FIDE Laws of Chess

FIDE Laws of Chess on Arxo

Explore a chess arbiter's decision through a pinned model, explicit facts and an inspectable answer. Ten retained fictional cases cover illegal moves, flag fall, rapid/blitz exceptions, recording and arbiter judgment.

Node.js 20+ · ESM · canon package 0.1.0 · model 0.1.1 · FIDE edition effective 1 January 2023

Start in Node.js · Cases · Evidence · Depth · Submit a case · Source repository

This is an independent open-source teaching and research demonstration. For an actual chess ruling, use the authentic FIDE Laws and the competent arbiter. Source text and model scope are pinned; the package does not claim general chess-engine capability or complete formalization.

Start in Node.js

Requirements: Node.js 20+. The example uses ES modules; keep the .mjs extension. This recipe was checked on macOS arm64 with Node.js 20.20.2 on 14 October 2026. No global CLI, Git clone or MCP client is needed for this route.

Create a fresh directory, open a terminal there, then run:

npm init -y

npm install --ignore-scripts @arxo/canon-fide-chess@0.1.0 @arxo/law@0.4.3

Save this as example.mjs, then run node example.mjs:

import { open } from '@arxo/law';

import { loadModel } from '@arxo/canon-fide-chess/chess'; import { game, white, getCaseInput } from '@arxo/canon-fide-chess/examples/arbiter';

const model = await loadModel(); const chess = await open(model.spec, { offline: true, local: { packages: { [model.spec]: model.package } }, });

const input = getCaseInput('SecondIllegal'); const loss = await chess.truth('game_lost_by', [game, white], input); const draw = await chess.truth('game_drawn', [game], input); console.log(loss.evaluationStatus, loss.truthStatus); // COMPUTED, TRUE_ONLY console.log(draw.evaluationStatus, draw.truthStatus); // COMPUTED, NEITHER

The two output lines are loss, then draw:

COMPUTED TRUE_ONLY

COMPUTED NEITHER

COMPUTED means the query was evaluated. TRUE_ONLY means it has positive support and no negative support. NEITHER means neither polarity is supported; it does not establish that the proposition is false.

Now replace only getCaseInput('SecondIllegal') with getCaseInput('SecondIllegalNoMate') and run node example.mjs again:

REQUIRES_JUDGMENT NEITHER

COMPUTED TRUE_ONLY

The second line now establishes a draw. This case supplies the fact that the opponent cannot checkmate by any legal series. The separate loss query still requests an arbiter's assessment because other reachable loss grounds remain unresolved. Read both status fields; REQUIRES_JUDGMENT identifies an assessment the model does not decide on its own.

To try another retained episode, choose a name from the case table. To propose your own episode, open an issue with the case facts, legal date, query, package/runtime versions, output and the FIDE clause or model rule you think should give a different answer. Use a fictional or anonymized case.

The SDK executes the rules through @arxo/law-engine@0.5.3. Installation alone does not register this distribution in the SDK's automatic canon resolver: keep the explicit local.packages handoff above. The loader implements no chess decisions or formulas.

Use as an MCP server

The separate native MCP wrapper runs the same pinned model over stdio:

npx -y @arxo/mcp-fide-chess@0.1.0

Node.js 20+, Linux x64 or macOS arm64. Client configuration, HTTP endpoint, inputs and limitations · OCI recipe · npm recipe · Accepted delivery. This MCP delivery is separate from the Node.js route above.

Choose a case

All 18 original case/query expectations are retained and checked. The fixture export provides caseNames, game, white, black, king, CaseName and getCaseInput(name). Each call returns an independent copy; an unknown name throws RangeError.

Facts and arbiter judgments

getCaseInput preserves the CLI's canonical assertions, constants, context and options. The loader supplies neither absent facts nor adjudications. The model's explicitly declared closures are part of its rules; they are not a general assumption that a missing fact is false.

For touch-move, use the two declared arguments — player and piece:

import { open } from '@arxo/law';

import { loadModel } from '@arxo/canon-fide-chess/chess'; import { white, king, getCaseInput } from '@arxo/canon-fide-chess/examples/arbiter';

const model = await loadModel(); const chess = await open(model.spec, { offline: true, local: { packages: { [model.spec]: model.package } }, }); const answer = await chess.truth( 'must_move_first_touched_piece', [white, king], getCaseInput('TouchIntentUnknown'), ); const document = JSON.parse(new TextDecoder().decode(answer.document)); console.log(answer.evaluationStatus, answer.truthStatus); // REQUIRES_JUDGMENT, NEITHER console.log(document.results[0].judgmentRequests);

A positive, explicitly adjudicated intentional-touch assertion establishes this model's obligation; that path is checked separately. The original missing-judgment fixture stays unchanged.

Missing empirical facts, negative facts and contradictory support retain NEITHER, FALSE_ONLY and BOTH. For the checked empirical conflict, SDK 0.4.3 returns COMPUTED / BOTH: the query was computed and both polarities are supported. Read both fields; do not collapse them to a Boolean. Exact Decimal inputs, such as recorded points, are strings ('0.75').

Complete canonical input for SecondIllegal

{

"legalTime": "2023-06-01", "assertions": [ { "contentHash": "sha256:696ba690ec316ad1654c94c94886b3d6e9764573460a0ed89f76280496c904fa", "evidence": [], "id": "urn:law:examples:chess:arbiter#player", "kind": "assertion", "literal": { "args": [ { "id": "urn:example:chess:game", "kind": "entity_ref" }, { "id": "urn:example:chess:white", "kind": "entity_ref" } ], "kind": "literal", "polarity": "positive", "predicate": "urn:fide:laws:clir:chess#player_of" }, "origin": "case_input", "package": "urn:law:examples:chess:arbiter" }, { "contentHash": "sha256:9f7706ed8482cd04f869afdd849b90eea3c2e42ca246bf932c8a20f77a1fcc15", "evidence": [], "id": "urn:law:examples:chess:arbiter#complete-mate-facts", "kind": "assertion", "literal": { "args": [ { "id": "urn:example:chess:white", "kind": "entity_ref" } ], "kind": "literal", "polarity": "positive", "predicate": "urn:fide:laws:clir:chess#checked_player" }, "origin": "case_input", "package": "urn:law:examples:chess:arbiter" }, { "contentHash": "sha256:cd688c83624c70688f10ead443e5c46c32ad7d3310696caf6dad9b80be7c1cb0", "evidence": [], "id": "urn:law:examples:chess:arbiter#illegal-count", "kind": "assertion", "literal": { "args": [ { "id": "urn:example:chess:white", "kind": "entity_ref" }, { "kind": "value", "type": { "name": "urn:law:std#Integer" }, "value": 2 } ], "kind": "literal", "polarity": "positive", "predicate": "urn:fide:laws:clir:chess#completed_illegal_move_count" }, "origin": "case_input", "package": "urn:law:examples:chess:arbiter" } ], "context": { "decisionTime": "2026-09-08T12:00:00Z", "knowledgeTime": "2026-09-08T12:00:00Z", "legalTime": "2023-06-01", "timezone": "UTC" }, "options": {} }

This input was captured by released CLI 0.2.3 from the retained .lawcase; the fixture generator does not reproduce chess semantics. Fixture bytes · Capture provenance.

Inspect the evidence

Preserve answer.document as the canonical bytes returned by Arxo. With the same runtime version, pinned model and dependencies, repeated executions of identical complete inputs (query, facts, judgments, temporal context and options) reproduce those bytes and their hashes. answer.proof.nodes identifies the applied rules and their premises.

The current SDK's answer.sources and proof-node source-anchor arrays are empty for these queries. Source links below are an explicit static join of executed producer IDs with anchors in the hash-checked model; they are not additional citations emitted by the evaluator. The pinned publication is the FIDE Handbook, Laws 2023.

This complete example saves the loss answer and prints its model anchor links:

import { writeFile } from 'node:fs/promises';

import { open } from '@arxo/law'; import { loadModel } from '@arxo/canon-fide-chess/chess'; import { game, white, getCaseInput } from '@arxo/canon-fide-chess/examples/arbiter';

const model = await loadModel(); const chess = await open(model.spec, { offline: true, local: { packages: { [model.spec]: model.package } }, }); const answer = await chess.truth('game_lost_by', [game, white], getCaseInput('SecondIllegal')); await writeFile('answer.json', answer.document); const nodes = new Map(model.package.ir.nodes.map(node => [node.id, node])); const sourceLinks = answer.proof.nodes.filter(node => node.rule).map(node => ({ rule: node.rule, anchors: (nodes.get(node.rule)?.anchors ?? []).map(id => ({ id, locator: nodes.get(id)?.locator, })), })); console.log(answer.hashes, sourceLinks); // includes SecondIllegalMoveLoses → article/7

What the model knows: native touch-move producer tree

![Native touch-move producer tre