# ADR-0020: we do not build an extensive-form port, and here is what to do instead

status: accepted
date: 2026-08-08
decided by: agent, under the standing loop authorization
tier: M (a decision *not* to create a capability port; creating one would
have been L). chaos d8 = 7 → no override. **Declaration 3 of chaos
window 4.**
references: [CB-WP-0041](../workplans/CB-WP-0041-the-extensive-form-foundation.md),
[CB-RES-0009](../research/CB-RES-0009-extensive-form-is-the-lingua-franca.md),
[`specs/Positioning.md`](../specs/Positioning.md) §4 Track B,
[`simulators/openspiel.md`](../simulators/openspiel.md)

## Context

[CB-RES-0009](../research/CB-RES-0009-extensive-form-is-the-lingua-franca.md)
found the extensive-form game is the interchange format between
describing a game and analysing it, and that clay-borg already has most
of one. CB-WP-0041 T01–T03 then asked what is actually true of the
engine, and the answers changed the decision.

## D1 — the three findings this rests on

**T01. Perfect recall fails on the projection and holds on the history.**
22 violations in 44,938 sampled decision points when the information set
is taken to be `project(Viewer::Player(seat))`; none when it is the
seat's observation history. **`project()` is an observation, not an
information state** — OpenSpiel's own `ObservationString` /
`InformationStateString` split, found here by measurement.

**T02. All chance is one root node, and the reshuffle is correlated.**
Three chance points, all reading the root seed; the Problems deal is not
chance at all. The reshuffle permutation is a pure function of
`(seed, round)` **because K5 requires replay to be deterministic**, where
a real table would reshuffle independently.

**T03. Commit/reveal is the EFG simultaneity encoding**, and now checked
in both directions rather than assumed.

## D2 — we do not build an EFG export port

**Refused, and the reason is T02, not T01.**

T01 looked like the blocker and is not: it is a *constraint* with a known
answer — key information sets on observation histories. That is a
specification, and a cheap one.

**T02 is the blocker.** Making chance explicit means the reshuffle
becomes a genuine chance node, and **that breaks the purity that makes
every recording in this repo replay**. Our scenarios, the replay bundles,
`record.rs`'s round trip, the trial notes' state hashes — all of them rest
on chance being a function of state (GameKernel K5).

> **A port would trade the property this project is built on for a
> property it has never needed.** Replay is load-bearing for the evidence
> discipline; equilibrium computation is, so far, a research interest.

**And the port is not the cheap part anyway.** Enumeration is impossible —
24 Solution cards give 24! root branches — so any analysis samples chance,
which is what our seed sweeps already do.

## D3 — what we do instead, in order

1. **Write the mapping down, do not implement it.** The EFG correspondence
   is now established and tested; it belongs in the taxonomy as a stated
   relationship, so a later pass starts from a specification rather than a
   memory.
2. **When Track B opens, take the question to OpenSpiel, not our kernel.**
   Express a *small* GROUND-shaped game against its API and ask whether
   exploitability says anything useful for a co-operative game with a
   shared threshold. **That is a question about game theory, not about our
   engine**, and answering it in our engine would cost the replay property
   to learn something a toy model answers.
3. **If the answer is yes, revisit this ADR.** The cost in D2 is real but
   it is not infinite: an EFG export that *samples* chance rather than
   representing it would preserve K5 and might be enough.

## D4 — what this decision would look like if it were wrong

**If exploitability turns out to be the instrument that settles design
questions** — if it answers *"does ATTACK ever pay"* in a way three review
rounds of policy sweeps could not — then refusing the port will look like
protecting a process property at the cost of the product's purpose.

**The tell would be**: OpenSpiel answering, on a toy model, a question our
panels have been unable to answer. **Watch for it. That is D3 step 2, and
it is deliberately the next thing.**

## Consequences

- No EFG port, no OpenSpiel dependency, no equilibrium computation.
- **The AM-4 dependency budget is untouched**, which is worth stating
  because adopting OpenSpiel would have been a large, non-Rust dependency.
- Track B's first move is a **question**, not a build.
- The three properties T01–T03 established are now regression-tested, so a
  later port starts from checked ground rather than re-deriving them.

## What was rejected

| rejected | why |
|---|---|
| build an EFG export now | breaks K5 replay purity for a research interest (D2) |
| key information sets on `project()` | measured wrong — 22 perfect-recall violations (T01) |
| adopt OpenSpiel as an engine dependency | large non-Rust dependency, for a question a toy model answers |
| declare Track B blocked | it is not blocked, it is *specified*: the constraint is known and the next step is a question |
| say nothing and revisit later | the three findings would be re-derived from memory, which is how this project produces wrong-subject errors |