diff --git a/src/stac_auth_proxy/handlers/reverse_proxy.py b/src/stac_auth_proxy/handlers/reverse_proxy.py
index 40863d8c..f7feb8dd 100644
--- a/src/stac_auth_proxy/handlers/reverse_proxy.py
+++ b/src/stac_auth_proxy/handlers/reverse_proxy.py
@@ -82,6 +82,15 @@ def _prepare_headers(self, request: Request) -> MutableHeaders:
 
     async def proxy_request(self, request: Request) -> Response:
         """Proxy a request to the upstream STAC API."""
+        # An encoded "?" or "#" in the path (e.g. item id "a%3Fb") is decoded by
+        # the server, so request.url.path is truncated at it ("a") and the
+        # request would act on a different record than the client addressed.
+        if "?" in request.scope["path"] or "#" in request.scope["path"]:
+            return Response(
+                status_code=400,
+                content='Path must not contain encoded "?" or "#"',
+            )
+
         headers = self._prepare_headers(request)
 
         # https://github.com/fastapi/fastapi/discussions/7382#discussioncomment-5136466
diff --git a/tests/test_proxy.py b/tests/test_proxy.py
index 1ce9c904..515ed8e2 100644
--- a/tests/test_proxy.py
+++ b/tests/test_proxy.py
@@ -1,5 +1,6 @@
 """Test authentication cases for the proxy app."""
 
+import pytest
 from fastapi.testclient import TestClient
 from utils import AppFactory, get_upstream_request
 
@@ -38,3 +39,14 @@ async def test_proxied_headers_with_encoding(source_api_server, mock_upstream):
 
     proxied_request = await get_upstream_request(mock_upstream)
     assert proxied_request.headers.get("accept-encoding") == "gzip"
+
+
+@pytest.mark.parametrize("encoded", ["%3F", "%23"])
+async def test_encoded_path_delimiters_rejected(
+    source_api_server, mock_upstream, encoded
+):
+    """DELETE of item 'a?b' must not be truncated to, and applied to, item 'a'."""
+    client = TestClient(app_factory(upstream_url=source_api_server))
+    response = client.delete(f"/collections/c/items/a{encoded}b")
+    assert response.status_code == 400
+    assert mock_upstream.call_count == 0