Hi,

If I use Baratheon house with City of shadows agenda and I put Farwynd explorer and I am the first player, can I cancel a triggered effect without paying the additional 1 gold to use it?

Also another question on this agenda whether I can use Meera Reed to blank a card during eg. Challenge phase by paying only 1 gold (and not 2).

Yes to both. The City of Shadows penalty is in addition to the cost of pulling a card out of shadows (using the shadows framework action). For the same reason that Hidden Chambers doesn't affect either of these, the cost of triggering the ability (the card text) isn't modified.

Thanks for the clarification - same would be true for The Long Plan too then ~ rather handy

I knew about Hidden Chambers but only because I have read about it - it's confusing that the same wording is used re bringing out of shadows whether it is a framework or player action

Why does Hidden Chambers only work for framework actions?

The specific reason isn't that it works only for framework actions (although that's how it works out), it's that it decreases the cost to bring a card out of shadows (a specific game term).

The gold cost on Meera, for instance, isn't the gold cost to bring her out of shadows - it's the gold cost to trigger her card ability (which happens to bring her out of shadows).

What about Young Griff - If I brought him out of shadows and stood my targ characters, do I pay the extra 1 or just the gold cost for the next card ... I don't this is a framework action?

you may immediately bring another card out of Shadows by paying the rest of its gold cost

Cost modifiers do not apply to the 2nd card: you are paying for Griff's effect, not for the card itself.

"the rest of its gold cost" is the printed cost of bringing it out of the shadows. This is not the shadows framework action bringing the card out.