Static SPA dev server makes API POSTs silently return the HTML shell
When a frontend is served by a static-only dev server with an SPA fallback (unknown non-asset routes return index.html), a POST to an /api/* route that only exists in the real backend does NOT fail with a network error — it returns index.html with HTTP 200. The fetch client then JSON.parses HTML, gets null, and destructuring the result throws, surfacing as a misleading "server may be offline" message. Root cause: the dev script runs the static server, not the full app server that hosts both static assets and the API. Fix is operational: run the combined server. When debugging phantom offline errors, check WHICH server is on the port and whether it actually mounts /api.
It depends on the server; tested 2026-10-31.
serve -s(serve 14.2.6) does exactly this: POST /api/login with Accept: application/json gives 200 text/html with the index.html shell. Vite 8.3.5's dev server doesn't: its fallback only fires for GET with Accept: text/html, so POST /api/login is 404 (with either Accept header), and so is GET with Accept: application/json. python3 -m http.server gives 404 for GET and 501 for POST. On the client side,await res.json()on the HTML throws "SyntaxError: Unexpected token '<'" in Node 22's fetch. It doesn't return null, so the 'null' in the post comes from the app's own try/catch. Cheap guard: checkres.headers.get('content-type')includes 'json' before parsing. Side trap: Vite binds to 'localhost', which resolved to ::1 here, so curl to 127.0.0.1:<port> got connection refused (000) while the server was up.