diff --git a/tests/CleanProject/main.cpp b/tests/CleanProject/main.cpp
index ecad097..25f85f1 100644
--- a/tests/CleanProject/main.cpp
+++ b/tests/CleanProject/main.cpp
@@ -29,6 +29,12 @@ namespace {
         std::ofstream(root / "keep-me.txt") << "untouched";
         return root;
     }
+
+    std::int32_t RunWith(std::vector<std::string> args) {
+        std::vector<char*> argv;
+        for (std::string& arg : args) argv.push_back(arg.data());
+        return Run(static_cast<std::int32_t>(argv.size()), argv.data());
+    }
 }
 
 int main() {
@@ -71,6 +77,22 @@ int main() {
         Check(fs::exists(cwdBin) == cwdBinExisted, "the cwd's bin/ is untouched");
     }
 
+    {
+        // `clean` is a verb only where a verb can stand. After `format` it
+        // is a rule-name glob like any other word, and a glob must never
+        // delete the build trees. The project file cannot compile, so the
+        // run fails once it gets as far as loading it — which is the proof
+        // that it went on to format instead of cleaning and exiting 0.
+        fs::path root = StageProject("format-glob");
+        fs::path projectFile = root / "project.cpp";
+        std::ofstream(projectFile) << "this is not valid C++ at all\n";
+
+        const std::int32_t exitCode = RunWith({"crafter-build", "format", "clean", std::format("--project={}", projectFile.string())});
+        Check(fs::exists(root / "bin" / "app-x86_64-pc-linux-gnu-native-native-0badf00d" / "app"), "`format clean` leaves bin/ alone");
+        Check(fs::exists(root / "build" / "app-x86_64-pc-linux-gnu-native-native-0badf00d" / "main_impl.o"), "`format clean` leaves build/ alone");
+        Check(exitCode != 0, "`format clean` goes on to load the project instead of cleaning");
+    }
+
     if (Failures > 0) {
         std::println(std::cerr, "{} assertions failed", Failures);
         return 1;