Timing of Suggested actions and a target's "best ability"

Question

The first level enchantment spell Command specifies that the action the target is forced to take happens

on its next turn

The second level enchantment spell Suggestion, on the other hand, says that for the target,

On a failed save, it pursues the course of action you described to the best of its ability.

Since the action is not specified as starting on its next turn, it is possible that it could be started or even completed before its next turn, provided the action was one which it could take even while not on its turn. Furthermore,

the spell ends when the subject finishes what it was asked to do.

How should the GM, or possibly the GM playing as the target of the spell, interpret "to the best of its ability"? Is it better to act sooner, before its turn, or wait until its turn so as to have more options or more powerful options available in order to complete the suggested course of action?

As a specific example, suppose a target with Extra Attack was subject to the suggestion "attack the guard". The guard then leaves the reach of the target before the target's turn. The target could choose to use its reaction to make a single attack, which would end the suggestion effect. The target could also choose to wait until its turn, allowing it to take the Attack Action, attack multiple times with Extra Attack, and possibly use other abilities such as bonus actions or movement to make its attacks more effective.

Of these two choices (less sooner or more later), which would be considered pursuing the course of the action to "the best of its ability"?

This question is related, and asks (in part) whether the target will perform actions before its first turn. However, my question is different in that asks about the resolution of the "to the best of its ability" clause by supposing a tradeoff between acting sooner for less effect or later for more effect.

Answer 1

It's undefined, subjective to the target, and mixes natural language use with game-mechanical terms anyhow

1. It's not well defined, so the DM makes the call

Whenever there is ambiguity in a published rule, the DM resolves it. The very nature of such a judgment call is that there isn't a clear-cut, obvious interpretation. If there were, then the DM's judgment would not be needed. So the answer to this part of the question is pretty firmly opinion based. A given DM at a particular table in a specific situation has to decide if attacking sooner is better than attacking harder.

In that same vein, it's possible that a DM could reject that entire suggestion. If the target can't understand, or the suggestion is not phrased in a way that makes the action seem reasonable, the DM is within their authority to ask for clarification or simply rule that the spell has failed:

You suggest a course of activity (limited to a sentence or two) and magically influence a creature you can see within range that can hear and understand you. [...] The suggestion must be worded in such a manner as to make the course of action sound reasonable.

2. It's subjective to the target

There are elements to the spell which require subjective interpretation, most specifically the "target must understand you" and "the course of action must sound reasonable" pieces suggest that this is not one of those spells in 5e which directly interpret the true condition of the world. This, in turn, suggests that the spell target's interpretation of the suggestion is critical. That's a useful guide to how a DM may prefer to rule.

What is the target's idea of "the best of its ability"? Do they think that the suggestion is best satisfied by an immediate attack, or a more effective one slightly later? Might two different characters respond to the same suggestion wording in different ways? Do they have to consider the possibility that the opportunity attack might be the only chance they have to fulfill the suggestion?

3. Natural language and game-mechanical terms

The final element to resolve is whether or not a character can, in-game, use terms which define game mechanics in a way that other characters would understand. If so, there would be a distinct and operational difference between "attack the guard" and "Attack the guard". It is a common interpretation, in my experience, that game-mechanical terms are meaningless within the game but are useful for defining in-game operations more precisely for people that are not in the game themselves (the players). For example, a character in-game doesn't understand the abstractions of class levels, or six-second combat rounds with slightly asynchronous turns. Would a character note a difference between an opportunity attack and a full, on-turn Attack action?

This is also important for resolving whether or not a single attack (lower-case "a") satisfies the suggestion. "Attack", in natural English, doesn't necessarily resolve to a single, discrete event. Throwing a single punch might not fully define an "attack", and I certainly wouldn't consider such an action to satisfy the suggestion to the best of a typical person's ability.

Answer 2

Suggestion changes your intent.

I'm not sure where you're getting the idea that suggestion would end after making one attack. That's not the best of your ability.

You don't get to say "Well, I did it to the best of my ability for five whole seconds, so now I'm done." You attacked him once. You could do better at attacking him by doing it again next turn, so you're obligated to do that. The spell ends early if you can't do any better, say because the guy is dead, or he escapes and you can't find him. Otherwise you're on the hook for 8 hours.

Suggestion makes you pursue a course of action. If it's to attack that guy, then you have latitude to attack him in the way that you think best, so long as you're actually doing that. This requires a lot of trust at the table, even more than for mind control spells in general. If you're going to rules lawyer about whether attacking once counts as a course of action, then you should probably not include this spell in your game.

Answer 3

Frame challenge: The DM should not just let a player make such a vague suggestion

Most of the time when a player casts suggestion they have a specific course of action in mind. The issue in the case of "attack the guard" as per the question is that the player likely has one understanding in their head, and the DM is left wondering what that understanding is.

This is where the collaborative effort of roleplaying comes into effect and as a DM you should question the player for their full intent, and help them word the suggestion accordingly.

With a bit of prompting most players would opt for something like "Keep attacking that guard until you have successfully knocked them unconscious". Sometimes a player might want to roleplay their potentially lesser mental stats and leave the suggestion vague, but that is less likely.

In the event that did want to leave the suggestion vague

We come to the natural language, and it has to be interpreted according to the specificity of the target.

For example:

If someone suggested to me to "build a house", then I would probably use my ability to spend money to pay a builder and not really lift a finger myself, because I know that I am not good at building houses

If I had no money then maybe I would consider trying, but knowing that I could not do what I would consider a good enough job I would still likely spend the 8 hours researching how to do it, because the best of my ability would include more than just my hands.

Alternatively, because I am not manually inclined, and would consider someone telling me to spend that amount of money to be unreasonable, the spell would probably fail on me entirely.

Each of the above would come into effect depending on something such as if I had just been paid, if I had just been given a book entitled 'how to build a house', or if I was using my own senses rather than trying my best to not let someone waste their spell on me (IE: the DM was trying to not just say no).

This gives the DM a nice opportunity to get into the head of the NPC and roleplay the reaction!