diff --git a/internal/mailbox/bot.go b/internal/mailbox/bot.go
index 8391c95..7f4e83a 100644
--- a/internal/mailbox/bot.go
+++ b/internal/mailbox/bot.go
@@ -137,7 +137,11 @@ func RunBot(ctx context.Context, c *Client, args []string, in io.Reader, out io.
 		}
 		on := true
 		if len(args) > 3 {
-			on = !strings.EqualFold(args[3], "off") && args[3] != "false" && args[3] != "0"
+			var ok bool
+			on, ok = parseBotBool(args[3])
+			if !ok {
+				return fail(fmt.Errorf("invalid %s value %q; use on/off", args[0], args[3]))
+			}
 		}
 		if strings.ToLower(args[0]) == "flag" {
 			err = c.Flag(ctx, mailbox, uid, on)
@@ -175,3 +179,14 @@ func mailboxUID(args []string) (string, uint32, error) {
 	}
 	return args[1], uint32(uid), nil
 }
+
+func parseBotBool(value string) (bool, bool) {
+	switch strings.ToLower(strings.TrimSpace(value)) {
+	case "on", "true", "1":
+		return true, true
+	case "off", "false", "0":
+		return false, true
+	default:
+		return false, false
+	}
+}
diff --git a/internal/mailbox/mailbox_test.go b/internal/mailbox/mailbox_test.go
index 45d595b..c5bdd72 100644
--- a/internal/mailbox/mailbox_test.go
+++ b/internal/mailbox/mailbox_test.go
@@ -230,3 +230,30 @@ func TestRunBotListRejectsInvalidLimit(t *testing.T) {
 		}
 	}
 }
+
+func TestRunBotFlagSeenRejectInvalidToggle(t *testing.T) {
+	tr := seeded()
+	c := paidClient(tr)
+
+	for _, args := range [][]string{
+		{"flag", Inbox, "1", "maybe"},
+		{"seen", Inbox, "1", "maybe"},
+	} {
+		var out bytes.Buffer
+		err := RunBot(context.Background(), c, args, strings.NewReader(""), &out)
+		if err == nil {
+			t.Fatalf("RunBot(%v) expected error", args)
+		}
+		if !strings.Contains(out.String(), "use on/off") {
+			t.Fatalf("RunBot(%v) output = %q, want on/off error", args, out.String())
+		}
+	}
+
+	msg, ok, err := tr.ReadMessage(context.Background(), Inbox, 1)
+	if err != nil || !ok {
+		t.Fatalf("ReadMessage: ok=%v err=%v", ok, err)
+	}
+	if msg.Flagged || msg.Seen {
+		t.Fatalf("invalid flag/seen toggles should not mutate message: %+v", msg.MessageSummary)
+	}
+}