Mallory, msg #24 in Chapter 4-C: The Host Tower of the Arcane:
Tressym can innately detect the presence of poison through scent, taste, and touch, and see Invisible things, so Astre is doing his usual job of sniffing out anything amiss from the tea set Harai prepared or if there's anything Invisible (such as a Scrying Orb) in the room with them, reporting to Mallory telepathically, and then going back to his pocket space regardless of the outcome. Not sure if a Perception check is required here given the proximity, but Astre would roll for poison detection at Advantage from his Keen Smell ability.
The Tressym can detect whether a substance is Poisonous by Taste, Touch, or Smell.
Range
While it is true that a Tressym can detect if something is Poisonous through either Taste, Touch, or Smell without the need for a roll, a range to the ability is not specified, nor is there any mention on the matter of checking many different items at the same time to determine which (if any) items are poisonous. There are two different interpretations of the rule the way I see it:
- The Tressym can determine whether or not one specific item is Poisonous by being in close proximity to it and sensing it specifically.
- The Tressym can determine whether or not any item in the range of its sensing is Poisonous.
2 is obviously more powerful than 1. One is a close-proximety detector where each item has to be sensed in turn, while the other is an ambient Poison detector running continously. In the case of Taste or Touch, it is obvious 1 is the correct interpretation, because you have to be in direct contact with something Taste or Touch to sense it specifically. Smell is different, but my initial inclination is to interpret it to be limited in the same way Taste or Touch is, since they are all mentioned with equal emphasis. But Smell is obviously different from Taste or Touch and can be better compared to Hearing or Sight. should we accept 2, two outcomes are possible in the case where Poison is detected:
- The Tressym knows there is one or several Poisonous item in the range of its Smelling ability without knowing how many or which specifically.
- The Tressym knows exactly which item or items in the range of its Smelling ability is Poisonous and how many there are of them without the need for further investigation.
Now, you do mentioned Astre is only checking the tea (specific), but that the casting is done subtly and out of Harai's line of sight (distant). It is logical that unless Harai's line of sight is diverted through distraction, the tea will be within it. Hence, Astre is not able to stick his snout into the cup without being discovered.
But he might not have to depending on the range of the ability. It's not specified, so it's probably just up to each DM. I'll have to arbitrarily settle on a number as a cats range of smelling in the real world is too far to inform my decision (4 miles is probably too far). This is also a Magical ability we can assume is inherent only of Tressyms, so it's direct transcription to a cat's ordinary smell is not given.
I'll rule that the Tressym can detect if any items within 15' are Poisonous, as long as the smell is not restricted or blocked from reaching Astre (e.g. kept in a container which fully prevents the smell from leaking). Astre is certainly within 15' in this specific circumstance.
Accuracy (Proximity and Direction)
Is Smell a directional sense? If Astre moves, he can probably determine where the scent is coming from, but can he if he remains stationary? Hearing and seeing is directional because we (and Tressyms) have two eyes and two ears, each of which registers the sensory input slightly differently (angle in the case of eyes, and time in the case of ears). We (and cats) also have two nostrils, but they are so close together that we humans, at least, are incapable of immediately determining the proximity and direction of the smell without multiple samples (i.e. changing our reference point by moving). It is possible a cat can.
A Hammerhead shark can, because its nostrils are spaced so far apart. This is an aside anyway, as Astre can probably accurately determine the proximity and direction by moving his head slightly.
With this in mind, I'll rule that the
Tressym can accurately determine within 15' which items are Poisonous and where they are located.
Confidence
Does it matter how strong the smell of the Poison is? RAW, no. In the context of realism of the fictional world? Probably. Different Poisons have different chemical compositions and would emit different odors. Those can be stronger or weaker, depending on. They can also be masked by overlapping odors, such as the one of the tea. How weak does a Poison have to be for a Tressym to be unsure if there is any close by?
But I don't want a roll to be necessary in every circumstance where a Poison is not present, as it would severely inhibit the strength of the ability. I'd also argue it wouldn't be RAW to gate the certainty behind the results of a roll, as it contains no mentions of any rolls.
Conclusion
A Tressym is always able to determine precisely which items are Poisonous and which aren't within 15', so long as their scent is able to reach him.
No rolls are necessary.
This also means it is never necessary to for a Tressym to sense a Poison by Taste or Touch (barring the exceptional case of the loss of its Smelling ability), but that's an inevitable consequence of how the ability is written . . .