diff --git a/stac_fastapi/pgstac/core.py b/stac_fastapi/pgstac/core.py
index 3d01fb7a..626897a3 100644
--- a/stac_fastapi/pgstac/core.py
+++ b/stac_fastapi/pgstac/core.py
@@ -150,7 +150,7 @@ async def all_collections(  # noqa: C901
             prev=prev_link,
         ).get_links()
 
-        return Collections(
+        collections = Collections(
             collections=linked_collections or [],
             links=links,
             numberMatched=collections_result.get(
@@ -161,6 +161,14 @@ async def all_collections(  # noqa: C901
             ),
         )
 
+        # If we have the `fields` extension enabled
+        # we need to avoid Pydantic validation because the
+        # Items might not be a valid STAC Item objects
+        if fields:
+            return JSONResponse(collections)  # type: ignore
+
+        return collections
+
     async def get_collection(
         self,
         collection_id: str,
@@ -617,7 +625,7 @@ def _clean_search_args(  # noqa: C901
                 else:
                     includes.add(field)
 
-            base_args["fields"] = {"include": includes, "exclude": excludes}
+            base_args["fields"] = {"include": list(includes), "exclude": list(excludes)}
 
         if q:
             base_args["q"] = q
diff --git a/tests/api/test_api.py b/tests/api/test_api.py
index 0c9057dc..7aa33d5f 100644
--- a/tests/api/test_api.py
+++ b/tests/api/test_api.py
@@ -271,6 +271,47 @@ async def test_app_query_extension_gte(load_test_data, app_client, load_test_col
     assert len(resp_json["features"]) == 1
 
 
+async def test_app_collection_fields_extension(
+    load_test_data, app_client, load_test_collection, app
+):
+    fields = ["title"]
+    resp = await app_client.get("/collections", params={"fields": ",".join(fields)})
+
+    assert resp.status_code == 200
+
+    resp_json = resp.json()
+    resp_collections = resp_json["collections"]
+
+    assert len(resp_collections) > 0
+    # NOTE: It's a bug that 'collection' is always included; see #327
+    constant_fields = ["id", "links", "collection"]
+    for collection in resp_collections:
+        assert set(collection.keys()) == set(fields + constant_fields)
+
+
+async def test_app_item_fields_extension(
+    load_test_data, app_client, load_test_collection, load_test_item, app
+):
+    coll = load_test_collection
+    fields = ["id", "geometry"]
+    resp = await app_client.get(
+        f"/collections/{coll['id']}/items", params={"fields": ",".join(fields)}
+    )
+
+    assert resp.status_code == 200
+
+    resp_json = resp.json()
+    features = resp_json["features"]
+
+    assert len(features) > 0
+    # These fields are always included in items
+    constant_fields = ["id", "links"]
+    if not app.state.settings.use_api_hydrate:
+        constant_fields.append("collection")
+    for item in features:
+        assert set(item.keys()) == set(fields + constant_fields)
+
+
 async def test_app_sort_extension(load_test_data, app_client, load_test_collection):
     coll = load_test_collection
     first_item = load_test_data("test_item.json")