diff --git a/scripts/smoke-test.sh b/scripts/smoke-test.sh
index ca27edcc6..31a69b9f0 100755
--- a/scripts/smoke-test.sh
+++ b/scripts/smoke-test.sh
@@ -22,6 +22,8 @@ they stage the release fixture, start the fixture server, and sandbox
 HOME/TEMP/agent-config destinations. Called bare, the download/checksum/
 install-script phases (12-13) SKIP for lack of a fixture server, and the run
 mutates the REAL profile — the venue-parity contract forbids that in any venue.
+The daemon runtime and cache are private to the run either way: every product
+process is started under a CBM_RUNTIME_DIR/CBM_CACHE_DIR this harness owns.
 
 Arguments:
   <binary-path>         product binary to smoke
@@ -41,6 +43,15 @@ if [ -n "$SMOKE_MODE" ] && [ "$SMOKE_MODE" != "--agent-config-only" ]; then
 fi
 REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]:-$0}")/.." && pwd)"
 
+# Every product process below — the phases, the install/update E2E and the
+# daemon retirements — must reach a daemon rendezvous this run owns. Only
+# CBM_RUNTIME_DIR moves that rendezvous; the wrappers' HOME/TMPDIR/CBM_CACHE_DIR
+# sandbox does not, so without this the retirements land on the operator's live
+# account daemon (#1691, #1696).
+# shellcheck source=test-runtime.sh
+source "$REPO_ROOT/scripts/test-runtime.sh"
+cbm_test_runtime_init
+
 smoke_mktemp_file() {
   if [ -n "${SMOKE_TEMP_ROOT:-}" ]; then
     mktemp "$SMOKE_TEMP_ROOT/cbm-smoke.XXXXXX"
@@ -100,8 +111,8 @@ copy_smoke_binary() {
   cp "$BINARY" "$destination"
 }
 
-# Retire the shared account daemon (if one is running) and wait until it
-# reports not-running. Install/uninstall flows leave an ephemeral daemon
+# Retire this run's private account daemon (if one is running) and wait until
+# it reports not-running. Install/uninstall flows leave an ephemeral daemon
 # draining asynchronously whose mapped generation backing and open logs
 # block rm on Windows (POSIX rm doesn't care) — so every cleanup of a
 # fixture HOME that received an install, and the final cache removal, must
@@ -149,7 +160,7 @@ CODEX_LIFECYCLE_HOME=""
 if command -v cygpath &>/dev/null; then
     TMPDIR=$(cygpath -m "$TMPDIR")
 fi
-trap 'smoke_rmtree "$TMPDIR" "${DRYRUN_HOME:-}" "${CODEX_LIFECYCLE_HOME:-}"' EXIT
+trap 'smoke_rmtree "$TMPDIR" "${DRYRUN_HOME:-}" "${CODEX_LIFECYCLE_HOME:-}"; cbm_test_runtime_cleanup "$BINARY"' EXIT
 
 CLI_STDERR=$(smoke_mktemp_file)
 # 10 of the cli call sites assign directly (VAR=$(cli ...)). Under
diff --git a/scripts/test.sh b/scripts/test.sh
index ecfc26998..a20a8526b 100755
--- a/scripts/test.sh
+++ b/scripts/test.sh
@@ -266,6 +266,9 @@ bash "$ROOT/tests/test_release_gate_chain_contract.sh"
 echo "=== Step 0t: test runtime isolation contract (#1691) ==="
 bash "$ROOT/tests/test_runtime_isolation_contract.sh"
 
+echo "=== Step 0t2: smoke harness runtime isolation contract (#1696) ==="
+bash "$ROOT/tests/test_smoke_runtime_isolation_contract.sh"
+
 echo "=== Step 0u: shell line-ending contract ==="
 bash "$ROOT/tests/test_shell_line_endings.sh"
 
diff --git a/tests/test_smoke_runtime_isolation_contract.sh b/tests/test_smoke_runtime_isolation_contract.sh
new file mode 100755
index 000000000..e91e1d38d
--- /dev/null
+++ b/tests/test_smoke_runtime_isolation_contract.sh
@@ -0,0 +1,82 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Runtime-isolation contract for the smoke harness (#1696, follow-up to #1691).
+#
+# scripts/smoke-test.sh is the process that actually starts the product during
+# a smoke run, and it retires "the account daemon" seven times through
+# `daemon stop`. Its wrappers sandbox HOME/XDG/TMPDIR and CBM_CACHE_DIR, but
+# only CBM_RUNTIME_DIR moves the daemon rendezvous (docs/CONFIGURATION.md), so
+# without a private runtime every one of those stops lands on the operator's
+# live daemon. Drive the harness with an environment-probe fixture and require
+# that no product process ever receives the caller's runtime or cache.
+
+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' "${path//\\//}"
+}
+
+ENV_PROBE="$WORKDIR/environment-probe"
+cat > "$ENV_PROBE" <<'EOF'
+#!/usr/bin/env bash
+printf '%s\t%s\n' "${CBM_CACHE_DIR-}" "${CBM_RUNTIME_DIR-}" >> "$CBM_SMOKE_ENV_PROBE"
+case "${1-}" in
+    --version) echo "v0.0.0-probe"; exit 0 ;;
+    daemon) [[ "${2-}" == status ]] && exit 1; exit 0 ;;
+esac
+echo '{}'
+exit 0
+EOF
+chmod +x "$ENV_PROBE"
+
+CALLER_CACHE="$WORKDIR/caller-cache"
+CALLER_RUNTIME="$WORKDIR/caller-runtime"
+ENV_LOG="$WORKDIR/environment.log"
+mkdir -p "$CALLER_CACHE" "$CALLER_RUNTIME"
+
+# The fixture answers nothing beyond --version, so the smoke fails early; only
+# the environment it handed to the product is under test here.
+CBM_CACHE_DIR="$CALLER_CACHE" \
+CBM_RUNTIME_DIR="$CALLER_RUNTIME" \
+CBM_SMOKE_ENV_PROBE="$ENV_LOG" \
+    "$ROOT/scripts/smoke-test.sh" "$ENV_PROBE" > "$WORKDIR/smoke.out" 2>&1 || true
+
+[[ -s "$ENV_LOG" ]] || fail "smoke-test did not execute the environment-probe fixture"
+
+CALLER_CACHE_NORMALIZED=$(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" == "$CALLER_RUNTIME_NORMALIZED" ]]; then
+        fail "smoke-test exposed the caller CBM_RUNTIME_DIR to a product process"
+    fi
+    if [[ -z "$child_cache" || "$child_cache" == "$CALLER_CACHE_NORMALIZED" ]]; then
+        fail "smoke-test exposed the caller CBM_CACHE_DIR to a product process"
+    fi
+    if [[ "${child_runtime%/*}" != "${child_cache%/*}" ||
+          "${child_runtime##*/}" != "runtime" || "${child_cache##*/}" != "cache" ]]; then
+        fail "smoke runtime/cache were not isolated beneath one private root"
+    fi
+    if [[ -n "$private_root" && "$private_root" != "${child_runtime%/*}" ]]; then
+        fail "smoke-test switched private roots mid-run"
+    fi
+    private_root="${child_runtime%/*}"
+done < "$ENV_LOG"
+
+[[ ! -e "$private_root" ]] || fail "smoke-test left its private root behind: $private_root"
+
+echo "PASS: smoke harness isolates its daemon runtime and cache from the caller"