Fixes Minecraft client lag from receiving chat messages ### YOU DO NOT NEED THIS MOD ANYMORE ON 1.18
Mojang has fixed the bug in their API that was causing it to return an error instead of an empty list. Minecraft will fetch the block-list once when joining the world. This mod may still improve world loading times by the time it takes to make that HTTP request; especially if you have a slow internet connection.

This comment by a Mojang employee confirms that the bug was fixed: https://bugs.mojang.com/browse/WEB-5587?focusedCommentId=1134973&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-1134973

This bug still affects 1.17 and 1.16 but only on the first chat message received: https://bugs.mojang.com/browse/MC-218167

Technical Details: When receiving a chat message, Minecraft sends a HTTP request in the render thread to check your player block list. The current frame does not finish rendering until this request has finished, causing a lag spike. This mod lets chat messages through until a block-list can be fetched.

Why is this happening now and not before?: something happened to Mojang's API. When joining a world, the blocklist would usually load fine, but Mojang's API has started returning errors for users who have migrated to Microsoft accounts, causing the block list to be fetched in 2 minute intervals when receiving a message.

A commented walkthrough of why the bug occurs

// A simplified view of the code.
private static final long BLOCKLIST_REQUEST_COOLDOWN = 120;
private Instant nextRequest;
private final blockList Set;

// This method is called when receiving a chat message
public boolean isBlockedPlayer(UUID playerID) {
    // If we don't have the blocklist yet, fetch it
    // Note that when there's an error, fetchBlockList returns null
    // 
    if (this.blockList == null) {
        this.blockList = fetchBlockList();
        // If we still don't have it, assume the player is not blocked
        if (this.blockList == null) {
            return false;
        }
    }
    return this.blockList.contains(playerID);
}

public Set fetchBlockList() {
    // Only check at least every 2 minutes.
    // This is why the lagspike only occurs every 2 minutes or later
    if (this.nextRequest == null || Instant.now().isAfter(this.nextRequest)) {
        return null;
    }
    // Reset the 2 minute timer
    this.nextRequest = Instant.now().plusSeconds(BLOCKLIST_REQUEST_COOLDOWN);
    try {
        // Make the HTTP request
        BlockListResponse response = minecraftClient.get(routeBlocklist, BlockListResponse.class);
        return response.getBlockedProfiles();
    } catch (/* exception */) {
        // If there's an error return null
        return null;
    }
}

If you'd like to see the unmodified for yourself, check out com.mojang.authlib.yggdrasil.YggdrasilUserApiService, remember that these methods get called on the render thread, thus blocking the frame from finishing.

GIF demonstration of the bug

Chat Lag Fix

External resources

Project members

Technical information