diff --git a/pyproject.toml b/pyproject.toml
index 7ff388f1840..4d1b59d61f0 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -261,6 +261,7 @@ markers = [
     "asyncio: mark test as an asyncio test",
     "limit_leaks: mark test with memory limit for leak detection (e.g., '40 MB')",
     "no_parallel: mark test to run sequentially (not in parallel) - typically for memory measurement tests",
+    "slow: mark test as a slow real-cloud test (e.g., real EC2 RunInstances). Skipped by default; opt in with `pytest -m slow`.",
 ]
 filterwarnings = [
     # Suppress Pydantic serializer warnings from mock server responses (non-critical for memory tests)