SOTA Async & Concurrency

Purpose

Concurrency bugs are the most expensive class of defect: they pass tests, ship, and then corrupt data or hang production under load. This skill encodes the 2026 state of the art for concurrent design and the bug catalog auditors need to spot defects by reading code, without reproducing them. Concepts are cross-language; per-runtime notes are inlined where semantics genuinely differ (GIL, goroutine scheduling, tokio executors, Node's single loop).

Two operating modes. Pick one explicitly before starting.

BUILD mode

When writing new concurrent code: