Local LAN Co-op for Slay the Spire 2
Play with friends on your home network or VPN. Works alongside Steam multiplayer.

Quick Start · Features · Screenshots · Install · Usage · FAQ · Build

---

Quick Start

1. Download the latest release
2. Drop the LanSpire folder into your game's mods/ directory
3. Launch the game → Multiplayer → Host LAN or Join LAN

That's it. The mod auto-configures on first launch.

Features

Core Multiplayer

Feature	Description
Host & Join LAN	Direct LAN connection via IP or auto-discovery scan
Auto-Discovery	Click "Scan LAN" to find games on your network
Quick Rejoin	Pin favorite host IP for one-click reconnect (separate from 5-entry history)
Latency Display	Ping indicator in top-right corner, updates every 2 seconds

Network & Compatibility

Feature	Description
VPN Support	Works with common VPNs (ZeroTier, Tailscale, WireGuard, etc.). VPN adapters detected and prioritized
Steam Compatible	Steam multiplayer still works. LAN buttons added alongside, not replacing
Update Check	Checks GitHub for new releases on startup, shows toast if update available

Advanced

Feature	Description
Version Tagging	LAN discovery includes mod version. Warns if host runs different version
Optional Passphrase	Set lan_host_passphrase in config to require password for joining
Connection Resilience	Auto-retry on transient disconnects, sync stall recovery, 8s grace period for disconnected players

Screenshots

Compatibility

Status	Details
⚠️ Game Version	Built against STS2 v0.107.1 (gameplay testing ongoing)
✅ Mod Loader	Requires game's built-in mod loader (mods/ folder)
⚠️ Game Updates	Can break the mod. Check for updates after game patches

Install

1. Download the latest release zip from Releases
2. Open your Slay the Spire 2 game folder
3. Open the mods folder (create it if it doesn't exist)
4. Copy the LanSpire folder from the zip into mods/
5. Launch the game

Expected structure:

Slay the Spire 2/
  mods/
    LanSpire/
      LanSpire.dll
      LanSpire.json

Usage

Host LAN

1. Main menu → Multiplayer → Host LAN
2. Select game mode (Standard, Daily, Custom)
3. Popup shows your IP(s) with VPN labels -- click any IP to copy to clipboard
4. A small "..." button appears in the top-right corner -- click anytime to reopen IP popup
5. Share the IP with your friend, or tell them to Scan LAN

Join LAN

1. Main menu → Multiplayer → Join LAN
2. Pinned Host -- pin frequently-used IP (e.g. your VPN group's fixed IP) for one-click reconnect
3. Recent Connections -- up to 5 saved hosts as quick-access buttons
4. Scan LAN -- finds games on your network with live player count [2/4]
   • Full rooms greyed out with [Full] badge
5. Manual -- type host IP (supports IP:port format)
6. Auto-retries on flaky connections, 15s timeout for unreachable hosts

VPN (ZeroTier / Tailscale / WireGuard)

• Works over common VPNs -- use VPN IP instead of LAN IP
• VPN adapters detected by name and prioritized in host popup
• Auto-retry helps with VPN route establishment (routes can take a few seconds)

In-Game Settings

Settings → General → LAN Multiplayer:

• Max LAN Players -- 2-8 players
• Player Name -- custom display name (optional)
• Open Game Log -- opens godot.log in Notepad for debugging

Firewall

If friends can't connect, ensure these ports are open:

Port	Protocol	Purpose
33771	UDP	Game traffic (required)
33772-33775	UDP	LAN discovery (optional, for Scan feature)

Config

Settings saved automatically on first launch at %APPDATA%/SlayTheSpire2/LanSpire/lan_config.json. Most users won't need to touch this.

View default config

{
  "lan_multiplayer_enabled": true,
  "lan_use_custom_player_id": false,
  "lan_custom_player_id": "",
  "lan_player_id": "",
  "lan_multiplayer_save_player_id": "",
  "lan_join_host": "",
  "lan_join_port": 33771,
  "lan_join_history": [],
  "lan_pinned_host": "",
  "lan_host_passphrase": "",
  "lan_debug_logging": false,
  "max_multiplayer_players": 4,
  "max_multiplayer_enabled": true,
  "lan_compatibility_mod_names": [],
  "resilience_sync_stall_recovery": true,
  "resilience_timing_failsafes": true,
  "resilience_disconnect_prevention": true
}

Config field reference

Field	Default	Description
lan_multiplayer_enabled	true	Master switch. If false, no patches applied.
lan_use_custom_player_id	false	Use custom player name instead of auto-generated.
lan_custom_player_id	""	Custom player name (hashed to ulong peer ID).
lan_player_id	auto	Persistent random peer ID. Auto-generated on first use.
lan_multiplayer_save_player_id	auto	Last used multiplayer save player ID.
lan_join_host	auto	Last joined host IP.
lan_join_port	33771	Last joined port.
lan_join_history	[]	Recent connections (up to 5).
lan_pinned_host	""	Pinned/favorite host (IP:port). Shown as separate quick-connect button.
lan_host_passphrase	""	Optional passphrase. If set, joining clients must send same passphrase. Host gets toast on mismatch.
lan_debug_logging	false	Enable verbose diagnostic logs in godot.log. Useful for troubleshooting.
max_multiplayer_players	4	Max players in lobby (2-8).
max_multiplayer_enabled	true	Show max players setting in-game.
lan_compatibility_mod_names	[]	Extra mod names for multiplayer compatibility list.
resilience_sync_stall_recovery	true	30s timeout on sync waits. Prevents freeze on lost UDP packets.
resilience_timing_failsafes	true	Guard against null-ref on early data arrival.
resilience_disconnect_prevention	true	Auto-mark disconnected players as ready after 8s grace.

How It Works

LanSpire adds direct LAN connection alongside the game's existing Steam networking, so you can play with friends on the same network without needing Steam's multiplayer system.

Steam still works normally -- the LAN buttons are added on top, and only activate when you specifically choose Host LAN or Join LAN. Switching between Steam and LAN multiplayer is seamless.

Build

Prerequisites: .NET SDK 9.0+, STS2 game files

# Set game directory (where data_sts2_windows_x86_64/ lives)
export STS2_GAME_DIR="/path/to/Slay the Spire 2"

# Build
cd LanSpire
dotnet build -c Release

Output: bin/Release/net9.0/LanSpire.dll

STS2_GAME_DIR is required -- the build references game assemblies from the game directory.

Game Logs

If something goes wrong, check the log at %APPDATA%/SlayTheSpire2/logs/godot.log. Messages from this mod are tagged with [LanSpire].

FAQ

Does this replace Steam multiplayer?

No. LanSpire adds LAN buttons alongside the existing Steam multiplayer buttons. Both work independently -- you can use either depending on your network setup.

Does it work over VPN?

Yes. Use common VPNs (ZeroTier, Tailscale, WireGuard, etc.) and enter the VPN IP instead of your LAN IP. VPN adapters are detected automatically and prioritized.

Scan LAN doesn't detect my friend's room. Why?

Scan LAN only works between two PCs running LanSpire. It will NOT detect:

• Native Steam multiplayer rooms -- these use a different system entirely
• Any other non-LanSpire PC mod -- discovery requires both sides to speak the same protocol

What to do instead:

You want to join	Use
Friend on PC with LanSpire	Scan LAN
Friend on VPN (ZeroTier, Tailscale, etc.)	Manual IP -- enter the VPN IP
Friend on Steam multiplayer	Not supported by this mod

If Scan LAN returns empty even when your PC friend is hosting:

1. Make sure both PCs are on the same network (same Wi-Fi / subnet)
2. Make sure port 33772 UDP is not blocked by firewall on the host
3. Try Manual IP -- ask them for their local IP (shown in Host popup)

My friend can't connect. What do I check?

1. Make sure port 33771 UDP is allowed through your firewall
2. If using VPN, make sure both peers can ping each other's VPN IP
3. Try entering IP manually instead of using Scan
4. Check godot.log for [LanSpire] messages

Will this break when the game updates?

Possibly. The mod patches specific game methods that may change between updates. Always check for an updated build after the game patches. Built against v0.107.1.

Uninstall

1. Close the game
2. Delete the LanSpire folder from your game's mods/ directory
3. (Optional) Delete config: %APPDATA%/SlayTheSpire2/LanSpire/

That's it. No registry entries, no background services.

Known Issues

• Game updates can break this mod. LanSpire patches specific game methods that may change between STS2 updates. If the mod stops working after a game patch, check Releases for an updated build.
• LAN Scan is PC-to-PC only. It will not detect native Steam multiplayer rooms. Use Manual IP for VPN play.
• VPN route delays. When using VPNs like ZeroTier or Tailscale, the first connection attempt may fail while routes are being established. The mod auto-retries, but if it still fails, try connecting again after a few seconds.
• Reflected field names. The mod relies on private game internals accessed via reflection. A game update that renames these fields will cause silent failures logged in godot.log.

How It Was Made

LanSpire adapts the LAN multiplayer protocol for PC, using Harmony patches to intercept and redirect the game's networking layer. The core approach:

• Kept: Message protocol, host/join logic, LAN join UI, player naming, save ID handling
• Replaced: Platform-specific settings with file-based JSON configuration (lan_config.json)
• Added: LAN auto-discovery, connection history, toast notifications, player count display, connection retry, timeout, VPN detection, Quick Rejoin

Credits

• Harmony - runtime patching library for .NET applications

Related Projects

LanSpire is compatible with:

• sts2-android-compat - Mobile client compatibility layer
• STS2MobileLauncher - Mobile launcher for STS2

License

MIT -- same as the original project.