diff --git a/modules/restic/README.md b/modules/restic/README.md
index 1ec851b8..c415262e 100644
--- a/modules/restic/README.md
+++ b/modules/restic/README.md
@@ -18,6 +18,12 @@ Against mistakes, not disasters: nothing leaves the machine.
   kept for good. That is a choice: deleting a module's last restore point is a person's act, and
   until then it costs only space.
 
+**A repository another restic holds** (a listing of the restore points, say, while a delivery plan backs up
+the machine's modules before their moves) is waited for: the thinning asks restic to retry the lock for up to
+ten minutes (`--retry-lock`, novox/hq issue 620), and fails only after that. A failed thinning keeps every
+restore point, and the module's health stays unhealthy until a thinning succeeds: the next night's, or one
+after `node-backup.now`.
+
 `node-backup.backed-up` lists each module's restore points taken by hand under `handTaken`: when
 each was taken, the time it is kept at least until, whether it still waits for a later good night,
 and a sentence saying so.
diff --git a/modules/restic/cmd/restic-backups/backups.go b/modules/restic/cmd/restic-backups/backups.go
index 4485b9d5..876c73ac 100644
--- a/modules/restic/cmd/restic-backups/backups.go
+++ b/modules/restic/cmd/restic-backups/backups.go
@@ -406,6 +406,15 @@ func (b *Backups) backUp(ctx context.Context, only string, hand bool) (map[strin
 	return outcome, nil
 }
 
+// thinningWaitsForALock is how long the thinning waits for a repository another restic holds, with restic's own
+// --retry-lock, before it fails as it did before (novox/hq issue 620). The night's lock keeps out this module's own
+// backups and restores, not a restic outside it: a listing of the restore points, asked while a delivery plan backs
+// up a machine's data modules before their moves (ADR 0233), held the lock 246 ms before a thinning asked for it.
+// Ten minutes outlasts such a plan's backups of every data module on a machine (a whole night of fifteen modules
+// took under five minutes) and stays well within the half hour after which restic takes a lock as stale; a lock
+// held longer than this is not a passing collision, and is said the same night.
+const thinningWaitsForALock = 10 * time.Minute
+
 // thin forgets the restore points taken by hand that are past their days and have a later night of their
 // module, by id, then thins the rest by the counts, keeping every one still tagged as taken by hand (novox/hq
 // issue 453). It answers the short ids it let go — on an error of the counts' thinning too, for those are gone. A list that does not read thins nothing: a restore point
@@ -420,14 +429,14 @@ func (b *Backups) thin(ctx context.Context, keep Retention) ([]string, error) {
 		ids, letGo = append(ids, s.ID), append(letGo, s.ShortID)
 	}
 	if len(ids) > 0 {
-		if _, err := b.restic(ctx, append([]string{"forget"}, ids...)...); err != nil {
+		if _, err := b.restic(ctx, append([]string{"forget", "--retry-lock", thinningWaitsForALock.String()}, ids...)...); err != nil {
 			return nil, err
 		}
 		b.Say("let go the restore points taken by hand past %d days with a later night: %s", keep.Hand, strings.Join(letGo, ", "))
 	}
 	_, err = b.restic(ctx, "forget", "--prune", "--group-by", "host,tags",
 		"--keep-daily", fmt.Sprint(keep.Daily), "--keep-weekly", fmt.Sprint(keep.Weekly), "--keep-monthly", fmt.Sprint(keep.Monthly),
-		"--keep-tag", HandTag)
+		"--keep-tag", HandTag, "--retry-lock", thinningWaitsForALock.String())
 	return letGo, err
 }
 
diff --git a/modules/restic/cmd/restic-backups/backups_test.go b/modules/restic/cmd/restic-backups/backups_test.go
index 24930540..3a545185 100644
--- a/modules/restic/cmd/restic-backups/backups_test.go
+++ b/modules/restic/cmd/restic-backups/backups_test.go
@@ -108,7 +108,7 @@ func TestANightDumpsBeforeEachSnapshotAndOneFailingModuleFailsOnlyItself(t *test
 		"sh -c fail-it",
 		"restic backup --json --tag module=mail /",
 		"restic snapshots --json",
-		"restic forget --prune --group-by host,tags --keep-daily 14 --keep-weekly 8 --keep-monthly 6 --keep-tag hand",
+		"restic forget --prune --group-by host,tags --keep-daily 14 --keep-weekly 8 --keep-monthly 6 --keep-tag hand --retry-lock 10m0s",
 	}
 	if !reflect.DeepEqual(calls, want) {
 		t.Errorf("ran\n%s\nwant\n%s", strings.Join(calls, "\n"), strings.Join(want, "\n"))
diff --git a/modules/restic/cmd/restic-backups/hand_test.go b/modules/restic/cmd/restic-backups/hand_test.go
index 22af7160..afd4637b 100644
--- a/modules/restic/cmd/restic-backups/hand_test.go
+++ b/modules/restic/cmd/restic-backups/hand_test.go
@@ -74,7 +74,7 @@ func TestAHandTakenBackupIsTaggedAndTheThinningKeepsThatTag(t *testing.T) {
 	if got := h.with("restic backup"); len(got) != 2 || got[1] != "restic backup --json --tag module=pg /" {
 		t.Errorf("the night backed up with %q, want it tagged with its module alone", got)
 	}
-	want := "restic forget --prune --group-by host,tags --keep-daily 14 --keep-weekly 8 --keep-monthly 6 --keep-tag hand"
+	want := "restic forget --prune --group-by host,tags --keep-daily 14 --keep-weekly 8 --keep-monthly 6 --keep-tag hand --retry-lock 10m0s"
 	for _, f := range h.with("restic forget") {
 		if f != want {
 			t.Errorf("thinned with %q, want %q", f, want)
@@ -134,7 +134,7 @@ func TestTheThinningForgetsAnExpiredHandTakenRestorePointByItsId(t *testing.T) {
 	_, err := b.BackUp(context.Background(), "")
 	must(t, err)
 	forgets := h.with("restic forget")
-	if len(forgets) != 2 || forgets[0] != "restic forget aaaaaaaa11" || !strings.HasPrefix(forgets[1], "restic forget --prune") {
+	if len(forgets) != 2 || forgets[0] != "restic forget --retry-lock 10m0s aaaaaaaa11" || !strings.HasPrefix(forgets[1], "restic forget --prune") {
 		t.Fatalf("thinned with %q, want the expired one forgotten by id, then the rotation", forgets)
 	}
 	th := b.LastRun().Thinned
diff --git a/modules/restic/cmd/restic-backups/lock_test.go b/modules/restic/cmd/restic-backups/lock_test.go
new file mode 100644
index 00000000..c474f30c
--- /dev/null
+++ b/modules/restic/cmd/restic-backups/lock_test.go
@@ -0,0 +1,100 @@
+package main
+
+// The thinning waits for a repository another restic holds (novox/hq issue 620): met three times, each when a
+// restic process outside the night's lock — a listing of the restore points while a delivery plan backs up a
+// machine's data modules before their moves (ADR 0233) — held the repository's lock a few hundred milliseconds
+// before the thinning asked for it, and the thinning failed at once.
+
+import (
+	"context"
+	"errors"
+	"slices"
+	"strings"
+	"testing"
+	"time"
+)
+
+// lockedRestic is a fake restic whose repository another process holds for heldFor, as restic answers it: a
+// forget without --retry-lock fails at once; one with it waits up to its bound, and succeeds when the lock is
+// let go within it, else fails with restic's words. waited is how long the forgets it was asked waited.
+type lockedRestic struct {
+	handRestic
+	heldFor time.Duration
+	waited  []time.Duration
+}
+
+const lockSaid = "unable to create lock in backend: repository is already locked by PID 4242 on the-machine by root " +
+	"(UID 0, GID 0) / lock was created at 2026-10-22 18:14:22 (246 ms ago)"
+
+func (l *lockedRestic) run(ctx context.Context, name string, args ...string) (string, error) {
+	if name != "restic" || args[5] != "forget" {
+		return l.handRestic.run(ctx, name, args...)
+	}
+	l.calls = append(l.calls, "restic "+strings.Join(args[5:], " "))
+	bound := time.Duration(0)
+	if i := slices.Index(args, "--retry-lock"); i >= 0 && i+1 < len(args) {
+		d, err := time.ParseDuration(args[i+1])
+		if err != nil {
+			return "", errors.New("invalid argument for --retry-lock: " + args[i+1])
+		}
+		bound = d
+	}
+	if l.heldFor > bound {
+		l.waited = append(l.waited, bound)
+		return "", errors.New(lockSaid)
+	}
+	l.waited = append(l.waited, l.heldFor)
+	l.heldFor = 0
+	return "", nil
+}
+
+func TestTheThinningWaitsForALockHeldBriefly(t *testing.T) {
+	for _, held := range []time.Duration{3 * time.Second, 5 * time.Minute} {
+		l := &lockedRestic{heldFor: held}
+		b := &Backups{Where: placed(t, "# pg\npath /\n"), Run: l.run, Now: time.Now, Say: quiet}
+		_, err := b.BackUp(context.Background(), "")
+		must(t, err)
+		if th := b.LastRun().Thinned; th == nil || !th.OK {
+			t.Errorf("held for %s: recorded %+v, want the thinning to wait for the lock and succeed", held, th)
+		}
+		if why := b.Health(context.Background()).Why; strings.Contains(why, "Thinning the restore points failed") {
+			t.Errorf("held for %s: the health says %q", held, why)
+		}
+	}
+}
+
+func TestTheThinningStillFailsOnALockHeldPastItsBound(t *testing.T) {
+	l := &lockedRestic{heldFor: thinningWaitsForALock + time.Minute}
+	b := &Backups{Where: placed(t, "# pg\npath /\n"), Run: l.run, Now: time.Now, Say: quiet}
+	_, err := b.BackUp(context.Background(), "")
+	must(t, err)
+	th := b.LastRun().Thinned
+	if th == nil || th.OK || !strings.Contains(th.Error, "already locked") {
+		t.Fatalf("recorded %+v, want the thinning failed on the lock", th)
+	}
+	if !slices.Equal(l.waited, []time.Duration{thinningWaitsForALock}) {
+		t.Errorf("waited %v, want once, for %s", l.waited, thinningWaitsForALock)
+	}
+	why := b.Health(context.Background()).Why
+	for _, want := range []string{"Thinning the restore points failed at", "already locked", "Nothing was forgotten",
+		"node-backup.now on this machine thins again"} {
+		if !strings.Contains(why, want) {
+			t.Errorf("the health says %q, want %q in it", why, want)
+		}
+	}
+}
+
+// A restore point taken by hand that is let go by its id waits for the lock as well: it is the thinning too.
+func TestTheThinningForgetsByIdWaitingForTheLock(t *testing.T) {
+	now := time.Date(2026, 10, 20, 3, 0, 0, 0, time.UTC)
+	l := &lockedRestic{heldFor: 3 * time.Second, handRestic: handRestic{made: "bbbbbbbb11", snaps: []Snapshot{
+		{ID: "aaaaaaaa11", ShortID: "aaaaaaaa", Time: now.AddDate(0, 0, -9), Tags: []string{"module=pg", HandTag}},
+		{ID: "bbbbbbbb11", ShortID: "bbbbbbbb", Time: now, Tags: []string{"module=pg"}},
+	}}}
+	b := &Backups{Where: placed(t, "# pg\npath /\n"), Run: l.run, Now: func() time.Time { return now }, Say: quiet}
+	_, err := b.BackUp(context.Background(), "")
+	must(t, err)
+	if th := b.LastRun().Thinned; th == nil || !th.OK || !slices.Equal(th.LetGo, []string{"aaaaaaaa"}) {
+		t.Errorf("recorded %+v, want aaaaaaaa let go after waiting for the lock", th)
+	}
+}
diff --git a/modules/restic/cmd/restic-backups/retention_test.go b/modules/restic/cmd/restic-backups/retention_test.go
index 2e0ce490..58a7ced4 100644
--- a/modules/restic/cmd/restic-backups/retention_test.go
+++ b/modules/restic/cmd/restic-backups/retention_test.go
@@ -51,7 +51,7 @@ func thinning(t *testing.T, retention *string) (string, []string) {
 func TestANightThinsByTheRetentionTheMeshWrote(t *testing.T) {
 	conf := "# Written by the mesh.\ndaily=7\nweekly=4\nmonthly=24\nhand=10\n"
 	got, _ := thinning(t, &conf)
-	if want := "restic forget --prune --group-by host,tags --keep-daily 7 --keep-weekly 4 --keep-monthly 24 --keep-tag hand"; got != want {
+	if want := "restic forget --prune --group-by host,tags --keep-daily 7 --keep-weekly 4 --keep-monthly 24 --keep-tag hand --retry-lock 10m0s"; got != want {
 		t.Errorf("ran %q, want %q", got, want)
 	}
 }