diff --git a/test/test_wbi_helpers.py b/test/test_wbi_helpers.py
index fbad77e0..1d16152f 100644
--- a/test/test_wbi_helpers.py
+++ b/test/test_wbi_helpers.py
@@ -3,6 +3,7 @@
 mapping), search, merge, SPARQL and the various pure helper functions.
 """
 import logging
+from types import MappingProxyType
 
 import pytest
 import requests
@@ -144,6 +145,46 @@ def test_assert_user_and_bot_flags(self, wikibase, item_q582):
         assert wikibase.last_request['assert'] == 'bot'
 
 
+class TestRequestDataNotMutated:
+    """The caller's request payload must be left untouched by the API helpers."""
+
+    def test_helper_does_not_mutate_caller_data(self, wikibase, item_q582):
+        login = FakeLogin(mediawiki_api_url=wikibase.mediawiki_api_url)
+        data = {'action': 'wbgetentities', 'ids': 'Q582'}
+
+        mediawiki_api_call_helper(data=data, login=login, is_bot=True)
+
+        assert data == {'action': 'wbgetentities', 'ids': 'Q582'}
+        # The copy sent over the wire still gets the automatic parameters
+        assert wikibase.last_request['token'] == login.edit_token
+        assert wikibase.last_request['assert'] == 'bot'
+        assert wikibase.last_request['maxlag'] == '5'
+        assert wikibase.last_request['format'] == 'json'
+
+    def test_api_call_does_not_mutate_caller_data(self, requests_mock):
+        url = 'https://example.org/w/api.php'
+        requests_mock.post(url, json={'success': 1})
+        data = {'action': 'query'}
+
+        mediawiki_api_call('POST', mediawiki_api_url=url, data=data)
+
+        assert data == {'action': 'query'}
+        assert 'format=json' in requests_mock.last_request.text
+
+    def test_api_call_accepts_read_only_mapping(self, requests_mock):
+        url = 'https://example.org/w/api.php'
+        requests_mock.post(url, json={'success': 1})
+
+        assert mediawiki_api_call('POST', mediawiki_api_url=url, data=MappingProxyType({'action': 'query'})) == {'success': 1}
+        assert 'format=json' in requests_mock.last_request.text
+
+    def test_helper_accepts_none_data(self, requests_mock):
+        url = 'https://example.org/w/api.php'
+        requests_mock.post(url, json={'success': 1})
+
+        assert mediawiki_api_call_helper(data=None, mediawiki_api_url=url, allow_anonymous=True) == {'success': 1}
+
+
 class TestUserAgent:
     def test_no_warning_when_user_agent_set(self, wikibase, item_q582, caplog):
         data = {'action': 'wbgetentities', 'ids': 'Q582', 'format': 'json'}
diff --git a/wikibaseintegrator/wbi_helpers.py b/wikibaseintegrator/wbi_helpers.py
index 18e86f7e..e48b239d 100644
--- a/wikibaseintegrator/wbi_helpers.py
+++ b/wikibaseintegrator/wbi_helpers.py
@@ -66,6 +66,7 @@ def mediawiki_api_call(method: str, mediawiki_api_url: str | None = None, sessio
     # TODO: Add support for 'multipart/form-data' when using POST (https://www.mediawiki.org/wiki/API:Edit#Large_edits)
 
     if 'data' in kwargs and kwargs['data']:
+        kwargs['data'] = dict(kwargs['data'])
         if 'format' not in kwargs['data']:
             kwargs['data'].update({'format': 'json'})
         elif kwargs['data']['format'] != 'json':
@@ -165,6 +166,7 @@ def mediawiki_api_call_helper(data: dict[str, Any], login: _Login | None = None,
     """
     mediawiki_api_url = str(mediawiki_api_url or config['MEDIAWIKI_API_URL'])
     user_agent = user_agent or (str(config['USER_AGENT']) if config['USER_AGENT'] is not None else None)
+    data = dict(data) if data is not None else None
 
     hostname = urlparse(mediawiki_api_url).hostname
     if hostname is not None and hostname.endswith(('wikidata.org', 'wikipedia.org', 'wikimedia.org')) and user_agent is None: