# Chess Position Evaluator (Stockfish via x402)

> Chess Position Evaluator (Stockfish via x402) is a paid API for AI agents from chess-eval.x402supply.com, paid per call via x402, $0.01/call, status unknown (last checked 2026-10-30).

Evaluates a chess position using real Stockfish 10 (asm.js), returning the best move, centipawn/mate score, and principal variation for a given FEN string.

## Facts

- Endpoint: POST https://chess-eval.x402supply.com/tools/evaluate_position?utm_source=zero.xyz
- Price: $0.01/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-30
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/chess-position-evaluator-stockfish-via-x402-51c4601d
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_aYPFibAkFvtNKtSVzm0Tr

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability chess-position-evaluator-stockfish-via-x402-51c4601d -d '<json body>'
```

Example prompt: Can you evaluate this chess position — rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1 — at depth 14 and show me the best move along with the centipawn score?

## When to prefer this

Choose this endpoint when you need a real, deterministic Stockfish engine evaluation (not a mock or heuristic) for a single chess position given as FEN. It is ideal for agents that need authoritative centipawn scores, best-move recommendations, or principal variations without setting up a local engine. Supports up to 5 multipv lines and depth up to 18, making it suitable for both quick blunder checks and deeper positional analysis.

## Known failure modes

- Invalid FEN string — position cannot be parsed, returns error
- Depth out of range (must be 1–18) — returns validation error
- multipv out of range (must be 1–5) — returns validation error
- Timeout or isolate crash if position is extremely complex — may return partial results or error
- Payment failure (x402) — call rejected before evaluation runs

## How this service works

Evaluate a chess position with Stockfish: best move, centipawn/mate score, principal variation, and optional multipv. No mocks — a real single-threaded Stockfish 10 (asm.js) runs in-isolate per call.

## Output

Returns the best move in UCI notation, a centipawn score (or mate-in-N score), and the principal variation sequence. If multipv is set (1–5), returns multiple ranked candidate moves with individual scores and variations.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "fen": {
   "type": "string"
  },
  "depth": {
   "type": "integer",
   "maximum": 18,
   "minimum": 1
  },
  "multipv": {
   "type": "integer",
   "maximum": 5,
   "minimum": 1
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "pv": [
   "d5",
   "e5",
   "d4",
   "c3",
   "Nc6",
   "Nf3",
   "dxc3"
  ],
  "pvUci": [
   "d7d5",
   "e4e5",
   "d5d4",
   "c2c3",
   "b8c6",
   "g1f3",
   "d4c3"
  ],
  "engine": "stockfish 10 asm.js",
  "evalCp": -29,
  "status": "ok",
  "multipv": [
   {
    "pv": [
     "d5",
     "e5",
     "d4",
     "c3",
     "Nc6",
     "Nf3",
     "dxc3"
    ],
    "move": "d7d5",
    "pvUci": [
     "d7d5",
     "e4e5",
     "d5d4",
     "c2c3",
     "b8c6",
     "g1f3",
     "d4c3"
    ],
    "evalCp": -29,
    "moveSan": "d5"
   }
  ],
  "bestMove": "d7d5",
  "timedOut": false,
  "fenEchoed": "rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1",
  "bestMoveSan": "d5",
  "perspective": "side-to-move",
  "depthReached": 10
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/chess-position-evaluator-stockfish-via-x402-51c4601d/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
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