feat(audit): log backend routing decisions and fallback events

src/daemon/routing.ts

@@ -919,9 +919,25 @@ export function createMessageRouter(deps: {
       // buildUserMessage() in the agent will create native audio content parts
 
       const requestedBackend = agentConfig?.backend ?? deps.defaultName;
+      const sessionIdForAudit = `${msg.channel}:${msg.senderId}`;
       const selectedBackend = requestedBackend && requestedBackend !== 'native'
         ? deps.externalBackends?.[requestedBackend]
         : undefined;
+      const selectedBackendForAudit: 'native' | ExternalBackendName = selectedBackend && requestedBackend
+        ? requestedBackend
+        : 'native';
+
+      auditLogger?.backendRoute?.({
+        session_id: sessionIdForAudit,
+        channel: msg.channel,
+        sender: msg.senderId,
+        selected_backend: selectedBackendForAudit,
+        source: agentConfig?.backend
+          ? 'agent_override'
+          : selectedBackend
+            ? 'default_external'
+            : 'native',
+      });
 
       if (selectedBackend && (!attachments || attachments.length === 0)) {
         try {
@@ -935,8 +951,18 @@ export function createMessageRouter(deps: {
           await reply({ text: response, replyTo: msg.id });
           return;
         } catch (error) {
-          const message = error instanceof Error ? error.message : String(error);
-          console.warn(`External backend "${selectedBackend.name}" failed, falling back to native: ${message}`);
+          const detail = error instanceof Error ? error.message : String(error);
+          console.warn(`External backend "${selectedBackend.name}" failed, falling back to native: ${detail}`);
+          auditLogger?.backendFallback?.({
+            session_id: sessionIdForAudit,
+            channel: msg.channel,
+            sender: msg.senderId,
+            from_backend: (requestedBackend && requestedBackend !== 'native')
+              ? requestedBackend
+              : (selectedBackend.name as ExternalBackendName),
+            to_backend: 'native',
+            reason: detail,
+          });
         }
       }