supprised rider on steel defender

[Aurich]

Hi,

I'm playing a kobold artificer battlesmith who is riding his steel defender.

I'd like some help with figuring out what a steel defender can do if the party would get surprised.

in 3 scenarios:

the steel defender is not ridden:
the steel defender is ridden as a controlled mount
the steel defender is ridden as an intelligent mount.

[JackPhoenix]

It would get surprised too.
If not, it can move and Dodge unless given different order (which the artificer can't do while surprised), as normal.
Steel Defender's rules override mounted combat rules, as specific (class ability) beats general (general rules for mounted combat). That means SD is pretty meh mount, as it ALWAYS takes its turn after the artificer.

[Dualight]

While the Steel Defender's Vigilant ability means that it cannot be subject to surprise, what it can do is dependent solely on what happens to the Battlesmith.

The three scenarios are the same in as far as the Steel Defender's options are concerned.

Surprise is determined individually, not party wide, per RAW: "A member of a group can be surprised even if the other members aren't"(Player's Handbook p. 189.
This means that we only need to consider what happens if the Battlesmith is among the surprised, since otherwise everything plays out as during any other turn.
If the Battlesmith is surprised, they cannot take any actions that turn, thus they cannot take a Bonus Action to command the Steel Defender, which means that the Steel Defender is limited to moving and taking the Dodge Action. That said, it could use its Deflect Attack reaction even before the Battlesmith's turn rolls around due to Vigilant, since Steel Defenders can use reactions freely without needing a command.

Pseudo-edit: I underlined the main conclusion, as I noticed that it ended up buried within the paragraph, but I am currently too tired to clean it up.

[Joe the Rat]

Dualight has it summed up pretty well - if the controller is surprised, it can move and Dodge action, and use its deflect.

Most likely behavior would be to close to melee with an attacker and take the dodge action, so it can bog down an enemy and use Deflect if necessary.

Unless you tend to work reaaaaaly hard at staying out of melee, in which case it'll act as partial cover to some part of the party, and Dodge.