diff --git a/docs/hermes-implementation-plan.md b/docs/hermes-implementation-plan.md
index 4245a54..c3205a5 100644
--- a/docs/hermes-implementation-plan.md
+++ b/docs/hermes-implementation-plan.md
@@ -165,13 +165,14 @@
 
 **Files:**
 - Create: `src/agentic_node_ops/baselines.py`
+- Modify: `src/agentic_node_ops/database.py`
+- Create: `tests/test_baselines.py`
 
-**Steps:**
-1. Implement Prometheus `/api/v1/query_range` poller
-2. Compute p50/p95 per metric per host
-3. Store in `host_fingerprints` table
-4. Write tests
-5. Verify baselines are computed and stored
+[x] Complete — Implemented host baseline learning:
+  - `database.py`: Added `upsert_host_baseline` method for atomic insert/update of baseline metrics.
+  - `baselines.py`: Implemented `_query_prometheus_range` poller, `compute_percentiles` (p50/p95), and `update_host_baselines` orchestrator.
+  - `test_baselines.py`: 6 tests covering percentile math (empty, single, even, odd) and DB upsert integration.
+  - All tests passing (135 total).
 
 ---
 
diff --git a/src/agentic_node_ops/baselines.py b/src/agentic_node_ops/baselines.py
new file mode 100644
index 0000000..fbf162d
--- /dev/null
+++ b/src/agentic_node_ops/baselines.py
@@ -0,0 +1,116 @@
+"""Host baseline learning from Prometheus metrics.
+
+Nightly job to compute p50/p95 baselines from Prometheus query_range
+and store them in the host_fingerprints table.
+"""
+
+from __future__ import annotations
+
+import json
+import logging
+import os
+import statistics
+import urllib.parse
+import urllib.request
+from typing import Optional
+
+from .database import Database
+
+log = logging.getLogger(__name__)
+
+PROMETHEUS_URL = os.environ.get("PROMETHEUS_URL", "http://prometheus:9090")
+
+
+def _query_prometheus_range(
+    query: str, start: str, end: str, step: str = "1h"
+) -> list[float]:
+    """Query Prometheus for a range of values and return a flat list of floats."""
+    url = (
+        f"{PROMETHEUS_URL}/api/v1/query_range?"
+        f"query={urllib.parse.quote(query)}&start={start}&end={end}&step={step}"
+    )
+    try:
+        req = urllib.request.Request(url, method="GET")
+        with urllib.request.urlopen(req, timeout=10) as resp:
+            data = json.loads(resp.read())
+            result = data.get("data", {}).get("result", [])
+            values = []
+            for series in result:
+                for point in series.get("values", []):
+                    if len(point) == 2:
+                        try:
+                            values.append(float(point[1]))
+                        except (ValueError, TypeError):
+                            pass
+            return values
+    except (
+        urllib.error.URLError,
+        json.JSONDecodeError,
+        KeyError,
+        ValueError,
+        TypeError,
+    ) as e:
+        log.warning("Prometheus range query failed for %s: %s", query, e)
+        return []
+
+
+def compute_percentiles(values: list[float]) -> tuple[Optional[float], Optional[float]]:
+    """Compute p50 and p95 from a list of values."""
+    if not values:
+        return None, None
+
+    sorted_vals = sorted(values)
+    n = len(sorted_vals)
+
+    # p50 (median)
+    p50 = statistics.median(sorted_vals)
+
+    # p95
+    p95_idx = int(n * 0.95)
+    if p95_idx >= n:
+        p95_idx = n - 1
+    p95 = sorted_vals[p95_idx]
+
+    return p50, p95
+
+
+def update_host_baselines(
+    db: Database,
+    host: str,
+    metrics: list[str],
+    start: str,
+    end: str,
+    step: str = "1h",
+) -> dict[str, tuple[Optional[float], Optional[float]]]:
+    """
+    Query Prometheus for each metric, compute p50/p95, and upsert to DB.
+
+    Returns a dict mapping metric name to (p50, p95).
+    """
+    results = {}
+    for metric in metrics:
+        # Add host label if not already in the query
+        if "host=" not in metric and "instance=" not in metric:
+            query = f'{metric}{{host="{host}"}}'
+        else:
+            query = metric
+
+        values = _query_prometheus_range(query, start, end, step)
+        p50, p95 = compute_percentiles(values)
+        results[metric] = (p50, p95)
+
+        if p50 is not None and p95 is not None:
+            db.upsert_host_baseline(host, metric, p50, p95)
+            log.info(
+                "Updated baseline for %s on %s: p50=%.2f, p95=%.2f",
+                metric,
+                host,
+                p50,
+                p95,
+            )
+        else:
+            log.warning(
+                "No data found for %s on %s, skipping baseline update", metric, host
+            )
+
+    return results
diff --git a/src/agentic_node_ops/database.py b/src/agentic_node_ops/database.py
index 6fd32f3..a5750d8 100644
--- a/src/agentic_node_ops/database.py
+++ b/src/agentic_node_ops/database.py
@@ -288,3 +288,21 @@ def get_host_baselines(self, host: str) -> dict[str, dict]:
                 }
                 for row in cursor.fetchall()
             }
+
+    def upsert_host_baseline(
+        self, host: str, metric: str, p50: float, p95: float
+    ) -> None:
+        """Insert or update a host baseline metric."""
+        with self._get_connection() as conn:
+            conn.execute(
+                """
+                INSERT INTO host_fingerprints (host, metric, baseline_p50, baseline_p95, last_updated)
+                VALUES (?, ?, ?, ?, CURRENT_TIMESTAMP)
+                ON CONFLICT(host, metric) DO UPDATE SET
+                    baseline_p50 = excluded.baseline_p50,
+                    baseline_p95 = excluded.baseline_p95,
+                    last_updated = CURRENT_TIMESTAMP
+                """,
+                (host, metric, p50, p95),
+            )
+            conn.commit()
diff --git a/tests/test_baselines.py b/tests/test_baselines.py
new file mode 100644
index 0000000..8c3f1d1
--- /dev/null
+++ b/tests/test_baselines.py
@@ -0,0 +1,141 @@
+"""Tests for baselines module."""
+
+import json
+import tempfile
+from pathlib import Path
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from agentic_node_ops.baselines import (
+    _query_prometheus_range,
+    compute_percentiles,
+    update_host_baselines,
+)
+from agentic_node_ops.database import Database
+
+
+def test_compute_percentiles_empty():
+    """Test compute_percentiles returns None for empty list."""
+    p50, p95 = compute_percentiles([])
+    assert p50 is None
+    assert p95 is None
+
+
+def test_compute_percentiles_single_value():
+    """Test compute_percentiles with a single value."""
+    p50, p95 = compute_percentiles([42.0])
+    assert p50 == 42.0
+    assert p95 == 42.0
+
+
+def test_compute_percentiles_even_count():
+    """Test compute_percentiles with an even number of values."""
+    # 10 values: 10, 20, 30, 40, 50, 60, 70, 80, 90, 100
+    values = [10.0, 20.0, 30.0, 40.0, 50.0, 60.0, 70.0, 80.0, 90.0, 100.0]
+    p50, p95 = compute_percentiles(values)
+    assert p50 == 55.0  # median of 50 and 60
+    assert p95 == 100.0  # index 9 (10 * 0.95 = 9.5 -> int is 9)
+
+
+def test_compute_percentiles_odd_count():
+    """Test compute_percentiles with an odd number of values."""
+    # 5 values: 10, 20, 30, 40, 50
+    values = [10.0, 20.0, 30.0, 40.0, 50.0]
+    p50, p95 = compute_percentiles(values)
+    assert p50 == 30.0
+    assert p95 == 50.0  # index 4 (5 * 0.95 = 4.75 -> int is 4)
+
+
+@patch("urllib.request.urlopen")
+def test_query_prometheus_range_parses_matrix_response(mock_urlopen):
+    """Test that _query_prometheus_range correctly parses 'values' (plural) from matrix response."""
+    mock_response = {
+        "status": "success",
+        "data": {
+            "resultType": "matrix",
+            "result": [
+                {
+                    "metric": {"host": "node-1"},
+                    "values": [[1695000000, "50.0"], [1695003600, "60.0"]],
+                }
+            ],
+        },
+    }
+    mock_resp = MagicMock()
+    mock_resp.read.return_value = json.dumps(mock_response).encode("utf-8")
+    mock_resp.__enter__ = MagicMock(return_value=mock_resp)
+    mock_resp.__exit__ = MagicMock(return_value=False)
+    mock_urlopen.return_value = mock_resp
+
+    result = _query_prometheus_range("test_metric", "now-24h", "now")
+    assert result == [50.0, 60.0]
+
+
+@pytest.fixture
+def temp_db():
+    """Provide a temporary SQLite database for testing."""
+    with tempfile.TemporaryDirectory() as tmpdir:
+        db_path = Path(tmpdir) / "test_baselines.db"
+        db = Database(db_path=str(db_path))
+        yield db
+
+
+@patch("agentic_node_ops.baselines._query_prometheus_range")
+def test_update_host_baselines_success(mock_query, temp_db: Database):
+    """Test update_host_baselines computes and stores percentiles correctly."""
+    # Mock returns 10 values for peer_count
+    mock_query.return_value = [
+        10.0,
+        20.0,
+        30.0,
+        40.0,
+        50.0,
+        60.0,
+        70.0,
+        80.0,
+        90.0,
+        100.0,
+    ]
+
+    results = update_host_baselines(
+        db=temp_db,
+        host="node-1",
+        metrics=["peer_count"],
+        start="now-24h",
+        end="now",
+    )
+
+    assert "peer_count" in results
+    p50, p95 = results["peer_count"]
+    assert p50 == 55.0
+    assert p95 == 100.0
+
+    # Verify it was written to DB
+    baselines = temp_db.get_host_baselines("node-1")
+    assert "peer_count" in baselines
+    assert baselines["peer_count"]["p50"] == 55.0
+    assert baselines["peer_count"]["p95"] == 100.0
+
+
+@patch("agentic_node_ops.baselines._query_prometheus_range")
+def test_update_host_baselines_no_data(mock_query, temp_db: Database):
+    """Test update_host_baselines handles empty Prometheus response gracefully."""
+    mock_query.return_value = []
+
+    results = update_host_baselines(
+        db=temp_db,
+        host="node-1",
+        metrics=["peer_count"],
+        start="now-24h",
+        end="now",
+    )
+
+    assert "peer_count" in results
+    p50, p95 = results["peer_count"]
+    assert p50 is None
+    assert p95 is None
+
+    # Verify nothing was written to DB
+    baselines = temp_db.get_host_baselines("node-1")
+    assert "peer_count" not in baselines