diff --git a/implementations/Crafter.Build-Platform.cpp b/implementations/Crafter.Build-Platform.cpp
index 5d6a493..5ffaa17 100644
--- a/implementations/Crafter.Build-Platform.cpp
+++ b/implementations/Crafter.Build-Platform.cpp
@@ -554,6 +554,12 @@ Configuration* Crafter::ProjectDependency(std::string_view name) {
     throw std::runtime_error(std::format("ProjectDependency('{}'): not declared by {} (declared: {})", name, frame.project.string(), declared.empty() ? "none" : declared));
 }
 
+bool Crafter::IsTopLevelProject() {
+    // One frame is the project file crafter-build was pointed at; a second
+    // is only ever pushed for a dependency's project file.
+    return LoadingFrames.size() <= 1;
+}
+
 #if defined(CRAFTER_BUILD_CONFIGURATION_TARGET_x86_64_pc_windows_msvc) || defined(CRAFTER_BUILD_CONFIGURATION_TARGET_x86_64_w64_mingw32)
 CommandResult Crafter::RunCommandChecked(std::string_view cmd) {
     std::array<char, 128> buffer;
diff --git a/interfaces/Crafter.Build-Platform.cppm b/interfaces/Crafter.Build-Platform.cppm
index c120cc0..0b37177 100644
--- a/interfaces/Crafter.Build-Platform.cppm
+++ b/interfaces/Crafter.Build-Platform.cppm
@@ -41,6 +41,15 @@ namespace Crafter {
     // Throws when no such dependency was declared, or when called from
     // anywhere other than inside LoadProject.
     export CRAFTER_API Configuration* ProjectDependency(std::string_view name);
+    // True while the project file crafter-build was pointed at runs its
+    // CrafterBuildProject, and when no project file is running at all; false
+    // inside a dependency's project file (a `#pragma crafter dependency`,
+    // GitProject or LocalProject target). Wire anything only the project's
+    // own build or test run needs behind it. The case that forced it: a
+    // library's test links crafter-build itself, so when crafter-build loads
+    // that library as a dependency, the test's GitProject loads crafter-build's
+    // project file in turn and the two recurse until the cycle guard throws.
+    export CRAFTER_API bool IsTopLevelProject();
     // Resolves the directory holding distributed runtime assets (Crafter.Build
     // module sources, wasi-runtime/, etc). Honors CRAFTER_BUILD_HOME; otherwise
     // derives <prefix>/share/crafter-build from the running executable's path.
diff --git a/tests/ProjectDependency/fixture/app/project.cpp b/tests/ProjectDependency/fixture/app/project.cpp
index 329f662..b6744d4 100644
--- a/tests/ProjectDependency/fixture/app/project.cpp
+++ b/tests/ProjectDependency/fixture/app/project.cpp
@@ -16,5 +16,8 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view>)
     // Once by the module it exports, once by its configuration name; the
     // test expects the same object back for both.
     cfg.dependencies = { ProjectDependency("Greeter"), ProjectDependency("greeter") };
+    // Reported through a flag the test can read back: this file is the one
+    // LoadProject was pointed at, the greeter's is not.
+    cfg.compileFlags.push_back(IsTopLevelProject() ? "-DTOP_LEVEL=1" : "-DTOP_LEVEL=0");
     return cfg;
 }
diff --git a/tests/ProjectDependency/fixture/greeter/project.cpp b/tests/ProjectDependency/fixture/greeter/project.cpp
index 82eb6c1..f9dcbbf 100644
--- a/tests/ProjectDependency/fixture/greeter/project.cpp
+++ b/tests/ProjectDependency/fixture/greeter/project.cpp
@@ -19,5 +19,7 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
     std::array<fs::path, 1> ifaces = { "interfaces/Greeter" };
     std::array<fs::path, 0> impls = {};
     cfg.GetInterfacesAndImplementations(ifaces, impls);
+    // Loaded as a dependency, so this must come out false (see app/).
+    cfg.compileFlags.push_back(IsTopLevelProject() ? "-DTOP_LEVEL=1" : "-DTOP_LEVEL=0");
     return cfg;
 }
diff --git a/tests/ProjectDependency/main.cpp b/tests/ProjectDependency/main.cpp
index 3d80370..fcf1c57 100644
--- a/tests/ProjectDependency/main.cpp
+++ b/tests/ProjectDependency/main.cpp
@@ -92,11 +92,14 @@ int main(int argc, char** argv) {
     const fs::path appSo = scratch / "app" / "build" / "project.so";
     std::array<std::string_view, 0> noArgs = {};
 
+    Check(IsTopLevelProject(), "outside any project file counts as top level");
+
     // The dependency is fetched, built and linked before project.cpp runs,
     // and ProjectDependency() hands it back by either name.
     try {
         Configuration cfg = LoadProject(app, noArgs);
         Check(cfg.name == "hello from greeter", std::format("project.cpp imported and called the dependency's module (got '{}')", cfg.name));
+        Check(std::ranges::contains(cfg.compileFlags, "-DTOP_LEVEL=1"), "the project LoadProject was pointed at sees itself as top level");
         Check(cfg.dependencies.size() == 2, "ProjectDependency resolved by module name and by configuration name");
         if (cfg.dependencies.size() == 2) {
             Check(cfg.dependencies[0] == cfg.dependencies[1], "both names resolve to the same dependency");
@@ -105,6 +108,7 @@ int main(int argc, char** argv) {
             Check(dep.target == HostTarget(), "the dependency was built for the host triple LoadProject uses");
             Check(dep.march == "native", "the environment's march was overridden for host tooling");
             Check(std::ranges::contains(dep.compileFlags, "-fPIC"), "the dependency archive was built position-independent");
+            Check(std::ranges::contains(dep.compileFlags, "-DTOP_LEVEL=0"), "a dependency's project file does not see itself as top level");
             Check(fs::exists(ExpectedOutput(dep)), "the dependency archive exists");
         }
     } catch (const std::exception& e) {