diff --git a/docs/client/development.md b/docs/client/development.md
index 0120b4e..cd58267 100644
--- a/docs/client/development.md
+++ b/docs/client/development.md
@@ -6,6 +6,12 @@ own tooling. Runtime dependency versions match the environment used to capture
 the pre-migration server; framework upgrades require a separate compatibility
 review.
 
+Pytest filters only the known `anyio.abc.BlockingPortal` deprecation emitted by
+Starlette's test client ([upstream issue](https://github.com/Kludex/starlette/issues/3497)).
+The filter matches the exact message, warning category, and module; other
+warnings remain visible. Remove it when a compatible Starlette upgrade replaces
+the deprecated alias. Runtime dependencies and warning handling are unchanged.
+
 ## Commands
 
 From the repository root:
diff --git a/s2s-client/CHANGELOG.md b/s2s-client/CHANGELOG.md
index f9b1abd..6918067 100644
--- a/s2s-client/CHANGELOG.md
+++ b/s2s-client/CHANGELOG.md
@@ -9,6 +9,8 @@ the implementation at the recorded date; current operating instructions live in
 
 ### Changed
 
+- Filter the specific Starlette TestClient / AnyIO `BlockingPortal` deprecation
+  in pytest while retaining the runtime versions used for compatibility captures.
 - Added a uv-managed Python 3.12 environment and Ruff, mypy, and pytest quality
   checks, all contained in `s2s-client/`. Startup and Docker use the committed
   lockfile. Split the FastAPI service into configuration, route, provider,
diff --git a/s2s-client/pyproject.toml b/s2s-client/pyproject.toml
index f221e33..295d6e3 100644
--- a/s2s-client/pyproject.toml
+++ b/s2s-client/pyproject.toml
@@ -67,3 +67,9 @@ warn_untyped_fields = true
 [tool.pytest.ini_options]
 testpaths = ["tests"]
 pythonpath = ["."]
+# Starlette 0.41.3 uses an alias deprecated by AnyIO 4.15. Keep the runtime
+# versions used for compatibility captures; remove this test-only exception
+# when upgrading Starlette. Upstream: https://github.com/Kludex/starlette/issues/3497
+filterwarnings = [
+  'ignore:^The anyio\.abc\.BlockingPortal alias is deprecated, use anyio\.from_thread\.BlockingPortal instead\.$:DeprecationWarning:^starlette\.testclient$',
+]