diff --git a/apps/site/next-data/generators/__tests__/partners.test.mjs b/apps/site/next-data/generators/__tests__/partners.test.mjs
new file mode 100644
index 0000000000000..f614e11690e19
--- /dev/null
+++ b/apps/site/next-data/generators/__tests__/partners.test.mjs
@@ -0,0 +1,55 @@
+import assert from 'node:assert/strict';
+import { describe, it, mock } from 'node:test';
+
+// The shuffle reads a different hash byte depending on the size of the weighted
+// list, so these timestamps only apply to a list of exactly 20 entries. This
+// fixture keeps them meaningful when the real partner list changes.
+const PARTNERS = [
+  { id: 'A', weight: 3 },
+  { id: 'B', weight: 2 },
+  { id: 'C', weight: 3 },
+  { id: 'D', weight: 3 },
+  { id: 'E', weight: 2 },
+  { id: 'F', weight: 2 },
+  { id: 'G', weight: 3 },
+  { id: 'H', weight: 2 },
+];
+
+mock.module('#site/next.json.mjs', {
+  namedExports: { partners: PARTNERS },
+});
+
+const { default: generatePartners } =
+  await import('#site/next-data/generators/partners.mjs');
+
+// The shuffle seed is `Math.floor(Date.now() / 300000)`, so the result can only
+// change on a 5-minute boundary. Each incident below is a real build failure,
+// and each neighbouring time is the same code on either side of that window.
+const TIMES = [
+  ['2026-09-09T19:59:59.999Z', 'just before the 20:00 window'],
+  ['2026-09-09T20:00:00.000Z', 'start of the 20:00 window'],
+  ['2026-09-09T20:00:26.000Z', 'failed build: Windows, /en/download'],
+  ['2026-09-09T20:01:26.000Z', 'failed build: Ubuntu, /en'],
+  ['2026-09-09T20:04:59.999Z', 'end of the 20:00 window'],
+  ['2026-09-09T20:05:00.000Z', 'just after the 20:00 window'],
+  ['2026-11-06T17:14:51.000Z', 'passed build: Ubuntu, same commit as below'],
+  ['2026-11-06T17:14:59.999Z', 'just before the 17:15 window'],
+  ['2026-11-06T17:15:00.000Z', 'start of the 17:15 window'],
+  ['2026-11-06T17:17:26.000Z', 'failed build: Windows, /en'],
+  ['2026-11-06T17:19:59.999Z', 'end of the 17:15 window'],
+  ['2026-11-06T17:20:00.000Z', 'just after the 17:15 window'],
+];
+
+describe('generatePartners', () => {
+  for (const [now, description] of TIMES) {
+    it(`returns every partner at ${now} (${description})`, async t => {
+      t.mock.timers.enable({ apis: ['Date'], now: new Date(now) });
+
+      const partners = await generatePartners();
+
+      assert.ok(!partners.includes(undefined), 'result contains undefined');
+      assert.equal(partners.length, PARTNERS.length);
+      assert.deepEqual(new Set(partners), new Set(PARTNERS));
+    });
+  }
+});
diff --git a/apps/site/util/__tests__/array.test.mjs b/apps/site/util/__tests__/array.test.mjs
new file mode 100644
index 0000000000000..7dbe672297f4c
--- /dev/null
+++ b/apps/site/util/__tests__/array.test.mjs
@@ -0,0 +1,45 @@
+import assert from 'node:assert/strict';
+import { describe, it } from 'node:test';
+
+import { shuffle } from '#site/util/array';
+
+const range = length => Array.from({ length }, (_, i) => i);
+
+const sorted = array => [...array].sort((a, b) => a - b);
+
+describe('shuffle', () => {
+  it('returns the same order for the same seed', async () => {
+    const input = range(20);
+
+    assert.deepEqual(await shuffle(input, 42), await shuffle(input, 42));
+  });
+
+  it('does not mutate the input', async () => {
+    const input = range(20);
+
+    await shuffle(input, 42);
+
+    assert.deepEqual(input, range(20));
+  });
+
+  // In these seeds, the first hash byte that the shuffle reads is 255
+  for (const seed of [5963280, 5979951]) {
+    it(`keeps every entry when the hash byte is 255 (seed ${seed})`, async () => {
+      const input = range(20);
+
+      assert.deepEqual(sorted(await shuffle(input, seed)), input);
+    });
+  }
+
+  // About 1 in 256 seeds has a 255 byte at any given position, so this range
+  // covers that case for each length without relying on hand-picked seeds
+  for (const length of [2, 3, 20, 32, 33, 100]) {
+    it(`returns a permutation of ${length} entries for any seed`, async () => {
+      const input = range(length);
+
+      for (let seed = 5979000; seed < 5981000; seed++) {
+        assert.deepEqual(sorted(await shuffle(input, seed)), input);
+      }
+    });
+  }
+});
diff --git a/apps/site/util/array.ts b/apps/site/util/array.ts
index f6aec1ed9c97b..21e3b8beb211f 100644
--- a/apps/site/util/array.ts
+++ b/apps/site/util/array.ts
@@ -12,8 +12,8 @@ export const shuffle = async <T>(
   for (let i = shuffled.length - 1; i > 0; i--) {
     // Use hash bytes to generate deterministic "random" index
     const hashIndex = (i + seed) % 32;
-    // Normalize to 0-1
-    const randomValue = hash[hashIndex] / 255;
+    // Normalize to [0, 1), since a value of 1 would put `j` out of bounds
+    const randomValue = hash[hashIndex] / 256;
 
     const j = Math.floor(randomValue * (i + 1));
     [shuffled[i], shuffled[j]] = [shuffled[j], shuffled[i]];