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diff --git a/src/launchselectors.h b/src/launchselectors.h
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+/*
+ * qtmaildir - a Qt6 mail client for notmuch-indexed Maildirs
+ * Copyright (C) 2026 Danilo M. <danix@danix.xyz>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#pragma once
+
+#include <QByteArray>
+#include <QMetaType>
+#include <QString>
+#include <QStringList>
+
+/// What a launch asked the window to show (item 200).
+///
+/// A value type with no Qt GUI dependency, deliberately: it is parsed before
+/// QApplication exists, it crosses a socket, and both halves need tests that
+/// no window has to be built for.
+///
+/// The three selectors COMPOSE rather than excluding each other. "Open this
+/// message, in this account's view" is one request, and nothing about it is
+/// contradictory.
+struct LaunchSelectors
+{
+    /// An account key as written in the config, e.g. `work` from
+    /// `[account.work]`. Validated against the configured accounts by the
+    /// window, not here: this unit knows nothing about a Config.
+    QString account;
+
+    /// A notmuch thread id.
+    QString threadId;
+
+    /// A Message-ID, with or without the angle brackets.
+    QString messageId;
+
+    bool isEmpty() const
+    {
+        return account.isEmpty() && threadId.isEmpty() && messageId.isEmpty();
+    }
+
+    /// Longest payload accepted off the socket.
+    ///
+    /// A local socket hands over whatever the peer sends, and the peer here is
+    /// another copy of this program running as the same user, so this is not a
+    /// defence against an attacker. It is what stops a confused or truncated
+    /// writer from being read as an unbounded selector. Three ids and an
+    /// account key are a few hundred bytes; 64 KiB is room to spare.
+    static constexpr int kMaxPayloadBytes = 64 * 1024;
+
+    /// Parses a command line, `arguments[0]` being the program name.
+    ///
+    /// Takes a QStringList rather than argc/argv so it can be called before
+    /// QCoreApplication exists, which is what lets --version keep answering on
+    /// a machine where the GUI cannot open.
+    ///
+    /// On an unknown option, returns an empty result and sets \p error. The
+    /// caller prints it; it is NOT fatal to the window, since a typo should
+    /// not cost the user their mail client.
+    static LaunchSelectors parse(const QStringList &arguments, QString *error);
+
+    /// The help text, for `--help`. Translatable prose; the option NAMES are
+    /// wire format and are never translated.
+    static QString helpText(const QString &versionDisplay);
+
+    /// Serialises for the socket. The inverse of fromPayload().
+    QByteArray toPayload() const;
+
+    /// Parses a socket payload. On anything malformed, oversized or truncated,
+    /// returns an empty result and sets \p error.
+    static LaunchSelectors fromPayload(const QByteArray &payload,
+                                       QString *error);
+};
+
+/// Declared in the header that DEFINES the type, as types.h and threaddigest.h
+/// do for theirs. A consumer declaring it instead would leave any other
+/// consumer without it, and QSignalSpy needs it to carry the type.
+Q_DECLARE_METATYPE(LaunchSelectors)