I don't believe that I'm misreading anything.I think you're misreading the situation somehow, but trying to parse your posts is difficult sometimes, so I can't pinpoint exactly where you may be going awry. The player is portrayed as believing the gloves make them safe from whatever it is they touched on the chest, so they raised their existence when the DM called for the Con save. The player is acting within their role. It would have been better perhaps if they said "I put on my gloves before I touch the chest," but the player may have simply believed the gloves were always on.
Where the DM would be stepping outside of their role is, as some suggest, the DM agrees the player has gloves, but presses on with the saving throw anyway because the roll will determine whether the character rubbed their eyes or picked their nose or whatever. That is the DM establishing what the character is doing which is not in the DM's role.
Unbeknownst to an unarmored character and despite the DM's sufficient telegraphing, they touched a chest that has been smeared with a dangerous contact poison. The DM describes the greasy feel of the poison and asks for a Constitution saving throw.
"Wait just a minute!" exclaims the player. "I imagine my character is wearing gloves. They have traveler's clothes on."
The DM considers this. There is nothing in the rules that says any clothing set comes with gloves, nor any armor for that matter except scale mail, chain mail, or plate which come with gauntlets at least (none of which the PC is wearing). There are no gloves in the equipment section to purchase, and the character has no magical gloves.
Is it reasonable that the player believed the clothing set they have comes with gloves that they are wearing even though it's not specifically listed on their character sheet? Does the timing of establishing this fact - after touching contact poison - matter to resolving this issue? Do you as DM side with the player's seemingly good faith belief that the character is wearing gloves or are they making that saving throw?
In short, how does this get resolved at your table?
- The DM gave "sufficient telegraphing". It doesn't matter if this is a description a failed perception/investigate check or a failed save. All that matters is that it was "sufficient"
- ↑ The GM has described the environment (5e PHB pg6/181)
- "will my gloves protect me?" is a perfectly reasonable question for a player to ask at this point. The player did not do so however
- "Despite the DM's sufficient telegraphing" [the Player] touched the chest [with their character].
- The player has described what they want to do(5e PHB 6/181)
- The next step in the play loop is to move straight to resolution of the "sufficiently telegraphed" contact poison because the player chose to come into contact with it. It is too late to ask how much protection the gloves offer because that rule no longer exists in 5e.
- ↑ The GM narrates the results of the adventurer's actions (5e PHB6/181)
A player believing that gloves are protective does not make them so in all situations & the onus is on them to ensure that their character determines if they will help or not before uttering the words "hold my beer"