server-state-tanstack-query

Query keys are arrays, most-generic segment first, defined once as a const ... as const next to the query itself (see SUBSCRIBERS_KEY in src/lib/queries.ts) — never inline a raw array literal at each call site, or cache invalidation will silently miss.
Default to useSuspenseQuery (not useQuery) for data a component cannot render without — it removes the isLoading/data is possibly undefined branch entirely at the type level. Wrap the consuming component in <Suspense fallback={...}> one level up (see src/routes/index.tsx). Use plain useQuery only when the component has a meaningful "no data yet" UI state of its own.
Mutations validate their input against the same Zod schema used at the API boundary (src/lib/schemas.ts) inside mutationFn, not in the calling component — the mutation hook is the trust boundary, not the form.
On mutation success, prefer queryClient.setQueryData with the server's actual response (see useSubscribeMutation) over invalidateQueries when the mutation response already contains the new state — one fewer network round-trip.
Every fetch goes through fetch() against a relative /api/... path with an MSW handler in src/mocks/handlers.ts backing it in dev and tests (testing-rtl-msw skill) — never call an absolute external URL directly from a component or hook.

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