{"uid":"cap_nfroJ117Pa6qDUbw7DLMq","slug":"chess-fen-validator-ffc1cc22","name":"Chess FEN Validator","description":"Validate a chess FEN: syntax of all six fields, king and pawn counts, castling and en passant consistency, piece totals.","url":"https://chess-notation.x402tookit.com/fen-parse?utm_source=zero.xyz","method":"GET","headers":{},"bodySchema":{"type":"object","$schema":"https://json-schema.org/draft/2020-12/schema","required":["input"],"properties":{"input":{"type":"object","required":["type","method"],"properties":{"type":{"type":"string","const":"http"},"method":{"enum":["GET"],"type":"string"},"queryParams":{"type":"object","required":["fen"],"properties":{"fen":{"type":"string","description":"A FEN string, for example the start position rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"}}}},"additionalProperties":false},"output":{"type":"object","required":["type"],"properties":{"type":{"type":"string"},"example":{"type":"object","properties":{"asOf":{"type":["null","string"]},"black":{"type":["null","object"],"properties":{"kings":{"type":["null","number"]},"pawns":{"type":["null","number"]},"rooks":{"type":["null","number"]},"queens":{"type":["null","number"]},"bishops":{"type":["null","number"]},"knights":{"type":["null","number"]}}},"valid":{"type":["boolean","null"]},"white":{"type":["null","object"],"properties":{"kings":{"type":["null","number"]},"pawns":{"type":["null","number"]},"rooks":{"type":["null","number"]},"queens":{"type":["null","number"]},"bishops":{"type":["null","number"]},"knights":{"type":["null","number"]}}},"source":{"type":["null","string"]},"reasons":{"type":["array","null"]},"summary":{"type":["null","string"]},"verdict":{"type":["null","string"]},"castling":{"type":["null","object"],"properties":{"blackKingside":{"type":["boolean","null"]},"whiteKingside":{"type":["boolean","null"]},"blackQueenside":{"type":["boolean","null"]},"whiteQueenside":{"type":["boolean","null"]}}},"degraded":{"type":["boolean","null"]},"problems":{"type":["array","null"]},"disclaimer":{"type":["null","string"]},"notChecked":{"type":["array","null"],"items":{"type":"string"}},"pieceCount":{"type":["null","object"],"properties":{"black":{"type":["null","number"]},"white":{"type":["null","number"]}}},"sideToMove":{"type":["null","string"]},"halfmoveClock":{"type":["null","number"]},"fullmoveNumber":{"type":["null","number"]},"enPassantSquare":{},"fiftyMoveRuleReached":{"type":["boolean","null"]}}}}}}},"responseSchema":null,"example":null,"exampleRequest":null,"tags":["x402"],"displayCostAmount":"0.02","displayCostAsset":"USDC","priceDynamic":false,"priceHint":null,"priceStatus":"priced","priceSource":"probe","requiresHandshake":false,"reviewCount":0,"rating":{"score":"0.00","successRate":"0.00","reviews":0,"stars":null,"state":"unrated"},"availabilityStatus":"unknown","priceObserved":null,"sessionDeposit":null,"pricing":{"kind":"static","summary":"$0.02/call","primary":{"kind":"static","protocol":"x402","network":"base","amountUsd":"0.02","per":"call","confidence":"exact"},"accepted":[{"kind":"static","protocol":"x402","network":"base","amountUsd":"0.02","per":"call","confidence":"exact"}]},"paymentMethods":[{"uid":"pm_WkvMyhOAd79fGdEYmGflK","protocol":"x402","methodType":"crypto","chain":"base","mode":"charge","costAmount":"0.02","costPer":"request","priority":0,"asset":"0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913","unit":"request","depositMicros":null,"planRef":null}],"brandName":null,"brandSlug":null,"brandBaseUrl":null,"brandDocsUrl":null,"whatItDoes":"Validates a chess FEN string by checking syntax of all six fields, king and pawn counts, castling rights, en passant consistency, and piece totals.","exampleAgentPrompt":"Can you validate this chess FEN for me and tell me if it's legal — rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1 — and flag any problems?","exampleUseCases":[{"title":"Verify imported game position","prompt":"I imported this FEN from a database: r1bqk2r/pppp1ppp/2n2n2/2b1p3/2B1P3/2N2N2/PPPP1PPP/R1BQK2R w KQkq - 4 4 — can you check if it's a valid chess position and tell me if the castling rights and piece counts look right?"},{"title":"Debug a custom chess puzzle","prompt":"I built a puzzle position and encoded it as this FEN: 8/8/4k3/8/8/4K3/8/4R3 w - - 0 1, but my chess engine is rejecting it. Can you validate the FEN and explain exactly what's wrong with it?"},{"title":"Validate user-submitted FEN in chess app","prompt":"A user submitted this FEN to our chess app: rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq e6 0 1 — check whether it's syntactically correct and legally consistent, especially the en passant field."}],"resultDescription":"Returns a validity boolean, a verdict string, a human-readable summary, a list of detected problems and reasons for failure, castling availability flags for both sides, side to move, halfmove clock, fullmove number, per-side piece counts (kings, queens, rooks, bishops, knights, pawns), total piece counts, and an array of fields not checked.","failureModes":["Missing or malformed FEN query parameter returns an error","FEN with fewer than 6 space-separated fields fails syntax check","Invalid piece placement characters cause parse failure","Positions with zero or multiple kings per side flagged as invalid","En passant target square inconsistent with side to move flagged","Service degraded flag returned when partial validation only is possible"],"whenToPreferThis":"Choose this endpoint when you need to programmatically verify that a FEN string is both syntactically well-formed and logically legal before passing it to a chess engine, storing it in a database, or displaying it to users. It is more thorough than a simple regex check because it also validates semantic constraints like king counts, pawn limits, castling eligibility, and en passant square consistency.","instructions":null,"reviewSummary":null,"reviewSummaryHighlights":null,"reviewSummaryConcerns":null,"reviewSummaryGeneratedAt":null,"activationCount":0,"lastUsedAt":null,"lastSuccessfullyRanAt":null,"lastHealthCheckAt":"2026-10-29T13:12:39.696Z","isFirstParty":false,"canonicalSlug":"chess-fen-validator-ffc1cc22"}