I noticed the preview (new) Device Builder (used ESPHome version 2026.5.1), which is looking great!
To enable it:
1 - Go to: Configuration --> Apps
2 - Select: ESPHome Device Builder --> Configuration
3 - Enable: Use new Device Builder Preview --> Save
To use the Build server:
1 - Go to ESPHome builder in Home Assistant (HA)
2 - Select the vertical 3-dots on the top right of the screen
3 - Choose: Settings
4 - New open window Settings - Appearance --> Build server (experimental)
5 - Enable remote build (default enabled)
From another HA do the same and setup Send builds, Pairing request (both HA instances) and so on.
So far so good, BUT, it giving me an Last connection error: auth rejected Error.
I read somewhere I needed a proper ssh setup (don't know where though...), so Installed the Terminal & SSH App and configured using an Authorized Key, which is working, even I can access it by a remote local-LAN Linux client.
FYI: https://lazyadmin.nl/smart-home/enable-ssh-home-assistant/
Now the question remains, the error stil exists, I guess because I need to setup the Authorisation Key some where on the Send builds HA instance.
Can anyone help me with some information how and where I put the autorisation-file/info?
The SSH add-on and authorized_keys are a red herring here, so you can undo that part. The Build server pairing does not use Home Assistant's SSH at all. It has its own handshake: a Noise-encrypted WebSocket (port 6055, path /remote-build/peer-link) with an emoji-fingerprint confirmation. There is no authorization file to drop anywhere.
"auth rejected" almost always means one specific thing: the receiving (Build server) side only accepts a pairing request while its "Pairing requests" screen is actually open. If you click Pair on the sending instance while that screen isn't mounted on the receiver, the receiver refuses the request and you get exactly this error.
Do it in this order:
On the Build server instance (the one lending CPU): Settings > Build server > Pairing requests. Open that screen and leave it open.
On the Send builds instance: Settings > Send builds > Known dashboards, find the receiver, click Pair. (If they're on different subnets, use "Pair with a build server" and type the receiver's hostname and port instead.)
Both sides show an emoji fingerprint grid. Check that they match (there's a hex view behind a disclosure if you prefer).
Back on the receiver's still-open Pairing requests screen, click Accept.
The trick is step 1 before step 2, and keeping that screen open through step 4. Close it or let the request sit too long and you're back to "auth rejected."
One more thing if your two instances aren't on the same flat LAN: if anything sits between them (reverse proxy, firewall, VLAN), it has to allow WebSocket upgrades on port 6055 for /remote-build/peer-link, or the link can't establish even with pairing done right.