G.I. Joe: Wrath of Cobra — Adapting an Existing Action Game for Multiplayer with Photon Fusion

Adding multiplayer to a game that already worked offline

The existing game already contained its gameplay systems, but multiplayer had not been implemented when I joined the engagement. The task was therefore not to build a new game around networking; it was to understand an existing action-game codebase and adapt its gameplay systems to a coherent multiplayer model without unnecessarily rewriting the original product.

Client-host topology with Photon Fusion

Client-host topology using Photon Fusion. Players held Input Authority for their player input while the host held State Authority over authoritative networked state and simulation.

Implementation themes

• NetworkRunner/session setup with host and client modes
• Input provisioning and tick-based input consumption
• State/input authority checks
• NetworkBehaviour, NetworkObject, and networked properties
• Player state replication
• Network-aware spawn/despawn and network object pooling
• Networked AI conversion
• Host migration
• Disconnect/shutdown handling and online-to-offline fallback

Adapting existing gameplay to authority-based networking

Existing gameplay code assumed local execution. Multiplayer required deciding which actions could remain responsive on the player's machine and which state had to be authoritative on the host.

Player input was captured locally, packed into Fusion network input, and consumed in the network simulation. The host/state-authoritative side controlled important gameplay state such as health and other replicated player values, while players retained responsive input through Fusion's predicted, tick-based model.

Converting AI and enemies without redesigning the game

Enemy and boss behavior already existed. The multiplayer layer had to preserve that behavior while preventing every client from independently simulating AI and diverging.

AI simulation was gated so that the authoritative instance drove behavior/state while clients received the synchronized result. Enemy health/state and network-aware spawning/despawning were adapted around this model.

In short: I converted existing enemy and boss gameplay systems for host-authoritative multiplayer simulation. The AI design itself was part of the original game.

Session failure without immediately ending the player's game

One of the more distinctive requirements was handling loss of the multiplayer session without forcing the player to abandon the current run.

The multiplayer module contains explicit shutdown/error handling that snapshots local player state, recreates an offline player, restores relevant state, and recreates enemies locally so play can continue outside the network session.

This required thinking about the boundary between network-owned state and the underlying offline gameplay model rather than treating disconnect as a simple error screen.

Host migration

The module implements Fusion host migration rather than relying only on a simple disconnect flow. It restarts the runner using a HostMigrationToken, iterates resume snapshot objects, recreates network objects, copies state, restores scene-object/networked behavior state, rebuilds player mappings, and reassigns input authority where needed.

This is one of the strongest pieces of direct networking evidence in the project because it deals with reconstructing a running session rather than only synchronizing ordinary gameplay.

Additional implementation highlights

• Networked player health/lives/score/combo/stamina and related state
• Projectile/network spawning paths
• Pickups/equipment/interactions adapted to networking
• Selected RPC-driven events/effects
• Custom INetworkObjectPool integration
• Animation/state synchronization work
• Host-side physics/gameplay interactions against AI

Outcome

Delivered a substantial multiplayer adaptation layer over an existing action game within a focused engagement, while coordinating with the original gameplay engineers and mentoring another developer on the multiplayer team.

A different kind of ownership

This project represents a different kind of ownership from a greenfield multiplayer game. The challenge was understanding a mature gameplay codebase quickly, identifying where networking boundaries belonged, and integrating Fusion without replacing systems that already worked offline.

Media & external evidence

Play video: G.I. Joe: Wrath of Cobra trailer (product media)

• G.I. Joe: Wrath of Cobra on Steam (opens in new tab)