How to Play Earth's Special Forces After the Half-Life 25th Anniversary Update

The short answer: for the current public build, switch Half-Life to its legacy beta branch in Steam. That is the only way to run the old Half-Life-based version after Valve's November 2023 25th anniversary update broke many mods. But this is a temporary fix, not a permanent one — the ESF team has confirmed the project is moving to Sven Co-op, and the legacy branch will eventually stop working for ESF because the mod depends on Steam connectivity.

Why the update broke ESF

Valve's 25th anniversary update changed how Half-Life uses memory. According to the ESF team's March 14, 2024 news post:

• Half-Life does not have the "LargeAddressAware" (LAA) header flagged, so it can only access 2GB of total memory.
• Had Valve enabled LAA with the update, the game could have used up to 4GB, and the problem would likely have been avoided.
• ESF's content is large enough that a single base Trunks model is roughly as big as all of Half-Life's models combined. The mod hit the memory wall and stopped working.

The team also considered enabling the LAA flag themselves, but rejected it over the risk of shipping a mod that could trigger a VAC ban.

Option 1: Run the legacy Half-Life branch (short-term)

This is the workaround for playing the existing Half-Life version of ESF:

1. Open Steam and go to your Library.
2. Right-click Half-Life and open Properties.
3. Go to the Betas tab.
4. Select the legacy beta branch from the dropdown.
5. Let Steam download the older build, then launch ESF as you normally would.

Expected result: Half-Life runs the pre-anniversary version, which restores compatibility with mods that the update broke.

The catch: ESF relies on Steam's connectivity functions. The legacy branch works fine for mods that don't need Steam features, but once the legacy version starts having trouble connecting to Steam servers, ESF:F will no longer work on this setup. Treat this as a stopgap, not a solution.

Option 2: Wait for the Sven Co-op release (long-term)

After years of trying to make everything fit within Half-Life's limits, the team decided to move the mod to Sven Co-op. The reasoning:

• Sven Co-op was given Half-Life's source code, so it can be altered to use 4GB of memory — something ESF's own requests for source code never achieved.
• The move fixed the major memory issues.

It also introduced new problems. The team notes that other outdated components, such as the old version of FMOD used for sound, are now causing their own issues and still need fixing.

Progress reported in the same update:

• Every model/character is now in the game, mostly animated and working.
• Missing pieces: each character's unique transformation animations and bonus tiers.
• You no longer have to close and reopen the game to change maps.
• The clutter system still needs cleanup so it works flawlessly — for example, rocks spawning in the air.
• Dalte is working on bonus tiers and transformation audio for each character.

What to do now

The team's own framing is that the legacy branch is a dead end for ESF specifically, because of the Steam dependency. The Sven Co-op migration is the path to a version that works without that constraint — but it isn't finished, and the update does not give a release date. Check esforces.com for the current status before planning around either option.