diff --git a/src/utils/tags.test.ts b/src/utils/tags.test.ts
new file mode 100644
index 0000000..43e1235
--- /dev/null
+++ b/src/utils/tags.test.ts
@@ -0,0 +1,45 @@
+import { assertEquals } from '@std/assert';
+
+import { addTag, deleteTag, findQuoteTag, findReplyTag, getTagSet, hasTag } from './tags.ts';
+
+Deno.test('addTag', () => {
+  const tags = [['p', 'alex']];
+  assertEquals(addTag(tags, ['p', 'alex']), [['p', 'alex']]);
+  assertEquals(addTag(tags, ['p', 'fiatjaf']), [['p', 'alex'], ['p', 'fiatjaf']]);
+});
+
+Deno.test('deleteTag', () => {
+  const tags = [['p', 'alex'], ['p', 'fiatjaf']];
+  assertEquals(deleteTag(tags, ['p', 'alex']), [['p', 'fiatjaf']]);
+  assertEquals(deleteTag(tags, ['p', 'fiatjaf']), [['p', 'alex']]);
+});
+
+Deno.test('findQuoteTag', () => {
+  assertEquals(findQuoteTag([['q', '123']]), ['q', '123']);
+  assertEquals(findQuoteTag([['e', '', '', 'mention', '456']]), ['e', '', '', 'mention', '456']);
+  assertEquals(findQuoteTag([['e', '', '', 'mention', '456'], ['q', '123']]), ['q', '123']);
+  assertEquals(findQuoteTag([['q', '123'], ['e', '', '', 'mention', '456']]), ['q', '123']);
+});
+
+Deno.test('findReplyTag', () => {
+  const root = ['e', '123', '', 'root'];
+  const reply = ['e', '456', '', 'reply'];
+
+  assertEquals(findReplyTag([root]), root);
+  assertEquals(findReplyTag([reply]), reply);
+  assertEquals(findReplyTag([root, reply]), reply);
+  assertEquals(findReplyTag([reply, root]), reply);
+  assertEquals(findReplyTag([['e', '321'], ['e', '789']]), ['e', '789']);
+  assertEquals(findReplyTag([reply, ['e', '789']]), reply);
+});
+
+Deno.test('getTagSet', () => {
+  const tags = [['p', 'alex'], ['p', 'fiatjaf'], ['p', 'alex']];
+  assertEquals(getTagSet(tags, 'p'), new Set(['alex', 'fiatjaf']));
+});
+
+Deno.test('hasTag', () => {
+  const tags = [['p', 'alex']];
+  assertEquals(hasTag(tags, ['p', 'alex']), true);
+  assertEquals(hasTag(tags, ['p', 'fiatjaf']), false);
+});
diff --git a/src/utils/tags.ts b/src/utils/tags.ts
index ecddaf4..6375e81 100644
--- a/src/utils/tags.ts
+++ b/src/utils/tags.ts
@@ -31,18 +31,40 @@ function addTag(tags: readonly string[][], tag: string[]): string[][] {
   }
 }
 
-const isReplyTag = (tag: string[]) => tag[0] === 'e' && tag[3] === 'reply';
-const isRootTag = (tag: string[]) => tag[0] === 'e' && tag[3] === 'root';
-const isLegacyReplyTag = (tag: string[]) => tag[0] === 'e' && !tag[3];
+/** Tag is a NIP-10 root tag. */
+function isRootTag(tag: string[]): tag is ['e', string, string, 'root', ...string[]] {
+  return tag[0] === 'e' && tag[3] === 'root';
+}
 
-const isQuoteTag = (tag: string[]) => tag[0] === 'q';
-const isLegacyQuoteTag = (tag: string[]) => tag[0] === 'e' && tag[3] === 'mention';
+/** Tag is a NIP-10 reply tag. */
+function isReplyTag(tag: string[]): tag is ['e', string, string, 'reply', ...string[]] {
+  return tag[0] === 'e' && tag[3] === 'reply';
+}
 
-function findReplyTag(tags: string[][]): string[] | undefined {
+/** Tag is a legacy "e" tag with a "mention" marker. */
+function isLegacyQuoteTag(tag: string[]): tag is ['e', string, string, 'mention', ...string[]] {
+  return tag[0] === 'e' && tag[3] === 'mention';
+}
+
+/** Tag is an "e" tag without a NIP-10 marker. */
+function isLegacyReplyTag(tag: string[]): tag is ['e', string, string] {
+  return tag[0] === 'e' && !tag[3];
+}
+
+/** Tag is a "q" tag. */
+function isQuoteTag(tag: string[]): tag is ['q', ...string[]] {
+  return tag[0] === 'q';
+}
+
+/** Get the "e" tag for the event being replied to, first according to the NIPs then falling back to the legacy way. */
+function findReplyTag(tags: string[][]): ['e', ...string[]] | undefined {
   return tags.find(isReplyTag) || tags.find(isRootTag) || tags.findLast(isLegacyReplyTag);
 }
 
-function findQuoteTag(tags: string[][]): string[] | undefined {
+/** Get the "q" tag, falling back to the legacy "e" tag with a "mention" marker. */
+function findQuoteTag(
+  tags: string[][],
+): ['q', ...string[]] | ['e', string, string, 'mention', ...string[]] | undefined {
   return tags.find(isQuoteTag) || tags.find(isLegacyQuoteTag);
 }