diff --git a/parts/packages.nix b/parts/packages.nix
index 13950a7..ef5f39a 100644
--- a/parts/packages.nix
+++ b/parts/packages.nix
@@ -36,7 +36,11 @@
           ruff
         ];
         text = ''
-          exec bash ${inputs.self}/scripts/mcp-server/run-tests.sh "$@"
+          if [ ! -x ./scripts/mcp-server/run-tests.sh ]; then
+            echo "Expected ./scripts/mcp-server/run-tests.sh to be executable from repo root." >&2
+            exit 1
+          fi
+          exec bash ./scripts/mcp-server/run-tests.sh "$@"
         '';
       };
     in
diff --git a/scripts/mcp-server/run-tests.sh b/scripts/mcp-server/run-tests.sh
index aa544b0..ad18d58 100755
--- a/scripts/mcp-server/run-tests.sh
+++ b/scripts/mcp-server/run-tests.sh
@@ -6,6 +6,13 @@ set -euo pipefail
 here="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 cd "$here"
 
+cache_root="${TMPDIR:-/tmp}/mcp-tests-cache"
+mkdir -p "$cache_root"
+export XDG_CACHE_HOME="$cache_root/xdg"
+export RUFF_CACHE_DIR="$cache_root/ruff"
+export MYPY_CACHE_DIR="$cache_root/mypy"
+export PYTEST_ADDOPTS="${PYTEST_ADDOPTS:-} -o cache_dir=$cache_root/pytest"
+
 fix=false
 for arg in "$@"; do
   if [ "$arg" = "--fix" ]; then