In Claude Code we measured what a spawned subagent pays before it reads a single file: about 54,000 tokens of system prompt, tool schemas and instruction files, written once into the prompt cache. We don't know the equivalent in your stack, but a cost like that never shows up in a call count, so the 2x you saw may be the cheaper half of the story. We came for the cost side and left thinking about the handoff contract instead.