NestJS Express Middleware and Fastify Runtime Interoperability in Version 10

NestJS 10 changed the default HTTP server from Express to Fastify, creating an integration boundary for teams that maintain Express-style middleware across version 9 and version 10 codebases. The precise goal is to determine which existing Express middleware patterns remain viable under the Fastify runtime without introducing runtime failures or hidden behavioral differences.

The relevant uncertainty centers on the absence of a definitive compatibility list in the official migration documentation. While the framework supports both server adapters, many third-party middlewares assume Express 4.x request and response objects, and it is unclear which of those assumptions are guaranteed to fail or silently degrade. The optional Express shim offers a bridge, but its long-term maintenance status and performance characteristics are not clearly documented.

Given this ambiguity, what validation strategy should teams apply before committing to either rewriting middleware for Fastify or relying on the Express compatibility layer? Which categories of Express middleware are most likely to break under the Fastify adapter, and how can those risks be identified without exhaustive trial and error? Is there an official compatibility matrix that distinguishes supported Express APIs from those that require replacement in NestJS 10?