Pondering about input and output randomness in ttrpgs

31 May, 2026

Have you all seen that Ben Brode talk about designing Marvel Snap? Have you also seen that Prismatic Wasteland is doing that thing he does where he keeps telling people to write blog posts? Well it inspired me to write a post, and look at that I rolled a 1. Twice.¹ No big deal, let's write the post then. I won't be rolling in the other topic and length tables since I already have some stuff in mind to talk about.

So, in that Ben Brode talk, he mentions the concept of input randomness versus output randomness. In the former, you first get a random number, then you get to make choices about it - like drawing a hand of cards, then choosing the one to play. In the latter, you first make a decision, then determine how it turns out using random numbers. Like saying what you're going to do then rolling a die to find out if it works. Both of them have their pros and cons, and some games lean harder on one over the other, while others try finding a balance (or having no randomness at all).

While remembering this talk, I was thinking about how your traditional ttrpgs generally favour output randomness a lot. Part of the fun is the supposed infinity of actions you can take, which then need to pass the trial of the dice to affect the world. I think it would be naive to say that input randomness has no part in it since, after every roll of dice, you generally see what happens, maybe suffer some consequences, and then regain control of your character, being able to act upon the random input which was the last action's output. Some would argue this makes your big bookstore ttrpgs strike a good balance between these two types of randomness, but I still think output randomness prevails. Despite a lot of these games not spelling it on the rulebook, people are playing to find out what happens.²

However, a more solid case of why this is not input-output balance is the fact that I think there is a game which works with this balance a lot better, Josh McCrowell's lovely dungeon crawler, His Majesty the Worm. In HMTW, you have your traditional "describe your action, then roll" (draw a random card, in this case) output randomness procedure for when you're doing most things, but during the Challenge Phase (generally combat), you actually get to draw four cards and assign them to actions during each turn, then redrawing after everyone has acted. This is probably my favorite combat procedure in all ttrpgs I've ever played, except for, well, not having a combat procedure.

I'm not going to write about the procedure in detail, but suffice to say that I think it's elegant enough to design a whole game around it. I tried to design something similar in the past with Magic: the Gathering cards, but never got around to playtesting it.

Ok then, here's my blogwagon post. "Posted as soon as I could", so it's not my fault if it's short, shallow or full of typos. Looking forward to reading the other ones.

anb

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