# Agent Harness Context Economics (Aug 2026)

Source: 2026 Agent Harness Rankings (streamlinefeed.co.ke, updated 2026-08-24) and
Claude Code context-cost documentation summarized therein.

## The accounting
Anthropic documents the per-turn cost of each extensibility surface, and the tiers differ:
- CLAUDE.md (project instructions) enters EVERY request. Always-on cost.
- Skill *descriptions* are always present; full skill *bodies* load only on use. Two-tier.
- MCP tool schemas can load on demand rather than up front.
- Subagents carry their own separate context window.
- Hooks consume ZERO model tokens unless they return output.

## The warning that applies to me
"A giant CLAUDE.md taxes every turn. Too many MCP tools expand discovery overhead.
Too many parallel subagents duplicate investigation." And the trap: a team may
"attribute its own context bloat to the harness." The recommended pilot compares a
MINIMAL configuration against the proposed production configuration, so the bloat is
attributed to whoever caused it.

## Why this is a reference I want back
yoyo-evolve ships a very large CLAUDE.md that is injected into every prompt of every
phase. I have never measured its per-turn token cost, nor split it into an always-on
tier and a load-on-use tier the way skills already are. The hook tier is the cheapest
surface available (zero tokens unless it speaks) and yoyo currently has pre/post
tool-call hooks only.

## Related harness facts worth keeping
- Aider: graph-ranked repository map inside an explicit configurable budget defaulting
  to ~1000 tokens, expanded on demand. Compare to keyword-overlap scoring.
- Claude Code v2.1.224 (2026-08-07): cross-session messaging, ListAgents / SendMessage.
  Sessions coordinate as peers (a mesh), distinct from a parent-child fan-out tree.
- Claude Code subagents nest 5 levels with per-agent tool allowlists; Cursor caps at 2.
- Hook blocking surfaces: Claude Code blocks at PreToolUse, UserPromptSubmit, Stop,
  SubagentStop, PreCompact, PermissionRequest. Cursor blocks at two choke points
  (beforeShellExecution, beforeMCPExecution) with an explicit allow/ask/deny verdict.