diff --git a/bin/fm-spawn.sh b/bin/fm-spawn.sh
index 705ac0f98fd..1e0d4dd6be4 100755
--- a/bin/fm-spawn.sh
+++ b/bin/fm-spawn.sh
@@ -5165,6 +5165,16 @@ if [ "$LAVISH_AXI_HOST_CONFIG_PRESENT" = 1 ]; then
   LAUNCH="export LAVISH_AXI_HOST=$(shell_quote "$LAVISH_AXI_HOST"); $LAUNCH"
 fi
 LAUNCH="export COMPACT_ADVISER_DISABLE=1; $LAUNCH"
+# When the live-harness gate has exported DISABLE_AUTOUPDATER into this spawn's
+# own environment, carry it into the launch command text so Claude Code's
+# auto-updater cannot rewrite the shared binary during a live run. Embedding the
+# assignment - like COMPACT_ADVISER_DISABLE above - rather than leaning on
+# ambient inheritance is what survives a pre-existing backend daemon that
+# constructs the pane command without the gate's environment. It is gated on the
+# value being set here so ordinary spawns are unchanged.
+if [ -n "${DISABLE_AUTOUPDATER:-}" ]; then
+  LAUNCH="export DISABLE_AUTOUPDATER=$(shell_quote "$DISABLE_AUTOUPDATER"); $LAUNCH"
+fi
 if [ -z "$SPAWN_TRACEPARENT" ] && [ "$RELAUNCH" -eq 1 ]; then
   LAUNCH="unset TRACEPARENT; $LAUNCH"
 fi
diff --git a/tests/fm-live-gate.test.sh b/tests/fm-live-gate.test.sh
index c2e3b4e1ca1..60be00cb4d0 100755
--- a/tests/fm-live-gate.test.sh
+++ b/tests/fm-live-gate.test.sh
@@ -15,8 +15,8 @@
 # cheap because a disabled gate exits before a guard touches a harness.
 set -u
 
-# shellcheck source=tests/lib.sh
-. "$(dirname "${BASH_SOURCE[0]}")/lib.sh"
+# shellcheck source=tests/fixtures.sh
+. "$(dirname "${BASH_SOURCE[0]}")/fixtures.sh"
 
 TMP_ROOT=$(fm_test_tmproot fm-live-gate)
 BIN="$TMP_ROOT/bin"
@@ -179,6 +179,111 @@ test_gate_lets_a_guard_drive_the_real_fleet_scripts_under_a_gate_marker() {
     "the shared gate must carry the test-suite bypass so a live guard can drive the real fleet scripts"
 }
 
+test_gate_exports_disable_autoupdater_for_a_proceeding_run() {
+  local path out rc
+  path="$TMP_ROOT/proceed-autoupdater.test.sh"
+  {
+    printf '#!/usr/bin/env bash\nset -u\n'
+    printf '. "%s/tests/lib.sh"\n' "$ROOT"
+    printf 'fm_live_gate default-on FM_FAKE_LIVE fmfakeharness\n'
+    # shellcheck disable=SC2016 # the written guard script expands this at its own runtime, not here
+    printf 'printf "autoupdater=%%s\\n" "${DISABLE_AUTOUPDATER:-unset}"\n'
+  } > "$path"
+  chmod +x "$path"
+  set +e
+  out=$(clean_env PATH="$BIN:/usr/bin:/bin" "$path" 2>&1)
+  rc=$?
+  set -e
+  expect_code 0 "$rc" "a proceeding guard must exit cleanly"
+  assert_contains "$out" "autoupdater=1" \
+    "a live run the gate lets proceed must export DISABLE_AUTOUPDATER=1 so Claude Code's auto-updater cannot run"
+}
+
+test_disable_autoupdater_reaches_the_claude_pane_on_the_fm_spawn_launch_path() {
+  # The gate exports DISABLE_AUTOUPDATER=1 into the ambient environment; this
+  # proves fm-spawn's claude launch construction preserves that ambient value
+  # all the way to the harness process, the inheritance a real pane relies on.
+  # It stages a real claude launch, then runs that exact command as a synthetic
+  # pane whose only claude is a stub recording the variable it inherited. (A
+  # backend daemon already running before the gate exported the variable is a
+  # separate case this cannot cover without launcher support.)
+  local case_dir home proj wt fakebin launchlog panebin panelog launch rc
+  case_dir="$TMP_ROOT/spawn-launch-path"
+  home="$case_dir/home"; proj="$case_dir/proj"; wt="$case_dir/wt"
+  launchlog="$case_dir/launch.log"
+  fakebin=$(make_spawn_fakebin "$case_dir/fake" gh gh-axi)
+  fm_test_spawn_home "$home" claude
+  fm_git_worktree "$proj" "$wt" "wt-autoupdater"
+  fm_test_spawn_brief "$home" AU-1
+  : > "$launchlog"
+  FM_FAKE_LAUNCH_LOG="$launchlog" \
+    fm_test_run_spawn "$home" "$wt" "$fakebin" AU-1 "$proj" --mode no-mistakes --yolo off \
+    >/dev/null 2>&1 || fail "the claude spawn must stage its launch command"
+  launch=$(cat "$launchlog")
+  [ -n "$launch" ] || fail "no claude launch command was captured"
+
+  # Synthetic pane: only claude is a recording stub, and DISABLE_AUTOUPDATER=1
+  # stands in for the value the live gate put in the ambient environment.
+  panebin="$case_dir/panebin"; mkdir -p "$panebin"
+  panelog="$case_dir/pane-autoupdater.log"
+  cat > "$panebin/claude" <<SH
+#!/usr/bin/env bash
+printf 'autoupdater=%s\n' "\${DISABLE_AUTOUPDATER:-unset}" > "$panelog"
+exit 0
+SH
+  chmod +x "$panebin/claude"
+  set +e
+  DISABLE_AUTOUPDATER=1 PATH="$panebin:/usr/bin:/bin" bash -c "$launch" >/dev/null 2>&1
+  rc=$?
+  set -e
+  expect_code 0 "$rc" "the staged claude launch must run cleanly in the synthetic pane"
+  assert_contains "$(cat "$panelog")" "autoupdater=1" \
+    "fm-spawn's claude launch must pass the ambient DISABLE_AUTOUPDATER through to the harness pane, so the auto-updater cannot run"
+}
+
+test_disable_autoupdater_survives_a_daemon_pane_that_never_inherited_it() {
+  # The finding: a live test exports DISABLE_AUTOUPDATER, but the pane is created
+  # by an already-running backend daemon that does not inherit the test process's
+  # environment, so ambient inheritance alone drops it and Claude's updater runs.
+  # This stages a real claude launch with DISABLE_AUTOUPDATER set in the spawn's
+  # own environment, then runs that exact command in a synthetic pane whose
+  # environment lacks the variable (standing in for the daemon). Claude must still
+  # see it, which only holds if fm-spawn embedded the assignment into the launch
+  # command text rather than relying on the pane inheriting it.
+  local case_dir home proj wt fakebin launchlog panebin panelog launch rc
+  case_dir="$TMP_ROOT/spawn-daemon-path"
+  home="$case_dir/home"; proj="$case_dir/proj"; wt="$case_dir/wt"
+  launchlog="$case_dir/launch.log"
+  fakebin=$(make_spawn_fakebin "$case_dir/fake" gh gh-axi)
+  fm_test_spawn_home "$home" claude
+  fm_git_worktree "$proj" "$wt" "wt-daemon-autoupdater"
+  fm_test_spawn_brief "$home" AU-2
+  : > "$launchlog"
+  DISABLE_AUTOUPDATER=1 FM_FAKE_LAUNCH_LOG="$launchlog" \
+    fm_test_run_spawn "$home" "$wt" "$fakebin" AU-2 "$proj" --mode no-mistakes --yolo off \
+    >/dev/null 2>&1 || fail "the claude spawn must stage its launch command"
+  launch=$(cat "$launchlog")
+  [ -n "$launch" ] || fail "no claude launch command was captured"
+
+  panebin="$case_dir/panebin"; mkdir -p "$panebin"
+  panelog="$case_dir/pane-autoupdater.log"
+  cat > "$panebin/claude" <<SH
+#!/usr/bin/env bash
+printf 'autoupdater=%s\n' "\${DISABLE_AUTOUPDATER:-unset}" > "$panelog"
+exit 0
+SH
+  chmod +x "$panebin/claude"
+  # The synthetic daemon-launched pane runs the staged command with the variable
+  # absent from its own environment; env -u strips any value the suite inherited.
+  set +e
+  env -u DISABLE_AUTOUPDATER PATH="$panebin:/usr/bin:/bin" bash -c "$launch" >/dev/null 2>&1
+  rc=$?
+  set -e
+  expect_code 0 "$rc" "the staged claude launch must run cleanly in the synthetic pane"
+  assert_contains "$(cat "$panelog")" "autoupdater=1" \
+    "fm-spawn must embed DISABLE_AUTOUPDATER in the launch command so a daemon-built pane that never inherited it still runs Claude with the updater off"
+}
+
 test_every_live_guard_is_wired_to_the_shared_gate() {
   local script out listing checked=0
   listing=$("$ROOT/bin/fm-test-run.sh" --family live-harness-optin --list) \
@@ -217,4 +322,10 @@ test_any_of_several_entry_points_turns_a_guard_on
 pass "any entry point of a multi-mode guard turns it on"
 test_gate_lets_a_guard_drive_the_real_fleet_scripts_under_a_gate_marker
 pass "the shared gate carries the gate-refusal bypass into every live guard"
+test_gate_exports_disable_autoupdater_for_a_proceeding_run
+pass "a proceeding live run exports DISABLE_AUTOUPDATER=1"
+test_disable_autoupdater_reaches_the_claude_pane_on_the_fm_spawn_launch_path
+pass "DISABLE_AUTOUPDATER rides fm-spawn's claude launch through to the harness pane"
+test_disable_autoupdater_survives_a_daemon_pane_that_never_inherited_it
+pass "DISABLE_AUTOUPDATER is embedded in the launch so a daemon-built pane keeps it"
 test_every_live_guard_is_wired_to_the_shared_gate
diff --git a/tests/lib.sh b/tests/lib.sh
index 68acd735148..0267f841277 100644
--- a/tests/lib.sh
+++ b/tests/lib.sh
@@ -320,6 +320,10 @@ fi
 # lets a live guard drive the real fm-spawn/fm-send/fm-teardown from inside a
 # no-mistakes gate worktree instead of being refused by
 # bin/fm-gate-refuse-lib.sh.
+#
+# Every path that lets a live run proceed also exports DISABLE_AUTOUPDATER=1,
+# so a live harness invocation never lets Claude Code's auto-updater rewrite
+# the installed binary out from under the host.
 
 fm_live_gate() {
   local policy=$1 vars=$2
@@ -383,6 +387,7 @@ fm_live_gate() {
     exit 0
   done
 
+  export DISABLE_AUTOUPDATER=1
   return 0
 }