Multiplayer Stability

Multiplayer Stability is a co-op mod for Warhammer 40,000: Rogue Trader. It contains targeted fixes for reproduced desynchronization paths, diagnostics for unresolved cases, and faster save transfer between players.

The current release is v1.0.0 for game build 1.6.1.514.

1.0 is the first stable release, following extended community testing and a tester-reported full campaign without a desync. It retains the runtime code and content from 0.9.8, with no new gameplay changes. Known limitations remain documented below. Use the same release on every machine and keep save backups.

Features

• Prevents confirmed paths where local camera, fog, view, UI, physics, or cache state changes synchronized simulation state.
• Isolates character-preview units from gameplay RNG, facts, auras, UUID allocation, and global rulebook subscriptions.
• Adds desync reports with state buckets, full RNG fingerprints, Player child hashes, per-entity hashes, and recent creation history.
• Uses Steam Networking Messages for bulk save data when exact-build compatibility is confirmed. Photon remains the control path and per-peer fallback.
• Adds framing, transfer identity, size checks, SHA-256 validation, receiver acceptance reporting, retries, and timeouts to the mod's custom network paths.
• Sequences selected loading barriers that could otherwise leave a peer waiting indefinitely.

The mod does not start a resync automatically and does not suppress Rogue Trader's desync dialog.

Requirements

• Rogue Trader build 1.6.1.514.
• The same Multiplayer Stability version on every player.
• The same gameplay-affecting mods on every player.
• Steam is required only for accelerated save transfer.

GOG and sessions with four to six players have not been field tested. There are no required mod dependencies.

Installation

ModFinder

ModFinder is the recommended installer. If Multiplayer Stability is not yet in its catalog:

1. Download MultiplayerStability-1.0.0.zip from GitHub Releases.
2. Drag the unchanged ZIP onto ModFinder's Drag zips here to install area.
3. Confirm that Multiplayer Stability is installed and enabled on every machine.
4. Launch the game, enable the mod from Mods if required, and restart when prompted.

Manual

1. Close the game and download the release ZIP.
2. Create: %USERPROFILE%\AppData\LocalLow\Owlcat Games\Warhammer 40000 Rogue Trader\Modifications\MultiplayerStability
3. Extract the ZIP contents into that folder.
4. Confirm this file exists: ...\MultiplayerStability\OwlcatModificationManifest.json
5. Open this file in a text editor: %USERPROFILE%\AppData\LocalLow\Owlcat Games\Warhammer 40000 Rogue Trader\OwlcatModificationManagerSettings.json
6. Add "MultiplayerStability" to the EnabledModifications array. Keep any existing mod names and other settings; separate names with commas. If this is your only enabled mod, the entry is "EnabledModifications": ["MultiplayerStability"].
7. Save the file, launch the game, and confirm the startup line in Multiplayer Check below. Repeat these steps on every player's machine.

If the settings file does not exist, create it with the following contents. Make sure its filename ends in .json, not .json.txt:

{
  "EnabledModifications": ["MultiplayerStability"]
}

Extracting the files alone does not enable the mod. Without the settings entry, it may not appear on the in-game Mods page or load its patches.

Do not extract directly into Modifications, and avoid a doubled MultiplayerStability\MultiplayerStability folder.

Multiplayer Check

At startup, search GameLogFull.txt (see log location) for:

[MPStability] [Init] Patches applied (

Do not continue if the line reports FAILED, or if any Multiplayer Stability line contains [Init][ERR], PATTERN NOT FOUND, or a component [ERR]. Patch classes load independently, so one startup failure can leave a multi-class component only partly active.

Before a save-transfer launch, peers exchange their compiled module IDs. The peer that starts the save checks those identities and the advertised manifest versions, then sends one reliable compatibility decision directly to each other peer before the game's LoadSave message. Decisions are keyed by sender so vanilla's non-owner and simultaneous save-start behavior remains available:

• matching manifest versions and compiled module IDs enable simulation fixes and custom protocols;
• a missing or different build selects vanilla simulation and transfer behavior for compatible clients that support this protocol (0.9.0 and later);
• if a peer advertises Multiplayer Stability but the save-sender decision is missing or invalid, the client refuses the load rather than enter play with a different policy.

Look for [Compat] Compatible on every peer when testing the fixes. Pre-0.9 builds do not understand this decision and remain unsupported in mixed-version sessions. The module ID identifies the compiled assembly but does not prove that every Harmony patch installed correctly, so clean startup logs are still required.

Known Limitations

• The weather combat-exit path and Tactician momentum remainder are instrumented, not fixed. The invalid Comfort in Conformity heal trigger is repaired separately, with post-fix field validation pending.
• Two consecutive Medikit uses preceded forks in the paired v0.9.2 capture. The second included one extra in-tick GlobalUuid draw. v0.9.5 records the exact UUID caller but does not alter Medikit behavior until that caller is identified.
• The achievement-reward, generalized all-unit combat-order, and Proving Ground extra-turn fixes need post-fix field validation. Sorting cannot repair peers that disagree about target membership.
• Dash delivery removes view-position timing from target delivery, but local movement completion can still select a different tick.
• Sorting range-query results cannot repair different physics candidate membership.
• Hidden AI turns run at 1x in multiplayer because the faster vanilla branch uses local visibility.
• Projectile mechanics use entity geometry instead of local view bones. Starship projectile geometry has limited field coverage.
• The augmentation screen omits its client-random bark in multiplayer.
• The nested-answer "new answers" marker is omitted in exact-parity multiplayer because the vanilla UI query can execute and persist an uncached party skill check. Synchronized answer commands use current offered-answer identity instead of vanilla's local transient drop guards.
• The There Is Only War... future-playthrough amulet is not granted while the exact-parity multiplayer fixes are active. The vanilla cabin action uses each peer's local platform achievement to decide a shared inventory write; solo reward behavior is unchanged.

Implementation status and evidence are listed in DESIGN_NOTES.md.

Troubleshooting and Reports

The full game log is:

%USERPROFILE%\AppData\LocalLow\Owlcat Games\Warhammer 40000 Rogue Trader\GameLogFull.txt

To find it, press Win+R, paste the folder path below, and press Enter:

%USERPROFILE%\AppData\LocalLow\Owlcat Games\Warhammer 40000 Rogue Trader

Report problems through GitHub Issues. For desyncs, freezes, or transfer failures:

1. Keep a copy of GameLogFull.txt from every player, including the host, before launching the game again. Collect logs from the same session and label them by player or host/client role.
2. Describe what happened, the area and action involved, and the approximate time. For a desync, record the action immediately before the first dialog.
3. Include the game version, installed mod versions, and startup patch result from every machine. ZIP the logs if they are large.
4. Keep the full logs private until account identifiers have been removed.

Logs from long sessions without desyncs are useful too: they show whether fixes were exercised without errors.

After a Rogue Trader update, treat the mod as unverified until its Harmony targets and reflected members have been reviewed against the new build.

Uninstalling

The mod adds no save-required content or blueprints. Disable or remove it on every machine before the next session. Existing saves then use vanilla multiplayer behavior.

Building

1. Use Modding/WhRtModificationTemplate.tar from the installed Rogue Trader build you are targeting. Do not assume that a previously extracted project was updated with the game.
2. When reusing an editor project, close Unity and refresh its Assets/RogueTraderAssemblies binaries from that template. Preserve existing .meta files. Do not copy WH40KRT_Data/Managed wholesale because the template contains editor-specific assembly variants.
3. Open the template in Unity 6000.0.64f1.
4. Copy Assets/Modifications/MultiplayerStability into the template's Assets/Modifications/ folder.
5. Run Assets > Modification Tools > Build.
6. Copy Blueprints/README.txt from the source tree into the built mod's Blueprints folder. The Owlcat build excludes the text placeholder, and an empty ZIP directory may be discarded by an installer.
7. Verify the packaged manifest version, DLL timestamp, and non-empty Blueprints folder before distribution. Timestamps alone do not identify a release; compare file hashes when checking an existing package.

The published 1.0.0 package reuses the 0.9.8 DLL and content byte-for-byte; only its external manifest version changed. Its DLL therefore retains the original build timestamp. Future code changes still require a fresh Unity build.

The template generates Generated; it is intentionally not tracked. No Python tooling is required.

Credits

• Built with Owlcat Games' Rogue Trader modification template.
• Developed from paired multiplayer logs, decompiled code review, and community testing.
• This is an unofficial fan modification. Warhammer 40,000 and related marks belong to Games Workshop. Rogue Trader belongs to Owlcat Games.

License

MIT. See LICENSE.