diff --git a/apps/web/src/app.js b/apps/web/src/app.js
index 623bf1b..dbafba4 100644
--- a/apps/web/src/app.js
+++ b/apps/web/src/app.js
@@ -181,7 +181,8 @@ app.get('/auth/coinpay/callback', async (c) => {
   let pass;
   try {
     ({ subject, pass } = await auth.coinPayIdentityFromCode(query.code, pending.codeVerifier));
-  } catch {
+  } catch (err) {
+    console.error(`[coinpay] code exchange failed: ${err?.message ?? err}`);
     return c.redirect(pending.userId ? '/app/account?coinpay=failed' : signInError('coinpay-failed'), 302);
   }
 
diff --git a/apps/web/src/auth.js b/apps/web/src/auth.js
index 03bb853..bb866d7 100644
--- a/apps/web/src/auth.js
+++ b/apps/web/src/auth.js
@@ -181,7 +181,7 @@ export async function coinPayIdentityFromCode(code, codeVerifier) {
   });
   const tokens = await tokenResponse.json().catch(() => null);
   if (!tokenResponse.ok || typeof tokens?.access_token !== 'string' || !tokens.access_token)
-    throw new Error('CoinPay sign-in failed');
+    throw new Error(`CoinPay sign-in failed: ${tokenResponse.status} ${tokens?.error ?? ''} ${tokens?.error_description ?? ''}`.trim());
 
   const userResponse = await fetch(config.coinpayOAuth.userInfoUrl, {
     headers: { authorization: `Bearer ${tokens.access_token}`, accept: 'application/json' },