fetch(url: URL) rejects with 'Invalid URL' (cc's MCP HTTP transport passes a URL instance); a DNS failure is a plain Error without node's 'fetch failed' + cause.code

[proggeramlug]

Found through the MCP debug logs that #9491 unlocked (#9500 / #9531): cc's HTTP MCP transport logs

node : HTTP Connection failed after 4ms: fetch failed (code: none, errno: none)   → TypeError: fetch failed
perry: HTTP Connection failed after 1ms: Invalid URL (code: none, errno: none)     → Error: Invalid URL

for https://example.invalid/mcp (the harness's unreachable HTTP server). The SDK's StreamableHTTPClientTransport calls fetch(this._url, init) where _url is the URL instance cc constructs (client/streamableHttp.js:94/311/449).

Minimal repro

const u = new URL("https://example.invalid/mcp");
for (const [label, go] of [
  ["string",               () => fetch("https://example.invalid/mcp", { method: "POST", body: "{}" })],
  ["URL object",           () => fetch(u, { method: "POST", body: "{}" })],
  ["URL object + Headers", () => fetch(u, { method: "POST", headers: new Headers({ "X-A": "1" }), body: "{}" })],
  ["Request object",       () => fetch(new Request(u, { method: "POST", body: "{}" }))],
  ["URL object, GET, signal", () => fetch(u, { signal: AbortSignal.timeout(5000) })],
] as const) {
  try { const r = await go(); console.log(label, "-> status", r.status); }
  catch (e: any) { console.log(label, "->", e?.name, JSON.stringify(e?.message), "cause:", e?.cause?.code ?? "none"); }
}

node 26.5.1: every shape   -> TypeError "fetch failed" cause: ENOTFOUND
perry main : string        -> Error "Fetch error: error sending request for url (https://example.invalid/mcp)" cause: none
             URL object    -> Error "Invalid URL" cause: none          <- the cc shape
             URL + Headers -> Error "Invalid URL" cause: none
             Request       -> Error "Fetch error: error sending request for url (...)" cause: none
             URL, signal   -> Error "Invalid URL" cause: none

Two divergences, in order of weight:

1. fetch(URL) is rejected as Invalid URL — the URL instance is not stringified (the spec's RequestInfo is USVString | Request, and a URL converts through toString(); node, Bun and browsers all accept it). Only the string and Request forms reach the network. This is what cc hits.
2. The DNS-failure error shape: node rejects with TypeError whose message is exactly fetch failed and whose cause carries the node-shaped error (code: 'ENOTFOUND', syscall, hostname); perry rejects with a plain Error("Fetch error: error sending request for url (...)") and no cause. Code that branches on e.cause.code (cc does for its "(code: ..., errno: ...)" log line, retry classifiers do for ENOTFOUND/ECONNREFUSED) sees none.

Byte-compare bar: the fixture above prints node's five lines.

[proggeramlug]

Closing: fixed in #9582 (fetch accepts WHATWG URL instances; transport failures reject with TypeError('fetch failed') and a cause carrying ENOTFOUND/errno/syscall/hostname), landed via merge train #9585 on 2026-09-03. Fixture test_gap_9536_fetch_url_error passes against node.