The short answer
Slay the Spire 2 supports online co-op for up to four players through Steam friends. There is no public matchmaking system.
Before you start
- Each participant needs access to the game. Kingy is not purchasing copies.
- Check everyone’s version and optional mods before comparing behavior.
Prepare the group
- Use Steam on Windows, macOS or Linux; Mega Crit also lists Steam Deck compatibility.
- Have the group ready on Steam friends before starting multiplayer.
- Use the game’s multiplayer lobby and invite controls.
- Compare game versions if one player cannot join. Keep everyone on the same Main or Beta branch for a controlled troubleshooting attempt.
Sources for this section
If an invitation does not work
Check whether Steam and its overlay are available. The v0.111.0 beta specifically adds an error when using Invite with the overlay disabled. Do not assume that beta behavior is present on Main.
For a repeatable failure, record branch, version, party size and the exact error. Test an unmodded session before blaming a game mechanic; the official Main update added Workshop support, and modded saves are a separate concern.
Sources for this section
Coordinate decisions before spending resources
Co-op has multiplayer-specific cards and team synergies. Announce which enemy you intend to finish and when you need support. Re-read the remaining intents after another player changes the fight. Solo enemy values are not a reliable party-size table.
Sources for this section
Your checklist
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Research checked 2026-09-12. This is a source-based guide; Kingy has not independently play-tested these steps. Evidence and coverage.