diff --git a/ollama/_client.py b/ollama/_client.py
index c1f5f95..3372af4 100644
--- a/ollama/_client.py
+++ b/ollama/_client.py
@@ -90,11 +90,16 @@ class BaseClient:
       base_url=_parse_host(host or os.getenv('OLLAMA_HOST')),
       follow_redirects=follow_redirects,
       timeout=timeout,
+      # Lowercase all headers to ensure override
       headers={
-        'Content-Type': 'application/json',
-        'Accept': 'application/json',
-        'User-Agent': f'ollama-python/{__version__} ({platform.machine()} {platform.system().lower()}) Python/{platform.python_version()}',
-      }.update(headers or {}),
+        k.lower(): v
+        for k, v in {
+          **(headers or {}),
+          'Content-Type': 'application/json',
+          'Accept': 'application/json',
+          'User-Agent': f'ollama-python/{__version__} ({platform.machine()} {platform.system().lower()}) Python/{platform.python_version()}',
+        }.items()
+      },
       **kwargs,
     )
 
diff --git a/tests/test_client.py b/tests/test_client.py
index 3bb451c..124ccfc 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -968,3 +968,19 @@ async def test_async_client_copy(httpserver: HTTPServer):
   client = AsyncClient(httpserver.url_for('/api/copy'))
   response = await client.copy('dum', 'dummer')
   assert response['status'] == 'success'
+
+
+def test_headers():
+  client = Client()
+  assert client._client.headers['content-type'] == 'application/json'
+  assert client._client.headers['accept'] == 'application/json'
+  assert client._client.headers['user-agent'].startswith('ollama-python/')
+
+  client = Client(
+    headers={
+      'X-Custom': 'value',
+      'Content-Type': 'text/plain',
+    }
+  )
+  assert client._client.headers['x-custom'] == 'value'
+  assert client._client.headers['content-type'] == 'application/json'