discoverHttpCapabilities

function discoverHttpCapabilities(
   baseUrl,
   prefix,
   authorization?,
   acceptedMaxResponseBytes?,
   fetchFn?
): Promise<HttpServerCapabilities>;

Defined in: src/client/capabilities.ts:172

Probe the server’s auth-exempt /health endpoint for HTTP capabilities.

The probe is HEAD, not OPTIONS, because OPTIONS cannot be used from a browser. It carries VGI-Accept-Max-Response-Bytes, which makes it a non-simple request, so the browser preflights it and asks Access-Control-Request-Method: OPTIONS — and a server answers a preflight with the methods its /health route actually implements, which is GET and HEAD. The probe is blocked before it is sent, and the error names a method nobody wrote. Every browser consumer hit this; each had to shim fetch to get a connection at all.

HEAD is equivalent for this purpose and is what the spec asks for: {prefix}/health answers GET, HEAD, OPTIONS and carries the capability headers on all three (WIRE_PROTOCOL.md §10). The C++ client already probes with HEAD, so this also removes a divergence between the ports rather than adding one.

Parameters

Parameter	Type	Default value
baseUrl	string	undefined
prefix	string	undefined
authorization?	string	undefined
acceptedMaxResponseBytes?	number	undefined
fetchFn?	typeof fetch	globalThis.fetch

Returns

Promise<HttpServerCapabilities>