diff --git a/pyproject.toml b/pyproject.toml
index 270c428..884a0e5 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -109,6 +109,9 @@ ban-relative-imports = "all"
 strict = true
 # restrict testpaths to speed up test discovery
 testpaths = ["test"]
+# Turn all warnings into errors,
+# except DeprecationWarnings (which we knowingly tolerate due to using old `protobuf` APIs).
+filterwarnings = ["error", "ignore::DeprecationWarning"]
 
 [tool.mypy]
 mypy_path = "gen"