Forty Thieves Solitaire: the two-pack patience that rarely gives in
Forty Thieves looks demanding on paper, and our tests confirmed it. It is a two-pack game with forty cards laid out in ten columns, a long stock you get to see only once, strict in-suit building and single-card moves. A computer player we built for it, which looks a few hundred positions ahead but cannot see the order of the stock, won just 5.8% of 3,000 deals.
That number needs a warning attached. Our player is simple, and a thoughtful person using Undo will certainly do better, so 5.8% is a floor, not a measure of how many deals can be won. And there is nothing published to compare it with: the University of St Andrews study that we check our other results against does not cover Forty Thieves.
The numbers, with their limits
| Measure | Result | Deals |
|---|---|---|
| Our computer player's win rate | 5.8% (95% range 5.0–6.7%) | 3,000 |
| Cards the player got home, on average | 34 of 104 | 3,000 |
| Moves in a won game, including stock turns | about 277 | 174 wins |
| Deals with an Ace playable from the opening layout | 57% | 3,000 |
| Published figure (St Andrews study) | None: game not covered | – |
For what it is worth, solitairex.io lists Forty Thieves among its hardest games, at a win rate of 11% or less. That site gives a range without saying how it was measured, so we mention it only as a rough point of comparison.
Why it is so hard
One card at a time
You may only move a single card. A tidy sequence of five spades cannot be shifted in one go; you would need somewhere to park four of them first.
In-suit building
A card can only go on the next higher card of its own suit. With two packs that means exactly two possible landing spots for each card — and often both are buried.
One pass through the stock
Sixty-four cards wait in the stock, and they come out one at a time with no second pass. A card you let slip past onto the waste can only be played from the top of the waste.
A slow grind
Even in the games our player won, it took around 277 moves. Across all games, it averaged only 34 of the 104 cards home before getting stuck.
How the game is played on this site
Layout
Two packs, 104 cards. Forty are dealt face up into ten columns of four. The other 64 form the stock.
Rules
- Build down in suit: the 8 of hearts on the 9 of hearts.
- Move one card at a time.
- Any card may go into an empty column.
- Turn cards from the stock one at a time onto the waste; the top waste card can be played. There is no redeal.
- Eight foundations, two per suit, each built from Ace to King.
Winning
Get all 104 cards onto the foundations.
Strategy for a tough game
Get the early Aces up
In 57% of our deals, at least one Ace was available in the opening layout. Play it, and look for the 2 that follows.
Empty a column as soon as you can
With single-card moves, an empty column is the only real room to manoeuvre. Emptying one is worth more than almost any other move.
Don't fill an empty column with just anything
Any card may go in, which is tempting. Put in a card that starts a useful run, or one that frees something important underneath.
Watch the waste
Because the stock comes round only once, think before turning the next card. A card that could have gone somewhere may end up buried in the waste.
Use Undo generously
With a game this unforgiving, trying a line and stepping back is the best way to find a route through.
Forty Thieves on solitaire-free.uk
The game is free in any modern browser on computer, tablet or phone, with nothing to download and no account needed. Cards are large and clear, and the zoom control helps with ten columns and a long stock. Hint, unlimited Undo, Restart and Auto Play are all there; you can drag or tap cards; and wins, time and moves are tracked for the current game. Sound can be switched off, and there are fullscreen and table-colour options.
How we ran the research, step by step
- Rules from the game code. Two packs, ten columns of four face up, a 64-card stock turned one at a time with no redeal, building down in suit, one card at a time, any card into a space.
- Rebuilding the game. We wrote Forty Thieves in Python, adapting the program we built for Diplomat, another two-pack game.
- Numbered deals. 3,000 deals (seeds 1 to 3,000), each shuffled from a numbered seed.
- The player. A look-ahead player that searches up to 300 positions per turn among the moves it can see. It does not know the order of the stock and never takes a move back.
- Why no solver. Unlike our FreeCell work, we did not build a solver that sees every card for this game, so we have no ceiling to set beside the floor.
- Checking against published research. The St Andrews study (Blake and Gent, Journal of Artificial Intelligence Research, 2026) does not cover Forty Thieves.
- Margin of error. Our 5.8% has a 95% range of 5.0% to 6.7%.
- Limits. The player is weak. Please don't read 5.8% as "how winnable" Forty Thieves is; it is what a simple program managed.
Behind the numbers: the code
The program is fortythieves.py, written in Python 3.11 with only the standard library and built from our Diplomat code with the rules changed. On each turn the player searches ahead through up to 300 positions it can reach with the cards it can see, and picks the move that leads to the best-scoring position. The 3,000 deals were shared among the cores of one machine using Python's multiprocessing, and the run took well under a minute. Ranges use the Wilson score interval. Claude, the AI assistant from Anthropic, helped us write and review the program and draft this page. The figures come from running the program, not from the AI.
Frequently asked questions
Is Forty Thieves really that hard?
It is one of the harder games on the site. Our simple player won fewer than one game in sixteen.
Can a person do better than your program?
Yes. It cannot undo moves or see the stock. A patient player with Undo should beat 5.8%.
How many deals can be won?
We don't know. We did not run a solver that sees all the cards, and the St Andrews study does not cover this game.
Can I go through the stock twice?
No. Forty Thieves here allows a single pass.
What is a gentler two-pack game?
Diplomat uses a similar layout and our player won far more often; Solitaire Double is gentler still. See also Spider Solitaire 4 Suits for another tough test.
By Admin, solitaire-free.uk. Study date: 19 September 2026.