Summary

Anthropic ran a controlled experiment: Opus 4.5 solo ($9) = 20% success . Add a Harness (5 subsystems) = 100% success at $200. OpenAI confirmed with a million-line repo: one AGENTS.md file changed everything. Stop swapping models. Build your harness first.

The Experiment

Config Cost Success Rate

Opus 4.5 solo $9 20%

Opus 4.5 + Harness $200 100%

The $191 premium was all verification loops: compile, test, lint, type check.

The 5 Harness Subsystems

Subsystem What It Prevents

Instructions Agent doesn't know project conventions

Tools Unauthorized operations, accidental deletes

Environment "Works on my machine" syndrome

State Cross-session amnesia

Feedback Premature victory declarations

The 3 Fatal Failure Modes

Premature Victory — Agent writes 500 lines, declares "done", CI goes red.

Fix : Pre-commit hook: npx tsc --noEmit

Context Amnesia — Agent adds feature but breaks existing one.

Fix : MEMORY.md — read previous state before acting.

Tool Abuse — Agent runs destructive commands…