What is the lossless mapping between UI workflow JSON v1.0 and API-format prompts, including subgraphs and reroutes?

The Workflow JSON spec defines the UI format and says nothing on the API format used by POST /prompt; ComfyScript, ComfyUI-to-Python and comfyui-api each convert or consume one of them, and subgraph blueprints add a nested structure. Which UI features (muted/bypassed nodes, subgraphs, groups, frontend-only nodes) cannot survive conversion? Why this is still open: The official docs omit the API format, and the custom-node overview notes that nodes needing client-server communication will not work through API mode.