# Chess Eval – Best Line (Principal Variation Playthrough)

> Chess Eval – Best Line (Principal Variation Playthrough) is a paid API for AI agents from chess-eval.x402supply.com, paid per call via x402, $0.01/call, status unknown (last checked 2026-10-31).

Plays out Stockfish's predicted principal variation move-by-move, returning each ply as a SAN move with its per-ply centipawn evaluation and depth reached.

## Facts

- Endpoint: POST https://chess-eval.x402supply.com/tools/best_line?utm_source=zero.xyz
- Price: $0.01/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-31
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/chess-eval-best-line-principal-variation-playthrough-804fe58e
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_kKCFaAG-ptitNVuJEfGdx

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-eval-best-line-principal-variation-playthrough-804fe58e -d '<json body>'
```

Example prompt: Analyze this chess position using Stockfish and play out the principal variation for up to 15 plies, showing the SAN move and centipawn eval at each step: FEN is rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1, search depth 12.

## When to prefer this

Choose this endpoint when you need the full move-by-move principal variation with per-ply centipawn scores, not just the single best move. It is the right choice for game analysis walkthroughs, opening preparation, and endgame technique study where seeing the engine's predicted sequence matters. Prefer the sibling best-move endpoint if you only need the top move and overall position score.

## Known failure modes

- Invalid FEN string causes rejection or undefined behavior
- Depth above 18 or below 1 is rejected by schema validation
- maxPlies above 40 is clamped or rejected
- Very deep searches on complex positions may time out (timedOut: true in response)
- Single-threaded asm.js engine is slower than native Stockfish — deep searches take longer
- Network or payment (x402) failure before engine runs returns no result

## 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.; Play out Stockfish&#39;s predicted principal variation as SAN moves with a per-ply eval, re-searching a

## Output

Returns an ordered array of plies, each containing the ply number, SAN move string, UCI move string, centipawn evaluation from the side-to-move perspective, and depth reached for that search. Also includes the engine identifier ('stockfish 10 asm.js'), a status field, a timedOut boolean, and the echoed FEN for verification.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "fen": {
   "type": "string"
  },
  "depth": {
   "type": "integer",
   "maximum": 18,
   "minimum": 1
  },
  "maxPlies": {
   "type": "integer",
   "maximum": 40,
   "minimum": 1
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "line": [
   {
    "ply": 1,
    "san": "d5",
    "move": "d7d5",
    "evalCp": -29,
    "depthReached": 10
   },
   {
    "ply": 2,
    "san": "exd5",
    "move": "e4d5",
    "evalCp": 100,
    "depthReached": 10
   },
   {
    "ply": 3,
    "san": "c6",
    "move": "c7c6",
    "evalCp": -52,
    "depthReached": 10
   },
   {
    "ply": 4,
    "san": "Nf3",
    "move": "g1f3",
    "evalCp": 96,
    "depthReached": 10
   }
  ],
  "engine": "stockfish 10 asm.js",
  "status": "ok",
  "timedOut": false,
  "fenEchoed": "rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1",
  "perspective": "side-to-move",
  "depthReached": 10
 }
}
```

## More

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