diff --git a/scripts/memlab.sh b/scripts/memlab.sh
index 373742556..4e441723a 100644
--- a/scripts/memlab.sh
+++ b/scripts/memlab.sh
@@ -24,6 +24,16 @@ if [ ! -x "$BINARY" ]; then
     exit 2
 fi
 
+# The profiled process must reach a daemon rendezvous and cache this run owns:
+# only CBM_RUNTIME_DIR moves the rendezvous, so a private CBM_CACHE_DIR alone
+# joined the operator's account daemon (#1691, #1696). The helper also stops
+# that daemon before its cache is removed and leaves the root for diagnosis
+# when it will not stop.
+# shellcheck source=test-runtime.sh
+source "$(dirname "$0")/test-runtime.sh"
+cbm_test_runtime_init || exit 1
+trap 'cbm_test_runtime_cleanup "$BINARY"' EXIT
+
 WORK=$(mktemp -d 2>/dev/null || mktemp -d -t memlab)
 
 # Native Windows: the server walks the full ancestor chain of both the binary
@@ -75,7 +85,12 @@ PROFILE_OUT="$PWD/memlab-${LABEL}.jsonl"
 RUN_LOG="$PWD/memlab-${LABEL}.log"
 rm -f "$PROFILE_OUT" "$RUN_LOG"
 
-cleanup() { rm -rf "$WORK" 2>/dev/null || true; rm -rf "${WIN_ROOT:-}" 2>/dev/null || true; }
+cleanup() {
+    # The helper probes with $BINARY, whose native-Windows copy lives in $WORK.
+    cbm_test_runtime_cleanup "$BINARY"
+    rm -rf "$WORK" 2>/dev/null || true
+    rm -rf "${WIN_ROOT:-}" 2>/dev/null || true
+}
 trap cleanup EXIT
 
 # A fixed corpus: same file count and content on every platform, so a
@@ -108,12 +123,15 @@ done
 echo "=== memlab: binary=$BINARY requests=$REQUESTS label=$LABEL ==="
 echo "corpus: $(find "$CORPUS" -name '*.py' | wc -l | tr -d ' ') files"
 
-# The Windows binary needs a native path here; an msys /c/... path is not one.
-if command -v cygpath >/dev/null 2>&1 && ! command -v winepath >/dev/null 2>&1; then
+# The native Windows binary needs a native path here, under its stamped root;
+# an msys /c/... path is not one. Everywhere else the helper's owner-only cache
+# (already exported as CBM_CACHE_DIR) is the cache.
+if [ -n "${WIN_ROOT:-}" ]; then
     mkdir -p "$WORK/cache"
     export CBM_CACHE_DIR="$(cygpath -w "$WORK/cache")"
+    CACHE_HOST="$WORK/cache"
 else
-    export CBM_CACHE_DIR="$WORK/cache"
+    CACHE_HOST="$CBM_TEST_CACHE_DIR_HOST"
 fi
 export CBM_MEM_PROFILE=1
 export CBM_MEM_PROFILE_OUT="$PROFILE_OUT"
@@ -139,7 +157,7 @@ python3 "$(dirname "$0")/memlab-drive.py" "$DRIVE_BINARY" "$DRIVE_CORPUS" "$REQU
 RC=$?
 # With CBM_CACHE_DIR set the process logs to its own file rather than stderr,
 # so fold that in or the census series is invisible.
-cat "$WORK"/cache/logs/*.log >> "$RUN_LOG" 2>/dev/null || true
+cat "$CACHE_HOST"/logs/*.log >> "$RUN_LOG" 2>/dev/null || true
 cat "$WORK/server-stderr.log" >> "$RUN_LOG" 2>/dev/null || true
 RESPONSES=$(sed -n "s/.*served=\\([0-9]*\\).*/\\1/p" "$WORK/drive.out" | head -1); RESPONSES=${RESPONSES:-0}
 CENSUS=$(grep -c "mem.census" "$RUN_LOG" 2>/dev/null | head -1); CENSUS=${CENSUS:-0}
diff --git a/scripts/test.sh b/scripts/test.sh
index ecfc26998..24902b39a 100755
--- a/scripts/test.sh
+++ b/scripts/test.sh
@@ -227,6 +227,9 @@ bash "$ROOT/tests/test_makefile_ts_runtime_dependencies.sh"
 echo "=== Step 0g: security fuzz harness self-test ==="
 bash "$ROOT/tests/test_security_fuzz_harness.sh"
 
+echo "=== Step 0g2: memlab harness runtime isolation contract (#1696) ==="
+bash "$ROOT/tests/test_memlab_runtime_isolation_contract.sh"
+
 echo "=== Step 0h: smoke release-fixture contract ==="
 bash "$ROOT/tests/test_smoke_fixture_contract.sh"
 
diff --git a/tests/test_memlab_runtime_isolation_contract.sh b/tests/test_memlab_runtime_isolation_contract.sh
new file mode 100755
index 000000000..f414c41c5
--- /dev/null
+++ b/tests/test_memlab_runtime_isolation_contract.sh
@@ -0,0 +1,92 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Runtime-isolation contract for the memlab harness (#1696, follow-up to #1691).
+#
+# scripts/memlab.sh starts the product over stdio to attribute retained memory
+# and then removes its work directory. It used to give the run a private
+# CBM_CACHE_DIR only, but only CBM_RUNTIME_DIR moves the daemon rendezvous
+# (docs/CONFIGURATION.md), so the profiled process joined the operator's
+# account daemon — refused with a cache-root conflict when one was live, or
+# left as the account daemon with its cache deleted underneath it otherwise.
+#
+# memlab launches the product through its Python driver, and the driver hands
+# the product the environment it inherited (memlab-drive.py passes no env= to
+# Popen). Recording the environment at the driver boundary therefore observes
+# exactly what the product receives, on every host — including Windows, whose
+# native Python cannot exec a shell fixture — so a python3 shim stands in for
+# the driver and no product process is started at all.
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+WORKDIR="$(mktemp -d)"
+trap 'rm -rf "$WORKDIR"' EXIT
+
+fail() {
+    echo "FAIL: $*" >&2
+    exit 1
+}
+
+normalize_path() {
+    local path=${1%$'\r'}
+    if command -v cygpath >/dev/null 2>&1; then
+        cygpath -u "$path" 2>/dev/null && return 0
+    fi
+    printf '%s\n' "${path//\\//}"
+}
+
+if grep -q 'env=' "$ROOT/scripts/memlab-drive.py"; then
+    fail "memlab-drive.py no longer passes the harness environment through unchanged"
+fi
+
+SHIM_DIR="$WORKDIR/bin"
+mkdir -p "$SHIM_DIR"
+cat > "$SHIM_DIR/python3" <<'EOF'
+#!/usr/bin/env bash
+printf '%s\t%s\n' "${CBM_CACHE_DIR-}" "${CBM_RUNTIME_DIR-}" >> "$CBM_MEMLAB_ENV_PROBE"
+echo "served=0 failed=0"
+exit 1
+EOF
+DUMMY_BINARY="$WORKDIR/dummy-binary"
+printf '#!/usr/bin/env bash\nexit 0\n' > "$DUMMY_BINARY"
+chmod +x "$SHIM_DIR/python3" "$DUMMY_BINARY"
+
+CALLER_CACHE="$WORKDIR/caller-cache"
+CALLER_RUNTIME="$WORKDIR/caller-runtime"
+ENV_LOG="$WORKDIR/environment.log"
+mkdir -p "$CALLER_CACHE" "$CALLER_RUNTIME" "$WORKDIR/cwd"
+
+# memlab writes its profile and log into $PWD, hence the cwd change. It is
+# tracked without an executable bit, so run it through bash.
+(
+    cd "$WORKDIR/cwd"
+    PATH="$SHIM_DIR:$PATH" \
+    CBM_CACHE_DIR="$CALLER_CACHE" \
+    CBM_RUNTIME_DIR="$CALLER_RUNTIME" \
+    CBM_MEMLAB_ENV_PROBE="$ENV_LOG" \
+        bash "$ROOT/scripts/memlab.sh" "$DUMMY_BINARY" 1 probe > "$WORKDIR/memlab.out" 2>&1 || true
+)
+
+[[ -s "$ENV_LOG" ]] || fail "memlab did not reach its driver"
+
+CALLER_CACHE_NORMALIZED=$(normalize_path "$CALLER_CACHE")
+CALLER_RUNTIME_NORMALIZED=$(normalize_path "$CALLER_RUNTIME")
+private_root=""
+while IFS=$'\t' read -r child_cache_raw child_runtime_raw; do
+    child_cache=$(normalize_path "$child_cache_raw")
+    child_runtime=$(normalize_path "$child_runtime_raw")
+    if [[ -z "$child_runtime" || "$child_runtime" == "$CALLER_RUNTIME_NORMALIZED" ]]; then
+        fail "memlab exposed the caller CBM_RUNTIME_DIR to the product"
+    fi
+    if [[ -z "$child_cache" || "$child_cache" == "$CALLER_CACHE_NORMALIZED" ]]; then
+        fail "memlab exposed the caller CBM_CACHE_DIR to the product"
+    fi
+    if [[ "${child_runtime%/*}" != "${child_cache%/*}" ||
+          "${child_runtime##*/}" != "runtime" || "${child_cache##*/}" != "cache" ]]; then
+        fail "memlab runtime/cache were not isolated beneath one private root"
+    fi
+    private_root="${child_runtime%/*}"
+done < "$ENV_LOG"
+
+[[ ! -e "$private_root" ]] || fail "memlab left its private root behind: $private_root"
+
+echo "PASS: memlab harness isolates its daemon runtime and cache from the caller"