Rootwire Amalgam — ruling 1

Use Rootwire Amalgam's power as it existed the last time it was on the battlefield (not the power of the creature card in the graveyard) to determine the value of X.

Rootwire Amalgam — ruling 2

A prototype card is a colorless card in every zone except the stack or the battlefield, as well as while on the stack or the battlefield if not cast as a prototyped spell. Ignore its alternative characteristics in those cases. For example, while it's in your graveyard, Blitz Automaton is a colorless creature card with mana value 7. It can't be the target of Recommission, a spell that targets an artifact or creature card with mana value 3 or less in your graveyard.

Rootwire Amalgam — ruling 3

When cast as a prototyped spell, that spell has the mana cost, power, and toughness characteristics shown in its colored, secondary text box rather than the normal values of those characteristics. Its color and mana value are determined by that mana cost. The permanent that spell becomes as it resolves has the same characteristics. If the spell leaves the stack in any other way, or the permanent it becomes leaves the battlefield, it immediately resumes using its normal characteristics.

Rootwire Amalgam — ruling 4

Regardless of how it was cast, a prototype card always has the same name, abilities, types, and so on. Only the mana cost, mana value, color, power, and toughness change depending on whether the card was cast as a prototyped spell.

Rootwire Amalgam — ruling 5

The prototype ability functions in any zone that the spell could be cast from. For example, if an effect allows you to cast artifact spells from your graveyard, you could cast a prototyped Blitz Automaton from your graveyard.

Rootwire Amalgam — ruling 6

Casting a prototyped spell isn't the same as casting it for an alternative cost, and an alternative cost may be applied to a spell cast this way. For example, if an effect allows you to cast an artifact card without paying its mana cost, you could either cast Blitz Automaton normally, or as a prototyped spell.

Rootwire Amalgam — ruling 7

When casting a prototyped spell, use only its prototype characteristics to determine whether it's legal to cast it. For example, if Blitz Automaton is exiled with the last ability of Chandra, Dressed to Kill, you would be able to cast it for {2}{R} (because it's a red spell), even though you wouldn't be able to cast it as a colorless spell for its normal cost.

Rootwire Amalgam — ruling 8

If an effect copies a prototyped spell, that copy (as well as the token it becomes on the battlefield) will have the same characteristics as the prototyped spell. Similarly, if an effect creates a token that's a copy of a prototyped permanent or causes another permanent to become a copy of it, the copy would have the same characteristics as the prototyped permanent.

Rootwire Amalgam : règles, fonctionnement et interactions (MTG)

Rootwire Amalgam 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

Prototype — 2/3 (You may cast this spell with different mana cost, color, and size. It keeps its abilities and types.) , Sacrifice this creature: Create an X/X colorless Golem artifact creature token, where X is three times this creature's power. It gains haste until end of turn. Activate only as a sorcery.

How it works : Rootwire Amalgam

Original English text — Wizards of the Coast / Scryfall

#1October 14, 2022· Wizards of the Coast
Use Rootwire Amalgam's power as it existed the last time it was on the battlefield (not the power of the creature card in the graveyard) to determine the value of X.
No other card is named in this ruling.
#2October 14, 2022· Wizards of the Coast
A prototype card is a colorless card in every zone except the stack or the battlefield, as well as while on the stack or the battlefield if not cast as a prototyped spell. Ignore its alternative characteristics in those cases. For example, while it's in your graveyard, Blitz Automaton is a colorless creature card with mana value 7. It can't be the target of Recommission, a spell that targets an artifact or creature card with mana value 3 or less in your graveyard.
No other card is named in this ruling.
#3October 14, 2022· Wizards of the Coast
When cast as a prototyped spell, that spell has the mana cost, power, and toughness characteristics shown in its colored, secondary text box rather than the normal values of those characteristics. Its color and mana value are determined by that mana cost. The permanent that spell becomes as it resolves has the same characteristics. If the spell leaves the stack in any other way, or the permanent it becomes leaves the battlefield, it immediately resumes using its normal characteristics.
No other card is named in this ruling.
#4October 14, 2022· Wizards of the Coast
Regardless of how it was cast, a prototype card always has the same name, abilities, types, and so on. Only the mana cost, mana value, color, power, and toughness change depending on whether the card was cast as a prototyped spell.
No other card is named in this ruling.
#5October 14, 2022· Wizards of the Coast
The prototype ability functions in any zone that the spell could be cast from. For example, if an effect allows you to cast artifact spells from your graveyard, you could cast a prototyped Blitz Automaton from your graveyard.
No other card is named in this ruling.
#6October 14, 2022· Wizards of the Coast
Casting a prototyped spell isn't the same as casting it for an alternative cost, and an alternative cost may be applied to a spell cast this way. For example, if an effect allows you to cast an artifact card without paying its mana cost, you could either cast Blitz Automaton normally, or as a prototyped spell.
No other card is named in this ruling.
#7October 14, 2022· Wizards of the Coast
When casting a prototyped spell, use only its prototype characteristics to determine whether it's legal to cast it. For example, if Blitz Automaton is exiled with the last ability of Chandra, Dressed to Kill, you would be able to cast it for (because it's a red spell), even though you wouldn't be able to cast it as a colorless spell for its normal cost.
No other card is named in this ruling.
#8October 14, 2022· Wizards of the Coast
If an effect copies a prototyped spell, that copy (as well as the token it becomes on the battlefield) will have the same characteristics as the prototyped spell. Similarly, if an effect creates a token that's a copy of a prototyped permanent or causes another permanent to become a copy of it, the copy would have the same characteristics as the prototyped permanent.
No other card is named in this ruling.