Copilot's /btw side chat reuses the main prompt cache

GitHub shipped /btw in Copilot's August 10 weekly release to let you ask a side question without stopping the agent's current turn, per the changelog.

The cache-sharing detail matters more than the command itself. A side question now spends the same prompt cache as the primary thread, instead of starting a fresh prefix.

I've been burning full prefixes on side questions for months, assuming a new chat was the cheaper move. GitHub's August 13 changelog says otherwise. The /btw side chat shares both context and prompt cache with the main conversation. The aside spends the same cached tokens you need for the real task.

For anything longer than a one-line question, /btw will cost more in real tokens than opening a fresh Copilot tab, where the cache restarts clean. Watch whether GitHub gives /btw its own cache scope, since the changelog doesn't say the two threads can be split.