6 files changed, 861 insertions, 0 deletions
diff --git a/meshmc/launcher/minecraft/legacy/LegacyInstance.cpp b/meshmc/launcher/minecraft/legacy/LegacyInstance.cpp
new file mode 100644
index 0000000000..c010974481
--- /dev/null
+++ b/meshmc/launcher/minecraft/legacy/LegacyInstance.cpp
@@ -0,0 +1,270 @@
+/* SPDX-FileCopyrightText: 2026 Project Tick
+ * SPDX-FileContributor: Project Tick
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ *   MeshMC - A Custom Launcher for Minecraft
+ *   Copyright (C) 2026 Project Tick
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   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, see <https://www.gnu.org/licenses/>.
+ *
+ *  This file incorporates work covered by the following copyright and
+ *  permission notice:
+ *
+ * Copyright 2013-2021 MultiMC Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <QFileInfo>
+#include <minecraft/launch/MeshMCPartLaunch.h>
+#include <QDir>
+#include <settings/Setting.h>
+
+#include "LegacyInstance.h"
+
+#include "minecraft/legacy/LegacyModList.h"
+#include "minecraft/WorldList.h"
+#include <MMCZip.h>
+#include <FileSystem.h>
+
+LegacyInstance::LegacyInstance(SettingsObjectPtr globalSettings,
+							   SettingsObjectPtr settings,
+							   const QString& rootDir)
+	: BaseInstance(globalSettings, settings, rootDir)
+{
+	settings->registerSetting("NeedsRebuild", true);
+	settings->registerSetting("ShouldUpdate", false);
+	settings->registerSetting("JarVersion", QString());
+	settings->registerSetting("IntendedJarVersion", QString());
+	/*
+	 * custom base jar has no default. it is determined in code... see the
+	 * accessor methods for it
+	 *
+	 * for instances that DO NOT have the CustomBaseJar setting (legacy
+	 * instances),
+	 * [.]minecraft/bin/mcbackup.jar is the default base jar
+	 */
+	settings->registerSetting("UseCustomBaseJar", true);
+	settings->registerSetting("CustomBaseJar", "");
+}
+
+QString LegacyInstance::mainJarToPreserve() const
+{
+	bool customJar = m_settings->get("UseCustomBaseJar").toBool();
+	if (customJar) {
+		auto base = baseJar();
+		if (QFile::exists(base)) {
+			return base;
+		}
+	}
+	auto runnable = runnableJar();
+	if (QFile::exists(runnable)) {
+		return runnable;
+	}
+	return QString();
+}
+
+QString LegacyInstance::baseJar() const
+{
+	bool customJar = m_settings->get("UseCustomBaseJar").toBool();
+	if (customJar) {
+		return customBaseJar();
+	} else
+		return defaultBaseJar();
+}
+
+QString LegacyInstance::customBaseJar() const
+{
+	QString value = m_settings->get("CustomBaseJar").toString();
+	if (value.isNull() || value.isEmpty()) {
+		return defaultCustomBaseJar();
+	}
+	return value;
+}
+
+bool LegacyInstance::shouldUseCustomBaseJar() const
+{
+	return m_settings->get("UseCustomBaseJar").toBool();
+}
+
+Task::Ptr LegacyInstance::createUpdateTask(Net::Mode)
+{
+	return nullptr;
+}
+
+std::shared_ptr<LegacyModList> LegacyInstance::jarModList() const
+{
+	if (!jar_mod_list) {
+		auto list = new LegacyModList(jarModsDir(), modListFile());
+		jar_mod_list.reset(list);
+	}
+	jar_mod_list->update();
+	return jar_mod_list;
+}
+
+QString LegacyInstance::gameRoot() const
+{
+	QFileInfo mcDir(FS::PathCombine(instanceRoot(), "minecraft"));
+	QFileInfo dotMCDir(FS::PathCombine(instanceRoot(), ".minecraft"));
+
+	if (mcDir.exists() && !dotMCDir.exists())
+		return mcDir.filePath();
+	else
+		return dotMCDir.filePath();
+}
+
+QString LegacyInstance::binRoot() const
+{
+	return FS::PathCombine(gameRoot(), "bin");
+}
+
+QString LegacyInstance::modsRoot() const
+{
+	return FS::PathCombine(gameRoot(), "mods");
+}
+
+QString LegacyInstance::jarModsDir() const
+{
+	return FS::PathCombine(instanceRoot(), "instMods");
+}
+
+QString LegacyInstance::libDir() const
+{
+	return FS::PathCombine(gameRoot(), "lib");
+}
+
+QString LegacyInstance::savesDir() const
+{
+	return FS::PathCombine(gameRoot(), "saves");
+}
+
+QString LegacyInstance::coreModsDir() const
+{
+	return FS::PathCombine(gameRoot(), "coremods");
+}
+
+QString LegacyInstance::resourceDir() const
+{
+	return FS::PathCombine(gameRoot(), "resources");
+}
+QString LegacyInstance::texturePacksDir() const
+{
+	return FS::PathCombine(gameRoot(), "texturepacks");
+}
+
+QString LegacyInstance::runnableJar() const
+{
+	return FS::PathCombine(binRoot(), "minecraft.jar");
+}
+
+QString LegacyInstance::modListFile() const
+{
+	return FS::PathCombine(instanceRoot(), "modlist");
+}
+
+QString LegacyInstance::instanceConfigFolder() const
+{
+	return FS::PathCombine(gameRoot(), "config");
+}
+
+bool LegacyInstance::shouldRebuild() const
+{
+	return m_settings->get("NeedsRebuild").toBool();
+}
+
+QString LegacyInstance::currentVersionId() const
+{
+	return m_settings->get("JarVersion").toString();
+}
+
+QString LegacyInstance::intendedVersionId() const
+{
+	return m_settings->get("IntendedJarVersion").toString();
+}
+
+bool LegacyInstance::shouldUpdate() const
+{
+	QVariant var = settings()->get("ShouldUpdate");
+	if (!var.isValid() || var.toBool() == false) {
+		return intendedVersionId() != currentVersionId();
+	}
+	return true;
+}
+
+QString LegacyInstance::defaultBaseJar() const
+{
+	return "versions/" + intendedVersionId() + "/" + intendedVersionId() +
+		   ".jar";
+}
+
+QString LegacyInstance::defaultCustomBaseJar() const
+{
+	return FS::PathCombine(binRoot(), "mcbackup.jar");
+}
+
+std::shared_ptr<WorldList> LegacyInstance::worldList() const
+{
+	if (!m_world_list) {
+		m_world_list.reset(new WorldList(savesDir()));
+	}
+	return m_world_list;
+}
+
+QString LegacyInstance::typeName() const
+{
+	return tr("Legacy");
+}
+
+QString LegacyInstance::getStatusbarDescription()
+{
+	return tr("Instance from previous versions.");
+}
+
+QStringList
+LegacyInstance::verboseDescription(AuthSessionPtr session,
+								   MinecraftServerTargetPtr serverToJoin)
+{
+	QStringList out;
+
+	auto alltraits = traits();
+	if (alltraits.size()) {
+		out << "Traits:";
+		for (auto trait : alltraits) {
+			out << "  " + trait;
+		}
+		out << "";
+	}
+
+	QString windowParams;
+	if (settings()->get("LaunchMaximized").toBool()) {
+		out << "Window size: max (if available)";
+	} else {
+		auto width = settings()->get("MinecraftWinWidth").toInt();
+		auto height = settings()->get("MinecraftWinHeight").toInt();
+		out << "Window size: " + QString::number(width) + " x " +
+				   QString::number(height);
+	}
+	out << "";
+	return out;
+}
diff --git a/meshmc/launcher/minecraft/legacy/LegacyInstance.h b/meshmc/launcher/minecraft/legacy/LegacyInstance.h
new file mode 100644
index 0000000000..6edcee96db
--- /dev/null
+++ b/meshmc/launcher/minecraft/legacy/LegacyInstance.h
@@ -0,0 +1,172 @@
+/* SPDX-FileCopyrightText: 2026 Project Tick
+ * SPDX-FileContributor: Project Tick
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ *   MeshMC - A Custom Launcher for Minecraft
+ *   Copyright (C) 2026 Project Tick
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   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, see <https://www.gnu.org/licenses/>.
+ *
+ *  This file incorporates work covered by the following copyright and
+ *  permission notice:
+ *
+ * Copyright 2013-2021 MultiMC Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include "BaseInstance.h"
+#include "launch/LaunchTask.h"
+
+class ModFolderModel;
+class LegacyModList;
+class WorldList;
+class Task;
+/*
+ * WHY: Legacy instances - from MultiMC 3 and 4 - are here only to provide a way
+ * to upgrade them to the current format.
+ */
+class LegacyInstance : public BaseInstance
+{
+	Q_OBJECT
+  public:
+	explicit LegacyInstance(SettingsObjectPtr globalSettings,
+							SettingsObjectPtr settings, const QString& rootDir);
+
+	virtual void saveNow() override {}
+
+	/// Path to the instance's minecraft.jar
+	QString runnableJar() const;
+
+	//! Path to the instance's modlist file.
+	QString modListFile() const;
+
+	////// Directories //////
+	QString libDir() const;
+	QString savesDir() const;
+	QString texturePacksDir() const;
+	QString jarModsDir() const;
+	QString coreModsDir() const;
+	QString resourceDir() const;
+
+	QString instanceConfigFolder() const override;
+
+	QString
+	gameRoot() const override; // Path to the instance's minecraft directory.
+	QString
+	modsRoot() const override; // Path to the instance's minecraft directory.
+	QString binRoot() const; // Path to the instance's minecraft bin directory.
+
+	/// Get the curent base jar of this instance. By default, it's the
+	/// versions/$version/$version.jar
+	QString baseJar() const;
+
+	/// the default base jar of this instance
+	QString defaultBaseJar() const;
+	/// the default custom base jar of this instance
+	QString defaultCustomBaseJar() const;
+
+	// the main jar that we actually want to keep when migrating the instance
+	QString mainJarToPreserve() const;
+
+	/*!
+	 * Whether or not custom base jar is used
+	 */
+	bool shouldUseCustomBaseJar() const;
+
+	/*!
+	 * The value of the custom base jar
+	 */
+	QString customBaseJar() const;
+
+	std::shared_ptr<LegacyModList> jarModList() const;
+	std::shared_ptr<WorldList> worldList() const;
+
+	/*!
+	 * Whether or not the instance's minecraft.jar needs to be rebuilt.
+	 * If this is true, when the instance launches, its jar mods will be
+	 * re-added to a fresh minecraft.jar file.
+	 */
+	bool shouldRebuild() const;
+
+	QString currentVersionId() const;
+	QString intendedVersionId() const;
+
+	QSet<QString> traits() const override
+	{
+		return {"legacy-instance", "texturepacks"};
+	};
+
+	virtual bool shouldUpdate() const;
+	virtual Task::Ptr createUpdateTask(Net::Mode mode) override;
+
+	virtual QString typeName() const override;
+
+	bool canLaunch() const override
+	{
+		return false;
+	}
+	bool canEdit() const override
+	{
+		return true;
+	}
+	bool canExport() const override
+	{
+		return false;
+	}
+	shared_qobject_ptr<LaunchTask>
+	createLaunchTask(AuthSessionPtr account,
+					 MinecraftServerTargetPtr serverToJoin) override
+	{
+		return nullptr;
+	}
+	IPathMatcher::Ptr getLogFileMatcher() override
+	{
+		return nullptr;
+	}
+	QString getLogFileRoot() override
+	{
+		return gameRoot();
+	}
+
+	QString getStatusbarDescription() override;
+	QStringList
+	verboseDescription(AuthSessionPtr session,
+					   MinecraftServerTargetPtr serverToJoin) override;
+
+	QProcessEnvironment createEnvironment() override
+	{
+		return QProcessEnvironment();
+	}
+	QMap<QString, QString> getVariables() const override
+	{
+		return {};
+	}
+
+  protected:
+	mutable std::shared_ptr<LegacyModList> jar_mod_list;
+	mutable std::shared_ptr<WorldList> m_world_list;
+};
diff --git a/meshmc/launcher/minecraft/legacy/LegacyModList.cpp b/meshmc/launcher/minecraft/legacy/LegacyModList.cpp
new file mode 100644
index 0000000000..32158bf59e
--- /dev/null
+++ b/meshmc/launcher/minecraft/legacy/LegacyModList.cpp
@@ -0,0 +1,150 @@
+/* SPDX-FileCopyrightText: 2026 Project Tick
+ * SPDX-FileContributor: Project Tick
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ *   MeshMC - A Custom Launcher for Minecraft
+ *   Copyright (C) 2026 Project Tick
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   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, see <https://www.gnu.org/licenses/>.
+ *
+ *  This file incorporates work covered by the following copyright and
+ *  permission notice:
+ *
+ * Copyright 2013-2021 MultiMC Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "LegacyModList.h"
+#include <FileSystem.h>
+#include <QString>
+#include <QDebug>
+
+LegacyModList::LegacyModList(const QString& dir, const QString& list_file)
+	: m_dir(dir), m_list_file(list_file)
+{
+	FS::ensureFolderPathExists(m_dir.absolutePath());
+	m_dir.setFilter(QDir::Readable | QDir::NoDotAndDotDot | QDir::Files |
+					QDir::Dirs);
+	m_dir.setSorting(QDir::Name | QDir::IgnoreCase | QDir::LocaleAware);
+}
+
+struct OrderItem {
+	QString id;
+	bool enabled = false;
+};
+typedef QList<OrderItem> OrderList;
+
+static void internalSort(QList<LegacyModList::Mod>& what)
+{
+	auto predicate = [](const LegacyModList::Mod& left,
+						const LegacyModList::Mod& right) {
+		return left.fileName().localeAwareCompare(right.fileName()) < 0;
+	};
+	std::sort(what.begin(), what.end(), predicate);
+}
+
+static OrderList readListFile(const QString& m_list_file)
+{
+	OrderList itemList;
+	if (m_list_file.isNull() || m_list_file.isEmpty())
+		return itemList;
+
+	QFile textFile(m_list_file);
+	if (!textFile.open(QIODevice::ReadOnly | QIODevice::Text))
+		return OrderList();
+
+	QTextStream textStream;
+	textStream.setAutoDetectUnicode(true);
+	textStream.setDevice(&textFile);
+	while (true) {
+		QString line = textStream.readLine();
+		if (line.isNull() || line.isEmpty())
+			break;
+		else {
+			OrderItem it;
+			it.enabled = !line.endsWith(".disabled");
+			if (!it.enabled) {
+				line.chop(9);
+			}
+			it.id = line;
+			itemList.append(it);
+		}
+	}
+	textFile.close();
+	return itemList;
+}
+
+bool LegacyModList::update()
+{
+	if (!m_dir.exists() || !m_dir.isReadable())
+		return false;
+
+	QList<Mod> orderedMods;
+	QList<Mod> newMods;
+	m_dir.refresh();
+	auto folderContents = m_dir.entryInfoList();
+
+	// first, process the ordered items (if any)
+	OrderList listOrder = readListFile(m_list_file);
+	for (auto item : listOrder) {
+		QFileInfo infoEnabled(m_dir.filePath(item.id));
+		QFileInfo infoDisabled(m_dir.filePath(item.id + ".disabled"));
+		int idxEnabled = folderContents.indexOf(infoEnabled);
+		int idxDisabled = folderContents.indexOf(infoDisabled);
+		bool isEnabled;
+		// if both enabled and disabled versions are present, it's a special
+		// case...
+		if (idxEnabled >= 0 && idxDisabled >= 0) {
+			// we only process the one we actually have in the order file.
+			// and exactly as we have it.
+			// THIS IS A CORNER CASE
+			isEnabled = item.enabled;
+		} else {
+			// only one is present.
+			// we pick the one that we found.
+			// we assume the mod was enabled/disabled by external means
+			isEnabled = idxEnabled >= 0;
+		}
+		int idx = isEnabled ? idxEnabled : idxDisabled;
+		QFileInfo& info = isEnabled ? infoEnabled : infoDisabled;
+		// if the file from the index file exists
+		if (idx != -1) {
+			// remove from the actual folder contents list
+			folderContents.takeAt(idx);
+			// append the new mod
+			orderedMods.append(info);
+		}
+	}
+	// if there are any untracked files... append them sorted at the end
+	if (folderContents.size()) {
+		for (auto entry : folderContents) {
+			newMods.append(entry);
+		}
+		internalSort(newMods);
+		orderedMods.append(newMods);
+	}
+	mods.swap(orderedMods);
+	return true;
+}
diff --git a/meshmc/launcher/minecraft/legacy/LegacyModList.h b/meshmc/launcher/minecraft/legacy/LegacyModList.h
new file mode 100644
index 0000000000..4a5627e7c8
--- /dev/null
+++ b/meshmc/launcher/minecraft/legacy/LegacyModList.h
@@ -0,0 +1,69 @@
+/* SPDX-FileCopyrightText: 2026 Project Tick
+ * SPDX-FileContributor: Project Tick
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ *   MeshMC - A Custom Launcher for Minecraft
+ *   Copyright (C) 2026 Project Tick
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   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, see <https://www.gnu.org/licenses/>.
+ *
+ *  This file incorporates work covered by the following copyright and
+ *  permission notice:
+ *
+ * Copyright 2013-2021 MultiMC Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <QList>
+#include <QString>
+#include <QDir>
+
+class LegacyModList
+{
+  public:
+	using Mod = QFileInfo;
+
+	LegacyModList(const QString& dir, const QString& list_file = QString());
+
+	/// Reloads the mod list and returns true if the list changed.
+	bool update();
+
+	QDir dir()
+	{
+		return m_dir;
+	}
+
+	const QList<Mod>& allMods()
+	{
+		return mods;
+	}
+
+  protected:
+	QDir m_dir;
+	QString m_list_file;
+	QList<Mod> mods;
+};
diff --git a/meshmc/launcher/minecraft/legacy/LegacyUpgradeTask.cpp b/meshmc/launcher/minecraft/legacy/LegacyUpgradeTask.cpp
new file mode 100644
index 0000000000..89b3506153
--- /dev/null
+++ b/meshmc/launcher/minecraft/legacy/LegacyUpgradeTask.cpp
@@ -0,0 +1,151 @@
+/* SPDX-FileCopyrightText: 2026 Project Tick
+ * SPDX-FileContributor: Project Tick
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ *   MeshMC - A Custom Launcher for Minecraft
+ *   Copyright (C) 2026 Project Tick
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   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, see <https://www.gnu.org/licenses/>.
+ */
+
+#include "LegacyUpgradeTask.h"
+#include "settings/INISettingsObject.h"
+#include "FileSystem.h"
+#include "NullInstance.h"
+#include "pathmatcher/RegexpMatcher.h"
+#include <QtConcurrentRun>
+#include "LegacyInstance.h"
+#include "minecraft/MinecraftInstance.h"
+#include "minecraft/PackProfile.h"
+#include "LegacyModList.h"
+#include "classparser.h"
+
+LegacyUpgradeTask::LegacyUpgradeTask(InstancePtr origInstance)
+{
+	m_origInstance = origInstance;
+}
+
+void LegacyUpgradeTask::executeTask()
+{
+	setStatus(tr("Copying instance %1").arg(m_origInstance->name()));
+
+	FS::copy folderCopy(m_origInstance->instanceRoot(), m_stagingPath);
+	folderCopy.followSymlinks(true);
+
+	m_copyFuture = QtConcurrent::run(QThreadPool::globalInstance(), folderCopy);
+	connect(&m_copyFutureWatcher, &QFutureWatcher<bool>::finished, this,
+			&LegacyUpgradeTask::copyFinished);
+	connect(&m_copyFutureWatcher, &QFutureWatcher<bool>::canceled, this,
+			&LegacyUpgradeTask::copyAborted);
+	m_copyFutureWatcher.setFuture(m_copyFuture);
+}
+
+static QString decideVersion(const QString& currentVersion,
+							 const QString& intendedVersion)
+{
+	if (intendedVersion != currentVersion) {
+		if (!intendedVersion.isEmpty()) {
+			return intendedVersion;
+		} else if (!currentVersion.isEmpty()) {
+			return currentVersion;
+		}
+	} else {
+		if (!intendedVersion.isEmpty()) {
+			return intendedVersion;
+		}
+	}
+	return QString();
+}
+
+void LegacyUpgradeTask::copyFinished()
+{
+	auto successful = m_copyFuture.result();
+	if (!successful) {
+		emitFailed(tr("Instance folder copy failed."));
+		return;
+	}
+	auto legacyInst = std::dynamic_pointer_cast<LegacyInstance>(m_origInstance);
+
+	auto instanceSettings = std::make_shared<INISettingsObject>(
+		FS::PathCombine(m_stagingPath, "instance.cfg"));
+	instanceSettings->registerSetting("InstanceType", "Legacy");
+	instanceSettings->set("InstanceType", "OneSix");
+	// NOTE: this scope ensures the instance is fully saved before we
+	// emitSucceeded
+	{
+		MinecraftInstance inst(m_globalSettings, instanceSettings,
+							   m_stagingPath);
+		inst.setName(m_instName);
+
+		QString preferredVersionNumber = decideVersion(
+			legacyInst->currentVersionId(), legacyInst->intendedVersionId());
+		if (preferredVersionNumber.isNull()) {
+			// try to decide version based on the jar(s?)
+			preferredVersionNumber =
+				classparser::GetMinecraftJarVersion(legacyInst->baseJar());
+			if (preferredVersionNumber.isNull()) {
+				preferredVersionNumber = classparser::GetMinecraftJarVersion(
+					legacyInst->runnableJar());
+				if (preferredVersionNumber.isNull()) {
+					emitFailed(tr("Could not decide Minecraft version."));
+					return;
+				}
+			}
+		}
+		auto components = inst.getPackProfile();
+		components->buildingFromScratch();
+		components->setComponentVersion("net.minecraft", preferredVersionNumber,
+										true);
+
+		QString jarPath = legacyInst->mainJarToPreserve();
+		if (!jarPath.isNull()) {
+			qDebug() << "Preserving base jar! : " << jarPath;
+			// FIXME: handle case when the jar is unreadable?
+			// TODO: check the hash, if it's the same as the upstream jar, do
+			// not do this
+			components->installCustomJar(jarPath);
+		}
+
+		auto jarMods = legacyInst->jarModList()->allMods();
+		for (auto& jarMod : jarMods) {
+			QString modPath = jarMod.absoluteFilePath();
+			qDebug() << "jarMod: " << modPath;
+			components->installJarMods({modPath});
+		}
+
+		// remove all the extra garbage we no longer need
+		auto removeAll = [&](const QString& root, const QStringList& things) {
+			for (auto& thing : things) {
+				auto removePath = FS::PathCombine(root, thing);
+				QFileInfo stat(removePath);
+				if (stat.isDir()) {
+					FS::deletePath(removePath);
+				} else {
+					QFile::remove(removePath);
+				}
+			}
+		};
+		QStringList rootRemovables = {"modlist", "version", "instMods"};
+		QStringList mcRemovables = {"bin", "MeshMCLauncher.jar", "icon.png"};
+		removeAll(inst.instanceRoot(), rootRemovables);
+		removeAll(inst.gameRoot(), mcRemovables);
+	}
+	emitSucceeded();
+}
+
+void LegacyUpgradeTask::copyAborted()
+{
+	emitFailed(tr("Instance folder copy has been aborted."));
+	return;
+}
diff --git a/meshmc/launcher/minecraft/legacy/LegacyUpgradeTask.h b/meshmc/launcher/minecraft/legacy/LegacyUpgradeTask.h
new file mode 100644
index 0000000000..a407cb8df8
--- /dev/null
+++ b/meshmc/launcher/minecraft/legacy/LegacyUpgradeTask.h
@@ -0,0 +1,49 @@
+/* SPDX-FileCopyrightText: 2026 Project Tick
+ * SPDX-FileContributor: Project Tick
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ *   MeshMC - A Custom Launcher for Minecraft
+ *   Copyright (C) 2026 Project Tick
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   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, see <https://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include "InstanceTask.h"
+#include "net/NetJob.h"
+#include <QUrl>
+#include <QFuture>
+#include <QFutureWatcher>
+#include "settings/SettingsObject.h"
+#include "BaseVersion.h"
+#include "BaseInstance.h"
+
+class LegacyUpgradeTask : public InstanceTask
+{
+	Q_OBJECT
+  public:
+	explicit LegacyUpgradeTask(InstancePtr origInstance);
+
+  protected:
+	//! Entry point for tasks.
+	virtual void executeTask() override;
+	void copyFinished();
+	void copyAborted();
+
+  private: /* data */
+	InstancePtr m_origInstance;
+	QFuture<bool> m_copyFuture;
+	QFutureWatcher<bool> m_copyFutureWatcher;
+};