Client side (lib/udp.mjs): Geckos on the wire serializes via {"<event>": payload}. When the server emits an object (as arena-manager does with snaps), the client's `on` handler receives the payload already as a parsed object — not a string. respond() was calling JSON.parse(content) unconditionally, which throws on objects ("[object Object]" is invalid JSON) and silently dropped every arena:snap. That's why remote figures appeared at their first WS-delivered position and then went stale once UDP bound and the server stopped falling back to WS. Guard the JSON.parse behind a typeof check. Server side (arena-manager.mjs): Add minimal telemetry to diagnose the matching cmd-direction issue: log the first cmd received per handle, count cmd rx + snap tx per player, and dump a 5-second rate summary. Cheap to leave in — ~1 line every 5s when the arena is occupied. Also bump sw cache to v7 so clients pick up the fresh udp.mjs on reload instead of riding out another stale-while-revalidate cycle. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>