diff --git a/hermes_cli/gateway_migrate.py b/hermes_cli/gateway_migrate.py
index bd1c7ae860..6dca0ba71b 100644
--- a/hermes_cli/gateway_migrate.py
+++ b/hermes_cli/gateway_migrate.py
@@ -88,8 +88,10 @@ class MigrationPlan:
     profiles: list[ProfileGateway]
     multiplex_flag_on: bool
     live_served: Optional[list[str]]  # served_profiles the live default gateway recorded, if any
-    # A manifest with the flag on and no live default gateway: an earlier apply died between flipping
-    # the flag and bringing the multiplexer up (#110850). Not "already multiplexed" — resumable.
+    # A manifest with the flag on and no LIVE default gateway: an earlier apply died between flipping
+    # the flag and the multiplexer confirming it is up (#110850). An installed unit is not proof of
+    # anything — `systemd_install` writes the unit before the start that can still fail or be killed.
+    # Not "already multiplexed" — resumable.
     interrupted: bool = False
     blockers: list[str] = field(default_factory=list)
     notices: list[str] = field(default_factory=list)
@@ -449,7 +451,7 @@ def build_migration_plan() -> MigrationPlan:
         multiplex_flag_on=_read_multiplex_flag(default_home),
         live_served=recorded_served_profiles(default_home),
     )
-    plan.interrupted = plan.multiplex_flag_on and not plan.default.has_gateway and _read_manifest(default_home) is not None
+    plan.interrupted = plan.multiplex_flag_on and _manifest_not_yet_served(_read_manifest(default_home), plan.live_served)
     if len(plan.profiles) < 2:
         plan.notices.append("Only one profile exists: nothing to multiplex.")
         return plan
@@ -487,7 +489,7 @@ def format_plan(plan: MigrationPlan, *, dry_run: bool) -> list[str]:
         return lines
     if plan.interrupted:
         lines.append(f"  ↻ An earlier migration was interrupted before the default gateway came up "
-                     f"(flag on, no gateway; manifest {plan.default_home / MANIFEST_NAME}); this run resumes it.")
+                     f"(flag on, no live multiplexer; manifest {plan.default_home / MANIFEST_NAME}); this run resumes it.")
     steps = []
     for p in plan.standalone_secondaries:
         what = " + ".join(x for x in (f"stop pid {p.pid}" if p.pid else "", f"uninstall {p.service_label()}" if p.services else "") if x)
@@ -615,6 +617,19 @@ def _read_manifest(default_home: Path) -> Optional[dict]:
     return data if isinstance(data, dict) else None
 
 
+def _manifest_not_yet_served(manifest: Optional[dict], live_served: Optional[list[str]]) -> bool:
+    """The postcondition ``apply_migration`` waits for, re-derived from live state: a LIVE default that
+    recorded serving every profile the manifest migrated. Anything less — no live gateway, an
+    installed-but-dead unit (``systemd_install`` writes the unit before the start that can still fail),
+    a standalone default never restarted — is a half-applied migration, not "already multiplexed".
+    Profiles created after the migration are not in the manifest, so they cannot flag it as interrupted."""
+    if manifest is None:
+        return False
+    recs = [r for r in (manifest.get("default"), *(_manifest_secondaries(manifest) or [])) if isinstance(r, dict)]
+    migrated = {str(r.get("profile") or "default") for r in recs} | {"default"}
+    return not migrated <= set(live_served or [])
+
+
 def _write_manifest(default_home: Path, data: dict) -> None:
     from utils import atomic_json_write
 
@@ -691,14 +706,48 @@ def _remove_secondary_gateways(plan: MigrationPlan) -> None:
             print(f"  ✓ {p.name}: stopped standalone gateway (pid {p.pid})")
 
 
+def _preflight_apply(plan: MigrationPlan, target: Optional[tuple[str, bool]], run_as_user: Optional[str]) -> Optional[str]:
+    """A failure of the destructive phase that is knowable from the plan alone, refused BEFORE any
+    working per-profile gateway is stopped: rollback is the fallback for surprises, not the plan.
+    Mirrors the checks ``systemd_install``/``_service_call`` make on a system unit (root, resolvable
+    ``User=``) and the config write's read-guard."""
+    from hermes_cli import gateway as gw
+    from hermes_cli.config import require_readable_config_before_write
+    try:
+        require_readable_config_before_write(plan.default_home / "config.yaml")
+    except Exception as exc:
+        return f"default: config.yaml cannot be updated ({exc})"
+    touches_system_unit = target == ("systemd", True) or any(p.has_system_unit for p in plan.standalone_secondaries)
+    if touches_system_unit:
+        try:
+            gw._require_root_for_system_service("migration")
+        except Exception as exc:
+            return str(exc)
+    if plan.default.service is None and target == ("systemd", True):
+        if run_as_user is None:
+            try:
+                gw._system_service_identity()  # the #110850 refusal (implicit root), before anything is removed
+            except ValueError as exc:
+                return f"default: {exc}"
+        else:
+            import pwd
+            try:
+                pwd.getpwnam(run_as_user)
+            except KeyError:
+                return f"default: the recorded service user '{run_as_user}' does not exist on this host"
+    return None
+
+
 def apply_migration(plan: MigrationPlan, *, served_wait: float = _SERVED_WAIT_SECONDS) -> bool:
-    """Stop/uninstall every secondary gateway, flip the flag, bring up the multiplexer, verify.
+    """Flip the flag, stop/uninstall every secondary gateway, bring up the multiplexer, verify.
     Returns True when the multiplexer verifiably serves every profile.
 
-    Bringing the default up is the one step that can fail after the destructive ones (a system unit
-    that needs ``--run-as-user``, an unreachable user bus). It runs inside a rollback: on failure the
-    manifest written before the first destructive step restores the flag and every recorded per-profile
-    gateway (#110850), so the fleet never ends with the flag on and no gateway at all."""
+    Every step after the manifest write is fallible (a config write, a secondary's stop or its
+    unit's daemon-reload, a system unit that needs ``--run-as-user``, an unreachable user bus) and
+    runs inside ONE compensating boundary: on failure the manifest written before the first
+    destructive step restores the flag and every recorded per-profile gateway (#110850), so the
+    fleet never ends half-migrated. The flag goes on first so an apply killed anywhere after it is
+    resumable from the manifest (flag on + manifest + no live multiplexer = interrupted)."""
     if plan.blocked:
         _print(["✗ Migration refused:", *[f"  • {b}" for b in plan.blockers]])
         return False
@@ -707,18 +756,22 @@ def apply_migration(plan: MigrationPlan, *, served_wait: float = _SERVED_WAIT_SE
         return True
     target, run_as_user = plan.target_service_kind(), plan.target_run_as_user()
     if plan.interrupted:
-        # An earlier apply removed the secondaries and flipped the flag but never brought the default
-        # up; the manifest is the only record of the units that existed. Finish from it, don't rewrite it.
+        # An earlier apply flipped the flag (and removed some or all secondaries) but the default never
+        # came up; the manifest is the only record of the units that existed. Finish from it, don't rewrite it.
         manifest = _read_manifest(plan.default_home) or {}
         target, run_as_user = _target_from_manifest(manifest)
         print(f"  ↻ resuming an interrupted migration recorded in {_manifest_path(plan.default_home)}")
     elif _read_manifest(plan.default_home) is not None:
-        # Flag off + manifest present = a rollback that did not finish. Overwriting the manifest would
-        # discard the only record of the units that rollback still has to restore.
+        # Flag off + manifest present = a rollback (or an apply killed before its flag write) that did
+        # not finish. Overwriting the manifest would discard the only record of the units to restore.
         _print([f"✗ A previous migration's manifest is still at {_manifest_path(plan.default_home)} (its rollback did not finish).",
                 "  Finish it with: hermes gateway migrate --standalone   (or delete the manifest to start over)"])
         return False
     else:
+        blocker = _preflight_apply(plan, target, run_as_user)
+        if blocker is not None:
+            _print(["✗ Migration refused before changing anything:", f"  • {blocker}"])
+            return False
         manifest = {
             "version": 1, "migrated_at": time.strftime("%Y-%m-%dT%H:%M:%S%z"),
             "flag_was": plan.multiplex_flag_on,
@@ -728,13 +781,13 @@ def apply_migration(plan: MigrationPlan, *, served_wait: float = _SERVED_WAIT_SE
         # Recovery metadata must exist before the first destructive operation; the manifest never
         # changes afterwards, so this is the only write it needs.
         _write_manifest(plan.default_home, manifest)
-    _remove_secondary_gateways(plan)
-    _write_multiplex_flag(plan.default_home, True)
-    print(f"  ✓ default: gateway.multiplex_profiles: true ({plan.default_home / 'config.yaml'})")
     try:
+        _write_multiplex_flag(plan.default_home, True)
+        print(f"  ✓ default: gateway.multiplex_profiles: true ({plan.default_home / 'config.yaml'})")
+        _remove_secondary_gateways(plan)  # on resume: whatever an apply killed mid-removal left installed
         print(f"  ✓ {_restart_default(plan.default, target, plan.default_home, run_as_user=run_as_user)}")
     except Exception as exc:
-        _print([f"  ✗ default: could not bring up the multiplexed gateway ({exc})",
+        _print([f"  ✗ migration failed ({exc})",
                 "  ↩ Rolling back to per-profile gateways so no profile is left without one..."])
         rolled_back = rollback_migration(plan.default_home)
         if not rolled_back:
diff --git a/hermes_cli/gateway_migrate_guards.py b/hermes_cli/gateway_migrate_guards.py
index a7a04b3d99..cadbb14ac7 100644
--- a/hermes_cli/gateway_migrate_guards.py
+++ b/hermes_cli/gateway_migrate_guards.py
@@ -105,6 +105,11 @@ def _guard_service_domain(plan: MigrationPlan, profile: ProfileGateway) -> Optio
 
 def _guard_unix_user(plan: MigrationPlan, profile: ProfileGateway) -> Optional[str]:
     default_uid = plan.default.uid
+    if default_uid is None and plan.default.has_system_unit:
+        # Consolidating INTO a principal this host cannot identify is the same unknown boundary
+        # from the other side: the default's system unit names an account NSS does not resolve.
+        return ("The default gateway runs a system unit whose User= cannot be resolved on this host: "
+                "an unknown service principal is not folded into automatically.")
     if profile.uid is None and profile.has_system_unit:
         # Unknown principal is not "same user": the unit names an account this host cannot resolve.
         return (f"Profile '{profile.name}' runs a system unit whose User= cannot be resolved on this host: "
@@ -134,12 +139,13 @@ _AUTO_MIGRATION_GUARDS: tuple[Callable[[MigrationPlan, ProfileGateway], Optional
 def auto_migration_blockers(plan: MigrationPlan) -> list[str]:
     """Every boundary a standalone secondary sits behind; empty when the fleet is one user, one service
     domain, one profiles/ tree — the only shape ``hermes update`` may fold on its own."""
-    return [
+    findings = [
         finding
         for profile in plan.standalone_secondaries
         for guard in _AUTO_MIGRATION_GUARDS
         if (finding := guard(plan, profile)) is not None
     ]
+    return list(dict.fromkeys(findings))  # a default-side finding repeats per secondary
 
 
 # --------------------------------------------------------------------------- opt-out
diff --git a/tests/hermes_cli/test_gateway_migrate_multiplex.py b/tests/hermes_cli/test_gateway_migrate_multiplex.py
index d1f1fe008f..8952fd28ba 100644
--- a/tests/hermes_cli/test_gateway_migrate_multiplex.py
+++ b/tests/hermes_cli/test_gateway_migrate_multiplex.py
@@ -78,6 +78,8 @@ def fleet(tmp_path, monkeypatch):
     monkeypatch.setattr(gm, "_service_op", _service_op)
     monkeypatch.setattr(gm, "_stop_gateway_process", lambda home: state.pids.pop(_name(home), None))
     monkeypatch.setattr(gm, "_host_supports_migration", lambda: None)
+    # Part of the faked service layer: the real check asks systemd's questions (root, NSS user).
+    monkeypatch.setattr(gm, "_preflight_apply", lambda plan, target, run_as_user: None)
     state.root = root
     return state
 
@@ -92,6 +94,9 @@ def _units(recorded) -> list:
     return list(recorded) if isinstance(recorded, list) else [recorded]
 
 
+_real_preflight = gm._preflight_apply
+
+
 def _config_flag(root: Path):
     import yaml
     raw = yaml.safe_load((root / "config.yaml").read_text(encoding="utf-8")) or {}
@@ -564,3 +569,111 @@ def test_opt_out_reads_effective_config_managed_false_wins_and_string_false_is_f
     assert auto_migration_opted_out(fleet.root) is True
     gm.maybe_auto_migrate_after_update()
     assert fleet.ops == [] and _config_flag(fleet.root) is None
+
+
+@pytest.mark.parametrize("default_unit_preinstalled", [False, True])
+def test_interruption_after_the_default_unit_exists_is_still_interrupted_not_already_multiplexed(
+    fleet, monkeypatch, capsys, default_unit_preinstalled,
+):
+    """``systemd_install`` writes the unit before the start that can be killed; an existing stopped
+    default unit can be interrupted mid-restart. Either way the flag is on and a default unit exists
+    but nothing serves anyone: that is an interrupted migration to resume, not a completed one. Only
+    a LIVE multiplexer that recorded ``served_profiles`` counts as already multiplexed."""
+    if default_unit_preinstalled:
+        fleet.services["default"] = ("systemd", False)
+    real_op = gm._service_op
+
+    def _killed_at_start(kind, system, verb, home, *, run_as_user=None):
+        if verb in ("start", "restart") and _name(home) == "default":
+            raise KeyboardInterrupt()
+        real_op(kind, system, verb, home, run_as_user=run_as_user)
+
+    with pytest.MonkeyPatch.context() as dying:
+        dying.setattr(gm, "_service_op", _killed_at_start)
+        with pytest.raises(KeyboardInterrupt):
+            gm.apply_migration(gm.build_migration_plan(), served_wait=0.1)
+    assert _config_flag(fleet.root) is True and fleet.services == {"default": ("systemd", False)}
+    assert (fleet.root / gm.MANIFEST_NAME).exists()
+
+    plan = gm.build_migration_plan()
+    assert plan.interrupted and not plan.already_multiplexed
+    fleet.ops.clear()
+    assert gm.apply_migration(plan, served_wait=5.0) is True
+    assert fleet.ops[-1] == ("default", "restart") and "serves 3 profiles" in capsys.readouterr().out
+    # Postcondition met: the next plan sees the live multiplexer and stops.
+    assert gm.build_migration_plan().already_multiplexed
+
+
+@pytest.mark.parametrize("failing_op", [("ops", "stop"), ("coder", "uninstall"), ("default", "flag")])
+def test_failure_anywhere_in_the_destructive_phase_restores_the_removed_secondaries(fleet, monkeypatch, capsys, failing_op):
+    """The compensation boundary covers the whole destructive phase, not only the default bring-up:
+    a later secondary's stop, a unit unlink/daemon-reload, or the flag write failing after an earlier
+    secondary was removed must put that secondary back (the manifest alone is not a restored fleet)."""
+    name, verb = failing_op
+    real_op = gm._service_op
+
+    def _failing(kind, system, verb_, home, *, run_as_user=None):
+        if (_name(home), verb_) == (name, verb):
+            raise RuntimeError(f"{name} {verb} failed")
+        real_op(kind, system, verb_, home, run_as_user=run_as_user)
+
+    monkeypatch.setattr(gm, "_service_op", _failing)
+    if verb == "flag":
+        real_flag = gm._write_multiplex_flag
+        monkeypatch.setattr(gm, "_write_multiplex_flag",
+                            lambda home, value: (_ for _ in ()).throw(OSError("read-only config")) if value else real_flag(home, value))
+    assert gm.apply_migration(gm.build_migration_plan(), served_wait=0.1) is False
+    out = capsys.readouterr().out
+    assert "Rolling back" in out and "Rolled back" in out
+    assert _config_flag(fleet.root) is not True
+    assert fleet.services == {"coder": ("systemd", False), "ops": ("systemd", False)}
+    assert not (fleet.root / gm.MANIFEST_NAME).exists()
+    assert gm.build_migration_plan().eligible_for_migration()
+
+
+def test_known_bringup_refusal_is_rejected_before_any_secondary_is_touched(fleet, monkeypatch, capsys):
+    """A system-unit fleet with no recorded User= run by root is the #110850 refusal: known from the plan,
+    so it is refused before a working gateway is stopped rather than discovered and rolled back."""
+    from hermes_cli import gateway as gw
+    fleet.services.update({"coder": ("systemd", True), "ops": ("systemd", True)})
+    monkeypatch.setattr(gm, "_systemd_service_user", lambda home, services: None)
+    monkeypatch.setattr(gm, "_preflight_apply", _real_preflight)
+    monkeypatch.setattr(gw, "_require_root_for_system_service", lambda action: None)  # we are "root"
+    for var in ("SUDO_USER", "USER", "LOGNAME"):
+        monkeypatch.setenv(var, "root")
+    assert gm.apply_migration(gm.build_migration_plan(), served_wait=0.1) is False
+    out = capsys.readouterr().out
+    assert "before changing anything" in out and "--run-as-user root" in out
+    assert fleet.ops == [] and _config_flag(fleet.root) is None and not (fleet.root / gm.MANIFEST_NAME).exists()
+
+
+def test_unknown_default_system_principal_blocks_the_update_hook(fleet, tmp_path, monkeypatch, capsys):
+    """Mirror of the unknown-secondary case: the default's system unit names a User= this host cannot
+    resolve while both secondaries are known root system units. Folding INTO an unidentifiable
+    principal is the same boundary; known-same uid still folds, known-different still refuses."""
+    from hermes_cli import gateway as gw
+    from hermes_cli.gateway_migrate_guards import auto_migration_blockers, gateway_identity
+    unit_dir = tmp_path / "system"; unit_dir.mkdir()
+    monkeypatch.setattr(gw, "_SYSTEM_UNIT_DIR", unit_dir)
+    with gm._home_env(fleet.root):
+        gw.get_systemd_unit_path(system=True).write_text("[Service]\nUser=no-such-pr111062-user\n", encoding="utf-8")
+    for name in ("coder", "ops"):
+        with gm._home_env(fleet.root / "profiles" / name):
+            gw.get_systemd_unit_path(system=True).write_text("[Service]\nUser=root\n", encoding="utf-8")
+    fleet.services.update({"default": ("systemd", True), "coder": ("systemd", True), "ops": ("systemd", True)})
+    fleet.pids.clear()  # stopped units everywhere: identity comes from User=, resolved for real
+    plan = gm.build_migration_plan()
+    assert plan.default.uid is None and {p.uid for p in plan.standalone_secondaries} == {0}
+    assert gateway_identity(fleet.root, None, [("systemd", True)])[0] is None
+    blockers = auto_migration_blockers(plan)
+    assert len(blockers) == 1 and "default gateway" in blockers[0] and "cannot be resolved" in blockers[0]
+    gm.maybe_auto_migrate_after_update()
+    out = capsys.readouterr().out
+    assert "cannot be resolved" in out and gm.MIGRATE_COMMAND in out
+    assert fleet.ops == [] and _config_flag(fleet.root) is None
+
+    # Controls: same known uid folds; a different known uid refuses.
+    monkeypatch.setattr(gm, "_gateway_identity", lambda home, pid, services: (0, home))
+    assert auto_migration_blockers(gm.build_migration_plan()) == []
+    monkeypatch.setattr(gm, "_gateway_identity", lambda home, pid, services: (0 if _name(home) == "default" else 1000, home))
+    assert any("UNIX privilege boundary" in b for b in auto_migration_blockers(gm.build_migration_plan()))
diff --git a/website/docs/user-guide/multi-profile-gateways.md b/website/docs/user-guide/multi-profile-gateways.md
index ecb4333e3d..2398580b58 100644
--- a/website/docs/user-guide/multi-profile-gateways.md
+++ b/website/docs/user-guide/multi-profile-gateways.md
@@ -854,7 +854,7 @@ A standalone secondary behind any of these boundaries stops the automatic path:
 |---|---|
 | different service manager or scope | default on user systemd, a secondary on **system** systemd (or launchd), or the default detached with a service-managed secondary |
 | more than one installed unit on a profile | a user **and** a system unit for the same profile (the explicit command removes both) |
-| different UNIX user | a system unit with its own `User=`, or a live gateway owned by another uid; a system unit whose `User=` this host cannot resolve counts as unknown, never as "same user" |
+| different UNIX user | a system unit with its own `User=`, or a live gateway owned by another uid; a system unit whose `User=` this host cannot resolve — on the secondary **or** on the default — counts as unknown, never as "same user" |
 | `HERMES_HOME` outside `<default home>/profiles/` | a unit pinning `HERMES_HOME=/opt/hermes/profiles/emma` |
 
 In that case `hermes update` prints the boundary it found plus
@@ -957,13 +957,17 @@ previous value, restarts the default gateway, and reinstalls/starts every
 recorded per-profile service (a system unit comes back with the `User=` it had).
 The manifest is removed once everything is back.
 
-The forward migration is transactional in the same way: if bringing the default
-gateway up fails after the per-profile gateways were removed (for example a
-system unit that has to run as root), `--multiplex` rolls back through the
-manifest on the spot so no profile is left without a gateway. Should the
-process die between flipping the flag and starting the default, the next
-`hermes gateway migrate --multiplex` sees the manifest with no live gateway and
-resumes from it instead of reporting "already multiplexed".
+The forward migration is transactional in the same way. Failures it can see
+coming from the plan (a system unit that would have to run as root without a
+recorded `User=`, a config file it cannot rewrite) are refused before any
+per-profile gateway is stopped. Anything that fails after the manifest is
+written — the flag write, a later secondary's stop or unit removal, the
+default's install or start — rolls back through the manifest on the spot, so no
+profile is left without a gateway. Should the process die anywhere in that
+window, the next `hermes gateway migrate --multiplex` sees the flag on, the
+manifest, and no live multiplexer serving the migrated profiles (an installed
+but stopped default unit does not count) and resumes from the manifest instead
+of reporting "already multiplexed".
 If no manifest exists (you enabled multiplexing by hand), leave multiplex mode
 with `hermes config set gateway.multiplex_profiles false && hermes gateway restart`
 and reinstall the per-profile services you want.