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Factorio will let you join a modded server only when the client and server match in four ways: the Factorio release, every mod and its exact version, each mod’s enabled or disabled state, and the save’s startup settings. In the multiplayer connection dialog, choose Sync mods with server; for a local save, use Sync mods with save and hold Ctrl while clicking it when you need the versions recorded by that save. If synchronization still fails, compare the complete lists, remove newly added mods, roll back recently updated ones, and restart the server between tests.
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What a Factorio mod-mismatch error means
Factorio calculates checksums for the installed game and mods when a player connects. The server compares those values with the client’s values before allowing the join. This prevents a player with different files from entering a game that could simulate a different world state.
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“The same mods” means more than matching names in the mod browser. A client can fail the check if a file was edited, manually copied, renamed, or replaced with another release that has the same displayed name. A server save can also preserve the mod state that was active when the save was created, so the newest release currently installed on a client may be incompatible with that save.
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Use the mismatch as a controlled comparison problem rather than repeatedly reinstalling everything. First establish which game version and mod state the server is actually using, then make the client identical to it.
The four things that must match
| Comparison | What to verify | Why it matters |
|---|---|---|
| Factorio release | The version shown in the client and the version used by the dedicated-server binary | Different game releases can produce different simulation behavior even when the mod list is identical. |
| Mod files and versions | Every mod filename, release number, and file content on both sides | Factorio checks mod contents, not just the name shown in the interface. An edited or replaced file changes its checksum. |
| Enabled state and dependencies | Which mods are enabled, which are disabled, and whether required dependencies are present at the expected versions | A disabled mod on one instance or a different dependency chain changes the effective installation. |
| Startup settings | The startup settings used by the save and the settings selected on the client or server | Different startup values can alter how the game and mods initialize, so a matching filename list is not sufficient. |
Check the Factorio release before comparing individual mods. A version mismatch can make every subsequent mod comparison misleading. Once the game releases match, compare the complete mod list, not only the mod you most recently installed.
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For a server that is visible in the multiplayer browser or a connection dialog, Factorio provides an official synchronization path. It downloads and enables the exact mod versions the server reports.
- Open the multiplayer browser or start the connection to the server.
- When Factorio offers Sync mods with server, accept it.
- Allow the download and enable process to finish. Do not substitute a similarly named file from a manual download; the purpose of this control is to obtain the server’s exact versions.
- If Factorio asks you to restart, restart the game before trying to connect again.
- Reconnect after the restart. Every player who receives the mismatch must perform the same synchronization, because one corrected client does not repair another player’s local installation.
This route is normally preferable to manually hunting through mod archives because it uses the server’s reported state as the comparison target. It also changes the local enabled list to match what the server expects.
Synchronize a local save
A local save can contain the mod state that was active when it was created. Use that recorded state when loading the save instead of assuming that the latest release of each mod is correct.
- Open Load game in Factorio.
- Select the save you intend to load.
- Choose Sync mods with save.
- For the normal action, Factorio uses the latest available mod versions.
- To use the exact versions recorded by the save, hold Ctrl while clicking Sync mods with save.
- Restart Factorio if prompted, then load the save again.
The Ctrl-click distinction is important. “Latest” is a moving target: a mod can have been updated since the save was made. The save’s recorded versions are the safer choice when you are restoring an older game or trying to reproduce a known working setup.
Check the server before changing the client
Synchronization is only reliable when the server is using the save and mod set you think it is. Establish the server’s actual state before deleting files or changing versions.
Confirm the server’s Factorio release
Read the version reported by the dedicated-server binary and compare it with the version displayed by the client. Do this first. If the releases differ, bring them to the same release before diagnosing individual mods.
Record the complete mod list
Write down every mod the server has installed, its version, and whether it is enabled. Include dependencies and any mod that appears inactive. A short list containing only the mod you remember changing can hide a second mismatch.
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Confirm the intended save
A command-line server can be associated with a different save than the one you expect. The save determines the mod state that was active when it was created. If the server starts another save, a client synchronized against the wrong file can remain mismatched even though its local list looks correct.
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Check startup settings
Compare startup settings as part of the mod state. Two installations can contain the same mod files and still initialize differently if their startup values do not match the save and server configuration.
When automatic synchronization still fails
Do not keep accepting the same prompt without recording what changes. Switch to a controlled isolation loop: capture the server list, compare it with the client list, change one variable, restart where required, and test again.
Look for manually copied or edited files
Inspect mods that were copied outside Factorio’s normal download flow, edited locally, renamed, or replaced with a file from another installation. A same-named mod with different contents produces a different checksum. Restore the server’s exact file rather than trying to make the filename look identical.
Identify recently added mods
The official desynchronization guidance recommends removing newly added mods and testing again. Remove the newest addition from the affected installation, restart the server if it was changed there, and retry the connection. If the mismatch disappears, add that mod back only after confirming the release and settings required by the save.
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Roll back recently updated mods
A mod update can be the trigger even when nobody deliberately changed the server configuration. Roll back mods that were updated recently and test the previous known state. This is especially relevant for an older save whose recorded mod version is no longer the latest release.
Restart between isolation tests
Restarting the server is part of the official isolation loop. A restart ensures the next test loads the changed enabled list and startup configuration rather than continuing with a previously initialized process.
Compare dependencies and enabled state
Check that required dependencies are installed at the versions expected by the server and that both sides enable the same set. A dependency that is present but disabled, or enabled at a different release, can keep the effective mod graph from matching.
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Factorio’s command-line reference documents two relevant options:
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--sync-modssynchronizes mods with a save.--start-serverlaunches a multiplayer server.
The server must be associated with a save when you use this workflow. Make sure the path or save selection points to the intended file; otherwise you may synchronize against one save and launch another.
A practical command-line sequence is:
- Stop the running server so no process is using the save.
- Identify the save that should define the mod state.
- Run the server’s documented command with
--sync-modsfor that save. - Start the multiplayer process with
--start-serverand the intended save. - Have each client connect and accept Sync mods with server if prompted.
The option names are exact; do not replace the hyphens with typographic dashes. If your launcher or hosting panel wraps the command, verify that it passes both options to the Factorio binary and that the selected save is the one you expect.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choosing between the main fixes
| Fix | Restores exact version/checksum equality? | Preserves the save’s recorded state? | Restart or rollback needed? | Best use |
|---|---|---|---|---|
| Sync mods with server | Yes, when the server is reporting the correct installation | It follows the server’s active state | Restart if Factorio prompts | A client joining a reachable multiplayer server |
| Sync mods with save | Yes, after downloading the selected versions | Only the Ctrl-click form explicitly selects versions recorded by the save | Restart if prompted | Loading a local save |
| Remove a newly added mod | Can, if that mod caused the difference | May require restoring the save’s prior state | Restart the server for a clean test | Isolating a recent configuration change |
| Roll back an updated mod | Can, if the previous release matches the server or save | Usually better for older saves than accepting the newest release | Server restart is part of the test loop | Diagnosing a mismatch that appeared after an update |
--sync-mods with the intended save |
Yes, when the command targets the correct save | Uses the save as the synchronization reference | Stop and restart the dedicated server | Command-line or automated server administration |
The deciding question is not which action is fastest in isolation; it is which action uses the correct reference. For a live server, the server’s active list is the reference. For a local or archived save, the save’s recorded versions—selected with Ctrl-click—are the reference.
A complete troubleshooting procedure
- Stop making simultaneous changes. Note the time the mismatch began and the last mod added or updated. This gives you a useful first suspect without assuming it is the only cause.
- Compare Factorio releases. Check the client and dedicated-server binary. Align them before changing mods.
- Use the built-in sync path once. For a server, choose Sync mods with server. For a local save, choose Sync mods with save; use Ctrl-click when the save’s exact versions matter.
- Restart when prompted. A restart applies downloaded files and changed startup state.
- Compare every mod. Record filenames, versions, enabled state, dependencies, and startup settings on both sides.
- Inspect manual changes. Replace edited, renamed, or manually copied files with the exact server versions.
- Remove the newest addition. Restart the server and test. Keep the result: either the mismatch remains or the change identified the offending mod.
- Roll back recent updates. Test the previous release of each recently updated mod one at a time where possible.
- Verify the save selection. For command-line operation, confirm that
--start-serverlaunches the same save used with--sync-mods. - Synchronize every player. Once the server state is known to be correct, have all clients repeat synchronization rather than distributing a mixture of old and new files.
Common mistakes that keep the mismatch alive
- Matching names instead of contents: A same-named but edited file has a different checksum.
- Assuming “latest” is always correct: A save may require an older recorded version.
- Checking only one changed mod: Dependencies, disabled entries, and startup settings also affect the effective state.
- Changing the client while the server runs another save: Synchronization cannot fix a wrong save selection.
- Updating one player only: Every client must be rebuilt from the same server state.
- Skipping a restart: The process may still be running with the prior configuration.
- Using a manual replacement without checking the file: A matching filename does not prove matching content.
How to prevent future mismatches
Keep a known-good save and mod state
When a multiplayer session works, preserve the save and note the Factorio release, mod versions, enabled state, dependencies, and startup settings associated with it. This gives you a reference for later updates.
Update deliberately
Do not update the server and only one client. Update the server, verify the resulting mod state, and have every player synchronize before the next session. If an older save is still in use, select its recorded versions rather than blindly taking the latest releases.
Use one source of truth
For a live multiplayer game, treat the server’s active list as authoritative. For an archived or local save, treat that save’s recorded list as authoritative. Avoid mixing manually copied files with automatically synchronized files unless you have verified their contents and versions.
Document command-line launches
Record which save is passed to the dedicated server and how --sync-mods and --start-server are used. A written launch procedure reduces the chance that an update silently starts a different save.
What a successful repair looks like
The repair is complete when the client and server show the same Factorio release, the same complete mod filenames and versions, the same enabled and disabled state, matching dependencies, and matching startup settings. Factorio should then calculate equal checksums during the join check, allowing the player to enter without a synchronization warning.
If the warning returns after a later update, repeat the comparison from the last known-good state. The official isolation method—remove newly added mods, roll back updated mods, and restart the server between tests—turns an otherwise vague error into a specific version or configuration change that can be managed safely.
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