diff --git a/tests/conftest.py b/tests/conftest.py
index f50a313..de58e0f 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -29,8 +29,17 @@ def load_handler(relative_path, module_name):
     across Lambdas. The same loader serves the ``ses_auth.py`` modules,
     which are likewise duplicated per pipeline and not importable as
     packages.
+
+    The Lambda runtime puts each function's own directory on ``sys.path``,
+    so handler modules import their siblings by bare name (e.g.
+    ``from ses_auth import authenticate_inbound_email``). The function
+    directory is added to ``sys.path`` here so the module executes the
+    same way under test as it does in the Lambda.
     """
     path = REPO_ROOT / relative_path
+    package_dir = str(path.parent)
+    if package_dir not in sys.path:
+        sys.path.insert(0, package_dir)
     spec = importlib.util.spec_from_file_location(module_name, path)
     module = importlib.util.module_from_spec(spec)
     sys.modules[module_name] = module