diff --git a/start.sh b/start.sh
index a1406663..8a37bf40 100755
--- a/start.sh
+++ b/start.sh
@@ -40,10 +40,14 @@ if [[ -f "${REPO_ROOT}/.env" ]]; then
   # both `source` and `.env`.
   # Sourced from PR #1686 (@binhpt310) — Cluster 1 (operational hardening),
   # extracted to a focused follow-up after the parent PR was deferred.
+  _hermes_env_filtered="$(mktemp "${TMPDIR:-/tmp}/hermes-webui-env.XXXXXX")"
+  grep -vE '^[[:space:]]*(export[[:space:]]+)?(UID|GID|EUID|EGID|PPID)=' "${REPO_ROOT}/.env" > "${_hermes_env_filtered}" || true
   set -a
   # shellcheck source=/dev/null
-  source <(grep -vE '^[[:space:]]*(export[[:space:]]+)?(UID|GID|EUID|EGID|PPID)=' "${REPO_ROOT}/.env")
+  source "${_hermes_env_filtered}"
   set +a
+  rm -f "${_hermes_env_filtered}"
+  unset _hermes_env_filtered
 fi
 
 PYTHON="${HERMES_WEBUI_PYTHON:-}"
diff --git a/tests/test_docker_env_readonly_vars.py b/tests/test_docker_env_readonly_vars.py
index 226f0b8d..42d07dd1 100644
--- a/tests/test_docker_env_readonly_vars.py
+++ b/tests/test_docker_env_readonly_vars.py
@@ -66,23 +66,16 @@ class TestStartShReadonlyEnvFilter:
                 f"'{var}: readonly variable'"
             )
 
-    def test_filter_pattern_uses_grep_or_equivalent(self):
+    def test_filter_pattern_uses_grep_before_source(self):
         """Filter must use a pattern that strips readonly-var lines before
-        the bash `source` consumes them.  `grep -vE` is the canonical form;
-        the assertion accepts any process-substitution-into-source shape."""
-        # Look for `source <(...UID...)` pattern. Note that the inner shell
-        # expression can contain its own parens (e.g. `(export[[:space:]]+)`),
-        # so we use a non-greedy `.*?` rather than `[^)]*`.
-        assert re.search(
-            r"source\s+<\(.*?UID.*?\)",
-            START_SH,
-            re.DOTALL,
-        ), (
-            "start.sh's .env loader must filter readonly bash vars "
-            "(UID/GID/EUID/EGID/PPID) via `source <(grep -vE ...)` or "
-            "equivalent process-substitution form before `source`-ing "
-            "the .env file"
-        )
+        the bash `source` consumes them.  The loader may use a temporary file
+        rather than process substitution because some bash/macOS combinations
+        can source an empty `/dev/fd/*` stream from `source <(grep ...)`."""
+        grep_idx = START_SH.find("grep -vE")
+        source_idx = START_SH.find("source", grep_idx)
+        assert grep_idx != -1, "start.sh must filter readonly vars with grep -vE or equivalent"
+        assert source_idx != -1, "start.sh must source the filtered .env stream"
+        assert grep_idx < source_idx, "readonly-var filtering must happen before source"
 
     def test_filter_handles_optional_export_prefix(self):
         """The ``export`` prefix on env vars is optional but common.  The