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

src/audit/logger.ts

@@ -19,6 +19,8 @@ import type {
   SessionCheckpointEvent,
   SessionAutoCompactEvent,
   UserActionEvent,
+  BackendRouteEvent,
+  BackendFallbackEvent,
   CronTriggerEvent,
   WebhookReceiveEvent,
   HeartbeatCycleEvent,
@@ -192,6 +194,16 @@ export class AuditLogger {
     this.write({ level: 'info', event_type: 'user.action', event: event as unknown as Record<string, unknown> });
   }
 
+  backendRoute(event: BackendRouteEvent): void {
+    if (!this.shouldLog('sessions', 'info')) {return;}
+    this.write({ level: 'info', event_type: 'backend.route', event: event as unknown as Record<string, unknown> });
+  }
+
+  backendFallback(event: BackendFallbackEvent): void {
+    if (!this.shouldLog('sessions', 'warn')) {return;}
+    this.write({ level: 'warn', event_type: 'backend.fallback', event: event as unknown as Record<string, unknown> });
+  }
+
   sessionTransfer(from: string, to: string, messageCount: number): void {
     if (!this.shouldLog('sessions', 'debug')) {return;}
     this.write({

src/audit/types.ts

@@ -11,6 +11,7 @@ export type AuditEventType =
   | 'skills.installer.execution_blocked' | 'skills.installer.command_result' | 'skills.registry_install'
   // Session lifecycle
   | 'session.create' | 'session.message' | 'session.delete' | 'session.transfer' | 'session.compact' | 'session.checkpoint' | 'session.auto_compact' | 'user.action'
+  | 'backend.route' | 'backend.fallback'
   // Automation - Cron
   | 'cron.trigger' | 'cron.sent' | 'cron.add' | 'cron.remove'
   // Automation - Webhook
@@ -209,6 +210,23 @@ export interface UserActionEvent {
   command?: string;
 }
 
+export interface BackendRouteEvent {
+  session_id: string;
+  channel: string;
+  sender: string;
+  selected_backend: 'native' | 'claude_code' | 'opencode' | 'codex' | 'gemini';
+  source: 'agent_override' | 'default_external' | 'native';
+}
+
+export interface BackendFallbackEvent {
+  session_id: string;
+  channel: string;
+  sender: string;
+  from_backend: 'claude_code' | 'opencode' | 'codex' | 'gemini';
+  to_backend: 'native';
+  reason: string;
+}
+
 export interface CronTriggerEvent {
   job_name: string;
   schedule: string;

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,
+          });
         }
       }