---
title: Letting a long look-ahead overrule the search
family: lifetime-objective
summary: Plays every candidate 25 moves forward and overrides the reference only when one column is clearly better.
status: rejected
evidence: reproduced
reads: public
---

The reference search sees four moves. This candidate looks twenty-five moves
ahead with a much weaker player, and is allowed to overrule the search when the
long view disagrees strongly enough.

<EvidenceLabel status="rejected" evidence="reproduced" reads="public" />

It was tested on 32 paired games and **it lost**, not narrowly, and not only on
score. It made the game's two survival flow rates *worse*, which is the one
thing it could not afford to do.

## The intuition

A four-move look-ahead is very good at not dying in the next four moves and
completely blind to a board slowly filling up over the next thirty. The idea
here is a division of labour: keep the strong short-range search as the default,
and add a second opinion that trades accuracy for reach.

The second opinion works like this. Take each column you could legally play.
Imagine seven possible futures: seven sequences of discs and hidden numbers.
Play each column forward for twenty-five moves in each future, using a fast
two-move search to make the intervening decisions. Now you have seven paired
comparisons for every column against the column the reference chose. If a
challenger wins consistently enough across all seven, take it; otherwise stay
with the reference.

The seven futures are shared across all the columns, and each future is seeded
from the visible board alone — never from the game's seed, so the comparison is
between columns rather than between lucky and unlucky imagined futures.

## How it works, step by step

1. **Input.** The visible board, the visible next disc, and the drops remaining
   before the next rise. The imagined futures are generated from a hash of the
   visible position, so nothing about the real game's randomness leaks in, and
   the score, level and move number are structurally unreachable by the
   continuation player.
2. **Default.** The unmodified reference: full-width, four-move
   [fair expectimax](/learn/concepts/chance-vs-choice) with five chance samples.
   Its chosen column is the incumbent.
3. **Routing.** The long look-ahead only runs when the board is already tall —
   when some column holds four or more discs. In the recorded cohort that fired
   on 77.3% of decisions.
4. **Challengers.** Every other legal column. There is no shortlist and no width
   limit.
5. **Continuation.** For each column and each of the seven futures, play the
   column, then let a fresh two-move search play the next twenty-four moves. Add
   up the points; if the line dies, charge it the same large death penalty the
   reference uses; if it survives all twenty-five moves, add one leaf score for
   where it ended up.
6. **The veto test.** A challenger replaces the incumbent only if it passes all
   four conditions: it survives at least as many of the seven futures; it clears
   at least as many numbered discs on average; its paired advantage over the
   incumbent clears a confidence bound across the seven futures; and the
   reference search itself does not rate it worse than the incumbent by more
   than one row-rise bonus.
7. **Output.** A column: the best passing challenger, or the reference's own
   column if none passes.

## What happened

**It was rejected on every one of its five preregistered conditions.** On 32
games played by both, the reference averaged 339,290 points and 98.66 moves; the
candidate averaged 292,780 points and 86.34 moves. Game by game it lost 19,
won 9 and tied 4, and the confidence bound on the difference sat around
−92,000 — comfortably on the wrong side of zero.

The rule genuinely fired: 68 vetoes across 2,137 opportunities, so this is not a
policy that quietly did nothing. Three games drew no vetoes at all, and in all
three the two arms produced *exactly* identical games, which is a free check
that the fallback works. Attributing the whole cohort's loss to the vetoes that
caused it gives an average of about −21,900 points per veto taken — and
individual vetoes were near coin flips with a strongly negative mean, one game
gaining nearly 300,000 and another losing over 440,000.

The decisive part is not the score. It is that the candidate **cleared fewer
discs and revealed fewer gray discs per move than the policy it was trying to
improve**. For a long look-ahead to buy survival it has to buy flow, and this one
sold it — despite one of the four veto conditions being an explicit demand for
more clears. The estimate of a column's clear advantage, measured twenty-five
moves deep under a two-move continuation, **did not transfer to a game that was
actually going to be played by the four-move search**. The rollout was measuring
a different policy's future.

<Callout title="This retest exists because a single game looked spectacular" tone="warn">
An earlier version of this mechanism produced one game worth 404,047 points in
250 moves, under the repository's old, incorrect 7,000-point rise bonus. Rescored
at the corrected 17,000-point bonus that becomes 894,047 — 89% of the
qualification bar, from a legal public-information policy. An internal audit
called it "the single largest unexploited number in the repository". Properly
powered, with the correction the audit itself specified, **the mechanism did not
reproduce.** The original seed was deliberately not re-run, because re-running
the exact game that produced a headline number and reporting it would be
selection on the outcome.
</Callout>

<TechnicalDetails title="The technical record">

Source: [`finding-03-rollout-veto-17k`](/docs/exploratory/finding-03-rollout-veto-17k).
Run validity **`valid`**; scientific outcome **`fail`**; **evidence tier
`development`** (SCREEN, 32 paired whole games); provenance **reproduced**,
executed in this checkout. Seed lease `SEEDLEASE-A51D-VETO`, cohort
`0xa51e0000`–`0xa51e001f`, 600-move declared cap, 8 threads. 0 runner failures,
0 illegal moves, 0 score-identity violations, 0 censored games in either arm.

Gate frozen in `PREREGISTRATION.md` before any cohort seed was read.

| Arm | Mean | Median | Min / max | Mean moves | Clears/move | Reveals/move |
| --- | ---: | ---: | ---: | ---: | ---: | ---: |
| fair depth-4 (comparator) | 339,290 | 257,270 | 120,279 / 725,934 | 98.66 | 2.0200 | 1.1308 |
| rollout veto (candidate) | 292,780 | 250,794 | 120,286 / 799,183 | 86.34 | 1.9515 | 1.0749 |

| Statistic | Score | Moves |
| --- | ---: | ---: |
| Paired mean delta | −46,510.5 | −12.31 |
| Paired median delta | −9,264.0 | −2.00 |
| One-sided 95% bootstrap lower bound (20,000 resamples) | −91,924.6 | −24.34 |
| Wins–ties–losses | 9–4–19 | 7–9–16 |
| Paired delta standard deviation | 159,175 | — |

An exact one-sided sign test on the 28 non-tied games gives p = 0.0436. All five
preregistered conditions failed: positive mean score delta, positive bootstrap
lower bound, positive mean move delta, at least 20 score wins of 32, and flow
rates not below the reference's.

Mechanism counters: 2,763 decisions, 2,137 routed, 12,314 alternatives scored,
103 passing all four conditions, **68 vetoes taken**. The binding condition is
the confidence test on the paired return, which rejected **12,203 of 12,314
alternatives (99.1%)** because a single imagined future in which one column
dies and another does not moves the difference by the whole million-point death
penalty, so the spread across seven futures is enormous relative to the mean.
A diagnostic on one probe game puts the number of futures the estimator would
actually need at roughly **16 to 51, against the 7 in use**.

An ablation ran the whole cohort again with the pre-correction band of 7,000
points instead of 17,000 on the fourth condition. It tripled rejections on that
condition (720 to 2,266) and changed the number of vetoes actually taken by one
(68 to 67); **29 of 32 games were identical between the two bands**, and both
bands failed all five conditions. The audit's specified repair was applied,
measured, and did not rescue the mechanism.

**Runtime, and why the original was shelved for the wrong reason.** The
historical version was paused on a projected 15,341 seconds against a 2,700-second
ceiling. Measured here, the long look-ahead adds about 1.92 times to total
decision CPU and 1.61 times to arm wall time: a factor of two, not a factor of
six. The runtime gate was never the real obstacle. **These timings were taken on
a heavily contended machine** (other jobs held 20 to 70 of 32 logical CPUs
throughout); the absolute seconds are upper bounds and are not benchmark-grade,
and only the ratios transfer.

**Correctness.** The port is proven byte-identical to the frozen historical
source at the rollout level: a canonical digest of per-column mean return,
surviving futures, mean clears, legal-action count, transitions, calls, work and
nodes over 10 public states matched over 80 digest lines. That gate caught a real
transcription error on its first run. Self-tests cover corrected scoring, repeat
determinism, horizontal-reflection equivalence, blindness to score, level and
move index, legality, and seed-lease containment. No cross-engine
TypeScript parity sweep was run for this candidate, which is a weaker claim than
full engine parity.

**Other limitations as recorded.** n = 32, with a paired delta standard
deviation of 159,175 — enough to decline promotion, not enough to quantify how
much worse the mechanism is. One machine, one compiler. The cohort and probe
seeds are now permanently development data; no protected or final seed was
touched.

</TechnicalDetails>

## What this taught us, and what is still open

**Ruled out:** this exact configuration — twenty-five moves, seven futures,
danger threshold of four discs, a two-move continuation, the confidence test as
written, every legal column as a challenger, at both settings of the fourth
condition, on 32 fresh paired games.

**Not ruled out:** the family. A valid negative rejects a configuration, not an
idea. Two live alternatives survive it. The first is simply that seven imagined
futures is too few to decide anything: the estimator is under-sampled by
something like two to seven times, and cost grows linearly in the number of
futures, so the cheapest valid next test is more futures over a shorter horizon,
preregistered as a new configuration rather than as a repair of this one. The
second is harder: even the vetoes that *were* accepted made realized flow worse,
which under-sampling alone does not explain. A continuation player that is much
weaker than the policy that will really be playing may simply be answering the
wrong question.

The wider lesson is about single spectacular games. This cohort contains a
+299,746 game and a −446,294 game side by side. That is what a one-game result
from this policy looks like, and it is why the repository counts
[means over paired whole games](/learn/glossary), not best games.