diff --git a/src/google/adk/tools/mcp_tool/mcp_session_manager.py b/src/google/adk/tools/mcp_tool/mcp_session_manager.py
index 1d9e5c80a0..90edbf1f34 100644
--- a/src/google/adk/tools/mcp_tool/mcp_session_manager.py
+++ b/src/google/adk/tools/mcp_tool/mcp_session_manager.py
@@ -261,7 +261,7 @@ async def before_request(
         )
         return
 
-    if 'Authorization' in headers:
+    if any(key.lower() == 'authorization' for key in headers):
       logger.debug('Authorization header already present, not overwriting')
       return
 
diff --git a/tests/unittests/tools/mcp_tool/test_mcp_session_manager.py b/tests/unittests/tools/mcp_tool/test_mcp_session_manager.py
index 52d9ebf5ca..5cfc4a6d57 100644
--- a/tests/unittests/tools/mcp_tool/test_mcp_session_manager.py
+++ b/tests/unittests/tools/mcp_tool/test_mcp_session_manager.py
@@ -1298,6 +1298,25 @@ def mock_refresh(req):
 
     assert headers["Authorization"] == "Bearer refreshed_token"
 
+  @pytest.mark.skipif(not AIO_SUPPORTED, reason="google.auth.aio not supported")
+  @pytest.mark.asyncio
+  async def test_before_request_preserves_lowercase_authorization_header(self):
+    """An existing lowercase authorization header prevents token injection."""
+    from google.adk.tools.mcp_tool.mcp_session_manager import _RefreshableAsyncCredentials
+
+    mock_creds = Mock()
+    mock_creds.expired = True
+    mock_creds.token = "service_account_token"
+    mock_creds.refresh = Mock()
+
+    credentials = _RefreshableAsyncCredentials(mock_creds)
+    headers = {"authorization": "Bearer user_token"}
+
+    await credentials.before_request(None, "GET", "http://example.com", headers)
+
+    assert headers == {"authorization": "Bearer user_token"}
+    mock_creds.refresh.assert_not_called()
+
 
 class TestGoogleAuthAsyncByteStream: