If tModLoader hangs on “Syncing Mods,” refuses a connection, or rejects you after a mod update, fix the setup in this order: launch tModLoader on every device, match the client and server tModLoader builds exactly, install one shared modpack with identical versions and load order, update the server correctly, then use the server log to identify any remaining dependency or download failure. A vanilla Terraria client cannot join a tModLoader server.
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Do not start by repeatedly clicking Join. A sync failure is usually a disagreement about the game build, enabled content, dependency set, or a file the client cannot obtain. Work through these checks in order and restart both sides after you change a version or mod.
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- Confirm the game mode. Every participant must start tModLoader, not vanilla Terraria. The two clients use different content and connection expectations.
- Compare builds. Read the tModLoader build in the client’s main menu and the build reported by the dedicated server console or log. They must match exactly. If the server did not receive a recent Steam update, install the server release that matches the clients.
- Create one shared modpack. Have the host save the enabled mods as a tModLoader modpack and distribute that pack. Each player should enable it, reload mods, wait for the reload to finish, and only then connect.
- Update the server using its deployment method. A Docker deployment normally requires rebuilding the image so it pulls the new tModLoader build. A native or managed server needs the matching release installed and then a full restart.
- Verify server dependencies. Required library mods must be installed and enabled on the server. Use a modpack or the server-management tooling instead of copying files based only on the names shown in the Workshop.
- Remove extra client content. If the build and shared pack agree, temporarily disable client-only or additional mods one at a time until the connection succeeds.
- Read the server log. Find the first mod name, dependency error, download error, or stack-trace line associated with the failed connection. That line is a better diagnosis than the generic “Syncing Mods” label.
- Perform a clean retry. Restart the server and every client after changing the build, mod list, or load order, then try the connection again.
What “syncing mods” actually does
tModLoader’s multiplayer exchange is not a single file-copy operation. The server sends the identifiers for its enabled mods. When downloading is permitted, a client can request missing files, reload its mod set, and send the SyncMods message again. A connection can appear to hang at this stage when the download is missing or blocked, the client’s download setting prevents the request, a dependency cannot load, or the client and server contain different content.
The status text does not identify which of those conditions occurred. The server log records where the process failed, so use it as the authoritative starting point once the basic version and modpack checks are complete. The tModLoader MessageID reference documents this SyncMods exchange; the multiplayer-debugging guidance likewise directs administrators to the server log for the failing location.
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Step-by-step fixes
1. Make sure everyone is running tModLoader
Open the game through the tModLoader installation or launcher, not the ordinary Terraria entry. This applies to the host, dedicated server and every player. A vanilla Terraria client has no tModLoader mod environment and cannot join a tModLoader server, even when the world and Terraria versions otherwise look compatible.
If one player launched the wrong entry, exit completely and start tModLoader before trying to troubleshoot individual mods. Do not use a vanilla client as a test of whether the server is healthy; that test can never succeed.
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2. Match the tModLoader build exactly
Read the build string shown on the tModLoader main menu for a client. On the server, inspect the startup console or the log header for its build. Compare the complete values rather than only the major version. The official multiplayer-debugging guidance is explicit: the client and server tModLoader versions must match exactly.
Commonly, a server remains on its previous release while a Steam client updates. In that case, either update the server to the client’s release or deliberately install the matching client release on every player. Do not assume that a mod update can compensate for different tModLoader builds; the protocol and loader behavior must agree before mod files are considered.
- Record the client build from the main menu.
- Record the server build from its console or log.
- Compare every part of the build identifier.
- After installing the matching release, restart the server and all clients before testing.
If different players have different builds, fix that before comparing their mod lists. Otherwise, a version error can mask the real mod or dependency problem.
3. Distribute a single modpack instead of rebuilding a list by hand
The host should save the working enabled-mod set as a tModLoader modpack and give that pack to every participant. Each player should enable the pack, reload mods, wait for the reload to complete, and then join. This controls three variables that manual installation often changes accidentally: which mods are enabled, which versions are installed, and the order in which they load.
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The multiplayer guide requires the client and server to use identical mods, versions and load order. A list of similarly named Workshop entries is not a reliable substitute. As the Dedicated Server Utils documentation puts it: “The steam workshop does not use mod names to identify mods, so a modpack should be made to install mods from the workshop.”
When the host changes the pack, treat it as a new release for the group:
- Disable or remove the old set only as needed to avoid stale entries.
- Enable the revised pack on the host and reload until tModLoader reports completion.
- Give the same pack to every player and the server administrator.
- Have every client reload before anyone attempts to connect.
- Restart the server after the pack is changed.
Do not add a “small extra” mod to one client while diagnosing. Even a quality-of-life or visual mod can alter the enabled set enough to expose a multiplayer incompatibility.
4. Update the server according to its deployment method
Docker deployments
For a Docker-based server, updating a tag or editing a configuration value is not enough if the running image still contains the old loader. Rebuild the image using the procedure documented for your server image so it pulls the new tModLoader build, then recreate or restart the container. Confirm the resulting build in the container’s startup output before asking players to connect.
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Native or managed servers
Install the tModLoader release that matches the clients, then perform a complete server restart. Managed panels may expose separate controls for Terraria and tModLoader; select the tModLoader release rather than the vanilla server. After startup, check the console or log for the actual build and enabled-mod list.
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When a hosting panel updates the game automatically, wait for the update to finish before clients launch. If the panel cannot provide the build your group uses, either move every client to the server’s build or choose a deployment that allows the required release. A managed host can reduce maintenance, but it does not remove the requirement for identical builds and mod content.
5. Check required libraries and Workshop identity
Many content mods depend on library or framework mods. The dependency must be installed and enabled on the server as well as on the clients. If the server log names a missing dependency, install the exact required Workshop item through the modpack or the server-management tooling, enable it, and restart.
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Workshop installation is identity-based rather than display-name-based. Two entries can have similar names while representing different items, and a renamed item can retain the same underlying identity. This is why the Dedicated Server Utils documentation recommends a modpack or management tooling instead of copying files selected only by their visible names.
Check the following when a dependency error appears:
- The required library is present on the server.
- It is enabled, not merely downloaded.
- The same version is included in the shared client pack.
- The load order places prerequisites where tModLoader expects them.
- No older duplicate of the library is enabled.
6. Remove client-only or extra mods methodically
Once the build and shared pack match, look for content that exists only on one client. The tModLoader multiplayer discussion specifically recommends checking for client-side mods that differ from the rest of the group. Some visual or interface additions may be safe in a particular setup, but you should not assume that every “client-only” label guarantees compatibility.
Disable one suspect mod at a time, reload, restart the client and test. Changing several mods together prevents you from identifying the cause. If removing one mod resolves the connection, keep it out of the shared session and check its documentation for a multiplayer-compatible configuration before reintroducing it.
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Do not delete the entire working pack as a first resort. Preserve a copy of the host’s known-good pack, then test a minimal change against that baseline. This makes it possible to restore the previous state if the suspected mod was unrelated.
7. Use the server log to identify the real failure
Open the log produced by the failed connection and start at the first relevant error, not the last cascade of messages. The multiplayer-debugging wiki notes that the server log should contain the error location. The first named mod, dependency, download failure or stack-trace line is usually the best next investigation target.
Interpret the evidence in categories:
- Build or protocol line: return to the exact-version comparison and update the lagging side.
- Missing dependency: install and enable the named library on the server and in the shared pack.
- Mod load exception: test the named mod at the same version on both sides, then check its load order and dependencies.
- Download or Workshop error: verify that downloads are allowed, the server tooling selected the correct Workshop identity, and the client can obtain the file.
- Network or timeout message: distinguish a file-transfer problem from a content mismatch before changing mods.
Keep the log from the server, not only a screenshot of the client’s loading screen. The client may show “Syncing Mods” while the server has already recorded a precise cause.
8. Cleanly reload after every change
tModLoader keeps an in-memory mod set during a session. After changing a build, enabling a pack, replacing a dependency or disabling an extra mod, reload the mods as requested by the client, then exit and restart both the server and clients. The troubleshooting guidance recommends restarting both sides after mod changes.
A clean cycle is:
- Stop the connection attempt and leave the world.
- Apply one controlled change.
- Reload mods on the client and wait for completion.
- Restart the server and confirm its startup build and mod list.
- Restart each client, enable the same pack, and connect.
Diagnose the symptom before changing anything
| What you see | Most likely area | First action |
|---|---|---|
| Vanilla client cannot join | Wrong game mode | Launch tModLoader on that client. |
| Immediate version or protocol rejection | Different tModLoader builds | Compare the full client and server build strings. |
| Endless “Syncing Mods” with no useful client message | Download setting, missing file, dependency, or content mismatch | Read the first related server-log error. |
| Connection fails after one player updates | Different mod version or load order | Reinstall the host’s shared modpack and reload. |
| Server starts but one player is rejected | Extra or client-only mod on that player | Disable the extra mod one at a time and retry. |
| Docker server reports an old loader after an update | Old image or container still running | Rebuild the image, recreate the container and verify the startup log. |
| Dependency named in the stack trace | Missing or disabled server library | Install the exact Workshop identity through a pack or management tool. |
Why manual mod lists fail more often than shared packs
Hand-maintained lists can differ in subtle ways: one player may have an older file, an identically named but different Workshop item, a disabled prerequisite, or a different load order. Those differences are difficult to see from a simple list of display names. A shared pack gives the host one reproducible definition of the session and gives the server administrator an installation source that preserves Workshop identities.
Keep the pack with the server’s configuration and label changes clearly. When a player reports a failure, compare the pack and build first rather than asking everyone to reinstall arbitrary files. This reduces troubleshooting to a known baseline and makes rollback possible.
When downloading during sync is the problem
The SyncMods process can request files only when downloading is allowed and the client can reach the required source. If the server sends its identifiers but the client never completes the request, check the download setting in the tModLoader setup and inspect the server log for a blocked, missing or failed download. A disabled download option can look like a mod mismatch because the client never obtains the content it needs.
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A repeatable procedure for groups and administrators
Use this workflow whenever you create a new world or change the mod set:
- The host chooses the tModLoader build and records it.
- The host enables the required mods, confirms dependencies, reloads successfully and saves a modpack.
- The server administrator installs the same tModLoader build and imports the same pack using the server’s supported tooling.
- The administrator starts the server and records the build and enabled-mod list from the console or log.
- Every player installs the pack, reloads mods and confirms the same build before joining.
- After a change, update the pack and server together, restart both sides, and preserve the previous pack for rollback.
This process separates three kinds of change that are often confused: a loader update, a mod-content update and a deployment update. If a connection fails, you can identify which layer changed and inspect the corresponding evidence.
What not to do
- Do not let one player launch vanilla Terraria while everyone else uses tModLoader.
- Do not assume a similar-looking tModLoader build is close enough; the requirement is an exact match.
- Do not ask players to recreate a pack by searching Workshop names manually.
- Do not update a Docker configuration without rebuilding the image and checking the running container’s build.
- Do not copy a client’s files to the server solely because their display names match.
- Do not remove many mods at once when the log identifies one likely culprit.
- Do not rely on the “Syncing Mods” label as a diagnosis; use the server log.
When the issue is outside the mod list
If the build, pack, dependencies and load order all match, and the log shows no mod or download error, the remaining evidence may point to a network or deployment problem rather than a content mismatch. Capture the server log from the failed attempt, note whether the client was allowed to download, and test again after a full restart. If a managed server cannot install the required loader release or Workshop identity, its administrator may need to change the server configuration or move to a deployment that provides those controls.
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Quick Recap
Final checklist
- Every participant launched tModLoader.
- The client and server build strings match exactly.
- The host’s modpack is enabled on every client and the server.
- Mods, versions and load order are identical.
- Required libraries are installed and enabled on the server.
- The server was rebuilt if it runs in Docker, or updated and restarted if it is native or managed.
- Client-only or extra mods were removed during isolation testing.
- The server log was checked for the first meaningful error.
- Both server and clients were restarted after the final change.
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