diff --git a/cmd/felis/offsite.go b/cmd/felis/offsite.go
index 7f15b4b..5863ad2 100644
--- a/cmd/felis/offsite.go
+++ b/cmd/felis/offsite.go
@@ -33,12 +33,17 @@ const offsiteUsage = `usage:
   felis offsite fetch-worlds [-config path] [-archive-dir dir]
   felis offsite fetch-images [-config path] [-registry host:port] [-at version]
   felis offsite fetch-uploads [-config path] [-uploads-dir dir] [-at version]
+  felis offsite check-key    [-config path]
   felis offsite keygen
 
 Every verb but keygen reads the bucket credentials and the encryption key from
 the variables [offsite] names (default FELIS_OFFSITE_ACCESS_KEY,
 FELIS_OFFSITE_SECRET_KEY, FELIS_OFFSITE_KEY), taking any that are unset from
 -env-file (default /etc/felis/offsite.env).
+
+check-key tells whether the key is the one the bucket's objects are sealed
+with, writing nothing; it exits 3 when they are sealed with another key, and
+sync then refuses to write or prune anything in the bucket.
 `
 
 // defaultOffsiteEnvFile is where bootstrap keeps the [offsite] secrets; the
@@ -74,6 +79,8 @@ func cmdOffsite(args []string, stdout, stderr io.Writer) int {
 		return offsiteFetchImages(fs, rest, stdout, stderr)
 	case "fetch-uploads":
 		return offsiteFetchUploads(fs, rest, stdout, stderr)
+	case "check-key":
+		return offsiteCheckKey(fs, rest, stdout, stderr)
 	case "keygen":
 		k, err := offsite.NewKey()
 		if err != nil {
@@ -218,12 +225,7 @@ func offsiteSync(fs *flag.FlagSet, args []string, stdout, stderr io.Writer) int
 		registry:   offsiteRegistryEndpoint(*registry, cfg.Registry),
 		uploadsDir: *uploadsDir, uploadsPVC: *uploadsPVC,
 	}, stderr)
-	st.Result = res
-	if err != nil {
-		st.LastError = err.Error()
-	} else {
-		st.LastSuccess = st.LastAttempt
-	}
+	recordRun(&st, res, err)
 	if werr := offsite.WriteStatus(*statusFile, st); werr != nil {
 		fmt.Fprintf(stderr, "felis offsite sync: record status: %v\n", werr)
 	}
@@ -246,6 +248,17 @@ func offsiteSync(fs *flag.FlagSet, args []string, stdout, stderr io.Writer) int
 	return 0
 }
 
+// recordRun puts one pass's outcome into its status record.
+func recordRun(st *offsite.Status, res offsite.Result, err error) {
+	st.Result = res
+	if err != nil {
+		st.LastError = err.Error()
+		st.KeyMismatch = errors.Is(err, offsite.ErrKeyMismatch)
+	} else {
+		st.LastSuccess = st.LastAttempt
+	}
+}
+
 // offsiteSources is where one sync pass reads from: the world archive volume
 // (archiveDir, or the backupPVC's directory), the bundle directory, the
 // registry's loopback endpoint and the uploads volume (uploadsDir, or the
@@ -438,6 +451,10 @@ func offsiteStatus(fs *flag.FlagSet, args []string, stdout, stderr io.Writer) in
 	if st.LastError != "" {
 		fmt.Fprintf(stdout, "last error:   %s\n", st.LastError)
 	}
+	if st.KeyMismatch {
+		fmt.Fprintf(stdout, "\nThe last run was refused: the bucket's objects are sealed with another key than this host's (key id %s). No sync copies or prunes anything there until FELIS_OFFSITE_KEY in %s is theirs (sudo felis offsite check-key).\n", st.KeyID, defaultOffsiteEnvFile)
+		return 1
+	}
 	r := st.Result
 	fmt.Fprintf(stdout, "bucket holds: %d world archives (%s), %d database bundles, newest %s\n",
 		r.RemoteWorlds, offsite.HumanBytes(r.RemoteBytes), r.RemoteDB, orNone(r.NewestDB))
@@ -552,6 +569,62 @@ func printDBBundles(ctx context.Context, b offsite.Bucket, key []byte, bundles [
 	}
 }
 
+func offsiteCheckKey(fs *flag.FlagSet, args []string, stdout, stderr io.Writer) int {
+	cfgPath := fs.String("config", "/etc/felis/felis.toml", "path to felis.toml")
+	envFile := fs.String("env-file", defaultOffsiteEnvFile, "file with the [offsite] secrets, for variables not already set")
+	if err := fs.Parse(args); err != nil {
+		return 2
+	}
+	_, env, err := loadOffsite(*cfgPath, *envFile)
+	if err != nil {
+		fmt.Fprintf(stderr, "felis offsite check-key: %v\n", err)
+		return 1
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Minute)
+	defer cancel()
+	if err := env.bucket.Check(ctx); err != nil {
+		fmt.Fprintf(stderr, "felis offsite check-key: %v\n", err)
+		return 1
+	}
+	return checkKey(ctx, env.bucket, env.key, stdout, stderr)
+}
+
+// checkKey is check-key once the bucket is open: 0 when the key fits, 3 when
+// the bucket's objects are sealed with another one, 1 when it cannot tell.
+func checkKey(ctx context.Context, b offsite.Bucket, key []byte, stdout, stderr io.Writer) int {
+	fit, err := offsite.CheckKey(ctx, b, key)
+	if err != nil {
+		fmt.Fprintf(stderr, "felis offsite check-key: %v\n", err)
+		if errors.Is(err, offsite.ErrKeyMismatch) {
+			return 3
+		}
+		return 1
+	}
+	id := offsite.KeyID(key)
+	switch fit {
+	case offsite.KeyRecorded:
+		fmt.Fprintf(stdout, "felis offsite check-key: the bucket records key id %s, this key's\n", id)
+	case offsite.KeyOpens:
+		fmt.Fprintf(stdout, "felis offsite check-key: the bucket's newest objects open with this key (key id %s); the next sync records it\n", id)
+	case offsite.KeyUnused:
+		fmt.Fprintf(stdout, "felis offsite check-key: the bucket holds no sealed object yet; the first sync records key id %s\n", id)
+	}
+	return 0
+}
+
+// keyHint explains an object the key cannot open when the bucket records
+// another key's id, "" otherwise.
+func keyHint(ctx context.Context, b offsite.Bucket, key []byte, err error) string {
+	if !errors.Is(err, offsite.ErrAuth) {
+		return ""
+	}
+	id, ierr := offsite.BucketKeyID(ctx, b)
+	if ierr != nil || id == "" || id == offsite.KeyID(key) {
+		return ""
+	}
+	return fmt.Sprintf("\n  the bucket records key id %s, and this key is %s: set FELIS_OFFSITE_KEY to the key the bucket was written with", id, offsite.KeyID(key))
+}
+
 func offsiteFetchDB(fs *flag.FlagSet, args []string, stdout, stderr io.Writer) int {
 	cfgPath := fs.String("config", "/etc/felis/felis.toml", "path to felis.toml; on a host with no install yet, give -endpoint and -bucket instead")
 	envFile := fs.String("env-file", defaultOffsiteEnvFile, "file with the [offsite] secrets, for variables not already set")
@@ -603,7 +676,7 @@ func fetchDB(ctx context.Context, b offsite.Bucket, key []byte, arg, dir string,
 	if name == "latest" {
 		var err error
 		if name, _, err = offsite.ChooseDB(ctx, b, key); err != nil {
-			fmt.Fprintf(stderr, "felis offsite fetch-db: %v\n", err)
+			fmt.Fprintf(stderr, "felis offsite fetch-db: %v%s\n", err, keyHint(ctx, b, key, err))
 			return 1
 		}
 	}
@@ -617,7 +690,7 @@ func fetchDB(ctx context.Context, b offsite.Bucket, key []byte, arg, dir string,
 	}
 	dst := filepath.Join(dir, name)
 	if err := offsite.FetchObject(ctx, b, key, offsite.DBKey(name), dst, 0o600); err != nil {
-		fmt.Fprintf(stderr, "felis offsite fetch-db: %v\n", err)
+		fmt.Fprintf(stderr, "felis offsite fetch-db: %v%s\n", err, keyHint(ctx, b, key, err))
 		return 1
 	}
 	m, err := dbbackup.Verify(dst)
diff --git a/cmd/felis/offsite_test.go b/cmd/felis/offsite_test.go
index 314d31f..001b523 100644
--- a/cmd/felis/offsite_test.go
+++ b/cmd/felis/offsite_test.go
@@ -8,6 +8,7 @@ import (
 	"encoding/hex"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"io"
 	"os"
 	"path/filepath"
@@ -285,6 +286,120 @@ func TestOffsiteFetchDB(t *testing.T) {
 	})
 }
 
+func TestOffsiteCheckKey(t *testing.T) {
+	newKey := func() []byte {
+		raw, _ := offsite.NewKey()
+		k, _ := offsite.ParseKey(raw)
+		return k
+	}
+	key, other := newKey(), newKey()
+	marked := func(k []byte) mapBucket { return mapBucket{"felis-key-id": []byte(offsite.KeyID(k) + "\n")} }
+	unmarked := func(k []byte) mapBucket {
+		b := mapBucket{}
+		putBundle(t, b, k, 0, nil)
+		return b
+	}
+	for _, tc := range []struct {
+		what   string
+		bucket mapBucket
+		code   int
+		says   []string
+	}{
+		{"the recorded key", marked(key), 0, []string{"records key id " + offsite.KeyID(key)}},
+		{"an unmarked bucket the key opens", unmarked(key), 0, []string{"open with this key", "the next sync records it"}},
+		{"an empty bucket", mapBucket{}, 0, []string{"no sealed object yet", "records key id " + offsite.KeyID(key)}},
+		{"another recorded key", marked(other), 3, []string{offsite.KeyID(other), offsite.KeyID(key), "FELIS_OFFSITE_KEY"}},
+		{"an unmarked bucket under another key", unmarked(other), 3, []string{"opens none of db/felis-db-"}},
+		{"a marker Felis did not write", mapBucket{"felis-key-id": []byte("hello")}, 1, []string{"not a key id"}},
+	} {
+		t.Run(tc.what, func(t *testing.T) {
+			before := len(tc.bucket)
+			var out, errb bytes.Buffer
+			code := checkKey(context.Background(), tc.bucket, key, &out, &errb)
+			if code != tc.code {
+				t.Fatalf("exit %d, want %d; stdout %q, stderr %q", code, tc.code, out.String(), errb.String())
+			}
+			said := out.String() + errb.String()
+			for _, s := range tc.says {
+				if !strings.Contains(said, s) {
+					t.Errorf("output lacks %q: %s", s, said)
+				}
+			}
+			if len(tc.bucket) != before {
+				t.Errorf("check-key wrote to the bucket: %d objects, had %d", len(tc.bucket), before)
+			}
+		})
+	}
+
+	// fetch-db names both ids when the bucket records another key.
+	b := marked(other)
+	name := putBundle(t, b, other, 0, &dbbackup.Counts{Users: 5, Servers: 3})
+	for _, arg := range []string{"latest", name} {
+		var out, errb bytes.Buffer
+		if code := fetchDB(context.Background(), b, key, arg, t.TempDir(), fetchT0, &out, &errb); code != 1 ||
+			!strings.Contains(errb.String(), "the bucket records key id "+offsite.KeyID(other)+", and this key is "+offsite.KeyID(key)) {
+			t.Errorf("fetch-db %s under another key: exit %d, stderr %q", arg, code, errb.String())
+		}
+	}
+	// A bundle the bucket lacks is not the key's fault.
+	var missOut, missErr bytes.Buffer
+	if code := fetchDB(context.Background(), b, key, "felis-db-20200101T000000Z-daily.tar", t.TempDir(), fetchT0, &missOut, &missErr); code != 1 || strings.Contains(missErr.String(), "records key id") {
+		t.Errorf("missing bundle: exit %d, stderr %q", code, missErr.String())
+	}
+	// A bundle damaged under the recorded key gets no such hint.
+	b = marked(key)
+	name = putBundle(t, b, key, 0, &dbbackup.Counts{Users: 5, Servers: 3})
+	b[offsite.DBKey(name)][60] ^= 1
+	var out, errb bytes.Buffer
+	if code := fetchDB(context.Background(), b, key, name, t.TempDir(), fetchT0, &out, &errb); code != 1 || strings.Contains(errb.String(), "records key id") {
+		t.Errorf("damaged bundle: exit %d, stderr %q", code, errb.String())
+	}
+}
+
+func TestRecordRunMarksAKeyMismatch(t *testing.T) {
+	t0 := time.Date(2026, 9, 27, 12, 0, 0, 0, time.UTC)
+	for _, tc := range []struct {
+		err      error
+		mismatch bool
+		success  bool
+	}{
+		{nil, false, true},
+		{errors.New("list worlds/ in the bucket: connection reset"), false, false},
+		{fmt.Errorf("%w: the bucket records key id 0123456789abcdef", offsite.ErrKeyMismatch), true, false},
+	} {
+		st := offsite.Status{LastAttempt: t0}
+		recordRun(&st, offsite.Result{RemoteDB: 2}, tc.err)
+		if st.KeyMismatch != tc.mismatch || st.LastSuccess.Equal(t0) != tc.success || st.Result.RemoteDB != 2 {
+			t.Errorf("err %v: status %+v", tc.err, st)
+		}
+	}
+}
+
+func TestOffsiteStatusSaysTheKeyWasRefused(t *testing.T) {
+	dir := t.TempDir()
+	cfg := filepath.Join(dir, "felis.toml")
+	writeTestFile(t, cfg, installerTOML("example.com", "127.0.0.1")+"\n[offsite]\nendpoint = \"https://s3.example.com\"\nbucket = \"felis-backups\"\n", 0o600)
+	statusFile := filepath.Join(dir, "status.json")
+	st := offsite.Status{LastAttempt: time.Now().Add(-time.Minute), LastSuccess: time.Now().Add(-time.Hour), KeyID: "0123456789abcdef"}
+	status := func() (int, string) {
+		t.Helper()
+		if err := offsite.WriteStatus(statusFile, st); err != nil {
+			t.Fatal(err)
+		}
+		var out, errb bytes.Buffer
+		code := cmdOffsite([]string{"status", "-config", cfg, "-status-file", statusFile}, &out, &errb)
+		return code, out.String() + errb.String()
+	}
+	if code, out := status(); code != 0 || strings.Contains(out, "refused") {
+		t.Fatalf("a recent success: exit %d\n%s", code, out)
+	}
+	st.LastError, st.KeyMismatch = "offsite: the bucket's objects are sealed with another key", true
+	code, out := status()
+	if code != 1 || !strings.Contains(out, "The last run was refused") || !strings.Contains(out, "key id 0123456789abcdef") || strings.Contains(out, "bucket holds:") {
+		t.Fatalf("a refused run: exit %d\n%s", code, out)
+	}
+}
+
 func TestPrintDBBundlesSaysWhatEachHolds(t *testing.T) {
 	rawKey, _ := offsite.NewKey()
 	key, _ := offsite.ParseKey(rawKey)
diff --git a/deploy/bootstrap.sh b/deploy/bootstrap.sh
index 0347fde..b47a183 100644
--- a/deploy/bootstrap.sh
+++ b/deploy/bootstrap.sh
@@ -4541,8 +4541,9 @@ quiet_watchdog() {
 }
 
 # The hourly off-site copy. The bucket is checked now, in the install, so wrong
-# credentials or an unreachable endpoint show up here; the first copy itself runs in the
-# background, since a host with many archives can take a long while to upload them.
+# credentials, an unreachable endpoint or a key other than the one its objects are sealed
+# with show up here; the first copy itself runs in the background, since a host with many
+# archives can take a long while to upload them.
 # With no bucket configured the timer is removed (the operator deleted [offsite]) and the
 # install says loudly that every backup is on this machine only.
 install_offsite_timer() {
@@ -4585,12 +4586,33 @@ WantedBy=timers.target
 EOF
   systemctl daemon-reload
   systemctl enable --now felis-offsite.timer
-  if "$HOST_BIN" offsite list -config "${STATE_DIR}/felis.host.toml" -env-file "$OFFSITE_ENV" >/dev/null; then
-    systemctl start --no-block felis-offsite.service
-    ok "off-site copy: hourly to the [offsite] bucket, first copy started (sudo felis offsite status; journalctl -u felis-offsite)"
-  else
-    warn "the [offsite] bucket did not answer (error above); nothing is copied off this machine until it does: fix ${OFFSITE_ENV} or [offsite] in ${STATE_DIR}/felis.host.toml, then sudo systemctl start felis-offsite.service"
-  fi
+  local rc=0
+  "$HOST_BIN" offsite check-key -config "${STATE_DIR}/felis.host.toml" -env-file "$OFFSITE_ENV" >/dev/null || rc=$?
+  case "$rc" in
+    0)
+      systemctl start --no-block felis-offsite.service
+      ok "off-site copy: hourly to the [offsite] bucket, first copy started (sudo felis offsite status; journalctl -u felis-offsite)"
+      ;;
+    3)
+      # The timer stays: every run is refused (and reported by the watchdog) until the
+      # key is fixed, and the first run after that needs no re-run of the installer.
+      OFFSITE_KEY_MISMATCH=1
+      warn "================================================================================"
+      warn "The [offsite] bucket's objects are sealed with another key than the one in"
+      warn "${OFFSITE_ENV} (felis offsite check-key, above). Nothing is copied off this"
+      warn "machine, and nothing in the bucket is written or pruned, until the keys match."
+      if [ "${OFFSITE_KEY_NEW:-0}" = 1 ]; then
+        warn "This run generated that key: neither FELIS_OFFSITE_KEY nor ${OFFSITE_ENV} had one."
+      fi
+      warn "Set FELIS_OFFSITE_KEY in ${OFFSITE_ENV} to the key the bucket was written with and run"
+      warn "sudo systemctl start felis-offsite.service, or give [offsite] an empty bucket or prefix"
+      warn "and re-run the installer (docs/troubleshooting.md §16)."
+      warn "================================================================================"
+      ;;
+    *)
+      warn "the [offsite] bucket did not answer (error above); nothing is copied off this machine until it does: fix ${OFFSITE_ENV} or [offsite] in ${STATE_DIR}/felis.host.toml, then sudo systemctl start felis-offsite.service"
+      ;;
+  esac
 }
 
 # summary_offsite is the installer's last word on where the backups live.
@@ -4601,6 +4623,13 @@ summary_offsite() {
     warn "FELIS_OFFSITE_ACCESS_KEY and FELIS_OFFSITE_SECRET_KEY and re-run (docs/troubleshooting.md §16)."
     return 0
   fi
+  if [ "${OFFSITE_KEY_MISMATCH:-0}" = 1 ]; then
+    # A key the bucket refuses is no key to store; this run's is in OFFSITE_ENV if the
+    # operator moves to an empty bucket instead.
+    warn "OFF-SITE COPY STOPPED: the bucket's objects are sealed with another key (see above)."
+    warn "Every backup is on this machine only until ${OFFSITE_ENV} holds that key (sudo felis offsite check-key)."
+    return 0
+  fi
   if [ "${OFFSITE_KEY_NEW:-0}" = 1 ]; then
     echo
     warn "================================================================================"
@@ -4789,10 +4818,12 @@ EOF
 # replace the file's (rotating the bucket credentials is a re-run); the encryption key is
 # generated when neither has one. A different key than the file's is refused: every object
 # already in the bucket is sealed with the old one, and swapping it would make them
-# unreadable without a word.
+# unreadable without a word. Whether the bucket's objects agree with the key is checked once
+# the binary is installed (install_offsite_timer).
 configure_offsite() {
   OFFSITE_ENABLED=0
   OFFSITE_KEY_NEW=0
+  OFFSITE_KEY_MISMATCH=0
   offsite_enabled || return 0
   OFFSITE_ENABLED=1
   local env_ak="${FELIS_OFFSITE_ACCESS_KEY:-}" env_sk="${FELIS_OFFSITE_SECRET_KEY:-}" env_key="${FELIS_OFFSITE_KEY:-}"
@@ -4806,7 +4837,7 @@ configure_offsite() {
   FELIS_OFFSITE_ACCESS_KEY="${env_ak:-$FELIS_OFFSITE_ACCESS_KEY}"
   FELIS_OFFSITE_SECRET_KEY="${env_sk:-$FELIS_OFFSITE_SECRET_KEY}"
   if [ -n "$env_key" ] && [ -n "$file_key" ] && [ "$env_key" != "$file_key" ]; then
-    die "FELIS_OFFSITE_KEY differs from the key in ${OFFSITE_ENV}; the objects already in the bucket are sealed with that one. Unset FELIS_OFFSITE_KEY to keep it (docs/troubleshooting.md §16)"
+    die "FELIS_OFFSITE_KEY differs from the key in ${OFFSITE_ENV}, which sealed what this host copied to the bucket. Unset FELIS_OFFSITE_KEY to keep it; when felis offsite check-key says the bucket's objects are sealed with another key, set FELIS_OFFSITE_KEY in ${OFFSITE_ENV} by hand instead (docs/troubleshooting.md §16)"
   fi
   FELIS_OFFSITE_KEY="${env_key:-$file_key}"
   if [ -z "$FELIS_OFFSITE_ACCESS_KEY" ] || [ -z "$FELIS_OFFSITE_SECRET_KEY" ]; then
diff --git a/deploy/bootstrap_test.sh b/deploy/bootstrap_test.sh
index 97fe94e..b2802e8 100644
--- a/deploy/bootstrap_test.sh
+++ b/deploy/bootstrap_test.sh
@@ -1997,7 +1997,8 @@ run_offsite() { # script; runs with the off-site functions sourced
     die() { printf "DIE: %s\n" "$*"; exit 1; }
     log() { printf "LOG: %s\n" "$*"; }; ok() { printf "OK: %s\n" "$*"; }; warn() { printf "WARN: %s\n" "$*"; }
     systemctl() { printf "SYSTEMCTL: %s\n" "$*" >&2; }
-    fakefelis() { printf "RUN: %s\n" "$*" >&2; [ -z "${CHECK_FAILS:-}" ] || { echo "bucket: access denied" >&2; return 1; }; }
+    fakefelis() { printf "RUN: %s\n" "$*" >&2; [ -z "${CHECK_FAILS:-}" ] || { echo "bucket: access denied" >&2; return 1; }
+      [ -z "${KEY_MISMATCH:-}" ] || { echo "the bucket records key id 1111111111111111, this key is 2222222222222222" >&2; return 3; }; }
     FELIS_OFFSITE_ENDPOINT="${FELIS_OFFSITE_ENDPOINT:-}" FELIS_OFFSITE_BUCKET="${FELIS_OFFSITE_BUCKET:-}"
     FELIS_OFFSITE_REGION="${FELIS_OFFSITE_REGION:-}" FELIS_OFFSITE_PREFIX="${FELIS_OFFSITE_PREFIX:-}"
     FELIS_OFFSITE_DB_KEEP="${FELIS_OFFSITE_DB_KEEP:-}"
@@ -2108,7 +2109,7 @@ expect "a pass over a big archive is not cut off at the hour" "TimeoutStartSec=2
 expect "the copy runs hourly" "OnCalendar=hourly" "$timer"
 expect "a missed run catches up at boot" "Persistent=true" "$timer"
 expect "the timer is enabled" "SYSTEMCTL: enable --now felis-offsite.timer" "$out"
-expect "the bucket is checked during the install" "RUN: offsite list -config $odir/felis.host.toml -env-file $odir/offsite.env" "$out"
+expect "the bucket and its key are checked during the install" "RUN: offsite check-key -config $odir/felis.host.toml -env-file $odir/offsite.env" "$out"
 expect "the first copy runs in the background" "SYSTEMCTL: start --no-block felis-offsite.service" "$out"
 
 out="$(OFFSITE_ENABLED=1 CHECK_FAILS=1 run_offsite install_offsite_timer)"
@@ -2119,6 +2120,28 @@ case "$out" in
   *) echo "PASS a failed check does not start the copy" ;;
 esac
 
+# A reinstall that lost offsite.env generates a fresh key; a bucket sealed with the old one
+# must stop the install's copy and say what to do, not read as an unreachable bucket.
+out="$(OFFSITE_ENABLED=1 OFFSITE_KEY_NEW=1 FELIS_OFFSITE_KEY=NEWKEY KEY_MISMATCH=1 run_offsite 'install_offsite_timer; summary_offsite')"
+expect "a key mismatch shows both ids" "the bucket records key id 1111111111111111, this key is 2222222222222222" "$out"
+expect "a key mismatch is a loud warning" "WARN: The [offsite] bucket's objects are sealed with another key than the one in" "$out"
+expect "a key mismatch says the key was generated by this run" "WARN: This run generated that key" "$out"
+expect "a key mismatch says how to fix it" "WARN: Set FELIS_OFFSITE_KEY in $odir/offsite.env to the key the bucket was written with" "$out"
+expect "the summary says the copy stopped" "WARN: OFF-SITE COPY STOPPED" "$out"
+expect "the timer stays for the run after the fix" "SYSTEMCTL: enable --now felis-offsite.timer" "$out"
+case "$out" in
+  *"start --no-block"* | *"did not answer"* | *"Store it NOW"* | *NEWKEY*)
+    echo "FAIL a key mismatch started the copy, read as an unreachable bucket or showed the refused key: $out"; fails=$((fails + 1)) ;;
+  *) echo "PASS a key mismatch neither starts the copy nor asks to store the refused key" ;;
+esac
+out="$(OFFSITE_ENABLED=1 KEY_MISMATCH=1 run_offsite install_offsite_timer)"
+case "$out" in
+  *"This run generated"*) echo "FAIL a kept key was called generated"; fails=$((fails + 1)) ;;
+  *) echo "PASS a kept key that mismatches is not called generated" ;;
+esac
+out="$(OFFSITE_ENABLED=1 OFFSITE_KEY_NEW=1 FELIS_OFFSITE_KEY=NEWKEY run_offsite 'install_offsite_timer; summary_offsite')"
+expect "a key the bucket takes is shown once" "WARN:     FELIS_OFFSITE_KEY=NEWKEY" "$out"
+
 out="$(OFFSITE_ENABLED=0 run_offsite 'systemctl() { printf "SYSTEMCTL: %s\n" "$*" >> "$STATE_DIR/systemctl.log"; }; install_offsite_timer')"
 expect "removing [offsite] disables the timer" "SYSTEMCTL: disable --now felis-offsite.timer" "$(cat "$odir/systemctl.log")"
 expect "removing [offsite] says so" "WARN: no [offsite] bucket is configured any more" "$out"
diff --git a/docs/troubleshooting.md b/docs/troubleshooting.md
index fc2174f..cf9e34b 100644
--- a/docs/troubleshooting.md
+++ b/docs/troubleshooting.md
@@ -2148,7 +2148,8 @@ host yourself, plus the off-site encryption key if the copy is in the bucket.
    checks it (`felis db verify`) and names it, with when it was taken, the
    release and schema that took it, and how many accounts and servers its
    database holds. Read those before going on. A wrong key fails with
-   `object does not decrypt with this key` and writes nothing. For a copy you
+   `object does not decrypt with this key` (naming the bucket's key id and this
+   key's, once the bucket records one) and writes nothing. For a copy you
    made yourself, check it with `sha256sum -c felis-db-....tar.sha256`.
 
    `latest` is the newest bundle, unless that one holds no servers and at
@@ -2266,6 +2267,21 @@ host that has a bucket, and removed `felis-offsite.timer` as they did; a re-run
 of the current installer puts it back. `systemctl list-timers felis-offsite.timer`
 shows whether the timer is there. [SH-TESTED] [VM-TESTED: a re-run that lost the race]
 
+The bucket records which key sealed it: `felis-key-id`, the one object kept in
+the clear, holds the key's id (the `key id:` of `felis offsite status`), which
+names the key without revealing it. The first sync writes it; a bucket from
+before it was recorded is judged by whether the key opens its newest objects.
+The installer runs `felis offsite check-key`, and every sync checks before
+anything else. A key other than the bucket's (a reinstall that lost
+`offsite.env` and was given no `FELIS_OFFSITE_KEY` generates a new one) stops
+the sync before it copies or prunes anything, the installer ends with
+`OFF-SITE COPY STOPPED`, `status` exits 1 and the watchdog mails the owners
+at once. Set `FELIS_OFFSITE_KEY` in `/etc/felis/offsite.env` to the bucket's
+key and `sudo systemctl start felis-offsite.service`, or give `[offsite]` an
+empty bucket or prefix and re-run the installer.
+[GO-TESTED: `TestSyncRefusesAnotherKeysBucket`, `TestCheckKey`] [SH-TESTED]
+[VM-TESTED: MinIO, the other key's sync refused with the bucket unchanged, check-key 0/3, fetch-db naming both ids]
+
 What runs:
 
 - **`felis-offsite.timer`** runs `felis offsite sync` hourly (plus up to
@@ -2306,6 +2322,7 @@ What runs:
   `registry/index/<stamp>.json.fenc`, `uploads/blobs/<sha256>.fenc` and
   `uploads/index/<stamp>.json.fenc`: AES-256-GCM in 64 KiB segments, so
   truncation, reordering and a wrong key are all refused on the way back.
+  `felis-key-id` next to them holds the key's id in the clear.
 - A pass sends the database bundles first, then world archives, images and
   uploads. Each object has its own time limit: 10 minutes plus its size at
   512 KiB/s (about 6 hours for 10 GiB). An archive the uplink cannot send in
@@ -2323,6 +2340,7 @@ Checking it:
 ```
 sudo felis offsite status        # last run, errors, what the bucket holds, what waits
 sudo felis offsite list          # the bundles, image lists and upload lists in the bucket, newest first
+sudo felis offsite check-key     # whether offsite.env holds the key the bucket was sealed with
 sudo journalctl -u felis-offsite -n 50 --no-pager
 sudo systemctl start felis-offsite.service   # run one now
 ```
diff --git a/internal/offsite/keymark.go b/internal/offsite/keymark.go
new file mode 100644
index 0000000..6c4fa16
--- /dev/null
+++ b/internal/offsite/keymark.go
@@ -0,0 +1,152 @@
+package offsite
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"regexp"
+	"sort"
+	"strings"
+)
+
+// keyMark is the one object the bucket holds in the clear: the KeyID of the
+// key every other object is sealed with. The id names the key without
+// revealing it, so a host holding another key (a reinstall that generated a
+// fresh one, an offsite.env from elsewhere) is refused before it writes an
+// object or prunes one the right key still opens.
+const keyMark = "felis-key-id"
+
+// ErrKeyMismatch is a bucket whose objects are sealed with another key.
+var ErrKeyMismatch = errors.New("offsite: the bucket's objects are sealed with another key")
+
+const keyMismatchFix = "set FELIS_OFFSITE_KEY in /etc/felis/offsite.env to the key they were sealed with, or point [offsite] at an empty bucket or prefix (docs/troubleshooting.md §16)"
+
+var keyIDPattern = regexp.MustCompile(`^[0-9a-f]{16}$`)
+
+// KeyFit is how CheckKey matched a key to a bucket. The zero value is no match:
+// what CheckKey returns with an error.
+type KeyFit int
+
+const (
+	// KeyRecorded: the bucket records this key's id.
+	KeyRecorded KeyFit = iota + 1
+	// KeyOpens: the bucket records no id, and the key opens its newest objects.
+	KeyOpens
+	// KeyUnused: the bucket holds no sealed object yet.
+	KeyUnused
+)
+
+// A bucket without a recorded id is judged by its newest sealed objects: the
+// key must open the first segment of one of them. keyRefusals objects that
+// refuse the key decide a mismatch; at most keyTries are read, so a run of
+// damaged objects cannot stall the check.
+const (
+	keyRefusals = 3
+	keyTries    = 10
+)
+
+// BucketKeyID reads the key id the bucket records, "" when it records none.
+func BucketKeyID(ctx context.Context, b Bucket) (string, error) {
+	rc, err := b.Get(ctx, keyMark)
+	if errors.Is(err, ErrNotFound) {
+		return "", nil
+	}
+	if err != nil {
+		return "", fmt.Errorf("read %s: %w", keyMark, err)
+	}
+	defer rc.Close()
+	raw, err := io.ReadAll(io.LimitReader(rc, 256))
+	if err != nil {
+		return "", fmt.Errorf("read %s: %w", keyMark, err)
+	}
+	id := strings.TrimSpace(string(raw))
+	if !keyIDPattern.MatchString(id) {
+		return "", fmt.Errorf("offsite: %s in the bucket is not a key id Felis wrote", keyMark)
+	}
+	return id, nil
+}
+
+// errOpened stops Decrypt once the first segment has authenticated.
+var errOpened = errors.New("offsite: first segment opened")
+
+type firstSegment struct{}
+
+func (firstSegment) Write([]byte) (int, error) { return 0, errOpened }
+
+// CheckKey tells whether key is the one the bucket's objects are sealed with.
+// It writes nothing; a mismatch is ErrKeyMismatch.
+func CheckKey(ctx context.Context, b Bucket, key []byte) (KeyFit, error) {
+	mine := KeyID(key)
+	id, err := BucketKeyID(ctx, b)
+	if err != nil {
+		return 0, err
+	}
+	if id != "" {
+		if id != mine {
+			return 0, fmt.Errorf("%w: the bucket records key id %s, this key is %s; %s", ErrKeyMismatch, id, mine, keyMismatchFix)
+		}
+		return KeyRecorded, nil
+	}
+	var sealed []Object
+	for _, dir := range []string{dbDir, worldsDir, registryDir, uploadsDir} {
+		objs, err := b.List(ctx, dir)
+		if err != nil {
+			return 0, fmt.Errorf("list %s: %w", dir, err)
+		}
+		for _, o := range objs {
+			if strings.HasSuffix(o.Key, objExt) {
+				sealed = append(sealed, o)
+			}
+		}
+	}
+	if len(sealed) == 0 {
+		return KeyUnused, nil
+	}
+	sort.Slice(sealed, func(i, j int) bool { return sealed[i].Modified.After(sealed[j].Modified) })
+	var refused []string
+	var unjudged error
+	for i, o := range sealed {
+		if i == keyTries || len(refused) == keyRefusals {
+			break
+		}
+		rc, err := b.Get(ctx, o.Key)
+		if errors.Is(err, ErrNotFound) {
+			continue // pruned since the listing
+		}
+		if err != nil {
+			return 0, fmt.Errorf("%s: %w", o.Key, err)
+		}
+		err = Decrypt(firstSegment{}, rc, key)
+		rc.Close()
+		switch {
+		case err == nil || errors.Is(err, errOpened):
+			return KeyOpens, nil
+		case errors.Is(err, ErrAuth):
+			refused = append(refused, o.Key)
+		case unjudged == nil:
+			unjudged = fmt.Errorf("%s: %w", o.Key, err)
+		}
+	}
+	if len(refused) > 0 {
+		return 0, fmt.Errorf("%w: this key (key id %s) opens none of %s; %s", ErrKeyMismatch, mine, strings.Join(refused, ", "), keyMismatchFix)
+	}
+	if unjudged != nil {
+		return 0, fmt.Errorf("offsite: cannot tell which key sealed the bucket's objects: %w", unjudged)
+	}
+	return KeyUnused, nil
+}
+
+// ClaimKey is CheckKey, then records key's id in a bucket that has none, so
+// that every later check reads the id.
+func ClaimKey(ctx context.Context, b Bucket, key []byte) error {
+	fit, err := CheckKey(ctx, b, key)
+	if err != nil || fit == KeyRecorded {
+		return err
+	}
+	id := KeyID(key) + "\n"
+	if err := b.Put(ctx, keyMark, strings.NewReader(id), int64(len(id))); err != nil {
+		return fmt.Errorf("record the key id in %s: %w", keyMark, err)
+	}
+	return nil
+}
diff --git a/internal/offsite/keymark_test.go b/internal/offsite/keymark_test.go
new file mode 100644
index 0000000..b831b5e
--- /dev/null
+++ b/internal/offsite/keymark_test.go
@@ -0,0 +1,248 @@
+package offsite
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"strings"
+	"testing"
+	"time"
+)
+
+// putAt stores data under key as modified at t0 plus min minutes.
+func putAt(b *memBucket, key string, data []byte, min int) {
+	if b.modified == nil {
+		b.modified = map[string]time.Time{}
+	}
+	b.objs[key] = data
+	b.modified[key] = now.Add(time.Duration(min) * time.Minute)
+}
+
+func TestCheckKey(t *testing.T) {
+	key, other := testKey(t), testKey(t)
+	big := make([]byte, 3*segmentSize)
+	tailDamaged := seal(t, big, key)
+	tailDamaged[len(tailDamaged)-1] ^= 1
+	for _, tc := range []struct {
+		what   string
+		bucket func(b *memBucket)
+		fit    KeyFit
+		err    string   // "" for none
+		named  []string // what a refusal names
+		skip   []string // what it must leave out
+	}{
+		{what: "an empty bucket", bucket: func(b *memBucket) {}, fit: KeyUnused},
+		{what: "nothing Felis sealed", bucket: func(b *memBucket) { putAt(b, "db/notes.txt", []byte("hi"), 0) }, fit: KeyUnused},
+		{
+			what: "the recorded id", fit: KeyRecorded,
+			bucket: func(b *memBucket) {
+				b.objs[keyMark] = []byte(KeyID(key) + "\n")
+				putAt(b, "worlds/a.tar.gz.fenc", seal(t, []byte("a"), other), 0) // the id is the judge
+			},
+		},
+		{
+			what:   "another recorded id",
+			bucket: func(b *memBucket) { b.objs[keyMark] = []byte(KeyID(other) + "\n") },
+			err:    "sealed with another key", named: []string{KeyID(other), KeyID(key), "offsite.env"},
+		},
+		{
+			what:   "a marker Felis did not write",
+			bucket: func(b *memBucket) { b.objs[keyMark] = []byte("hello") },
+			err:    "not a key id Felis wrote",
+		},
+		{
+			what: "an unmarked bucket the key opens", fit: KeyOpens,
+			bucket: func(b *memBucket) {
+				putAt(b, "db/felis-db-20260101T030000Z-daily.tar.fenc", seal(t, []byte("old"), other), 0)
+				putAt(b, "registry/blobs/aa.fenc", seal(t, []byte("new"), key), 10)
+			},
+		},
+		{
+			what: "only the first segment is read", fit: KeyOpens,
+			bucket: func(b *memBucket) { putAt(b, "uploads/blobs/bb.fenc", tailDamaged, 0) },
+		},
+		{
+			what: "an unmarked bucket sealed with another key",
+			bucket: func(b *memBucket) {
+				for i, k := range []string{"worlds/1.fenc", "worlds/2.fenc", "db/3.fenc", "uploads/index/4.json.fenc"} {
+					putAt(b, k, seal(t, []byte(k), other), i)
+				}
+			},
+			err: "sealed with another key", named: []string{"uploads/index/4.json.fenc", "db/3.fenc", "worlds/2.fenc", KeyID(key)}, skip: []string{"worlds/1.fenc"},
+		},
+		{
+			what: "two refusals, then an object the key opens", fit: KeyOpens,
+			bucket: func(b *memBucket) {
+				putAt(b, "worlds/1.fenc", seal(t, []byte("1"), key), 0)
+				putAt(b, "worlds/2.fenc", seal(t, []byte("2"), other), 1)
+				putAt(b, "worlds/3.fenc", seal(t, []byte("3"), other), 2)
+			},
+		},
+		{
+			what: "damaged objects are passed over", fit: KeyOpens,
+			bucket: func(b *memBucket) {
+				putAt(b, "worlds/1.fenc", seal(t, []byte("1"), key), 0)
+				putAt(b, "worlds/2.fenc", []byte("partial"), 1)
+			},
+		},
+		{
+			what: "a refusal among damaged objects",
+			bucket: func(b *memBucket) {
+				putAt(b, "worlds/1.fenc", seal(t, []byte("1"), other), 0)
+				putAt(b, "worlds/2.fenc", []byte("partial"), 1)
+			},
+			err: "sealed with another key", named: []string{"worlds/1.fenc"},
+		},
+		{
+			what: "nothing it can judge",
+			bucket: func(b *memBucket) {
+				putAt(b, "worlds/1.fenc", seal(t, []byte("1"), key), 0)
+				for i := range keyTries {
+					putAt(b, fmt.Sprintf("worlds/junk-%d.fenc", i), []byte("partial"), 1+i)
+				}
+			},
+			err: "cannot tell which key", named: []string{"worlds/junk-"},
+		},
+	} {
+		t.Run(tc.what, func(t *testing.T) {
+			b := newMemBucket()
+			tc.bucket(b)
+			fit, err := CheckKey(context.Background(), b, key)
+			if b.puts != 0 {
+				t.Errorf("CheckKey wrote %d objects", b.puts)
+			}
+			if tc.err == "" {
+				if err != nil || fit != tc.fit {
+					t.Fatalf("CheckKey = %v, %v; want %v", fit, err, tc.fit)
+				}
+				return
+			}
+			if err == nil || !strings.Contains(err.Error(), tc.err) {
+				t.Fatalf("CheckKey = %v, %v; want an error saying %q", fit, err, tc.err)
+			}
+			if mismatch := tc.err == "sealed with another key"; errors.Is(err, ErrKeyMismatch) != mismatch {
+				t.Errorf("errors.Is(err, ErrKeyMismatch) = %v, want %v: %v", !mismatch, mismatch, err)
+			}
+			for _, s := range tc.named {
+				if !strings.Contains(err.Error(), s) {
+					t.Errorf("error lacks %q: %v", s, err)
+				}
+			}
+			for _, s := range tc.skip {
+				if strings.Contains(err.Error(), s) {
+					t.Errorf("error names %q: %v", s, err)
+				}
+			}
+		})
+	}
+
+	// A bucket that does not answer is not a mismatch.
+	b := newMemBucket()
+	putAt(b, "worlds/1.fenc", seal(t, []byte("1"), other), 0)
+	down := errors.New("connection reset")
+	b.getErr = map[string]error{"worlds/1.fenc": down}
+	if _, err := CheckKey(context.Background(), b, key); !errors.Is(err, down) || errors.Is(err, ErrKeyMismatch) {
+		t.Errorf("unreachable object: err = %v, want the read error", err)
+	}
+	b.getErr = map[string]error{keyMark: down}
+	if _, err := CheckKey(context.Background(), b, key); !errors.Is(err, down) || errors.Is(err, ErrKeyMismatch) {
+		t.Errorf("unreachable marker: err = %v, want the read error", err)
+	}
+
+	// An object pruned between the listing and the read is passed over.
+	b = newMemBucket()
+	putAt(b, "worlds/1.fenc", seal(t, []byte("1"), key), 0)
+	putAt(b, "db/2.fenc", seal(t, []byte("2"), key), 1)
+	b.getErr = map[string]error{"db/2.fenc": fmt.Errorf("%w: db/2.fenc", ErrNotFound)}
+	if fit, err := CheckKey(context.Background(), b, key); fit != KeyOpens || err != nil {
+		t.Errorf("an object gone since the listing: CheckKey = %v, %v; want KeyOpens", fit, err)
+	}
+}
+
+func TestClaimKey(t *testing.T) {
+	key, other := testKey(t), testKey(t)
+	marker := func(b *memBucket) string { return string(b.objs[keyMark]) }
+
+	b := newMemBucket()
+	if err := ClaimKey(context.Background(), b, key); err != nil {
+		t.Fatal(err)
+	}
+	if marker(b) != KeyID(key)+"\n" {
+		t.Fatalf("empty bucket: marker = %q, want %s", marker(b), KeyID(key))
+	}
+	if err := ClaimKey(context.Background(), b, key); err != nil || b.puts != 1 {
+		t.Errorf("second claim: err %v, puts %d; want the marker written once", err, b.puts)
+	}
+	if fit, err := CheckKey(context.Background(), b, key); fit != KeyRecorded || err != nil {
+		t.Errorf("after the claim: CheckKey = %v, %v", fit, err)
+	}
+	if err := ClaimKey(context.Background(), b, other); !errors.Is(err, ErrKeyMismatch) || marker(b) != KeyID(key)+"\n" {
+		t.Errorf("another key: err %v, marker %q; want a refusal that leaves the marker", err, marker(b))
+	}
+
+	b = newMemBucket()
+	putAt(b, "worlds/1.fenc", seal(t, []byte("1"), key), 0)
+	if err := ClaimKey(context.Background(), b, key); err != nil || marker(b) != KeyID(key)+"\n" {
+		t.Errorf("unmarked bucket the key opens: err %v, marker %q", err, marker(b))
+	}
+
+	b = newMemBucket()
+	putAt(b, "worlds/1.fenc", seal(t, []byte("1"), other), 0)
+	if err := ClaimKey(context.Background(), b, key); !errors.Is(err, ErrKeyMismatch) || b.puts != 0 {
+		t.Errorf("unmarked bucket under another key: err %v, puts %d; want a refusal that writes nothing", err, b.puts)
+	}
+}
+
+// TestSyncRefusesAnotherKeysBucket: a host whose key does not match the
+// bucket's objects neither copies into it nor prunes what only the right key
+// opens.
+func TestSyncRefusesAnotherKeysBucket(t *testing.T) {
+	for _, marked := range []bool{true, false} {
+		t.Run(fmt.Sprintf("marked=%v", marked), func(t *testing.T) {
+			cat := &fakeCatalog{rows: []*row{
+				{WorldBackup: WorldBackup{ID: "b1", Server: "alpha", Ref: "/a/alpha-1.tar.gz"}, status: "present"},
+			}}
+			s, b := newSyncer(t, cat)
+			other := testKey(t)
+			delete(b.objs, keyMark)
+			if marked {
+				b.objs[keyMark] = []byte(KeyID(other) + "\n")
+			}
+			writeFile(t, s.ArchiveDir, "alpha-1.tar.gz", 100)
+			writeFile(t, s.DBDir, "felis-db-20260924T030000Z-daily.tar", 50)
+			for i, name := range []string{"felis-db-20260901T030000Z-daily.tar", "felis-db-20260902T030000Z-daily.tar", "felis-db-20260903T030000Z-daily.tar"} {
+				putAt(b, DBKey(name), seal(t, []byte(name), other), i)
+			}
+			before := len(b.objs)
+
+			_, err := s.Run(context.Background())
+			if !errors.Is(err, ErrKeyMismatch) {
+				t.Fatalf("Run error = %v, want ErrKeyMismatch", err)
+			}
+			if len(b.started) != 0 || len(b.removed) != 0 || len(b.objs) != before {
+				t.Errorf("the refused run began puts %v and removed %v", b.started, b.removed)
+			}
+			if !cat.rows[0].offsite.IsZero() {
+				t.Error("the refused run recorded alpha as copied")
+			}
+		})
+	}
+}
+
+// TestSyncRecordsTheKey: the first run into a bucket records its key before
+// the first upload.
+func TestSyncRecordsTheKey(t *testing.T) {
+	s, b := newSyncer(t, &fakeCatalog{})
+	delete(b.objs, keyMark)
+	writeFile(t, s.DBDir, "felis-db-20260924T030000Z-daily.tar", 50)
+	if _, err := s.Run(context.Background()); err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(b.objs[keyMark], []byte(KeyID(s.Key)+"\n")) {
+		t.Fatalf("marker = %q, want %s", b.objs[keyMark], KeyID(s.Key))
+	}
+	if len(b.started) != 2 || b.started[0] != keyMark {
+		t.Errorf("puts began in the order %v, want the marker, then the bundle", b.started)
+	}
+}
diff --git a/internal/offsite/offsite_test.go b/internal/offsite/offsite_test.go
index 84088d9..80d2d4b 100644
--- a/internal/offsite/offsite_test.go
+++ b/internal/offsite/offsite_test.go
@@ -122,6 +122,10 @@ type memBucket struct {
 	stall map[string]bool
 	// started lists every Put in the order it began.
 	started []string
+	// modified is what List reports as each key's modification time.
+	modified map[string]time.Time
+	// getErr fails Get for a key the way an unreachable bucket would.
+	getErr map[string]error
 }
 
 func newMemBucket() *memBucket {
@@ -157,6 +161,9 @@ func (b *memBucket) Put(ctx context.Context, key string, r io.Reader, size int64
 func (b *memBucket) Get(_ context.Context, key string) (io.ReadCloser, error) {
 	b.mu.Lock()
 	defer b.mu.Unlock()
+	if err := b.getErr[key]; err != nil {
+		return nil, err
+	}
 	data, ok := b.objs[key]
 	if !ok {
 		return nil, fmt.Errorf("%w: %s", ErrNotFound, key)
@@ -170,7 +177,7 @@ func (b *memBucket) List(_ context.Context, prefix string) ([]Object, error) {
 	var out []Object
 	for k, v := range b.objs {
 		if strings.HasPrefix(k, prefix) {
-			out = append(out, Object{Key: k, Size: int64(len(v))})
+			out = append(out, Object{Key: k, Size: int64(len(v)), Modified: b.modified[k]})
 		}
 	}
 	sort.Slice(out, func(i, j int) bool { return out[i].Key < out[j].Key })
@@ -245,11 +252,15 @@ func writeFile(t *testing.T, dir, name string, size int) []byte {
 
 var now = time.Date(2026, 9, 24, 12, 0, 0, 0, time.UTC)
 
+// newSyncer syncs into a bucket that already records its key, as every bucket
+// does after its first run.
 func newSyncer(t *testing.T, cat *fakeCatalog) (*Syncer, *memBucket) {
 	t.Helper()
 	b := newMemBucket()
+	key := testKey(t)
+	b.objs[keyMark] = []byte(KeyID(key) + "\n")
 	return &Syncer{
-		Bucket: b, Catalog: cat, Key: testKey(t),
+		Bucket: b, Catalog: cat, Key: key,
 		ArchiveDir: t.TempDir(), DBDir: t.TempDir(), DBKeep: 2,
 		Now: func() time.Time { return now },
 	}, b
@@ -393,7 +404,7 @@ func TestSyncDBKeepsNewest(t *testing.T) {
 		keys = append(keys, k)
 	}
 	sort.Strings(keys)
-	want := "db/felis-db-20260923T030000Z-manual.tar.fenc db/felis-db-20260924T030000Z-daily.tar.fenc db/notes.txt"
+	want := "db/felis-db-20260923T030000Z-manual.tar.fenc db/felis-db-20260924T030000Z-daily.tar.fenc db/notes.txt " + keyMark
 	if strings.Join(keys, " ") != want {
 		t.Fatalf("bucket = %v, want %s", keys, want)
 	}
diff --git a/internal/offsite/status.go b/internal/offsite/status.go
index 81568d6..3a6b174 100644
--- a/internal/offsite/status.go
+++ b/internal/offsite/status.go
@@ -28,6 +28,10 @@ type Status struct {
 	Prefix      string    `json:"prefix,omitempty"`
 	KeyID       string    `json:"key_id"`
 	Result      Result    `json:"result"`
+	// KeyMismatch is a run refused because the bucket's objects are sealed
+	// with another key (ErrKeyMismatch): no later run copies anything until
+	// the key is fixed, so the watchdog reports it at once.
+	KeyMismatch bool `json:"key_mismatch,omitempty"`
 }
 
 // ReadStatus reads the status file. A missing file is (nil, nil): no sync has
diff --git a/internal/offsite/sync.go b/internal/offsite/sync.go
index 3b4d5c5..d468d62 100644
--- a/internal/offsite/sync.go
+++ b/internal/offsite/sync.go
@@ -200,6 +200,11 @@ func (s *Syncer) Run(ctx context.Context) (Result, error) {
 		s.logf("%s", msg)
 	}
 
+	// Before anything is written or pruned: objects sealed with another key
+	// are copies only that key opens, and DBKeep would prune them.
+	if err := ClaimKey(ctx, s.Bucket, s.Key); err != nil {
+		return res, err
+	}
 	remoteWorlds, err := s.listSizes(ctx, worldsDir)
 	if err != nil {
 		return res, fmt.Errorf("list %s in the bucket: %w", worldsDir, err)
diff --git a/internal/watchdog/probes.go b/internal/watchdog/probes.go
index fce910f..a2dc404 100644
--- a/internal/watchdog/probes.go
+++ b/internal/watchdog/probes.go
@@ -360,6 +360,14 @@ func OffsiteFinding(statusFile string, now time.Time) *Finding {
 			Hint:      hint,
 		}
 	}
+	if st != nil && st.KeyMismatch {
+		return &Finding{
+			Key: "offsite", Severity: Warning, For: backupFor,
+			Summary:   "异地备份已停止：桶里的对象是用另一把密钥加密的，本机不往桶里写入也不清理任何对象（" + st.LastError + "）",
+			SummaryEN: "the off-site copy has stopped: the bucket's objects are sealed with another key, and this host writes and prunes nothing there (" + st.LastError + ")",
+			Hint:      "`sudo felis offsite check-key`; set FELIS_OFFSITE_KEY in /etc/felis/offsite.env to the key the bucket was written with (docs/troubleshooting.md §16)",
+		}
+	}
 	if st != nil && !st.LastSuccess.IsZero() && now.Sub(st.LastSuccess) <= offsite.StaleAfter {
 		return nil
 	}
diff --git a/internal/watchdog/probes_test.go b/internal/watchdog/probes_test.go
index 72d7f04..26f9fa8 100644
--- a/internal/watchdog/probes_test.go
+++ b/internal/watchdog/probes_test.go
@@ -183,6 +183,11 @@ func TestOffsiteFinding(t *testing.T) {
 	if f := OffsiteFinding(path, t0); f != nil {
 		t.Fatalf("a success within %s reported: %+v", offsite.StaleAfter, f)
 	}
+	// A key mismatch stops every later run too: reported without waiting out StaleAfter.
+	write(offsite.Status{LastAttempt: t0.Add(-time.Hour), LastSuccess: t0.Add(-2 * time.Hour), LastError: "sealed with another key", KeyMismatch: true})
+	if f := OffsiteFinding(path, t0); f == nil || !strings.Contains(f.SummaryEN, "has stopped") || !strings.Contains(f.SummaryEN, "(sealed with another key)") || !strings.Contains(f.Hint, "check-key") {
+		t.Fatalf("key mismatch: %+v", f)
+	}
 }
 
 func TestMemoryFinding(t *testing.T) {