Park a model and get back a short handle (model_ref). Every authoring tool, and solve_model, accepts model_ref instead of model — so a multi-step build stops shipping the whole document through your context on every call, which on a real model is thousands of tokens per step. Tools called with a handle return the same handle, so the chain stays cheap. Handles are private to you and expire after 24 hours; they are NOT saved models and never appear in list_models (use save_model for that). Free.