internal/mcp: add a command transport for connecting to a sidecar

Add a missing CommandTransport that communicates with a sidecar process
over newline-delimited stdin/stdout, the counterpart to the existing
StdIOTransport.

Add a test using the usual TestMain trick.

Change-Id: I170ba78faf6b6d7fca7da1af11bf92e919b11ed5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/668895
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

mcp/cmd.go

@@ -0,0 +1,106 @@
+// Copyright 2025 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 mcp
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"os/exec"
+	"syscall"
+	"time"
+)
+
+// A CommandTransport is a [Transport] that runs a command and communicates
+// with it over stdin/stdout, using newline-delimited JSON.
+type CommandTransport struct {
+	cmd *exec.Cmd
+}
+
+// NewCommandTransport returns a [CommandTransport] that runs the given command
+// and communicates with it over stdin/stdout.
+//
+// The resulting transport takes ownership of the command, starting it during
+// [CommandTransport.Connect], and stopping it when the connection is closed.
+func NewCommandTransport(cmd *exec.Cmd) *CommandTransport {
+	return &CommandTransport{cmd}
+}
+
+// Connect starts the command, and connects to it over stdin/stdout.
+func (t *CommandTransport) Connect(ctx context.Context) (Stream, error) {
+	stdout, err := t.cmd.StdoutPipe()
+	if err != nil {
+		return nil, err
+	}
+	stdout = io.NopCloser(stdout) // close the connection by closing stdin, not stdout
+	stdin, err := t.cmd.StdinPipe()
+	if err != nil {
+		return nil, err
+	}
+	if err := t.cmd.Start(); err != nil {
+		return nil, err
+	}
+	return newIOStream(&pipeRWC{t.cmd, stdout, stdin}), nil
+}
+
+// A pipeRWC is an io.ReadWriteCloser that communicates with a subprocess over
+// stdin/stdout pipes.
+type pipeRWC struct {
+	cmd    *exec.Cmd
+	stdout io.ReadCloser
+	stdin  io.WriteCloser
+}
+
+func (s *pipeRWC) Read(p []byte) (n int, err error) {
+	return s.stdout.Read(p)
+}
+
+func (s *pipeRWC) Write(p []byte) (n int, err error) {
+	return s.stdin.Write(p)
+}
+
+// Close closes the input stream to the child process, and awaits normal
+// termination of the command. If the command does not exit, it is signalled to
+// terminate, and then eventually killed.
+func (s *pipeRWC) Close() error {
+	// Spec:
+	// "For the stdio transport, the client SHOULD initiate shutdown by:...
+
+	// "...First, closing the input stream to the child process (the server)"
+	if err := s.stdin.Close(); err != nil {
+		return fmt.Errorf("closing stdin: %v", err)
+	}
+	resChan := make(chan error, 1)
+	go func() {
+		resChan <- s.cmd.Wait()
+	}()
+	// "...Waiting for the server to exit, or sending SIGTERM if the server does not exit within a reasonable time"
+	wait := func() (error, bool) {
+		select {
+		case err := <-resChan:
+			return err, true
+		case <-time.After(5 * time.Second):
+		}
+		return nil, false
+	}
+	if err, ok := wait(); ok {
+		return err
+	}
+	// Note the condition here: if sending SIGTERM fails, don't wait and just
+	// move on to SIGKILL.
+	if err := s.cmd.Process.Signal(syscall.SIGTERM); err == nil {
+		if err, ok := wait(); ok {
+			return err
+		}
+	}
+	// "...Sending SIGKILL if the server does not exit within a reasonable time after SIGTERM"
+	if err := s.cmd.Process.Kill(); err != nil {
+		return err
+	}
+	if err, ok := wait(); ok {
+		return err
+	}
+	return fmt.Errorf("unresponsive subprocess")
+}

mcp/cmd_test.go

@@ -0,0 +1,67 @@
+// Copyright 2025 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 mcp_test
+
+import (
+	"context"
+	"log"
+	"os"
+	"os/exec"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+)
+
+const runAsServer = "_MCP_RUN_AS_SERVER"
+
+func TestMain(m *testing.M) {
+	if os.Getenv(runAsServer) != "" {
+		os.Unsetenv(runAsServer)
+		runServer()
+		return
+	}
+	os.Exit(m.Run())
+}
+
+func runServer() {
+	ctx := context.Background()
+
+	server := mcp.NewServer("greeter", "v0.0.1", nil)
+	server.AddTools(mcp.MakeTool("greet", "say hi", SayHi))
+
+	if err := server.Run(ctx, mcp.NewStdIOTransport(), nil); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func TestCmdTransport(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	exe, err := os.Executable()
+	if err != nil {
+		t.Fatal(err)
+	}
+	cmd := exec.Command(exe)
+	cmd.Env = append(os.Environ(), runAsServer+"=true")
+
+	client := mcp.NewClient("client", "v0.0.1", nil)
+	serverConn, err := client.Connect(ctx, mcp.NewCommandTransport(cmd), nil)
+	if err != nil {
+		log.Fatal(err)
+	}
+	got, err := serverConn.CallTool(ctx, "greet", SayHiParams{Name: "user"})
+	if err != nil {
+		log.Fatal(err)
+	}
+	want := []mcp.Content{mcp.TextContent{Text: "Hi user"}}
+	if diff := cmp.Diff(want, got); diff != "" {
+		t.Errorf("greet returned unexpected content (-want +got):\n%s", diff)
+	}
+	if err := serverConn.Close(); err != nil {
+		t.Fatalf("closing server: %v", err)
+	}
+}

mcp/transport.go

@@ -227,13 +227,11 @@ type rwc struct {
 }
 
 func (r rwc) Read(p []byte) (n int, err error) {
-	n, err = r.rc.Read(p)
+	return r.rc.Read(p)
-	return n, err
 }
 
 func (r rwc) Write(p []byte) (n int, err error) {
-	n, err = r.wc.Write(p)
+	return r.wc.Write(p)
-	return n, err
 }
 
 func (r rwc) Close() error {