diff --git a/tests/comment-avatar.test.mjs b/tests/comment-avatar.test.mjs
new file mode 100644
index 00000000..7cb40364
--- /dev/null
+++ b/tests/comment-avatar.test.mjs
@@ -0,0 +1,156 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import {
+  normalizeCommentAvatarKey,
+  resolveCommentAvatarAsset,
+  registerCommentAvatarFailure,
+} from "../js/ui/components/video-modal/utils/commentAvatar.js";
+
+test("normalizeCommentAvatarKey returns empty string for non-string inputs", () => {
+  assert.equal(normalizeCommentAvatarKey(null), "");
+  assert.equal(normalizeCommentAvatarKey(undefined), "");
+  assert.equal(normalizeCommentAvatarKey(123), "");
+  assert.equal(normalizeCommentAvatarKey({}), "");
+  assert.equal(normalizeCommentAvatarKey([]), "");
+});
+
+test("normalizeCommentAvatarKey returns empty string for empty or whitespace-only strings", () => {
+  assert.equal(normalizeCommentAvatarKey(""), "");
+  assert.equal(normalizeCommentAvatarKey("   "), "");
+  assert.equal(normalizeCommentAvatarKey("\t\n"), "");
+});
+
+test("normalizeCommentAvatarKey returns normalized lowercased string", () => {
+  assert.equal(normalizeCommentAvatarKey("FooBar"), "foobar");
+  assert.equal(normalizeCommentAvatarKey("  BazQux  "), "bazqux");
+  assert.equal(normalizeCommentAvatarKey("ABC"), "abc");
+});
+
+test("resolveCommentAvatarAsset returns default avatar if source is in failures", () => {
+  const failures = new Set(["https://example.com/failed.jpg"]);
+  const result = resolveCommentAvatarAsset({
+    cache: new Map(),
+    failures,
+    defaultAvatar: "default.png",
+    pubkey: "somekey",
+    sanitizedPicture: "https://example.com/failed.jpg",
+  });
+
+  assert.deepEqual(result, { url: "default.png", source: "" });
+});
+
+test("resolveCommentAvatarAsset returns cached asset if pubkey matches and source matches", () => {
+  const cache = new Map();
+  cache.set("somekey", { url: "cached.jpg", source: "https://example.com/pic.jpg" });
+
+  const result = resolveCommentAvatarAsset({
+    cache,
+    failures: new Set(),
+    defaultAvatar: "default.png",
+    pubkey: "somekey",
+    sanitizedPicture: "https://example.com/pic.jpg",
+  });
+
+  assert.deepEqual(result, { url: "cached.jpg", source: "https://example.com/pic.jpg" });
+});
+
+test("resolveCommentAvatarAsset returns cached asset if source is empty", () => {
+  const cache = new Map();
+  cache.set("somekey", { url: "cached.jpg", source: "https://example.com/pic.jpg" });
+
+  const result = resolveCommentAvatarAsset({
+    cache,
+    failures: new Set(),
+    defaultAvatar: "default.png",
+    pubkey: "somekey",
+    sanitizedPicture: "",
+  });
+
+  assert.deepEqual(result, { url: "cached.jpg", source: "https://example.com/pic.jpg" });
+});
+
+test("resolveCommentAvatarAsset updates cache and returns new asset if not cached", () => {
+  const cache = new Map();
+  const result = resolveCommentAvatarAsset({
+    cache,
+    failures: new Set(),
+    defaultAvatar: "default.png",
+    pubkey: "newkey",
+    sanitizedPicture: "https://example.com/new.jpg",
+  });
+
+  assert.deepEqual(result, { url: "https://example.com/new.jpg", source: "https://example.com/new.jpg" });
+  assert.ok(cache.has("newkey"));
+  assert.deepEqual(cache.get("newkey"), { url: "https://example.com/new.jpg", source: "https://example.com/new.jpg" });
+});
+
+test("resolveCommentAvatarAsset updates cache when source changes for existing key", () => {
+  const cache = new Map();
+  cache.set("somekey", { url: "old.jpg", source: "https://example.com/old.jpg" });
+
+  const result = resolveCommentAvatarAsset({
+    cache,
+    failures: new Set(),
+    defaultAvatar: "default.png",
+    pubkey: "somekey",
+    sanitizedPicture: "https://example.com/new.jpg",
+  });
+
+  assert.deepEqual(result, { url: "https://example.com/new.jpg", source: "https://example.com/new.jpg" });
+  assert.deepEqual(cache.get("somekey"), { url: "https://example.com/new.jpg", source: "https://example.com/new.jpg" });
+});
+
+test("resolveCommentAvatarAsset uses default avatar when no source provided", () => {
+  const cache = new Map();
+  const result = resolveCommentAvatarAsset({
+    cache,
+    failures: new Set(),
+    defaultAvatar: "default.png",
+    pubkey: "somekey",
+    sanitizedPicture: null,
+  });
+
+  assert.deepEqual(result, { url: "default.png", source: "" });
+  assert.deepEqual(cache.get("somekey"), { url: "default.png", source: "" });
+});
+
+test("registerCommentAvatarFailure ignores invalid or default source urls", () => {
+  const failures = new Set();
+  const cache = new Map();
+  const defaultAvatar = "default.png";
+
+  registerCommentAvatarFailure({ cache, failures, defaultAvatar, sourceUrl: null });
+  assert.equal(failures.size, 0);
+
+  registerCommentAvatarFailure({ cache, failures, defaultAvatar, sourceUrl: "" });
+  assert.equal(failures.size, 0);
+
+  registerCommentAvatarFailure({ cache, failures, defaultAvatar, sourceUrl: "default.png" });
+  assert.equal(failures.size, 0);
+});
+
+test("registerCommentAvatarFailure adds source to failures", () => {
+  const failures = new Set();
+  const cache = new Map();
+  const defaultAvatar = "default.png";
+  const failedUrl = "https://example.com/fail.jpg";
+
+  registerCommentAvatarFailure({ cache, failures, defaultAvatar, sourceUrl: failedUrl });
+  assert.ok(failures.has(failedUrl));
+});
+
+test("registerCommentAvatarFailure updates cache entries matching the failed source", () => {
+  const failures = new Set();
+  const cache = new Map();
+  const defaultAvatar = "default.png";
+  const failedUrl = "https://example.com/fail.jpg";
+
+  cache.set("user1", { url: failedUrl, source: failedUrl });
+  cache.set("user2", { url: "other.jpg", source: "other.jpg" });
+
+  registerCommentAvatarFailure({ cache, failures, defaultAvatar, sourceUrl: failedUrl });
+
+  assert.ok(failures.has(failedUrl));
+  assert.deepEqual(cache.get("user1"), { url: defaultAvatar, source: "" });
+  assert.deepEqual(cache.get("user2"), { url: "other.jpg", source: "other.jpg" });
+});