platform/telegram/internal/bot/bot.go

@@ -191,6 +191,13 @@ func (t *Bot) handleStart(ctx context.Context, api *tgbot.Bot, update *models.Up
 	if update.Message == nil {
 		return
 	}
+	// Reply only in a private chat: the Mini App launch button is an inline web_app
+	// button, which Telegram permits only in private chats — replying to a group message
+	// (the bot is an admin in the moderated chat and now receives its messages) fails with
+	// BUTTON_TYPE_INVALID. In the group the bot only manages permissions, it never chats.
+	if update.Message.Chat.Type != models.ChatTypePrivate {
+		return
+	}
 	startParam := startPayload(update.Message.Text)
 	if _, err := api.SendMessage(ctx, &tgbot.SendMessageParams{
 		ChatID:      update.Message.Chat.ID,

platform/telegram/internal/bot/bot_test.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/go-telegram/bot/models"
 	"go.uber.org/zap"
 )
 
@@ -103,6 +104,29 @@ func TestTestEnvironmentRoutesGetMe(t *testing.T) {
 	}
 }
 
+func TestHandleStartRepliesPrivateOnly(t *testing.T) {
+	t.Run("private replies", func(t *testing.T) {
+		api := &fakeBotAPI{}
+		b := newTestBot(t, api)
+		b.handleStart(context.Background(), b.api, &models.Update{Message: &models.Message{
+			Chat: models.Chat{ID: 42, Type: models.ChatTypePrivate}, Text: "/start g7",
+		}})
+		if api.chatID != "42" || !strings.Contains(api.replyMarkup, "web_app") {
+			t.Errorf("private /start: chat=%q markup=%q, want a web_app reply", api.chatID, api.replyMarkup)
+		}
+	})
+	t.Run("group ignored", func(t *testing.T) {
+		api := &fakeBotAPI{}
+		b := newTestBot(t, api)
+		b.handleStart(context.Background(), b.api, &models.Update{Message: &models.Message{
+			Chat: models.Chat{ID: -100, Type: models.ChatTypeSupergroup}, Text: "/start",
+		}})
+		if api.chatID != "" {
+			t.Errorf("group /start got a reply (chat=%q); an inline web_app button is invalid in groups", api.chatID)
+		}
+	})
+}
+
 func TestStartPayload(t *testing.T) {
 	cases := map[string]string{
 		"/start g123":  "g123",

platform/telegram/internal/promobot/promobot.go

@@ -92,6 +92,11 @@ func (t *Bot) handleStart(ctx context.Context, api *tgbot.Bot, update *models.Up
 	if update.Message == nil {
 		return
 	}
+	// Only respond to a private /start: the promo bot is a one-on-one onboarding entry
+	// point and should never reply to group messages.
+	if update.Message.Chat.Type != models.ChatTypePrivate {
+		return
+	}
 	if err := t.throttle(ctx); err != nil {
 		return
 	}

platform/telegram/internal/promobot/promobot_test.go

@@ -85,7 +85,7 @@ func TestHandleStartReplies(t *testing.T) {
 	}
 
 	b.handleStart(context.Background(), b.api, &models.Update{Message: &models.Message{
-		Chat: models.Chat{ID: 42},
+		Chat: models.Chat{ID: 42, Type: models.ChatTypePrivate},
 		From: &models.User{LanguageCode: "ru"},
 		Text: "/start f99",
 	}})
@@ -103,3 +103,19 @@ func TestHandleStartReplies(t *testing.T) {
 		t.Errorf("reply_markup = %q, want startapp=f99 (the /start payload forwarded)", api.replyMarkup)
 	}
 }
+
+func TestHandleStartIgnoresGroup(t *testing.T) {
+	api := &fakeAPI{}
+	srv := httptest.NewServer(api)
+	t.Cleanup(srv.Close)
+	b, err := New(Config{Token: "1:2", APIBaseURL: srv.URL, BotUsername: "B", BotLinkURL: "https://t.me/b/a"}, zap.NewNop())
+	if err != nil {
+		t.Fatalf("new: %v", err)
+	}
+	b.handleStart(context.Background(), b.api, &models.Update{Message: &models.Message{
+		Chat: models.Chat{ID: -100, Type: models.ChatTypeSupergroup}, Text: "/start",
+	}})
+	if api.chatID != "" {
+		t.Errorf("replied to a group message (chat=%q); want none", api.chatID)
+	}
+}