Harden wallet mints: browser-side CORS reachability probe

- Add mintReachability.ts: probe a mint's /v1/info from the user's browser to detect CORS-blocked ('cors-blocked') vs down ('unreachable') vs 'ok'
- useMintManager.handleAddMint: probe before adding; reject CORS-blocked/unreachable mints with a clear message instead of letting them fail later on invoice
- useCashu default mint: instead of hardcoding one (unverifiable at build time), probe an ordered list of web-friendly candidates and add the first that's reachable in-browser; minibits excluded (mobile-app only)

Co-authored-by: shakespeare.diy <assistant@shakespeare.diy>

src/hooks/wallet/useCashu.ts

@@ -5,6 +5,7 @@ import { wordlist } from '@scure/bip39/wordlists/english';
 import { initializeCoco, ConsoleLogger, getEncodedToken, type Repositories } from 'coco-cashu-core';
 import { IndexedDbRepositories } from 'coco-cashu-indexeddb';
 import type { Mint } from 'coco-cashu-core';
+import { probeMintReachability } from '@/lib/mintReachability';
 
 // Types from coco-cashu-core
 type CocoManager = Awaited<ReturnType<typeof initializeCoco>>;
@@ -166,22 +167,32 @@ export function useCashu(): UseCashuReturn {
         console.warn('[Cashu] Failed to enable some coco services:', err);
       }
       
-      // Add default mint if no mints exist.
-      // NOTE: the mint MUST send permissive CORS headers, otherwise the browser
-      // blocks every request with a NetworkError ("Failed to fetch mint").
-      // mint.minibits.cash does NOT set Access-Control-Allow-Origin, so it can't
-      // be used directly from a browser — we default to a CORS-enabled mint.
+      // Add a default mint if the user has none yet.
+      //
+      // A mint MUST send permissive CORS headers or the browser blocks every
+      // request with a NetworkError ("Failed to fetch mint"). We can't know from
+      // build time which mint is reachable in THIS browser, so we probe an
+      // ordered list of web-friendly candidates and add the first that works.
+      // (mint.minibits.cash is intentionally excluded — it's mobile-app only.)
       try {
-        const DEFAULT_MINT_URL = 'https://mint.coinos.io';
         const existingMints = await coco.mint.getAllTrustedMints();
-        const hasDefaultMint = existingMints.some((m: Mint) => 
-          m.mintUrl.toLowerCase() === DEFAULT_MINT_URL.toLowerCase()
-        );
-        
-        if (!hasDefaultMint && existingMints.length === 0) {
-          console.log('[Cashu] Adding default mint:', DEFAULT_MINT_URL);
-          await coco.mint.addMint(DEFAULT_MINT_URL, { trusted: true });
-          console.log('[Cashu] Default mint added successfully');
+        if (existingMints.length === 0) {
+          const CANDIDATE_MINTS = [
+            'https://mint.coinos.io',
+            'https://21mint.me',
+            'https://stablenut.cashu.network',
+            'https://mint.lnvoltz.com',
+          ];
+          for (const url of CANDIDATE_MINTS) {
+            const probe = await probeMintReachability(url);
+            if (probe.reachability === 'ok') {
+              console.log('[Cashu] Adding default mint:', url);
+              await coco.mint.addMint(url, { trusted: true });
+              console.log('[Cashu] Default mint added successfully');
+              break;
+            }
+            console.warn('[Cashu] Skipping unreachable default candidate:', url, probe.reachability);
+          }
         }
       } catch (err) {
         console.warn('[Cashu] Failed to add default mint:', err);

src/hooks/wallet/useMintManager.ts

@@ -1,6 +1,7 @@
 import { useState, useCallback } from 'react';
 import type { Manager, Repositories } from 'coco-cashu-core';
 import { useToast } from '@/hooks/useToast';
+import { probeMintReachability } from '@/lib/mintReachability';
 
 export interface CashuMint {
   url: string;
@@ -84,6 +85,20 @@ export function useMintManager({
         throw new Error('This mint is already in your list');
       }
 
+      // Probe reachability from the browser first — a CORS-blocked mint would
+      // otherwise be added but fail on every invoice with a cryptic error.
+      const probe = await probeMintReachability(normalizedUrl);
+      if (probe.reachability === 'cors-blocked') {
+        throw new Error(
+          `This mint can't be used from a web browser — it doesn't allow web requests (no CORS). It may work in a mobile app, but pick a web-friendly mint here.`,
+        );
+      }
+      if (probe.reachability === 'unreachable') {
+        throw new Error(
+          `Couldn't reach this mint. Check the URL and that the mint is online, then try again.`,
+        );
+      }
+
       // Add mint and automatically trust it
       await coco.mint.addMint(normalizedUrl, { trusted: true });
       

src/lib/mintReachability.ts

@@ -0,0 +1,75 @@
+/**
+ * Browser-side Cashu mint reachability check.
+ *
+ * A Cashu mint is only usable from a web wallet if it answers `GET /v1/info`
+ * with permissive CORS headers. Many popular mints (e.g. mint.minibits.cash)
+ * work great in mobile apps but do NOT set `Access-Control-Allow-Origin`, so the
+ * browser blocks them with a `NetworkError` — which otherwise only surfaces
+ * later as a cryptic "Failed to create invoice".
+ *
+ * This probe runs in the USER's browser (where CORS actually applies), so it's
+ * the only reliable place to detect the problem.
+ */
+export type MintReachability = 'ok' | 'cors-blocked' | 'unreachable';
+
+export interface MintProbeResult {
+  reachability: MintReachability;
+  /** Mint name from /v1/info when reachable. */
+  name?: string;
+}
+
+function normalizeMintUrl(url: string): string {
+  let u = url.trim().replace(/\/+$/, '');
+  if (!/^https?:\/\//.test(u)) u = `https://${u}`;
+  return u;
+}
+
+/**
+ * Probe a mint's `/v1/info` from the browser. Distinguishes a CORS block
+ * (fetch rejects with a TypeError and the request never yields a response)
+ * from a genuinely down mint. Best-effort — treats ambiguous failures as
+ * `unreachable` rather than falsely flagging CORS.
+ */
+export async function probeMintReachability(
+  mintUrl: string,
+  timeoutMs = 7000,
+): Promise<MintProbeResult> {
+  const base = normalizeMintUrl(mintUrl);
+  const controller = new AbortController();
+  const timer = setTimeout(() => controller.abort(), timeoutMs);
+
+  try {
+    const res = await fetch(`${base}/v1/info`, {
+      method: 'GET',
+      headers: { Accept: 'application/json' },
+      signal: controller.signal,
+      mode: 'cors',
+    });
+
+    if (!res.ok) {
+      return { reachability: 'unreachable' };
+    }
+
+    let name: string | undefined;
+    try {
+      const info = await res.json();
+      if (info && typeof info.name === 'string') name = info.name;
+    } catch {
+      // info body wasn't JSON — still counts as reachable/CORS-ok
+    }
+    return { reachability: 'ok', name };
+  } catch (err) {
+    // A CORS failure throws a TypeError with a "Failed to fetch"-style message
+    // and no response object. An abort (timeout) is a genuine unreachable.
+    if (controller.signal.aborted) {
+      return { reachability: 'unreachable' };
+    }
+    const msg = err instanceof Error ? err.message : String(err);
+    if (/failed to fetch|networkerror|load failed|cors/i.test(msg)) {
+      return { reachability: 'cors-blocked' };
+    }
+    return { reachability: 'unreachable' };
+  } finally {
+    clearTimeout(timer);
+  }
+}