for reference for those who are interested in testing the HEAD:

diff --git c/lix/libstore/build/local-derivation-goal.cc i/lix/libstore/build/local-derivation-goal.cc
index 68442d839..bd5cf4147 100644
--- c/lix/libstore/build/local-derivation-goal.cc
+++ i/lix/libstore/build/local-derivation-goal.cc
@@ -457,12 +457,13 @@ try {
                 "Failed to use the system-wide build directory '%s', falling back to a temporary "
                 "directory inside '%s'",
                 globalBuildDir,
-                worker.buildDir
+                *worker.buildDir
             );
+        }
     }
     /* Create a temporary directory where the build will take
        place. */
-    tmpDirRoot = createTempSubdir(buildDir, std::nullopt, 0700);
+    tmpDirRoot = createTempSubdir(*worker.buildDir, std::nullopt, 0700);
     /* The TOCTOU between the previous mkdir call and this open call is unavoidable due to
      * POSIX semantics.*/
     tmpDirRootFd = AutoCloseFD{open(tmpDirRoot.c_str(), O_RDONLY | O_NOFOLLOW | O_DIRECTORY)};
diff --git c/lix/libutil/file-system.cc i/lix/libutil/file-system.cc
index f85202956..750d64cb4 100644
--- c/lix/libutil/file-system.cc
+++ i/lix/libutil/file-system.cc
@@ -677,7 +677,7 @@ Path makeTempPath(const Path & root, const std::optional<Path> & prefix)
     const uint64_t entropy[2] = {uniform_dist(generator), uniform_dist(generator)};
     const std::string unique = base32Encode(std::string_view(
         reinterpret_cast<const char *>(entropy),
-        sizeof(entropy),
+        sizeof(entropy)
     ));
     if (prefix) {
         return fmt("%s%s-%s", root, *prefix, unique);

should be sufficient on the top of the chain

In reply to @emilazy:matrix.org

something like

       For full logs, run:
           nix log /nix/store/r0729886rq9nvzsngj2dnwwdl5q5dcq0-test.drv
       The build directory has been kept in:
           /nix/var/nix/b/3qxda0xkh4wllkxh182kvrza5n

fwiw this is the code that propagates the remote error

    std::shared_ptr<Error> remoteError;
    if (result.getResult().isBad()) {
        remoteError = std::make_shared<Error>(from(result.getResult().getBad()));
        logger->logEI(remoteError->info());
    }
    co_return HookResult::Accept{TRY_AWAIT(buildDone(remoteError))};