diff --git a/docs/DEPLOYMENT.md b/docs/DEPLOYMENT.md
index 7e7bffb..49c8697 100644
--- a/docs/DEPLOYMENT.md
+++ b/docs/DEPLOYMENT.md
@@ -257,6 +257,7 @@ A stop mid-reclaim finishes the command in hand and leaves the rest pending. A c
 | unstamped: the release has no `REVISION` | `alert` (`unstamped`) | `live` names something that isn't a finished release: deploy |
 | GitHub can't answer: a rate limit, a 5xx, a timeout, a wrong-shaped answer | none; the run exits 1 | nothing. The monitor's grace absorbs one missed run |
 
+- **Each `alert` names its kind as `metadata.fault`** (#58; status#28, the contract of `co-archiver-drift` and `co-broker-drift`); an `ok` sends no `metadata`. Status groups a run of alerts into one fault by it, so a change of kind (`lag` → `off_main`, say) reports at once, not at the next daily reminder.
 - **What runs:** `src/`, `deploy/`, `scripts/deploy.sh`, `pyproject.toml`, `uv.lock`. A rename counts by either name. Docs, tests, CI, skills and the other scripts never count, however old.
 - **The check is itself runtime:** it lives in `src/` and its units in `deploy/`. A change to it needs a deploy, and lags like any other.
 - **The clock** starts at the push, from its CI run's `created_at`, never the commit date. Within the grace, the body names the oldest push since live, which may be docs only.
@@ -283,7 +284,7 @@ The level is INFO for `ok`, WARNING for an alert or a silent GitHub, and ERROR w
 
 **Silence a lag you mean to keep.** It alerts hourly until deployed. `sudo systemctl stop processor-drift.timer` stops the alerts, and the monitor then goes `missing`, which is honest. `start` resumes them. A deploy never re-enables a timer it installed before, so `disable --now` keeps it stopped across reboots too.
 
-**A deliberate alert** proves the channels end to end. It sends `kind: test`, and asks GitHub nothing:
+**A deliberate alert** proves the channels end to end. It sends `kind: test`, and asks GitHub nothing. Its fault is `test`, so sent while a real fault is open it reports at once, as a fault of its own; the next hourly run's alert, back to the real kind, reports at once too:
 
 ```bash
 sudo systemd-run --quiet --pipe --wait --collect -p User=processor -p Group=processor \
diff --git a/docs/plans/2026-10-21-58-drift-fault.md b/docs/plans/2026-10-21-58-drift-fault.md
new file mode 100644
index 0000000..c19e274
--- /dev/null
+++ b/docs/plans/2026-10-21-58-drift-fault.md
@@ -0,0 +1,35 @@
+---
+title: Drift alerts carry metadata.fault (#58)
+date: 2026-10-21
+status: per #58's hand-off from epic #24 (2026-10-21)
+---
+
+# Drift alerts carry `metadata.fault`
+
+## Problem
+
+`post_checkin` sends `{"status", "variables"}` only. Since status#28 co-status groups a run of `alert`s into one fault by `metadata.fault`; a change of it reports at once. Without the key a change of drift `kind` inside one fault (`lag` → `off_main`, `unstamped`) waits for the daily reminder, up to ~23.5 h. Same for a `--test-alert` sent while a real fault is open.
+
+## Approach
+
+The sibling contract (status#32, #51; broker#110): every `alert` sends `"metadata": {"fault": <kind>}`; an `ok` sends no `metadata` key at all.
+
+- `checkin.post_checkin` gains `metadata: dict[str, str] | None = None`; the key enters the body only when given. Liveness's call is unchanged (it only checks in `ok`).
+- `drift.Verdict.metadata()` beside `variables()`, as broker's: `{"fault": kind}` when `status == "alert"`, else `None`. The fault is the kind token only, never `body` or a URL.
+- `__main__._drift` passes `metadata=verdict.metadata()`.
+- Docs: `checkin.py`'s module docstring, `drift.py`'s `Verdict`, DEPLOYMENT.md § The drift check (each `alert` names its kind as `metadata.fault`; a `--test-alert` during an open fault reports at once as its own fault, `test`).
+
+## Tests (red first)
+
+- `test_checkin.py`: no `metadata` → exactly today's body (no key); `metadata` given → in the body.
+- `test_drift.py::TestVerdict`: every alerting kind's `metadata()` is `{"fault": kind}`; `ok` and `silent` are `None`.
+- `test_drift.py::TestTheCommand`: a `lag` check-in carries `metadata.fault == "lag"`; an `ok` has no `metadata` key; `--test-alert`'s carries `"test"`.
+- `test_liveness.py`: body already pinned by equality (no `metadata`); unchanged.
+
+## Ship
+
+PR `Refs #58` → FF merge → pull → `scripts/deploy.sh` → next hourly drift run `ok`/202 from the new release → `--test-alert` only with the operator's approval.
+
+## Open questions
+
+None.
diff --git a/src/processor/__main__.py b/src/processor/__main__.py
index 04f151c..49c4930 100644
--- a/src/processor/__main__.py
+++ b/src/processor/__main__.py
@@ -115,7 +115,12 @@ def _drift(settings: DriftSettings, *, test_alert: bool) -> int:
         return 1
     try:
         code = post_checkin(
-            settings.status_url, settings.drift_monitor_id, key, verdict.status, verdict.variables()
+            settings.status_url,
+            settings.drift_monitor_id,
+            key,
+            verdict.status,
+            verdict.variables(),
+            metadata=verdict.metadata(),
         )
     except CheckinFailed as exc:
         logger.error("drift check", extra=fields | {"checkin": f"failed: {exc}"})
diff --git a/src/processor/checkin.py b/src/processor/checkin.py
index 78a71fd..99877ee 100644
--- a/src/processor/checkin.py
+++ b/src/processor/checkin.py
@@ -5,6 +5,9 @@
 "variables": {...}}`` and an ``X-API-Key``, answered 202. An ``ok`` records the
 check-in; an ``alert`` renders the monitor's templates against ``variables``
 and dispatches them; silence past the interval and grace goes ``missing``.
+``"metadata": {"fault": ...}`` rides along only when given (status#28): co-status
+groups a run of alerts into one fault by it, and a change of it reports at once.
+With none, the key is absent, never ``null`` or ``{}`` (the siblings', status#51).
 
 **One attempt, never a loop** (#35 trap 4): the next run is the retry, and the
 monitor's grace absorbs one missed check-in. A failure raises
@@ -53,6 +56,7 @@ def post_checkin(
     status: Literal["ok", "alert"],
     variables: dict[str, str],
     *,
+    metadata: dict[str, str] | None = None,
     timeout: float = TIMEOUT_SECONDS,
 ) -> int:
     """Check in once; return Status's answer code (202), or raise :class:`CheckinFailed`."""
@@ -62,10 +66,13 @@ def post_checkin(
             "reinstall it (docs/DEPLOYMENT.md § The drift check)"
         )
     url = f"{base_url.rstrip('/')}/api/v1/monitors/{monitor_id}/checkin"
+    body: dict[str, object] = {"status": status, "variables": variables}
+    if metadata:
+        body["metadata"] = metadata
     try:
         response = requests.post(
             url,
-            json={"status": status, "variables": variables},
+            json=body,
             headers={"X-API-Key": key},
             timeout=timeout,
         )
diff --git a/src/processor/drift.py b/src/processor/drift.py
index cfcb574..b1af357 100644
--- a/src/processor/drift.py
+++ b/src/processor/drift.py
@@ -90,6 +90,14 @@ def variables(self) -> dict[str, str]:
         """Every variable on every check-in, so the monitor's template always renders."""
         return {"kind": self.kind, "live": self.live, "main": self.main, "body": self.body}
 
+    def metadata(self) -> dict[str, str] | None:
+        """An alert's fault, its kind; ``None`` on an ``ok``, so the key is absent (#58).
+
+        co-status compares ``metadata.fault`` and opens a new fault, reported at
+        once, when it changes (status#28). The kind token only: never ``body``.
+        """
+        return {"fault": self.kind} if self.status == "alert" else None
+
 
 @dataclass(frozen=True)
 class Push:
diff --git a/tests/test_checkin.py b/tests/test_checkin.py
index afccf67..7b38d6e 100644
--- a/tests/test_checkin.py
+++ b/tests/test_checkin.py
@@ -1,8 +1,8 @@
 """``processor.checkin``: one check-in to a Status monitor (#35), against a local stub.
 
 Status's contract (CannObserv/status spec D6, notifier's unchanged):
-``POST /api/v1/monitors/{id}/checkin`` with ``{"status", "variables"}`` and an
-``X-API-Key``, answered 202.
+``POST /api/v1/monitors/{id}/checkin`` with ``{"status", "variables"}``, and
+``metadata`` only when given (status#28), and an ``X-API-Key``, answered 202.
 """
 
 import json
@@ -25,6 +25,27 @@ def test_a_check_in_posts_status_and_variables_with_the_key_as_a_header(http_stu
     assert request["headers"]["Content-Type"] == "application/json"
 
 
+def test_metadata_given_is_sent_beside_status_and_variables(http_stub):
+    """co-status compares ``metadata.fault``: a change of it reports at once (status#28)."""
+    http_stub.route("POST", PATH, 202)
+    post_checkin(http_stub.url, MONITOR, KEY, "alert", {"kind": "lag"}, metadata={"fault": "lag"})
+    (request,) = http_stub.requests
+    assert json.loads(request["body"]) == {
+        "status": "alert",
+        "variables": {"kind": "lag"},
+        "metadata": {"fault": "lag"},
+    }
+
+
+@pytest.mark.parametrize("metadata", [None, {}], ids=["none", "empty"])
+def test_no_metadata_is_no_key_at_all(http_stub, metadata):
+    """Absent, never ``null`` or ``{}``: the siblings' contract on an ``ok`` (status#51)."""
+    http_stub.route("POST", PATH, 202)
+    post_checkin(http_stub.url, MONITOR, KEY, "ok", {}, metadata=metadata)
+    (request,) = http_stub.requests
+    assert json.loads(request["body"]) == {"status": "ok", "variables": {}}
+
+
 def test_a_trailing_slash_on_the_base_url_is_harmless(http_stub):
     http_stub.route("POST", PATH, 202)
     assert post_checkin(http_stub.url + "/", MONITOR, KEY, "ok", {}) == 202
diff --git a/tests/test_drift.py b/tests/test_drift.py
index d3fe034..cab45d1 100644
--- a/tests/test_drift.py
+++ b/tests/test_drift.py
@@ -169,6 +169,15 @@ def test_every_check_in_carries_every_variable(self):
             "body": "b",
         }
 
+    @pytest.mark.parametrize("kind", ["lag", "off_main", "unstamped", "test"])
+    def test_every_alert_s_fault_is_its_kind(self, kind):
+        """The one value co-status compares: a change of fault reports at once (status#28)."""
+        assert Verdict(kind, "b", LIVE).metadata() == {"fault": kind}
+
+    @pytest.mark.parametrize("kind", ["ok", "silent"])
+    def test_an_ok_carries_no_metadata(self, kind):
+        assert Verdict(kind, "b", LIVE).metadata() is None
+
 
 class TestFirstLook:
     def test_unstamped_alerts(self):
@@ -611,6 +620,7 @@ def test_a_runtime_lag_past_the_grace_alerts(self, world, capsys):
         assert checkin["variables"]["live"] == LIVE
         assert checkin["variables"]["main"] == sha(1)[:12]
         assert "main's CI: success — scripts/deploy.sh" in checkin["variables"]["body"]
+        assert checkin["metadata"] == {"fault": "lag"}
         record = outcome(capsys)
         assert record["level"] == "WARNING"
         assert (record["build"], record["kind"], record["checkin"]) == (LIVE, "lag", 202)
@@ -621,6 +631,7 @@ def test_a_docs_only_lag_is_ok_however_old(self, world, capsys):
         (checkin,) = world.checkins()
         assert checkin["status"] == "ok"
         assert checkin["variables"]["kind"] == "ok"
+        assert "metadata" not in checkin
         assert outcome(capsys)["level"] == "INFO"
 
     def test_a_runtime_lag_within_the_grace_is_ok(self, world):
@@ -723,4 +734,5 @@ def test_test_alert_sends_one_alert_and_asks_github_nothing(self, world, capsys)
         assert checkin["status"] == "alert"
         assert checkin["variables"]["kind"] == "test"
         assert set(checkin["variables"]) == {"kind", "live", "main", "body"}
+        assert checkin["metadata"] == {"fault": "test"}
         assert outcome(capsys)["kind"] == "test"