# ADR-0023: a policy is bound by what its seat can see

status: accepted
date: 2026-08-08
decided by: agent, under the standing loop authorization
tier: S (states a rule that already held in practice, and makes it
checkable; no interface moved)
references: [ADR-0013](ADR-0013-the-retrospective-search.md) (why the
retrospective search may see everything), [CB-RES-0009](../research/CB-RES-0009-extensive-form-is-the-lingua-franca.md)
(information sets), [specs/GameDesign.md](../specs/GameDesign.md) §1.4
(sensitivity), F27

## Context

`Policy::choose` takes `&GroundState`. `GroundState` carries every
face-down Problem's suit and value and every seat's hand.
`GroundState::project` exists precisely to withhold those:
`ProblemView::FaceDown` carries nothing but the fact that a Problem is
there.

**So the trait hands a bot exactly the information the projection was
written to hide.** No shipped policy reads it — `GreedyPolicy::rank`
touches only `stress`, `claimed_by` and the legal set — but nothing
prevents it, and nothing would notice.

This stops being latent immediately. CB-WP-0049 writes policies whose job
is to **value Problems** in order to test F27, and a Problem's value is
the field the projection hides. A seat that ranks a claim by the value of
a face-down card is playing a game nobody can play, and every number
measured with it would be about that game.

## D1 — a policy may not depend on what its seat cannot see

A measurement made with a peeking policy is not a measurement of GROUND.

**This is the mirror of ADR-0013 D1**, and the pairing is the point. The
retrospective search is *allowed* to see everything, because after the
game there is one world and a line found in it was executable in the only
world there was. A policy plays **during** the game, from inside an
information set, so the same permission would be a different claim
entirely — the classic strategy-fusion error.

Same repository, two searches over the same kernel, opposite permissions,
and the discriminator is **when the question is asked**.

## D2 — checked behaviourally, not by the type system

The obvious enforcement is to change `choose` to take `&GroundView`. We do
not do that first, because a **behavioural control catches more**:

> Vary only what the seat cannot see, and assert the policy's choice does
> not move.

- It binds **every** policy, including ones written later and ones written
  outside this crate, without their cooperation.
- It survives a policy that gets the view and reconstructs hidden state by
  other means.
- It is the sensitivity discipline the project already runs on
  (GameDesign §1.4): name the variable, vary it, watch the number.

A type change makes one route unavailable; the control makes the property
false-if-violated. **The type change is still worth doing** and is not
refused here — it is simply not the thing that establishes the property.

## D3 — the control must be shown to fail

A peek control that no policy can fail is decoration (ADR-0006 D3). The
test carries a deliberately peeking policy — one that ranks by the value
of face-down Problems — and asserts **that** policy is caught. A control
proven only against compliant policies has not been proven at all.

## Consequences

- Every policy is covered by the peek control, and a new one that peeks
  goes red on arrival rather than on review.
- Hidden-information variation becomes a fixture: the same helper that
  proves policies blind can measure how much a line depended on what was
  hidden.
- Policies that must reason about unseen Problems have to do it as
  *inference from what is visible* (a face-down Problem exists, and the
  scenario states the deal), which is the honest form of the same
  reasoning.
- Bots remain unable to test claims about mechanisms they do not attend to
  — an orthogonal limit, recorded as F27, and **not** addressed by this.

## What was rejected

| rejected | why |
|---|---|
| changing `choose` to `&GroundView` first | closes one route; the property still would not be checked, and outside policies could still peek |
| trusting review | four reviews on this repo found five, three, four and two FATAL items; peeking is invisible in a diff that looks like ranking |
| allowing peeking for "stronger" bots | a stronger bot at a game nobody can play measures that game; ADR-0013 D1's permission is retrospective and does not transfer |