diff --git a/src/jasper/modules/world/include/jasper/world/worldstatic.hpp b/src/jasper/modules/world/include/jasper/world/worldstatic.hpp
index b4a44a6..b78b88b 100644
--- a/src/jasper/modules/world/include/jasper/world/worldstatic.hpp
+++ b/src/jasper/modules/world/include/jasper/world/worldstatic.hpp
@@ -110,13 +110,17 @@ void loadTile(ObjectCache const&objCache, TileStatic &dst, TileDoc const&src) no
 ox::Result<WorldStatic> loadWorldStatic(ObjectCache const&objCache, WorldDoc const&doc) noexcept;
 
 [[nodiscard]]
-auto &tile(ox::CommonRefWith<WorldStatic> auto &ws, size_t lyr, ox::Integer_c auto col, ox::Integer_c auto row) noexcept {
+auto &tile(
+		ox::CommonRefWith<WorldStatic> auto&ws,
+		size_t lyr,
+		ox::Integer_c auto col,
+		ox::Integer_c auto row) noexcept {
 	auto const idx = static_cast<size_t>(row) * static_cast<size_t>(ws.columns) + static_cast<size_t>(col);
 	return ws.map[lyr].tiles[idx];
 }
 
 [[nodiscard]]
-auto &cbb(ox::CommonRefWith<WorldStatic> auto &ws, size_t lyr) noexcept {
+auto &cbb(ox::CommonRefWith<WorldStatic> auto&ws, size_t lyr) noexcept {
 	return ws.map[lyr].cbb;
 }
 
diff --git a/src/jasper/modules/world/src/studio/studiomodule.cpp b/src/jasper/modules/world/src/studio/studiomodule.cpp
index f9f5cdd..9bfaee2 100644
--- a/src/jasper/modules/world/src/studio/studiomodule.cpp
+++ b/src/jasper/modules/world/src/studio/studiomodule.cpp
@@ -11,7 +11,7 @@
 
 namespace jasper::world {
 
-class StudioModule: public studio::Module {
+static class: public studio::Module {
 	public:
 		ox::Vector<studio::EditorMaker> editors(studio::StudioContext &ctx) const noexcept override {
 			return {
@@ -21,13 +21,14 @@ class StudioModule: public studio::Module {
 		}
 		ox::Vector<ox::UPtr<studio::ItemMaker>> itemMakers(studio::StudioContext&) const noexcept override {
 			ox::Vector<ox::UPtr<studio::ItemMaker>> out;
-			out.emplace_back(ox::make<studio::ItemMakerT<WorldObjectSet>>("World Object Set", "WorldObjectSets", FileExt_jwob, ox::ClawFormat::Organic));
-			out.emplace_back(ox::make<studio::ItemMakerT<WorldDoc>>("World", "Worlds", FileExt_jwld, ox::ClawFormat::Organic));
+			out.emplace_back(ox::make<studio::ItemMakerT<WorldObjectSet>>(
+					"World Object Set", "WorldObjectSets", FileExt_jwob, ox::ClawFormat::Organic));
+			out.emplace_back(ox::make<studio::ItemMakerT<WorldDoc>>(
+					"World", "Worlds", FileExt_jwld, ox::ClawFormat::Organic));
 			return out;
 		}
-};
+} mod;
 
-static StudioModule const mod;
 studio::Module const*studioModule() noexcept {
 	return &mod;
 }
diff --git a/src/jasper/modules/world/src/studio/worldeditor/commands/addrmobjectset.cpp b/src/jasper/modules/world/src/studio/worldeditor/commands/addrmobjectset.cpp
index c5ce28a..e024c09 100644
--- a/src/jasper/modules/world/src/studio/worldeditor/commands/addrmobjectset.cpp
+++ b/src/jasper/modules/world/src/studio/worldeditor/commands/addrmobjectset.cpp
@@ -7,14 +7,14 @@
 
 namespace jasper::world {
 
-AddObjectSet::AddObjectSet(WorldDoc &doc, ox::StringView objSetPath):
+AddObjectSet::AddObjectSet(WorldDoc &doc, ox::UUID const&objSetUuid):
 		m_doc(doc),
-		m_path(ox::String(objSetPath)) {
+		m_path(ox::sfmt("uuid://{}", objSetUuid.toString())) {
 	for (auto const&o : m_doc.objSets) {
-		if (o.path > objSetPath) {
+		if (o.path > m_path) {
 			break;
-		} else if (o.path == objSetPath) {
-			throw ox::Exception(OxError(1, "Path already exists in doc"));
+		} else if (o.path == m_path) {
+			throw OxException(1, "Path already exists in doc");
 		}
 		++m_insertIdx;
 	}
diff --git a/src/jasper/modules/world/src/studio/worldeditor/commands/addrmobjectset.hpp b/src/jasper/modules/world/src/studio/worldeditor/commands/addrmobjectset.hpp
index d9846cd..288716f 100644
--- a/src/jasper/modules/world/src/studio/worldeditor/commands/addrmobjectset.hpp
+++ b/src/jasper/modules/world/src/studio/worldeditor/commands/addrmobjectset.hpp
@@ -18,7 +18,7 @@ class AddObjectSet: public studio::UndoCommand {
 		size_t m_insertIdx{};
 		ox::String m_path;
 	public:
-		AddObjectSet(WorldDoc &doc, ox::StringView objSetPath);
+		AddObjectSet(WorldDoc &doc, ox::UUID const&objSetUuid);
 		void redo() noexcept override;
 		void undo() noexcept override;
 		[[nodiscard]]
diff --git a/src/jasper/modules/world/src/studio/worldeditor/commands/modifytiles.cpp b/src/jasper/modules/world/src/studio/worldeditor/commands/modifytiles.cpp
index 91ee67a..257623c 100644
--- a/src/jasper/modules/world/src/studio/worldeditor/commands/modifytiles.cpp
+++ b/src/jasper/modules/world/src/studio/worldeditor/commands/modifytiles.cpp
@@ -7,7 +7,8 @@
 
 namespace jasper::world {
 
-ModifyTilesCommand::ModifyTilesCommand(WorldDoc &doc,
+ModifyTilesCommand::ModifyTilesCommand(
+		WorldDoc &doc,
 		WorldStatic &worldStatic,
 		ObjectCache &objCache,
 		ox::Vector<Mod> mods):
@@ -15,6 +16,17 @@ ModifyTilesCommand::ModifyTilesCommand(WorldDoc &doc,
 		m_worldStatic(worldStatic),
 		m_objCache(objCache),
 		m_mods(std::move(mods)) {
+	bool noChanges = true;
+	for (auto const&mod : m_mods) {
+		auto const&docTile = tile(m_doc, mod.layer, mod.tileAddr.x, mod.tileAddr.y);
+		if (docTile.obj.worldObjectId != mod.objId || docTile.obj.worldObjectSetId != mod.setId) {
+			noChanges = false;
+			break;
+		}
+	}
+	if (noChanges) {
+		throw studio::NoChangesException();
+	}
 }
 
 void ModifyTilesCommand::redo() noexcept {
diff --git a/src/jasper/modules/world/src/studio/worldeditor/worldeditor-imgui.cpp b/src/jasper/modules/world/src/studio/worldeditor/worldeditor-imgui.cpp
index 43a5b34..c38457e 100644
--- a/src/jasper/modules/world/src/studio/worldeditor/worldeditor-imgui.cpp
+++ b/src/jasper/modules/world/src/studio/worldeditor/worldeditor-imgui.cpp
@@ -47,6 +47,14 @@ static WorldDoc makeValid(WorldDoc doc) noexcept {
 }
 
 [[nodiscard]]
+static ox::Vec2 dropPos(ox::Vec2 dropPos) noexcept {
+    auto const winPos = ImGui::GetWindowPos();
+    dropPos.x -= winPos.x;
+    dropPos.y -= winPos.y;
+    return dropPos;
+}
+
+    [[nodiscard]]
 constexpr ox::Point fbPtToTileAddr(
 		ox::Vec2 const&fbPt,
 		ox::Vec2 const&mapSz) noexcept {
@@ -98,7 +106,7 @@ void WorldEditorImGui::onActivated() noexcept {
 }
 
 ox::Error WorldEditorImGui::saveItem() noexcept {
-	return m_sctx.project->writeObj(itemPath(), m_doc, ox::ClawFormat::Organic);
+	return m_sctx.project->writeObj(itemPath(), m_doc, ox::ClawFormat::Metal);
 }
 
 void WorldEditorImGui::drawObjSetSelector() noexcept {
@@ -111,14 +119,23 @@ void WorldEditorImGui::drawObjSetSelector() noexcept {
 		rmObjSet();
 	}
 	ig::ListBox("Object Sets", [this](size_t i) -> ox::CStringView {
-		return m_doc.objSets[i].path;
+		auto const&uuidUrl = m_doc.objSets[i].path;
+		auto [setName, err] = uuidUrlToPath(keelCtx(m_sctx.tctx), uuidUrl);
+		if (err) {
+			setName = uuidUrl;
+		}
+		return setName;
 	}, m_doc.objSets.size(), m_palMgr.selectedIdx);
 }
 
 void WorldEditorImGui::drawObjSelector() noexcept {
 	ig::IDStackItem const idStackItem("ObjSelector");
-	for (auto const&[setName, setId, set] : m_objSets) {
+	for (auto const&[uuidUrl, setId, set] : m_objSets) {
 		constexpr auto flags = ImGuiTreeNodeFlags_OpenOnArrow | ImGuiTreeNodeFlags_OpenOnDoubleClick;
+		auto [setName, err] = uuidUrlToPath(keelCtx(m_sctx.tctx), uuidUrl);
+		if (err) {
+			setName = uuidUrl;
+		}
 		if (ImGui::TreeNodeEx(setName.c_str(), flags)) {
 			for (auto const&obj : set->objects) {
 				if (ImGui::TreeNodeEx(obj.name.c_str(), ImGuiTreeNodeFlags_Leaf)) {
@@ -155,14 +172,6 @@ void WorldEditorImGui::drawResources() noexcept {
 	drawObjSelector();
 }
 
-[[nodiscard]]
-static ox::Vec2 dropPos(ox::Vec2 dropPos) noexcept {
-	auto const winPos = ImGui::GetWindowPos();
-	dropPos.x -= winPos.x;
-	dropPos.y -= winPos.y;
-	return dropPos;
-}
-
 void WorldEditorImGui::drawWorldView() noexcept {
 	ig::IDStackItem const idStackItem("WorldView");
 	auto const paneSize = ImGui::GetContentRegionAvail();
@@ -221,7 +230,8 @@ void WorldEditorImGui::drawWorldView() noexcept {
 }
 
 ox::Error WorldEditorImGui::addObjSet(ox::StringView path) noexcept {
-	pushCommand<AddObjectSet>(m_doc, path);
+	oxRequire(uuid, keel::pathToUuid(keelCtx(m_sctx.tctx), path));
+	pushCommand<AddObjectSet>(m_doc, uuid);
 	return {};
 }
 
@@ -247,7 +257,7 @@ ox::Error WorldEditorImGui::loadObjectSets() noexcept {
 	return {};
 }
 
-ox::Error WorldEditorImGui::undoStackChanged(const studio::UndoCommand *) {
+ox::Error WorldEditorImGui::undoStackChanged(studio::UndoCommand const*) {
 	oxReturnError(m_view.setupWorld());
 	return {};
 }
diff --git a/src/jasper/modules/world/src/studio/worldeditor/worldeditor-imgui.hpp b/src/jasper/modules/world/src/studio/worldeditor/worldeditor-imgui.hpp
index c060fa4..501d955 100644
--- a/src/jasper/modules/world/src/studio/worldeditor/worldeditor-imgui.hpp
+++ b/src/jasper/modules/world/src/studio/worldeditor/worldeditor-imgui.hpp
@@ -19,6 +19,10 @@ namespace jasper::world {
 class WorldEditorImGui: public studio::Editor {
 
 	private:
+		struct Selection {
+			ox::Point begin, end;
+		};
+		ox::Optional<Selection> m_selection;
 		uint8_t m_activeLayer{};
 		studio::StudioContext &m_sctx;
 		studio::ig::FilePicker m_objSetPicker{
diff --git a/src/jasper/modules/world/src/world.cpp b/src/jasper/modules/world/src/world.cpp
index 8500aa5..07d7499 100644
--- a/src/jasper/modules/world/src/world.cpp
+++ b/src/jasper/modules/world/src/world.cpp
@@ -8,8 +8,6 @@
 
 namespace jasper::world {
 
-namespace ncore = nostalgia::core;
-
 World::World(WorldStatic const&worldStatic) noexcept:
 	m_worldStatic(worldStatic) {
 }
@@ -37,9 +35,11 @@ void World::setupLayer(
 		uint_t cbb) const noexcept {
 	ncore::setBgStatus(ctx, lyr, true);
 	ncore::setBgCbb(ctx, lyr, cbb);
-	for (auto y = 0; y < m_worldStatic.rows; ++y) {
-		for (auto x = 0; x < m_worldStatic.columns; ++x) {
-			auto &t = tile(m_worldStatic, lyr, static_cast<size_t>(x), static_cast<size_t>(y));
+	auto const rows = ox::min<int>(m_worldStatic.rows, ncore::tileRows(ctx) / 2);
+	auto const columns = ox::min<int>(m_worldStatic.columns, ncore::tileColumns(ctx) / 2);
+	for (auto y = 0; y < rows; ++y) {
+		for (auto x = 0; x < columns; ++x) {
+			auto &t = tile(m_worldStatic, lyr, x, y);
 			auto const tx = x * 2;
 			auto const ty = y * 2;
 			ncore::setBgTile(ctx, lyr, tx + 0, ty + 0, {