diff --git a/MonteCarloMarginalizeCode/Code/bin/util_ILEdagPostprocess.sh b/MonteCarloMarginalizeCode/Code/bin/util_ILEdagPostprocess.sh
index c764cd6a6..b5107c1f5 100755
--- a/MonteCarloMarginalizeCode/Code/bin/util_ILEdagPostprocess.sh
+++ b/MonteCarloMarginalizeCode/Code/bin/util_ILEdagPostprocess.sh
@@ -5,6 +5,23 @@
 #   For NR-based DAGs, (a) consolidates the output, (b) runs ILE simplification, then (c) creates an NR-indexed version.
 #   The second format uses a *portable* name, which is stable to me changing the underlying relationship between spins and label.
 
+set -o pipefail
+
+SCRIPT_DIR="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd -P)"
+
+resolve_helper() {
+    local helper=$1
+    if [ -x "${SCRIPT_DIR}/${helper}" ]; then
+        printf '%s\n' "${SCRIPT_DIR}/${helper}"
+    elif command -v "${helper}" >/dev/null 2>&1; then
+        command -v "${helper}"
+    else
+        echo "ERROR: unable to locate required helper ${helper}" >&2
+        return 127
+    fi
+}
+
+CLEAN_ILE="$(resolve_helper util_CleanILE.py)" || exit $?
 
 DIR_PROCESS=$1
 BASE_OUT=$2
@@ -24,12 +41,23 @@ find ${DIR_PROCESS} -name 'CME*.dat' -exec cat {} \; > ${RND}_tmp.dat
 echo " Consolidating multiple instances of the monte carlo  .... "
 if [ "$3" == '--eccentricity' ]; then
     if [ "$4" == '--meanPerAno' ]; then
-	util_CleanILE.py ${RND}_tmp.dat $3 $4 | sort -rg -k12 > $BASE_OUT.composite
+	"${CLEAN_ILE}" ${RND}_tmp.dat $3 $4 | sort -rg -k12 > $BASE_OUT.composite
     else
-	util_CleanILE.py ${RND}_tmp.dat $3 | sort -rg -k11 > $BASE_OUT.composite
+	"${CLEAN_ILE}" ${RND}_tmp.dat $3 | sort -rg -k11 > $BASE_OUT.composite
     fi
 else
-    util_CleanILE.py ${RND}_tmp.dat $3 | sort -rg -k10 > $BASE_OUT.composite
+    "${CLEAN_ILE}" ${RND}_tmp.dat $3 | sort -rg -k10 > $BASE_OUT.composite
+fi
+clean_status=$?
+if [ ${clean_status} -ne 0 ]; then
+    echo "ERROR: ILE consolidation failed with status ${clean_status}" >&2
+    rm -f "$BASE_OUT.composite"
+    exit ${clean_status}
+fi
+if [ ! -s "$BASE_OUT.composite" ]; then
+    echo "ERROR: ILE consolidation produced an empty composite: $BASE_OUT.composite" >&2
+    rm -f "$BASE_OUT.composite"
+    exit 1
 fi
 
 # Manifest
@@ -43,6 +71,9 @@ cat ${DIR_PROCESS}/command-single.sh >>  ${BASE_OUT}.manifest
 env >> ${BASE_OUT}.environment  
 
 # tar file
-tar cvzf ${BASE_OUT}.tgz ${BASE_OUT}.composite  ${BASE_OUT}.manifest ${BASE_OUT}.environment
+if ! tar cvzf ${BASE_OUT}.tgz ${BASE_OUT}.composite  ${BASE_OUT}.manifest ${BASE_OUT}.environment; then
+    echo "ERROR: failed to create ${BASE_OUT}.tgz" >&2
+    exit 1
+fi
 
-exit 0 ;  # force end on success, for DAG
+exit 0
diff --git a/MonteCarloMarginalizeCode/Code/bin/util_NRdagPostprocess.sh b/MonteCarloMarginalizeCode/Code/bin/util_NRdagPostprocess.sh
index 50037f79c..2733ac38b 100755
--- a/MonteCarloMarginalizeCode/Code/bin/util_NRdagPostprocess.sh
+++ b/MonteCarloMarginalizeCode/Code/bin/util_NRdagPostprocess.sh
@@ -5,6 +5,24 @@
 #   For NR-based DAGs, (a) consolidates the output, (b) runs ILE simplification, then (c) creates an NR-indexed version.
 #   The second format uses a *portable* name, which is stable to me changing the underlying relationship between spins and label.
 
+set -o pipefail
+
+SCRIPT_DIR="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd -P)"
+
+resolve_helper() {
+    local helper=$1
+    if [ -x "${SCRIPT_DIR}/${helper}" ]; then
+        printf '%s\n' "${SCRIPT_DIR}/${helper}"
+    elif command -v "${helper}" >/dev/null 2>&1; then
+        command -v "${helper}"
+    else
+        echo "ERROR: unable to locate required helper ${helper}" >&2
+        return 127
+    fi
+}
+
+CLEAN_ILE="$(resolve_helper util_CleanILE.py)" || exit $?
+RELABEL_ILE="$(resolve_helper util_NRRelabelILE.py)" || exit $?
 
 DIR_PROCESS=$1
 BASE_OUT=$2
@@ -21,9 +39,20 @@ find ${DIR_PROCESS} -name 'CME*.dat' -exec cat {} \; > ${DIR_PROCESS}_tmp.dat
 echo " Consolidating multiple instances of the monte carlo  .... "
 if [ "$4" == '--eccentricity' ]
 then
-    util_CleanILE.py ${DIR_PROCESS}_tmp.dat $4 | sort -rg -k11 > $BASE_OUT.composite
+    "${CLEAN_ILE}" ${DIR_PROCESS}_tmp.dat $4 | sort -rg -k11 > $BASE_OUT.composite
 else
-    util_CleanILE.py ${DIR_PROCESS}_tmp.dat $4 | sort -rg -k10 > $BASE_OUT.composite
+    "${CLEAN_ILE}" ${DIR_PROCESS}_tmp.dat $4 | sort -rg -k10 > $BASE_OUT.composite
+fi
+clean_status=$?
+if [ ${clean_status} -ne 0 ]; then
+    echo "ERROR: NR consolidation failed with status ${clean_status}" >&2
+    rm -f "$BASE_OUT.composite"
+    exit ${clean_status}
+fi
+if [ ! -s "$BASE_OUT.composite" ]; then
+    echo "ERROR: NR consolidation produced an empty composite: $BASE_OUT.composite" >&2
+    rm -f "$BASE_OUT.composite"
+    exit 1
 fi
 
 # index them
@@ -32,9 +61,20 @@ echo " Reindexing the data to   .... "
 if [ "$4" == '--eccentricity' ]
 then
     #    util_NRRelabelILE.py --group ${GROUP} --fname ${BASE_OUT}.composite --eccentricity | grep '^-1*' > ${BASE_OUT}.indexed
-    util_NRRelabelILE.py --group Sequence-RIT-All --fname ${BASE_OUT}.composite --eccentricity | sed -n '/ -----  BEST MATCHES ------ /,$p' > ${BASE_OUT}.indexed
+    "${RELABEL_ILE}" --group Sequence-RIT-All --fname ${BASE_OUT}.composite --eccentricity | sed -n '/ -----  BEST MATCHES ------ /,$p' > ${BASE_OUT}.indexed
 else
-    util_NRRelabelILE.py --group ${GROUP} --fname ${BASE_OUT}.composite | grep '^-1*' > ${BASE_OUT}.indexed
+    "${RELABEL_ILE}" --group ${GROUP} --fname ${BASE_OUT}.composite | grep '^-1*' > ${BASE_OUT}.indexed
+fi
+relabel_status=$?
+if [ ${relabel_status} -ne 0 ]; then
+    echo "ERROR: NR relabeling failed with status ${relabel_status}" >&2
+    rm -f "$BASE_OUT.indexed"
+    exit ${relabel_status}
+fi
+if [ ! -s "$BASE_OUT.indexed" ]; then
+    echo "ERROR: NR relabeling produced an empty index: $BASE_OUT.indexed" >&2
+    rm -f "$BASE_OUT.indexed"
+    exit 1
 fi
 
 # Manifest
@@ -50,4 +90,7 @@ cat ${DIR_PROCESS}/integrate.sub >>  ${BASE_OUT}.submit
 env >> ${BASE_OUT}.environment  
 
 # tar file
-tar cvzf ${BASE_OUT}.tgz ${BASE_OUT}.composite ${BASE_OUT}.indexed ${BASE_OUT}.manifest ${BASE_OUT}.environment ${BASE_OUT}.submit
+if ! tar cvzf ${BASE_OUT}.tgz ${BASE_OUT}.composite ${BASE_OUT}.indexed ${BASE_OUT}.manifest ${BASE_OUT}.environment ${BASE_OUT}.submit; then
+    echo "ERROR: failed to create ${BASE_OUT}.tgz" >&2
+    exit 1
+fi
diff --git a/MonteCarloMarginalizeCode/Code/test/test_dag_postprocess_fail_closed.py b/MonteCarloMarginalizeCode/Code/test/test_dag_postprocess_fail_closed.py
new file mode 100644
index 000000000..3aedf7728
--- /dev/null
+++ b/MonteCarloMarginalizeCode/Code/test/test_dag_postprocess_fail_closed.py
@@ -0,0 +1,115 @@
+import os
+from pathlib import Path
+import shutil
+import subprocess
+
+import pytest
+
+
+BIN_DIR = Path(__file__).resolve().parents[1] / "bin"
+SANITIZED_PATH = "/usr/bin:/bin"
+
+
+def _copy_wrapper(tmp_path, name):
+    wrapper = tmp_path / name
+    shutil.copy2(BIN_DIR / name, wrapper)
+    return wrapper
+
+
+def _write_helper(directory, name, body):
+    helper = directory / name
+    helper.write_text("#!/bin/sh\n" + body)
+    helper.chmod(0o755)
+    return helper
+
+
+def _run_ile(wrapper, tmp_path):
+    input_dir = tmp_path / "ile"
+    input_dir.mkdir()
+    (input_dir / "CME_test.dat").write_text(
+        "0 1 2 3 4 5 6 7 8 9 10 11 12\n"
+    )
+    (input_dir / "test.psd.xml.gz").write_bytes(b"psd")
+    (input_dir / "command-single.sh").write_text("command\n")
+    base_out = tmp_path / "consolidated"
+    env = os.environ.copy()
+    env["PATH"] = SANITIZED_PATH
+    result = subprocess.run(
+        [str(wrapper), str(input_dir), str(base_out)],
+        cwd=tmp_path,
+        env=env,
+        text=True,
+        capture_output=True,
+    )
+    return result, base_out
+
+
+def _run_nr(wrapper, tmp_path):
+    input_dir = tmp_path / "nr"
+    input_dir.mkdir()
+    (input_dir / "CME_test.dat").write_text(
+        "0 1 2 3 4 5 6 7 8 9 10 11 12\n"
+    )
+    (input_dir / "test.psd.xml.gz").write_bytes(b"psd")
+    (input_dir / "command-single.sh").write_text("command\n")
+    (input_dir / "integrate.sub").write_text("queue 1\n")
+    base_out = tmp_path / "consolidated_nr"
+    env = os.environ.copy()
+    env["PATH"] = SANITIZED_PATH
+    result = subprocess.run(
+        [str(wrapper), str(input_dir), str(base_out), "Sequence-RIT-All"],
+        cwd=tmp_path,
+        env=env,
+        text=True,
+        capture_output=True,
+    )
+    return result, base_out
+
+
+@pytest.mark.parametrize("name", ["util_ILEdagPostprocess.sh", "util_NRdagPostprocess.sh"])
+def test_postprocess_fails_when_cleaner_is_unavailable(tmp_path, name):
+    assert shutil.which("util_CleanILE.py", path=SANITIZED_PATH) is None
+    wrapper = _copy_wrapper(tmp_path, name)
+    env = os.environ.copy()
+    env["PATH"] = SANITIZED_PATH
+    args = [str(wrapper), "missing-input", str(tmp_path / "out")]
+    if name == "util_NRdagPostprocess.sh":
+        args.append("Sequence-RIT-All")
+    result = subprocess.run(args, cwd=tmp_path, env=env, text=True, capture_output=True)
+    assert result.returncode != 0
+    assert "unable to locate required helper util_CleanILE.py" in result.stderr
+
+
+def test_ile_postprocess_resolves_sibling_cleaner_with_sanitized_path(tmp_path):
+    wrapper = _copy_wrapper(tmp_path, "util_ILEdagPostprocess.sh")
+    _write_helper(tmp_path, "util_CleanILE.py", 'cat "$1"\n')
+    result, base_out = _run_ile(wrapper, tmp_path)
+    assert result.returncode == 0, result.stderr
+    assert base_out.with_suffix(".composite").stat().st_size > 0
+
+
+def test_ile_postprocess_rejects_empty_composite(tmp_path):
+    wrapper = _copy_wrapper(tmp_path, "util_ILEdagPostprocess.sh")
+    _write_helper(tmp_path, "util_CleanILE.py", "exit 0\n")
+    result, base_out = _run_ile(wrapper, tmp_path)
+    assert result.returncode != 0
+    assert "produced an empty composite" in result.stderr
+    assert not base_out.with_suffix(".composite").exists()
+
+
+def test_ile_postprocess_propagates_cleaner_failure(tmp_path):
+    wrapper = _copy_wrapper(tmp_path, "util_ILEdagPostprocess.sh")
+    _write_helper(tmp_path, "util_CleanILE.py", "exit 42\n")
+    result, base_out = _run_ile(wrapper, tmp_path)
+    assert result.returncode == 42
+    assert not base_out.with_suffix(".composite").exists()
+
+
+def test_nr_postprocess_rejects_empty_relabel_output(tmp_path):
+    wrapper = _copy_wrapper(tmp_path, "util_NRdagPostprocess.sh")
+    _write_helper(tmp_path, "util_CleanILE.py", 'cat "$1"\n')
+    _write_helper(tmp_path, "util_NRRelabelILE.py", "exit 0\n")
+    result, base_out = _run_nr(wrapper, tmp_path)
+    assert result.returncode != 0
+    assert "NR relabeling failed" in result.stderr
+    assert not base_out.with_suffix(".indexed").exists()