diff --git a/interfaces/Crafter.Build-Clang.cppm b/interfaces/Crafter.Build-Clang.cppm
index 1149661..8a306be 100644
--- a/interfaces/Crafter.Build-Clang.cppm
+++ b/interfaces/Crafter.Build-Clang.cppm
@@ -7,6 +7,7 @@ import :Shader;
 import :Interface;
 import :Implementation;
 import :External;
+import :Toolchain;
 namespace fs = std::filesystem;
 
 export namespace Crafter {
@@ -488,10 +489,21 @@ export namespace Crafter {
         void AddAstLintRule(std::string name, std::function<void(LintContext&)> check);
         // Suffix that uniquely identifies this Configuration's compile state.
         // target+march+mtune are spelled out for readability; the rest
-        // (type, debug, sysroot, defines, compileFlags) collapse into a short
-        // hash so two Configurations sharing a path but with different
-        // compile state can't clobber each other's outputs.
-        std::string VariantId() const {
+        // (type, debug, sysroot, defines, compileFlags, the toolchain)
+        // collapse into a short hash so two Configurations sharing a path but
+        // with different compile state can't clobber each other's outputs.
+        std::string VariantId() const { return VariantId(GetToolchain()); }
+        // The variant under an explicit toolchain; VariantId() is this for
+        // the running one. The compiler is compile state like any flag: a
+        // PCM can only be read by the clang release that wrote it, and
+        // staleness is decided by mtimes, which a compiler update does not
+        // touch. Before the toolchain was hashed, a project built with clang
+        // 22 kept its PCMs when crafter-build moved to clang 23, and the
+        // first edit afterwards failed with "module file ... uses an older
+        // format that is no longer supported" until the project was cleaned.
+        // Keyed here, an update starts an empty directory and rebuilds; the
+        // old one stays until `crafter-build clean`.
+        std::string VariantId(const Toolchain& toolchain) const {
             std::string compileKey;
             compileKey += std::to_string(static_cast<std::int32_t>(type));
             compileKey += '|';
@@ -521,6 +533,8 @@ export namespace Crafter {
                 compileKey += "|A:";
                 compileKey += a;
             }
+            compileKey += "|T:";
+            compileKey += toolchain.Key();
             std::size_t configHash = std::hash<std::string>{}(compileKey);
             return std::format("{}-{}-{}-{}-{:08x}", name, target, march, mtune, configHash);
         }