---
title: The shared game harness
family: lifetime-objective
summary: One instrumented game loop and one results writer, shared by every experiment in this family so their numbers are directly comparable and every game's score is checked against the rules.
status: support-only
evidence: repository-verified
reads: diagnostic
---

A single header that plays whole games, counts the same things every time, and
writes the same file format, so that two experiments in this family can be
compared without anyone having to ask whether they measured the same way.

<EvidenceLabel status="support-only" evidence="repository-verified" reads="diagnostic" />

It contains **no strategy at all.** A policy is handed in from outside, one per
worker thread; the harness only runs it and watches.

## What it provides

1. **One game loop.** Plays a game from a seed to its end or to a declared move
   cap, recording per-move information as it goes.
2. **One cohort runner.** Plays an ordered list of seeds across threads, so a
   candidate and its comparator see exactly the same games.
3. **One results writer.** Emits the same fields for every experiment: score,
   moves, whether the game hit the cap, row rises survived, board clears, the
   three point sources, numbered discs cleared, gray discs revealed, deepest
   chain, a histogram of chain depths, mean board occupancy, and the mean height
   of the stack at each rise.
4. **The same statistics.** Quantiles, and a one-sided lower confidence bound on
   the mean computed by resampling **whole games**: the repository's required
   statistical unit, with a fixed number of resamples and a fixed seed, so the
   bound is reproducible.

## The one thing it refuses to let through

Every game's score must satisfy the game's own arithmetic exactly:

> score = 17,000 × row rises + 70,000 × board clears + the points paid for every
> chain wave

The harness asserts this on **every** game. A run whose scores do not decompose
is a run whose engine or instrumentation is wrong, and it stops rather than
reporting a number. That check is not decorative: several results in this family
are quoted with "0 score-identity violations" as part of their validity
statement, and a 32-game cohort in the
[rollout veto](/approaches/lifetime-objective/rollout-veto-17k) work reports 0
runner failures, 0 illegal moves and 0 identity violations directly from it.

It also flags games stopped at the move cap as **censored**, so that a capped
game's score is never quietly treated as a finished one.

## Why one shared harness matters here

The results in this family are mostly small differences between similar
policies, decided on paired whole games. That comparison only means anything if
both arms were run by the same loop, with the same cap, counting clears and
reveals the same way, and reporting the same bound. Reimplementing the runner
per experiment is the cheapest way to produce two numbers that look comparable
and are not.

<TechnicalDetails title="Where it is used, and what is recorded">

Source: `approaches/lifetime-objective/common/harness.hpp`. It has no experiment
of its own, no row in the [experiment index](/docs/research/experiment-index) and
no entry in the [ledger](/docs/research/history). It is support code, and there
is **no retained result** to report for it.

It is named as a retained artifact in the contribution record
`CT-20260820T100249Z-b1ea4b1a`, whose recorded validation includes the score
decomposition identity holding for **64 of 64** reference-policy games and
**192 of 192** weak-control games in the
[score decomposition](/approaches/lifetime-objective/score-decomposition) run.

Experiments in this family that adopt it report the identity check and the
censoring flag as part of their own run-validity statements — for example the
[rollout veto](/approaches/lifetime-objective/rollout-veto-17k) cohort, which
records the harness's game loop and artifact writer as a deliberate substitute
for the historical program's bespoke one, specifically so its output would carry
the same fields and the same per-game assertion as every other arm.

</TechnicalDetails>