I told you not to go for Kokushi

We’ve all been there: you’re dealt a hand with a lot of yaochuu tiles — ones, nines, and characters — and you find yourself asking, “Do I go for the stupid thing?”

Despite being one of the most common yakuman, Kokushi Musou still boasts a wide arsenal of ways to betray you. Those last few tiles never seem to come; your discards give away your strategy; you have no real defense; and if the other players discard all four of a tile you need, you can’t easily switch to something else.

So when do you go for it? An advanced player might say that you can do it when you start with 11 different yaochuu tiles, but if you’re playing a 3-player game on Tenhou, you can do it with 8. Aside from that, are there any situations in which it’s worth being more aggressive? Are there any strategic uses for the forced redeal (kyuushukyuuhai)? In this post, I will look at the strategies employed by players when they had the chance to go for Kokushi. What choice did they make? Were they successful? What situational information influences the decision to be more or less aggressive?

As usual, this data comes from querying the Tenhou statistics database maintained by bps. For this post, I looked at just over 9000 hands, each of which started with 9 or more distinct yaochuu tiles. All results came from 4-player games in the Houou lobby.

First off, let’s identify the strategies you can use when dealt a lot of yaochuu tiles:

  1. Redeal the hand (kyuushukyuuhai)
  2. Go for yakuman
  3. Try to make some other hand

Now let’s look at what choices people made when dealt 9, 10, 11, or 12 different yaochuu tiles:

Starting Yaochuu Redeals Kokushi Attempts Other Attempts
9 82.02% 4.72% 13.26%
10 44.16% 31.59% 24.25%
11 10.67% 64% 25.33%
12 0% 66.67% 33.33%

Let’s see the success rate. Did they get their Kokushi?

Starting Yaochuu Kokushi Gets Other Agari
9 5.5% 13.26%
10 13.55% 1.96%
11 35.42% 0%
12 50% 0%

A note on that 12-tile case: I only found 3 games in which that happened. That means there’s not enough data to make any meaningful analysis of that situation. Of course, 12 tiles is already 1 shanten if not tenpai for yakuman — you’d have to be crazy not to go for it! As for the “other” attempts — they include the cases in which they originally were trying for Kokushi but had to bail (either to defend or because Kokushi became impossible). As you can see, bailing from a Kokushi attempt doesn’t offer much hope of making any other hands.

What about speed? How many turns does it take to complete your Kokushi?

Starting Yaochuu Average Turns to Kokushi Std Dev
9 11.25 3.46
10 10.23 3.44
11 9.18 4.11
12 4 0

This doesn’t seem too helpful, particularly in light of the success rate highlighted above. I wouldn’t go saying you can turn 9 tiles into Kokushi in just 11 turns when only 5.5% of attempts to do so actually succeeded. Maybe instead we should look at how many turns elapsed between getting each new tile.

Progress to Kokushi Average Turns since Previous Tile Std Dev
10th tile 4.3 2.47
11th tile 4.91 2.93
12th tile 4.07 2.96
13th tile 2.24 2.26

This is slightly better since it takes into consideration all the attempts, including the ones that failed. Expect it to take 2-6 turns to get each new tile. Of course, if you have a pair, getting the 13th tile is much easier, since you can Ron it off someone else rather than waiting to draw it yourself.

We can assume from this data that it makes sense to go for Kokushi when you start with at least 11 of the tiles needed. A success rate of 1 in 3 is phenomenal for a 4-player game. Anything less than 11 and you might as well force a redeal. Or should you? Even a 10% chance of success could be worth the tremendous Yakuman payout. We can see that people did sometimes try for Kokushi even when dealt only 9 or 10 of the tiles they needed. Since this is the houou table, we can assume that any silly “limit maker” or other showy playstyles don’t come into play here. There must have been some situational factors that made these players decide the risk was worth the reward. If we take a look at these factors, maybe we can identify if there is a consensus on when to be aggressive and when to play it safe.

One obvious factor would be placement by score. Here’s the frequency of forced redeals separated by place:

Score Placement Ryuukyoku with 9 tiles Ryuukyoku with 10 tiles
1st 84.52% 51.52%
2nd 84.14% 49.61%
3rd 84.27% 43.54%
4th 73.18% 32.29%

We can clearly see a strong preference to avoid last place here. The desperation to get out of last is so powerful that some players are even trying to make Kokushi with a “bad” starting hand.

How about early vs. late game? Here’s the frequency of forced redeals separated by phase:

Phase of game Ryuukyoku with 9 tiles Ryuukyoku with 10 tiles
Early 83.75% 39.69%
Early Mid 82.38% 42.32%
Late Mid 82.20% 44.79%
Late 79.29% 50.94%

This unveils some interesting information. The placement chart showed something like “desperation” at play, but this seems to show more of a calculated gamble, and highlights the difference between a 9-tile and a 10-tile start. This chart tells me that going for Kokushi with only 9 tiles is a “bad” move, but trying with 10 tiles could be called “edgy”. While your chance of agari is maybe half what it would be for a typical hand, the payout is four times or more! In addition, when you take a gamble in the first round and fail, you have the whole game to recover. If you take a gamble in the final round, however, there is no recovery should you fail.

What about seat? Does being Oya make players more or less likely to try for Kokushi? Here’s the frequency of forced redeals separated by seat:

Seat Ryuukyoku with 9 tiles Ryuukyoku with 10 tiles
East 90.36% 62%
South 81.8% 43.74%
West 79.24% 35.7%
North 76.57% 34.94%

This shows a huge preference to avoid risk when Oya. This makes sense: as dealer, you don’t want to invest too heavily in a slow and risky hand. The chance of someone else getting a tsumo can be pretty high when you are waiting for the last few tiles, and you stand to lose more points as dealer. I think that most players would prefer to make fast and cheap hands during their dealer turn. Interestingly enough, people are more likely to take the risk and go for Kokushi when they are North, moreso than when they are South or West.

So, to recap:

  • Go for Kokushi when you start with 11 or more tiles. The odds are actually better than your odds with a typical starting hand.
  • Go for Kokushi with 10 tiles only when the points are important in the long run — for example, in a tournament where points carry over from each game.
  • Don’t attempt Kokushi if you only have 9 tiles.
  • Players who are in a position to take risks will often go for Kokushi with “bad” hands.

Finally, feel free to post any interpretations you might have for the data shared here. I’m sure there are quite a few strategic angles that I’ve overlooked. I also haven’t covered defense. Maybe that will be a future article?

12 thoughts on “I told you not to go for Kokushi

  1. “Finally, feel free to post any interpretations you might have for the data shared here.”

    Can you get any info about the actual starting hand? As in:
    -already have head?
    -missing more numbers or kanjis? Seems to me kanjis are more likely to be discarded early in the game so there is less chance to get a chance at ron to get the last one -although if other players do read your attempt you won’t get any either, but kokushi can be deceiving as you can discard the double tiles.

  2. Hi,
    Yeah, that’s me, I’m still working on it although for a lot of different reasons (free work vs. paid work, difficulty of getting translations, getting more material) it is very slow. By the way, if someone’s interested in watching interviews and providing me with translations, that’d be awesome :)

  3. OK, where do I send them? Also, if you don’t have time for a translation, a transcription would help, my Japanese level is enough to translate written material!

  4. Here’s my take on Kokushi – by which, I’ll completely ignore all those fancy numbers, by establishing some kind of general principles as to when to actually go for it, based on these two questions:

    1. Can you afford to go for it?
    2. Do you want to go for the kill, right then and there?

    Of course, these two questions can apply to any hand; but the subject matter of this entry is Kokushi itself. Yet, before I continue, I will say this: “I hate Kokushi”. With this, most of my kokushi attempts its tied with the thought of “Am I really gonna go for it? Fine, whatever.” Now going back to the fancy numbers, most Kokushi attempts tend to fail anyways.

    Now, if anyone has never gotten Kokushi before, might as well go for it – just for the sake of saying: “I’ve completed it at least once.” After that, be happy with it.

  5. My first Kokushi happened in a live game and started
    with only 7 terminal/honors but since then with the
    digital ones i have made it 4-5 times!

    It is one of my favorite yakumans but if we count that
    in my ~2500 hanchans in tenhou i am going for it
    maybe once in every hanchan the rate is 2-3 out of
    2500 so yes the precentage is so low you don’t go
    “to win” but just “for fun”

    I have paid Kokushi when i was really focusing on my
    tiles that i did not notice the kamicha’s discards which would
    have made it obvious XD

  6. I once saw a hand with 5 tiles become kokushi (only super late game, 2 tiles left in the wall, but still…)

    Kokushi is my favorite yakuman, but nowadays I almost always click the draw button and only go for it when 2-shanten (point difference is a factor, too)

  7. When it comes to Kokushi, I’m more open-minded than most people. Sure, it’s easy for your opponents to figure out your game plan by looking at your discards. But at the same time, since your hand is entirely concealed, it’s impossible to tell whether or not you’re already in tenpai. Without this information, most people are unwilling to break-apart their hand to defend against a Kokushi, because the odds of dealing into a Kokushi is extremely low.

    The downside of attempting Kokushi of course is that once the plan fails, transitioning into another winning hand is nearly impossible. You’re pretty much left with no other option but to forfeit the round. Still, with this many unwanted Yaochuus under your possession, you should never have to worry about dealing into another player’s hand. So even if you don’t win with your big hand, at least you have nothing big to lose.

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