cmd/go: set a http user agent

go.dev/issue/35699 was a declined proposal to add the go toolchain version
as the user agent on the grounds of privacy.
However, it'd still be useful to identify cmd/go from the sea
of other web clients written in Go with a fixed string user agent.

Updates #35699
Fixes #78891

Change-Id: I872ce567995bdf3f9e968e7aa80e227e6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/769500
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>
Auto-Submit: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/cmd/go/internal/web/http.go

@@ -31,6 +31,8 @@ import (
 	"cmd/internal/browser"
 )
 
+const userAgent = "GoCommand/1 (+https://go.dev/cmd/go)"
+
 // impatientInsecureHTTPClient is used with GOINSECURE,
 // when we're connecting to https servers that might not be there
 // or might be using self-signed certificates.
@@ -146,6 +148,7 @@ func get(security SecurityMode, url *urlpkg.URL) (*Response, error) {
 			req.Host = req.URL.Host
 			req.URL.Host = t.ToHost
 		}
+		req.Header.Set("User-Agent", userAgent)
 
 		release, err := base.AcquireNet()
 		if err != nil {

src/cmd/go/internal/web/http_test.go

@@ -0,0 +1,38 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package web
+
+import (
+	"bytes"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"testing"
+)
+
+func TestUserAgent(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Write([]byte(r.UserAgent()))
+	}))
+	defer ts.Close()
+
+	u, err := url.Parse(ts.URL)
+	if err != nil {
+		t.Fatal("parse httptest url:", err)
+	}
+	res, err := Get(Insecure, u)
+	if err != nil {
+		t.Error("http get:", err)
+	}
+	b, err := io.ReadAll(res.Body)
+	if err != nil {
+		t.Error("read response body:", err)
+	}
+	gotUserAgent := string(bytes.TrimSpace(b))
+	if gotUserAgent != userAgent {
+		t.Errorf("User-Agent: %s, want %s", gotUserAgent, userAgent)
+	}
+}