Supporting Paper too

A plugin that extends FeatherPlugin is Feather-only by default. On Paper it logs a red message and disables itself. To make it work on both FeatherMC and Paper servers, opt in with one method, then keep the FeatherMC-specific code behind a check.

1. Allow Paper

Override allowPaper() and return true. Without this override the plugin stays Feather-only.

import dev.feathermc.feather.FeatherPlugin;

public final class MyPlugin extends FeatherPlugin {

    @Override
    protected boolean allowPaper() {
        return true;
    }

    @Override
    protected void onFeatherEnable() {
        // runs on FeatherMC and on Paper
    }
}

onFeatherEnable() and onFeatherDisable() still replace onEnable() and onDisable().

2. Keep FeatherMC features behind a check

Branch on FeatherSupport.isFeather() for anything that uses the FeatherMC API (Feather, the events or the config classes). Paper does not have those classes, so loading them there throws a NoClassDefFoundError.

import dev.feathermc.feather.FeatherSupport;

@Override
protected void onFeatherEnable() {
    if (FeatherSupport.isFeather()) {
        // Register the FeatherMC listener only here, so Paper never loads it.
        getServer().getPluginManager().registerEvents(new FeatherListener(), this);
        getLogger().info("FeatherMC features enabled.");
    } else {
        getLogger().info("Running on Paper, FeatherMC features are off.");
    }
}

Put all FeatherMC API use in separate classes, such as FeatherListener above, and only create or register them inside the isFeather() branch. A class that merely mentions an API type in a field, parameter or event handler can be loaded early and fail on Paper.

public final class FeatherListener implements Listener {
    @EventHandler
    public void onFlag(final AntiCheatFlagEvent event) {
        // FeatherMC-only behaviour
    }
}

3. Bundle the gate classes

Required for Paper

Your main class extends FeatherPlugin, and Paper does not ship it. Bundle FeatherPlugin and FeatherSupport into your jar, or the plugin cannot load on Paper at all. Do not bundle Feather, the events or the config classes.

build.gradle.kts

tasks.shadowJar {
    include("dev/feathermc/feather/FeatherSupport*.class")
    include("dev/feathermc/feather/FeatherPlugin*.class")
    // also include your own plugin classes and resources
}

Use compileOnly for feather-api as in the setup guide. Shadow only includes what you list, so list your own packages too, for example include("com/example/**"), and your resources such as plugin.yml.

Summary

You want	Do this
Runs on FeatherMC only	Extend FeatherPlugin and change nothing else.
Runs on FeatherMC and Paper	Override allowPaper(), bundle the two gate classes and guard FeatherMC code with FeatherSupport.isFeather().