clay-borg/workplans/CB-WP-0041-the-extensive-form-foundation.md

id	kind	title	status
CB-WP-0041	product	The extensive-form foundation	active

Purpose

structural tier  M   (states what the kernel claims about itself as a
                      game-theoretic object, and prepares — but does not
                      build — a capability port)
chaos            d8 = 5  → no override
declared tier    M

Declaration 1 of chaos window 4. Window 3 closed at 12 declarations with one override that changed nothing, and its verdict is still owed (ChaosRollHistory.md).

Why

CB-RES-0009 found that the extensive-form game is the interchange format between describing a game and analysing it — Ludii's universality result grounds its language in EFGs, and OpenSpiel's CFR, best-response and exploitability consume them.

And that we already have most of one, under other names:

EFG component	ours
histories	the journal — Applied { actor, command, events } in order
actions	bot::legal_commands(state, seat)
information sets	state.project(Viewer::Player(seat))
payoffs	Outcome / score()
terminal	outcome.is_some()

This workplan does not build the port. It answers what is true of the engine today, because a port built on an unchecked assumption is worse than no port — and one of the three gaps invalidates every equilibrium concept if it turns out badly.

Task: does the engine satisfy perfect recall?

id: CB-WP-0041-T01
status: todo
priority: high

Perfect recall — every player remembers their own past actions and observations — is the assumption CFR and exploitability both rest on. Nobody has checked whether ours holds.

Formally, for a seat i and any two histories h, h' in the same information set of i: the sequence of i's own actions and information sets along h and h' must be identical.

Controls:

• derived from the journal and the projection, not asserted — the check must be able to say NO;
• a positive control: a deliberately forgetful projection must fail it, or the check proves nothing;
• stated per seat count, since the partition depends on how many seats there are to hide from;
• the answer may be that we do NOT have perfect recall, and that is reported as plainly as the other outcome. It would be a real finding and would make Track B's adoption unsound as it stands.

Task: say precisely what our chance is

id: CB-WP-0041-T02
status: todo
priority: high

setup shuffles, deals and draws Lead from a seeded RNG, and the mid-game reshuffle derives from seed and round. So a clay-borg game is one chance realisation, not a game with chance nodes, and the panels approximate the distribution by sampling seeds.

Controls:

• state it, do not fix it. Monte Carlo over seeds is legitimate and is what we do; the defect would be calling it an EFG;
• name every point where chance enters, from the code, not from memory;
• say what an explicit chance player would cost — that is the input to T04's decision, and guessing it is how a port gets built on a hope.

Task: state the simultaneity encoding, and check it

id: CB-WP-0041-T03
status: todo
priority: medium

Commit/reveal is the textbook EFG encoding of simultaneous moves: sequence them, and hide the earlier move in an information set.

Control: it is not enough to say so. The projection must actually hide another seat's selection before Reveal, and a test must fail if it stops doing that. That property is load-bearing for every claim in §1 of the research note and is currently only implied by SelectionView::Hidden.

Task: decide whether to build the port at all

id: CB-WP-0041-T04
status: todo
priority: medium

decisions/ADR-*.md, written after T01–T03 and not before.

It must be able to conclude "no". Options include exporting an EFG, adopting OpenSpiel's API directly for analysis only, or deciding the gaps are too expensive and Track B borrows vocabulary rather than machinery.

Controls:

• the decision cites T01's answer, because if perfect recall fails, most of the option space closes;
• it states what it would cost to be wrong;
• a port is declared separately, at its own tier. Creating a capability port is a tier-L trigger and this workplan is M — it may not smuggle one in.

Not in this workplan

• No EFG export, no OpenSpiel integration, no equilibrium computation.
• No answer to "is exploitability meaningful for a co-operative game with a shared threshold" — that is the other open question from CB-RES-0009 §6, it is a question about game theory rather than about our engine, and it wants its own pass.