Slay the Spire 2 Connection Error: Why It Happens and How to Fix It

Slay the Spire 2 introduced something the original never had: online co-op. Up to four players can climb the spire together, combining their card builds and coordinating strategies in real time. It's a fantastic addition — when it works.

When it doesn't, you're staring at a connection error screen, wondering why you can see your friend in the lobby but can't actually start a game together. The answer lies in how Slay the Spire 2 handles multiplayer networking, and understanding that is the key to fixing it.

How Slay the Spire 2 Multiplayer Actually Works

Unlike games that use dedicated servers (where all players connect to a central server managed by the developer), Slay the Spire 2 uses peer-to-peer (P2P) connections. This means one player acts as the host, and other players connect directly to that host's machine.

There are no dedicated servers for Slay the Spire 2 co-op. Every multiplayer session is a direct connection between players' devices.

This matters because P2P connections are more fragile than dedicated server connections. They depend on each player's network configuration, and when those configurations don't agree, connections fail.

Why Connections Fail: The Main Causes

NAT Type Conflicts

NAT (Network Address Translation) determines how your device communicates with devices outside your local network. There are generally three types:

• Open NAT (Type 1) — Can connect to anyone
• Moderate NAT (Type 2) — Can connect to Open and Moderate, but may have issues with Strict
• Strict NAT (Type 3) — Can only connect to Open NAT

If two players both have Strict NAT, they cannot establish a P2P connection. This is the single most common cause of Slay the Spire 2 connection errors.

Host Network Quality

In a P2P setup, the host's network quality affects everyone. If the host has an unstable connection, high latency, or limited bandwidth, all connected players experience problems — even if their own connections are fine.

Geographic Distance

The farther apart players are, the more network hops their connection must traverse and the higher the latency. Playing across continents introduces delay that can cause timeouts and disconnects.

Firewall and Port Blocking

Firewalls (Windows Firewall, third-party security software, or your router's built-in firewall) can block the ports that Slay the Spire 2 uses for P2P connections. If the game can't communicate through the right ports, it can't establish a connection.

ISP Routing Issues

Some ISPs route traffic inefficiently, especially for P2P connections. Your ISP might also use carrier-grade NAT (CGNAT), which adds another layer of NAT translation that can break P2P connectivity entirely.

What the Errors Look Like

Connection problems in Slay the Spire 2 co-op manifest in several ways:

• Friends not showing online — You can see them in your friends list, but they appear offline in-game, or vice versa
• Endless loading screen — You accept an invite or try to join, and the game hangs on a loading screen indefinitely
• Black screen after connecting — The game transitions to a black screen instead of loading the run
• Sudden disconnects mid-game — You're playing normally, then get kicked back to the menu with a connection error
• Failed to join game — A direct error message when attempting to connect

Each of these points to a slightly different networking issue, but the fixes overlap significantly.

How to Fix Slay the Spire 2 Connection Errors

1. Let the Most Stable Player Host

Since the host's network quality affects everyone, choose your host wisely. The player with the fastest, most stable connection — ideally wired, with open NAT — should host the session. If you're getting connection errors with one friend hosting, try switching who hosts.

2. Enable UPnP on Your Router

UPnP (Universal Plug and Play) allows applications to automatically open the ports they need on your router. Without it, your router may block Slay the Spire 2's P2P traffic.

To enable UPnP:

1. Open a browser and go to your router's admin page (usually 192.168.1.1 or 192.168.0.1)
2. Log in with your router credentials
3. Find the UPnP setting (usually under Advanced Settings, NAT, or Firewall)
4. Enable it
5. Restart your router

If UPnP is already enabled, try disabling and re-enabling it to refresh the port mappings.

3. Check Your Firewall

Windows Firewall or third-party antivirus software might be blocking Slay the Spire 2:

1. Open Windows Security > Firewall & network protection
2. Click Allow an app through firewall
3. Find Slay the Spire 2 in the list and ensure both Private and Public are checked
4. If it's not listed, click Allow another app and browse to the game's executable

For third-party firewalls (Norton, McAfee, Kaspersky), add Slay the Spire 2 as an exception in the firewall settings.

4. Optimize Your Connection with GearUP

The root cause of many P2P connection failures is the network path between players — NAT conflicts, inefficient routing, and firewall issues that are hard to diagnose manually.

GearUP addresses this with several targeted features:

• NAT optimization — Helps resolve NAT type conflicts that prevent P2P connections from forming
• Network node routing — Routes your connection through optimized nodes that are more likely to support stable P2P traffic
• Real-time dynamic multi-path routing — Monitors your connection and switches to better paths when it detects instability, preventing the sudden disconnects that plague P2P sessions

Important: For the best results in P2P co-op, both players should run GearUP. Since the connection quality depends on both ends, optimizing only one side may not be enough to resolve the issue.

5. Use a Virtual Network Adapter

If NAT conflicts persist even after enabling UPnP, a virtual network adapter can sometimes help by changing how your traffic appears to the other player's network. This is a more advanced fix, but some network tools (including GearUP) use virtual adapter technology under the hood to facilitate better P2P connections.

What Won't Help

VPNs — A VPN adds encryption overhead and typically increases latency. For P2P connections, it can actually make things worse by introducing another layer of NAT. Some gaming VPNs claim to help, but the encryption cost almost always outweighs any routing benefit for P2P games like Slay the Spire 2.

Restarting the game repeatedly — If a connection fails due to NAT conflicts or firewall blocking, restarting the game won't fix it. The underlying network configuration issue persists.

Switching from Wi-Fi to wired — While this improves stability and reduces jitter, it doesn't address NAT conflicts or firewall issues. It's a good practice but not a fix for P2P connection errors specifically.

The Bottom Line

Slay the Spire 2's P2P networking means connection quality depends on both players' network configurations. NAT conflicts, firewall blocking, and inefficient routing are the primary causes of connection errors. Start with the host check and UPnP, verify your firewall isn't blocking the game, and use GearUP on both ends to optimize the connection path. P2P co-op is inherently more fragile than dedicated server multiplayer, but with the right setup, you can make it reliable.