diff --git a/test/relay-health.test.mjs b/test/relay-health.test.mjs
new file mode 100644
index 0000000..21d4984
--- /dev/null
+++ b/test/relay-health.test.mjs
@@ -0,0 +1,133 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { summarizeHistory } from '../src/relay-health.mjs';
+
+function createEntry(timestamp, healthyRelays, totalRelays = 3) {
+  return {
+    timestamp,
+    summary: {
+      healthyRelays,
+      totalRelays,
+    },
+  };
+}
+
+test('summarizeHistory - basic statistics calculation', () => {
+  const now = Date.parse('2024-01-01T12:00:00Z');
+  const history = [
+    // Chronological order (oldest to newest)
+    createEntry('2024-01-01T11:45:00Z', 0, 3), // 0% healthy
+    createEntry('2024-01-01T11:50:00Z', 1, 3), // 33% healthy
+    createEntry('2024-01-01T11:55:00Z', 3, 3), // 100% healthy
+  ];
+
+  const result = summarizeHistory(history, { nowMs: now, windowMinutes: 60 });
+
+  assert.equal(result.sampleCount, 3);
+  // Total probes: 9, Total healthy: 4. Rate: 4/9
+  assert.equal(result.successRate, 4 / 9);
+
+  // Last healthy was at 11:55 (5 minutes ago)
+  assert.equal(result.allDownDurationMinutes, 5);
+});
+
+test('summarizeHistory - handles empty history', () => {
+  const result = summarizeHistory([], { nowMs: Date.now() });
+  assert.equal(result.sampleCount, 0);
+  assert.equal(result.successRate, 1); // Default when sampleCount is 0
+  assert.equal(result.allDownDurationMinutes, null);
+});
+
+test('summarizeHistory - respects time window for stats', () => {
+  const now = Date.parse('2024-01-01T12:00:00Z');
+  const history = [
+    // Chronological order
+    createEntry('2024-01-01T10:00:00Z', 0, 3), // Outside 60m window (120m ago)
+    createEntry('2024-01-01T11:55:00Z', 3, 3), // Inside 60m window (5m ago)
+  ];
+
+  const result = summarizeHistory(history, { nowMs: now, windowMinutes: 60 });
+
+  assert.equal(result.sampleCount, 1);
+  assert.equal(result.successRate, 1); // Only the recent healthy entry counts
+});
+
+test('summarizeHistory - allDownDurationMinutes considers outside window entries', () => {
+  const now = Date.parse('2024-01-01T12:00:00Z');
+  const history = [
+    // Chronological order
+    createEntry('2024-01-01T10:00:00Z', 3, 3), // Outside window, healthy
+    createEntry('2024-01-01T11:55:00Z', 0, 3), // Inside window, unhealthy
+  ];
+
+  const result = summarizeHistory(history, { nowMs: now, windowMinutes: 60 });
+
+  assert.equal(result.sampleCount, 1); // Only recent one counted
+  assert.equal(result.successRate, 0); // 0/3 = 0
+
+  // Last healthy was at 10:00 (120 minutes ago)
+  assert.equal(result.allDownDurationMinutes, 120);
+});
+
+test('summarizeHistory - allDownDurationMinutes is null if never healthy', () => {
+  const now = Date.parse('2024-01-01T12:00:00Z');
+  const history = [
+    createEntry('2024-01-01T11:55:00Z', 0, 3),
+    createEntry('2024-01-01T10:00:00Z', 0, 3),
+  ];
+
+  const result = summarizeHistory(history, { nowMs: now, windowMinutes: 60 });
+  assert.equal(result.allDownDurationMinutes, null);
+});
+
+test('summarizeHistory - allDownDurationMinutes is 0 if currently healthy', () => {
+  const now = Date.parse('2024-01-01T12:00:00Z');
+  const history = [
+    createEntry('2024-01-01T12:00:00Z', 3, 3),
+  ];
+
+  const result = summarizeHistory(history, { nowMs: now, windowMinutes: 60 });
+  assert.equal(result.allDownDurationMinutes, 0);
+});
+
+test('summarizeHistory - handles malformed entries gracefully', () => {
+  const now = Date.now();
+  const history = [
+    {}, // Missing everything
+    { timestamp: 'invalid-date' },
+    { timestamp: new Date(now).toISOString(), summary: null },
+  ];
+
+  const result = summarizeHistory(history, { nowMs: now, windowMinutes: 60 });
+
+  // Should calculate stats safely (ignoring malformed or treating as 0)
+  assert.equal(typeof result.successRate, 'number');
+  assert.equal(result.sampleCount, 1); // Only the valid timestamp one is counted in 'recent'
+  // That one has summary: null, so totalRelays=0, healthyRelays=0.
+  // relayProbeCount = 0. So successRate = 1.
+  assert.equal(result.successRate, 1);
+});
+
+test('summarizeHistory - handles valid timestamp but missing summary', () => {
+  const now = Date.parse('2024-01-01T12:00:00Z');
+  const history = [
+    { timestamp: '2024-01-01T11:55:00Z' } // Missing summary
+  ];
+
+  const result = summarizeHistory(history, { nowMs: now, windowMinutes: 60 });
+
+  assert.equal(result.sampleCount, 1);
+  assert.equal(result.successRate, 1); // totalRelays defaults to 0 -> relayProbeCount=0 -> rate=1
+  assert.equal(result.allDownDurationMinutes, null); // healthyRelays defaults to 0 -> never healthy
+});
+
+test('summarizeHistory - healthy entry with invalid timestamp results in NaN duration', () => {
+  const now = Date.now();
+  const history = [
+    { timestamp: 'invalid-date', summary: { healthyRelays: 1 } }
+  ];
+
+  const result = summarizeHistory(history, { nowMs: now });
+
+  assert.equal(Number.isNaN(result.allDownDurationMinutes), true);
+});