diff --git a/pyproject.toml b/pyproject.toml
index 236ad24..c6e1aeb 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -7,7 +7,7 @@ readme = "README.md"
 packages = [{include = "src"}]
 
 [tool.poetry.dependencies]
-python = "^3.12.3"
+python = "^3.12"
 flask = "^3.0"
 
 [tool.poetry.group.dev.dependencies]
@@ -49,7 +49,7 @@ known-first-party = ["src"]
 
 # ── Mypy ───────────────────────────────────────────────────────────────────
 [tool.mypy]
-python_version = "3.12.3"
+python_version = "3.12"
 strict = true
 warn_return_any = true
 warn_unused_configs = true
diff --git a/src/app.py b/src/app.py
index 8d8c3bd..ff7316f 100644
--- a/src/app.py
+++ b/src/app.py
@@ -1,16 +1,17 @@
-from flask import Flask, jsonify
-from datetime import datetime, timezone
+from datetime import UTC, datetime
+
+from flask import Flask, Response, jsonify
 
 app = Flask(__name__)
 
 
 @app.get("/heartbeat")
-def heartbeat() -> tuple:
+def heartbeat() -> tuple[Response, int]:
     return (
         jsonify(
             {
                 "status": "ok",
-                "timestamp": datetime.now(timezone.utc).isoformat(),
+                "timestamp": datetime.now(UTC).isoformat(),
             }
         ),
         200,