The framing is the establishment of some shared fiction, which speaks to the PCs' dramatic needs. It doesn't dictate options.
The players learning that it will be hard for the PCs both to befriend the baron and deal with the leader of the hobgoblins doesn't dictate their options. It does establish a context for making choices that will tell us something about these protagonists . . .
. . . As for the bit about the ability of the players to author things into the fiction - I will repeat again that the games I GM generally do not involve player fiat authorship, and in my view that this is largely a red herring as far as player agency in respect of the shared fiction is concerned.
"Is there a vessel in the room that would allow me to catch the mage's blood?" isn't "authoring things into the fiction." It's just an action declaration.
You've lost me here for a bit, @
pemerton --- "Is there a vessel in the room that would allow me to catch the mage's blood?" isn't an action declaration, it's a query --- I'd call it a "scene frame query." It's asking for confirmation from the GM about a particular element within the scene frame.
An action declaration would be, "I find a vessel in the room that would allow me to catch the mage's blood, and I start capturing the blood." An action declaration of this type does include direct player authorship of something into the fiction---the presence of the vessel.
So how, then, do we move from "scene frame query" to the actual action declaration, "I find a vessel in the room that would allow me to catch the mage's blood, and I start capturing the blood"? From a practical, in-game perspective of actually GM-ing, this is where it starts to get interesting for me.
1) The GM can simply agree with the player without any consultation to game mechanics. "Oh yes, of course, there's a small ceramic pot with a plant in it, you can easily toss out the plant and collect some of the blood." (In some cases the GM may not even acknowledge the act of finding the vessel, but move immediately to the narration---"Okay, you're now catching the dripping liquid into an urn.")
2) The GM can consult a chart or establish an ad hoc probability and roll against it. "Oh, you know what, there might be a vessel --- roll a d20, on 7+ there is, on 6 or less, there isn't." Or perhaps, "Hmmm, I don't know, let me consult this 'stuff in a typical wizard's tower' chart and see what we get."
3) The GM asks the player to process the frame query through some relevant PC mechanics. "Sure, roll a search check, on a success, you've found a vessel, on a success of 5 or more, you find a perfectly clean, pristine pot or urn that will not taint the blood."
Which, if any of these, are you advocating for?
Then let's say a session later, the player who captured the blood says, "Hey, you know what? I bet that tainted sorcerer's blood has some cool magical properties. I bet if I sprinkle it on food, the food will become toxic." This would definitely be player-authored fiction. How as a GM would you determine whether this is "allowable" fiction?
Of course, the PCs may also be unaware of the "secret backstory"---"The PCs can take the blood of the dead sorcerer to Goulash the Dread Witch of Lower Sobovia, who can only be found in sector X55 of the map and the PCs will only discover sector X55 by singing a bardic tune to the stone giantess. Upon receiving the blood, Goulash will grant them 15,000 gold pieces and bake them apple pies to give to the winter wolf." <<<< If THIS is the kind of worldbuilding you're against, I totally see your point; this is rather useless trivia to the players, that will likely never be useful or speak to any sort of dramatic need. It's mere window dressing included only because the GM really likes the metaphorical "sound of their own imagination."
But this is a different play component from querying the scene frame and determining the state of the fiction through either agreement or mechanical/probabilistic outcome resolution.
The more I consider it, the more I'm not sure that the concept of "secret backstory" is really all that helpful. The principle of the matter is, "Be open to letting players really advocate for their characters, and leave elements of the fiction open to letting them do that." As a GM, I'm simply not inclined to ask myself about every single thing I note down about the game world and get all up in arms and anxious about, "Oh crap, have I introduced the dreaded HIDDEN BACKSTORY!!!????" If it turns out later that some piece of pre-authored fiction could be altered in response to what the players are doing and the stakes in the fiction, then just
change it. I don't think it does much good to worry about it before hand.
The question each group has to answer is, at what scale or level of specificity are the players allowed to either A) outright introduce elements to the fiction, or B) query the scene frame to determine if a fictional element can appropriately be introduced?
And then find a system that successfully meets the criteria of their answer.