diff --git a/server.py b/server.py
index 99a20a7..9bae643 100644
--- a/server.py
+++ b/server.py
@@ -766,6 +766,11 @@ if __name__ == "__main__":
     args = parser.parse_args()
     if args.host:
         mcp.settings.host = args.host
+        if args.host not in ("127.0.0.1", "localhost", "::1"):
+            # FastMCP auto-sets restrictive transport_security for localhost at init time.
+            # Reset it when binding to a non-localhost address so external hosts
+            # (e.g. host.docker.internal) are not rejected with 421.
+            mcp.settings.transport_security = None
     if args.port:
         mcp.settings.port = args.port
     mcp.run(transport=args.transport)