Next.js 16 Custom Cache Handler and Partial Prerendering (PPR) Races and Crashes

Developers using Next.js 16 with custom async cache handlers (such as Redis) alongside Partial Prerendering (PPR), cacheComponents, and cachedNavigations encounter severe runtime issues. These include client navigation crashes due to unresumable fallback shells (React error #412), build output and resume protocol failures resulting in 500 errors (E592), and asynchronous cache handler race conditions causing cache misses and demotions to runtime holes (E1163).

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