Mishra's Foundry — ruling 1

If Mishra's Foundry becomes a creature and you haven't controlled it continuously since the beginning of your most recent turn, you won't be able to activate its first or last abilities, and it won't be able to attack (unless it somehow gains haste).

Mishra's Foundry — ruling 2

If Mishra's Foundry is already a creature, activating the second ability will override any previous effects that set its power and/or toughness to specific values. Other effects that are affecting its power and/or toughness, including +1/+1 counters and the effects of spells like Giant Growth, will continue to apply.

Mishra's Foundry : règles, fonctionnement et interactions (MTG)

Mishra's Foundry is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

: Add . : This land becomes a 2/2 Assembly-Worker artifact creature until end of turn. It's still a land. , : Target attacking Assembly-Worker gets +2/+2 until end of turn.

How it works : Mishra's Foundry

Original English text — Wizards of the Coast / Scryfall

#1October 14, 2022· Wizards of the Coast
If Mishra's Foundry becomes a creature and you haven't controlled it continuously since the beginning of your most recent turn, you won't be able to activate its first or last abilities, and it won't be able to attack (unless it somehow gains haste).
No other card is named in this ruling.
#2October 14, 2022· Wizards of the Coast
If Mishra's Foundry is already a creature, activating the second ability will override any previous effects that set its power and/or toughness to specific values. Other effects that are affecting its power and/or toughness, including +1/+1 counters and the effects of spells like Giant Growth, will continue to apply.
No other card is named in this ruling.