Replace the entire project with the given source JSON, returning its project_id. This is the PRIMARY way to build: use it to create a composition or to add/change many elements at once. (To tweak a single element in an existing project, use edit_element / add_element / delete_element instead.) REVISION RULE: if you are iterating on a video that already exists in this conversation — the user asked for changes, you are fixing your own work — you MUST pass its project_id so the project is updated in place and keeps one editor link and its version history. Omitting project_id creates a NEW, unrelated project and strands the old one; only omit for a genuinely different video. The input is validated against the @clipkit/protocol before being accepted; invalid inputs return an error. Shape: { width, height, duration, frame_rate, output_format, background_color?, defaults? (house easing, e.g. a spring), fonts?, camera?, lights?, elements:[…] }; every element has a `type` plus base fields (id, layer, x, y, width, height, time, duration, opacity, rotation, animations, keyframe_animations) and type-specific fields. CKP/1.1: keyframe easings accept a spring object {type:"spring", duration?, bounce?}; channels take stagger {each, from, ease, split, seed} to spread across text units / set copies / group children; groups take layout {direction, gap, align} (stack — no hand-computed positions); text takes a count-up {expr, format}. For exact field names + types call get_schema (optionally with an element_type) — the runtime ignores unrecognized keys, and this tool flags any it does not recognize.