Record Linux starter project validation

apps/editor_main.cpp

@@ -4,6 +4,7 @@
 #include <faset/editor/mcp.hpp>
 #include <faset/editor/session.hpp>
 #include <iostream>
+#include <memory>
 #include <thread>
 #ifdef FASET_HAS_EDITOR_UI
 #include <faset/editor/editor_ui.hpp>
@@ -40,7 +41,7 @@ void help() {
     std::cout
         << "Faset Editor\n"
            "  faset_editor                         Open the project launcher\n"
-           "  faset_editor --project PATH [--new NAME --dimension 2|3] [--scene RELATIVE_PATH]\n"
+           "  faset_editor --project PATH [--new NAME --dimension 2|3 [--language cpp|lua]] [--scene RELATIVE_PATH]\n"
            "  faset_editor --project PATH --mcp [--gui]\n"
            "  faset_editor --project PATH --command JSON [--wait]\n"
            "Options: --engine SDK_SOURCE, --headless, --frames N, --capture PATH.ppm\n"
@@ -52,9 +53,10 @@ int editor_main(int argc, char** argv) {
     using namespace faset::editor;
     try {
         std::filesystem::path project, engine = path_from_utf8(FASET_ENGINE_SOURCE), scene, capture;
-        std::string new_name, command;
+        std::string new_name, command, language = "cpp";
         int dimension = 3;
         bool mcp = false, gui = true, explicit_gui = false, wait = false;
+        bool explicit_language = false;
         std::uint64_t frames = 0;
         for (int i = 1; i < argc; ++i) {
             const std::string arg = argv[i];
@@ -74,6 +76,10 @@ int editor_main(int argc, char** argv) {
                 new_name = value();
             else if (arg == "--dimension")
                 dimension = std::stoi(value());
+            else if (arg == "--language") {
+                language = value();
+                explicit_language = true;
+            }
             else if (arg == "--scene")
                 scene = path_from_utf8(value());
             else if (arg == "--mcp")
@@ -100,30 +106,65 @@ int editor_main(int argc, char** argv) {
                 throw Error("cli.option", "Unknown option: " + arg);
         }
         require(!(mcp && !command.empty()), "cli.mode", "Choose MCP or a single command");
+        require(!explicit_language || !new_name.empty(), "cli.language",
+                "--language requires --new NAME");
+        require(language == "cpp" || language == "lua", "cli.language",
+                "Language must be cpp or lua");
         if (mcp && !explicit_gui)
             gui = false;
         require(!project.empty() || (gui && !mcp && new_name.empty()), "cli.project",
                 "Use --project PATH for command, MCP, or --new modes");
         auto previous_project = project;
+#ifdef FASET_HAS_EDITOR_UI
+        std::optional<ProjectSelection> retry_create;
+        std::string creation_error;
+#endif
         for (;;) {
             if (project.empty()) {
 #ifdef FASET_HAS_EDITOR_UI
                 const auto selection =
-                    run_project_launcher(engine, previous_project, frames, capture);
+                    run_project_launcher(engine, previous_project, frames, capture,
+                                         retry_create, creation_error);
+                retry_create.reset();
+                creation_error.clear();
                 if (!selection)
                     return 0;
                 project = selection->path;
                 new_name = selection->create ? selection->name : std::string();
                 dimension = selection->dimension;
+                language = selection->language;
+                explicit_language = selection->create;
 #else
                 throw Error("editor.gui_unavailable",
                             "This build has no graphical project launcher; use --project PATH");
 #endif
             }
-            Session session({std::filesystem::absolute(project), std::filesystem::absolute(engine),
+            std::unique_ptr<Session> session_holder;
-                             executable_directory(argv[0])});
+            try {
-            if (!new_name.empty())
+                session_holder = std::make_unique<Session>(SessionConfig{
-                session.scaffold(new_name, dimension);
+                    std::filesystem::absolute(project), std::filesystem::absolute(engine),
+                    executable_directory(argv[0])});
+                if (!new_name.empty()) {
+                    if (explicit_language)
+                        session_holder->scaffold(new_name, dimension, language);
+                    else
+                        session_holder->scaffold(new_name, dimension);
+                }
+            } catch (const std::exception& error) {
+#ifdef FASET_HAS_EDITOR_UI
+                if (gui && !new_name.empty()) {
+                    retry_create = ProjectSelection{std::filesystem::absolute(project),
+                                                    new_name, dimension, true, language};
+                    creation_error = error.what();
+                    project.clear();
+                    new_name.clear();
+                    scene.clear();
+                    continue;
+                }
+#endif
+                throw;
+            }
+            Session& session = *session_holder;
             const auto settings = session.project();
 #ifdef FASET_HAS_EDITOR_UI
             if (gui && std::filesystem::exists(project / "project.faset.json"))
@@ -167,6 +208,8 @@ int editor_main(int argc, char** argv) {
                 project.clear();
                 scene.clear();
                 new_name.clear();
+                explicit_language = false;
+                language = "cpp";
                 continue;
 #else
                 throw Error(

apps/project_launcher.cpp

@@ -75,6 +75,22 @@ ProjectSelection open_project(const fs::path& path) {
         throw std::runtime_error("Project name or scene type is invalid.");
     return {path, std::move(name), dimension, false};
 }
+bool fresh_destination(const fs::path& path) {
+    if (!fs::exists(path))
+        return true;
+    if (!fs::is_directory(path) || fs::is_symlink(path))
+        return false;
+    for (const auto& entry : fs::directory_iterator(path)) {
+        if (entry.path().filename() != ".faset" || !entry.is_directory() ||
+            entry.is_symlink())
+            return false;
+        for (const auto& internal : fs::directory_iterator(entry.path()))
+            if (internal.path().filename() != "cache" || !internal.is_directory() ||
+                internal.is_symlink() || !fs::is_empty(internal.path()))
+                return false;
+    }
+    return true;
+}
 Json recent_records(const fs::path& file) {
     try {
         auto j = read_json(file);
@@ -124,9 +140,11 @@ struct ProjectLauncher::Impl {
     std::optional<ProjectSelection> selected;
     bool create = false, cancelled = false, browsing = false, browser_valid = false;
     int dimension = 3;
+    std::string language = "cpp";
     std::string error;
     Impl(render::Renderer& r, const fs::path& engine, const fs::path& initial,
-         const fs::path& recents)
+         const fs::path& recents, const std::optional<ProjectSelection>& retry,
+         std::string creation_error)
         : renderer(r), ui(engine / "assets/fonts/NotoSans.ttf"),
           recents_file(recents.empty() ? recent_path() : recents) {
         auto theme = ui::Theme::load(engine / "assets/ui/dark.json");
@@ -147,13 +165,22 @@ struct ProjectLauncher::Impl {
         ui.set_ime(
             [this](bool enabled) { renderer.set_text_input(enabled); },
             [this](ui::Rect r) { renderer.set_text_input_area(r.x, r.y, r.width, r.height); });
-        const auto start = initial.empty() ? user_home() / "FasetProjects/MyGame" : initial;
+        const auto start = retry ? retry->path
+                                 : initial.empty() ? user_home() / "FasetProjects/MyGame" : initial;
         ui.update_text("launcher-path", path_to_utf8(start));
-        create = initial.empty();
+        create = retry ? retry->create : initial.empty();
+        if (retry) {
+            ui.update_text("launcher-name", retry->name);
+            dimension = retry->dimension;
+            language = retry->language;
+            error = std::move(creation_error);
+        }
         ui.find("launcher-open")->on_click = [this](Widget&) { set_mode(false); };
         ui.find("launcher-create")->on_click = [this](Widget&) { set_mode(true); };
         ui.find("launcher-2d")->on_click = [this](Widget&) { dimension = 2; };
         ui.find("launcher-3d")->on_click = [this](Widget&) { dimension = 3; };
+        ui.find("launcher-cpp")->on_click = [this](Widget&) { language = "cpp"; };
+        ui.find("launcher-lua")->on_click = [this](Widget&) { language = "lua"; };
         ui.find("launcher-submit")->on_click = [this](Widget&) { submit(); };
         ui.find("launcher-cancel")->on_click = [this](Widget&) { cancelled = true; };
         ui.find("launcher-name")->on_preview = [this](Widget&) { error.clear(); };
@@ -209,14 +236,14 @@ struct ProjectLauncher::Impl {
                     throw std::runtime_error("Enter a project name.");
                 if (name.size() > 256 || name.find_first_of("\r\n\t") != std::string::npos)
                     throw std::runtime_error("Use a short project name on one line.");
-                if (fs::exists(path) && (!fs::is_directory(path) || !fs::is_empty(path)))
+                if (!fresh_destination(path))
                     throw std::runtime_error("Create requires a new or empty directory.");
                 auto parent = path.parent_path();
                 while (!parent.empty() && !fs::exists(parent))
                     parent = parent.parent_path();
                 if (parent.empty() || !fs::is_directory(parent))
                     throw std::runtime_error("Project parent directory is unavailable.");
-                selected = ProjectSelection{path, name, dimension, true};
+                selected = ProjectSelection{path, name, dimension, true, language};
             } else
                 selected = open_project(path);
             error.clear();
@@ -351,14 +378,24 @@ struct ProjectLauncher::Impl {
         ui.find("launcher-create")->selected = create;
         ui.find("launcher-2d")->selected = dimension == 2;
         ui.find("launcher-3d")->selected = dimension == 3;
+        ui.find("launcher-cpp")->selected = language == "cpp";
+        ui.find("launcher-lua")->selected = language == "lua";
         ui.find("launcher-2d")->appearance =
             dimension == 2 ? Appearance::Quiet : Appearance::Default;
         ui.find("launcher-3d")->appearance =
             dimension == 3 ? Appearance::Quiet : Appearance::Default;
+        ui.find("launcher-cpp")->appearance =
+            language == "cpp" ? Appearance::Quiet : Appearance::Default;
+        ui.find("launcher-lua")->appearance =
+            language == "lua" ? Appearance::Quiet : Appearance::Default;
+        ui.find("launcher-language-hint")->text =
+            language == "cpp" ? "Compiled gameplay. Add Lua later if you need quick reloads."
+                              : "Lua gameplay reloads during development without a C++ rebuild.";
         for (const auto* id : {"launcher-name-label", "launcher-name", "launcher-dimension-label",
                                "launcher-dimensions", "launcher-name-group",
                                "launcher-name-space", "launcher-path-space",
-                               "launcher-dimension-group"})
+                               "launcher-dimension-group", "launcher-language-group",
+                               "launcher-language-space"})
             ui.find(id)->visible = create;
         ui.find("launcher-heading")->text = create ? "Create project" : "Open project";
         ui.find("launcher-submit")->text = create ? "Create project" : "Open project";
@@ -424,8 +461,11 @@ struct ProjectLauncher::Impl {
     }
 };
 ProjectLauncher::ProjectLauncher(render::Renderer& r, const fs::path& engine,
-                                 const fs::path& initial, const fs::path& recents)
+                                 const fs::path& initial, const fs::path& recents,
-    : impl_(std::make_unique<Impl>(r, engine, initial, recents)) {}
+                                 const std::optional<ProjectSelection>& retry,
+                                 std::string creation_error)
+    : impl_(std::make_unique<Impl>(r, engine, initial, recents, retry,
+                                   std::move(creation_error))) {}
 ProjectLauncher::~ProjectLauncher() = default;
 void ProjectLauncher::frame(const std::vector<render::Event>& events) {
     impl_->frame(events);
@@ -445,9 +485,11 @@ bool ProjectLauncher::cancelled() const {
 std::optional<ProjectSelection> run_project_launcher(const fs::path& engine,
                                                      const fs::path& initial,
                                                      std::uint64_t max_frames,
-                                                     const fs::path& capture) {
+                                                     const fs::path& capture,
+                                                     const std::optional<ProjectSelection>& retry,
+                                                     std::string creation_error) {
     render::Renderer renderer({1100, 720, "Faset Engine — Projects", false, true});
-    ProjectLauncher launcher(renderer, engine, initial);
+    ProjectLauncher launcher(renderer, engine, initial, {}, retry, std::move(creation_error));
     std::uint64_t frames = 0;
     while (!renderer.should_close() && !launcher.cancelled() && !launcher.selection() &&
            (max_frames == 0 || frames < max_frames)) {

assets/ui/project-launcher-layout.json

@@ -41,6 +41,15 @@
                 {"id": "launcher-3d", "kind": "button", "text": "3D", "layout": {"width": 116}}
               ]}
             ]},
+            {"id": "launcher-language-space", "kind": "row", "layout": {"height": 18}},
+            {"id": "launcher-language-group", "kind": "column", "layout": {"gap": 6}, "children": [
+              {"id": "launcher-language-label", "kind": "label", "text": "Gameplay language", "layout": {"height": 24}},
+              {"id": "launcher-languages", "kind": "row", "layout": {"height": 42, "gap": 4}, "children": [
+                {"id": "launcher-cpp", "kind": "button", "text": "C++", "layout": {"width": 116}},
+                {"id": "launcher-lua", "kind": "button", "text": "Lua", "layout": {"width": 116}}
+              ]},
+              {"id": "launcher-language-hint", "kind": "label", "text": "Compiled gameplay. Add Lua later if you need quick reloads.", "font_size": 12, "layout": {"height": 30}}
+            ]},
             {"id": "launcher-hint", "kind": "label", "text": "Open a directory containing project.faset.json.", "font_size": 13, "layout": {"height": 36}},
             {"id": "launcher-actions-space", "kind": "row", "layout": {"height": 22}},
             {"id": "launcher-actions-rule", "kind": "row", "layout": {"height": 1}},

cmake/BuildService.cmake

@@ -1,8 +1,18 @@
-add_library(faset_build_service STATIC ${PROJECT_SOURCE_DIR}/src/editor/build_service.cpp)
+add_library(faset_build_service STATIC
+  ${PROJECT_SOURCE_DIR}/src/editor/build_service.cpp
+  ${PROJECT_SOURCE_DIR}/src/editor/build_cache.cpp
+  ${PROJECT_SOURCE_DIR}/src/editor/build_diagnostics.cpp)
 target_include_directories(faset_build_service PUBLIC ${PROJECT_SOURCE_DIR}/include)
 target_compile_features(faset_build_service PUBLIC cxx_std_20)
 target_link_libraries(faset_build_service PUBLIC faset_core faset_scripting_project PRIVATE faset_assets faset_authoring Threads::Threads)
 if(BUILD_TESTING)
+  add_executable(faset_build_diagnostics_tests ${PROJECT_SOURCE_DIR}/tests/build_diagnostics_tests.cpp)
+  target_link_libraries(faset_build_diagnostics_tests PRIVATE faset_build_service)
+  add_test(NAME build_diagnostics COMMAND faset_build_diagnostics_tests)
+  add_executable(faset_build_cache_tests ${PROJECT_SOURCE_DIR}/tests/build_cache_tests.cpp)
+  target_link_libraries(faset_build_cache_tests PRIVATE faset_build_service)
+  target_compile_definitions(faset_build_cache_tests PRIVATE FASET_ENGINE_SOURCE="${PROJECT_SOURCE_DIR}")
+  add_test(NAME build_cache COMMAND faset_build_cache_tests)
   add_executable(faset_build_service_tests ${PROJECT_SOURCE_DIR}/tests/build_service_tests.cpp)
   target_link_libraries(faset_build_service_tests PRIVATE faset_build_service faset_assets)
   target_compile_definitions(faset_build_service_tests PRIVATE FASET_ENGINE_SOURCE="${PROJECT_SOURCE_DIR}")

docs/manual/getting-started/build.md

@@ -31,17 +31,27 @@ cmake --build --preset linux-debug --parallel
 ctest --preset linux-debug
 ```
 
-Create a project and open the native Editor:
+Create a runnable C++ or Lua starter and open the native Editor:
 
 ```sh
-build/linux-debug/faset_editor --project "$PWD/MyGame" --new MyGame --dimension 3
+build/linux-debug/faset_editor --project "$PWD/MyGame" --new MyGame --dimension 3 --language cpp
+build/linux-debug/faset_editor --project "$PWD/LuaGame" --new LuaGame --dimension 2 --language lua
 ```
 
 You can also run `build/linux-debug/faset_editor` without arguments to open the
-project launcher and create or select a project using the native interface.
+project launcher. Its Create view offers C++ 2D, C++ 3D, Lua 2D, and Lua 3D.
+Each explicit starter includes `Scenes/main.scene.json` with a player, ground,
+and a behavior bound to `Scripts/Gameplay.cpp` or `Scripts/main.lua`. 3D starters
+also include a camera and light. Existing files in a destination directory are
+never replaced by starter creation. The older `--new NAME --dimension 2|3`
+command without `--language` retains its original scene-free C++ scaffold.
 
 Use **Build** after changing `MyGame/Scripts/Gameplay.cpp`, then **Play**.
 The Player runs separately. Stop it before changing and rebuilding C++ gameplay.
+Lua starters declare their entry script in `project.faset.json`, install LuaLS
+annotations/configuration, and need no `Gameplay.cpp`. Use **Refresh Lua** after
+editing a Lua behavior declaration; development **Reload Lua** can apply a valid
+script edit to a running Player without recompiling C++.
 See [MCP and CLI](../editor/mcp.md) for headless authoring and automation.
 
 For an optimized build use `linux-release`. The `linux-sanitize` preset enables

docs/manual/scripting/lua.md

@@ -5,6 +5,23 @@ behaviors share the same runtime lifecycle, typed entity operations, scene compo
 and Inspector metadata. The Editor does not run gameplay code in its own process.
 There is no built-in script editor: edit `.lua` files in Zed or another external editor.
 
+## Start with a runnable Lua project
+
+Choose **Create project → Lua → 2D or 3D** in the native launcher, or run:
+
+```sh
+build/linux-debug/faset_editor --project "$PWD/LuaGame" --new LuaGame --dimension 2 --language lua
+```
+
+The starter creates `Scripts/main.lua`, a scene with a player and ground, and a
+manifest entry for that script. The player moves horizontally and jumps with the
+default input actions. The 3D choice also provides a camera and directional light.
+Open `Scripts/main.lua` in your external editor and change `speed` or
+`fixed_update`. **Refresh Lua** updates declared Inspector fields; **Reload Lua**
+updates a development Player after a valid edit without rebuilding native gameplay.
+The starter installs LuaLS declarations and `.luarc.json`; reopening it in Zed or
+another LuaLS-enabled editor provides completions for the Faset API.
+
 ## Enable Lua in a project
 
 Add explicit entry scripts to `project.faset.json`:

docs/validation/p1-starters-linux-2026-09-24.md

@@ -0,0 +1,42 @@
+# P1 starter projects: Linux checkpoint (2026-09-24)
+
+This is a focused validation of the four explicit new-project choices in commit
+`cbb8b4508723e1a3a0bbd53c884e116a47b121dc`. It is not the final P1 acceptance
+record. The run used Linux 7.0.0-31-generic x86_64, Clang 21.1.8, CMake 4.2.3,
+and an NVIDIA GeForce RTX 2080 Ti with driver 595.84.
+
+The Editor CLI created C++ 2D, C++ 3D, Lua 2D, and Lua 3D projects in disposable
+directories with `--new`, `--dimension`, and `--language`. The test checked the
+project manifest, authored start scene, behavior binding, Lua script declaration,
+LuaLS files, and the absence of a C++ gameplay stub in Lua-only projects. A real
+Debug `faset_build` succeeded for C++ 2D and Lua 3D. Their built Players then
+validated and rendered **both** corresponding scene dimensions using the same
+language module, so all four templates exercised runtime schema and rendering.
+Each headless one-frame render reported `validation_errors: 0` on the physical GPU.
+
+| Project | Built Player used | `--validate` | One-frame headless render |
+| --- | --- | --- | --- |
+| C++ 2D | C++ 2D | passed | passed |
+| C++ 3D | C++ 2D | passed | passed |
+| Lua 2D | Lua 3D | passed | passed |
+| Lua 3D | Lua 3D | passed | passed |
+
+The C++ and Lua source files are identical within each language choice; the 2D/3D
+difference is the authored scene. Reusing one built Player per language therefore
+checks scene validity without repeating an identical native compile. The focused
+CTest cases `process_and_cook` and `editor_ui_launcher` passed 2/2. They include a
+two-service concurrent creation test: exactly one starter wins, and the other
+cannot overwrite it. A fresh Lua starter was initialized as a Git repository;
+`git check-ignore` confirmed `.faset/cache` is ignored while the LuaLS API
+declaration `.faset/lua/faset.lua` can be committed. A strict MkDocs build passed.
+
+These captures are representative output from the C++ 2D and 3D scenes. Lua
+scenes rendered the same layout with their own behavior module.
+
+![C++ 2D starter after one frame](p1-starters-linux-2026-09-24/cpp-2.png)
+
+![C++ 3D starter after one frame](p1-starters-linux-2026-09-24/cpp-3.png)
+
+The run did not test Windows, a relocated Release export, multi-frame gameplay
+input, or the final P1 build-cache integration. Those belong to the P1 acceptance
+matrix and must be recorded separately.

include/faset/editor/build_cache.hpp

@@ -0,0 +1,52 @@
+#pragma once
+
+#include <faset/editor/build_service.hpp>
+#include <faset/scripting/project.hpp>
+#include <filesystem>
+#include <optional>
+#include <string>
+
+namespace faset::editor {
+
+struct BuildInputs {
+    std::string source_hash;
+    std::string recipe_hash;
+    std::string toolchain_hash;
+    std::string fingerprint() const;
+};
+
+// Hash all files in Scripts and the active Lua declaration, not only the two
+// conventional gameplay files. Both Editor stale-state and builds use this key.
+std::string gameplay_source_hash(const std::filesystem::path& project_root,
+                                 const scripting::LuaProject& lua);
+BuildInputs capture_build_inputs(const BuildConfig& config, const scripting::LuaProject& lua);
+std::optional<std::string> configured_cmake_value(const BuildConfig& config,
+                                                   std::string_view key);
+
+// Copy a content-addressed, immutable Scripts tree for the native compiler.
+// Returning the same path for equal source bytes preserves Ninja incrementality.
+std::filesystem::path stage_gameplay_sources(const BuildConfig& config,
+                                              const BuildInputs& inputs,
+                                              const scripting::LuaProject& lua,
+                                              std::string_view job_id);
+
+// A compiler replaced at the same path may be invisible to Ninja. Invalidate
+// only this generated native tree; published build generations stay intact.
+void ensure_native_toolchain_stamp(const BuildConfig& config,
+                                   const std::filesystem::path& native_directory,
+                                   const BuildInputs& inputs);
+
+// Native build completes before package reuse is considered. Hash the actual
+// generated outputs, not their timestamps or just the gameplay sources.
+std::string build_package_key(const BuildInputs& inputs,
+                              const std::filesystem::path& native_directory,
+                              const std::string& configuration,
+                              const std::filesystem::path& player,
+                              const std::filesystem::path& exporter);
+
+// A hit is accepted only if the complete published file set and schema match
+// the manifest. Malformed or old manifests are safe cache misses.
+bool validate_build_generation(const std::filesystem::path& directory,
+                               const std::string& package_key);
+
+} // namespace faset::editor

include/faset/editor/build_diagnostics.hpp

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <faset/core/json.hpp>
+#include <filesystem>
+#include <string_view>
+
+namespace faset::editor {
+
+// Parse compiler, MSVC/clang-cl and Lua output into bounded Editor-facing rows.
+// Only sources under the current project's Scripts directory receive a file
+// key, so clients cannot offer navigation to unrelated filesystem paths.
+Json parse_build_diagnostics(std::string_view raw_log, std::string_view phase,
+                             const std::filesystem::path& project_root);
+
+} // namespace faset::editor

include/faset/editor/build_service.hpp

@@ -3,6 +3,7 @@
 #include <filesystem>
 #include <memory>
 #include <string>
+#include <string_view>
 #include <vector>
 
 namespace faset::editor {
@@ -22,6 +23,7 @@ struct JobStatus {
     std::string id, kind, state{"queued"}, stage{"queued"};
     double progress{};
     std::string log, error;
+    Json diagnostics = Json::array();
     Json result = Json::object();
     bool finished() const {
         return state == "succeeded" || state == "failed" || state == "cancelled";
@@ -37,6 +39,9 @@ class BuildService {
     BuildService(const BuildService&) = delete;
     BuildService& operator=(const BuildService&) = delete;
     void scaffold(const std::string& name, int dimension);
+    // Explicit starter choice creates a runnable start scene. The legacy overload above
+    // remains intentionally scene-free for existing command-line and API callers.
+    void scaffold(const std::string& name, int dimension, std::string_view language);
     std::string start_build();
     std::string start_cook(Json resolved_scene);
     // Publishes output/generations/<id>; current.json changes only after all validation succeeds.

include/faset/editor/project_launcher.hpp

@@ -8,6 +8,7 @@ struct ProjectSelection {
     std::string name;
     int dimension = 3;
     bool create = false;
+    std::string language = "cpp";
 };
 // Records only existing, valid projects. Call after successful Session setup.
 void remember_project(const std::filesystem::path& project);
@@ -15,7 +16,9 @@ class ProjectLauncher {
   public:
     ProjectLauncher(render::Renderer&, const std::filesystem::path& engine_root,
                     const std::filesystem::path& initial_project = {},
-                    const std::filesystem::path& recent_store = {});
+                    const std::filesystem::path& recent_store = {},
+                    const std::optional<ProjectSelection>& retry = {},
+                    std::string creation_error = {});
     ~ProjectLauncher();
     void frame(const std::vector<render::Event>&);
     const render::Snapshot& snapshot() const;
@@ -30,5 +33,7 @@ class ProjectLauncher {
 std::optional<ProjectSelection>
 run_project_launcher(const std::filesystem::path& engine_root,
                      const std::filesystem::path& initial_project = {},
-                     std::uint64_t max_frames = 0, const std::filesystem::path& capture = {});
+                     std::uint64_t max_frames = 0, const std::filesystem::path& capture = {},
+                     const std::optional<ProjectSelection>& retry = {},
+                     std::string creation_error = {});
 } // namespace faset::editor

include/faset/editor/session.hpp

@@ -6,6 +6,7 @@
 #include <faset/editor/plugins.hpp>
 #include <memory>
 #include <mutex>
+#include <string_view>
 #include <thread>
 
 namespace faset::editor {
@@ -33,6 +34,7 @@ class Session {
         return plugins_ ? plugins_->panels() : Json::array();
     }
     void scaffold(const std::string& name, int dimension);
+    void scaffold(const std::string& name, int dimension, std::string_view language);
     const SessionConfig& config() const {
         return config_;
     }

src/editor/build_cache.cpp

@@ -0,0 +1,393 @@
+#include <faset/editor/build_cache.hpp>
+
+#include <faset/authoring/schema.hpp>
+#include <faset/core/hash.hpp>
+#include <faset/core/io.hpp>
+#include <faset/core/process.hpp>
+#include <cctype>
+#include <cstdlib>
+#include <set>
+#include <stdexcept>
+
+namespace faset::editor {
+namespace fs = std::filesystem;
+namespace {
+std::string normalized_hash(const Json& value) {
+    return sha256(value.dump());
+}
+std::pair<std::string, std::string> parse_define(const std::string& argument) {
+    if (!argument.starts_with("-D"))
+        throw std::invalid_argument("Gameplay configure arguments must be -DNAME=value options");
+    const auto equal = argument.find('=', 2);
+    if (equal == std::string::npos || equal == 2)
+        throw std::invalid_argument("Invalid gameplay configure definition");
+    const auto colon = argument.find(':', 2);
+    const auto key = argument.substr(2, colon < equal ? colon - 2 : equal - 2);
+    if (key.empty())
+        throw std::invalid_argument("Invalid gameplay configure definition");
+    return {key, argument.substr(equal + 1)};
+}
+void validate_configure_arguments(const BuildConfig& config) {
+    static const std::set<std::string> controlled = {
+        "FASET_GAMEPLAY_SOURCE_DIR", "FASET_ENABLE_LUA", "CMAKE_BUILD_TYPE", "BUILD_TESTING",
+        "FASET_BUILD_EDITOR", "FASET_BUILD_RENDERER", "FASET_BUILD_RUNTIME", "FASET_BUILD_ASSETS"};
+    for (const auto& argument : config.configure_arguments)
+        if (controlled.contains(parse_define(argument).first))
+            throw std::invalid_argument("Cannot override an engine-controlled CMake definition");
+}
+std::string configured_tool(const BuildConfig& config, std::string_view key,
+                            std::string fallback) {
+    if (auto value = configured_cmake_value(config, key))
+        return *value;
+    return fallback;
+}
+fs::path resolve_tool(const std::string& name, const fs::path& working_directory) {
+    const auto supplied = path_from_utf8(name);
+    if (supplied.has_parent_path())
+        return (supplied.is_absolute() ? supplied : working_directory / supplied).lexically_normal();
+    try {
+        return find_executable(name);
+    } catch (const std::exception&) {
+        // CMake reports a missing compiler. Keep the identity deterministic so
+        // fixture build tools can exercise publication without native compiling.
+        return supplied;
+    }
+}
+Json tool_identity(const std::string& name, const fs::path& working_directory) {
+    const auto resolved = resolve_tool(name, working_directory);
+    Json result = {{"path", path_to_utf8(resolved)}};
+    if (fs::is_regular_file(resolved))
+        result["sha256"] = sha256_file(resolved);
+    else
+        result["missing"] = true;
+    return result;
+}
+std::string slang_name(const BuildConfig& config) {
+    auto configured = configured_tool(config, "SLANGC_EXECUTABLE", "");
+    if (!configured.empty())
+        return configured;
+#ifdef _WIN32
+    const auto local = config.engine_root / ".cache/slang/bin/slangc.exe";
+#else
+    const auto local = config.engine_root / ".cache/slang/bin/slangc";
+#endif
+    if (fs::is_regular_file(local))
+        return path_to_utf8(local);
+    if (const char* sdk = std::getenv("VULKAN_SDK")) {
+#ifdef _WIN32
+        const auto sdk_tool = path_from_utf8(sdk) / "Bin/slangc.exe";
+#else
+        const auto sdk_tool = path_from_utf8(sdk) / "bin/slangc";
+#endif
+        if (fs::is_regular_file(sdk_tool))
+            return path_to_utf8(sdk_tool);
+    }
+    return "slangc";
+}
+Json recipe_files(const fs::path& engine_root) {
+    Json files = Json::object();
+    for (const auto& relative : {"CMakeLists.txt", "dependencies.lock.json",
+                                 "tools/compile_shader.py"}) {
+        const auto path = engine_root / relative;
+        files[relative] = fs::is_regular_file(path) ? Json(sha256_file(path)) : Json(nullptr);
+    }
+    const auto cmake = engine_root / "cmake";
+    if (fs::is_directory(cmake))
+        for (const auto& entry : fs::directory_iterator(cmake))
+            if (entry.is_regular_file() && entry.path().extension() == ".cmake")
+                files[generic_path_to_utf8(entry.path().lexically_relative(engine_root))] =
+                    sha256_file(entry.path());
+    return files;
+}
+bool within(const fs::path& path, const fs::path& directory) {
+    const auto relative = path.lexically_relative(directory);
+    if (relative.empty() || relative.is_absolute())
+        return false;
+    for (const auto& component : relative)
+        if (component == "..")
+            return false;
+    return true;
+}
+void refuse_symlink_ancestors(const fs::path& path) {
+    fs::path prefix;
+    for (const auto& component : path) {
+        prefix /= component;
+        std::error_code error;
+        if (fs::is_symlink(fs::symlink_status(prefix, error)))
+            throw std::runtime_error("Native build path contains a symlink: " +
+                                     path_to_utf8(prefix));
+    }
+}
+} // namespace
+
+std::optional<std::string> configured_cmake_value(const BuildConfig& config,
+                                                   std::string_view key) {
+    std::optional<std::string> result;
+    for (const auto& argument : config.configure_arguments) {
+        const auto [name, value] = parse_define(argument);
+        if (name == key)
+            result = value;
+    }
+    return result;
+}
+
+std::string BuildInputs::fingerprint() const {
+    return normalized_hash({{"format", "faset.build-inputs.v1"},
+                            {"source", source_hash},
+                            {"recipe", recipe_hash},
+                            {"toolchain", toolchain_hash}});
+}
+
+std::string gameplay_source_hash(const fs::path& project_root, const scripting::LuaProject& lua) {
+    Json files = Json::object();
+    const auto scripts = project_root / "Scripts";
+    if (fs::is_symlink(fs::symlink_status(scripts)))
+        throw std::runtime_error("Scripts directory must not be a symlink");
+    if (fs::is_directory(scripts))
+        for (const auto& entry : fs::recursive_directory_iterator(scripts)) {
+            if (entry.is_symlink())
+                throw std::runtime_error("Scripts source cannot be a symlink: " +
+                                         path_to_utf8(entry.path()));
+            if (entry.is_regular_file())
+                files[generic_path_to_utf8(entry.path().lexically_relative(project_root))] =
+                    sha256_file(entry.path());
+        }
+    return normalized_hash({{"format", "faset.gameplay-sources.v1"},
+                            {"files", files},
+                            {"lua", lua.fingerprint}});
+}
+
+BuildInputs capture_build_inputs(const BuildConfig& config, const scripting::LuaProject& lua) {
+    validate_configure_arguments(config);
+    BuildInputs result;
+    result.source_hash = gameplay_source_hash(config.project_root, lua);
+    result.recipe_hash = normalized_hash({{"format", "faset.build-recipe.v1"},
+                                          {"generator", config.generator},
+                                          {"configuration", config.configuration},
+                                          {"export_configuration", config.export_configuration},
+                                          {"configure_arguments", config.configure_arguments},
+                                          {"engine_files", recipe_files(config.engine_root)}});
+#ifdef _WIN32
+    constexpr auto default_c = "clang-cl", default_cxx = "clang-cl";
+#else
+    constexpr auto default_c = "clang", default_cxx = "clang++";
+#endif
+    result.toolchain_hash = normalized_hash(
+        {{"format", "faset.native-toolchain.v1"},
+         {"cmake", tool_identity(config.cmake, config.project_root)},
+         {"c", tool_identity(configured_tool(config, "CMAKE_C_COMPILER", default_c),
+                             config.project_root)},
+         {"cxx", tool_identity(configured_tool(config, "CMAKE_CXX_COMPILER", default_cxx),
+                               config.project_root)},
+         {"slang", tool_identity(slang_name(config), config.project_root)},
+         {"toolchain_file", tool_identity(configured_tool(config, "CMAKE_TOOLCHAIN_FILE", ""),
+                                          config.project_root)}});
+    return result;
+}
+
+fs::path stage_gameplay_sources(const BuildConfig& config, const BuildInputs& inputs,
+                                const scripting::LuaProject& lua, std::string_view job_id) {
+    const auto cache = config.cache_root.empty() ? config.project_root / ".faset" / "cache"
+                                                  : config.cache_root;
+    const auto snapshots = cache / "source-snapshots";
+    const auto destination = snapshots / inputs.source_hash;
+    const auto verified = [&](const fs::path& root) {
+        return fs::is_directory(root / "Scripts") && !fs::is_symlink(root) &&
+               gameplay_source_hash(root, lua) == inputs.source_hash;
+    };
+    if (fs::exists(destination)) {
+        if (!verified(destination))
+            throw std::runtime_error("Cached gameplay source snapshot is corrupt");
+        return destination / "Scripts";
+    }
+    const auto source = config.project_root / "Scripts";
+    if (!fs::is_directory(source) || fs::is_symlink(source))
+        throw std::runtime_error("Gameplay Scripts directory is missing or a symlink");
+    const auto staging = snapshots / (".staging-" + std::string(job_id));
+    if (fs::exists(staging))
+        throw std::runtime_error("Gameplay source staging directory already exists");
+    fs::create_directories(staging / "Scripts");
+    try {
+        for (const auto& entry : fs::recursive_directory_iterator(source)) {
+            if (entry.is_symlink())
+                throw std::runtime_error("Gameplay source snapshot contains a symlink");
+            const auto target = staging / "Scripts" / entry.path().lexically_relative(source);
+            if (entry.is_directory())
+                fs::create_directories(target);
+            else if (entry.is_regular_file()) {
+                fs::create_directories(target.parent_path());
+                fs::copy_file(entry.path(), target);
+            }
+        }
+        if (!verified(staging))
+            throw std::runtime_error("Gameplay sources changed while creating the build snapshot");
+        fs::rename(staging, destination);
+        return destination / "Scripts";
+    } catch (...) {
+        std::error_code ignored;
+        fs::remove_all(staging, ignored);
+        if (fs::exists(destination) && verified(destination))
+            return destination / "Scripts";
+        throw;
+    }
+}
+
+void ensure_native_toolchain_stamp(const BuildConfig& config, const fs::path& native_directory,
+                                   const BuildInputs& inputs) {
+    const auto project = fs::absolute(config.project_root).lexically_normal();
+    const auto base = fs::absolute(config.build_directory).lexically_normal();
+    const auto native = fs::absolute(native_directory).lexically_normal();
+    const auto cache = fs::absolute(config.cache_root.empty()
+                                        ? project / ".faset" / "cache"
+                                        : config.cache_root).lexically_normal();
+    const auto engine = fs::absolute(config.engine_root).lexically_normal();
+    if (native.parent_path() != base ||
+        (native.filename() != "Debug" && native.filename() != "Release" &&
+         native.filename() != "RelWithDebInfo"))
+        throw std::runtime_error("Native build path is outside its configured managed root");
+    const auto default_base = project / ".faset" / "build";
+    if ((within(base, project) && base != default_base) || within(project, base) ||
+        within(base, cache) || within(cache, base) || within(base, engine) ||
+        within(engine, base))
+        throw std::runtime_error("Native build root overlaps project, cache or engine files");
+    refuse_symlink_ancestors(native);
+    const auto stamp = native_directory / ".faset-toolchain.json";
+    bool matching = false, owned = false;
+    if (fs::is_regular_file(stamp) && !fs::is_symlink(stamp))
+        try {
+            const auto existing = read_json(stamp);
+            const auto format = existing.value("format", std::string()) ==
+                                "faset.toolchain-stamp";
+            const auto version = existing.value("version", 0);
+            owned = format &&
+                    ((version == 2 &&
+                      existing.value("project_root", std::string()) == path_to_utf8(project) &&
+                      existing.value("native_directory", std::string()) == path_to_utf8(native)) ||
+                     (version == 1 && base == default_base));
+            matching = owned &&
+                       existing.value("toolchain_hash", std::string()) == inputs.toolchain_hash;
+        } catch (const std::exception&) {
+            owned = false;
+        }
+    if (fs::exists(native_directory) && !owned)
+        throw std::runtime_error("Refusing to clear an unowned native build directory: " +
+                                 path_to_utf8(native));
+    if (!matching && fs::exists(native_directory))
+        fs::remove_all(native_directory);
+    fs::create_directories(native_directory);
+    atomic_write_json(stamp, {{"format", "faset.toolchain-stamp"},
+                              {"version", 2},
+                              {"project_root", path_to_utf8(project)},
+                              {"native_directory", path_to_utf8(native)},
+                              {"toolchain_hash", inputs.toolchain_hash}});
+}
+
+std::string build_package_key(const BuildInputs& inputs, const fs::path& native_directory,
+                              const std::string& configuration, const fs::path& player,
+                              const fs::path& exporter) {
+    Json files = Json::object();
+    for (const auto& [name, path] :
+         {std::pair{"player", player}, std::pair{"exporter", exporter},
+          std::pair{"cmake_cache", native_directory / "CMakeCache.txt"}}) {
+        if (!fs::is_regular_file(path) || fs::is_symlink(path))
+            throw std::runtime_error("Native build artifact is missing: " + path_to_utf8(path));
+        files[name] = sha256_file(path);
+    }
+    const auto shaders = native_directory / "shaders";
+    if (!fs::is_directory(shaders))
+        throw std::runtime_error("Native shader directory is missing");
+    for (const auto& entry : fs::recursive_directory_iterator(shaders)) {
+        if (entry.is_symlink())
+            throw std::runtime_error("Native shader is a symlink");
+        if (entry.is_regular_file())
+            files[generic_path_to_utf8(entry.path().lexically_relative(native_directory))] =
+                sha256_file(entry.path());
+    }
+    // On Windows, a changed runtime DLL must also invalidate a package hit.
+    for (const auto& root : {player.parent_path(), native_directory,
+                             native_directory / configuration}) {
+        if (!fs::is_directory(root))
+            continue;
+        for (const auto& entry : fs::directory_iterator(root)) {
+            auto extension = path_to_utf8(entry.path().extension());
+            for (auto& character : extension)
+                character = static_cast<char>(std::tolower(static_cast<unsigned char>(character)));
+            if (extension != ".dll")
+                continue;
+            if (!entry.is_regular_file() || entry.is_symlink())
+                throw std::runtime_error("Native runtime DLL is not a regular file");
+            files["dll:" + path_to_utf8(entry.path().filename())] = sha256_file(entry.path());
+        }
+    }
+    return normalized_hash({{"format", "faset.package-key.v1"},
+                            {"inputs", inputs.fingerprint()},
+                            {"configuration", configuration},
+                            {"files", files}});
+}
+
+bool validate_build_generation(const fs::path& directory, const std::string& package_key) {
+    try {
+        if (!fs::is_directory(directory) || fs::is_symlink(directory))
+            return false;
+        const auto manifest_file = directory / "manifest.json";
+        if (!fs::is_regular_file(manifest_file) || fs::is_symlink(manifest_file))
+            return false;
+        const auto manifest = read_json(manifest_file);
+        if (manifest.at("format") != "faset.build" || manifest.at("version") != 2 ||
+            manifest.at("package_key") != package_key || !manifest.at("files").is_array())
+            return false;
+        std::set<std::string> expected;
+        for (const auto& record : manifest.at("files")) {
+            const auto name = record.at("path").get<std::string>();
+            const auto relative = path_from_utf8(name);
+            if (relative.empty() || relative.is_absolute() || name == "manifest.json" ||
+                generic_path_to_utf8(relative) != name)
+                return false;
+            for (const auto& component : relative)
+                if (component == "." || component == "..")
+                    return false;
+            if (!expected.insert(name).second)
+                return false;
+            auto file = directory;
+            for (const auto& component : relative) {
+                file /= component;
+                if (fs::is_symlink(file))
+                    return false;
+            }
+            if (!fs::is_regular_file(file) || fs::is_symlink(file) ||
+                sha256_file(file) != record.at("sha256").get<std::string>() ||
+                fs::file_size(file) != record.at("size").get<std::uintmax_t>())
+                return false;
+        }
+        for (const auto& entry : fs::recursive_directory_iterator(directory)) {
+            if (entry.is_symlink())
+                return false;
+            if (entry.is_regular_file()) {
+                const auto name = generic_path_to_utf8(entry.path().lexically_relative(directory));
+                if (name != "manifest.json" && !expected.contains(name))
+                    return false;
+            }
+        }
+        const auto player_name = manifest.at("player").get<std::string>();
+        const auto schema_name = manifest.at("schema").get<std::string>();
+        if (!expected.contains(player_name) || !expected.contains(schema_name) ||
+            !expected.contains("faset_schema_exporter" + fs::path(player_name).extension().string()))
+            return false;
+        for (const auto* shader : {"vertexMain", "fragmentMain", "shadowMain",
+                                   "gpuVertexMain", "gpuShadowMain", "gpuCullMain",
+                                   "gpuHzbMain", "gpuPostCullMain"})
+            for (const auto* extension : {".spv", ".reflection.json"})
+                if (!expected.contains("shaders/" + std::string(shader) + extension))
+                    return false;
+        const auto schema = read_json(directory / schema_name);
+        if (schema.at("format") != "faset.schema" || schema.at("version") != 1 ||
+            schema.at("build_fingerprint") != manifest.at("fingerprint"))
+            return false;
+        (void)authoring::gameplay_schemas(schema);
+        return true;
+    } catch (const std::exception&) {
+        return false;
+    }
+}
+
+} // namespace faset::editor

src/editor/build_diagnostics.cpp

@@ -0,0 +1,147 @@
+#include <faset/editor/build_diagnostics.hpp>
+
+#include <faset/core/io.hpp>
+#include <algorithm>
+#include <cctype>
+#include <regex>
+#include <sstream>
+
+namespace faset::editor {
+namespace {
+constexpr std::size_t max_rows = 200;
+constexpr std::size_t max_line = 8192;
+constexpr std::size_t max_message = 2048;
+
+std::string strip_ansi(std::string_view text) {
+    std::string clean;
+    clean.reserve(text.size());
+    for (std::size_t index = 0; index < text.size(); ++index) {
+        if (text[index] == '\x1b' && index + 1 < text.size() && text[index + 1] == '[') {
+            index += 2;
+            while (index < text.size() && !(text[index] >= '@' && text[index] <= '~'))
+                ++index;
+            continue;
+        }
+        clean += text[index];
+    }
+    return clean;
+}
+std::string normalize_path(std::string path) {
+    std::replace(path.begin(), path.end(), '\\', '/');
+    path = std::filesystem::path(path).lexically_normal().generic_string();
+    while (path.size() > 1 && path.back() == '/')
+        path.pop_back();
+    return path;
+}
+bool has_windows_drive(std::string_view path) {
+    return path.size() >= 2 && std::isalpha(static_cast<unsigned char>(path[0])) &&
+           path[1] == ':';
+}
+std::string lower_ascii(std::string value) {
+    for (auto& character : value)
+        character = static_cast<char>(std::tolower(static_cast<unsigned char>(character)));
+    return value;
+}
+std::string project_source(std::string raw, const std::filesystem::path& project_root) {
+    if (raw.starts_with("lua: "))
+        raw.erase(0, 5);
+    std::replace(raw.begin(), raw.end(), '\\', '/');
+    for (const auto& component : std::filesystem::path(raw))
+        if (component == "..")
+            return {};
+    const auto path = normalize_path(std::move(raw));
+    const auto root = normalize_path(path_to_utf8(project_root));
+    std::string relative;
+    if (!path.empty() && path[0] != '/' && !has_windows_drive(path))
+        relative = path;
+    else {
+        const auto windows = has_windows_drive(path) && has_windows_drive(root);
+        const auto comparable_path = windows ? lower_ascii(path) : path;
+        const auto comparable_root = windows ? lower_ascii(root) : root;
+        if (comparable_path.size() <= comparable_root.size() ||
+            !comparable_path.starts_with(comparable_root) ||
+            comparable_path[comparable_root.size()] != '/')
+            return {};
+        relative = path.substr(root.size() + 1);
+    }
+    if (!relative.starts_with("Scripts/") || relative.size() <= 8)
+        return {};
+    if (relative.find("/../") != std::string::npos || relative.ends_with("/.."))
+        return {};
+    return relative;
+}
+int positive_number(const std::string& text) {
+    try {
+        const auto value = std::stoll(text);
+        return value > 0 && value <= 100000000 ? static_cast<int>(value) : 0;
+    } catch (const std::exception&) {
+        return 0;
+    }
+}
+} // namespace
+
+Json parse_build_diagnostics(std::string_view raw_log, std::string_view phase,
+                             const std::filesystem::path& project_root) {
+    static const std::regex msvc(
+        R"(^(.+)\(([0-9]+)(?:,([0-9]+))?\)\s*:\s*(fatal error|error|warning|note)\s*([A-Za-z]+[0-9]+)?\s*:\s*(.*)$)");
+    static const std::regex clang_column(
+        R"(^(.+):([0-9]+):([0-9]+):\s*(fatal error|error|warning|note)(?:\s+([A-Za-z]+[0-9]+))?\s*:\s*(.*)$)");
+    static const std::regex clang_line(
+        R"(^(.+):([0-9]+):\s*(fatal error|error|warning|note)(?:\s+([A-Za-z]+[0-9]+))?\s*:\s*(.*)$)");
+    static const std::regex lua(R"(^(.+\.lua):([0-9]+):\s*(.*)$)");
+    Json rows = Json::array();
+    std::istringstream stream{std::string(raw_log)};
+    std::string raw;
+    while (rows.size() < max_rows && std::getline(stream, raw)) {
+        if (raw.size() > max_line)
+            raw.resize(max_line);
+        auto line = strip_ansi(raw);
+        if (!line.empty() && line.back() == '\r')
+            line.pop_back();
+        std::smatch match;
+        std::string file, message, severity, code;
+        int row = 0, column = 0;
+        bool has_column = false;
+        if (std::regex_match(line, match, msvc) ||
+            std::regex_match(line, match, clang_column)) {
+            file = match[1].str();
+            row = positive_number(match[2].str());
+            has_column = !match[3].str().empty();
+            column = positive_number(match[3].str());
+            severity = match[4].str();
+            code = match[5].str();
+            message = match[6].str();
+        } else if (std::regex_match(line, match, clang_line)) {
+            file = match[1].str();
+            row = positive_number(match[2].str());
+            severity = match[3].str();
+            code = match[4].str();
+            message = match[5].str();
+        } else if (std::regex_match(line, match, lua)) {
+            file = match[1].str();
+            row = positive_number(match[2].str());
+            severity = "error";
+            message = match[3].str();
+        } else
+            continue;
+        if (row == 0 || (has_column && column == 0))
+            continue;
+        if (severity == "fatal error")
+            severity = "error";
+        if (message.size() > max_message)
+            message.resize(max_message);
+        Json diagnostic{{"severity", severity},
+                        {"phase", std::string(phase)},
+                        {"message", message},
+                        {"line", row}};
+        if (column > 0)
+            diagnostic["column"] = column;
+        if (!code.empty())
+            diagnostic["code"] = code;
+        if (auto relative = project_source(file, project_root); !relative.empty())
+            diagnostic["file"] = std::move(relative);
+        rows.push_back(std::move(diagnostic));
+    }
+    return rows;
+}
+} // namespace faset::editor

src/editor/build_service.cpp

@@ -1,3 +1,4 @@
+#include <algorithm>
 #include <atomic>
 #include <cctype>
 #include <chrono>
@@ -6,9 +7,12 @@
 #include <faset/assets/asset_data.hpp>
 #include <faset/assets/asset_pipeline.hpp>
 #include <faset/authoring/schema.hpp>
+#include <faset/authoring/service.hpp>
 #include <faset/core/hash.hpp>
 #include <faset/core/io.hpp>
 #include <faset/core/process.hpp>
+#include <faset/editor/build_cache.hpp>
+#include <faset/editor/build_diagnostics.hpp>
 #include <faset/editor/build_service.hpp>
 #include <faset/scripting/project.hpp>
 #include <fstream>
@@ -89,11 +93,107 @@ fs::path build_executable(const fs::path& build, const std::string& configuratio
     }
     throw std::runtime_error("Build did not produce " + target);
 }
+Json starter_scene(const std::string& name, int dimension, std::string_view language) {
+    const auto schemas = authoring::builtin_schemas();
+    auto scene = authoring::make_scene(name + " — Starter", dimension);
+    scene["simulation"] = authoring::default_simulation_settings();
+    auto add_component = [&](Json& entity, std::string type, Json fields) {
+        entity["components"].push_back({{"id", new_id()},
+                                        {"type", std::move(type)},
+                                        {"version", 1},
+                                        {"fields", std::move(fields)}});
+    };
+    auto ground = authoring::make_entity(schemas, "Ground");
+    auto& ground_pose = ground["components"][0]["fields"];
+    ground_pose["position"] = {0, -1, 0};
+    ground_pose["scale"] = dimension == 2 ? Json::array({12, 1, 1}) : Json::array({12, 1, 12});
+    if (dimension == 2) {
+        add_component(ground, "faset.sprite", {{"size", {1, 1}},
+                                                {"color", {0.20, 0.25, 0.31, 1}}});
+    } else {
+        add_component(ground, "faset.mesh", {{"asset", "builtin:cube"},
+                                              {"color", {0.20, 0.25, 0.31, 1}}});
+    }
+    auto ground_body = schemas.default_fields("faset.rigid_body_" + std::to_string(dimension) +
+                                              "d");
+    ground_body["body_type"] = "static";
+    add_component(ground, "faset.rigid_body_" + std::to_string(dimension) + "d",
+                  std::move(ground_body));
+    scene["entities"].push_back(std::move(ground));
+
+    auto actor = authoring::make_entity(schemas, "Player");
+    actor["components"][0]["fields"]["position"] = {0, 1, 0};
+    if (dimension == 2) {
+        add_component(actor, "faset.sprite", {{"size", {0.8, 0.8}},
+                                               {"color", {0.25, 0.72, 0.85, 1}}});
+    } else {
+        actor["components"][0]["fields"]["scale"] = {0.8, 0.8, 0.8};
+        add_component(actor, "faset.mesh", {{"asset", "builtin:cube"},
+                                             {"color", {0.25, 0.72, 0.85, 1}}});
+    }
+    add_component(actor, "faset.rigid_body_" + std::to_string(dimension) + "d",
+                  schemas.default_fields("faset.rigid_body_" + std::to_string(dimension) + "d"));
+    add_component(actor, language == "lua" ? "starter.player" : "gameplay.character",
+                  {{"speed", 4.0}, {"jump_speed", 5.0}});
+    scene["entities"].push_back(std::move(actor));
+    if (dimension == 3) {
+        auto camera = authoring::make_entity(schemas, "Camera");
+        camera["components"][0]["fields"]["position"] = {0, 2.5, 8};
+        camera["components"][0]["fields"]["rotation"] = {-0.18, 0, 0};
+        add_component(camera, "faset.camera", schemas.default_fields("faset.camera"));
+        scene["entities"].push_back(std::move(camera));
+        auto sun = authoring::make_entity(schemas, "Sun");
+        add_component(sun, "faset.light", schemas.default_fields("faset.light"));
+        scene["entities"].push_back(std::move(sun));
+    }
+    // Validate every built-in component now; the behavior component is validated by
+    // SchemaExporter from the selected C++/Lua module during the first build.
+    auto builtins = scene;
+    for (auto& entity : builtins["entities"]) {
+        auto& components = entity["components"].get_ref<Json::array_t&>();
+        std::erase_if(components, [](const Json& component) {
+            return !component.at("type").get<std::string>().starts_with("faset.");
+        });
+    }
+    authoring::validate_scene(builtins, schemas);
+    return scene;
+}
+struct StarterCreateLock {
+    fs::path path;
+    explicit StarterCreateLock(const fs::path& root) : path(root / ".faset/starter-create.lock") {
+        if (!fs::create_directory(path))
+            throw std::runtime_error("Another project creation is in progress; remove a stale " +
+                                     path_to_utf8(path) + " only after closing that Editor");
+    }
+    ~StarterCreateLock() {
+        std::error_code ignored;
+        fs::remove_all(path, ignored);
+    }
+    StarterCreateLock(const StarterCreateLock&) = delete;
+    StarterCreateLock& operator=(const StarterCreateLock&) = delete;
+};
+void require_new_starter_directory(const fs::path& root) {
+    // BuildService creates .faset/cache in its constructor. Any other content is
+    // user-owned, including a previous starter, and must be left intact.
+    for (const auto& entry : fs::directory_iterator(root)) {
+        if (entry.path().filename() != ".faset" || !entry.is_directory() ||
+            entry.is_symlink())
+            throw std::runtime_error("Create requires a new or empty project directory");
+        for (const auto& internal : fs::directory_iterator(entry.path())) {
+            if (internal.path().filename() == "starter-create.lock" &&
+                internal.is_directory() && !internal.is_symlink())
+                continue;
+            if (internal.path().filename() != "cache" || !internal.is_directory() ||
+                internal.is_symlink() || !fs::is_empty(internal.path()))
+                throw std::runtime_error("Create requires a new or empty project directory");
+        }
+    }
+}
 } // namespace
 Json JobStatus::json() const {
     return {{"id", id},       {"kind", kind},         {"state", state},
             {"stage", stage}, {"progress", progress}, {"log", log},
-            {"error", error}, {"result", result}};
+            {"error", error}, {"diagnostics", diagnostics}, {"result", result}};
 }
 void write_cooked_scene(const fs::path& path, const Json& scene) {
     validate_scene(scene);
@@ -198,6 +298,13 @@ struct BuildService::Impl {
     std::string run(Job& job, std::vector<std::string> arguments, const fs::path& cwd) {
         if (job.cancelled)
             throw Cancelled{};
+        std::string phase;
+        std::size_t diagnostics_before;
+        {
+            std::lock_guard lock(job.mutex);
+            phase = job.status.stage;
+            diagnostics_before = job.status.diagnostics.size();
+        }
         std::string description = "$";
         for (const auto& argument : arguments)
             description += " " + Json(argument).dump();
@@ -205,6 +312,35 @@ struct BuildService::Impl {
         log(job, description);
         Process process({std::move(arguments), cwd, {}});
         std::string output;
+        std::string pending;
+        const auto collect_diagnostics = [&](std::string_view chunk, bool final) {
+            pending.append(chunk);
+            std::size_t newline;
+            while ((newline = pending.find('\n')) != std::string::npos) {
+                const auto line = pending.substr(0, newline + 1);
+                pending.erase(0, newline + 1);
+                const auto found = parse_build_diagnostics(line, phase, config.project_root);
+                if (found.empty())
+                    continue;
+                std::lock_guard lock(job.mutex);
+                for (const auto& row : found) {
+                    if (job.status.diagnostics.size() >= 200)
+                        break;
+                    job.status.diagnostics.push_back(row);
+                }
+            }
+            if (final && !pending.empty()) {
+                const auto found = parse_build_diagnostics(pending, phase, config.project_root);
+                std::lock_guard lock(job.mutex);
+                for (const auto& row : found) {
+                    if (job.status.diagnostics.size() >= 200)
+                        break;
+                    job.status.diagnostics.push_back(row);
+                }
+                pending.clear();
+            } else if (pending.size() > 8192)
+                pending.erase(0, pending.size() - 8192);
+        };
         while (true) {
             if (job.cancelled) {
                 process.cancel();
@@ -214,14 +350,29 @@ struct BuildService::Impl {
             }
             auto poll = process.poll();
             log(job, poll.output);
+            collect_diagnostics(poll.output, !poll.running);
             output += poll.output;
             if (output.size() > max_log_bytes)
                 output.erase(0, output.size() - max_log_bytes);
             if (!poll.running) {
-                if (poll.exit_code.value_or(1) != 0)
+                if (poll.exit_code.value_or(1) != 0) {
+                    std::lock_guard lock(job.mutex);
+                    bool has_error = false;
+                    for (std::size_t index = diagnostics_before;
+                         index < job.status.diagnostics.size(); ++index)
+                        if (job.status.diagnostics[index].value("severity", "") == "error")
+                            has_error = true;
+                    if (!has_error && job.status.diagnostics.size() < 200)
+                        job.status.diagnostics.push_back(
+                            {{"severity", "error"},
+                             {"phase", phase},
+                             {"message", "Process exited with code " +
+                                             std::to_string(poll.exit_code.value_or(1)) +
+                                             "; see job log"}});
                     throw std::runtime_error("Process exited with code " +
                                              std::to_string(poll.exit_code.value_or(1)) +
                                              "; see job log");
+                }
                 return output;
             }
             std::this_thread::sleep_for(std::chrono::milliseconds(15));
@@ -240,19 +391,23 @@ struct BuildService::Impl {
         }
     }
     Json build(Job& job, bool exporting = false) {
+        const auto started = std::chrono::steady_clock::now();
+        const auto milliseconds = [](auto from, auto to) {
+            return std::chrono::duration_cast<std::chrono::milliseconds>(to - from).count();
+        };
         const auto& configuration = exporting ? config.export_configuration : config.configuration;
         const auto native_directory = config.build_directory / configuration;
         checkpoint(job, "Configuring gameplay", .05);
         job.lua = scripting::loadLuaProject(config.project_root);
-        const auto cpp = config.project_root / "Scripts" / "Gameplay.cpp";
+        const auto inputs = capture_build_inputs(config, job.lua);
-        const auto hpp = config.project_root / "Scripts" / "Gameplay.hpp";
+        const auto staged_scripts = stage_gameplay_sources(config, inputs, job.lua, job.status.id);
+        const auto cpp = staged_scripts / "Gameplay.cpp";
+        const auto hpp = staged_scripts / "Gameplay.hpp";
         const bool has_cpp = fs::is_regular_file(cpp), has_hpp = fs::is_regular_file(hpp);
         if (has_cpp != has_hpp || (!has_cpp && !job.lua.enabled()))
             throw std::runtime_error("Project requires Scripts/Gameplay.cpp and Gameplay.hpp, "
                                      "or Lua entry scripts declared in project.faset.json");
-        const auto cpp_source = has_cpp ? read_text(cpp) : std::string{};
+        ensure_native_toolchain_stamp(config, native_directory, inputs);
-        const auto hpp_source = has_hpp ? read_text(hpp) : std::string{};
-        fs::create_directories(native_directory);
         std::vector<std::string> arguments = {config.cmake,
                                               "-S",
                                               path_to_utf8(config.engine_root),
@@ -267,37 +422,100 @@ struct BuildService::Impl {
                                               "-DFASET_BUILD_RUNTIME=ON",
                                               "-DFASET_BUILD_ASSETS=ON",
                                               "-DFASET_GAMEPLAY_SOURCE_DIR=" +
-                                                  path_to_utf8(config.project_root / "Scripts")};
+                                                  path_to_utf8(staged_scripts)};
-        bool compiler_overridden = false;
+        if (!configured_cmake_value(config, "CMAKE_C_COMPILER")) {
-        for (const auto& arg : config.configure_arguments)
-            if (arg.starts_with("-DCMAKE_CXX_COMPILER="))
-                compiler_overridden = true;
-        if (!compiler_overridden) {
 #ifdef _WIN32
             auto compiler = find_executable("clang-cl");
             arguments.push_back("-DCMAKE_C_COMPILER=" + path_to_utf8(compiler));
-            arguments.push_back("-DCMAKE_CXX_COMPILER=" + path_to_utf8(compiler));
 #else
             arguments.push_back("-DCMAKE_C_COMPILER=" + path_to_utf8(find_executable("clang")));
+#endif
+        }
+        if (!configured_cmake_value(config, "CMAKE_CXX_COMPILER")) {
+#ifdef _WIN32
+            arguments.push_back("-DCMAKE_CXX_COMPILER=" +
+                                path_to_utf8(find_executable("clang-cl")));
+#else
             arguments.push_back("-DCMAKE_CXX_COMPILER=" + path_to_utf8(find_executable("clang++")));
 #endif
         }
-        arguments.insert(arguments.end(), config.configure_arguments.begin(),
+        for (const auto& argument : config.configure_arguments) {
-                         config.configure_arguments.end());
+            const auto equal = argument.find('=');
+            const auto colon = argument.find(':', 2);
+            const auto key = argument.substr(2, colon < equal ? colon - 2 : equal - 2);
+            if (key == "CMAKE_C_COMPILER" || key == "CMAKE_CXX_COMPILER" ||
+                key == "SLANGC_EXECUTABLE" || key == "CMAKE_TOOLCHAIN_FILE") {
+                const auto value = path_from_utf8(argument.substr(equal + 1));
+                if (value.has_parent_path() && value.is_relative()) {
+                    arguments.push_back(argument.substr(0, equal + 1) +
+                                        path_to_utf8((config.project_root / value).lexically_normal()));
+                    continue;
+                }
+            }
+            arguments.push_back(argument);
+        }
         arguments.push_back("-DCMAKE_BUILD_TYPE=" + configuration);
         // Project declarations, not a stale cache or a user-supplied override, determine
         // whether the packaged game has a Lua VM linked into it.
         arguments.push_back(std::string("-DFASET_ENABLE_LUA=") +
                             (job.lua.enabled() ? "ON" : "OFF"));
         run(job, std::move(arguments), config.project_root);
+        const auto configured = std::chrono::steady_clock::now();
         checkpoint(job, "Compiling and linking Player", .25);
         run(job,
             {config.cmake, "--build", path_to_utf8(native_directory), "--config", configuration,
              "--parallel", "4", "--target", "faset_player", "faset_schema_exporter"},
             config.project_root);
-        checkpoint(job, "Exporting gameplay schema", .58);
+        const auto compiled = std::chrono::steady_clock::now();
         auto player = build_executable(native_directory, configuration, "faset_player");
         auto exporter = build_executable(native_directory, configuration, "faset_schema_exporter");
+        const auto package_key =
+            build_package_key(inputs, native_directory, configuration, player, exporter);
+        const auto source_unchanged = [&] {
+            if (gameplay_source_hash(config.project_root,
+                                     scripting::loadLuaProject(config.project_root)) !=
+                inputs.source_hash)
+                throw std::runtime_error("Gameplay sources changed during the build; build again");
+        };
+        const auto result_for = [&](const std::string& id, const std::string& fingerprint,
+                                    bool reused) -> Json {
+            const auto directory = config.cache_root / "builds" / id;
+            const auto finished = std::chrono::steady_clock::now();
+            return {{"generation", id},
+                    {"directory", path_to_utf8(directory)},
+                    {"build_directory", path_to_utf8(native_directory)},
+                    {"configuration", configuration},
+                    {"player", path_to_utf8(directory / ("faset_player" + executable_suffix()))},
+                    {"schema", path_to_utf8(directory / "schema.json")},
+                    {"lua_enabled", job.lua.enabled()},
+                    {"lua_fingerprint", job.lua.fingerprint},
+                    {"fingerprint", fingerprint},
+                    {"schema_cache_hit", reused},
+                    {"generation_reused", reused},
+                    {"phase_times_ms",
+                     {{"configure", milliseconds(started, configured)},
+                      {"native_build", milliseconds(configured, compiled)},
+                      {"schema_package", reused ? 0 : milliseconds(compiled, finished)},
+                      {"total", milliseconds(started, finished)}}}};
+        };
+        const auto pointer_file = config.cache_root / "last_build.json";
+        if (fs::is_regular_file(pointer_file))
+            try {
+                const auto pointer = read_json(pointer_file);
+                const auto id = pointer.at("generation").get<std::string>();
+                const auto candidate =
+                    project_path(config.cache_root, fs::path("builds") / id);
+                if (validate_build_generation(candidate, package_key)) {
+                    source_unchanged();
+                    checkpoint(job, "Reusing verified build generation", .68);
+                    const auto manifest = read_json(candidate / "manifest.json");
+                    log(job, "Verified schema/package cache hit: " + id + "\n");
+                    return result_for(id, manifest.at("fingerprint").get<std::string>(), true);
+                }
+            } catch (const std::exception&) {
+                // A bad pointer or old/corrupt generation is a cache miss.
+            }
+        checkpoint(job, "Exporting gameplay schema", .58);
         const auto staging = config.cache_root / "builds" / (".staging-" + job.status.id);
         const auto generation = config.cache_root / "builds" / job.status.id;
         fs::create_directories(staging);
@@ -322,10 +540,7 @@ struct BuildService::Impl {
             // Validate the complete metadata before publishing either the schema or
             // its Player generation. Session uses this same authoring contract.
             (void)authoring::gameplay_schemas(schema);
-            std::string fingerprint = sha256_file(player) + sha256_file(exporter) +
+            const auto fingerprint = package_key;
-                                      read_text(native_directory / "CMakeCache.txt");
-            fingerprint += cpp_source + hpp_source + job.lua.fingerprint;
-            fingerprint = sha256(fingerprint);
             schema["build_fingerprint"] = fingerprint;
             if (job.lua.enabled())
                 schema["lua_fingerprint"] = job.lua.fingerprint;
@@ -341,36 +556,39 @@ struct BuildService::Impl {
                                      "gpuHzbMain.reflection.json", "gpuPostCullMain.reflection.json"})
                 copy_required_file(native_directory / "shaders" / file, staging / "shaders" / file);
             copy_runtime_libraries(job, player, staging, native_directory, configuration);
+            Json files = Json::array();
+            for (const auto& entry : fs::recursive_directory_iterator(staging)) {
+                if (entry.is_symlink())
+                    throw std::runtime_error("Build generation contains a symlink");
+                if (entry.is_regular_file())
+                    files.push_back(
+                        {{"path", generic_path_to_utf8(entry.path().lexically_relative(staging))},
+                         {"sha256", sha256_file(entry.path())},
+                         {"size", entry.file_size()}});
+            }
             Json manifest{{"format", "faset.build"},
-                          {"version", 1},
+                          {"version", 2},
                           {"id", job.status.id},
                           {"fingerprint", fingerprint},
+                          {"package_key", package_key},
                           {"configuration", configuration},
                           {"lua_enabled", job.lua.enabled()},
                           {"lua_fingerprint", job.lua.fingerprint},
                           {"player", "faset_player" + executable_suffix()},
-                          {"schema", "schema.json"}};
+                          {"schema", "schema.json"},
+                          {"files", files}};
             atomic_write_json(staging / "manifest.json", manifest);
             checkpoint(job, "Publishing build generation", .68);
             if (job.lua.enabled() &&
                 scripting::loadLuaProject(staging).fingerprint != job.lua.fingerprint)
                 throw std::runtime_error("Lua build snapshot changed during schema export");
-            if (scripting::loadLuaProject(config.project_root).fingerprint != job.lua.fingerprint ||
+            source_unchanged();
-                has_cpp != fs::is_regular_file(cpp) || has_hpp != fs::is_regular_file(hpp) ||
+            if (!validate_build_generation(staging, package_key))
-                (has_cpp && (read_text(cpp) != cpp_source || read_text(hpp) != hpp_source)))
+                throw std::runtime_error("Build generation failed final integrity validation");
-                throw std::runtime_error("Gameplay sources changed during the build; build again");
             fs::rename(staging, generation);
             atomic_write_json(config.cache_root / "last_build.json",
                               {{"generation", job.status.id}, {"fingerprint", fingerprint}});
-            return {{"generation", job.status.id},
+            return result_for(job.status.id, fingerprint, false);
-                    {"directory", path_to_utf8(generation)},
-                    {"build_directory", path_to_utf8(native_directory)},
-                    {"configuration", configuration},
-                    {"player", path_to_utf8(generation / ("faset_player" + executable_suffix()))},
-                    {"schema", path_to_utf8(generation / "schema.json")},
-                    {"lua_enabled", job.lua.enabled()},
-                    {"lua_fingerprint", job.lua.fingerprint},
-                    {"fingerprint", fingerprint}};
         } catch (...) {
             std::error_code error;
             fs::remove_all(staging, error);
@@ -785,6 +1003,97 @@ void BuildService::scaffold(const std::string& name, int dimension) {
     if (!fs::exists(c.project_root / ".gitignore"))
         atomic_write(c.project_root / ".gitignore", ".faset/\nExports/\n");
 }
+void BuildService::scaffold(const std::string& name, int dimension,
+                            std::string_view language) {
+    if (name.empty() || (dimension != 2 && dimension != 3) ||
+        (language != "cpp" && language != "lua"))
+        throw std::invalid_argument("Starter requires a name, 2D/3D and cpp/lua language");
+    const auto& c = impl_->config;
+    StarterCreateLock lock(c.project_root);
+    require_new_starter_directory(c.project_root);
+    const auto scene = starter_scene(name, dimension, language);
+    const auto source = c.engine_root / "tools/project_templates" /
+                        (language == "lua" ? "lua-main.lua" : "Gameplay.cpp");
+    const auto module = read_text(source);
+    const auto header = language == "cpp"
+                            ? read_text(c.engine_root / "tools/project_templates/Gameplay.hpp")
+                            : std::string();
+    const auto lua_annotations = language == "lua"
+                                     ? read_text(c.engine_root / "tools/lua/faset.lua")
+                                     : std::string();
+    const auto lua_config = language == "lua"
+                                ? read_json(c.engine_root / "tools/lua/luarc.json")
+                                : Json();
+    const auto lua_scripts_config =
+        language == "lua" ? read_json(c.engine_root / "tools/lua/luarc-scripts.json") : Json();
+    const auto stage = lock.path / "staged";
+    fs::create_directories(stage / "Scripts");
+    fs::create_directories(stage / "Scenes");
+    fs::create_directories(stage / "Assets");
+    if (language == "cpp") {
+        atomic_write(stage / "Scripts/Gameplay.cpp", module);
+        atomic_write(stage / "Scripts/Gameplay.hpp", header);
+    } else {
+        atomic_write(stage / "Scripts/main.lua", module);
+        atomic_write(stage / ".faset/lua/faset.lua", lua_annotations);
+        atomic_write_json(stage / ".luarc.json", lua_config);
+        atomic_write_json(stage / "Scripts/.luarc.json", lua_scripts_config);
+    }
+    atomic_write_json(stage / "Scenes/main.scene.json", scene);
+    Json project = {{"format", "faset.project"},
+                    {"version", 1},
+                    {"id", new_id()},
+                    {"name", name},
+                    {"dimension", dimension},
+                    {"start_scene", "Scenes/main.scene.json"}};
+    if (language == "lua")
+        project["scripting"] = {{"lua", {{"scripts", Json::array({"Scripts/main.lua"})}}}};
+    atomic_write_json(stage / "project.faset.json", project);
+    atomic_write(stage / ".gitignore",
+                 ".faset/*\n!.faset/lua/\n.faset/lua/*\n!.faset/lua/faset.lua\nExports/\n");
+    std::vector<std::pair<fs::path, std::string>> created;
+    auto publish = [&](const fs::path& relative) {
+        const auto source = stage / relative;
+        const auto target = c.project_root / relative;
+        fs::create_directories(target.parent_path());
+        if (!fs::copy_file(source, target, fs::copy_options::none))
+            throw std::runtime_error("Starter destination appeared during creation: " +
+                                     path_to_utf8(target));
+        created.emplace_back(target, sha256_file(source));
+    };
+    try {
+        publish(".gitignore");
+        if (language == "cpp") {
+            publish("Scripts/Gameplay.cpp");
+            publish("Scripts/Gameplay.hpp");
+        } else {
+            publish("Scripts/main.lua");
+            publish(".faset/lua/faset.lua");
+            publish(".luarc.json");
+            publish("Scripts/.luarc.json");
+        }
+        publish("Scenes/main.scene.json");
+        fs::create_directory(c.project_root / "Assets");
+        // The manifest is the final commit marker: an interrupted create has no
+        // valid project record and never replaces an existing project file.
+        publish("project.faset.json");
+    } catch (...) {
+        for (auto it = created.rbegin(); it != created.rend(); ++it) {
+            try {
+                std::error_code ignored;
+                if (fs::is_regular_file(it->first, ignored) &&
+                    sha256_file(it->first) == it->second)
+                    fs::remove(it->first, ignored);
+            } catch (...) { /* Preserve the original create failure. */
+            }
+        }
+        for (const auto& relative : {"Assets", "Scenes", "Scripts", ".faset/lua"}) {
+            std::error_code ignored;
+            fs::remove(c.project_root / relative, ignored); // Empty directories only.
+        }
+        throw;
+    }
+}
 std::string BuildService::start_build() {
     return impl_->enqueue("build");
 }

src/editor/session.cpp

@@ -2,6 +2,7 @@
 #include <chrono>
 #include <faset/core/hash.hpp>
 #include <faset/core/io.hpp>
+#include <faset/editor/build_cache.hpp>
 #include <faset/editor/session.hpp>
 #include <faset/scripting/project.hpp>
 
@@ -112,6 +113,10 @@ void Session::scaffold(const std::string& name, int dimension) {
     builds_.scaffold(name, dimension);
     log("Created project: " + name);
 }
+void Session::scaffold(const std::string& name, int dimension, std::string_view language) {
+    builds_.scaffold(name, dimension, language);
+    log("Created " + std::string(language) + " starter project: " + name);
+}
 Json Session::assets_list() const {
     Json list = Json::array();
     const auto directory = assets_.cache_root() / "assets";
@@ -132,20 +137,7 @@ Json Session::assets_list() const {
 }
 std::string Session::source_signature() const {
     const auto lua = scripting::loadLuaProject(config_.project_root);
-    const auto directory = config_.project_root / "Scripts";
+    return gameplay_source_hash(config_.project_root, lua);
-    std::vector<std::filesystem::path> files;
-    if (std::filesystem::exists(directory))
-        for (const auto& file : std::filesystem::recursive_directory_iterator(directory))
-            if (file.is_regular_file() && (!lua.enabled() || file.path().extension() != ".lua"))
-                files.push_back(file.path());
-    std::sort(files.begin(), files.end());
-    std::string contents;
-    for (const auto& file : files)
-        contents += generic_path_to_utf8(file.lexically_relative(directory)) + ":" +
-                    sha256_file(file) + "\n";
-    if (lua.enabled())
-        contents += "lua:" + lua.fingerprint + "\n";
-    return sha256(contents);
 }
 Json Session::schema_status() const {
     try {
@@ -514,51 +506,102 @@ void Session::register_commands() {
                         {"configuration_created", create_configuration},
                         {"scripts_configuration_created", create_scripts_configuration}};
         });
-    commands_.add(
+    const auto open_source = [this](const Json& args, bool lua_only) -> Json {
-        "faset_script_open",
+        const auto path_error = lua_only ? "lua.source" : "source.path";
-        "Open a project Lua source in an external editor, never as an executable. Optional editor "
+        const auto editor_error = lua_only ? "lua.editor" : "source.editor";
-        "is an argv array (default: project editor.script_editor, then zed). Exact {file} and "
+        const auto requested = path_from_utf8(args.at("path").get<std::string>());
-        "{project} arguments are replaced; no shell expansion is performed.",
+        require(!requested.empty() && !requested.is_absolute(), path_error,
-        schema({{"path", text}, {"editor", {{"type", "array"}, {"items", text}, {"minItems", 1}}}},
+                "Choose an existing source inside Scripts");
-               {"path"}),
+        for (const auto& part : requested)
-        [&](const Json& args) {
+            require(part != "..", path_error, "Source path cannot traverse outside Scripts");
-            const auto file = project_path(config_.project_root,
+        std::filesystem::path file;
-                                           path_from_utf8(args.at("path").get<std::string>()));
+        try {
-            require(file.extension() == ".lua" && std::filesystem::is_regular_file(file),
+            file = project_path(config_.project_root, requested);
-                    "lua.source", "Select an existing .lua file inside the project");
+        } catch (const std::exception&) {
-            Json command = Json::array({"zed", "{file}"});
+            throw Error(path_error, "Source path must stay inside project Scripts");
-            const auto settings = project();
+        }
-            if (settings.contains("editor") && settings.at("editor").is_object() &&
+        const auto project_root = std::filesystem::weakly_canonical(config_.project_root);
-                settings.at("editor").contains("script_editor"))
+        const auto relative = file.lexically_relative(project_root);
-                command = settings.at("editor").at("script_editor");
+        const auto name = generic_path_to_utf8(relative);
-            if (args.contains("editor"))
+        const auto extension = path_to_utf8(file.extension());
-                command = args.at("editor");
+        const bool supported = extension == ".cpp" || extension == ".hpp" ||
-            require(command.is_array() && !command.empty(), "lua.editor",
+                               extension == ".h" || extension == ".cc" ||
-                    "Configure editor.script_editor as a nonempty executable/argument array");
+                               extension == ".cxx" || extension == ".lua";
-            std::vector<std::string> arguments;
+        require(name.starts_with("Scripts/") && supported &&
-            bool has_file = false;
+                    (!lua_only || extension == ".lua") &&
-            for (const auto& part : command) {
+                    std::filesystem::is_regular_file(file),
-                require(part.is_string(), "lua.editor", "Editor arguments must be strings");
+                path_error, "Choose an existing source file inside Scripts");
-                auto argument = part.get<std::string>();
+        const auto line = args.value("line", 1);
-                require(argument.find('\0') == std::string::npos, "lua.editor",
+        const auto column = args.value("column", 1);
-                        "Editor arguments cannot contain NUL");
+        require(line > 0 && column > 0, path_error, "Source line and column must be positive");
-                if (argument == "{file}") {
+        Json command = Json::array({"zed", "{file}:{line}:{column}"});
-                    require(!arguments.empty(), "lua.editor", "The first argument is the editor");
+        const auto settings = project();
-                    argument = path_to_utf8(file);
+        if (settings.contains("editor") && settings.at("editor").is_object() &&
-                    has_file = true;
+            settings.at("editor").contains("script_editor"))
-                } else if (argument == "{project}") {
+            command = settings.at("editor").at("script_editor");
-                    require(!arguments.empty(), "lua.editor", "The first argument is the editor");
+        if (args.contains("editor"))
-                    argument = path_to_utf8(std::filesystem::absolute(config_.project_root));
+            command = args.at("editor");
-                }
+        require(command.is_array() && !command.empty(), editor_error,
-                arguments.push_back(std::move(argument));
+                "Configure editor.script_editor as a nonempty executable/argument array");
+        auto replace = [](std::string& target, std::string_view token,
+                          const std::string& replacement) {
+            bool found = false;
+            std::size_t position = 0;
+            while ((position = target.find(token, position)) != std::string::npos) {
+                target.replace(position, token.size(), replacement);
+                position += replacement.size();
+                found = true;
             }
-            require(!arguments.front().empty(), "lua.editor", "Choose an editor executable");
+            return found;
-            if (!has_file)
+        };
-                arguments.push_back(path_to_utf8(file));
+        std::vector<std::string> arguments;
+        bool has_file = false;
+        for (const auto& part : command) {
+            require(part.is_string(), editor_error, "Editor arguments must be strings");
+            auto argument = part.get<std::string>();
+            require(argument.find('\0') == std::string::npos, editor_error,
+                    "Editor arguments cannot contain NUL");
+            if (arguments.empty())
+                require(!argument.empty() && argument.find('{') == std::string::npos,
+                        editor_error, "The first argument must be an editor executable");
+            else {
+                has_file |= replace(argument, "{file}", path_to_utf8(file));
+                replace(argument, "{line}", std::to_string(line));
+                replace(argument, "{column}", std::to_string(column));
+                replace(argument, "{project}", path_to_utf8(project_root));
+            }
+            arguments.push_back(std::move(argument));
+        }
+        if (!has_file)
+            arguments.push_back(path_to_utf8(file));
+        try {
             launch_detached(arguments, config_.project_root);
-            log("Opened Lua source in external editor: " + args.at("path").get<std::string>());
+        } catch (const std::exception& error) {
-            return Json{{"opened", args.at("path")}};
+            throw Error(editor_error, "Could not launch external source editor",
-        });
+                        {{"reason", error.what()}});
+        }
+        log("Opened source in external editor: " + name);
+        return {{"opened", name}, {"path", name}, {"line", line}, {"column", column}};
+    };
+    const auto source_schema = schema(
+        {{"path", text},
+         {"line", {{"type", "integer"}, {"minimum", 1}}},
+         {"column", {{"type", "integer"}, {"minimum", 1}}},
+         {"editor", {{"type", "array"}, {"items", text}, {"minItems", 1}}}},
+        {"path"});
+    commands_.add("faset_source_open",
+                  "Open a C++ or Lua source under Scripts at a one-based line and column. "
+                  "The editor is an argv array; {file}, {line}, {column} and {project} are "
+                  "substituted without a shell.",
+                  source_schema, [open_source](const Json& args) {
+                      return open_source(args, false);
+                  });
+    commands_.add("faset_script_open",
+                  "Open a project Lua source in an external editor. This compatibility command "
+                  "retains Lua-only validation and uses the same safe source launcher.",
+                  source_schema, [open_source](const Json& args) {
+                      return open_source(args, true);
+                  });
     commands_.add("faset_export",
                   "Build, validate and export a resolved authoring snapshot to a project-relative "
                   "output directory. Returns a job ID.",

tests/build_cache_tests.cpp

@@ -0,0 +1,196 @@
+#include <faset/core/io.hpp>
+#include <faset/editor/build_cache.hpp>
+#include <faset/editor/build_service.hpp>
+#include <faset/scripting/project.hpp>
+#include <iostream>
+
+using namespace faset;
+namespace fs = std::filesystem;
+
+namespace {
+void check(bool condition, std::string_view message) {
+    if (!condition)
+        throw std::runtime_error(std::string(message));
+}
+void run(const fs::path& root) {
+    const auto project = root / "project";
+    const auto scripts = project / "Scripts";
+    atomic_write(scripts / "Gameplay.cpp", "#include \"Gameplay.hpp\"\n");
+    atomic_write(scripts / "Gameplay.hpp", "#pragma once\n");
+    atomic_write(scripts / "Extensions/Extra.hpp", "#define SPEED 1\n");
+    const auto compiler = root / "fake-compiler";
+    const auto cmake = root / "fake-cmake";
+    const auto slang = root / "fake-slangc";
+    atomic_write(compiler, "compiler-v1\n");
+    atomic_write(cmake, "cmake-v1\n");
+    atomic_write(slang, "slang-v1\n");
+    editor::BuildConfig config;
+    config.project_root = project;
+    config.engine_root = path_from_utf8(FASET_ENGINE_SOURCE);
+    config.build_directory = project / ".faset/build";
+    config.cmake = path_to_utf8(cmake);
+    config.configure_arguments = {
+        "-DCMAKE_C_COMPILER=" + path_to_utf8(compiler),
+        "-DCMAKE_CXX_COMPILER=" + path_to_utf8(compiler),
+        "-DSLANGC_EXECUTABLE=" + path_to_utf8(slang)};
+
+    const scripting::LuaProject none;
+    const auto initial = editor::capture_build_inputs(config, none);
+    atomic_write(scripts / "Extensions/Extra.hpp", "#define SPEED 2\n");
+    const auto changed_header = editor::capture_build_inputs(config, none);
+    check(initial.source_hash != changed_header.source_hash &&
+              initial.fingerprint() != changed_header.fingerprint(),
+          "Nested gameplay header changes the complete source snapshot");
+
+    atomic_write(scripts / "main.lua", "return {}\n");
+    atomic_write_json(project / "project.faset.json",
+                      {{"format", "faset.project"},
+                       {"version", 1},
+                       {"scripting", {{"lua", {{"scripts", {"Scripts/main.lua"}}}}}}});
+    auto lua = scripting::loadLuaProject(project);
+    const auto with_lua = editor::capture_build_inputs(config, lua);
+    atomic_write(scripts / "other.lua", "return {}\n");
+    auto manifest = read_json(project / "project.faset.json");
+    manifest["scripting"]["lua"]["scripts"].push_back("Scripts/other.lua");
+    atomic_write_json(project / "project.faset.json", manifest);
+    lua = scripting::loadLuaProject(project);
+    const auto changed_declaration = editor::capture_build_inputs(config, lua);
+    check(with_lua.source_hash != changed_declaration.source_hash,
+          "Lua entry declaration changes the source snapshot");
+
+    const auto native = config.build_directory / "Debug";
+    editor::ensure_native_toolchain_stamp(config, native, changed_declaration);
+    atomic_write(native / "sentinel", "keep\n");
+    editor::ensure_native_toolchain_stamp(config, native, changed_declaration);
+    check(fs::exists(native / "sentinel"), "Unchanged toolchain preserves the native tree");
+    atomic_write(compiler, "compiler-v2\n");
+    const auto changed_tool = editor::capture_build_inputs(config, lua);
+    check(changed_tool.source_hash == changed_declaration.source_hash &&
+              changed_tool.toolchain_hash != changed_declaration.toolchain_hash,
+          "Changing compiler bytes at the same path changes toolchain identity");
+    editor::ensure_native_toolchain_stamp(config, native, changed_tool);
+    check(!fs::exists(native / "sentinel"),
+          "Changed toolchain invalidates only the generated native tree");
+
+    const auto user_directory = root / "important-user-data";
+    atomic_write(user_directory / "do-not-delete.txt", "valuable\n");
+    bool refused_unowned{};
+    try {
+        editor::ensure_native_toolchain_stamp(config, user_directory, changed_tool);
+    } catch (const std::exception&) {
+        refused_unowned = true;
+    }
+    check(refused_unowned && fs::is_regular_file(user_directory / "do-not-delete.txt"),
+          "Unowned native directory is never cleared");
+    std::error_code native_link_error;
+    fs::create_directory_symlink(user_directory, config.build_directory / "Release",
+                                 native_link_error);
+    if (!native_link_error) {
+        bool refused_link{};
+        try {
+            editor::ensure_native_toolchain_stamp(config, config.build_directory / "Release",
+                                                  changed_tool);
+        } catch (const std::exception&) {
+            refused_link = true;
+        }
+        check(refused_link && fs::is_regular_file(user_directory / "do-not-delete.txt"),
+              "Symlinked native directory cannot redirect cleanup into user data");
+    }
+
+    const auto relative_tool = project / "tools/cmake-fixture";
+    atomic_write(relative_tool, "relative-cmake-v1\n");
+    auto relative_config = config;
+    relative_config.cmake = "tools/cmake-fixture";
+    const auto relative_before = editor::capture_build_inputs(relative_config, lua);
+    atomic_write(relative_tool, "relative-cmake-v2\n");
+    check(relative_before.toolchain_hash !=
+              editor::capture_build_inputs(relative_config, lua).toolchain_hash,
+          "Relative CMake executable resolves from the project working directory");
+    auto typed_config = config;
+    const auto typed_compiler = root / "typed-compiler";
+    atomic_write(typed_compiler, "typed-v1\n");
+    typed_config.configure_arguments = {
+        "-DCMAKE_C_COMPILER:FILEPATH=" + path_to_utf8(typed_compiler)};
+    const auto typed_before = editor::capture_build_inputs(typed_config, lua);
+    atomic_write(typed_compiler, "typed-v2\n");
+    check(typed_before.toolchain_hash !=
+              editor::capture_build_inputs(typed_config, lua).toolchain_hash,
+          "Typed CMake compiler overrides are included in toolchain identity");
+    auto malicious_config = config;
+    malicious_config.configure_arguments.push_back(
+        "-DFASET_GAMEPLAY_SOURCE_DIR:PATH=" + path_to_utf8(root / "outside"));
+    bool refused_source_override{};
+    try {
+        (void)editor::capture_build_inputs(malicious_config, lua);
+    } catch (const std::exception&) {
+        refused_source_override = true;
+    }
+    check(refused_source_override,
+          "Configure arguments cannot redirect gameplay compilation away from hashed Scripts");
+
+    const auto staged = editor::stage_gameplay_sources(config, changed_tool, lua,
+                                                        "snapshot-fixture");
+    check(staged != scripts && fs::is_regular_file(staged / "Gameplay.cpp") &&
+              read_text(staged / "Extensions/Extra.hpp") == "#define SPEED 2\n",
+          "Native gameplay compilation uses a complete staged Scripts snapshot");
+    atomic_write(scripts / "Extensions/Extra.hpp", "#define SPEED 999\n");
+    check(read_text(staged / "Extensions/Extra.hpp") == "#define SPEED 2\n",
+          "A live edit cannot alter the source bytes of an in-flight native build");
+    atomic_write(scripts / "Extensions/Extra.hpp", "#define SPEED 2\n");
+    check(editor::stage_gameplay_sources(config, changed_tool, lua, "snapshot-again") ==
+              staged,
+          "Identical sources retain a stable native path for incremental Ninja builds");
+
+    atomic_write(native / "faset_player", "player-v1\n");
+    atomic_write(native / "faset_schema_exporter", "exporter-v1\n");
+    atomic_write(native / "CMakeCache.txt", "recipe-v1\n");
+    atomic_write(native / "shaders/vertexMain.spv", "shader-v1\n");
+    atomic_write(native / "faset_runtime.dll", "runtime-v1\n");
+    auto package_key = [&] (const editor::BuildInputs& inputs) {
+        return editor::build_package_key(inputs, native, "Debug", native / "faset_player",
+                                         native / "faset_schema_exporter");
+    };
+    const auto first_key = package_key(changed_tool);
+    check(first_key != package_key(changed_declaration) &&
+              first_key != package_key(with_lua),
+          "Toolchain and Lua source changes invalidate package identity");
+    auto option_config = config;
+    option_config.configure_arguments.push_back("-DFASET_TEST_OPTION=ON");
+    check(first_key != package_key(editor::capture_build_inputs(option_config, lua)),
+          "Configure option changes invalidate package identity");
+    atomic_write(native / "shaders/vertexMain.spv", "shader-v2\n");
+    check(first_key != package_key(changed_tool),
+          "Shader bytes invalidate package identity");
+    atomic_write(native / "shaders/vertexMain.spv", "shader-v1\n");
+    atomic_write(native / "faset_runtime.dll", "runtime-v2\n");
+    check(first_key != package_key(changed_tool),
+          "Runtime DLL bytes invalidate package identity");
+
+    fs::create_directories(root / "outside");
+    std::error_code link_error;
+    fs::create_directory_symlink(root / "outside", scripts / "linked", link_error);
+    if (!link_error) {
+        bool rejected = false;
+        try {
+            (void)editor::capture_build_inputs(config, lua);
+        } catch (const std::exception&) {
+            rejected = true;
+        }
+        check(rejected, "Source snapshot refuses a symlink escaping Scripts");
+    }
+}
+} // namespace
+
+int main() {
+    const auto root = fs::temp_directory_path() / path_from_utf8("Faset build cache Café 世界 " + new_id());
+    try {
+        run(root);
+        fs::remove_all(root);
+        std::cout << "Source, toolchain and native package identity contracts passed\n";
+        return 0;
+    } catch (const std::exception& error) {
+        std::cerr << error.what() << '\n';
+        fs::remove_all(root);
+        return 1;
+    }
+}

tests/build_diagnostics_tests.cpp

@@ -0,0 +1,75 @@
+#include <faset/editor/build_diagnostics.hpp>
+#include <iostream>
+#include <stdexcept>
+
+using namespace faset;
+namespace fs = std::filesystem;
+
+namespace {
+void check(bool condition, std::string_view message) {
+    if (!condition)
+        throw std::runtime_error(std::string(message));
+}
+void contracts() {
+    const auto project = fs::path("/project");
+    auto rows = editor::parse_build_diagnostics(
+        "/project/Scripts/Game.cpp:17:4: error: bad field\n", "compile", project);
+    check(rows.size() == 1 && rows[0].at("file") == "Scripts/Game.cpp" &&
+              rows[0].at("line") == 17 && rows[0].at("column") == 4 &&
+              rows[0].at("severity") == "error" && rows[0].at("phase") == "compile",
+          "Clang error has a project-relative source location");
+    rows = editor::parse_build_diagnostics(
+        "\x1b[31mScripts/player.lua:6: unexpected symbol near '='\x1b[0m\n",
+        "schema", project);
+    check(rows.size() == 1 && rows[0].at("file") == "Scripts/player.lua" &&
+              rows[0].at("line") == 6 && rows[0].at("severity") == "error" &&
+              rows[0].at("message") == "unexpected symbol near '='",
+          "Lua syntax error and ANSI stripping work");
+    const auto windows = fs::path(R"(C:\Café)");
+    rows = editor::parse_build_diagnostics(
+        R"(C:\Café\Scripts\Game.cpp(17,4): error C2143: syntax error)" "\n"
+        R"(C:\Café\Scripts\Game.cpp:19:2: warning: suspicious conversion)" "\n",
+        "compile", windows);
+    check(rows.size() == 2 && rows[0].at("file") == "Scripts/Game.cpp" &&
+              rows[0].at("code") == "C2143" && rows[0].at("line") == 17 &&
+              rows[1].at("severity") == "warning" && rows[1].at("line") == 19,
+          "Windows drive colon and Unicode directory remain intact");
+    rows = editor::parse_build_diagnostics(
+        "Scripts/Game.cpp:9:3: error: missing value\n"
+        "    broken call\n"
+        "    ^~~~~~\n"
+        "Scripts/Game.cpp:4:1: note: declared here\n",
+        "compile", project);
+    check(rows.size() == 2 && rows[0].at("severity") == "error" &&
+              rows[1].at("severity") == "note" && rows[1].at("file") == "Scripts/Game.cpp",
+          "Caret and source excerpts do not become duplicate diagnostics");
+    rows = editor::parse_build_diagnostics(
+        "/outside/Scripts/Game.cpp:4:2: error: external\n"
+        "../Scripts/Escape.cpp:5:2: error: escaped\n",
+        "compile", project);
+    check(rows.size() == 2 && !rows[0].contains("file") && !rows[1].contains("file"),
+          "External and traversing sources are never navigable");
+    rows = editor::parse_build_diagnostics(
+        "Scripts/../Scripts/Game.cpp:4:2: error: disguised traversal\n"
+        "Scripts/Game.cpp:4:0: error: invalid column\n",
+        "compile", project);
+    check(rows.size() == 1 && !rows[0].contains("file"),
+          "Traversal stays non-navigable and zero columns are rejected");
+    std::string noisy;
+    for (int index = 0; index < 300; ++index)
+        noisy += "Scripts/Game.cpp:2:1: warning: repeated\n";
+    check(editor::parse_build_diagnostics(noisy, "compile", project).size() <= 200,
+          "Structured diagnostic count is bounded");
+}
+} // namespace
+
+int main() {
+    try {
+        contracts();
+        std::cout << "Build diagnostic parsing contracts passed\n";
+        return 0;
+    } catch (const std::exception& error) {
+        std::cerr << error.what() << '\n';
+        return 1;
+    }
+}

tests/build_schema_tests.cpp

@@ -142,11 +142,72 @@ int test_main(int argc, char** argv) {
         config.configure_arguments = {"-DCMAKE_CXX_COMPILER=fixture-does-not-compile"};
         editor::BuildService builds(config);
         builds.scaffold("Schema publication", 2);
+        atomic_write(config.project_root / "Scripts/Extensions/BuildOnly.hpp",
+                     "#define BUILD_ONLY 1\n");
         authoring::AuthoringService authoring(config.project_root, authoring::builtin_schemas());
         const auto valid = manifest();
         atomic_write_json(config.project_root / "schema-fixture.json", valid);
-        const auto first = builds.wait(builds.start_build());
+        {
+            auto compiler_config = config;
+            compiler_config.project_root = root / "c-only-compiler-project";
+            compiler_config.configure_arguments = {
+                "-DCMAKE_C_COMPILER:FILEPATH=fixture-c-only"};
+            editor::BuildService compiler_build(compiler_config);
+            compiler_build.scaffold("Compiler argument fixture", 2);
+            atomic_write_json(compiler_config.project_root / "schema-fixture.json", valid);
+            const auto compiler_result = compiler_build.wait(compiler_build.start_build());
+            check(compiler_result.state == "succeeded",
+                  "C-only compiler override fixture builds: " + compiler_result.error);
+            const auto compiler_args =
+                read_json(compiler_config.project_root / "configure-fixture.json");
+            std::size_t c_overrides{};
+            bool cxx_default{};
+            for (const auto& arg : compiler_args) {
+                const auto text = arg.get<std::string>();
+                if (text.starts_with("-DCMAKE_C_COMPILER")) {
+                    ++c_overrides;
+                    check(text == "-DCMAKE_C_COMPILER:FILEPATH=fixture-c-only",
+                          "Typed C override is not replaced by a default compiler");
+                }
+                if (text.starts_with("-DCMAKE_CXX_COMPILER="))
+                    cxx_default = true;
+            }
+            check(c_overrides == 1 && cxx_default,
+                  "C and C++ compiler defaults are selected independently");
+        }
+        auto first = builds.wait(builds.start_build());
         check(first.state == "succeeded", "Valid custom schema v2 publishes: " + first.error);
+        fs::path compiled_scripts;
+        for (const auto& arg : read_json(config.project_root / "configure-fixture.json")) {
+            const auto text = arg.get<std::string>();
+            constexpr std::string_view prefix = "-DFASET_GAMEPLAY_SOURCE_DIR=";
+            if (text.starts_with(prefix))
+                compiled_scripts = path_from_utf8(text.substr(prefix.size()));
+        }
+        check(!compiled_scripts.empty() &&
+                  compiled_scripts != config.project_root / "Scripts" &&
+                  read_text(compiled_scripts / "Extensions/BuildOnly.hpp") ==
+                      "#define BUILD_ONLY 1\n",
+              "Native C++ build receives the immutable source snapshot");
+        const auto repeated = builds.wait(builds.start_build());
+        check(repeated.state == "succeeded" &&
+                  repeated.result.at("generation") == first.result.at("generation") &&
+                  repeated.result.at("schema_cache_hit") == true &&
+                  repeated.result.at("generation_reused") == true &&
+                  read_text(config.project_root / "schema-export-count.txt") == "1",
+              "Unchanged native build reuses a verified schema generation");
+        atomic_write(path_from_utf8(first.result.at("schema").get<std::string>()), "truncated");
+        first = builds.wait(builds.start_build());
+        check(first.state == "succeeded" && first.result.at("schema_cache_hit") == false &&
+                  read_text(config.project_root / "schema-export-count.txt") == "2",
+              "Corrupt schema cannot be a cache hit");
+        atomic_write(path_from_utf8(first.result.at("directory").get<std::string>()) /
+                         "shaders/vertexMain.spv",
+                     "corrupt");
+        first = builds.wait(builds.start_build());
+        check(first.state == "succeeded" && first.result.at("schema_cache_hit") == false &&
+                  read_text(config.project_root / "schema-export-count.txt") == "3",
+              "Corrupt shader cannot be a cache hit");
         const auto directory = path_from_utf8(first.result.at("directory").get<std::string>());
         for (const auto* entry : {"gpuVertexMain", "gpuShadowMain", "gpuCullMain",
                                   "gpuHzbMain", "gpuPostCullMain"})
@@ -161,6 +222,34 @@ int test_main(int argc, char** argv) {
         const auto previous_player = sha256_file(player);
         const auto previous_schema = read_text(schema);
         const auto previous_manifest = read_text(directory / "manifest.json");
+        atomic_write(config.project_root / "emit-clang-error-and-fail-build", "fixture\n");
+        const auto compile_failed = builds.wait(builds.start_build());
+        check(compile_failed.state == "failed" &&
+                  read_text(last_build) == previous_pointer &&
+                  compile_failed.json().at("diagnostics").size() == 1 &&
+                  compile_failed.json().at("diagnostics")[0].at("file") ==
+                      "Scripts/Gameplay.cpp" &&
+                  compile_failed.log.find("fixture compile failure") != std::string::npos,
+              "Failed compiler output retains raw log and structured source location");
+        fs::remove(config.project_root / "emit-clang-error-and-fail-build");
+        atomic_write(config.project_root / "fail-unparseable-build", "fixture\n");
+        const auto generic_failed = builds.wait(builds.start_build());
+        check(generic_failed.state == "failed" &&
+                  generic_failed.json().at("diagnostics").size() == 1 &&
+                  generic_failed.json().at("diagnostics")[0].at("severity") == "error" &&
+                  !generic_failed.json().at("diagnostics")[0].contains("file") &&
+                  read_text(last_build) == previous_pointer,
+              "Unparseable process failure has a non-navigable diagnostic");
+        fs::remove(config.project_root / "fail-unparseable-build");
+        atomic_write(config.project_root / "Scripts/Extensions/BuildOnly.hpp",
+                     "#define BUILD_ONLY 3\n");
+        atomic_write(config.project_root / "mutate-cpp-header-during-build", "fixture\n");
+        const auto raced_header = builds.wait(builds.start_build());
+        check(raced_header.state == "failed" && read_text(last_build) == previous_pointer,
+              "Header changed during schema export cannot publish a mixed build");
+        fs::remove(config.project_root / "mutate-cpp-header-during-build");
+        atomic_write(config.project_root / "Scripts/Extensions/BuildOnly.hpp",
+                     "#define BUILD_ONLY 1\n");
         check(!first.result.at("lua_enabled").get<bool>() &&
                   !fs::exists(directory / "project.faset.json"),
               "C++-only build publishes no Lua sources or project manifest");
@@ -169,6 +258,11 @@ int test_main(int argc, char** argv) {
               "C++-only build explicitly disables the Lua VM in CMake");
         authoring.replace_external_schemas(read_json(schema));
         const auto previous_registry = authoring.schemas().manifest();
+        std::size_t published_generations{};
+        for (const auto& entry : fs::directory_iterator(directory.parent_path())) {
+            (void)entry;
+            ++published_generations;
+        }
         check(authoring.schemas().schema("game.mover").at("version") == 2,
               "Matching custom v2 metadata reaches authoring");
         migration_contracts(config.project_root / "migration-contracts", read_json(schema));
@@ -252,9 +346,11 @@ int test_main(int argc, char** argv) {
             std::size_t generations{};
             for (const auto& entry : fs::directory_iterator(directory.parent_path())) {
                 ++generations;
-                check(entry.path() == directory, "Invalid build leaves no staging or generation");
+                check(!entry.path().filename().string().starts_with(".staging-"),
+                      "Invalid build leaves no staging generation");
             }
-            check(generations == 1, "Only the validated build generation remains");
+            check(generations == published_generations,
+                  "Invalid build publishes no new generation");
             bool rejected{};
             try {
                 authoring.replace_external_schemas(invalid[index]);
@@ -305,6 +401,8 @@ int test_main(int argc, char** argv) {
               "Later edits never mutate an already published Lua generation");
         const auto lua_pointer = read_text(last_build);
         for (const auto* marker : {"mutate-lua-during-build", "mutate-lua-snapshot"}) {
+            atomic_write(config.project_root / "Scripts/main.lua",
+                         std::string(lua_source) + "-- force a schema export\n");
             atomic_write(config.project_root / marker, "fixture\n");
             const auto raced = builds.wait(builds.start_build());
             check(raced.state == "failed" && read_text(last_build) == lua_pointer,

tests/build_schema_tool.cpp

@@ -8,6 +8,9 @@ using namespace faset;
 int tool_main(int argc, char** argv) {
     try {
         if (argc >= 3 && std::string_view(argv[1]) == "--output") {
+            const auto count_file = fs::path("schema-export-count.txt");
+            const auto count = fs::exists(count_file) ? std::stoi(read_text(count_file)) : 0;
+            atomic_write(count_file, std::to_string(count + 1));
             if (argc == 5 && std::string_view(argv[3]) == "--project") {
                 const auto snapshot = path_from_utf8(argv[4]);
                 const auto project = read_json(snapshot / "project.faset.json");
@@ -21,6 +24,8 @@ int tool_main(int argc, char** argv) {
                 if (fs::exists("mutate-lua-snapshot"))
                     atomic_write(snapshot / "Scripts/main.lua", "-- corrupt snapshot\n");
             }
+            if (fs::exists("mutate-cpp-header-during-build"))
+                atomic_write("Scripts/Extensions/BuildOnly.hpp", "#define BUILD_ONLY 2\n");
             atomic_write_json(path_from_utf8(argv[2]), read_json("schema-fixture.json"));
             return 0;
         }
@@ -34,8 +39,18 @@ int tool_main(int argc, char** argv) {
             std::cout << "Native packaging fixture validated\n";
             return 0;
         }
-        if (argc > 2 && std::string_view(argv[1]) == "--build")
+        if (argc > 2 && std::string_view(argv[1]) == "--build") {
+            if (fs::exists("emit-clang-error-and-fail-build")) {
+                std::cerr << path_to_utf8(fs::current_path() / "Scripts/Gameplay.cpp")
+                          << ":7:3: error: fixture compile failure\n";
+                return 1;
+            }
+            if (fs::exists("fail-unparseable-build")) {
+                std::cerr << "Synthetic native build failed without a source location\n";
+                return 1;
+            }
             return 0;
+        }
         fs::path build;
         for (int i = 1; i + 1 < argc; ++i)
             if (std::string_view(argv[i]) == "-B")

tests/build_service_tests.cpp

@@ -1,8 +1,12 @@
 #include "assets_image_fixtures.hpp"
+#include <algorithm>
+#include <atomic>
 #include <bit>
 #include <chrono>
 #include <cstdlib>
+#include <functional>
 #include <faset/assets/asset_pipeline.hpp>
+#include <faset/authoring/service.hpp>
 #include <faset/core/hash.hpp>
 #include <faset/core/io.hpp>
 #include <faset/core/process.hpp>
@@ -42,6 +46,102 @@ Json scene(int dimension) {
             {"name", "Build test"},      {"dimension", dimension}, {"entities", Json::array()},
             {"instances", Json::array()}};
 }
+void starter_contracts(const fs::path& root) {
+    for (const int dimension : {2, 3}) {
+        for (const char* language : {"cpp", "lua"}) {
+            const auto project = root / path_from_utf8(std::string("Starter café ") + language +
+                                                        std::to_string(dimension));
+            editor::BuildConfig config;
+            config.project_root = project;
+            config.engine_root = path_from_utf8(FASET_ENGINE_SOURCE);
+            editor::BuildService service(config);
+            service.scaffold("Starter", dimension, language);
+            const auto manifest = read_json(project / "project.faset.json");
+            const auto start = read_json(project / "Scenes/main.scene.json");
+            require(manifest.at("dimension") == dimension &&
+                        manifest.at("start_scene") == "Scenes/main.scene.json",
+                    "Explicit starter has the requested scene type and startup path");
+            require(start.at("dimension") == dimension && !start.at("entities").empty(),
+                    "Starter scene is valid and has visible contents");
+            auto builtins = start;
+            bool has_behavior = false;
+            for (auto& entity : builtins["entities"]) {
+                auto& components = entity["components"].get_ref<Json::array_t&>();
+                std::erase_if(components, [&](const Json& component) {
+                    const auto custom =
+                        !component.at("type").get<std::string>().starts_with("faset.");
+                    has_behavior |= custom;
+                    return custom;
+                });
+            }
+            require(has_behavior, "Starter scene binds its gameplay behavior");
+            authoring::validate_scene(builtins, authoring::builtin_schemas());
+            if (std::string_view(language) == "lua") {
+                require(!fs::exists(project / "Scripts/Gameplay.cpp") &&
+                            !fs::exists(project / "Scripts/Gameplay.hpp"),
+                        "Lua starter has no C++ gameplay stub");
+                require(manifest.at("scripting").at("lua").at("scripts") ==
+                            Json::array({"Scripts/main.lua"}) &&
+                            scripting::loadLuaProject(project).enabled() &&
+                            fs::is_regular_file(project / ".luarc.json") &&
+                            fs::is_regular_file(project / "Scripts/.luarc.json") &&
+                            fs::is_regular_file(project / ".faset/lua/faset.lua"),
+                        "Lua starter declares an actual runnable module");
+            } else {
+                require(fs::is_regular_file(project / "Scripts/Gameplay.cpp") &&
+                            fs::is_regular_file(project / "Scripts/Gameplay.hpp") &&
+                            !manifest.contains("scripting"),
+                        "C++ starter contains its compiled gameplay module");
+            }
+            const auto marker = read_text(project / "Scenes/main.scene.json");
+            bool rejected = false;
+            try {
+                service.scaffold("Again", dimension, language);
+            } catch (const std::exception&) {
+                rejected = true;
+            }
+            require(rejected && read_text(project / "Scenes/main.scene.json") == marker,
+                    "Explicit creation rejects existing project without overwriting it");
+        }
+    }
+    editor::BuildConfig legacy;
+    legacy.project_root = root / "legacy";
+    legacy.engine_root = path_from_utf8(FASET_ENGINE_SOURCE);
+    editor::BuildService(legacy).scaffold("Legacy", 3);
+    require(!fs::exists(legacy.project_root / "Scenes/main.scene.json"),
+            "Two-argument scaffold retains its prior no-scene behavior");
+    const auto concurrent_root = root / "concurrent";
+    editor::BuildConfig concurrent;
+    concurrent.project_root = concurrent_root;
+    concurrent.engine_root = path_from_utf8(FASET_ENGINE_SOURCE);
+    editor::BuildService first(concurrent), second(concurrent);
+    std::atomic<int> ready = 0;
+    std::atomic<int> successes = 0;
+    auto attempt = [&](editor::BuildService& service, const char* name) {
+        ++ready;
+        while (ready.load() < 2)
+            std::this_thread::yield();
+        try {
+            service.scaffold(name, 2, "lua");
+            ++successes;
+        } catch (const std::exception&) {
+        }
+    };
+    std::thread a(attempt, std::ref(first), "First");
+    std::thread b(attempt, std::ref(second), "Second");
+    a.join();
+    b.join();
+    require(successes == 1, "Two concurrent creators publish exactly one starter");
+    const auto winner = read_json(concurrent_root / "project.faset.json");
+    require(winner.at("name") == "First" || winner.at("name") == "Second",
+            "Published manifest belongs to the successful creator");
+    require(fs::is_regular_file(concurrent_root / "Scripts/main.lua") &&
+                fs::is_regular_file(concurrent_root / "Scenes/main.scene.json"),
+            "The winning starter publishes all required files");
+    const auto ignore = read_text(concurrent_root / ".gitignore");
+    require(ignore.find("!.faset/lua/faset.lua") != std::string::npos,
+            "LuaLS declarations are commit-friendly in generated projects");
+}
 void lua_project_contracts(const fs::path& root) {
     fs::create_directories(root);
     require(!scripting::loadLuaProject(root).enabled(), "No manifest means no Lua dependency");
@@ -379,6 +479,7 @@ int test_main(int argc, char** argv) {
     try {
         fs::create_directories(temporary);
         lua_project_contracts(temporary / "lua-project");
+        starter_contracts(temporary / "starters");
         const auto original_executable = fs::absolute(path_from_utf8(argv[0]));
         const auto executable = temporary / original_executable.filename();
         fs::copy_file(original_executable, executable);

tests/editor_session_tests.cpp

@@ -1,9 +1,18 @@
 #include <faset/core/io.hpp>
 #include <faset/editor/session.hpp>
+#include <chrono>
 #include <iostream>
+#include <thread>
 
-int main() {
+int main(int argc, char** argv) {
     using namespace faset;
+    if (argc >= 3 && std::string_view(argv[1]) == "--editor-probe") {
+        Json arguments = Json::array();
+        for (int index = 3; index < argc; ++index)
+            arguments.push_back(argv[index]);
+        atomic_write_json(path_from_utf8(argv[2]), arguments);
+        return 0;
+    }
     const auto root = std::filesystem::temp_directory_path() /
                       path_from_utf8("Faset Café 世界 session " + new_id());
     try {
@@ -115,6 +124,32 @@ int main() {
                        {{"path", "Scripts/behavior.lua"}, {"editor", {"{file}"}}}, "lua.editor");
         reject_command("faset_script_open", {{"path", "Scripts/behavior.lua"}, {"editor", {""}}},
                        "lua.editor");
+        const auto probe = root / "editor-argv.json";
+        const auto opened = commands.call(
+            "faset_source_open",
+            {{"path", "Scripts/Gameplay.cpp"}, {"line", 17}, {"column", 4},
+             {"editor", Json::array({path_to_utf8(std::filesystem::absolute(path_from_utf8(argv[0]))),
+                                     "--editor-probe", path_to_utf8(probe),
+                                     "{file}:{line}:{column}", "{project}"})}});
+        for (int attempt = 0; attempt < 100 && !std::filesystem::exists(probe); ++attempt)
+            std::this_thread::sleep_for(std::chrono::milliseconds(10));
+        require(opened.at("path") == "Scripts/Gameplay.cpp" && opened.at("line") == 17 &&
+                    std::filesystem::is_regular_file(probe) &&
+                    read_json(probe) ==
+                        Json::array({path_to_utf8(root / "Scripts/Gameplay.cpp") + ":17:4",
+                                     path_to_utf8(root)}),
+                "test", "Source navigation passes a Unicode path and location as literal argv");
+        reject_command("faset_source_open", {{"path", "../outside.cpp"}}, "source.path");
+        reject_command("faset_source_open", {{"path", "Scripts/missing.cpp"}}, "source.path");
+        reject_command("faset_source_open", {{"path", "Scripts"}}, "source.path");
+        atomic_write(root / "Scripts/tool.exe", "not a source\n");
+        reject_command("faset_source_open", {{"path", "Scripts/tool.exe"}}, "source.path");
+        std::filesystem::create_directories(root / "Outside");
+        std::error_code link_error;
+        std::filesystem::create_symlink(root / "Outside/secret.cpp",
+                                        root / "Scripts/escape.cpp", link_error);
+        if (!link_error)
+            reject_command("faset_source_open", {{"path", "Scripts/escape.cpp"}}, "source.path");
         auto bad_lua_project = session.project();
         bad_lua_project["scripting"] = {{"lua", {{"scripts", {"Scripts/missing.lua"}}}}};
         atomic_write_json(root / "project.faset.json", bad_lua_project);

tests/editor_ui_launcher.cpp

@@ -111,6 +111,9 @@ int main() {
             text(launcher, "launcher-path", path_to_utf8(root / "." / new_project.filename()));
             click(launcher, "launcher-2d");
             check(launcher.widgets().find("launcher-2d")->selected, "2D project selection");
+            click(launcher, "launcher-lua");
+            check(launcher.widgets().find("launcher-lua")->selected,
+                  "Lua project language selection");
             renderer.render(launcher.snapshot());
             renderer.capture(root / "launcher-create.ppm");
             launcher.frame({key("Return", true)});
@@ -118,6 +121,7 @@ int main() {
                   "Create through keyboard rejected: " +
                       launcher.widgets().find("launcher-error")->text);
             check(launcher.selection()->create && launcher.selection()->dimension == 2 &&
+                      launcher.selection()->language == "lua" &&
                       launcher.selection()->name == "Тестовый проект",
                   "Create through keyboard preserves typed project metadata");
             check(launcher.selection()->path == new_project,
@@ -127,6 +131,19 @@ int main() {
             check(!std::filesystem::exists(new_project),
                   "Launcher does not create a partial project before Session scaffold");
         }
+        {
+            editor::ProjectSelection retry{new_project, "Retry starter", 3, true, "lua"};
+            editor::ProjectLauncher launcher(renderer, path_from_utf8(FASET_TEST_ENGINE), {},
+                                             recents, retry, "Template source is unavailable");
+            launcher.frame({});
+            check(launcher.widgets().find("launcher-create")->selected &&
+                      launcher.widgets().find("launcher-lua")->selected &&
+                      launcher.widgets().find("launcher-path")->text == path_to_utf8(new_project) &&
+                      launcher.widgets().find("launcher-name")->text == "Retry starter" &&
+                      launcher.widgets().find("launcher-error")->text ==
+                          "Template source is unavailable",
+                  "Failed starter returns to Create with the chosen options and error");
+        }
         {
             editor::ProjectLauncher launcher(renderer, path_from_utf8(FASET_TEST_ENGINE),
                                              root / "Missing", recents);

tools/project_templates/lua-main.lua

@@ -0,0 +1,22 @@
+-- Faset starter gameplay. Edit this file and use Refresh Lua or development reload;
+-- the native C++ engine does not need to recompile for a Lua-only edit.
+local Player = faset.behavior {
+    id = "starter.player",
+    version = 1,
+    name = "Player",
+    fields = {
+        speed = { name = "Move speed", type = "number", default = 4, min = 0 },
+        jump_speed = { name = "Jump speed", type = "number", default = 5, min = 0 }
+    }
+}
+
+function Player:fixed_update(delta)
+    local velocity = self.entity:velocity()
+    velocity.x = faset.input().horizontal * self.fields.speed
+    if faset.input().jump_pressed and self.entity:is_grounded() then
+        velocity.y = self.fields.jump_speed
+    end
+    self.entity:set_velocity(velocity)
+end
+
+return Player