diff --git a/project.cpp b/project.cpp
index a1e9fd1..89ab636 100644
--- a/project.cpp
+++ b/project.cpp
@@ -98,6 +98,15 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
     // Mirrors how downstream consumers link their own libraries into tests.
     if (cfg.target == "x86_64-pc-linux-gnu") {
         cfg.AddTest("HelloWorld").Dependencies({ crafterBuildLib.get() });
+        cfg.AddTest("StaticLib").Dependencies({ crafterBuildLib.get() });
+        cfg.AddTest("ModuleInterface").Dependencies({ crafterBuildLib.get() });
+        cfg.AddTest("DependencyLink").Dependencies({ crafterBuildLib.get() });
+        cfg.AddTest("ShaderCompile").Dependencies({ crafterBuildLib.get() });
+        cfg.AddTest("StandardArgs").Dependencies({ crafterBuildLib.get() });
+        cfg.AddTest("TestRunnerSpec").Dependencies({ crafterBuildLib.get() });
+        cfg.AddTest("VariantId").Dependencies({ crafterBuildLib.get() });
+        cfg.AddTest("WasiBrowserRuntime").Dependencies({ crafterBuildLib.get() });
+        cfg.AddTest("RunSingleTestExit").Dependencies({ crafterBuildLib.get() });
     }
 
     return cfg;
diff --git a/tests/DependencyLink/fixture/main.cpp b/tests/DependencyLink/fixture/main.cpp
new file mode 100644
index 0000000..93a058f
--- /dev/null
+++ b/tests/DependencyLink/fixture/main.cpp
@@ -0,0 +1,7 @@
+import std;
+import Calc;
+
+int main() {
+    std::print("{}", Add(3, 4));
+    return 0;
+}
diff --git a/tests/DependencyLink/fixture/mathlib/Calc.cppm b/tests/DependencyLink/fixture/mathlib/Calc.cppm
new file mode 100644
index 0000000..994e745
--- /dev/null
+++ b/tests/DependencyLink/fixture/mathlib/Calc.cppm
@@ -0,0 +1,4 @@
+export module Calc;
+import std;
+
+export int Add(int a, int b) { return a + b; }
diff --git a/tests/DependencyLink/main.cpp b/tests/DependencyLink/main.cpp
new file mode 100644
index 0000000..445f56e
--- /dev/null
+++ b/tests/DependencyLink/main.cpp
@@ -0,0 +1,90 @@
+import std;
+import Crafter.Build;
+namespace fs = std::filesystem;
+using namespace Crafter;
+
+// Two-config build: an exe linked against a sibling static library via
+// cfg.dependencies. Exercises cross-config module import resolution (the
+// exe imports Calc, which is defined in the lib's interface set) and the
+// dep-archive linking step.
+int main() {
+    fs::path fixtureRoot = fs::current_path() / "tests" / "DependencyLink" / "fixture";
+
+    auto lib = std::make_unique<Configuration>();
+    lib->path = fixtureRoot / "mathlib";
+    lib->name = "calc";
+    lib->outputName = "calc";
+    lib->target = HostTarget();
+    lib->type = ConfigurationType::LibraryStatic;
+    {
+        std::array<fs::path, 1> ifaces = { "Calc" };
+        std::array<fs::path, 0> impls = {};
+        lib->GetInterfacesAndImplementations(ifaces, impls);
+    }
+
+    Configuration app;
+    app.path = fixtureRoot;
+    app.name = "calc-app";
+    app.outputName = "calc-app";
+    app.target = HostTarget();
+    app.type = ConfigurationType::Executable;
+    app.dependencies = { lib.get() };
+    {
+        std::array<fs::path, 0> ifaces = {};
+        std::array<fs::path, 1> impls = { "main" };
+        app.GetInterfacesAndImplementations(ifaces, impls);
+    }
+
+    if (app.implementations.size() != 1) {
+        std::println(std::cerr, "expected 1 implementation, got {}", app.implementations.size());
+        return 1;
+    }
+    // The import resolves to the dependency's interface, not a local one.
+    if (!app.implementations[0].moduleDependencies.empty()) {
+        std::println(std::cerr, "expected no local module deps, got {}",
+            app.implementations[0].moduleDependencies.size());
+        return 1;
+    }
+    if (app.implementations[0].externalModuleDependencies.size() != 1) {
+        std::println(std::cerr, "expected 1 external module dep, got {}",
+            app.implementations[0].externalModuleDependencies.size());
+        return 1;
+    }
+    if (app.implementations[0].externalModuleDependencies[0].first->name != "Calc") {
+        std::println(std::cerr, "expected external dep 'Calc', got '{}'",
+            app.implementations[0].externalModuleDependencies[0].first->name);
+        return 1;
+    }
+
+    std::unordered_map<fs::path, std::shared_future<BuildResult>> depResults;
+    std::mutex depMutex;
+    BuildResult r = Build(app, depResults, depMutex);
+    if (!r.result.empty()) {
+        std::println(std::cerr, "build failed: {}", r.result);
+        return 1;
+    }
+
+    fs::path libArchive = lib->BinDir() / "libcalc.a";
+    if (!fs::exists(libArchive)) {
+        std::println(std::cerr, "dep archive not produced at {}", libArchive.string());
+        return 1;
+    }
+
+    fs::path bin = app.BinDir() / "calc-app";
+    if (!fs::exists(bin)) {
+        std::println(std::cerr, "binary not produced at {}", bin.string());
+        return 1;
+    }
+
+    auto run = RunCommandWithTimeout(bin.string(), std::chrono::seconds(10));
+    if (run.exitCode != 0 || run.timedOut || run.crashed) {
+        std::println(std::cerr, "exe did not exit cleanly: exit={} output={}",
+            run.exitCode, run.output);
+        return 1;
+    }
+    if (run.output != "7") {
+        std::println(std::cerr, "expected '7', got '{}'", run.output);
+        return 1;
+    }
+    return 0;
+}
diff --git a/tests/ModuleInterface/fixture/Greeter.cppm b/tests/ModuleInterface/fixture/Greeter.cppm
new file mode 100644
index 0000000..97ebc78
--- /dev/null
+++ b/tests/ModuleInterface/fixture/Greeter.cppm
@@ -0,0 +1,4 @@
+export module Greeter;
+import std;
+
+export std::string Greet() { return "ok-from-module"; }
diff --git a/tests/ModuleInterface/fixture/main.cpp b/tests/ModuleInterface/fixture/main.cpp
new file mode 100644
index 0000000..e4fc1b5
--- /dev/null
+++ b/tests/ModuleInterface/fixture/main.cpp
@@ -0,0 +1,7 @@
+import std;
+import Greeter;
+
+int main() {
+    std::print("{}", Greet());
+    return 0;
+}
diff --git a/tests/ModuleInterface/main.cpp b/tests/ModuleInterface/main.cpp
new file mode 100644
index 0000000..9173dc1
--- /dev/null
+++ b/tests/ModuleInterface/main.cpp
@@ -0,0 +1,65 @@
+import std;
+import Crafter.Build;
+namespace fs = std::filesystem;
+using namespace Crafter;
+
+// Build an executable that imports a project-local .cppm module, then run
+// it and check the produced output. Exercises the within-project module
+// dependency path (main.cpp imports Greeter, resolved through cfg.interfaces).
+int main() {
+    Configuration cfg;
+    cfg.path = fs::current_path() / "tests" / "ModuleInterface" / "fixture";
+    cfg.name = "greeter-app";
+    cfg.outputName = "greeter-app";
+    cfg.target = HostTarget();
+    cfg.type = ConfigurationType::Executable;
+
+    std::array<fs::path, 1> ifaces = { "Greeter" };
+    std::array<fs::path, 1> impls  = { "main" };
+    cfg.GetInterfacesAndImplementations(ifaces, impls);
+
+    // Implementation should have picked up the import Greeter; through the
+    // interface list resolution in GetInterfacesAndImplementations.
+    if (cfg.implementations.size() != 1) {
+        std::println(std::cerr, "expected 1 implementation, got {}", cfg.implementations.size());
+        return 1;
+    }
+    if (cfg.implementations[0].moduleDependencies.size() != 1) {
+        std::println(std::cerr,
+            "expected main.cpp to depend on 1 module, got {}",
+            cfg.implementations[0].moduleDependencies.size());
+        return 1;
+    }
+    if (cfg.implementations[0].moduleDependencies[0]->name != "Greeter") {
+        std::println(std::cerr,
+            "expected dep 'Greeter', got '{}'",
+            cfg.implementations[0].moduleDependencies[0]->name);
+        return 1;
+    }
+
+    std::unordered_map<fs::path, std::shared_future<BuildResult>> depResults;
+    std::mutex depMutex;
+    BuildResult r = Build(cfg, depResults, depMutex);
+    if (!r.result.empty()) {
+        std::println(std::cerr, "build failed: {}", r.result);
+        return 1;
+    }
+
+    fs::path bin = cfg.BinDir() / "greeter-app";
+    if (!fs::exists(bin)) {
+        std::println(std::cerr, "binary not produced at {}", bin.string());
+        return 1;
+    }
+
+    auto run = RunCommandWithTimeout(bin.string(), std::chrono::seconds(10));
+    if (run.exitCode != 0 || run.timedOut || run.crashed) {
+        std::println(std::cerr, "binary did not exit cleanly: exit={} output={}",
+            run.exitCode, run.output);
+        return 1;
+    }
+    if (run.output != "ok-from-module") {
+        std::println(std::cerr, "unexpected output: '{}'", run.output);
+        return 1;
+    }
+    return 0;
+}
diff --git a/tests/RunSingleTestExit/main.cpp b/tests/RunSingleTestExit/main.cpp
new file mode 100644
index 0000000..a76a0ab
--- /dev/null
+++ b/tests/RunSingleTestExit/main.cpp
@@ -0,0 +1,87 @@
+import std;
+import Crafter.Build;
+namespace fs = std::filesystem;
+using namespace Crafter;
+
+namespace {
+    int failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++failures;
+        }
+    }
+
+    // Write a tiny POSIX shell script that exits with `code` and return its
+    // path. The harness invokes `<binary> <args>` through the host shell, so
+    // a #!/bin/sh script works as a stand-in for a real test binary.
+    fs::path WriteExitScript(const fs::path& dir, std::string_view stem, int code) {
+        fs::path script = dir / stem;
+        std::ofstream out(script);
+        out << "#!/bin/sh\nexit " << code << "\n";
+        out.close();
+        fs::permissions(script,
+            fs::perms::owner_read | fs::perms::owner_write | fs::perms::owner_exec
+            | fs::perms::group_read | fs::perms::group_exec
+            | fs::perms::others_read | fs::perms::others_exec,
+            fs::perm_options::replace);
+        return script;
+    }
+}
+
+// Drives RunSingleTest against shell-script fixtures and asserts the
+// exit-code → TestOutcome mapping documented in the README:
+//   0  -> Pass
+//   77 -> Skipped (autoconf convention)
+//   anything else -> Fail
+int main() {
+    fs::path dir = fs::temp_directory_path() / "crafter-build-run-single-test";
+    std::error_code ec;
+    fs::remove_all(dir, ec);
+    fs::create_directories(dir);
+
+    Test t;
+    t.runner = TestRunner::Local();
+    t.config.outputName = "run-single";
+
+    {
+        fs::path script = WriteExitScript(dir, "pass.sh", 0);
+        TestResult r = RunSingleTest(t, script, std::chrono::seconds(5));
+        Check(r.outcome == TestOutcome::Pass, "exit 0 -> Pass");
+        Check(r.exitCode == 0, "Pass carries exitCode 0");
+    }
+    {
+        fs::path script = WriteExitScript(dir, "skip.sh", 77);
+        TestResult r = RunSingleTest(t, script, std::chrono::seconds(5));
+        Check(r.outcome == TestOutcome::Skipped, "exit 77 -> Skipped");
+        Check(r.exitCode == 77, "Skipped carries exitCode 77");
+    }
+    {
+        fs::path script = WriteExitScript(dir, "fail.sh", 3);
+        TestResult r = RunSingleTest(t, script, std::chrono::seconds(5));
+        Check(r.outcome == TestOutcome::Fail, "non-zero/non-77 exit -> Fail");
+        Check(r.exitCode == 3, "Fail carries the original exitCode");
+    }
+
+    // Cmd-prefix runner: wrap the script in `sh {bin} {args}` so we exercise
+    // the templated-exec path through RunSingleTest. Probes don't run here
+    // (RunSingleTest doesn't probe — RunTests does), so any prefix that
+    // resolves on PATH is fine.
+    {
+        Test ct;
+        ct.runner = TestRunner::Cmd("sh");
+        ct.config.outputName = "run-single-prefix";
+        fs::path script = WriteExitScript(dir, "prefixed.sh", 0);
+        TestResult r = RunSingleTest(ct, script, std::chrono::seconds(5));
+        Check(r.outcome == TestOutcome::Pass, "Cmd-prefixed exit 0 -> Pass");
+    }
+
+    fs::remove_all(dir, ec);
+
+    if (failures > 0) {
+        std::println(std::cerr, "{} assertions failed", failures);
+        return 1;
+    }
+    return 0;
+}
diff --git a/tests/ShaderCompile/fixture/triangle.frag b/tests/ShaderCompile/fixture/triangle.frag
new file mode 100644
index 0000000..7acc257
--- /dev/null
+++ b/tests/ShaderCompile/fixture/triangle.frag
@@ -0,0 +1,8 @@
+#version 450
+
+layout(location = 0) in vec2 inUV;
+layout(location = 0) out vec4 outColor;
+
+void main() {
+    outColor = vec4(inUV, 0.0, 1.0);
+}
diff --git a/tests/ShaderCompile/main.cpp b/tests/ShaderCompile/main.cpp
new file mode 100644
index 0000000..6fac150
--- /dev/null
+++ b/tests/ShaderCompile/main.cpp
@@ -0,0 +1,53 @@
+import std;
+import Crafter.Build;
+namespace fs = std::filesystem;
+using namespace Crafter;
+
+// Compile a tiny GLSL fragment shader directly via the Shader API. Verifies
+// the SPIR-V output lands in the target dir and starts with the SPIR-V magic
+// number, without dragging in the full Build() pipeline.
+int main() {
+    fs::path src = fs::current_path() / "tests" / "ShaderCompile" / "fixture" / "triangle.frag";
+    if (!fs::exists(src)) {
+        std::println(std::cerr, "fixture missing: {}", src.string());
+        return 1;
+    }
+
+    fs::path outDir = fs::temp_directory_path() / "crafter-build-shader-test";
+    std::error_code ec;
+    fs::remove_all(outDir, ec);
+    fs::create_directories(outDir);
+
+    Shader shader(fs::path(src), "main", ShaderType::Fragment);
+
+    std::array<fs::path, 0> includeDirs = {};
+    std::string err = shader.Compile(outDir, includeDirs);
+    if (!err.empty()) {
+        std::println(std::cerr, "shader compile failed: {}", err);
+        return 1;
+    }
+
+    fs::path spv = outDir / "triangle.spv";
+    if (!fs::exists(spv)) {
+        std::println(std::cerr, "spv not produced at {}", spv.string());
+        return 1;
+    }
+
+    // SPIR-V binaries start with the magic number 0x07230203 (little-endian
+    // when written by glslang on x86). Read the first four bytes to confirm
+    // the produced file is real SPIR-V, not a fluke empty file.
+    std::ifstream in(spv, std::ios::binary);
+    std::uint32_t magic = 0;
+    in.read(reinterpret_cast<char*>(&magic), sizeof(magic));
+    if (!in || magic != 0x07230203u) {
+        std::println(std::cerr, "spv has wrong magic: 0x{:08x}", magic);
+        return 1;
+    }
+
+    // Compiling again should be a no-op since the source isn't newer.
+    if (!shader.Check(outDir)) {
+        std::println(std::cerr, "Shader::Check returned false on an up-to-date spv");
+        return 1;
+    }
+    return 0;
+}
diff --git a/tests/StandardArgs/main.cpp b/tests/StandardArgs/main.cpp
new file mode 100644
index 0000000..200c196
--- /dev/null
+++ b/tests/StandardArgs/main.cpp
@@ -0,0 +1,75 @@
+import std;
+import Crafter.Build;
+using namespace Crafter;
+
+namespace {
+    int failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++failures;
+        }
+    }
+}
+
+// Pure-data tests for ApplyStandardArgs and ArgQuery — no build/run, just
+// confirms the documented arg parsing produces the documented mutations.
+int main() {
+    // --debug + --target= + --march= + --mtune=
+    {
+        Configuration cfg;
+        cfg.target = "ignored";
+        std::array<std::string_view, 4> raw = {
+            "--debug", "--target=aarch64-linux-gnu", "--march=armv8-a", "--mtune=cortex-a72",
+        };
+        ApplyStandardArgs(cfg, raw);
+        Check(cfg.debug, "--debug should set cfg.debug");
+        Check(cfg.target == "aarch64-linux-gnu", "--target= should overwrite cfg.target");
+        Check(cfg.march == "armv8-a", "--march= should overwrite cfg.march");
+        Check(cfg.mtune == "cortex-a72", "--mtune= should overwrite cfg.mtune");
+    }
+
+    // --lib promotes Executable -> LibraryStatic; --shared then promotes to Dynamic.
+    {
+        Configuration cfg;
+        cfg.type = ConfigurationType::Executable;
+        std::array<std::string_view, 1> raw = { "--lib" };
+        ApplyStandardArgs(cfg, raw);
+        Check(cfg.type == ConfigurationType::LibraryStatic, "--lib promotes Exe -> Static");
+    }
+    {
+        Configuration cfg;
+        cfg.type = ConfigurationType::Executable;
+        // Order shouldn't matter — promotions chain in priority order.
+        std::array<std::string_view, 2> raw = { "--shared", "--lib" };
+        ApplyStandardArgs(cfg, raw);
+        Check(cfg.type == ConfigurationType::LibraryDynamic, "--lib + --shared lands on Dynamic regardless of order");
+    }
+    {
+        Configuration cfg;
+        cfg.type = ConfigurationType::Executable;
+        std::array<std::string_view, 1> raw = { "--shared" };
+        ApplyStandardArgs(cfg, raw);
+        Check(cfg.type == ConfigurationType::Executable, "--shared alone on Executable is a no-op");
+    }
+
+    // ArgQuery is returned over the same args span and exposes Has/Get.
+    {
+        Configuration cfg;
+        std::array<std::string_view, 3> raw = { "--timing", "--prefix=/opt/x", "extra" };
+        ArgQuery q = ApplyStandardArgs(cfg, raw);
+        Check(q.Has("--timing"), "ArgQuery::Has true for present flag");
+        Check(!q.Has("--missing"), "ArgQuery::Has false for absent flag");
+        auto prefix = q.Get("--prefix=");
+        Check(prefix.has_value(), "ArgQuery::Get returns a value for known prefix");
+        Check(prefix.value_or("") == "/opt/x", "ArgQuery::Get returns the substring after =");
+        Check(!q.Get("--other=").has_value(), "ArgQuery::Get returns nullopt for absent prefix");
+    }
+
+    if (failures > 0) {
+        std::println(std::cerr, "{} assertions failed", failures);
+        return 1;
+    }
+    return 0;
+}
diff --git a/tests/StaticLib/fixture/Greet.cppm b/tests/StaticLib/fixture/Greet.cppm
new file mode 100644
index 0000000..b54de20
--- /dev/null
+++ b/tests/StaticLib/fixture/Greet.cppm
@@ -0,0 +1,4 @@
+export module Greet;
+import std;
+
+export std::string Hello() { return "hi"; }
diff --git a/tests/StaticLib/main.cpp b/tests/StaticLib/main.cpp
new file mode 100644
index 0000000..10dc5d4
--- /dev/null
+++ b/tests/StaticLib/main.cpp
@@ -0,0 +1,49 @@
+import std;
+import Crafter.Build;
+namespace fs = std::filesystem;
+using namespace Crafter;
+
+// Build a tiny module-only static library and confirm libgreet.a lands in
+// BinDir(). Exercises ConfigurationType::LibraryStatic, interface module
+// parsing via GetInterfacesAndImplementations, and the ar/llvm-lib archive
+// step in Crafter.Build-Clang.cpp.
+int main() {
+    Configuration cfg;
+    cfg.path = fs::current_path() / "tests" / "StaticLib" / "fixture";
+    cfg.name = "greet";
+    cfg.outputName = "greet";
+    cfg.target = HostTarget();
+    cfg.type = ConfigurationType::LibraryStatic;
+
+    std::array<fs::path, 1> ifaces = { "Greet" };
+    std::array<fs::path, 0> impls = {};
+    cfg.GetInterfacesAndImplementations(ifaces, impls);
+
+    if (cfg.interfaces.size() != 1) {
+        std::println(std::cerr, "expected 1 interface, got {}", cfg.interfaces.size());
+        return 1;
+    }
+    if (cfg.interfaces[0]->name != "Greet") {
+        std::println(std::cerr, "expected interface 'Greet', got '{}'", cfg.interfaces[0]->name);
+        return 1;
+    }
+
+    std::unordered_map<fs::path, std::shared_future<BuildResult>> depResults;
+    std::mutex depMutex;
+    BuildResult r = Build(cfg, depResults, depMutex);
+    if (!r.result.empty()) {
+        std::println(std::cerr, "build failed: {}", r.result);
+        return 1;
+    }
+
+    fs::path archive = cfg.BinDir() / "libgreet.a";
+    if (!fs::exists(archive)) {
+        std::println(std::cerr, "archive not produced at {}", archive.string());
+        return 1;
+    }
+    if (fs::file_size(archive) == 0) {
+        std::println(std::cerr, "archive {} is empty", archive.string());
+        return 1;
+    }
+    return 0;
+}
diff --git a/tests/TestRunnerSpec/main.cpp b/tests/TestRunnerSpec/main.cpp
new file mode 100644
index 0000000..0b94adc
--- /dev/null
+++ b/tests/TestRunnerSpec/main.cpp
@@ -0,0 +1,115 @@
+import std;
+import Crafter.Build;
+using namespace Crafter;
+
+// POSIX env helpers — libc++ exports only std::getenv via `import std`, so
+// forward-declare the setters we need against the C library directly. The
+// test only runs on POSIX hosts (the project.cpp gates new tests by
+// x86_64-pc-linux-gnu) so we don't need a Windows alternative here.
+extern "C" int setenv(const char* name, const char* value, int overwrite);
+extern "C" int unsetenv(const char* name);
+
+namespace {
+    int failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++failures;
+        }
+    }
+}
+
+// Pure-data tests for TestRunner factories. No tests are actually executed;
+// we just confirm the routing tables in ForTarget / FromSpec / FromEnv.
+int main() {
+    // FromSpec: empty -> nullopt; recognized -> populated; garbage -> throws.
+    {
+        auto empty = TestRunner::FromSpec("");
+        Check(!empty.has_value(), "FromSpec(\"\") returns nullopt");
+    }
+    {
+        auto local = TestRunner::FromSpec("local");
+        Check(local.has_value() && local->IsLocal(), "FromSpec(\"local\") returns Local");
+        Check(local.has_value() && local->name == "local", "Local name is 'local'");
+    }
+    {
+        auto cmd = TestRunner::FromSpec("cmd:wasmtime");
+        Check(cmd.has_value(), "FromSpec(\"cmd:wasmtime\") returns a runner");
+        Check(cmd.has_value() && cmd->name == "cmd:wasmtime", "Cmd name carries the binary");
+        Check(cmd.has_value() && cmd->exec.find("wasmtime") != std::string::npos, "Cmd exec carries the binary");
+        Check(cmd.has_value() && !cmd->probe.empty(), "Cmd probe is set so harness can detect missing tools");
+    }
+    {
+        bool threw = false;
+        try {
+            (void)TestRunner::FromSpec("garbage");
+        } catch (const std::exception&) {
+            threw = true;
+        }
+        Check(threw, "FromSpec rejects unrecognized non-empty specs");
+    }
+
+    // ForTarget routing.
+    {
+        Configuration cfg;
+        cfg.target = HostTarget();
+        TestRunner r = TestRunner::ForTarget(cfg);
+        Check(r.IsLocal(), "ForTarget(host) is Local");
+    }
+    {
+        Configuration cfg;
+        cfg.target = "wasm32-wasip1";
+        TestRunner r = TestRunner::ForTarget(cfg);
+        Check(r.name == "cmd:wasmtime", "ForTarget(wasm32-wasip1) routes through wasmtime");
+    }
+    {
+        Configuration cfg;
+        cfg.target = "aarch64-linux-gnu";
+        TestRunner r = TestRunner::ForTarget(cfg);
+        Check(r.name == "cmd:qemu-aarch64", "ForTarget(aarch64-linux-gnu) routes through qemu-aarch64");
+    }
+    {
+        Configuration cfg;
+        cfg.target = "i686-linux-gnu";
+        TestRunner r = TestRunner::ForTarget(cfg);
+        Check(r.name == "cmd:qemu-i386", "ForTarget(i686-linux-gnu) rewrites arch to i386");
+    }
+    {
+        // Sysroot propagates to QEMU_LD_PREFIX so the dynamic linker is reachable.
+        Configuration cfg;
+        cfg.target = "aarch64-linux-gnu";
+        cfg.sysroot = "/opt/alarm-sysroot";
+        TestRunner r = TestRunner::ForTarget(cfg);
+        Check(r.exec.find("QEMU_LD_PREFIX=/opt/alarm-sysroot") != std::string::npos,
+            "ForTarget propagates sysroot to QEMU_LD_PREFIX");
+    }
+    {
+        // From a Linux host the only sensible way to run x86_64-w64-mingw32 is wine.
+        Configuration cfg;
+        cfg.target = "x86_64-w64-mingw32";
+        TestRunner r = TestRunner::ForTarget(cfg);
+        if (HostTarget().find("linux") != std::string::npos) {
+            Check(r.name == "wine", "ForTarget(mingw) on Linux host uses wine");
+        }
+    }
+
+    // FromEnv: when the env var is set, it wins over the fallback.
+    {
+        // Use a unique fake triple so we don't stomp on a real env var the
+        // CI/dev shell may have set.
+        const char* name = "CRAFTER_BUILD_RUNNER_fake_target_for_unit_test";
+        setenv(name, "cmd:fake-tool", 1);
+        TestRunner r = TestRunner::FromEnv("fake-target-for-unit-test", TestRunner::Local());
+        Check(r.name == "cmd:fake-tool", "FromEnv reads CRAFTER_BUILD_RUNNER_<target>");
+        unsetenv(name);
+        TestRunner fallback = TestRunner::FromEnv("fake-target-for-unit-test", TestRunner::Local());
+        Check(fallback.IsLocal(), "FromEnv falls back when env var is unset");
+    }
+
+    if (failures > 0) {
+        std::println(std::cerr, "{} assertions failed", failures);
+        return 1;
+    }
+    return 0;
+}
diff --git a/tests/VariantId/main.cpp b/tests/VariantId/main.cpp
new file mode 100644
index 0000000..13331ce
--- /dev/null
+++ b/tests/VariantId/main.cpp
@@ -0,0 +1,120 @@
+import std;
+import Crafter.Build;
+using namespace Crafter;
+
+namespace {
+    int failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++failures;
+        }
+    }
+
+    Configuration MakeBase() {
+        Configuration cfg;
+        cfg.path = "/tmp/variant-id-test";
+        cfg.name = "vt";
+        cfg.outputName = "vt";
+        cfg.target = "x86_64-pc-linux-gnu";
+        cfg.march = "native";
+        cfg.mtune = "native";
+        cfg.type = ConfigurationType::Executable;
+        return cfg;
+    }
+}
+
+// VariantId is the cache key for build outputs. Two Configurations that
+// differ only in something that affects codegen must produce different
+// VariantId / BuildDir / BinDir, otherwise their .o files collide.
+int main() {
+    {
+        // Baseline against itself: VariantId is deterministic.
+        Configuration a = MakeBase();
+        Configuration b = MakeBase();
+        Check(a.VariantId() == b.VariantId(), "identical configs have identical VariantId");
+        Check(a.BuildDir() == b.BuildDir(), "identical configs have identical BuildDir");
+    }
+    {
+        // type differs -> VariantId differs.
+        Configuration a = MakeBase();
+        Configuration b = MakeBase();
+        b.type = ConfigurationType::LibraryStatic;
+        Check(a.VariantId() != b.VariantId(), "type change perturbs VariantId");
+    }
+    {
+        // debug differs -> VariantId differs.
+        Configuration a = MakeBase();
+        Configuration b = MakeBase();
+        b.debug = true;
+        Check(a.VariantId() != b.VariantId(), "debug change perturbs VariantId");
+    }
+    {
+        // sysroot differs -> VariantId differs.
+        Configuration a = MakeBase();
+        Configuration b = MakeBase();
+        b.sysroot = "/opt/sysroot";
+        Check(a.VariantId() != b.VariantId(), "sysroot change perturbs VariantId");
+    }
+    {
+        // defines differ -> VariantId differs.
+        Configuration a = MakeBase();
+        Configuration b = MakeBase();
+        b.defines.push_back({"FOO", "1"});
+        Check(a.VariantId() != b.VariantId(), "define added perturbs VariantId");
+    }
+    {
+        // Same define name, different value -> VariantId differs.
+        Configuration a = MakeBase();
+        a.defines.push_back({"FOO", "1"});
+        Configuration b = MakeBase();
+        b.defines.push_back({"FOO", "2"});
+        Check(a.VariantId() != b.VariantId(), "define value change perturbs VariantId");
+    }
+    {
+        // compileFlags differ -> VariantId differs.
+        Configuration a = MakeBase();
+        Configuration b = MakeBase();
+        b.compileFlags.push_back("-fno-omit-frame-pointer");
+        Check(a.VariantId() != b.VariantId(), "compile flag added perturbs VariantId");
+    }
+    {
+        // target differs -> VariantId differs.
+        Configuration a = MakeBase();
+        Configuration b = MakeBase();
+        b.target = "aarch64-linux-gnu";
+        Check(a.VariantId() != b.VariantId(), "target change perturbs VariantId");
+    }
+    {
+        // march differs -> VariantId differs.
+        Configuration a = MakeBase();
+        Configuration b = MakeBase();
+        b.march = "x86-64-v3";
+        Check(a.VariantId() != b.VariantId(), "march change perturbs VariantId");
+    }
+    {
+        // PcmDir() differs between Executable (in BuildDir) and Library
+        // (in BinDir) — Library PCMs land in the installable bin dir so
+        // downstream consumers can find them.
+        Configuration exe = MakeBase();
+        Configuration lib = MakeBase();
+        lib.type = ConfigurationType::LibraryStatic;
+        Check(exe.PcmDir() == exe.BuildDir(), "Executable PcmDir == BuildDir");
+        Check(lib.PcmDir() == lib.BinDir(), "Library PcmDir == BinDir");
+    }
+    {
+        // VariantId is embedded in the path so distinct ids produce distinct dirs.
+        Configuration a = MakeBase();
+        Configuration b = MakeBase();
+        b.debug = true;
+        Check(a.BuildDir() != b.BuildDir(), "different VariantId yields different BuildDir");
+        Check(a.BinDir() != b.BinDir(), "different VariantId yields different BinDir");
+    }
+
+    if (failures > 0) {
+        std::println(std::cerr, "{} assertions failed", failures);
+        return 1;
+    }
+    return 0;
+}
diff --git a/tests/WasiBrowserRuntime/main.cpp b/tests/WasiBrowserRuntime/main.cpp
new file mode 100644
index 0000000..ab61d0e
--- /dev/null
+++ b/tests/WasiBrowserRuntime/main.cpp
@@ -0,0 +1,97 @@
+import std;
+import Crafter.Build;
+namespace fs = std::filesystem;
+using namespace Crafter;
+
+// POSIX env helper — std::setenv is non-standard. Linux-only is fine here;
+// project.cpp gates this test on x86_64-pc-linux-gnu.
+extern "C" int setenv(const char* name, const char* value, int overwrite);
+
+// EnableWasiBrowserRuntime stamps runtime.js, a generated index.html, and a
+// files.json manifest onto cfg.files so the build step copies them next to
+// the .wasm. We don't run the wasm build here (avoids requiring wasi-sdk +
+// the wasi-libc++ PCM rebuild on top of every test); we just verify the
+// helper produced the right artifacts and registered them on the config.
+int main() {
+    Configuration cfg;
+    cfg.path = fs::temp_directory_path() / "crafter-build-wasi-runtime-test";
+    cfg.name = "wasi-helper";
+    cfg.outputName = "wasi-helper";
+    cfg.target = "wasm32-wasip1";
+    cfg.type = ConfigurationType::Executable;
+
+    std::error_code ec;
+    fs::remove_all(cfg.path, ec);
+    fs::create_directories(cfg.path);
+
+    // EnableWasiBrowserRuntime looks up runtime.js + index.html.in under
+    // GetCrafterBuildHome()/wasi-runtime. The test binary lives in
+    // bin/<variant>/, so the auto-detect walks parents and won't find a
+    // share/crafter-build/Crafter.Build.cppm anchor — point CRAFTER_BUILD_HOME
+    // at the repo root (cwd when crafter-build test runs) so it picks up
+    // ./wasi-runtime/. Probe both repo root and the system-installed share
+    // dir to also work from a packaged checkout.
+    fs::path repoWasi = fs::current_path() / "wasi-runtime";
+    fs::path systemHome = "/usr/local/share/crafter-build";
+    if (fs::exists(repoWasi / "runtime.js")) {
+        setenv("CRAFTER_BUILD_HOME", fs::current_path().c_str(), 1);
+    } else if (fs::exists(systemHome / "wasi-runtime" / "runtime.js")) {
+        setenv("CRAFTER_BUILD_HOME", systemHome.c_str(), 1);
+    } else {
+        std::println(std::cerr,
+            "wasi-runtime assets not found near {} or {}",
+            repoWasi.string(), systemHome.string());
+        return 77;  // skipped — environmental, not a code defect
+    }
+
+    EnableWasiBrowserRuntime(cfg);
+
+    // Three files registered: runtime.js, generated index.html, files.json.
+    if (cfg.files.size() != 3) {
+        std::println(std::cerr, "expected 3 cfg.files entries, got {}", cfg.files.size());
+        for (const auto& f : cfg.files) std::println(std::cerr, "  - {}", f.string());
+        return 1;
+    }
+
+    auto findByName = [&](std::string_view name) -> const fs::path* {
+        for (const fs::path& f : cfg.files) {
+            if (f.filename() == name) return &f;
+        }
+        return nullptr;
+    };
+
+    const fs::path* runtimeJs = findByName("runtime.js");
+    const fs::path* indexHtml = findByName("index.html");
+    const fs::path* manifest = findByName("files.json");
+
+    if (!runtimeJs) { std::println(std::cerr, "runtime.js not registered"); return 1; }
+    if (!indexHtml) { std::println(std::cerr, "index.html not registered"); return 1; }
+    if (!manifest)  { std::println(std::cerr, "files.json not registered"); return 1; }
+
+    // The generated index.html and manifest live under the project's build/
+    // wasi-runtime/<name>/ directory and must actually exist on disk.
+    if (!fs::exists(*indexHtml)) {
+        std::println(std::cerr, "index.html not produced at {}", indexHtml->string());
+        return 1;
+    }
+    if (!fs::exists(*manifest)) {
+        std::println(std::cerr, "files.json not produced at {}", manifest->string());
+        return 1;
+    }
+
+    // The template's {{WASM}} placeholder should have been replaced with
+    // the configured outputName.
+    std::ifstream in(*indexHtml);
+    std::string html{std::istreambuf_iterator<char>(in), std::istreambuf_iterator<char>()};
+    if (html.find("wasi-helper.wasm") == std::string::npos) {
+        std::println(std::cerr, "index.html missing wasi-helper.wasm reference");
+        return 1;
+    }
+    if (html.find("{{WASM}}") != std::string::npos) {
+        std::println(std::cerr, "index.html still contains unreplaced {{WASM}} placeholder");
+        return 1;
+    }
+
+    fs::remove_all(cfg.path, ec);
+    return 0;
+}