
Engine	SAGE (Generals lineage; C&C3 era), x86
Versions tested	Steam Tiberium Wars 1.10 (buildid 17447444, `RetailExe\1.10\cnc3game.dat` SHA-256 `af568a89...0d141`; the C&C Online variant `ade9f5b6...a3543` differs only in the PE header: large-address-aware flag and checksum). Kane's Wrath 1.3 identified, not worked on yet.
Projects	Campaign co-op prototype (proxy `dinput8.dll`), unpublished, owner's workspace; notes in NOTES.md
QA level	community-reports (owner + friend played most of GDI mission 1 online); automated two-client smoke tests on one PC
Last updated	2026-10-18

Where things stand

Two players can play a Tiberium Wars campaign mission together: one hosts a normal multiplayer game (C&C Online or LAN), the other joins, and both get the campaign mission with its own sides, scripts, AI, objectives and in-mission cutscenes. Both control the same campaign side (shared units, shared money). Tested by the owner and a friend over C&C Online on GDI mission 1 (prologue): both had full control of units, building and placement. Missions can be won and an experimental mission flow advances both PCs to the next mission (not yet tested by humans). Briefing movies and own factions per player are not done. Kane's Wrath has the same engine structure but no port yet.

Existing work

Searched 2026-10-17: this repo, web search, GitHub repo search; ModDB pages were behind a Cloudflare check (search-result text only).

No official PC campaign co-op for either game (Revora answer). The Xbox 360 version had co-op versus AI, not the story (Co-Optimus).
Community co-op missions exist as custom multiplayer maps, not the official campaign: COOP Evacuation, Boyle's Legacy pack, Co-Op Tiberian Sun Map Pack (needs Tiberium Essence).
Not found by those searches: any engine-level campaign co-op for C&C3 or Kane's Wrath.
Related engine source: EA's GPL release of Generals Zero Hour (electronicarts/CnC_Generals_Zero_Hour) and OpenSAGE (map format) were the main references.

How the game works (for co-op)

Install: CNC3.exe is a launcher; the real game is RetailExe\1.10\cnc3game.dat, started as cnc3game.dat -config "<install>\CNC3_english_1.10.SkuDef". -win, -xres, -yres work. The 2025 Steam exes are not packed. C&C Online ("Tacitus") is a dsound.dll proxy and needs no launcher.
Maps: campaign maps are in SP\1.0\SPMaps.big as raw .map (RefPack, EAR\0, RefPack, CkMp chunks; layout as in OpenSAGE) plus compiled map.bin. Each campaign map has one human side and several named AI sides with their own scripts.
Netcode: lockstep, Generals lineage. The LAN lobby is broadcast only (ports 8086-8093) and identifies players by IP. Commands carry a player index; a slot's commands are accepted only if no other slot already acts for the same player (anti-spoofing, see NOTES section 12).
Game start: startNewGame with a lobby GameInfo deletes flagged teams, strips every map side except civilian/creeps (prepareForMP_or_Skirmish) and builds Player_N sides from the slots. The slot -> Player mapping is the player name stored in the slot.
Victory: a skirmish victory check looks at the slot players; campaign maps decide victory and defeat in their scripts. After online games a results report is sent to the ladder service.

What we changed

All in a dinput8.dll proxy; every patch site is checked against its expected bytes and the DLL does nothing to an unknown build. Addresses are in NOTES.md.

Campaign map in a lobby game: the lobby map is swapped for the chosen mission on both clients; side stripping, slot sides and the team deletion step are skipped; the campaign's human side is marked local; every human slot is named after it; the skirmish victory check is off.
Guest orders were dropped: the receive-side anti-spoofing check flagged the second slot acting for the campaign side and discarded all its commands, even on the guest's own PC (found by logging UI-created, sent and per-frame commands on both clients). The check is relaxed for co-op games.
Missions could not be won: missions end the classic SAGE way (scripts kill the enemy sides, the victory check declares the last side standing). That check was off in co-op because the campaign side failed the multiplayer elimination rule at the start. Now the check runs, but in co-op a side counts as defeated only when the mission scripts kill it: the "nothing left" rule had also eliminated allied AI sides (and destroyed their buildings) at the first check. Test: enemy sides killed at a fixed frame on both clients -> VICTORY on both.
Mission flow (experimental): the current mission is kept in a file in the co-op profile; after a victory both clients advance it to the next mission, and the next lobby game loads it.
Shared selection: an order applies to the player's logic-side selection, which both humans share. Each client keeps its own selection messages and re-sends them before its next order when the partner's selection changed in between.
Crash to desktop when leaving an online game: the results report built one record per player and corrupted the heap with co-op's player layout (WER minidumps from both players). Co-op games are not reported.
Test harness: two game instances on one PC over LAN (per-instance loopback IP, broadcasts re-sent as unicast, SIO_UDP_CONNRESET off), each instance with its own AppData, kept minimized but "active", driven by posted window messages, with in-engine screenshots (D3D9 Present hook).

QA

Automated (two real clients on one PC, LAN): mission start in sync on both clients (screenshots match, logic frames advance, identical command stream), guest commands executed on both clients, shared-selection restore (host selects, guest selects, host orders: host's selection is re-sent first), exit from the game back to the lobby on both clients. These are smoke tests with logged evidence, not a pass/fail suite with negative controls yet; mouse move orders cannot be produced by the harness (keyboard orders can).
Human test, C&C Online, two PCs, GDI mission 1: full shared control for both players, no crashes, no mismatch. One oddity: Nod bombers damaged but did not destroy the starting power plant, so a tutorial step had to be finished by hand. Unknown whether this is random or caused by co-op (for example campaign difficulty not applied in lobby games).

How to play

Not published. The owner's package: run Setup.bat (finds the Steam install, makes a separate hard-linked copy of the game with the DLL, remembers folders), pick the same mission on both PCs, click Play, host a private unranked 2-player game on any map in C&C Online (or LAN).

Known issues

Mission flow: after a victory both players return to the lobby and host a new game for the next mission; no briefings or between-mission movies; no saves.
GDI mission 2 (Pentagon), nobody playing: lost after about 40 s when Nod destroys an allied power plant (the mission's own lose script). Probably normal for an idle player; not compared with a single-player run. (An earlier co-op bug that lost it after 3 s is fixed, see NOTES section 14.)
Control groups are shared; post-game stats show odd player names.

Next steps

Briefing movies between missions; saves.
Own faction per player: see mission-survey.md for the script survey of all 39 missions and the generic rules it suggests (and the <Local Player> desync trap).
Check which campaign difficulty lobby games run (prologue: Nod money and attack forces differ).
Kane's Wrath port (same seams, different addresses).