diff --git a/.agents/skills/lib.sh b/.agents/skills/lib.sh
index e98976e..4951ec4 100644
--- a/.agents/skills/lib.sh
+++ b/.agents/skills/lib.sh
@@ -3247,3 +3247,55 @@ mam_control_plane_prefix() {
   [ -n "${1:-}" ] || return 0
   PYTHONPATH="$SKILL_DIR:${PYTHONPATH:-}" "$(_delegate_py_bin)" -m lib_py.control_plane prefix "$1"
 }
+
+# ---------------------------------------------------------------------------
+# Issue #72 (decision 8): the update admission gate.
+#
+# The update skill publishes .mam/update-in-progress (pid + lstart, loop_lock.sh
+# rules) while it holds .mam/update-admission.lock exclusively, then runs its
+# pre-check. Every entry that starts something the pre-check must see - create,
+# resume, run_loop.sh, delegate submit - or writes the registry - stop,
+# orc-onboard - takes that lock shared on fd 7, refuses while the marker is
+# `live` or `unknown`, and keeps fd 7 until its record is written
+# (mam_update_admit_release). An update therefore either waits for that record
+# and sees it, or publishes first and the entry refuses. The lock is a BSD
+# flock taken by lib_py.pane_lock on the open fd: it ends when every process
+# holding fd 7 has ended, however that ends, so nothing long-lived may inherit
+# fd 7 (`7>&-` on such a child, as pane_lock's fd 9). The wait is bounded by
+# MAM_UPDATE_ADMISSION_WAIT_S (default 120).
+#
+# mam_update_admit [workspace]: rc 0 admitted (fd 7 held), 1 refused (stderr).
+mam_update_admit() {
+  local root="${1:-$WORKSPACE_ROOT}"
+  local lock="$root/.mam/update-admission.lock" marker="$root/.mam/update-in-progress" st
+  if ! { mkdir -p "$root/.mam" && exec 7>>"$lock"; }; then
+    echo "ERROR: cannot open the update admission lock $lock" >&2
+    return 1
+  fi
+  case "$(PYTHONPATH="$SKILL_DIR:${PYTHONPATH:-}" python3 -m lib_py.pane_lock acquire --fd 7 --shared --wait-env MAM_UPDATE_ADMISSION_WAIT_S)" in
+    locked) ;;
+    timeout)
+      exec 7>&-
+      echo "ERROR: an update of $root holds its admission lock (MAM_UPDATE_ADMISSION_WAIT_S passed); try again when it has finished" >&2
+      return 1 ;;
+    *)
+      exec 7>&-
+      echo "ERROR: the update admission lock $lock could not be decided (lib_py.pane_lock)" >&2
+      return 1 ;;
+  esac
+  [ -f "$marker" ] || return 0
+  declare -F _mam_lock_holder_state >/dev/null || source "$SKILL_DIR/multi-agent-mux-loop/scripts/loop_lock.sh"
+  st="$(_mam_lock_holder_state "$marker")"
+  case "$st" in
+    stale)
+      echo "NOTE: stale update marker $marker (its update is gone); ignored" >&2
+      return 0 ;;
+  esac
+  exec 7>&-
+  echo "ERROR: an update of $root is in progress (marker $marker, holder $st); try again when it has finished" >&2
+  return 1
+}
+
+mam_update_admit_release() {
+  exec 7>&-
+}
diff --git a/.agents/skills/lib_py/pane_lock.py b/.agents/skills/lib_py/pane_lock.py
index bfc2cc9..62a4824 100644
--- a/.agents/skills/lib_py/pane_lock.py
+++ b/.agents/skills/lib_py/pane_lock.py
@@ -18,6 +18,9 @@ one lock per server from the choice to the claim.
   pane_lock.py acquire --fd N     flock(LOCK_EX) on the inherited fd N; prints
                                   `locked`, or `timeout` once MAM_PANE_CLAIM_WAIT_S
                                   (default 120, minimum 1) has passed
+     [--shared] [--wait-env VAR]  Issue #72: LOCK_SH instead, and the wait read
+                                  from VAR (same default and minimum) - the
+                                  update admission lock (lib.sh mam_update_admit)
 
 The lock is a BSD flock, which belongs to the open file on fd 9: it outlives
 this helper and ends when the last process holding that fd ends, however it
@@ -63,11 +66,11 @@ def lock_path(session: Optional[str], env: Mapping[str, str]) -> str:
     return os.path.join(home, ".local", "state", "mam", "pane-lock", session + ".lock")
 
 
-def wait_s(env: Mapping[str, str]) -> int:
-    return ints.positive_int(env.get("MAM_PANE_CLAIM_WAIT_S"), 1) or DEFAULT_WAIT_S
+def wait_s(env: Mapping[str, str], var: str = "MAM_PANE_CLAIM_WAIT_S") -> int:
+    return ints.positive_int(env.get(var), 1) or DEFAULT_WAIT_S
 
 
-def acquire(fd: int, wait: float) -> str:
+def acquire(fd: int, wait: float, shared: bool = False) -> str:
     """`locked`, or `timeout` when the lock was not free within `wait` s. The
     deadline is on the monotonic clock; a step of the wall clock does not
     stretch or cut it. Only contention is waited out: any other error (a bad
@@ -75,7 +78,7 @@ def acquire(fd: int, wait: float) -> str:
     deadline = time.monotonic() + wait
     while True:
         try:
-            fcntl.flock(fd, fcntl.LOCK_EX | fcntl.LOCK_NB)
+            fcntl.flock(fd, (fcntl.LOCK_SH if shared else fcntl.LOCK_EX) | fcntl.LOCK_NB)
             return "locked"
         except BlockingIOError:
             if time.monotonic() >= deadline:
@@ -90,11 +93,13 @@ def main(argv: Optional[List[str]] = None) -> int:
     s.add_argument("--session", default="")
     s = sub.add_parser("acquire")
     s.add_argument("--fd", type=int, required=True)
+    s.add_argument("--shared", action="store_true")
+    s.add_argument("--wait-env", default="MAM_PANE_CLAIM_WAIT_S")
     a = p.parse_args(argv)
     if a.cmd == "path":
         print(lock_path(a.session, os.environ))
     else:
-        print(acquire(a.fd, wait_s(os.environ)))
+        print(acquire(a.fd, wait_s(os.environ, a.wait_env), a.shared))
     return 0
 
 
diff --git a/.agents/skills/multi-agent-mux-create/SKILL.md b/.agents/skills/multi-agent-mux-create/SKILL.md
index fecf5cc..f859216 100644
--- a/.agents/skills/multi-agent-mux-create/SKILL.md
+++ b/.agents/skills/multi-agent-mux-create/SKILL.md
@@ -181,7 +181,7 @@ bash .agents/skills/multi-agent-mux-create/scripts/create_session.sh --workspace
 | Exit | Meaning |
 |---|---|
 | 0 | Created; or `--dry-run` |
-| 1 | Pre-flight failure, nothing was spawned: workspace not a directory, herdr not installed, agent CLI not in `PATH`, agent not authenticated, or a same-cwd session of the same Class A agent did not record its conversation ID within `MAM_ID_ORDER_WAIT_S` (default 600). After spawn: the task brief could not be written or the instruction injection failed (the session is rolled back) |
+| 1 | Pre-flight failure, nothing was spawned: workspace not a directory, herdr not installed, agent CLI not in `PATH`, agent not authenticated, or a same-cwd session of the same Class A agent did not record its conversation ID within `MAM_ID_ORDER_WAIT_S` (default 600), or an update of the workspace is in progress (`multi-agent-mux-update`). After spawn: the task brief could not be written or the instruction injection failed (the session is rolled back) |
 | 2 | Bad arguments; control-plane binding refused for a worktree; whitespace in a control-plane path that must be forwarded |
 | 3 | A herdr session of that name already exists (`multi-agent-mux-resume`, or `multi-agent-mux-stop` first) |
 | 4 | The YAML already has a `running` row of that name. The YAML write raises it after spawn, and the herdr session is rolled back. A row the monitor registered for this very pane while it was being created is replaced instead |
diff --git a/.agents/skills/multi-agent-mux-create/scripts/create_session.sh b/.agents/skills/multi-agent-mux-create/scripts/create_session.sh
index 18cfa3b..5899963 100755
--- a/.agents/skills/multi-agent-mux-create/scripts/create_session.sh
+++ b/.agents/skills/multi-agent-mux-create/scripts/create_session.sh
@@ -260,7 +260,7 @@ spawn() {
         SESSION_UUID=""
         CMD_FULL="$("$(_delegate_py_bin)" -m lib_py.agents spawn-spec "$AGENT" "$RESOLVED_BIN" "" "true" 2>/dev/null || echo "${RESOLVED_BIN} --dangerously-skip-permissions")"
         [ -z "$CP_PREFIX" ] || echo "WARNING: the ~/.local/bin wrapper path bypasses herdr; the control-plane binding is recorded but not forwarded" >&2
-        nohup "$WRAPPER" >/dev/null 2>&1 &
+        nohup "$WRAPPER" >/dev/null 2>&1 7>&- &
         disown
       else
         HERDR_SESSION_NAME="$HERDR_SESSION_NAME" _herdr new-session -d -s "$SESSION_NAME" -x 140 -y 40 -c "$WORKSPACE" "$CP_PREFIX$CMD_FULL"
@@ -285,6 +285,9 @@ if [ "$DRY_RUN" = "1" ]; then
 fi
 # Issue #63: a same-cwd session of this post-discovered agent records its id first (lib.sh mam_id_order_wait).
 mam_id_order_wait "$WORKSPACE" "$AGENT" "$SESSION_NAME" || exit 1
+# Issue #72: no new session while an update runs; held until the row is written
+# (lib.sh mam_update_admit).
+mam_update_admit || exit 1
 HERDR_EPOCH=$(date +%s)
 NOW_ISO=$(date -u +'%Y-%m-%dT%H:%M:%SZ')
 spawn
@@ -638,6 +641,7 @@ snap['cwd'] = os.environ['PANE_CWD']
 print(f"appended: {name}", flush=True)
 PYEOF
 ROW_APPENDED=1
+mam_update_admit_release
 
 echo
 echo "=== created ==="
diff --git a/.agents/skills/multi-agent-mux-delegate-job/multi-agent-mux-delegate-job b/.agents/skills/multi-agent-mux-delegate-job/multi-agent-mux-delegate-job
index e77146a..6223140 100755
--- a/.agents/skills/multi-agent-mux-delegate-job/multi-agent-mux-delegate-job
+++ b/.agents/skills/multi-agent-mux-delegate-job/multi-agent-mux-delegate-job
@@ -301,6 +301,10 @@ cmd_submit() {
   local reg_dry=()
   if [[ "$DRY_RUN" == "1" ]]; then reg_dry=(--dry-run); fi
 
+  # Issue #72: no job is registered while an update of the target's workspace
+  # runs; held until the record is written (lib.sh mam_update_admit). A
+  # dry-run record is never dispatched and is not counted by the update.
+  if [[ "$DRY_RUN" != "1" ]]; then mam_update_admit "$WORKSPACE_ROOT" || exit 1; fi
   # 1) register job (prints the new job id)
   JOB_ID="$("$PY" "$SCRIPT_DIR/scripts/registry.py" --registry-dir "$REGISTRY_DIR" register \
       "${reg_prompt[@]}" --agent "$AGENT" --agent-session "$AGENT_SESSION" --role "$DELEGATE_ROLE" \
@@ -310,6 +314,7 @@ cmd_submit() {
       --job-type "$TYPE" --reviewer "$REVIEWER" --reviewer-session "$REVIEWER_SESSION" \
       --max-iterations "$MAX_ITERATIONS" ${reg_parent[@]+"${reg_parent[@]}"} ${reg_dry[@]+"${reg_dry[@]}"})"
   echo "registered job: $JOB_ID"
+  [[ "$DRY_RUN" == "1" ]] || mam_update_admit_release
 
   # 1-1) Provision job directory and write direct brief.md (MAM Job Restructuring)
   local job_dir="$REGISTRY_DIR/$JOB_ID"
diff --git a/.agents/skills/multi-agent-mux-loop/scripts/run_loop.sh b/.agents/skills/multi-agent-mux-loop/scripts/run_loop.sh
index 7c481b3..d8369ce 100644
--- a/.agents/skills/multi-agent-mux-loop/scripts/run_loop.sh
+++ b/.agents/skills/multi-agent-mux-loop/scripts/run_loop.sh
@@ -244,6 +244,9 @@ log_error() {
   echo -e "\033[1;31m[✗]\033[0m $1"
 }
 
+# Issue #72: no loop starts while an update runs; held until the loop marker
+# is published (lib.sh mam_update_admit), released before the loop body runs.
+mam_update_admit "$MAM_REAL_ROOT" || exit 1
 _mam_acquire_rc=0
 mam_acquire_loop_lock "$MAM_LOOP_MARKER" || _mam_acquire_rc=$?
 case "$_mam_acquire_rc" in
@@ -262,6 +265,7 @@ case "$_mam_acquire_rc" in
     ;;
 esac
 trap _mam_release_guard EXIT INT TERM HUP
+mam_update_admit_release
 rm -f "$MAM_REAL_ROOT/.agents/skills/multi-agent-mux-delegate-job/multi-agent-mux-delegate-job".*.tmp 2>/dev/null || true
 
 # --all-reviewer silently takes precedence over an explicit --reviewer list;
diff --git a/.agents/skills/multi-agent-mux-orc-onboard/scripts/orc_onboard.sh b/.agents/skills/multi-agent-mux-orc-onboard/scripts/orc_onboard.sh
index 9235ea1..ccd447d 100755
--- a/.agents/skills/multi-agent-mux-orc-onboard/scripts/orc_onboard.sh
+++ b/.agents/skills/multi-agent-mux-orc-onboard/scripts/orc_onboard.sh
@@ -257,6 +257,11 @@ if [ "$DRY_RUN" = true ]; then
   exit 0
 fi
 
+# Issue #72 (U2): no registry write while an update runs - a rollback would
+# restore the registry without it. Held until the write is done (lib.sh
+# mam_update_admit).
+mam_update_admit || exit 1
+
 # Issue #13: the orchestrator has no herdr_sessions row, so stop_session.sh
 # --purge-conversation can never reach its conversation. Purge FIRST, through
 # the adapter (`lib_py.agents purge`): a refusal because the conversation is
@@ -304,6 +309,7 @@ elif op == "remove":
     uuids = [u for u in uuids if u != target]
 d["orchestrator_uuids"] = uuids
 PYEOF
+mam_update_admit_release
 
 case "$MODE" in
   add) verb="added" ;;
diff --git a/.agents/skills/multi-agent-mux-resume/SKILL.md b/.agents/skills/multi-agent-mux-resume/SKILL.md
index 634345d..63f7974 100644
--- a/.agents/skills/multi-agent-mux-resume/SKILL.md
+++ b/.agents/skills/multi-agent-mux-resume/SKILL.md
@@ -75,11 +75,11 @@ bash .agents/skills/multi-agent-mux-resume/scripts/resume_session.sh --workspace
 | Exit | Meaning |
 |---|---|
 | 0 | Resumed; or already running, so only the row was updated (no attach); or `--dry-run` |
-| 1 | From the checks before spawn, nothing was changed: no saved session for claude/grok (use `multi-agent-mux-create` first), the row's id is also claimed by another row, a same-cwd Class A session did not record its id within `MAM_ID_ORDER_WAIT_S` (default 600), or agent binary missing/not executable. **The YAML update after spawn (`update_yaml_resumed.sh`) also exits 1** (YAML not found) |
+| 1 | From the checks before spawn, nothing was changed: no saved session for claude/grok (use `multi-agent-mux-create` first), the row's id is also claimed by another row, a same-cwd Class A session did not record its id within `MAM_ID_ORDER_WAIT_S` (default 600), agent binary missing/not executable, or an update of the workspace is in progress (`multi-agent-mux-update`). **The YAML update after spawn (`update_yaml_resumed.sh`) also exits 1** (YAML not found) |
 | 2 | From the checks before spawn, nothing was changed: bad arguments, `--workspace` differs from the original cwd or it no longer exists, control-plane binding refused, whitespace in the control-plane path. **The YAML update after spawn also exits 2** (bad arguments) |
 | other | A later step failed (herdr spawn, start-up dialogs, YAML update) and its code is passed through. resume does not roll back |
 
-A non-zero exit after the spawn can leave the herdr session running while the row is still `stopped`. Before retrying or creating a new session, check with `multi-agent-mux-status`. The `ERROR:` line on stderr tells the two apart: the pre-spawn checks' messages are in `resume_session.sh` (lines 50-243).
+A non-zero exit after the spawn can leave the herdr session running while the row is still `stopped`. Before retrying or creating a new session, check with `multi-agent-mux-status`. The `ERROR:` line on stderr tells the two apart: the pre-spawn checks' messages are in `resume_session.sh` (lines 50-259).
 
 ## Original cwd only (Issue #21 phase 2)
 
diff --git a/.agents/skills/multi-agent-mux-resume/scripts/resume_session.sh b/.agents/skills/multi-agent-mux-resume/scripts/resume_session.sh
index cd76362..0668256 100755
--- a/.agents/skills/multi-agent-mux-resume/scripts/resume_session.sh
+++ b/.agents/skills/multi-agent-mux-resume/scripts/resume_session.sh
@@ -172,12 +172,15 @@ if [ "$HERDR_ALIVE" = "1" ]; then
     echo "[dry-run] herdr '$SESSION_NAME' already running — nothing to validate"
     exit 0
   fi
+  # Issue #72: no registry write while an update runs (lib.sh mam_update_admit).
+  mam_update_admit || exit 1
   echo "herdr '$SESSION_NAME' already running."
   # Just update YAML to make sure it's set to running
   CONTROL_PLANE_JSON="$CP_JSON" bash "$(dirname "${BASH_SOURCE[0]}")/update_yaml_resumed.sh" \
     --session "$SESSION_NAME" --uuid "$UUID" --agent "$AGENT" --workspace "$WORKSPACE" \
     --herdr-session "$HERDR_SESSION_NAME" --already-running \
     ${HERDR_WORKSPACE_OPT:+--herdr-workspace "$HERDR_WORKSPACE_OPT"}
+  mam_update_admit_release
   _ensure_monitor_daemon
   exit 0
 fi
@@ -251,6 +254,10 @@ if [ "${DRY_RUN:-0}" = "1" ]; then
   exit 0
 fi
 
+# Issue #72: no new session while an update runs; held until the row is written
+# (lib.sh mam_update_admit).
+mam_update_admit || exit 1
+
 # Issue #63: wait if another same-cwd session of this post-discovered agent
 # is still being created or has not recorded its id yet (lib.sh mam_id_order_wait).
 mam_id_order_wait "$WORKSPACE" "$AGENT" "$SESSION_NAME" "$UUID" || exit 1
@@ -287,6 +294,7 @@ CONTROL_PLANE_JSON="$CP_JSON" bash "$(dirname "${BASH_SOURCE[0]}")/update_yaml_r
   --herdr-session "$HERDR_SESSION_NAME" --cmd-full "$CMD_FULL" \
   ${DROP_FLAG:+$DROP_FLAG} \
   ${HERDR_WORKSPACE_OPT:+--herdr-workspace "$HERDR_WORKSPACE_OPT"}
+mam_update_admit_release
 _ensure_monitor_daemon
 
 echo "Successfully resumed $SESSION_NAME ($AGENT)"
diff --git a/.agents/skills/multi-agent-mux-stop/SKILL.md b/.agents/skills/multi-agent-mux-stop/SKILL.md
index cd89e89..eb8db61 100644
--- a/.agents/skills/multi-agent-mux-stop/SKILL.md
+++ b/.agents/skills/multi-agent-mux-stop/SKILL.md
@@ -58,7 +58,7 @@ bash .agents/skills/multi-agent-mux-stop/scripts/stop_session.sh \
 | Exit | Meaning |
 |---|---|
 | 0 | Stopped; or already `stopped` (no-op, except with `--purge-conversation`, which still purges) |
-| 1 | From the checks before the stop, nothing was changed: YAML not found, its row could not be read, or the session is neither registered nor on the herdr server. **The registry update after the stop also exits 1** (the row disappeared from the YAML during the run) |
+| 1 | From the checks before the stop, nothing was changed: YAML not found, its row could not be read, the session is neither registered nor on the herdr server, or an update of the workspace is in progress (`multi-agent-mux-update`). **The registry update after the stop also exits 1** (the row disappeared from the YAML during the run) |
 | 2 | Bad arguments (including the retired `--mode` / `--capture-id` / `--graceful`), or `--agent` omitted and not resolvable |
 | 3 | `--purge-conversation` without `--yes` |
 | 4 | Purge aborted: the herdr session, or an agent process still holding the conversation, survived the kill chain. Records preserved: the registry row and the conversation files are untouched, though the session itself may already be down |
diff --git a/.agents/skills/multi-agent-mux-stop/scripts/stop_session.sh b/.agents/skills/multi-agent-mux-stop/scripts/stop_session.sh
index 50067a5..edeaed6 100755
--- a/.agents/skills/multi-agent-mux-stop/scripts/stop_session.sh
+++ b/.agents/skills/multi-agent-mux-stop/scripts/stop_session.sh
@@ -100,6 +100,11 @@ fi
 [ -n "$SESSION_NAME" ] || { echo "ERROR: --session required" >&2; usage; exit 2; }
 [ -f "$AGENT_SESSIONS_YAML" ] || { echo "ERROR: $AGENT_SESSIONS_YAML not found" >&2; exit 1; }
 
+# Issue #72 (U2): no registry write while an update runs - a rollback would
+# restore the registry without it. Held until the row is written (lib.sh
+# mam_update_admit).
+mam_update_admit || exit 1
+
 # Implement the purging-<session> file lock mechanism
 if [ "$PURGE" = "1" ]; then
   touch "$WORKSPACE_ROOT/.mam/purging-$SESSION_NAME"
@@ -453,6 +458,7 @@ if purge:
 else:
     print(f"updated: {name} status={target['status']}", flush=True)
 PYEOF
+mam_update_admit_release
 
 delegate_publish_event "$DELEGATE_JOB_ID" cancelled "session stopped by operator before job completion" "$DELEGATE_JOB_TOKEN" "$DELEGATE_JOB_ATTEMPT"
 
diff --git a/tests/data/skill_code_blocks.json b/tests/data/skill_code_blocks.json
index c04ea77..2e321dd 100644
--- a/tests/data/skill_code_blocks.json
+++ b/tests/data/skill_code_blocks.json
@@ -11,7 +11,7 @@
    "kind": "example",
    "evidence": [
     "multi-agent-mux-create/scripts/create_session.sh:99",
-    "multi-agent-mux-create/scripts/create_session.sh:356"
+    "multi-agent-mux-create/scripts/create_session.sh:359"
    ],
    "mismatch": null,
    "note": "HERDR_SESSION_NAME environment example"
@@ -67,7 +67,7 @@
    "class": "c",
    "kind": "example",
    "evidence": [
-    "multi-agent-mux-create/scripts/create_session.sh:514-541"
+    "multi-agent-mux-create/scripts/create_session.sh:517-544"
    ],
    "mismatch": null,
    "note": "Output format: the row create_session.sh writes"
@@ -122,7 +122,7 @@
    "class": "c",
    "kind": "invocation",
    "evidence": [
-    "multi-agent-mux-delegate-job/multi-agent-mux-delegate-job:1046-1057"
+    "multi-agent-mux-delegate-job/multi-agent-mux-delegate-job:1051-1062"
    ],
    "mismatch": null,
    "note": "CLI reference; subcommands submit/status/list/verify/wait/logs match main()"
@@ -433,7 +433,7 @@
    "class": "c",
    "kind": "diagram",
    "evidence": [
-    "multi-agent-mux-stop/scripts/stop_session.sh:245-259"
+    "multi-agent-mux-stop/scripts/stop_session.sh:250-264"
    ],
    "mismatch": null,
    "note": "state machine"
@@ -477,10 +477,10 @@
     "multi-agent-mux-resume/scripts/resume_session.sh:111-130",
     "multi-agent-mux-resume/scripts/resume_session.sh:133-144",
     "multi-agent-mux-resume/scripts/resume_session.sh:147-170",
-    "multi-agent-mux-resume/scripts/resume_session.sh:185-210",
-    "multi-agent-mux-resume/scripts/resume_session.sh:258",
-    "multi-agent-mux-resume/scripts/resume_session.sh:262",
-    "multi-agent-mux-resume/scripts/resume_session.sh:268-271",
+    "multi-agent-mux-resume/scripts/resume_session.sh:188-213",
+    "multi-agent-mux-resume/scripts/resume_session.sh:265",
+    "multi-agent-mux-resume/scripts/resume_session.sh:269",
+    "multi-agent-mux-resume/scripts/resume_session.sh:275-278",
     "lib_py/agents/adapters/claude.py:109"
    ],
    "mismatch": [
@@ -504,8 +504,8 @@
    "class": "a",
    "evidence": [
     "multi-agent-mux-stop/scripts/stop_session.sh:101",
-    "multi-agent-mux-stop/scripts/stop_session.sh:123-193",
-    "multi-agent-mux-stop/scripts/stop_session.sh:245-249"
+    "multi-agent-mux-stop/scripts/stop_session.sh:128-198",
+    "multi-agent-mux-stop/scripts/stop_session.sh:250-254"
    ],
    "mismatch": [
     "T1: the doc offered --force-no-yaml, which no script accepts"
@@ -547,8 +547,8 @@
     "multi-agent-mux-create/scripts/create_session.sh:166-168",
     "multi-agent-mux-create/scripts/create_session.sh:207",
     "multi-agent-mux-create/scripts/create_session.sh:259-274",
-    "multi-agent-mux-create/scripts/create_session.sh:361-398",
-    "multi-agent-mux-create/scripts/create_session.sh:410-412"
+    "multi-agent-mux-create/scripts/create_session.sh:364-401",
+    "multi-agent-mux-create/scripts/create_session.sh:413-415"
    ],
    "mismatch": [
     "C1: an existing session is exit 1 in the doc, exit 3 in the script",
@@ -568,7 +568,7 @@
    "base_sha": "f71be0f26fb8",
    "class": "a",
    "evidence": [
-    "multi-agent-mux-create/scripts/create_session.sh:486-604"
+    "multi-agent-mux-create/scripts/create_session.sh:489-607"
    ],
    "mismatch": [],
    "replacement": "bash .agents/skills/multi-agent-mux-create/scripts/create_session.sh --workspace <path> --agent <agent> --role <role>",
@@ -601,7 +601,7 @@
     "other"
    ],
    "passthrough": {
-    "1": "multi-agent-mux-stop/scripts/stop_session.sh:384"
+    "1": "multi-agent-mux-stop/scripts/stop_session.sh:389"
    }
   },
   "multi-agent-mux-create/scripts/create_session.sh": {
@@ -619,7 +619,7 @@
     "other"
    ],
    "passthrough": {
-    "4": "multi-agent-mux-create/scripts/create_session.sh:546"
+    "4": "multi-agent-mux-create/scripts/create_session.sh:549"
    }
   }
  },
diff --git a/tests/test_issue72_admission.py b/tests/test_issue72_admission.py
new file mode 100644
index 0000000..7df3e44
--- /dev/null
+++ b/tests/test_issue72_admission.py
@@ -0,0 +1,196 @@
+"""Issue #72 (decision 8, U2): while an update runs, the entries that start
+something or write the registry refuse - create, resume, run_loop.sh, delegate
+submit, stop (plain and --purge-conversation) and orc-onboard - and change
+nothing. A marker its update left behind (`stale`) refuses nothing.
+
+The marker is .mam/update-in-progress as the update skill publishes it (pid +
+lstart, loop_lock.sh); `live` here is a sleeping helper process's.
+"""
+from __future__ import annotations
+
+import hashlib
+import json
+import os
+import subprocess
+from pathlib import Path
+
+import pytest
+import yaml
+
+UUID = "11111111-2222-3333-4444-555555555555"
+ORC = "99999999-8888-7777-6666-555555555555"
+ORC_OLD = "aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee"
+
+
+@pytest.fixture
+def marker(mam_sandbox):
+    procs = []
+
+    def make(live: bool):
+        p = mam_sandbox / ".mam" / "update-in-progress"
+        if live:
+            proc = subprocess.Popen(["sleep", "120"], start_new_session=True)
+            procs.append(proc)
+            ls = " ".join(subprocess.run(["ps", "-p", str(proc.pid), "-o", "lstart="],
+                                         capture_output=True, text=True).stdout.split())
+            p.write_text("pid=%d\nlstart=%s\nstarted=x\n" % (proc.pid, ls))
+        else:
+            dead = subprocess.Popen(["true"])
+            dead.wait()
+            p.write_text("pid=%d\nlstart=Thu Jan  1 00:00:00 1970\nstarted=x\n" % dead.pid)
+        return p
+
+    yield make
+    for proc in procs:
+        proc.kill()
+        proc.wait()
+
+
+def _seed(sandbox, rows, **top):
+    lib = sandbox / ".agents" / "skills" / "lib.sh"
+    mutation = "d['herdr_sessions'] = %r\n" % (rows,) + "".join("d[%r] = %r\n" % kv for kv in top.items())
+    res = subprocess.run(["bash", "-c", 'source "$1" && atomic_dump_yaml "$AGENT_SESSIONS_YAML"', "_", str(lib)],
+                         input=mutation, capture_output=True, text=True)
+    assert res.returncode == 0, res.stderr
+
+
+def _registry(sandbox):
+    out = {}
+    for f in ("agent-sessions.yaml", "agent-sessions.db"):
+        p = sandbox / ".mam" / f
+        out[f] = hashlib.sha256(p.read_bytes()).hexdigest() if p.exists() else None
+    return out
+
+
+def _script(sandbox, rel):
+    return str(sandbox / ".agents" / "skills" / rel)
+
+
+def _run(sandbox, argv, env=None, timeout=120):
+    return subprocess.run(argv, capture_output=True, text=True, cwd=str(sandbox), timeout=timeout,
+                          env=dict(os.environ, **(env or {})), stdin=subprocess.DEVNULL)
+
+
+def _refused(res, sandbox):
+    assert res.returncode != 0, res.stdout + res.stderr
+    assert "an update of %s is in progress" % sandbox in res.stderr, res.stdout + res.stderr
+
+
+def _row(name, status="running", agent="claude"):
+    r = {"name": name, "status": status, "role": "creator", "herdr_session": "default", "herdr_server": "default",
+         "pane": {"cwd": "", "cmd": agent}, "agent": agent}
+    if agent == "claude":
+        r["claude_session_id_own"] = UUID
+    return r
+
+
+def _agent(cwd, pane):
+    return {"agent": "claude", "status": "running", "cwd": str(cwd), "workspace_id": "w1",
+            "pid": 9999, "pane_id": pane, "command": "claude", "buffer": "Ready"}
+
+
+def _set_mock(state_file, **kw):
+    st = json.loads(Path(state_file).read_text())
+    st.update(kw)
+    Path(state_file).write_text(json.dumps(st))
+
+
+def test_create_refuses_while_an_update_runs(mam_sandbox, mock_herdr, mock_agents, marker):
+    marker(True)
+    before = _registry(mam_sandbox)
+    res = _run(mam_sandbox, ["bash", _script(mam_sandbox, "multi-agent-mux-create/scripts/create_session.sh"),
+                             "--workspace", str(mam_sandbox), "--agent", "claude", "--role", "creator",
+                             "--session", "gated-create", "--no-onboard"], timeout=300)
+    _refused(res, mam_sandbox)
+    assert _registry(mam_sandbox) == before
+    assert json.loads(Path(mock_herdr).read_text())["agents"] == {}, "nothing was spawned"
+
+
+def test_resume_refuses_on_the_spawn_and_the_alive_path(mam_sandbox, mock_herdr, mock_agents, marker):
+    _seed(mam_sandbox, [dict(_row("gated-agy", status="stopped", agent="agy"), pane={"cwd": str(mam_sandbox), "cmd": "agy"})])
+    marker(True)
+    before = _registry(mam_sandbox)
+    script = _script(mam_sandbox, "multi-agent-mux-resume/scripts/resume_session.sh")
+    res = _run(mam_sandbox, ["bash", script, "--workspace", str(mam_sandbox), "--agent", "agy",
+                             "--session", "gated-agy"], timeout=300)
+    _refused(res, mam_sandbox)
+    assert _registry(mam_sandbox) == before
+    assert json.loads(Path(mock_herdr).read_text())["agents"] == {}, "nothing was spawned"
+
+    _set_mock(mock_herdr, agents={"gated-agy": _agent(mam_sandbox, "w1:p9")})
+    res = _run(mam_sandbox, ["bash", script, "--workspace", str(mam_sandbox), "--agent", "agy",
+                             "--session", "gated-agy"], timeout=300)
+    _refused(res, mam_sandbox)
+    assert _registry(mam_sandbox) == before
+
+
+def test_run_loop_refuses_before_its_loop_marker(mam_sandbox, marker):
+    marker(True)
+    loop_marker = mam_sandbox / ".mam" / "loop-guard-active"
+    res = _run(mam_sandbox, ["bash", _script(mam_sandbox, "multi-agent-mux-loop/scripts/run_loop.sh"),
+                             "--creator", "c", "--task", "t"],
+               env={"MAM_LOOP_NO_FREEZE": "1", "MAM_REAL_ROOT": str(mam_sandbox)})
+    _refused(res, mam_sandbox)
+    assert not loop_marker.exists()
+
+
+def test_delegate_submit_registers_no_job(mam_sandbox, mock_herdr, marker):
+    marker(True)
+    res = _run(mam_sandbox, ["bash", _script(mam_sandbox, "multi-agent-mux-delegate-job/multi-agent-mux-delegate-job"),
+                             "submit", "--agent", "claude-code", "--agent-session", "herdr:anyone", "--prompt", "hi"])
+    _refused(res, mam_sandbox)
+    jobs = mam_sandbox / ".mam" / "jobs"
+    assert not list(jobs.glob("*.json")) if jobs.exists() else True
+
+
+@pytest.mark.parametrize("purge", [False, True])
+def test_stop_refuses_and_changes_nothing(mam_sandbox, mock_herdr, marker, purge):
+    _seed(mam_sandbox, [_row("gated-stop")])
+    _set_mock(mock_herdr, agents={"gated-stop": _agent(mam_sandbox, "w1:p8")})
+    marker(True)
+    before = _registry(mam_sandbox)
+    extra = ["--purge-conversation", "--yes"] if purge else []
+    res = _run(mam_sandbox, ["bash", _script(mam_sandbox, "multi-agent-mux-stop/scripts/stop_session.sh"),
+                             "--session", "gated-stop", "--agent", "claude", *extra])
+    _refused(res, mam_sandbox)
+    assert _registry(mam_sandbox) == before
+    assert "gated-stop" in json.loads(Path(mock_herdr).read_text())["agents"], "the session was not touched"
+    assert not (mam_sandbox / ".mam" / "purging-gated-stop").exists()
+
+
+def test_orc_onboard_refuses_and_changes_nothing(mam_sandbox, marker):
+    _seed(mam_sandbox, [], orchestrator_uuids=[ORC_OLD])
+    marker(True)
+    before = _registry(mam_sandbox)
+    script = _script(mam_sandbox, "multi-agent-mux-orc-onboard/scripts/orc_onboard.sh")
+    for args in (["--uuid", ORC], ["--remove", ORC_OLD]):
+        res = _run(mam_sandbox, ["bash", script, *args])
+        _refused(res, mam_sandbox)
+    assert _registry(mam_sandbox) == before
+
+
+def test_a_stale_marker_refuses_nothing(mam_sandbox, mock_herdr, marker):
+    _seed(mam_sandbox, [_row("stale-stop")], orchestrator_uuids=[])
+    _set_mock(mock_herdr, agents={"stale-stop": _agent(mam_sandbox, "w1:p7")})
+    marker(False)
+    res = _run(mam_sandbox, ["bash", _script(mam_sandbox, "multi-agent-mux-orc-onboard/scripts/orc_onboard.sh"),
+                             "--uuid", ORC])
+    assert res.returncode == 0, res.stdout + res.stderr
+    assert "stale update marker" in res.stderr
+    res = _run(mam_sandbox, ["bash", _script(mam_sandbox, "multi-agent-mux-stop/scripts/stop_session.sh"),
+                             "--session", "stale-stop", "--agent", "claude"])
+    assert res.returncode == 0, res.stdout + res.stderr
+    d = yaml.safe_load((mam_sandbox / ".mam" / "agent-sessions.yaml").read_text())
+    assert d["orchestrator_uuids"] == [ORC]
+    assert {r["name"]: r["status"] for r in d["herdr_sessions"]} == {"stale-stop": "stopped"}
+
+
+def test_the_admission_lock_is_released_after_the_record(mam_sandbox, marker):
+    """An entry takes the lock shared and drops it once its record is written:
+    a later exclusive taker (the update) gets it at once."""
+    lib = mam_sandbox / ".agents" / "skills" / "lib.sh"
+    res = subprocess.run(["bash", "-c", 'source "$1" && mam_update_admit && mam_update_admit_release && '
+                          'exec 7>>"$WORKSPACE_ROOT/.mam/update-admission.lock" && '
+                          'MAM_UPDATE_ADMISSION_WAIT_S=1 python3 -m lib_py.pane_lock acquire --fd 7 --wait-env MAM_UPDATE_ADMISSION_WAIT_S',
+                          "_", str(lib)], capture_output=True, text=True, timeout=60)
+    assert res.returncode == 0 and res.stdout.strip() == "locked", res.stdout + res.stderr