Auto HDR - Series S/X
This article explains what's unique to Auto HDR on Series S/X compared to Auto HDR on Windows PC. So read my article about Auto HDR - Windows PC before this article. And a small comparison to PS5 at the end.
What it does
Auto HDR on Xbox does the same job as on PC. It takes an SDR game and expands the image into HDR. It can't bring back detail that was already crushed when the SDR image was made, it only redistributes and expands the luminance that survived. For the full explanation, see the PC article.
Highlights top out around 1000 nits. I measured the same 1000 nit peak on my Series X as on PC.
It's best known for old back compat games from the original Xbox, Xbox 360 and Xbox One, but it isn't limited to old games. A current game can ship in SDR and lean on Auto HDR instead of doing its own native HDR. Starfield is a well known example. Games that do their own native HDR are left alone.
It's on by default, and the only tuning is the HDR calibration tool where peak brightness from 1000 nits and lower has an effect on peak brightness used in Auto HDR. There is no intensity slider like the one in the PC Game Bar.
Setup
On the Xbox, go to Settings > General > TV & display options > Video modes and check Allow HDR10.
Run the Calibrate HDR for games in the same menu. Auto HDR tone maps to that peak brightness value (for 1000 nits and lower).
Not every SDR game gets it
When you turn Auto HDR on, the Xbox setting says the console will output HDR using Auto HDR in SDR only games. That's misleading. Not every SDR game gets Auto HDR on Series X. Microsoft decides which titles are covered, and most recent SDR only games are left out. In my testing, Clair Obscur: Expedition 33 and Kena Bridge of Spirits get no Auto HDR on Series X and just output SDR.
On PC you can apply Auto HDR to more DirectX 11 or DirectX 12 games, including both of these, but not to all.
The Windows black level bug does not apply here
The whole black level fix from the PC article is not needed on Xbox. You can't install color profiles on a console, and the console doesn't have the SDR in HDR desktop behavior that causes the bug. So there is nothing to fix.
Versus PS5
PS5 does not have Auto HDR. It has no feature that expands SDR into brighter HDR. What it has instead is HDR Always On. With it, the PS5 keeps sending an HDR signal to the TV even for SDR games. It doesn't add brightness or expand highlights, it simply wraps the SDR image in an HDR container.
The benefit is that the TV stays in HDR mode. You keep the TV's HDR features, like dynamic tone mapping and the brighter HDR picture features, instead of dropping into an SDR picture mode. It gives the PS5 an advantage in recent SDR only games, since it can stay in HDR while the Series X outputs plain SDR for the same game.
See my article about HDR Always On, where I go into more detail with this.