House Rules: Using Fate Assessment and Declaration rules in other systems (FATE, Cypher System, D&D 5e)

House Rule; A version of the Declarations rules for FATE are available as part of the Cypher system.

This is an expansion of the existing rules for spending XP to gain short/medium term benefits. Rather than gaining a skill or ability you declare a certain thing about the area, environment, or world to be true. This should have something to do with the nature of the character. For example, a character with the History skill might state that the decor of a palace dates is more than a hundred years old and that suggests a particular place where weapons would be displayed, and thus creates a place the characters can find arms in the heat of the moment, while a character who Employs Magnetism might determine that the table hidden under a tablecloth is made of a ferrous metal. Obviously, declarations cannot be made that relate to things already known to be false, such as trying to place such a weapons display in a location where the characters previously looked for them without success. While, generally, this will cost the player 2 experience points as a short/medium term benefit some declarations with more lasting significance may instead be 3 point long term benefits. Only rarely will a declaration will be instant, such as a fighter declaring that an opponent in the middle of an open ballroom has stepped directly below a chandelier.
GMs have the right to veto any suggestions that seem out of scope (or ask the player to revise them), especially if the rest of the group isn’t buying into it, or they can veto with an offer for the character to make an appropriate skill check to get alternate similarly useful information instead. Finally, the GM can simply veto by giving the player an experience point, as a player refusing a GM intrusion.

House Rule; A version of the Assessment and Declaration rules  are available in D&D.

The player can declare their character "notices" or "remembers" some piece of knowledge. Based on the obscurity of that knowledge the GM sets a DC and the player makes an Intelligence roll. Depending on the nature of the declared item, they may add their proficiency and other bonuses based on skills. For example, recognizing the god to which a now-defiled altar was once sanctified would be an Intelligence (Religion) roll or a warrior may use Intelligence(History) to know the background, and a particular weakness, of a fighting style an opponent uses.

If the DM has already determined a piece of information a player wants to declare, they treat the roll normally but provide the predetermined answer on a successful roll. Similarly, if a player attempts to determine information that the DM has not pre-decided after a successful roll the player may determine the answer as if they had made a declaration at the outset. As always, DMs have the right to veto any suggestions that seem out of scope, specifically asking the player to revise them, especially if the rest of the group isn’t buying into it.

Such pieces of information can be used to provide advantage or force disadvantage on a roll taken that they might influence. Only the character who made the discovery can use benefit unless they specifically share the information with another. After being used once it cannot be used again by any character with the party until after the character which used it takes a short rest.

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On one hand, it seems to me that making these things rules seems like it should be unnecessary. This seems, to me, like it's just good shared storytelling...

The Fate system, like many modern narrative systems, has a lot of interesting innovations. The one that absolutely caught my attention the very first time I read it, though, was the assessment and declarations rules as written in the Spirit of the Century. The idea of a formalized system where the players took part in worldbuilding was fantastic. I immediately started trying to figure out how to convert these rules for use in other systems - mostly 3.5 D&D and Requiem, as those were my major games at the time. Fast forward several years, and this is still a fantastic idea that needs more traction. Numenera is close with its multiple uses of XP. However, the idea of using XP to build and influence the world isn't quite present, preferring to limit itself to location-specific duration specific modifications to the character. This relationship between the FATE Core SRD writeup and the Numenera system made it almost unnecessary to do a translation.

D&D, on the other hand, has never had such a mechanism. It is designed much more from the origins of adversarialism than cooperative storytelling without any clear resource scaled to this sort of change. In fact, the only universal resource that could be expended in this manner without significantly distorting the system is hit points, which is both thematically completely wrong and still problematic in scale. The most rational thing to do was to lean back on the SotC rules with their skill checks. Of course, the same problems that make integrating these rules challenging mechanically extend into the application of the rules. Where areas of conflict in FATE and Numenera are generally very loosely defined spaces with connections between the typical Dungeon is meticulously mapped down to 5' squares where the characters stand. Every secret door and trap is carefully placed, and the features of interest are defined in advance. All of this combines to make the typical D&D experience pretty much antithetical to this sort of power sharing.

However, personally, I find that I'm constantly trying to draw my players into the exercise of worldbuilding when I GM. To get them to join me in writing their story, letting me add the improvisational theater "yes, but" to their statements, rather than simply fight through a story I tell them. I regularly take the ideas that float around the players and as far as my mysteries and go "why not?" Sometimes my players even know I've done that, because I may make it obvious to encourage this behavior. So, why not formalize rules that encourage that further? If the players are willing to sit and describe to me the evolution of the style of a minor group of enemies why shouldn't I reward them with some bonus die-rolls? In many ways this reminds me of a blog post I read once (and, sadly, cannot find now) where a 4th ed DM, lamenting that his players would never look further than their attack cards, created "improvised attack" basic attack cards and distributed them to his players. Things that shouldn't need to be said out loud, or codified, sometimes have to be approached in that technical manner to remind people to think about them.
And, if it makes knowledge skills more valuable (something I think they desperately need) then, double win.