Server-Side vs Client-Side Mods — Who Has to Install What

Before you install a mod for a group session, read the Side field on its Thunderstore page. It says one of three things — Client, Server, or Both — and it determines exactly who has to install it. Getting this wrong is the second most common cause of “my friends can’t join” (right after crossplay).

Honesty note (what we have and have not verified): the rule for reading the Side field and the desync behaviour below are stable and well established. We have not published a maintained, authoritative “mod X is Server, mod Y is Both” table for 1.0, because that mapping changes as mods are updated. The categories are the durable part; for any specific mod, trust its page’s Side field at the time you install.

The three categories

• Client-side only — cosmetics, UI tweaks, minimap, quality-of-life that only changes what you see or do. Only the player who wants it installs it. The server does not need it, and other players do not need it. This is the safe, no-coordination category.
• Server-side, or both — anything that changes shared world state: combat rebalances, loot tables, crafting rule changes, new items or mechanics. These need to be installed on the server and on every joining client. If one player runs a combat mod the others do not, the world state diverges — desync, stuck items, or failed joins.

Why “both” is the dangerous one

Valheim is a host-authoritative-ish game, and a mod that alters shared state has to agree on both ends of the connection. The server runs the logic; the clients render and input it. When the mod set or versions differ between them, the two sides stop agreeing, and you see the symptoms: items that do not exist for some players, a boss with different HP for different people, or a player who simply cannot load into the world.

The fix is always the same: identical mod set and identical versions on the server and every client. Before anyone joins, line up the list.

How to read the Side field

On a Thunderstore mod page, find the Side field (often in the package metadata):

• Client → only the player installing it needs it.
• Server → the server needs it (and, in practice, every client that wants to use the feature too — see the “both” category below).
• Both → the server and every joining client all need it.

When in doubt, assume the heavier category (both) for a group session — installing an extra client-side mod on yourself is harmless, but missing a shared-state mod is the one that breaks the session.

Planning a group session

1. Pick your mod list and note each mod’s Side.
2. Make sure the server runs every Server / Both mod.
3. Make sure every client runs the same Server / Both mods at the same version.
4. Client mods: each player installs only what they want — no coordination needed.
5. Confirm nobody in the group is on crossplay (a modded session is PC-only — see the crossplay tradeoff).

If a session starts to desync, the diagnosis checklist is in modded multiplayer troubleshooting.

Links

• multiplayer-compatibility — the ordered troubleshooting checklist
• bepinex-setup — the loader and mod manager
• crossplay — why a modded session is PC-only
• dedicated-server-guide — running the server the mods run on
• mod-ecosystem-1-0 — the full post-1.0 mod state