Fix: Mc-proxy Incompatible Client Error After Upgrade

Encountering an "Incompatible client! Please use 1.7.2-1.21" error after upgrading your itzg/mc-proxy Docker container from version 2025.6.0 to 2025.9.0 (or 2025.9.1) can be frustrating. This article provides a comprehensive guide to understanding and resolving this issue, ensuring a smooth Minecraft experience for you and your players. We'll explore potential causes and walk you through several troubleshooting steps to get your server back on track. Understanding the root cause is essential, as this error typically indicates a mismatch between the client version attempting to connect and the server version that mc-proxy is configured to support.

Understanding the Problem

When you upgrade mc-proxy, it might introduce changes in how it handles client connections or the Minecraft protocol versions it supports. This error message specifically tells you that the client attempting to connect is not compatible with the version that mc-proxy expects. This can happen due to several reasons:

• Minecraft Client Version Mismatch: The most common cause is that the Minecraft client version your players are using doesn't match the Minecraft server version that mc-proxy is configured to proxy. For example, if your mc-proxy is set up to support Minecraft 1.18, but a player tries to connect with a 1.16 client, they'll encounter this error.
• Configuration Issues: Sometimes, the configuration of mc-proxy itself might be incorrect after the upgrade. Certain settings related to protocol support or client version handling might not be properly configured or may have been reset during the upgrade process.
• Underlying Server Version Incompatibility: While mc-proxy is designed to handle different client versions, the underlying Minecraft server it proxies might have its own version restrictions. If the server's version is significantly different from what mc-proxy expects or what the connecting client is using, compatibility issues can arise.

Let's dive deep into how to troubleshoot and resolve this issue effectively.

Troubleshooting Steps

Here are several steps you can take to diagnose and fix the "Incompatible client" error:

1. Verify Minecraft Client and Server Versions

• Check Client Version: Ensure that the Minecraft client version your players are using is compatible with the Minecraft server version you intend to support. Communicate with your players to confirm they are using the correct version. It's crucial that all players connecting to your server use a compatible client version.
• Check Server Version: Determine the exact version of your Minecraft server. If you are using a server.properties file, you can find it there. If you're using a Docker image for your Minecraft server, check the image's documentation or environment variables to ascertain the version. Confirm that your Minecraft server version aligns with the intended client version.

2. Review mc-proxy Configuration

• Examine Environment Variables: Check the environment variables used to configure your mc-proxy Docker container. Pay close attention to variables that define the supported Minecraft versions or protocol handling. For example, variables like ALLOWED_VERSIONS or similar configurations may be relevant.
• Inspect Configuration Files: If you're using any custom configuration files for mc-proxy, review them for any settings that might be causing version conflicts. Look for deprecated settings or settings that need to be updated to align with the newer version of mc-proxy.
• Ensure Correct Proxy Settings: Confirm that mc-proxy is correctly configured to proxy the Minecraft server. Double-check the server address and port settings to ensure they are pointing to the correct Minecraft server instance. Incorrect proxy settings can lead to miscommunication and version incompatibility errors.

3. Update mc-proxy Configuration

• Adjust Supported Versions: If necessary, adjust the mc-proxy configuration to explicitly allow the Minecraft client version your players are using. This might involve modifying environment variables or configuration files to include the specific version.
• Consider Version Ranges: Some mc-proxy configurations allow you to specify a range of supported Minecraft versions. Ensure that this range includes the client versions your players are using. Using version ranges can provide flexibility and compatibility for a wider range of clients.

4. Check for Conflicts with Other Mods or Plugins

• Isolate the Issue: If you're using mods or plugins on your Minecraft server, there might be conflicts causing the version incompatibility error. Try disabling mods or plugins one by one to see if any of them are contributing to the problem. Sometimes, outdated or incompatible mods can interfere with the server's ability to handle client connections correctly.
• Update Mods/Plugins: Ensure that all your mods and plugins are up to date and compatible with the Minecraft server version you are using. Check the mod/plugin documentation for compatibility information and update instructions.

5. Recreate the Docker Container

• Clean Rebuild: In some cases, remnants of the old mc-proxy version might be causing conflicts. Try completely removing the Docker container and recreating it from scratch using the updated image. This ensures a clean installation and eliminates any potential conflicts from previous versions.
• Clear Docker Cache: Clearing the Docker cache can also help ensure that you're using the latest version of the image and that there are no cached files interfering with the upgrade. Use the docker system prune command to clear unused Docker resources, including cached images and containers.

6. Downgrade mc-proxy (Temporary Solution)

• Revert to Previous Version: If you're unable to resolve the issue quickly, you can temporarily downgrade to the previous version of mc-proxy (2025.6.0) that was working correctly. This will allow your players to continue connecting while you investigate the root cause of the incompatibility issue. Downgrading is a temporary workaround, and you should still aim to identify and resolve the underlying problem to ensure long-term stability.

Example Scenario and Solution

Let's say you've upgraded to itzg/mc-proxy:2025.9.1 and are encountering the "Incompatible client" error. Your Minecraft server is running version 1.19. You've verified that some players are using Minecraft client version 1.18.

In this scenario, you would need to configure mc-proxy to allow connections from both 1.18 and 1.19 clients. This might involve setting an environment variable like ALLOWED_VERSIONS to `