Harness Forge

Turn Claude Code into its own Meta-Harness — evolve the scaffolding around a fixed model, natively.

Harness Forge is a Claude Code skill that runs an end-to-end harness-optimization loop — propose → score → keep the Pareto-best → repeat — to improve the code around a fixed model: its memory, retrieval, context construction, summarization, prompt templates, and tool-selection logic. The model never changes; the scaffolding gets better.

It is a native reimplementation of the method in Meta-Harness: End-to-End Optimization of Model Harnesses (Lee, Nair, Zhang, Lee, Khattab & Finn, 2026). The original reference repo ships ~1,260 lines of Python (claude_wrapper.py + meta_harness.py) whose job is to drive a headless Claude: spawn a session, parse its output, track tool calls, log everything, loop. Inside Claude Code, that runtime already exists as first-class tools. So Harness Forge keeps only the irreducible domain logic — a cheap scorer — and expresses the entire outer loop as native orchestration. The whole search becomes ~75 lines instead of ~1,260.

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The idea in one picture

seed the frontier with the incumbent harness (the thing to beat)
repeat:
    PROPOSE   k candidate harness variants     ← parallel proposer agents write code
    VALIDATE  each imports / type-checks
    SCORE     each on a held-out-protected eval ← a $0, deterministic scorer
    FRONTIER  Pareto-merge: quality up, cost down, floor-respecting
final: score the frontier once on the untouched test split

The proposer is the mutation operator. The frontier is the search memory. The model is frozen throughout — which is exactly why this fits a fixed / off-the-shelf-API deployment, where you can't change the weights and the gain has to come from the harness.

The paper's headline result was +7.7 accuracy points at ~4× fewer context tokens on text classification — a pure harness-side win. Harness Forge reproduces that shape of result natively.

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Why native?

claude_wrapper.py is a hand-rolled agent runtime. Claude Code is an agent runtime. So every orchestration piece has a native equivalent, and the Python driver becomes redundant:

Meta-Harness (Python)	Harness Forge (native)
claude_wrapper.run() — drive a headless Claude	Agent / agent() inside a Workflow
meta_harness.py outer loop	a Workflow script (parallel / while)
pending_eval.json handshake	a typed schema return — no file round-trip
evolution_summary.jsonl / frontier.json	workflow variables + a results JSONL
SKILL.md proposer prior	a skill / prior file the proposer agent reads
"run N iterations"	the workflow loop, /loop, or CronCreate
3 candidates / iteration (serial)	parallel() — proposers run concurrently
inner_loop.py scorer	stays a script — the one irreducible piece

The only thing you still write is the cheap scorer + rubric + candidate interface. Everything orchestration-shaped is free.

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Quick start

1. Install the skill — one line:

curl -fsSL https://raw.githubusercontent.com/001TMF/harness-forge/main/install.sh | bash

Or as a Claude Code plugin (inside Claude Code):

/plugin marketplace add 001TMF/harness-forge
/plugin install harness-forge@tmf-skills

Other ways