diff --git a/tests/VariantId/main.cpp b/tests/VariantId/main.cpp
index 022b0d6..e4d2a36 100644
--- a/tests/VariantId/main.cpp
+++ b/tests/VariantId/main.cpp
@@ -133,6 +133,33 @@ int main() {
         Check(a.BuildDir() != b.BuildDir(), "different VariantId yields different BuildDir");
         Check(a.BinDir() != b.BinDir(), "different VariantId yields different BinDir");
     }
+    {
+        // The compiler is compile state as much as any flag: a PCM can only
+        // be read by the clang release that wrote it, and nothing in a
+        // project's sources changes when the compiler does. With the
+        // toolchain left out of the id, a project built with clang 22 kept
+        // its PCMs after crafter-build moved to clang 23; the build after
+        // the update found nothing stale, and the first edit failed with
+        // "module file ... uses an older format that is no longer supported"
+        // until the project was cleaned by hand.
+        Configuration cfg = MakeBase();
+        Toolchain older;
+        older.clangxx = "/app/bin/clang++";
+        older.clangVersion = "clang version 22.1.8";
+        Toolchain newer = older;
+        newer.clangVersion = "clang version 23.1.2";
+        Check(cfg.VariantId(older) != cfg.VariantId(newer), "a compiler update at the same path perturbs VariantId");
+        Check(cfg.VariantId(older) == cfg.VariantId(older), "VariantId under one toolchain is deterministic");
+
+        // Two installs of one release are still two toolchains: their
+        // libc++ need not be the same build.
+        Toolchain elsewhere = older;
+        elsewhere.clangxx = "/usr/bin/clang++";
+        Check(cfg.VariantId(older) != cfg.VariantId(elsewhere), "the same release installed elsewhere perturbs VariantId");
+
+        Check(cfg.VariantId() == cfg.VariantId(GetToolchain()), "VariantId() is the variant under the running toolchain");
+        Check(cfg.BuildDir().filename() == cfg.VariantId(GetToolchain()), "BuildDir is named for the running toolchain's variant");
+    }
 
     if (Failures > 0) {
         std::println(std::cerr, "{} assertions failed", Failures);