ModelResponse

What a model wrapper returns: the model's completion, and an update to keys the wrapper declares in its writes, committed with the model step's task. A wrapper adds its writes with addUpdate, which keeps those of the wrappers inside it - the combined update applies innermost first - and changes the completion with withCompletion, which keeps the update. A wrapper that answers without calling next builds its own with ModelResponse(completion). Only the response a wrapper returns counts: the updates of a response it discarded, calling next again, are never committed.