Subnautica 2 Coop Mod

Subnautica 2 Co-op Mod Skill

Skill by ara.so — Devtools Skills collection.

Overview

The Deep Synergy Multiplayer Mod transforms Subnautica 2 into a synchronized cooperative experience using BepInEx plugin architecture. It implements peer-to-peer multiplayer with deterministic state synchronization, shared inventory systems, and adaptive difficulty scaling based on player count.

Core Capabilities:

• Peer-to-peer WebRTC connections with NAT traversal
• Real-time world state synchronization (base building, inventory, creature AI)
• Host migration and session persistence
• Adaptive difficulty scaling (2-4 players)
• Cross-platform support (Windows, Linux, macOS)

Installation

Prerequisites

1. Install BepInEx 6.0.x for Subnautica 2:

# Download from https://github.com/BepInEx/BepInEx/releases
# Extract to Subnautica 2 game directory
# Structure should be: Subnautica2/BepInEx/

2. Verify BepInEx is working:

   • Launch game once, check for BepInEx/LogOutput.log
   • Should see "Chainloader initialized" message

3. Install Deep Synergy Mod:

# Extract mod archive to:
# Subnautica2/BepInEx/plugins/DeepSynergy/
# Files: DeepSynergy.dll, config/synergy_profile.json

Directory Structure

Subnautica2/
├── BepInEx/
│   ├── core/
│   ├── plugins/
│   │   └── DeepSynergy/
│   │       ├── DeepSynergy.dll
│   │       └── dependencies/
│   └── config/
│       └── synergy_profile.json

Configuration

Basic Profile Configuration

Create or edit BepInEx/config/synergy_profile.json:

{
  "session_name": "Ocean Explorers",
  "max_players": 4,
  "difficulty_scale": "adaptive",
  "resource_multiplier": 1.0,
  "oxygen_consumption": 1.0,
  "creature_spawn_divider": 1,
  "enable_pvp": false,
  "friendly_fire": false,
  "shared_blueprints": true,
  "ping_locations_shared": true,
  "time_of_day_sync": "host",
  "voice_chat_integration": "disabled",
  "network": {
    "port": 7777,
    "timeout_seconds": 30,
    "max_latency_ms": 250
  },
  "api_integration": {
    "openai": {
      "enabled": false,
      "api_key_env": "OPENAI_API_KEY",
      "role": "narrator"
    },
    "claude": {
      "enabled": false,
      "api_key_env": "ANTHROPIC_API_KEY",
      "role": "lore_engine"
    }
  }
}

Configuration Fields Reference

Console Commands

The mod adds in-game console commands accessible via BepInEx terminal (F5 by default):

Session Management

# Host a new session
/start_server

# Join existing session
/join_session <session-code>
# Example: /join_session 9B2A-4C7D-E8F1

# Leave current session
/leave_session

# Force disconnect player (host only)
/kick_player <player-id>

Session Monitoring

# Show connection status
/synergy_status
# Output:
# Connected peers: 3
# State sync: 100% complete
# Inventory hash: 0xFA342B1E
# Avg latency: 45ms

# List connected players
/list_players
# Output:
# [1] PlayerName (host) - 35ms
# [2] Player2 - 52ms
# [3] Player3 - 48ms

Gameplay Adjustments

# Temporarily adjust difficulty
/synergy_scale <multiplier>
# Example: /synergy_scale 1.5

# Override world seed (must do before starting)
/seed_override <seed-number>
# Example: /seed_override 8251

# Force state resync
/force_sync

# Debug inventory state
/debug_inventory

AI Integration (if enabled)

# Trigger narrative generation
/api_narrate "<context>"
# Example: /api_narrate "exploring thermal vents"

# Generate species lore
/api_lore_species <creature-name>
# Example: /api_lore_species "ghost_leviathan"

Code Examples

Creating a Custom Mod Plugin (C#)

If extending the Deep Synergy mod with custom functionality:

using BepInEx;
using BepInEx.IL2CPP;
using DeepSynergy.Core;
using UnityEngine;

namespace MyCustomExtension
{
    [BepInPlugin("com.example.subnautica2.extension", "Custom Extension", "1.0.0")]
    [BepInDependency("com.deepsynergy.subnautica2", BepInDependency.DependencyFlags.HardDependency)]
    public class CustomPlugin : BasePlugin
    {
        public override void Load()
        {
            // Hook into Deep Synergy events
            SessionManager.OnPlayerJoined += HandlePlayerJoin;
            SessionManager.OnStateSync += HandleStateSync;
            
            Log.LogInfo("Custom Extension loaded");
        }

        private void HandlePlayerJoin(PlayerInfo player)
        {
            Log.LogInfo($"Player {player.Name} joined with ID {player.Id}");
            
            // Send custom welcome message
            NetworkMessenger.SendToPlayer(player.Id, new WelcomeMessage
            {
                Text = $"Welcome {player.Name}!",
                Color = Color.cyan
            });
        }

        private void HandleStateSync(SyncEvent syncEvent)
        {
            // React to state changes
            if (syncEvent.Type == SyncType.InventoryUpdate)
            {
                Log.LogInfo($"Inventory synced: {syncEvent.Data}");
            }
        }
    }
}

Custom Network Message Handler

using DeepSynergy.Networking;

public class CustomMessageHandler : INetworkMessageHandler
{
    public void RegisterHandlers()
    {
        NetworkManager.RegisterHandler<CustomDataPacket>(OnCustomData);
    }

    private void OnCustomData(CustomDataPacket packet)
    {
        // Process custom multiplayer data
        Debug.Log($"Received from {packet.SenderId}: {packet.Payload}");
        
        // Broadcast to other clients
        if (NetworkManager.IsHost)
        {
            NetworkManager.BroadcastExcept(packet, packet.SenderId);
        }
    }
}

[Serializable]
public class CustomDataPacket : INetworkPacket
{
    public string SenderId { get; set; }
    public string Payload { get; set; }
    public long Timestamp { get; set; }
}

Session Configuration Override at Runtime

using DeepSynergy.Config;

public class DynamicConfigManager
{
    public static void AdjustDifficulty(float multiplier)
    {
        var config = SessionConfig.Current;
        config.ResourceMultiplier *= multiplier;
        config.CreatureSpawnDivider = (int)(config.CreatureSpawnDivider / multiplier);
        
        SessionConfig.ApplyChanges(config);
        NetworkManager.SyncConfig(); // Broadcast to all clients
    }

    public static void EnableAPI(string provider)
    {
        var config = SessionConfig.Current;
        
        if (provider == "openai")
        {
            config.ApiIntegration.OpenAI.Enabled = true;
            config.ApiIntegration.OpenAI.ApiKey = Environment.GetEnvironmentVariable("OPENAI_API_KEY");
        }
        else if (provider == "claude")
        {
            config.ApiIntegration.Claude.Enabled = true;
            config.ApiIntegration.Claude.ApiKey = Environment.GetEnvironmentVariable("ANTHROPIC_API_KEY");
        }
        
        SessionConfig.ApplyChanges(config);
    }
}

Common Patterns

Hosting a Session

# 1. Configure profile
vim BepInEx/config/synergy_profile.json

# 2. Launch game, open console (F5)
/start_server

# 3. Share session code with players
# Output: Session code: 9B2A-4C7D-E8F1

# 4. Wait for players to join
/list_players

# 5. Start playing when ready

Joining a Session

# 1. Launch game, open console
/join_session 9B2A-4C7D-E8F1

# 2. Wait for state sync
/synergy_status
# Should show "State sync: 100% complete"

# 3. Verify connection
/list_players

Handling Disconnects

The mod automatically handles host migration:

// Host migration is automatic, but you can monitor it:
SessionManager.OnHostMigration += (newHostId) =>
{
    Debug.Log($"Host migrated to player {newHostId}");
    // UI notification, state verification, etc.
};

// Manual reconnection attempt
if (!NetworkManager.IsConnected)
{
    NetworkManager.AttemptReconnect(lastSessionCode);
}

Custom Difficulty Presets

{
  "presets": {
    "easy": {
      "resource_multiplier": 1.5,
      "oxygen_consumption": 0.7,
      "creature_spawn_divider": 2
    },
    "normal": {
      "resource_multiplier": 1.0,
      "oxygen_consumption": 1.0,
      "creature_spawn_divider": 1
    },
    "hardcore": {
      "resource_multiplier": 0.7,
      "oxygen_consumption": 1.3,
      "creature_spawn_divider": 0.8,
      "friendly_fire": true
    }
  },
  "active_preset": "normal"
}

Troubleshooting

Connection Issues

Problem: Cannot join session / "Connection timeout"

Solutions:

# 1. Check firewall rules
sudo ufw allow 7777/udp  # Linux
# Windows: Allow BepInEx through Windows Defender Firewall

# 2. Verify NAT configuration
# Router should allow UDP hole punching
# Or manually forward port 7777 to host machine

# 3. Check console for errors
/synergy_status
# Look for "NAT traversal failed" or "Peer unreachable"

# 4. Increase timeout
# Edit synergy_profile.json:
"network": {
  "timeout_seconds": 60,
  "max_latency_ms": 500
}

# 5. Test direct connection (bypass NAT)
/join_session <host-ip>:7777

State Desync

Problem: Players see different base structures / inventory

Solutions:

# 1. Force resynchronization
/force_sync

# 2. Check hash integrity
/debug_inventory
# Compare hashes between clients

# 3. Restart session if corruption detected
/leave_session
# Host: /start_server (with same seed)
# Clients: /join_session <new-code>

# 4. Clear cache (last resort)
rm -rf BepInEx/cache/synergy_state.dat

Performance Degradation

Problem: Lag spikes, FPS drops with multiple players

Solutions:

# 1. Reduce sync frequency (trade responsiveness for performance)
# Edit synergy_profile.json:
"network": {
  "sync_interval_ms": 100,  # Default: 50ms
  "state_compression": true
}

# 2. Limit creature AI sync
"creature_sync_range": 100,  # Only sync creatures within 100m

# 3. Monitor bandwidth usage
/synergy_status
# Look for "Bandwidth: X MB/s"

# 4. Disable API integrations if enabled
"api_integration": {
  "openai": { "enabled": false },
  "claude": { "enabled": false }
}

Mod Conflicts

Problem: Crashes with other BepInEx mods

Solutions:

# 1. Check BepInEx logs
tail -f BepInEx/LogOutput.log

# 2. Disable conflicting mods
# Common conflicts: other multiplayer mods, world-state mods
mv BepInEx/plugins/ConflictingMod.dll BepInEx/plugins/disabled/

# 3. Load order matters - ensure Deep Synergy loads first
# Rename: DeepSynergy.dll -> 00_DeepSynergy.dll

# 4. Report incompatibilities
# Check wiki: https://github.com/maglin-jenebellah/Subnautica-II-Coop-Client/wiki

API Integration Errors

Problem: OpenAI/Claude features not working

Solutions:

# 1. Verify environment variables
echo $OPENAI_API_KEY
echo $ANTHROPIC_API_KEY

# 2. Test API connectivity
/api_narrate "test connection"
# Check logs for HTTP errors

# 3. Check rate limits
# APIs have usage quotas - verify account status

# 4. Fallback to local mode
"api_integration": {
  "openai": { "enabled": false },
  "fallback_to_local": true
}

Advanced Configuration

Custom Network Transport

using DeepSynergy.Networking;

public class CustomTransportLayer : INetworkTransport
{
    public void Initialize(TransportConfig config)
    {
        // Implement custom UDP/TCP logic
        // Example: Use QUIC instead of WebRTC
    }

    public void Send(byte[] data, string targetId)
    {
        // Custom send implementation
    }

    public void Broadcast(byte[] data)
    {
        // Broadcast to all peers
    }
}

// Register in plugin Load()
NetworkManager.RegisterTransport(new CustomTransportLayer());

Event Hooks

// Available event hooks
SessionManager.OnPlayerJoined += (player) => { /* ... */ };
SessionManager.OnPlayerLeft += (playerId) => { /* ... */ };
SessionManager.OnStateSync += (syncEvent) => { /* ... */ };
SessionManager.OnHostMigration += (newHostId) => { /* ... */ };
InventoryManager.OnItemPickup += (item, playerId) => { /* ... */ };
BaseBuilder.OnStructurePlaced += (structure, playerId) => { /* ... */ };

Best Practices

1. Always verify state sync before critical actions (e.g., blueprint unlocks)
2. Use console commands for debugging before modifying config files
3. Back up save files before enabling PvP or hardcore modes
4. Monitor network stats with /synergy_status regularly
5. Disable API integrations if not needed (reduces overhead)
6. Test with 2 players first before scaling to 3-4
7. Use environment variables for API keys, never hardcode

Resources

• Wiki: Project documentation and compatibility lists
• Discord: Real-time community support
• Issue Tracker: Bug reports and feature requests
• BepInEx Docs: https://docs.bepinex.dev/