diff --git a/app/features/entity_mentions.py b/app/features/entity_mentions.py
index e0ed134..f3b4b01 100644
--- a/app/features/entity_mentions.py
+++ b/app/features/entity_mentions.py
@@ -1,7 +1,9 @@
 import asyncio
+import datetime as dt
 import re
 from contextlib import suppress
 from types import SimpleNamespace
+from typing import Literal, Protocol, cast
 
 import discord
 import github
@@ -17,6 +19,10 @@
 IGNORED_MESSAGE_TYPES = frozenset(
     (discord.MessageType.thread_created, discord.MessageType.channel_name_change)
 )
+REPOSITORIES: dict[str, Repository] = {
+    kind: gh.get_repo(f"{config.GITHUB_ORG}/{name}", lazy=True)
+    for kind, name in config.GITHUB_REPOS.items()
+}
 
 DISCUSSION_QUERY = """
 query getDiscussion($number: Int!, $org: String!, $repo: String!) {
@@ -30,7 +36,50 @@
 }
 """
 
-repo_cache: dict[str, Repository] = {}
+RepoName = Literal["web", "bot", "main"]
+CacheKey = tuple[RepoName, int]
+EntityKind = Literal["Pull Request", "Issue", "Discussion"]
+
+
+class Entity(Protocol):
+    number: int
+    title: str
+    html_url: str
+
+
+class TTLCache:
+    def __init__(self, ttl: int) -> None:
+        self._ttl = dt.timedelta(seconds=ttl)
+        self._cache: dict[CacheKey, tuple[dt.datetime, EntityKind, Entity]] = {}
+
+    def _fetch_entity(self, key: CacheKey) -> None:
+        repo_name, entity_id = key
+        try:
+            entity = REPOSITORIES[repo_name].get_issue(entity_id)
+            kind = "Pull Request" if entity.pull_request else "Issue"
+        except github.UnknownObjectException:
+            try:
+                entity = get_discussion(REPOSITORIES[repo_name], entity_id)
+                kind = "Discussion"
+            except github.GithubException:
+                raise KeyError(key) from None
+        self._cache[key] = (dt.datetime.now(), kind, cast(Entity, entity))
+
+    def _refresh(self, key: CacheKey) -> None:
+        if key not in self._cache:
+            self._fetch_entity(key)
+            return
+        timestamp, *_ = self._cache[key]
+        if dt.datetime.now() - timestamp >= self._ttl:
+            self._fetch_entity(key)
+
+    def __getitem__(self, key: CacheKey) -> tuple[EntityKind, Entity]:
+        self._refresh(key)
+        _, kind, entity = self._cache[key]
+        return kind, entity
+
+
+entity_cache = TTLCache(1800)  # 30 minutes
 
 
 async def handle_entities(message: Message) -> None:
@@ -46,22 +95,9 @@ async def handle_entities(message: Message) -> None:
 
     entities: list[str] = []
     for match in ENTITY_REGEX.finditer(message.content):
-        repo_name = config.GITHUB_REPOS[match[1] or "main"]
-        if (repo := repo_cache.get(repo_name)) is None:
-            repo_cache[repo_name] = repo = gh.get_repo(
-                f"{config.GITHUB_ORG}/{repo_name}", lazy=True
-            )
-        entity_id = int(match[2])
-        try:
-            entity = repo.get_issue(entity_id)
-            kind = "Pull Request" if entity.pull_request else "Issue"
-        except github.UnknownObjectException:
-            try:
-                entity = get_discussion(repo, entity_id)
-                kind = "Discussion"
-            except github.GithubException:
-                continue
-        if entity_id < 10:
+        repo_name = cast(RepoName, match[1] or "main")
+        kind, entity = entity_cache[repo_name, int(match[2])]
+        if entity.number < 10:
             # Ignore single-digit mentions (likely a false positive)
             continue
         entities.append(ENTITY_TEMPLATE.format(kind=kind, entity=entity))