Three Lessons Building a Desktop-Control MCP Server, Plus Batching Benchmarks
_Ar5en1c_ · reddit · 2026-10-22
A developer built YouxPC, an MCP server (fork of Ghost OS) letting agents drive Mac apps via the accessibility tree, and shares three transferable lessons: (1) bind IDs to elements, not positions — positional IDs misfire when table rows shift; (2) refuse ambiguous names and list all matching IDs instead of guessing; (3) the format shown to the model is part of the product — [7] button "9" caused a model to press 13×79 instead of 47×19 on Calculator. Batching benchmark with Claude Haiku 5.5 across 17 tasks: with pcdo batching, 594s and 219 tool calls; without, 780s and 444 calls, both passing 51/51 — gains concentrated on button-heavy apps, no extra errors. Test code is open-sourced.
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