Marshland Bloodcaster — ruling 1

If you cast a spell for another cost "rather than pay its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, such as that of Draconic Intervention, those must be paid to cast the card.

Marshland Bloodcaster — ruling 2

If the next spell you cast has {X} in its mana cost, you must choose 0 as the value of X if you pay life rather than pay its mana cost.

Marshland Bloodcaster — ruling 3

Marshland Bloodcaster's ability affects the next spell you cast after the ability has resolved. If you cast any spells in response to the ability, those spells are cast as normal.

Marshland Bloodcaster — ruling 4

Even after the ability resolves, you may choose to not pay life rather than pay mana cost of the next spell you cast that turn. However, if you do this, you won't have the opportunity to pay life for any other spells you cast that turn (unless you activate the ability again).

Marshland Bloodcaster : règles, fonctionnement et interactions (MTG)

Marshland Bloodcaster 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

Flying , : Rather than pay the mana cost of the next spell you cast this turn, you may pay life equal to that spell's mana value.

How it works : Marshland Bloodcaster

Original English text — Wizards of the Coast / Scryfall

#1April 16, 2021· Wizards of the Coast
If you cast a spell for another cost "rather than pay its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, such as that of Draconic Intervention, those must be paid to cast the card.
No other card is named in this ruling.
#2April 16, 2021· Wizards of the Coast
If the next spell you cast has in its mana cost, you must choose 0 as the value of X if you pay life rather than pay its mana cost.
No other card is named in this ruling.
#3April 16, 2021· Wizards of the Coast
Marshland Bloodcaster's ability affects the next spell you cast after the ability has resolved. If you cast any spells in response to the ability, those spells are cast as normal.
No other card is named in this ruling.
#4April 16, 2021· Wizards of the Coast
Even after the ability resolves, you may choose to not pay life rather than pay mana cost of the next spell you cast that turn. However, if you do this, you won't have the opportunity to pay life for any other spells you cast that turn (unless you activate the ability again).
No other card is named in this ruling.