Add a hook to observe server function errors before serialization

[adipascu]

Problem

When a server function throws, the handler in packages/start/src/fns/handler.ts catches the error, sets X-Error, and serializes it into the response for the calling client. No server-side error hook ever fires, so genuine crashes inside server functions are invisible to monitoring (Sentry and friends): the platform error hooks (for example Nitro's error hook) only see errors that escape the handler, and these never do.

The result is that a production server function can crash on every call while dashboards stay green, and the only signal is users reporting a generic failure state.

Prior art in other frameworks

• SvelteKit has handleError, called only for unexpected errors (expected error() throws skip it), letting apps report to monitoring and return a sanitized user-safe representation. This expected-vs-unexpected split is exactly what server-function consumers need here too.
• Next.js has onRequestError in instrumentation.ts, invoked for server errors including Server Actions (routeType: 'action'), designed for custom observability providers.
• TanStack Start supports global server-function middleware via registerGlobalMiddleware, which wraps every server function and can observe failures.

Proposal

Call an app-registered global hook from the handler's catch, before serialization, skipping thrown Responses since those are control flow (redirects, forbidden):

if (!(x instanceof Response)) (globalThis as any).__reportServerFnError?.(x);

An app can then register globalThis.__reportServerFnError from its server entry (or a Nitro plugin) and forward to its monitoring SDK.

Status

Draft on purpose: the global hook is the minimal shape that solves the problem, and I am happy to rework it into whatever API surface you prefer (an option on the handler, an exported setServerFnErrorHandler, an event on the app instance, or a SvelteKit-style handleError in the server entry). We currently maintain this as a pnpm patch in production alongside #2159 and would love to drop both.

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