En Passant Legality Checker
Check standard FEN immediate en-passant assertion, side/pawn geometry and resulting own-king safety including discovered checks; plausible FEN history is trusted, not fully reconstructed.
Description
Legal en passant requires the FEN target and a pawn capture that leaves the moving king safe.
Legal en passant requires the FEN target and a pawn capture that leaves the moving king safe. Removing the captured pawn can uncover a line attack.1
Enter supported FEN and lowercase from/to squares. Check pawn geometry and FEN en-passant target, then simulate the move and test own-king attacks. Return legality.
When to use En Passant Legality Checker
Use En Passant Legality Checker when you need to evaluate the supplied records under the calculation and evidence rules below. The FEN is trusted for immediate history; earlier moves and complete game state are not reconstructed.
Prepare the supplied game state, observations and explicit variant settings. Follow the card, board, coordinate and state conventions in the calculation rules.
How to use En Passant Legality Checker and understand its results
- Prepare the supplied game state, observations and explicit variant settings. Follow the card, board, coordinate and state conventions in the calculation rules.
- Enter positionFen, fromSquare, toSquare using the field table and calculation rules, then run the tool.
- Read legalImmediateEnPassant. The explanation below defines the units, signs, ordering and any flags or placeholder values.
- If the tool reports an input issue, correct the values and run it again. Apply the stated assumptions when interpreting a successfully returned result.
| Input field | Meaning | Value type; see rules below | Requirement |
|---|---|---|---|
| positionFen | Position fen | string | Required |
| fromSquare | From square | string | Required |
| toSquare | To square | string | Required |
Enter supported FEN and lowercase from/to squares. Check pawn geometry and FEN en-passant target, then simulate the move and test own-king attacks. Return legality.
simulate pawn relocation and captured-pawn removal; reject own king attack Units, indexing, boundary conditions and exceptional cases are defined in the calculation rules above.
| Output field | Meaning | Value type |
|---|---|---|
| legalImmediateEnPassant | Legal immediate en passant | boolean |
{
"input": {
"positionFen": "4k3/8/8/3pP3/8/8/8/4K3 w - d6 0 1",
"fromSquare": "e5",
"toSquare": "d6"
},
"output": {
"legalImmediateEnPassant": true
}
}Limitations and assumptions
The FEN is trusted for immediate history; earlier moves and complete game state are not reconstructed.
Numeric inputs, where present, must be finite. Follow the calculation-specific rules for overflow, underflow and exceptional cases. Extremely large text inputs are rejected. Missing required fields, extra fields, wrong value types and violations of the calculation rules are input errors. A successfully returned false flag is an evaluated outcome; its meaning is explained above.
Alternative or Complementary methods
Use Castling Rights Checker: Standard chess castling needs rights, an empty path and safe king start/transit/destination squares. Queenside also needs an empty b-file square. For a complementary task, use Threefold Repetition Counter: Chess repetition compares position identity while ignoring move clocks. An en-passant target changes identity only when a legal capture exists.
References
-
En Passant Legality Checker — tool.io
Similar or alternative tools
- Castling Rights Checker
Standard chess castling needs rights, an empty path and safe king start/transit/destination squares. Queenside also needs an empty b-file square.
- Threefold Repetition Counter
Chess repetition compares position identity while ignoring move clocks. An en-passant target changes identity only when a legal capture exists.