chain. Callers using
errors.Is(err, io.EOF) to distinguish a clean host disconnect from a
real failure therefore always got false.

Use %w so the write error is preserved in the error chain.

Fixes #1098
---
 internal/jsonrpc2/conn.go      |  4 ++--
 internal/jsonrpc2/conn_test.go | 27 +++++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 2 deletions(-)
 create mode 100644 internal/jsonrpc2/conn_test.go

diff --git a/internal/jsonrpc2/conn.go b/internal/jsonrpc2/conn.go
index 4994c63b..03f2e6e4 100644
--- a/internal/jsonrpc2/conn.go
+++ b/internal/jsonrpc2/conn.go
@@ -163,7 +163,7 @@ func (s *inFlightState) shuttingDown(errClosing error) error {
 	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
 }
@@ -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)
diff --git a/internal/jsonrpc2/conn_test.go b/internal/jsonrpc2/conn_test.go
new file mode 100644
index 00000000..9ad094ae
--- /dev/null
+++ b/internal/jsonrpc2/conn_test.go
@@ -0,0 +1,27 @@
+// 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)
+	}
+}

From aecb26d0a2f04a56c25e6e9613c0e3b1c7e2de7d Mon Sep 17 00:00:00 2001
From: Bo Zhang <187063395+Tethys0@users.noreply.github.com>
Date: Fri, 21 Aug 2026 09:06:59 -0500
Subject: [PATCH 2/2] fix(jsonrpc2): wrap read errors on shutdown

---
 internal/jsonrpc2/conn.go      |  2 +-
 internal/jsonrpc2/conn_test.go | 11 +++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/internal/jsonrpc2/conn.go b/internal/jsonrpc2/conn.go
index 03f2e6e4..5a2253b1 100644
--- a/internal/jsonrpc2/conn.go
+++ b/internal/jsonrpc2/conn.go
@@ -158,7 +158,7 @@ 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
diff --git a/internal/jsonrpc2/conn_test.go b/internal/jsonrpc2/conn_test.go
index 9ad094ae..afeb4c02 100644
--- a/internal/jsonrpc2/conn_test.go
+++ b/internal/jsonrpc2/conn_test.go
@@ -25,3 +25,14 @@ func TestShuttingDownWrapsWriteError(t *testing.T) {
 		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)
+	}
+}