Sonic Frontiers is a premium title and is not available for free as a full legal download. While you can find free additional content (DLC), the base game requires a purchase on official platforms like Steam or the Epic Games Store.  Official Purchase Options 

Free Download Options

Sonic didn't want to argue semantics. He wanted to help. Echo asked for a runner to restore its lost loops — to recompile broken paths and reconnect severed servers with a cavalcade of speed. In return, Echo offered something that felt like a full release: a gateway back to the real world with a seed of its code safely archived, so unfinished dreams wouldn't have to vanish.

Epic Games Store: Available for digital purchase and instant download on the Epic Games Store.

While the full game isn't free, you can access certain content and similar experiences at no cost:  Sonic Frontiers on Steam

Sonic Frontiers is a premium high-speed, open-zone action-adventure game available for purchase on PC via authorized digital storefronts like the Steam Store Epic Games Store

The safest and only legal way to download the full game is through official digital storefronts.

In Sonic Frontiers, players will experience a new type of gameplay that combines the classic Sonic formula with open-world elements. The game features: