diff --git a/pyproject.toml b/pyproject.toml
index 354e042..940fad6 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -33,7 +33,13 @@ dependencies = [
 [project.optional-dependencies]
 dev = [
     "pytest>=8.3.0",
-    "pytest-asyncio>=0.24.0",
+    # <1.0: 1.x changed session-scoped async fixture/event-loop behavior
+    # across test modules, breaking the single-shared-engine assumption
+    # tests/conftest.py:engine and the asyncio_default_*_loop_scope
+    # settings below depend on (surfaced as sporadic "relation ... does
+    # not exist" failures in specific test files when run alongside others,
+    # despite passing standalone).
+    "pytest-asyncio>=0.24.0,<1.0.0",
     "pytest-cov>=5.0.0",
     "ruff>=0.6.9",
     "mypy>=1.11.2",