diff --git a/bin/fm-test-run.sh b/bin/fm-test-run.sh
index a4ef94500f..ec78cd0ec7 100755
--- a/bin/fm-test-run.sh
+++ b/bin/fm-test-run.sh
@@ -2002,6 +2002,23 @@ record_script_result() {
   TOTAL=$((TOTAL + 1))
 }
 
+# The fleet-home overrides a test script must never inherit from whoever
+# invoked the runner. A test builds its own home; an inherited one is a
+# second, invisible input that can decide a verdict - bin/fm-arm-pretool-
+# check.sh, for one, reads FM_HOME as a classification input. The serial
+# lane below and the parallel worker subshell further down both scrub this
+# same list so a script's verdict cannot depend on the lane it was
+# scheduled into.
+FM_TEST_INHERITED_OVERRIDES=(
+  FM_HOME
+  FM_STATE_OVERRIDE
+  FM_DATA_OVERRIDE
+  FM_ROOT_OVERRIDE
+  FM_PROJECTS_OVERRIDE
+  FM_CONFIG_OVERRIDE
+  FM_BACKEND
+)
+
 # Run <script>, capturing output to <out>. <stream> 1 also echoes it live.
 # <id> only has to be unique within this run. When PER_SCRIPT_TIMEOUT_SECS is
 # positive, a script that outruns it is terminated and reported as exit 124: a
@@ -2009,18 +2026,25 @@ record_script_result() {
 # because an unbounded suite is what silently outruns its caller's budget.
 run_script_bounded() {  # <script> <out> <stream> <id>
   local script=$1 out=$2 stream=$3 id=$4
-  local rc
+  local rc name
+  local -a scrub=()
   : "$id"
   set +e
   if [ "$stream" -eq 1 ]; then
+    # The serial lane runs the script from this shell, so the scrub is an
+    # env -u prefix on the child rather than the parallel worker subshell's
+    # unset of its own environment.
+    for name in "${FM_TEST_INHERITED_OVERRIDES[@]}"; do
+      scrub+=(-u "$name")
+    done
     if [ "$PER_SCRIPT_TIMEOUT_SECS" -gt 0 ]; then
       # Expansion is intentionally deferred to the child bash passed to -c.
       # shellcheck disable=SC2016
-      fm_run_timed "$PER_SCRIPT_TIMEOUT_SECS" bash -c \
+      fm_run_timed "$PER_SCRIPT_TIMEOUT_SECS" env "${scrub[@]}" bash -c \
         'bash "$1" 2>&1 | tee "$2"; exit "${PIPESTATUS[0]}"' _ "$script" "$out"
       rc=$?
     else
-      bash "$script" 2>&1 | tee "$out"
+      env "${scrub[@]}" bash "$script" 2>&1 | tee "$out"
       rc=${PIPESTATUS[0]}
     fi
   elif [ "$PER_SCRIPT_TIMEOUT_SECS" -gt 0 ]; then
@@ -2154,8 +2178,7 @@ else
       set +e
       export TMPDIR="$work/tmp"
       export TMP="$work/tmp"
-      unset FM_HOME FM_STATE_OVERRIDE FM_DATA_OVERRIDE FM_ROOT_OVERRIDE \
-        FM_PROJECTS_OVERRIDE FM_CONFIG_OVERRIDE FM_BACKEND 2>/dev/null || true
+      unset "${FM_TEST_INHERITED_OVERRIDES[@]}" 2>/dev/null || true
       cd "$ROOT" || exit 1
       begin_ms=$(now_ms)
       set +e
diff --git a/tests/fm-test-run.test.sh b/tests/fm-test-run.test.sh
index 79a847c372..ed7052d8c3 100755
--- a/tests/fm-test-run.test.sh
+++ b/tests/fm-test-run.test.sh
@@ -1118,6 +1118,91 @@ SH
   pass "jobs scheduler runs proven scripts; failure propagates; non-proven refused"
 }
 
+# A test script builds its own fleet home, so an FM_HOME (or any other
+# override) inherited from whoever invoked the runner is a second, invisible
+# input that can decide a verdict - bin/fm-arm-pretool-check.sh reads FM_HOME
+# as a classification input. The parallel worker subshell has always scrubbed
+# those; this pins that the serial path scrubs the identical list, so a
+# script cannot pass in one lane and fail in the other. The passthrough
+# marker proves the scrub is targeted and not a blanket wipe of the
+# environment a test still needs.
+test_fleet_home_overrides_are_scrubbed_in_both_lanes() {
+  local tmp repo runner evidence fake_bin a b rc lane name seen
+  tmp=$(mktemp -d "${TMPDIR:-/tmp}/fm-test-run-scrub.XXXXXX")
+  repo="$tmp/repo"
+  runner="$repo/bin/fm-test-run.sh"
+  evidence="$tmp/evidence"
+  fake_bin="$tmp/fake-bin"
+  a=tests/fm-brief.test.sh
+  b=tests/fm-composer-lib.test.sh
+  mkdir -p "$repo/bin" "$repo/tests" "$evidence" "$fake_bin"
+  cp "$RUNNER" "$runner"
+  cat >"$fake_bin/stat" <<'SH'
+#!/usr/bin/env bash
+if [ "$1" = "-c" ] && [ "$2" = "%a" ]; then
+  printf '700\n'
+  exit 0
+fi
+if [ "$1" = "-f" ] && [ "$2" = "%Lp" ]; then
+  printf '  File: "%s"\n    ID: fake Namelen: 255 Type: ext2/ext3\n700\n' "$3"
+  exit 0
+fi
+exit 1
+SH
+  cat >"$repo/$a" <<'SH'
+#!/usr/bin/env bash
+out="$SCRUB_EVIDENCE/$SCRUB_LANE"
+: >"$out"
+for name in FM_HOME FM_STATE_OVERRIDE FM_DATA_OVERRIDE FM_ROOT_OVERRIDE \
+  FM_PROJECTS_OVERRIDE FM_CONFIG_OVERRIDE FM_BACKEND; do
+  eval "value=\${$name:-absent}"
+  printf '%s=%s\n' "$name" "$value" >>"$out"
+done
+printf 'PASSTHROUGH=%s\n' "${SCRUB_PASSTHROUGH:-absent}" >>"$out"
+echo "ok - scrub fixture"
+SH
+  cat >"$repo/$b" <<'SH'
+#!/usr/bin/env bash
+echo "ok - filler fixture"
+SH
+  chmod +x "$runner" "$repo/$a" "$repo/$b" "$fake_bin/stat"
+
+  set +e
+  env FM_HOME="$tmp/leaked-home" FM_STATE_OVERRIDE="$tmp/leaked-state" \
+    FM_DATA_OVERRIDE="$tmp/leaked-data" FM_ROOT_OVERRIDE="$tmp/leaked-root" \
+    FM_PROJECTS_OVERRIDE="$tmp/leaked-projects" FM_CONFIG_OVERRIDE="$tmp/leaked-config" \
+    FM_BACKEND=leaked-backend SCRUB_EVIDENCE="$evidence" SCRUB_LANE=serial \
+    SCRUB_PASSTHROUGH=kept "$runner" "$a" >"$tmp/out-serial" 2>"$tmp/err-serial"
+  rc=$?
+  set -e
+  [ "$rc" -eq 0 ] || { cat "$tmp/out-serial" "$tmp/err-serial"; rm -rf "$tmp"; fail "serial scrub fixture run failed"; }
+
+  set +e
+  env FM_HOME="$tmp/leaked-home" FM_STATE_OVERRIDE="$tmp/leaked-state" \
+    FM_DATA_OVERRIDE="$tmp/leaked-data" FM_ROOT_OVERRIDE="$tmp/leaked-root" \
+    FM_PROJECTS_OVERRIDE="$tmp/leaked-projects" FM_CONFIG_OVERRIDE="$tmp/leaked-config" \
+    FM_BACKEND=leaked-backend SCRUB_EVIDENCE="$evidence" SCRUB_LANE=parallel \
+    SCRUB_PASSTHROUGH=kept PATH="$fake_bin:$PATH" \
+    "$runner" --jobs 2 "$a" "$b" >"$tmp/out-parallel" 2>"$tmp/err-parallel"
+  rc=$?
+  set -e
+  [ "$rc" -eq 0 ] || { cat "$tmp/out-parallel" "$tmp/err-parallel"; rm -rf "$tmp"; fail "parallel scrub fixture run failed"; }
+
+  for lane in serial parallel; do
+    seen="$evidence/$lane"
+    [ -s "$seen" ] || { rm -rf "$tmp"; fail "$lane lane produced no scrub evidence"; }
+    for name in FM_HOME FM_STATE_OVERRIDE FM_DATA_OVERRIDE FM_ROOT_OVERRIDE \
+      FM_PROJECTS_OVERRIDE FM_CONFIG_OVERRIDE FM_BACKEND; do
+      grep -qx "$name=absent" "$seen" \
+        || { rm -rf "$tmp"; fail "$lane lane leaked $name into the test script: $(grep "^$name=" "$seen")"; }
+    done
+    grep -qx 'PASSTHROUGH=kept' "$seen" \
+      || { rm -rf "$tmp"; fail "$lane lane wiped an unrelated variable the test still needs"; }
+  done
+  rm -rf "$tmp"
+  pass "both lanes scrub the same inherited fleet-home overrides and keep the rest of the environment"
+}
+
 test_herdr_ci_family_run_has_a_step_timeout() {
   # The required Herdr lane's hang tripwire is the family-run *step* bound, not
   # the 75-minute job cap. Parse the workflow as YAML so nested `with.name`
@@ -1218,5 +1303,6 @@ test_concurrent_runs_are_ordered_longest_first
 test_per_script_timeout_bounds_a_hang
 test_max_wall_ms_is_a_result_not_advice
 test_jobs_parallel_scheduler_and_failure_propagation
+test_fleet_home_overrides_are_scrubbed_in_both_lanes
 test_herdr_ci_family_run_has_a_step_timeout
 test_aggregate_json