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Chess Eval – Best Line (Principal Variation Playthrough)
x402
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.
Price / call$0.01
Success—
StatusUnknown
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'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.
Example use cases
Post-game blunder walkthrough
“I just finished a game and want to understand what Stockfish would have played from the position after move 18. Here's the FEN: r1bq1rk1/pp2ppbp/2np1np1/8/3NP3/2N1BP2/PPPQ2PP/R3KB1R w KQ - 0 10 — play out the best line for 20 plies at depth 14 and show me the eval at each move.”
Opening preparation line check
“I'm studying the Caro-Kann and want to see Stockfish's full predicted continuation from this position for 15 plies at depth 12: rnbqkbnr/pp1ppppp/2p5/8/4P3/8/PPPP1PPP/RNBQKBNR w KQkq - 0 2. Show me each move in SAN notation with its centipawn score.”
Endgame technique evaluation
“Can you run Stockfish on this king-and-pawn endgame FEN — 8/8/4k3/8/4K3/4P3/8/8 w - - 0 1 — and play out the principal variation for up to 30 plies at depth 16, so I can see the winning technique move by move with evals?”
Technical details
Endpoint
POSThttps://chess-eval.x402supply.com/tools/best_line?utm_source=zero.xyzWhen to prefer
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.
Side effects
- 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
Input
Schema · Request body
| Field | Type | Description |
|---|---|---|
| fen | string | |
| depth | integer | |
| maxPlies | integer |
Output
Schema · Response
{
"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
}
}Tags
x402
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