diff --git a/serpapi/core.py b/serpapi/core.py
index f24e089..be245f3 100644
--- a/serpapi/core.py
+++ b/serpapi/core.py
@@ -39,8 +39,7 @@ def search(self, params: dict = None, **kwargs):
 
         **Learn more**: https://serpapi.com/search-api
         """
-        if params is None:
-            params = {}
+        params = dict(params) if params is not None else {}
 
         # These are arguments that should be passed to the underlying requests.request call.
         request_kwargs = {}
@@ -63,8 +62,7 @@ def search_archive(self, params: dict = None, **kwargs):
 
         **Learn more**: https://serpapi.com/search-archive-api
         """
-        if params is None:
-            params = {}
+        params = dict(params) if params is not None else {}
 
         # These are arguments that should be passed to the underlying requests.request call.
         request_kwargs = {}
@@ -141,8 +139,7 @@ def locations(self, params: dict = None, **kwargs):
 
         **Learn more**: https://serpapi.com/locations-api
         """
-        if params is None:
-            params = {}
+        params = dict(params) if params is not None else {}
 
         # These are arguments that should be passed to the underlying requests.request call.
         request_kwargs = {}
@@ -171,8 +168,7 @@ def account(self, params: dict = None, **kwargs):
         **Learn more**: https://serpapi.com/account-api
         """
 
-        if params is None:
-            params = {}
+        params = dict(params) if params is not None else {}
 
         # These are arguments that should be passed to the underlying requests.request call.
         request_kwargs = {}
diff --git a/serpapi/http.py b/serpapi/http.py
index 4ea1a33..c6afd81 100644
--- a/serpapi/http.py
+++ b/serpapi/http.py
@@ -22,6 +22,9 @@ def __init__(self, *, api_key=None, timeout=None):
         self.session = requests.Session()
 
     def request(self, method, path, params, *, assert_200=True, **kwargs):
+        # Keep API key injection local to this request.
+        params = dict(params)
+
         # Inject the API Key into the params.
         request_data = kwargs.get("data")
         api_key_in_data = isinstance(request_data, dict) and "api_key" in request_data
diff --git a/tests/test_parameter_reuse.py b/tests/test_parameter_reuse.py
new file mode 100644
index 0000000..8842b54
--- /dev/null
+++ b/tests/test_parameter_reuse.py
@@ -0,0 +1,79 @@
+import copy
+from urllib.parse import parse_qs, urlsplit
+
+import pytest
+import requests
+
+from serpapi import Client
+
+
+PARAMS = {
+    "search": {"q": "coffee"},
+    "search_archive": {"search_id": "example-search"},
+    "locations": {"q": "Austin"},
+    "account": {},
+}
+
+
+def capture_requests(monkeypatch, client):
+    sent = []
+
+    def send(request, **kwargs):
+        sent.append(request)
+        response = requests.Response()
+        response.status_code = 200
+        response.headers["Content-Type"] = "application/json"
+        response._content = b'{}'
+        response.request = request
+        return response
+
+    monkeypatch.setattr(client.session, "send", send)
+    return sent
+
+
+@pytest.mark.parametrize("method", list(PARAMS))
+@pytest.mark.parametrize("explicit_key", [False, True])
+def test_reused_params_keep_each_clients_key(monkeypatch, method, explicit_key):
+    first = Client(api_key="client-one")
+    second = Client(api_key="client-two")
+    first_sent = capture_requests(monkeypatch, first)
+    second_sent = capture_requests(monkeypatch, second)
+    params = dict(PARAMS[method])
+    if explicit_key:
+        params["api_key"] = "caller-key"
+    original = copy.deepcopy(params)
+    getattr(first, method)(params)
+    getattr(second, method)(params)
+    expected = ["caller-key", "caller-key"] if explicit_key else ["client-one", "client-two"]
+    actual = [parse_qs(urlsplit(sent[0].url).query)["api_key"][0] for sent in (first_sent, second_sent)]
+    assert actual == expected
+    assert params == original
+
+
+@pytest.mark.parametrize("method,override", [
+    ("search", {"q": "tea"}),
+    ("search_archive", {"output": "json"}),
+    ("locations", {"limit": 2}),
+    ("account", {"api_key": "caller-key"}),
+])
+def test_keyword_overrides_do_not_mutate_params(monkeypatch, method, override):
+    client = Client(api_key="client-key")
+    sent = capture_requests(monkeypatch, client)
+    params = dict(PARAMS[method])
+    original = copy.deepcopy(params)
+    getattr(client, method)(params, **override)
+    query = parse_qs(urlsplit(sent[0].url).query)
+    for key, value in override.items():
+        assert query[key] == [str(value)]
+    assert params == original
+
+
+@pytest.mark.parametrize("explicit_key", [False, True])
+def test_direct_request_does_not_mutate_params(monkeypatch, explicit_key):
+    client = Client(api_key="client-key")
+    sent = capture_requests(monkeypatch, client)
+    params = {"api_key": "caller-key"} if explicit_key else {}
+    original = dict(params)
+    client.request("GET", "/search", params)
+    assert parse_qs(urlsplit(sent[0].url).query)["api_key"] == ["caller-key" if explicit_key else "client-key"]
+    assert params == original