diff --git a/lib/galaxy_test/api/test_libraries.py b/lib/galaxy_test/api/test_libraries.py
index 6cbfaa21e0fe..1fb51357301b 100644
--- a/lib/galaxy_test/api/test_libraries.py
+++ b/lib/galaxy_test/api/test_libraries.py
@@ -313,7 +313,7 @@ def test_legacy_upload_unknown_datatype(self):
             "upload_option": "upload_file",
             "files_0|url_paste": FILE_URL,
         }
-        create_response = self._post(f"libraries/{library['id']}/contents", payload)
+        create_response = self._post(f"libraries/{library['id']}/contents", payload, json=True)
         self._assert_status_code_is(create_response, 400)
         assert create_response.json() == "Requested extension 'xxx' unknown, cannot upload dataset."
 
@@ -552,7 +552,7 @@ def test_create_datasets_in_library_from_collection(self):
             history_id, contents=["xxx", "yyy"], direct_upload=True, wait=True
         ).json()["outputs"][0]["id"]
         payload = {"from_hdca_id": hdca_id, "create_type": "file", "folder_id": folder_id}
-        create_response = self._post(f"libraries/{library['id']}/contents", payload)
+        create_response = self._post(f"libraries/{library['id']}/contents", payload, json=True)
         self._assert_status_code_is(create_response, 200)
 
     @requires_new_library
@@ -588,7 +588,7 @@ def _create_folder(self, library):
             create_type="folder",
             name="New Folder",
         )
-        return self._post(f"libraries/{library['id']}/contents", data=create_data)
+        return self._post(f"libraries/{library['id']}/contents", data=create_data, json=True)
 
     def _create_subfolder(self, containing_folder_id):
         create_data = dict(
diff --git a/lib/galaxy_test/base/populators.py b/lib/galaxy_test/base/populators.py
index 82b458d0068e..1df5b0358c56 100644
--- a/lib/galaxy_test/base/populators.py
+++ b/lib/galaxy_test/base/populators.py
@@ -2740,7 +2740,7 @@ def raw_library_contents_create(self, library_id, payload, files=None):
             files = {}
 
         url_rel = f"libraries/{library_id}/contents"
-        return self.galaxy_interactor.post(url_rel, payload, files=files)
+        return self.galaxy_interactor.post(url_rel, payload, files=files, json=True)
 
     def show_ld_raw(self, library_id: str, library_dataset_id: str) -> Response:
         response = self.galaxy_interactor.get(f"libraries/{library_id}/contents/{library_dataset_id}")
@@ -2759,7 +2759,7 @@ def show_ldda(self, ldda_id):
     def new_library_dataset_in_private_library(self, library_name="private_dataset", wait=True):
         library = self.new_private_library(library_name)
         payload, files = self.create_dataset_request(library, file_type="txt", contents="create_test")
-        create_response = self.galaxy_interactor.post(f"libraries/{library['id']}/contents", payload, files=files)
+        create_response = self.galaxy_interactor.post(f"libraries/{library['id']}/contents", payload, files=files, json=True)
         api_asserts.assert_status_code_is(create_response, 200)
         library_datasets = create_response.json()
         assert len(library_datasets) == 1