diff --git a/tests/plugins/host_agent/test_agent_build_payload.py b/tests/plugins/host_agent/test_agent_build_payload.py
new file mode 100644
index 0000000..15c779e
--- /dev/null
+++ b/tests/plugins/host_agent/test_agent_build_payload.py
@@ -0,0 +1,55 @@
+from unittest.mock import patch
+from plugins.host_agent import agent as a
+
+
+def test_build_sample_shape():
+    fake_mem = {"total_bytes": 100, "used_bytes": 40,
+                "available_bytes": 60, "swap_used_bytes": 0}
+    fake_load = {"1m": 0.1, "5m": 0.2, "15m": 0.3}
+    fake_storage = [{"mount": "/", "total_bytes": 1000, "used_bytes": 400}]
+
+    with patch.object(a, "collect_cpu", return_value=12.5), \
+         patch.object(a, "collect_memory", return_value=fake_mem), \
+         patch.object(a, "collect_load", return_value=fake_load), \
+         patch.object(a, "collect_uptime", return_value=1234), \
+         patch.object(a, "collect_storage", return_value=fake_storage):
+        sample = a.build_sample(mounts=["/"])
+
+    assert sample["cpu_pct"] == 12.5
+    assert sample["mem"]["used_bytes"] == 40
+    assert sample["load"]["1m"] == 0.1
+    assert sample["uptime_secs"] == 1234
+    assert sample["storage"][0]["mount"] == "/"
+    assert "ts" in sample
+    # ISO-8601 UTC string
+    assert "T" in sample["ts"]
+
+
+def test_build_sample_tolerates_collector_failure():
+    # If a collector raises, the sample should still be built with None for that field.
+    def _boom():
+        raise RuntimeError("no /proc")
+
+    with patch.object(a, "collect_cpu", side_effect=_boom), \
+         patch.object(a, "collect_memory", return_value={"total_bytes": 1, "used_bytes": 0,
+                                                          "available_bytes": 1, "swap_used_bytes": 0}), \
+         patch.object(a, "collect_load", return_value={"1m": 0.0, "5m": 0.0, "15m": 0.0}), \
+         patch.object(a, "collect_uptime", return_value=0), \
+         patch.object(a, "collect_storage", return_value=[]):
+        sample = a.build_sample(mounts=["/"])
+    assert sample["cpu_pct"] is None
+    assert sample["mem"]["total_bytes"] == 1
+
+
+def test_build_payload_wraps_samples():
+    metadata = {"kernel": "6.8", "distro": "Ubuntu", "arch": "x86_64"}
+    payload = a.build_payload(
+        samples=[{"ts": "2026-04-14T00:00:00+00:00", "cpu_pct": 5.0}],
+        hostname="myhost",
+        metadata=metadata,
+    )
+    assert payload["agent_version"] == a.AGENT_VERSION
+    assert payload["hostname"] == "myhost"
+    assert payload["metadata"] == metadata
+    assert len(payload["samples"]) == 1
+    assert payload["samples"][0]["cpu_pct"] == 5.0
diff --git a/tests/plugins/host_agent/test_agent_ring_buffer.py b/tests/plugins/host_agent/test_agent_ring_buffer.py
new file mode 100644
index 0000000..f0d7a46
--- /dev/null
+++ b/tests/plugins/host_agent/test_agent_ring_buffer.py
@@ -0,0 +1,24 @@
+from plugins.host_agent.agent import RingBuffer
+
+
+def test_ring_buffer_preserves_order():
+    rb = RingBuffer(maxlen=3)
+    rb.push(1); rb.push(2); rb.push(3)
+    assert list(rb.drain()) == [1, 2, 3]
+    assert len(rb) == 0
+
+
+def test_ring_buffer_drops_oldest_when_full():
+    rb = RingBuffer(maxlen=3)
+    for v in (1, 2, 3, 4, 5):
+        rb.push(v)
+    assert list(rb.drain()) == [3, 4, 5]
+
+
+def test_ring_buffer_drain_clears_and_is_atomic():
+    rb = RingBuffer(maxlen=5)
+    rb.push("a"); rb.push("b")
+    out = list(rb.drain())
+    rb.push("c")
+    assert out == ["a", "b"]
+    assert list(rb.drain()) == ["c"]