diff --git a/agent/fc_agent/worker.py b/agent/fc_agent/worker.py
index e0c4cc5..8683ad2 100644
--- a/agent/fc_agent/worker.py
+++ b/agent/fc_agent/worker.py
@@ -342,15 +342,44 @@ class Worker:
 
     # --- background loops ---------------------------------------------------
     def _heartbeat_loop(self) -> None:
-        """Keep every held lease alive so buffered jobs waiting on the GPU aren't
-        reclaimed by curator's 180s TTL. Errors are swallowed by client.heartbeat;
-        a reclaimed lease just re-leases elsewhere — never fatal."""
+        """Keep every held lease alive, and say we are here even when holding none.
+
+        Leases: buffered jobs waiting on the GPU would otherwise be reclaimed by
+        curator's 180s TTL. Errors are swallowed by client.heartbeat; a reclaimed
+        lease just re-leases elsewhere — never fatal.
+
+        ## Why this sends with an EMPTY list rather than skipping
+
+        Curator's roster records a check-in on this call (and on `lease`), and
+        calls an agent stopped after 300s of silence. This loop used to be
+        gated on `if ids:` — so an agent holding no leases sent nothing at all,
+        and the only check-in left was the lease poll, which sleep mode backs
+        off exponentially to a 900s ceiling (see IDLE_POLL_MAX_SECONDS).
+
+        900 against 300: an IDLE agent was structurally guaranteed to read as
+        stopped. Operator, 2026-09-23: *"I'm running the gpu agent on my device
+        and it currently reads as 'offline' but it's running and has checked in
+        recently."* It had — twelve minutes ago, partway up the backoff ladder.
+
+        The two halves were written ten weeks apart and never reconciled: sleep
+        mode landed 2026-07-02, and the roster adopted the lease as its
+        check-in on 2026-09-02 without noticing the call it was piggybacking on
+        had been deliberately slowed.
+
+        An empty heartbeat extends nothing (`id.in_([])` matches no rows) and
+        costs one small POST every 45s — against the 6/min lease poll sleep
+        mode exists to avoid, that is not a cadence worth protecting, and it is
+        what makes "is the agent alive" answerable at all.
+
+        Still gated on `self._running`: a worker that has been stopped is not
+        checking in for work, and reporting it as present would be a different
+        lie.
+        """
         while True:
             if self._running:
                 with self._held_lock:
                     ids = list(self._held)
-                if ids:
-                    self.client.heartbeat(ids)
+                self.client.heartbeat(ids)
             time.sleep(HEARTBEAT_INTERVAL)
 
     def _queue_poll_loop(self):
diff --git a/tests/test_api_gpu.py b/tests/test_api_gpu.py
index 25f0a7b..db51353 100644
--- a/tests/test_api_gpu.py
+++ b/tests/test_api_gpu.py
@@ -314,3 +314,67 @@ async def test_cpu_embed_never_blocks_gpu_crop_backfills(db):
         select(GpuJob.task).where(GpuJob.image_record_id == img.id)
     )).scalars().all())
     assert tasks == {"ccip", "siglip"}
+
+
+# --- an idle agent still checks in -------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_a_heartbeat_with_no_jobs_still_records_the_check_in(client, db):
+    """Operator, 2026-09-23: *"I'm running the gpu agent on my device and it
+    currently reads as 'offline' but it's running and has checked in
+    recently."*
+
+    It had — twelve minutes ago. The roster takes its agent check-in from the
+    `lease` and `heartbeat` calls, and calls an agent stopped after 300s of
+    silence. The agent's heartbeat loop was gated on holding leases, so an
+    IDLE agent sent none; the only check-in left was the lease poll, which
+    sleep mode backs off to a 900s ceiling. 900 against 300 — an idle agent
+    was structurally guaranteed to read as stopped.
+
+    So the empty heartbeat has to be a real check-in on the server side, not
+    merely tolerated. Asserted on `last_seen_at` moving, because "it returned
+    200" would pass against an endpoint that recorded nothing.
+    """
+    from backend.app.models import ServiceSeen
+
+    token = (await (await client.post("/api/gpu/token/rotate")).get_json())["token"]
+    hdr = {"Authorization": f"Bearer {token}"}
+
+    resp = await client.post(
+        "/api/gpu/jobs/heartbeat",
+        json={"agent_id": "desktop-agent", "job_ids": []}, headers=hdr,
+    )
+
+    assert resp.status_code == 200
+    assert (await resp.get_json())["extended"] == 0, "it extends no lease"
+
+    row = (await db.execute(
+        select(ServiceSeen).where(ServiceSeen.key == "agent:desktop-agent")
+    )).scalar_one()
+    assert row.kind == "agent"
+    assert row.display_name == "GPU agent (desktop-agent)"
+    assert row.last_seen_at is not None
+
+
+def test_the_agent_heartbeats_whether_or_not_it_holds_a_lease():
+    """The other half, in the agent itself — the half that was actually wrong.
+
+    Read from the source rather than by running the loop: it is a `while True`
+    with a sleep, so exercising it means threads and timing, and the property
+    is simply that the call is not behind a `if ids:`.
+    """
+    from pathlib import Path
+
+    src = (
+        Path(__file__).resolve().parents[1] / "agent" / "fc_agent" / "worker.py"
+    ).read_text()
+    loop = src[src.index("def _heartbeat_loop"):]
+    loop = loop[:loop.index("\n    def ")]
+
+    assert "self.client.heartbeat(ids)" in loop
+    assert "if ids:" not in loop, (
+        "the heartbeat is gated on holding leases again; an idle agent then "
+        "reads as stopped after 300s while sleep mode backs its lease poll "
+        "off to 900s"
+    )