diff --git a/mpp-pay.js b/mpp-pay.js
index 14ca5e1..9d7f4a2 100644
--- a/mpp-pay.js
+++ b/mpp-pay.js
@@ -266,6 +266,7 @@ function createMppPay({ secret, realm, recipient, routes, facilitator, tempo = n
   }
 
   function refuse(req, res, route, reason) {
+    res.locals.mppRefused = String(reason).slice(0, 160); // for the usage log: why the credential was refused
     res.status(402);
     res.set('WWW-Authenticate', headersFor(route));
     res.set('Cache-Control', 'no-store');
diff --git a/usage-log.js b/usage-log.js
index 851adaf..1813e26 100644
--- a/usage-log.js
+++ b/usage-log.js
@@ -264,7 +264,7 @@ function createUsageLog({ service, env = process.env, fetchFn = globalThis.fetch
           ms: Date.now() - started,
           paid: Boolean(payment),
           ...(payment || {}),
-          ...(quote ? { quote: true, ...(offered ? { payment_failed: true, ...(mppOffered ? { protocol: 'mpp' } : {}), ...(attemptedNetwork(req) ? { network: attemptedNetwork(req) } : {}) } : {}) } : {}),
+          ...(quote ? { quote: true, ...(offered ? { payment_failed: true, ...(mppOffered ? { protocol: 'mpp', ...(res.locals && res.locals.mppRefused ? { reason: res.locals.mppRefused } : {}) } : {}), ...(attemptedNetwork(req) ? { network: attemptedNetwork(req) } : {}) } : {}) } : {}),
           agent: agentOf(req.headers['user-agent']),
           visitor: visitorOf(req.ip || (req.socket && req.socket.remoteAddress), visitorSecret),
           input: described.input,