Can a player exhaust a rune for no reason (without using the energy) in order to ready it again with Mana Flow and effectively gain an extra rune?

Question

Accepted Answer

No. You cannot arbitrarily exhaust a rune for no reason just to have Mana Flow ready it again and net an extra usable rune. There is no rule that allows players to exhaust runes as a free, discretionary action; actions that change runes or add resources happen only when game effects instruct them to, and Adds (resource gains) are limited and finalize immediately [429.4] [429.2].

A player may not exhaust a rune solely as a voluntary, purposeless action in order to later ready it with an effect and thereby gain an extra resource. Exhausting or readying runes must be performed by game effects (cards/abilities) that instruct you to do so; you can't take a free exhaust action to create a loop that nets resources.

Player wishes to exhaust a rune without any card or ability instructing them to do so (a voluntary exhaustion). There is no provided rule allowing a player to take a discretionary action to exhaust a rune; changes to runes and resource Adds are governed by game effects and the rules about Adding resources [429.4]. Because Adds and related rune/resource interactions finalize immediately and are limited to when effects direct them [429.2] [429.4], a player cannot create a loop by doing a free exhaust and then using Mana Flow to ready it and gain extra usable resources. If a card or ability explicitly instructs you to exhaust a rune (for example as a cost or as part of an effect), then you may do so; effects that Add Energy or otherwise interact with runes then proceed per their text and the normal add/finalize rules [165.1] [429.2].

If a specific card text explicitly reads 'Exhaust a rune: ...' or 'As an additional cost, exhaust a rune' then exhausting is being done as part of a game effect or cost and is legal. In that case the resulting interactions (including Adds or readying) are permitted. If an effect or spell says 'channel 1 rune exhausted' (e.g., Siphoning Strike), it requires an exhausted rune to channel. You cannot supply that exhausted rune by a free, non-effect exhaustion — it must have been exhausted by an effect or cost. Some cards (e.g., some with Reaction Add wording) can be used to add resources during the Pay Costs step of playing spells [357.1.a] [429.3]. The Core Rules excerpt shows Reaction Add abilities can be used to generate resources for costs, but it does not permit arbitrary exhaustion of runes outside of effect instructions. The parenthetical on Diana - Scorn of the Moon '(Abilities that add resources can't be reacted to.)' is printed on that card and would override other expectations for reactions to Adds where applicable, so specific card text may further constrain interactions.

The core point is that game-state changes to runes and the act of Adding resources are not free, discretionary actions players can take whenever they like; they are performed only when a card or effect tells you to do so. The rules explicitly state that Adding resources is a Limited Action and 'Players may only Add resources when Game Effects direct them to do so' [429.4]. Adds finalize immediately and are governed by their own resolution rules [429.2].

Because the provided rules do not show any allowance for a player to 'exhaust a rune for no reason' as a standalone action, the most defensible interpretation is that such an exhaustion is not legal unless a card or ability instructs it. That prevents the abusive loop you describe: you can't voluntarily exhaust a rune just to have Mana Flow ready it back and thereby net an extra usable rune. If a card or ability does instruct you to exhaust a rune (for example as a cost or effect), then that exhaustion is legal, and subsequent effects that ready runes or Add Energy will resolve normally and add resources to your Rune Pool per the rules [165.1] [429.2].

Also note relevant interactions: some effects require an exhausted rune to channel (for example, Siphoning Strike says 'channel 1 rune exhausted'), so you need an actual exhausted rune produced by an effect or cost to satisfy that. Reaction abilities that Add resources can be used when paying costs of spells in general [357.1.a], but the provided rules also show that Adds are limited and finalize immediately [429.2] [429.4]. Finally, specific printed text on cards can further restrict interactions — e.g., Diana - Scorn of the Moon includes '(Abilities that add resources can't be reacted to.)' — so always check exact card wording.

Therefore, absent a specific card or ability that instructs you to exhaust a rune, you cannot exhaust a rune 'for no reason' to create a ready/exhaust loop to gain extra resources.