Companion DLL: OnlineFix

Early / Unfinished Implementation

The companion DLL feature is still in early development. The [[inject]] array syntax is part of a draft pull request (PR #146) that has not been merged. The OnlineFix companion DLL itself must be built from source with no pre-built releases available. This page is provided for developers who want to experiment with the feature.

This page is located in the Upcoming Features section for a reason — expect breaking changes and incomplete functionality.

For games whose matchmaking runs outside Steam (e.g., Epic Online Services, custom P2P networking), the built-in -onlinefix alone may not be sufficient. The OnlineFix companion DLL (by @Ran-Mewo) fills this gap.

What It Does

While OST's built-in -onlinefix spoofs AppID 480 at the Steam client level (enabling Steamworks lobby matchmaking), some games handle matchmaking in-process without going through Steam APIs. The OnlineFix companion DLL is injected directly into the game process to intercept and redirect those calls.

How to Use

1. Build OnlineFix.dll from source (see build instructions below). No pre-built releases are published yet.
2. Place the DLL in your Steam root directory
3. Configure in opensteamtool.toml:

[[inject]]
path = "OnlineFix.dll"
when_cmdline = "-onlinefix"

4. Launch the game with -onlinefix in its Steam launch options

Note

The [[inject]] array syntax is part of a pending pull request (PR #146) and is still in draft. See Upcoming Features for details.

Build Instructions

The OnlineFix repo requires:

• CMake 3.20+
• Visual Studio 2022 (MSVC x64) — Windows primary build
• Or mingw-w64 (x86_64-w64-mingw32-*) for cross-compilation on Linux/macOS

cmake -B build -A x64
cmake --build build --config Release

Output: build/Release/OnlineFix.dll

Relationship to Built-in Online Fix

Layer	What it handles
OST built-in	Steam client level — AppID 480 spoofing for Steamworks P2P
OnlineFix DLL	Process level — in-game matchmaking that bypasses Steam (EOS, custom P2P, etc.)

Most games will work with the built-in feature alone. The companion DLL is only needed for titles whose multiplayer implementation does not use Steam APIs.

Related Pages

• Online Fix — the built-in -onlinefix feature
• Upcoming Features — [[inject]] array syntax and online fix improvements