diff --git a/pyproject.toml b/pyproject.toml
index e52d84be..13ee595d 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -148,6 +148,9 @@ envs.testing.dependencies = [
   "pytest-mock==3.12.0",
   "pytest-asyncio==0.23.5",
   "pytest-aiohttp==1.0.5",
+  # aiohttp 3.13 changed ClientResponse.__init__ (new stream_writer kwarg),
+  # which aioresponses (up to 0.7.8) cannot mock. Test-only pin.
+  "aiohttp<3.13",
   "aioresponses==0.7.6",
   "fastapi",
   "httpx",