🎮 Game Error Codes What it means & how to fix it

Astroneer Co-op Won't Connect? Here's the Fix

Most 'Astroneer multiplayer not working' cases are a store/crossplay account issue or a version mismatch between players. The fastest fix in 2026: both players update to the same build and use the invite flow for your platform (Steam invite for Steam-to-Steam; a linked Microsoft account for crossplay), rather than expecting the session to appear automatically.

Exact details for Astroneer

Port to forward (UDP)
8777
Protocol
UDP only — the game port must NOT be TCP
Config file path
C:\AstroneerServer\Astro\Saved\Config\WindowsServer\AstroServerSettings.ini
Also set
Engine.ini → [URL] Port=8777 ; edit only while server stopped

Error codes

Can't join friend / session not found The invite or join doesn't connect

Cause: Version mismatch, or crossplay requires a linked account.

Fix:
  1. Both players update Astroneer to the same version and verify files.
  2. Steam-to-Steam: host invites via the Steam overlay (Shift+Tab > Join Game).
  3. Crossplay (Steam <> Xbox/Microsoft Store): both link a Microsoft account in-game and use the in-game friends/invite system.
  4. Set the session to Friends and make sure the host has loaded into the save first.
Connection lost / kicked mid-game The P2P link dropped

Cause: Unstable host connection or strict NAT.

Fix:
  1. Host from the most stable/wired connection with the best upload.
  2. Enable UPnP on the router or switch networks.
  3. Allow Astroneer through the firewall on both machines.

Fixes that work for most Astroneer errors

  1. Both players update to the same Astroneer build and verify files.
  2. Steam-to-Steam: invite via the Steam overlay; crossplay: link a Microsoft account.
  3. Host loads into the save first, sets the session to Friends, then invites.
  4. Allow Astroneer through the firewall; enable UPnP for strict NAT.
  5. Host from the strongest/wired connection.
  6. Restart the store client (Steam/Xbox) before hosting.

FAQ

Why can't my friend join my Astroneer game?

Check that you're both on the same version and that you're using the right invite path — a direct Steam invite for Steam players, or a linked Microsoft account for crossplay between Steam and the Xbox/Microsoft Store version.

Does Astroneer support crossplay co-op?

Yes — Astroneer supports crossplay across Steam, Xbox and the Microsoft Store, but each player must link a Microsoft account in-game and invite through the in-game friends system for it to work.

Other games