Measuring MCP servers: 78% of fixed token cost comes from tool schemas
Lexeik · reddit · 2026-09-02
After squeezing an MCP server's memory block down to 2,542 tokens, the author measured what tools/list actually ships: 48,339 bytes (12,085 tokens) for 19 tool schemas, pushing total fixed session cost to 15,420 tokens before any request.
- 78% of that fixed cost is the author's own schemas, not the protocol
- Most weight sits in output schema descriptions, with sentences like "Absent unless pinned." repeated across eight tools that share one type
- His rule: input descriptions earn their bytes (they prevent wrong tool calls); output descriptions mostly don't, since the agent sees the value anyway — keep one only if the value can't explain itself
He asks the community for counter-evidence (has cutting output descriptions ever cost anyone calls?) and whether client-side tool filtering makes full-schema budgeting obsolete. He urges others to measure their own servers to see if 78% is normal.
More from coding & agent
- Sprite Fusion generates production-ready AI pixel art at native sizes in seconds — HugoDuprez · 2026-09-02
- Open-Source Cohub: Humans and Agents Build Together, Burning 100B Tokens a Week — rohanpaul_ai · 2026-09-02
- Free Lovable design skill promises premium Linear-level UI — damienghader · 2026-09-02
- Harness engineering is a core skill: build a minimal agent loop first, learn the knobs — omarsar0 · 2026-09-02
- Indie dev ships SaaS with AI coding but can't explain his own architecture — Excellent-Wheel7769 · 2026-09-02
- ORM-running MCP server catches coding agents confidently wrong on N+1 fixes — deusaquilus · 2026-09-02