diff --git a/tests/HostCacheSourceStamp/main.cpp b/tests/HostCacheSourceStamp/main.cpp
index b6904e8..4110c82 100644
--- a/tests/HostCacheSourceStamp/main.cpp
+++ b/tests/HostCacheSourceStamp/main.cpp
@@ -119,6 +119,17 @@ int main() {
     Check(fs::last_write_time(probePcm) == afterCold, "unchanged sources do not rebuild the cache");
     Check(ReadStamp(moduleCacheDir) == firstStamp, "unchanged sources keep the same stamp");
 
+    // The compiled project file is the other half of the same hazard: it is
+    // loaded into this process and must have been compiled against the
+    // declarations this process was built from.
+    fs::path projectSo;
+    for (const fs::directory_entry& entry : fs::recursive_directory_iterator(projectDir)) {
+        if (entry.path().filename() == "project.so") projectSo = entry.path();
+    }
+    Check(!projectSo.empty(), "the project file was compiled to a project.so");
+    if (projectSo.empty()) return 1;
+    const fs::file_time_type soBeforeEdit = fs::last_write_time(projectSo);
+
     // Now the case the mtime rule cannot see. The edit lands in a module the
     // probe does not name, because the cached PCMs import each other: a change
     // anywhere in the set invalidates all of them.
@@ -139,6 +150,11 @@ int main() {
 
     Check(fs::last_write_time(probePcm) > afterCold, "a content change rebuilds the cache even when the source's mtime is older");
     Check(ReadStamp(moduleCacheDir) != firstStamp, "the recorded stamp follows the sources");
+    // project.cpp itself is untouched, so only the stamp can force this. It
+    // once covered the version string alone (the rest sat behind a NUL in a
+    // format string): a project.so built against a class of another size was
+    // reused and corrupted the heap when it filled one in.
+    Check(fs::last_write_time(projectSo) > soBeforeEdit, "changed declarations recompile project.so even though project.cpp is unchanged");
 
     fs::remove_all(scratch, ec);