What are the rules of Slitherlink?

Each numbered cell has exactly that many Line edges around it. A 0 has none, and a 3 has exactly three. Each used dot has two lines, so branches and dead ends are invalid. All selected lines must form one nonempty closed loop; two separate loops fail even when the clues match. Mark every remaining edge Cross before completion, because Unknown is not a solved decision.

How do I mark edges with the keyboard?

Choose Line, Cross or Unknown and select an edge. You can also focus an edge and press L for Line, X for Cross, or U, Backspace or Delete for Unknown. Arrow keys move among edges, Home and End move within an edge row, and Enter or Space applies the selected mark. Horizontal edges are listed before vertical edges. The board scrolls inside its panel on smaller screens; every edge has a 44-pixel target and a spoken row, column and mark.

Are the generated puzzles uniquely solvable?

The original generator grows a cell region while retaining a single simple boundary, then keeps the clue in every cell. Exact edge search counts solutions up to two while checking cell counts, vertex degrees and one-loop connectivity. A board is offered only when exactly one solution is proved. Generation is bounded by 128 candidates, 500000 search nodes and an eight-second worker limit. Failure or cancellation keeps your previous puzzle.

How is a shared challenge scored?

The fixed challenge is a seeded 6 × 6 board with no timer. Score is max(0, 1000 minus final retained Line or Cross moves), within 0–1000; higher is better. There are 84 edges, so a perfect decisive history earns 916 points. Clearing to Unknown and repeating the existing mark add no decisive move. Undo removes the undone move and redo restores it. Hints, revealed answers, answer-key printing before completion, restored puzzles and previously opened equivalent boards are practice. Scores are self-reported.

What happens when someone opens my challenge link?

They see your claimed score and the fixed rules, then choose Start same challenge. After their fresh unassisted completion, the page shows both scores and the signed difference, says whether their score is better, tied or lower, and offers a new link with the same seed and their own score. Copy, native sharing and a result-card PNG become available only after an eligible completion. The link contains no solution or move history.

Can I print a puzzle without its solution?

Print puzzle prepares only the original numbered clues and dots, without current player marks. Print puzzle and answer key adds the unique loop on a separate page and makes an unfinished run assisted practice. Reviewing or printing the answer after a fresh completion preserves its earned score.

What is saved in this browser?

Touched settings and progress stay under this tool's local storage key. The save excludes the solution; resuming regenerates the seeded board and validates its clues, edge marks and retained history, then treats it as practice. Up to 1024 clue-grid exposure fingerprints also stay local, including equivalent rotations and reflections. Additional boards stay practice when that memory is full. Storage failure is shown without removing the puzzle currently in memory. Nothing is sent to a server.

Slitherlink Puzzles

Draw one closed loop around an original numbered grid. Each clue tells you how many of its four surrounding edges belong to the loop, and every used dot must connect exactly two lines. Generate a 3 × 3 through 8 × 8 board, mark edges as Line or Cross, inspect clue and connectivity feedback, and print the original clues. Every offered board is proved uniquely solvable before play. A fresh 6 × 6 completion can share its seed and self-reported move score for a same-puzzle comparison.

Original numbered-cell loop puzzles. Every offered board has exactly one proved solution.

Choose a puzzle3 × 3 through 8 × 8

Board sizePuzzle seed

Seed: whole number from 0 to 4294967295.

All three Slitherlink rulesCurrent feedback

Match each clue
Each numbered cell needs exactly that many surrounding line edges. Cross marks mean no line; unknown edges still need a decision.

Zero or two at every dot
A used vertex must have exactly two lines. A dead end or a branch is invalid.

Exactly one closed loop
All line edges must connect into one nonempty loop. Two separate loops fail even when every clue matches.

Original cell-region growth produces a simple boundary. All cell clues are retained, then exact edge search counts solutions up to two. Only a proved unique candidate is published, within 128 candidates, 500000 search nodes and an 8-second worker limit.

Choose a size and seed, then generate an original Slitherlink puzzle.

Board: Not started

Status: Not started

Decisive moves: 0

Unknown edges: Not started

Worked rules

A clue of 0No surrounding lines
Cross all four edges. Even one line exceeds the clue.
A clue of 3Three surrounding lines
Exactly three edges are Line; the fourth is Cross.
Two closed loopsCounts and degrees may still match
Invalid. All selected lines must form one connected loop.

A decisive move changes an edge to Line or Cross. Repeating its current mark or clearing to Unknown adds no decisive move. Undo removes the undone move, and redo restores it; only the final retained history is scored. A 6 × 6 board has 84 edges, so a perfect decisive history earns 916 points. Completed results remain fixed. Touched progress saves locally, without the solution, and is revalidated as practice on reload. Up to 1024 clue-grid exposure fingerprints stay local; additional boards become practice when that memory fills.

Accuracy. Each clue equals the number of surrounding loop edges; the result is exactly one nonbranching closed loop. Original generation must prove uniqueness under a bounded search budget.

Common questions

What are the rules of Slitherlink?: Each numbered cell has exactly that many Line edges around it. A 0 has none, and a 3 has exactly three. Each used dot has two lines, so branches and dead ends are invalid. All selected lines must form one nonempty closed loop; two separate loops fail even when the clues match. Mark every remaining edge Cross before completion, because Unknown is not a solved decision.
How do I mark edges with the keyboard?: Choose Line, Cross or Unknown and select an edge. You can also focus an edge and press L for Line, X for Cross, or U, Backspace or Delete for Unknown. Arrow keys move among edges, Home and End move within an edge row, and Enter or Space applies the selected mark. Horizontal edges are listed before vertical edges. The board scrolls inside its panel on smaller screens; every edge has a 44-pixel target and a spoken row, column and mark.
Are the generated puzzles uniquely solvable?: The original generator grows a cell region while retaining a single simple boundary, then keeps the clue in every cell. Exact edge search counts solutions up to two while checking cell counts, vertex degrees and one-loop connectivity. A board is offered only when exactly one solution is proved. Generation is bounded by 128 candidates, 500000 search nodes and an eight-second worker limit. Failure or cancellation keeps your previous puzzle.
How is a shared challenge scored?: The fixed challenge is a seeded 6 × 6 board with no timer. Score is max(0, 1000 minus final retained Line or Cross moves), within 0–1000; higher is better. There are 84 edges, so a perfect decisive history earns 916 points. Clearing to Unknown and repeating the existing mark add no decisive move. Undo removes the undone move and redo restores it. Hints, revealed answers, answer-key printing before completion, restored puzzles and previously opened equivalent boards are practice. Scores are self-reported.
What happens when someone opens my challenge link?: They see your claimed score and the fixed rules, then choose Start same challenge. After their fresh unassisted completion, the page shows both scores and the signed difference, says whether their score is better, tied or lower, and offers a new link with the same seed and their own score. Copy, native sharing and a result-card PNG become available only after an eligible completion. The link contains no solution or move history.
Can I print a puzzle without its solution?: Print puzzle prepares only the original numbered clues and dots, without current player marks. Print puzzle and answer key adds the unique loop on a separate page and makes an unfinished run assisted practice. Reviewing or printing the answer after a fresh completion preserves its earned score.
What is saved in this browser?: Touched settings and progress stay under this tool's local storage key. The save excludes the solution; resuming regenerates the seeded board and validates its clues, edge marks and retained history, then treats it as practice. Up to 1024 clue-grid exposure fingerprints also stay local, including equivalent rotations and reflections. Additional boards stay practice when that memory is full. Storage failure is shown without removing the puzzle currently in memory. Nothing is sent to a server.

Each clue equals the number of surrounding loop edges; the result is exactly one nonbranching closed loop. Original generation must prove uniqueness under a bounded search budget.