Must the DM be told before a game what magic items a PC owns?

Question

Recently in an Adventurers League game the DM prevented my PC from using a magic item because I had not mentioned to the DM before the game that my PC possessed it, but I didn't know I was supposed to mention it. He didn't ask to review my PC's magic items prior to the game, and other DMs I've played with have never asked to review my PC's magic items prior to their games.

By the Adventurers' League rules, must a player tell the DM what magic items the player's character possesses in order for the PC to to benefit from them in that game?

Note : to his defense, he did ask me, in a previous game a couple of weeks ago, about what magic items I had at the time, so I suppose he considered that we were aware that we should tell him of new items, but playing with so many different DMs with different procedures gets confusing over time.

Answer 1

The only official information I found about your question is in the FAQ for the Adventurer's League, page 14, in the chapter Gaining levels between sessions:

Since characters may adventure between sessions of a multiple-session adventure, they may also advance in levels between sessions.

[...]

This requires some suspension of disbelief on the part of the players and Dungeon Master, especially in the case of the character gaining new equipment, magic items, or class features between sessions.

So while this suggests the DM and the players should trust each others about magic items, there is no rule in the AL documents stating that a player has to annouce the magic items he possesses.

Also, the magic item you have should be duly recorded on your logsheet :

If you received a magic item generated randomly by the Dungeon Master, you must record the name of the adventure you received it in, the location where it was found, and the result of the roll that determined the item.

(source: AL player's guide, page 4, magic items)

Answer 2

From the Adventurer's League Dungeon Master's Guide (a newer version can be found on DMsGuild.com):

Ensure that each player has an adventure logsheet for their character [...] In addition, the player also fills in the starting values for XP, gold, downtime, renown, and number of permanent magic items.

Emphasis mine.

Here it is clear that it is the DM's responsibility to ensure that each player has a logsheet. It goes on:

If you have time, you can do a quick scan of a player’s character sheet to ensure that nothing looks out of order. If you see magic items of very high rarities or strange arrays of ability scores, you can ask players to provide documentation for the irregularities. If they cannot, feel free to restrict item use or ask them to use a standard ability score array.

Emphasis mine.

DM therefore has the authority to restrict magic items, but only if he deems them obtained dishonestly or if you cannot provide the proof through the logsheet.

Must a player tell the DM what magic items the player's character possesses?

If the DM ask for it, you need to not only tell him, but also provide proof through a logsheet. But nowhere is it said that it is the responsibility of the player to be upfront about this and the DM should not punish you for this. He can doubt the way you acquired those items and prevent you from using them, but that doesn't seem to be your case.

That being said...

It is usually good practice to warn a DM about powerful items on your PC. Same goes for feats or gimmicks that may affect the flow of the game.

Answer 3

Typically it is good practise to be upfront with the GM about the character's possessions, innate skills and traits, and no one should have second thoughts about handing over the character sheet for the GM to review it.

• It allows the GM to rebalance his campaign/adventure according to the player's strengths and weaknesses (Player can't read elvish? Let's make the discarded note in Common, instead)
• In more serious cases, it would allow the GM to rebalance the character in case it is too over- or underpowered or simply unsuitable for the setting.
• It prevents nasty surprises during the game session. ("Oh, I never mentioned the ring of featherfall? So then, I slip it on my finger and don't plunge into the lava...")
• (Connected to the previous bullet point) It prevents "problem players" to "slip in" feats/skills/items during the game session to avoid a disfavourable situation.

In any case, full disclosure would make for reliability and accountability on both sides and could prevent unpleasant discussions around the game table right when the game had been at full swing.

That's enough good reasons to go for full disclosure, but, like this answer says, AL guidelines don't require full disclosure upfront by the player per se; the guidelines only give the GM the 'right' to doubt and disallow any 'strange findings' on the character or log sheet.

In short, you may not need to pass over your character documentation at the start of the session, but you should at least offer it, and the GM might be well-advised to at least glance over it to avoid any game-stalling discussions later on.