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Client side features

While Polymer by itself is mainly server side api, it includes some client side functionality for mods to use. It allows you for example to display vanilla friendly item for normal clients and custom models if it's present on server.

Keeping modded item/block/entity on compatible client.

To keep client side model for loading, you need to implement PolymerKeepModel interface on your modded object. To enable it's decoding, just add PolymerClientDecoded interface for it.

After that, you just need to return server side items/block in corresponding player-aware getPolymerX methods.

To "sync" presence/version of your mod you can use Polymer's handshake feature.

You can do that by registering packets for which you should check Polymer Networking documentation

After that you can just validate if player supports it with this check it like this

SomeObject getPolymerX(ServerPlayerEntity player) {
    if (PolymerServerNetworking.getSupportedVersion(player.networkHandler, PACKET_ID) > 0) {
        // Client state for modded
        return this;
    } else {
        // Client state for vanilla
        return VanillaObjects.SOMETHING;

This section is incomplete... I hope in code comments will guide you well...