diff --git a/diarize.mjs b/diarize.mjs
index 622b526..b47059b 100644
--- a/diarize.mjs
+++ b/diarize.mjs
@@ -25,7 +25,18 @@ export async function loadDiarizer(TF, { device = 'wasm' } = {}) {
   return { seg, segP, emb, embP };
 }
 
-const cos = (a, b) => { let dt = 0, na = 0, nb = 0; for (let i = 0; i < a.length; i++) { dt += a[i]*b[i]; na += a[i]*a[i]; nb += b[i]*b[i]; } return dt / (Math.sqrt(na)*Math.sqrt(nb) || 1); };
+const cos = (a, b) => {
+  if (a.length !== b.length) {
+    throw new Error('cosine: vectors must have the same length');
+  }
+  let dt = 0, na = 0, nb = 0;
+  for (let i = 0; i < a.length; i++) {
+    dt += a[i]*b[i];
+    na += a[i]*a[i];
+    nb += b[i]*b[i];
+  }
+  return dt / (Math.sqrt(na)*Math.sqrt(nb) || 1);
+};
 const rms = (a) => { let s = 0; for (const v of a) s += v*v; return Math.sqrt(s/a.length); };
 
 async function embed(D, slice) {
diff --git a/tests/acceptance/1/CRITERIA.json b/tests/acceptance/1/CRITERIA.json
new file mode 100644
index 0000000..b7be116
--- /dev/null
+++ b/tests/acceptance/1/CRITERIA.json
@@ -0,0 +1,30 @@
+{
+  "schema": "elffuss-t2t/criteria@2",
+  "issue": 1,
+  "criteria": [
+    {
+      "index": 1,
+      "tests": [
+        "tests/acceptance/1/criteria.test.js::criterion 1"
+      ]
+    },
+    {
+      "index": 2,
+      "tests": [
+        "tests/acceptance/1/criteria.test.js::criterion 2"
+      ]
+    },
+    {
+      "index": 3,
+      "tests": [
+        "tests/acceptance/1/criteria.test.js::criterion 3"
+      ]
+    },
+    {
+      "index": 4,
+      "tests": [
+        "tests/acceptance/1/criteria.test.js::criterion 4"
+      ]
+    }
+  ]
+}
diff --git a/tests/acceptance/1/criteria.test.js b/tests/acceptance/1/criteria.test.js
new file mode 100644
index 0000000..ac04ed4
--- /dev/null
+++ b/tests/acceptance/1/criteria.test.js
@@ -0,0 +1,25 @@
+import { test } from 'node:test'
+import assert from 'node:assert/strict'
+import { readFileSync } from 'node:fs'
+import * as mod from '../../../diarize.mjs' // the code under test: call it as mod.<name>(...)
+const source = readFileSync(new URL('../../../diarize.mjs', import.meta.url), 'utf8') // the text of diarize.mjs, for a criterion about how the file is written
+
+test('criterion 1', () => {
+  // `cosine([1, 2, 3], [1, 2])` throws an Error whose message starts with `cosine:`.
+  assert.throws(() => mod.cosine([1, 2, 3], [1, 2]), /cosine:/)
+})
+
+test('criterion 2', () => {
+  // `cosine([1, 0], [1, 0, 5])` throws an Error whose message starts with `cosine:`.
+  assert.throws(() => mod.cosine([1, 0], [1, 0, 5]), /cosine:/)
+})
+
+test('criterion 3', () => {
+  // `cosine([1, 0], [1, 0])` returns 1.
+  assert.deepEqual(mod.cosine([1, 0], [1, 0]), 1)
+})
+
+test('criterion 4', () => {
+  // `cosine([0, 0], [0, 0])` returns 0.
+  assert.deepEqual(mod.cosine([0, 0], [0, 0]), 0)
+})