Strider Harness — ruling 1

If a creature enters the battlefield under your control and gains haste, but then loses it before attacking, it won’t be able to attack that turn. This means that you can’t use one Strider Harness to allow two new creatures to attack in the same turn.

Strider Harness : règles, fonctionnement et interactions (MTG)

Strider Harness 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

Equipped creature gets +1/+1 and has haste. Equip (: Attach to target creature you control. Equip only as a sorcery.)

How it works : Strider Harness

Original English text — Wizards of the Coast / Scryfall

#1January 19, 2018· Wizards of the Coast
If a creature enters the battlefield under your control and gains haste, but then loses it before attacking, it won’t be able to attack that turn. This means that you can’t use one Strider Harness to allow two new creatures to attack in the same turn.
No other card is named in this ruling.