A backend developer subagent is going to do the job ok, but then the supervisor agent will be missing useful context about what’s been done and will go off the rails.
The ideal sub agent is one that can take a simple question, use up massive amounts of tokens answering it, and then return a simple answer, dropping all those intermediate tokens as unnecessary.
Documentation Search is a good one - does X library have a Y function - the subagent can search the web, read doc MCPs, and then return a simple answer without the supervisor needing to be polluted with all the context