Problem Forge
These rules say what the site does. The terms are the binding text; where the two differ, the terms win.
Months
1. A month opens at 00:00 UTC on the 1st and ends at 00:00 UTC on the 1st of the next month. Every month has new rungs. Progress, clocks and the month's boards start empty.
2. The ten problems take answers while their month is open. The server moves to the new month at or within a minute after 00:00 UTC on the 1st; from then on the old month takes no answers. An answer received at or after 00:00 UTC on the 1st does not count, even if it arrives before the server has moved. The Problem of the Month opens and closes on the clock, at 00:00 UTC exactly.
The ten problems
3. Each problem has rungs 00 to 12. They become available in order: rung 00 first, and solving a rung makes the next one available.
4. Rung 00 is practice. You must solve it before rung 01 is available. It adds nothing to your score and nothing to your time.
5. Each rung is your own copy: a file made for your account alone. Another climber's answer does not solve it.
6. An answer is one integer, written in digits. Spaces and underscores in it are ignored. Type it, upload it as a text file, or upload a .tar.gz or .gz of that one file, on the rung's page or through the API.
7. Each answer gets one of three verdicts: accepted, not accepted or malformed. No reason is given. Your first accepted answer solves the rung; no more answers to it are taken. An answer that is not all digits (once spaces and underscores are dropped) is refused with a reason before it is sent, and costs no minute.
The clock
8. You open a rung by getting your copy for the first time, and that starts its clock: the file or its .tar.gz downloaded from the rung's page, or the instance fetched through the API, all count the same. Nothing else opens a rung: not looking at its page, not pressing "Show this problem" (which only shows the rung's name and download link), and not a copy we prepare ahead. Rung 00 needs no press; for most problems its copy is shown on its page, and that view opens it. Where it is not shown, your first answer to it opens it.
9. The clock stops at the moment we receive your first accepted answer. The time of receipt counts, not the time of checking.
10. Downloading the copy again does not restart the clock.
11. A rung's time counts only once it is solved. An unsolved rung, and rung 00, add no time. Times are kept to the millisecond.
Answers and limits
12. One answer a minute for each rung, counted from your last answer to it, whatever its verdict. The page tells you to wait; the API replies 429 with the seconds left.
13. A wrong answer costs nothing but the wait.
14. If our checker cannot be reached, the answer does not count and the minute is given back.
15. There are also technical limits against overload.
Score and order
16. Your score for a month is the number of rungs 01 to 12 you solved in it, on the ten problems and on the Problem of the Month: one point each.
17. Order on every board: more points first. On equal points, less total time, the sum of the solve times of the rungs that scored. On equal time, the earlier last scoring answer. After that, whoever first had an accepted answer on that board (rung 00 counts here).
18. You appear on a board once you have a point on it.
The boards
19. Per problem, this month: points and time on that problem, with the highest rung solved.
20. All problems, this month: points and time on the ten problems and the Problem of the Month together.
21. All time: the sum over every month, in the same order.
22. Problem of the Month: one board per edition (the month itself, or a preview month before it), the same measure, from that edition's rungs 01 to 12.
23. Each board shows the top 100. Signed in, you also see your own place, counted among every row of that board.
24. A month's boards stay as they were when it ended.
25. Honours: places 1 to 3 on a problem's board for a month, or on a Problem of the Month board, are listed on your profile.
26. We may correct a board after a bug or an outage, and we say so on the site.
Firsts
27. For each problem, each rung from 01 to 12 and each month, the First is the first accepted answer to any copy of that rung. Rung 00 has no First. The Firsts board shows, per rung: who sent it and when; the quickest time (the least solve time, a tie to the earlier answer); the average and median solve time; "opened by", which counts the climbers who pressed "Show this problem", downloaded the rung or solved it (wider than opening in rule 8); and percent solved (solvers as a whole percent of that count). The Problem of the Month editions that score in the month have their own rows, measured the same way; there "opened by" counts the climbers who got the graph or key, or solved it. Firsts change no score.
Problem of the Month
28. It has rungs 00 to 12, open in any order; no rung needs another solved first. Each rung has its own clock.
29. Its clock starts when you first get the rung's graph: "Show my graph" (rungs 00 and 01; in the preview, also "Show my key" on rungs 06 and 07), a download, the .tar.gz, or the API's instance call. It stops at your first accepted answer.
30. An answer is one integer that encodes the cycle's edges, in the same form as Hamiltonian Cycle and TSP answers: its bit i (bit 0 the lowest) is 1 exactly when edge i of your file is in the cycle. Exactly n bits are 1, each for an edge of your graph, and those edges form one cycle through every vertex. Any Hamiltonian cycle is accepted. A refused answer comes with the reason. In the preview, before December 2026, an answer is every vertex in cycle order.
31. Rungs 01 to 12 score one point each, with their times, on the edition's own board and on the all-problems and all-time boards of the month in which the edition runs. Rung 00 scores nothing.
32. A preview edition runs for one calendar month, on its own graphs and its own board, and counts toward that month. Nothing carries over to the next edition.
33. After an edition closes, no answer is taken. Your graphs can still be downloaded.
Staff, suspended and erased accounts
34. From the 2026-11 month on, staff accounts are on no board of that month and take no First; the First goes to the first solver who is not staff. From 2026-11-01 00:00 UTC they are off the all-time board. October 2026 keeps them on its boards, marked "staff". Staff get no honours in any month.
35. A suspended account is off every board and the Firsts while it is suspended. Its rows come back if it is unsuspended.
36. After a wipe or an erasure, the account's board rows and Firsts stay, shown as "<anonymous>". Its answers and times are deleted, so it leaves the Firsts board's time and count columns.
Released answers
37. When a month ends, an account made before the end gets, for each of the ten problems, the answer to its own copy of the rung that stopped it: the lowest rung it did not solve. Solve all twelve and there is nothing to release. See Released answers.
Statistics
38. The statistics pool every month, per problem and rung, without names: attempts (first downloads, or for rung 00 a first answer sent without one; one per climber per rung and month), successes (accepted answers), their ratio, the median solve time and a histogram of solve times. Rung 00 is included.
39. On the climb and rung pages, "answers sent" counts every answer sent to the rung by all climbers, whatever its verdict; "solved" counts the accepted ones.
Bounty problems
40. Bounty problems are outside the monthly score. Each has its own terms; see the outline. One claim a minute; the first claim received and verified wins; staff are not eligible. No bounty is open for claims yet.
Guidelines
41. Guidelines, not rules we enforce: one account per person; and do not post answers to a live month's rungs before the 1st of the next month (see the terms).