diff --git a/src/lib/utils/oidc-discovery-url.ts b/src/lib/utils/oidc-discovery-url.ts
new file mode 100644
index 0000000..2534d3c
--- /dev/null
+++ b/src/lib/utils/oidc-discovery-url.ts
@@ -0,0 +1,41 @@
+// The OIDC discovery URLs to try for an issuer, in order. The spec path is
+// `/.well-known/openid-configuration` WITHOUT a trailing slash, so that's tried first. Some
+// providers (e.g. FortiAuthenticator #1368) 404 the canonical path and only serve the
+// trailing-slash variant, so we fall back to it. A spec-compliant provider answers the first
+// URL and the fallback is never requested.
+export function oidcDiscoveryUrls(issuerUrl: string): string[] {
+	const base = issuerUrl.endsWith('/') ? issuerUrl.slice(0, -1) : issuerUrl;
+	const canonical = `${base}/.well-known/openid-configuration`;
+	return [canonical, `${canonical}/`];
+}
+
+type FetchLike = (url: string) => Promise<{ ok: boolean; statusText: string }>;
+
+/**
+ * Fetch the OIDC discovery document, trying each candidate URL in order and returning the first
+ * OK response. A not-ok (e.g. 404) falls through to the next URL; a NETWORK error is fatal (the
+ * host is unreachable either way) and is rethrown with the issuer + cause surfaced (#1293).
+ * Throws with the last not-ok statusText if every candidate was reachable but rejected.
+ * `fetchFn` is injected so the retry logic is testable without a process-global fetch mock.
+ */
+export async function fetchOidcDiscovery<R extends { ok: boolean; statusText: string }>(
+	urls: string[],
+	fetchFn: (url: string) => Promise<R>
+): Promise<R> {
+	let lastNotOk = '';
+	for (const url of urls) {
+		let r: R;
+		try {
+			r = await fetchFn(url);
+		} catch (err: any) {
+			const cause = err?.cause?.message || err?.cause?.code || err?.cause;
+			throw new Error(
+				`Failed to reach OIDC issuer at ${url}: ${err?.message || err}` +
+				(cause ? ` (${cause})` : '')
+			);
+		}
+		if (r.ok) return r;
+		lastNotOk = r.statusText;
+	}
+	throw new Error(`Failed to fetch OIDC discovery document: ${lastNotOk}`);
+}