Move middleware to proxy, and decide what the new runtime buys

next dev warns on every start that the middleware file convention is deprecated. Next 16 renamed it to proxy — the upgrade guide says "to clarify network boundary and routing focus" — and ships a codemod:

npx @next/codemod@canary middleware-to-proxy .

Nothing is broken. It is a deprecation, not a removal, and the warning is the whole of the damage today.

It is not only a rename

From the version 16 upgrade guide, in as many words:

The edge runtime is NOT supported in proxy. The proxy runtime is nodejs, and it cannot be configured. If you want to continue using the edge runtime, keep using middleware.

So renaming the file moves src/middleware.ts from the Edge runtime to Node, and there is no flag to keep it where it is.

Which makes one of our own comments untrue

src/middleware.ts explains why it verifies nothing before the page does:

Nothing is verified at this point, deliberately. Middleware runs on the Edge runtime with no node:crypto, and extending a cookie that turns out to be wrong costs nothing.

On Node, node:crypto is there. The constraint that shaped that decision is gone, so the comment cannot survive the rename unedited — it would be a statement about this code that is no longer true of it.

The decision this ticket is really for

Two things pull against each other and the rename forces a choice rather than settling one:

• The gate could verify in the proxy now. matches() and expectedKey() would run before the page. Whether that is worth doing is open — the page already refuses, and a second place that decides who gets in is a second place to get it wrong.
• It stops running at the edge. A Node function on Vercel, on every matched request, for what is currently a cookie refresh on one route. Fair trade, probably, but it should be a trade someone made rather than one that arrived with a mv.

Config flags carrying the middleware name were renamed too (skipMiddlewareUrlNormalize → skipProxyUrlNormalize). We set none of them, so that part costs nothing.